transdermal drug delivery by passive diffusion and iontophoresis: a review

Transdermal Drug Delivery by Passive Diffusion and Iontophoresis: A Review

S. Singh and J. Singh* Department of Pharmaceutics, lnstitute of Technology, Banaras Hindu University,

Varanasi 221 005, lndia

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Viable Epidermis _ _ . . . . . . , . , . . , . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . .

3. Solubility and Molecular Characteristics of the Drug . . . . . . . . . .

5. Effect of VehiclesiSolvents . . . . . . . . . . . . . . . . . . . . . . 4. Degree of Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. Cutaneous Drug Metabolism . . . . . . . _ . . . . . _ _ . . . . . . . , _ . . . . _ _ . . . . . _ . _ _ _ _ . . . . C . Miscellaneous Factors . . . . . . . . . . . . . . . . . . . . . . .

A. Dimethylsulfoxide . . . . . . . . . . . . . . . . . . . . . . . V. Permeation Enhancers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . VIII. Iontophoresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . D. Factors Affecting Iontophoresis . . . . . . . .

. . . . . . . . . . . .

3. Ionic Strength , . . . . . . . . . . . . . . . . . . . . 4. Concentration . . ......................... . . . . . . . . . . . . . . . . .

6. Resistance of Skin . . . . . . . . . . . . . . . . , . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Torresponding author

570 571 571 572 572 573 573 574 575 577 577 577 577 578 579 581 581 582 581 582 582 583 583 583 585 586 586 588 589 590 590 590 591 592 600 600 601 602 602 602 603

Medicinal Research Reviews, Vol. 13, No. 5, 569-621 (1993) 0 1993 John Wiley 81 Sons, Inc. CCC 0198-6325/931050569-53

570 SINGH AND SINGH

E. In Vi t ro and In Situ Studies . . . . . . . . . . . . . . . . . . . . . . . 1. In Vi t ro Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. In Vitro Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Iontophoretic Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Clinical Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Ear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Dental Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Muscles and Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Skin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

G. Peptides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

meation Enhancers . . . . . . . . . . . . . . . . . . . IX. Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

H. Nonelectrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . Iontophoresis in

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

603 603 605 606 608 609 609 609 609 609 609 610 610 613 613 614 614

I. INTRODUCTION

The penetration of substances through the skin is important from both toxicological and therapeutic viewpoints. The potential of using the intact skin as the port for drug delivery has been recognized for over several decades as evidenced by the development of medicated plasters, ointments, and complex inunctions. Historically, the medicated plasters could be viewed as the first development of human’s idea of transdermal drug delivery. Medi- cated plasters such as salicylic acid plaster have also been used in western medicine for several decades. They are indicated mainly for localized medication in the tissues directly underneath the site of application.’

The transdermal drug-delivery system has several advantages, particularly avoidance of gastrointestinal incompatability and hepatic “first-pass” effect. However, it is also associated with some specific limitations arising mainly from the excellent barrier properties of the stratum corneum, the outermost (dead) layer of the epidermis. The conventional topical delivery systems are therefore restricted to either substances for local effects or to highly potent, small, and lipophilic substances for systemic effects.* It is also difficult to deliver ionic and higher-molecular-weight drugs in therapeutically sufficient amount by conventional system^.^

Dr. Jagdish Singh is Reader in Pharmaceutics, Banaras Hindu University. He earned his B. Pharm. (1977), M . Pharm. (1979), and Ph.D. (1982) degrees from Banaras Hindu Univer- sity. He pursued his postdoctoral research (1985519881 at the University of Otago, New Zea- land, with Professor M . S . Roberts and was visiting scientist in 1990 at the College of Phar- macy, University of Kentucky. His current research interests are transdermal delivery of drugs and peptides using passive and iontophoresis techniques, passive targetting of of drug-loaded biodegradable microspheres to lungs, and controlled release oral products. Dr. Singh has 50 research publications to his credit.

Dr. Sanjay Singh is a research associate in Pharmaceutics at the Institute of Technology, Banaras Hindu University. He received his B . Pharm. (29851, M . Pharm. (1987), and Ph.D. (2992) from Banaras Hindu University. His research interest is in the area of novel drug- delivery systems and he is currently investigating the factors responsible and mechanisms involved in the transport of drugs and peptides through human skin during passive difusion and iontophoresis .

TRANSDERMAL DRUG DELIVERY 571

Iontophoresis has the potential to overcome many of these limitations and could be feasible for several compounds for delivery via the transdermal route in a controlled manner. Iontophoresis is a process which causes an increased penetration of ionized substances into or through a tissue by the application of an electric field.4-7 It is being increasingly investigated as a technique for enhancing the penetration of ionic drugs through the

This review presents the recent research and developments in the transder- ma1 delivery of drugs by passive diffusion and iontophoresis. The factors affecting this mode of drug delivery is one of the aspects of the review. The identification and understanding of these factors is necessary for the development of an efficient transdermal delivery system. The theories explaining the process of passive diffusion and iontophoresis are extensively elaborated in addition to a brief description of skin structures.

11. STRUCTURE OF THE HUMAN SKIN

It is necessary to understand the anatomy, physiological function, physicochemical characteristics, and biochemical properties of the skin to utilize successfully the phenomenon of percutaneous absorption.

The skin of an average adult human covers a surface area of nearly 2 m2 and receives about one-third of the blood circulating through the body.8

Microscopically, the skin is composed of three main histological layers: epidermis, dermis, and subcutaneous tissues1 (Fig. 1). The epidermis is further divided into two parts-the nonviable epidermis or the stratum corneum and the viable epidermis, which includes four other layers of the epidermis, viz., stratum lucidum, stratum granulosum, stratum spinosum, and stratum germinativum.

A. Stratum Corneum or Nonviable Epidermis

The stratum corneum consists of multilayer of horny dead cells which are compacted, flattened, dehydrated, and keratinized. The horny cells are stacked in highly interdigitated columns with 15 to 25 cells in the stack over most of the body.9 It has a density of 1.55 gicc. It is the end product of cell differentiation and comprises the principal skin protective or barrier layer. These cells are physiologically rather inactive and are continuously shed with constant replacement from the underlying viable epidermal tissue.1°

The stratum corneum has a water content of only 20% as compared to 70% in the case of physiologically active stratum germinativum. It exhibits regional differences in thickness over the body. The thickness of nonhydrated stratum corneum over most of the body is approximately 10-50 p,m. However, the thickness may be several hundred micrometers (300-400 pm) on friction surfaces such as the palms of the hand and soles of the feet.9,*1,*2

Keratin, present in the cells of the stratum corneum, is a fibrous protein which is poor in sulphur and forms a filamentous network to assure cohesion, flexibility, and recovery. The unique properties of stability, insolubility, and resistance observed in the stratum corneum are due to the thick cell membrane and cell matrix, which consists of amorphous proteins rich in sulphur content and lipids with many disulphide linkages.I3 The stratum corneum is described as the only rate-limiting barrier of the skin with regard to the viable

572 SINGH AND SINGH

Figure 1. Schematic cross section of the skin.17

epidermis and d e r m i ~ . ~ - * * , ' ~ The stratum corneum is a heterogeneous membrane consisting of alternating lipophilic and hydrophilic layers. Lipophilic layers consist of skin fat and dry keratin, while the hydrophilic property is due to the corneocytes, which are identified as natural moisturizing factors such as aminoacids, sugars, and their derivatives3 As the skin consists of a heterogeneous mixture of proteins, it can be charged in either direction because of their amphoteric character^.'^ The pH of the skin surface is between 3 and 4, which is about the isoelectric point of keratin in the stratum corneum layer. l6

B. Viable Epidermis

The viable epidermis is an aqueous solution of protein encapsulated into cellular compartments by thin cell membranes which are fused together by tonofibrils. The viable epidermis has a density near that of water.l7 The germinal (proliferative) layer above the dermis undergoes cell divisions produc- ing an outward displacement of the cell towards the surface. As the germinal layer moves upwards, it changes its shape into a more rounded form with spiny projections and appears as stratum spinosum. After the germinal layer has risen 12-15 layers above its point of origin, it becomes flattened and the


basophilic nuclear material is dispersed throughout the cell, as granules. This layer is referred to as stratum granulosum. 18-21

The stratum lucidum layer, which lies just below the stratum corneum, is the site of an increase in the stage of nuclei disintegration, keratinization, and sulphahydryl-rich matrix formation. It moves upwards to eventually form the stratum ~ o r n e u m . ~ ~ , ~ ~

It should be pointed out that the epidermis contains no vascular elements. The cells receive their nourishment from the capillary beds located in the papillary layers of the dermis by diffusion of plasma and serum compo- nentsZ1

C. Dermis

The dermis is 0.2-0.3 cm thick and is made of a fibrous protein matrix, mainly collagen, elastin, and reticulum, embedded in an amorphous colloidal ground substance (semigel matrix of micropolysaccharides). It is divided into two distinct zones: a superficial finely structured thin papillary layer adjacent to the epidermis and a deeper coarse recticular layer (the main structural layer of the skin).17 The dermis is also the locus of blood vessels, sensory nerves (pressure, temperature, and pain), and lymphatics. It also contains the inner segments of the sweat glands and pilosebaceous units. The blood vessels supply blood to the hair follicles, the glandular skin appendages, and the subcutaneous fat, as well as the dermis itself.24 It protects the body from injury, provides flexibility with strength, serves as a barrier to infection, and functions as a water-storage organ.

D. Subcutaneous Fatty Tissue

Cushioning the epidermis and dermis is the subcutaneous tissue or fat layer where fat is manufactured and stored. It acts as a heat insulator and a shock absorber. l8 It essentially has no effect on the percutaneous absorption of drugs because it lies below the vascular system.

E. Skin Appendages

The skin surface has several types of appendages. These include hair follicles with sebaceous glands, eccrine and apocrine sweat glands, and the nails.17 An average human skin surface is known to contain, on the average, 40-70 hair follicles and 200-250 sweat ducts per square centimeter of the skin. These skin appendages occupy only 0.1% of the total human skin surface.ls The eccrine sweat glands (2-5 million) produce sweat (pH 4.0-6.8) and may also secrete drugs, proteins, or antibodies. Their principal function is to aid heat control. 25 Approximately 400 glands per square centimeter are particularly concentrated in the palms and soles.24

The apocrine sweat glands develop at the pilosebaceous follicle to provide the characteristic adult distribution in the armpit (axilla), the breast areola, and the perianal region. The milky or oily secretions may be colored and contain proteins, lipids, lipoproteins, and saccharides. Surface bacteria me- tabolize this odorless liquid to produce the characteristic body

Hair follicles develop over all skin except the red part of the lips, the palms

574 SINGH AND SINGH

and soles, and parts of the sex organs. The hair (shaft) originates from the hair follicle housed beneath the surface in the epidermis. The follicles consist of concentric layers of cellular and noncellular components and are placed in the skin at an angle. The hair shaft is formed by a process of cellular division and migration of cells similar to that which forms the stratum corneum. Hairs are thus formed of keratinized cells compacted together into plates or ~ca1es. l~

Sebaceous glands are most numerous and largest on the face, forehead, in the ear, on the midline of the back, and on anogenital surfaces. The palms and soles usually lack them.25 The glands vary in size from 200 to 2000 Fm in diameter.24 The larger ones are found on the nose. They secrete an oily material known as sebum from cell disintegration. Its principal components are glycerides, free fatty acids, cholesterol, cholesterol esters, wax esters, and ~ e q u a l e n e . ~ ~ It acts as a skin lubricant and a source of stratum corneum plasticizing lipid and maintains an acidic condition on the skin's outer surface (pH 5).17

111. PERCUTANEOUS ABSORPTION

The process of percutaneous absorption can be defined as the mass movement of substance(s) from the skin surface to the general circulation. It includes penetration through the stratum corneum, diffusion through each layer of the skin, uptake by the capillary network at the dermo-epidermal junction, and finally transportation to the target tissues to achieve therapeutic action (Fig. 2).

The percutaneous absorption can occur through two different routes, viz., transfollicular (via hair follicles, sweat ducts, and secretory glands) and transepidermal (across the stratum corneum intracellularly and intercellularly) (Fig. 3). A circuit diagram (Fig. 4) can be used to explain the overall phenomenon of percutaneous absorption by recognizing that both the resistance in series and the diffusional currents in parallel are additive, just as they are in electrical circuits. The transepidermal and transfollicular routes are in parallel

S t r a t u m Papll iary Blood C or n e um Layer Circulation

I I

I I

I I I

D r u g in Delivery Systems ,

I I Capil lary I

' +Systemic T a r g e t Dist r ibut ion T i s s u e s

Transdermal D r u g i n Skin's S e c r e t i o n

~ T a r g e t T i s s u e s

D e r m i s Subdermal T issue Muscle

Figure 2. Schematic illustration of drug absorption across the skin tissues for localized therapeutic actions or for systemic medications.'


TRANSEPI D E R M AL I ROUTE

TRANSCELLULAR ROUTE

TRANSFOLLIC ULAR ROUTE

A N 0 SEBACEOUS GLANDS

PARTITIONING INTO SEBUM - CELLULAR MASSOF

FIBROUS MASS O F

Figure 3. Schematic representation of percutaneous absorption

and electric current (or a drug) will flow through the pathway of least resistance, with the overall driving force being the concentration gradient across the skin (Ac). The relative affinity of these routes for a particular drug, the fractional surface area of each of the routes, and the ease of diffusion through the respective phases will also determine the path of the drug through the

The permeation of drugs through biological or synthetic membranes may occur due to passive, active, or facilitated t r a n ~ p o r t . ' ~ , ~ ~ The occurrence of percutaneous absorption through human or animal skin is attributed to the passive diffusion of the drug from a carrier or a vehicle on the surface tissues of the skin to reach the systemic c i r ~ u l a t i o n . ~ , ~ ~

A topical product may be applied to the skin for a local effect or a systemic effect (Figs. 2 and 5). Hydrocortisone was used in dermatitis for localised action.33 The systemic effect via transdermal administration was reported by M a ~ h t , ~ ~ and later Alza Corporation developed transdermal therapeutic systems containing each of scopolamine, nitroglycerine, isosorbide dinitrate, clonidine, and estradiol for systemic e f f e ~ t . ~ ~ - ~ O

skin. 19,26-30

IV. FACTORS AFFECTING PASSIVE DIFFUSION

Passive diffusion of the drug through the skin is affected by physicochemical properties of the drug and physiologic and pathophysiologic conditions of the skin.

576 SINGH AND SINGH

Vehicle rrrirtonce

Viablr tirrur

Figure 4. Circuit diagram to explain the phenomenon of routes of skin penetrati0n.1~

SKIN SITE

n RATE LlMlTlNO PROCESSES

viable cpidcrmis-+ Diffusion

1 Blood supply and diffusion

Metabolism and excretion -

DISEASE EXAMPLES

Tinro,corns and COIIUWS

Psoriasis eczema and molipnancy

Urticaria

Hyprrtcnsion Angina Nausro

EFFECTS

locol

local

local

systrmic

Figure 5. Rate-limiting steps in the absorption of drugs and sites of action required for topical medication.75


A. Physicochemical Factors

The rate of transport through a transepidermal or transfollicular route can be influenced by altering various physicochemical factors which are related to the administered drug or barrier function of the stratum ~ o r n e u m . ~ ~

1. Drug Concentration

The amount of drug percutaneously absorbed per unit surface area increases as the concentration of the drug in the vehicle is i n ~ r e a s e d . ~ ~ , ~ ~ The amount of the drug absorbed at a constant drug concentration is greater when it is applied to a larger surface area.44-46 However, a few compounds have produced significant decrease in absorption rates with increasing concentra- t i o n ~ . ~ ~ , ~ ~

2 . SkinIDruglVehicle lnferactions

The interaction between drugiskin, vehiclelskin, and druglvehicle influ- ences the rate of the drug penetration through the skin.18,4x,49 Many substances interact with skin to either produce a strong chemical bond or Van der Waals type of attraction. These bonds could damage the skin or strongly bind the drug to the

A drug/skin interaction may result in the increase or decrease of the penetration rate of the drug, e.g., the vasoconstrictor effect of steroids not only slow their own penetration but might also affect the absorption of other concomitantly applied drug.^.^^,^^

Vehiclelskin interaction may affect the skin permeability, which depends upon the hydration state of the stratum corneum, temperature of the skin andlor nature of the vehicle being ~ s e d . ~ ~ , ~ ~ , ~ ~

A drug/vehicle interaction can result in a slow diffusion of the drug from the vehicle onto the skin surface and this will affect the rate of diffusion of the drug across the stratum corneum. The release rate of a drug from a vehicle can be manipulated by changing the drug concentration, varying the diffusion coefficient and increasing the solubility of the drug in the ~ e h i c l e . ~ ~ , ~ ~ , ~ ~

3 . Solubility and Molecular Characteristics of the Drug

The solubility characteristics of the drug greatly influence its ability to penetrate biological membranes. The Meyer-Overton theory of absorption states that the substances soluble in lipids pass through the cell membrane owing to its lipid content while water-soluble substances pass after the hydration of the protein portion of the cell wall, which leaves the cell permeable to water- soluble substances. 20,46,57

The aqueous solubility of a drug and the partition coefficient strongly influence the rate of transport across the absorption site.44-46,58 The importance of relative solubility of the drug in the skin, that is, the stratum corneum/vehicle partition coefficient, has been illustrated by several worker^.^^,^^-^^ It has been demonstrated that the drug solubilization in the vehicle, increased partitioning, increased solvent penetration, and barrier disruption can contribute to increased skin permeation rates in the presence of fatty acids and amines.62

The size and shape of a molecule may also be a significant factor and an

578 SINGH AND SINGH

inverse relationship may exist between the flux and the molecular weight of a substance.46 The small molecules penetrate more rapidly than large molecules.60 However, the molecular size of a solute of molecular weight up to 500 (probably up to 5000) daltons plays no role in the rate of percutaneous absorption of drugs.3

S ~ h e u p l e i n ~ ~ has estimated the diffusion coefficients by factoring out the stratum corneum/vehicle partition coefficient and suggests that the diffusion coefficient is a strong function of molecular size. Very subtle structural modifications such as the position of the hydroxy group on benzoic acid can have profound effects. For example, the p-isomer is just as soluble in propylene glycol as the o-isomer but the cutaneous transport of the p-isomer is an order of magnitude lower.64 An increase in the hydrophilicity of progesterone by incorporating one or more hydroxy substituents at different positions on the steroidal skeleton resulted in the decrease in its solubility in the stratum corneum.61 The role of molecular structure in determining cutaneous transport is one of the most intriguing problems in the area of transdermal drug delivery. There is a great need to modify the molecular structure to control the cutaneous transport with the goal of enhancing the transport of drugs and also for decreasing the absorption of toxic agents that contact the skin.65

4. Degree of Ionization

The unionized moiety can permeate the lipid membrane easily according to pH-partition hypothesis.66-68 The theory hypothesis66 is based on the assumption that the predominantly lipophilic nature of biological membranes, for example, the stratum corneum and the gastrointestinal tract, would act as barriers to the ionized species. However, it has been suggested that ionized species can permeate the lipid-membrane-like gastrointestinal tract through the pore^.^^,^^) A physical model is represented for the transport of a weak electrolyte across the hairless mouse skin7' (Fig. 6). The stratum corneum is modeled as parallel lipoidal and aqueous pore pathways for diffusion and is in series with a porous matrix representing the dermis-epidermis. It is assumed that the undissociated species only penetrate the lipid phase, while

PHYSICAL MODEL OF THE SKIN

DONOR CHAMBER

LIPID POROUS MATRIX

PORES-

LIPID

RECEIVER CHAMBER

*STRATUM CORNEUM+-DERMIS- EPIDERMIS-

Figure 6. Schematic diagram of the physical model for the diffusion of charged and uncharged species across the skin.71


both the charged and uncharged species. penetrate the pore route. On the other hand, the model proposed by Wagner and Sedman72 takes into consideration the partitioning of ionic species into the membrane and does not incorporate the concept of transport through the pores. However, a number of in uitro studies (with isolated intestinal membrane and excised skin) have shown that both ionized and unionized species of a drug can permeate a lipid

The transdermal delivery of narcotic analgesics, fentanyl and sufentanil, at pH 3 and 9 (at pH 3, drug exists as protonated species and at pH 9 as a free- base form) showed that the contributions of ionized forms to the total permeabilities of these drugs are so small that one can safely say that the free- base forms are almost entirely responsible for their permeation through the skin. Free-base forms of fentanyl and sufentanil are 218 and 100 times more permeable than their ionized forms, respectively. Data also suggest that the polar pathway is also available for the permeation of ions, albeit to a far lesser extent. 76

The transdermal flux of vasopressin (VP) is significantly higher at pH 5.0 (exists in ionized form) than at isoelectric pH 10.9.77 This anomaly may be due to that the VP is forming aggregates at pH values close to its isoelectric point. The 1-octanollbuffer partition also decreases with increasing pH and is significantly different at each pH.

Parry et al . 78 showed theoretically (mathematical model) and experimen- tally that only the undissociated species of benzoic acid partitions into and permeates across the skin.

Kuo et a1.80 found that permeabilities of the protonated form of oxycodone through intact skin of all the animal models used was about 7-15-fold lower than that of the nonionic form. The effect of pH on the percutaneous absorption of indomethacin was studied and found to increase with decreasing P H . ~ '

The flux of verapamil and nicotine across an artificial membrane as a function of pH of the reservoir was studied and found to be related with the solubility which changed as a function of pH.82 It is recognized, however, that the individual ions (sodium and potassium) and electrolytes (sodium chloride) can be easily absorbed through the human and animal skins.3,83-85 It is suggested that the ionized compounds may be absorbed from the lipid membrane by an ion-pair mechanism69,70,74,75,86,87 and an ion-exchange mechanism. 75,88,89

membrane. 68,70,73-75

5. Efect of VehicleslSolvents The primary requirement for topical therapy is that a drug incorporated in

a vehicle reaches the skin surface at an adequate rate and in sufficient amounts. Vehicles may increase penetration of the drug by including agents in the vehicle that affect the barrier function of the epidermis so as to promote penetration of therapeutic and alter the physical characteristics of the vehicle affecting diffusion of the drug from the vehicle into the ~ k i n . ~ ~ , 9 3 Therefore the choice of vehicle is an important factor in increasing the transport of the drug across the biological membrane. For example, the combination of propylene glycol and oleic acid increased the penetration of salicylic acid compared with each vehicle alone.94 Also, propylene glycol promoted the effect of 1-dodecylazacycloheptan-2-one on the cutaneous per-

580 SINGH AND SINGH

meation of metronidazole. Bronaugh and F r a n ~ ~ ~ measured the percutaneous absorption of benzoic acid, caffeine, and testosterone through human skin in three vehicles: petrolatum, ethylene glycol gel, and water gel. Caffeine pene- trated most readily from a petrolatum vehicle, and the greatest testosterone absorption was from a water gel. Shahi and Z a t ~ ~ ~ reported the flux of hydrocortisone from solutions containing propylene glycol, 2-propanol, and a poly- sorbate 80/2-propanol combination. Solutions containing 2-propanol gave flux higher than those obtained from propylene glycol solutions. Ethanol increased the nitroglycerine flux 5-10-fold under optimal condition^.^^

An aqueous gel, prepared with hydrogenated soya phospholipid, increased the in uitro transport of indomethacin across rat dorsal skin. The addition of various alkanols further accelerated the transport, with an increasing effect as the chain length of the alkanol increased. The addition of urea alone did not significantly affect the transport of indomethacin, but when it is included in an aqueous gel containing an alkanol, such as l-octanol, 1- decanol, or l-dodecanol, markedly accelerated the transport of indomethacin.99 The penetration enhancement of indomethacin by a series of primary alcohols with alkyl chains of C4 to C16 was evaluated through shed skin of black rat snake. A parabolic relationship between the enhancement and the carbon atom number of the alcohols was established with tridecanol showing the maximum effect.'OO

Ghanem et u Z . ~ O ' investigated the influence of ethanol on the transport behaviour of P-estradiol and other permeants in hairless mouse skin over a 0- 100% ethanol/saline concentration range. At high ethanol levels (>50%), there was a significant increase in the new pore formation in the stratum corneum component of the skin. With pure ethanol, pore pathway transport dominated the permeation for all solutes, irrespective of polarity. At low ethanol levels (<25%), ethanol had little or no effect on the pore pathway.

Lippold and Hackemuller102 treated flexor forearms of volunteers with 10% skin moisturing solutions, e.g., N-hydroxyethylactamide (OH-Lac), sodium lactate (Na-Lac), sodium pyroglutaminate (Na-PAC), sorbital, and urea and also with 10% of the hydrophilic penetration enhancer such as propylene glycol (P-glyc) and water, respectively. Significant differences in the biological activity of benzyl nicotinate were observed after its application as petrolatum ointment at different concentrations on the pretreated skin. The resulting concentration response curves demonstrate significant penetration decelera- tion with all moisturizers except urea. They suggested that the moisturizers compete for water of the keratin in the corneocytes and of the polar regions in the lipid bilayers, and thus decreases the fluidity of the lipids. As a conse- quence, the resistance of both the intracellular and intercellular penetration pathway is increased.

The ethanol water system have been found to enhance the permeation of ionic solutes through human stratum corneum. lo3 The increased skin permeation of the ionic permeant by the ethanol-water systems may be associated with alteration involving the polar pathway. Polar pathway alterations may occur in either or both the lipid polar head and proteinaceous regions of the stratum corneum. Ion-pair formation may also contribute to increased permeation. Ethanol has been used to increase the transdermal flux of rela- tively lipophilic drugs by other workers also. 101,104,105

The effects of vehicle and percutaneous penetration enhancer on the pen-


etration of acyclovir through excised hairless mouse and rat skins were investigated. lo6 Four solvents-propylene glycol, ethanol, isopropanol, and isopropyl myristate-were employed as vehicles, in combination with four different permeation enhancers. The combination of hydrophilic vehicle and hydrophobic enhancer resulted in a greater enhancing effect. Lauric acid and lauryl alcohol in isopropanol, polyethylene glycol 400, and mineral oil vehicles were not as effective in promoting naloxone skin penetration as when dissolved in propylene glycol. Sodium lauryl sulfate in propylene glycol slightly increased the flux, but a much greater effect was observed using a mineral oil vehicle.lo7

The penetration enhancement of diclofenac sodium salt by C8-Cl4 alcohols was evaluated by steady-state flux of diclofenac through rat abdominal skin. Decanol showed the greatest effect in this series. A more remarkable enhancing effect of the alcohols was observed for diclofenac sodium than diclofenac. log A series of binary vehicles was used to deliver physostigmine across dermatomed human skin. The vehicle consisted of isopropyl myristate and isopropyl alcohol mixed in various volume fractions. Optimized percutaneous delivery of physostigmine is possible by thermodynamic control of the penetration process.lo9

The penetration-enhancing effect of ethylacetate with or without ethanol as a cosolvent was evaluated in vitro on rat, hairless mouse, hairless guinea pig, and human cadaver skin using the contraceptive drug levonorgestrol. The steady-state flux of levonorgestrol was increased about sevenfold when neat ethylacetate was used in place of ethanol as donor vehicle.*1°

B. Physiological and Pathological Conditions of Skin

1. Degree of Skin Hydration

It is generally accepted that hydration of the skin increases the rate of penetration of any drug; however, the physicochemical properties of the drug would determine magnitude of its penetration enhancement. The degree of hydration of the stratum corneum and its importance in increasing the percutaneous absorption of major topical drugs has been investigated by several ~ o r k e r ~ . 3 , ~ ~ , ~ ~ , ~ ~ , ~ ~ - ~ ~ , ~ ~ , ~ ~ ~ Wiedmann112 suggested that there is an increase in the effective diffusion coefficient with an increase in the water content. With higher water content, there is an apparent increase in the dynamic motion of the epidermal tissue and this increase the effective diffusion coefficient of water. A change in the mobility of the skin constituents would also be expected similarly to affect the diffusion coefficients of solutes in the skin. The deterioration of the permeation barrier with increasing time of hydration was a very significant factor in the increasing permeation pattern of urea and water. 113

2 . Skin Temperature

Generally, an increase in temperature results in increase in the in vitro rate of permeation of the substance^.^^,^^^-^^^ Blank and Scheuplein19 observed little alterations of the permeability of the barrier after exposure for several hours to temperatures as high as 60 "C. Temperature studies in vitro on nude mouse skin indicated that a complete deterioration of the barrier to urea and

582 SINGH AND SINGH

water permeation occurred after 48 h (irrespective of temperature), with the most pronounced effect at 5O"C.ll3 Allenby et a1.120,121 showed an abrupt and irreversible structural changes in the excised stratum corneum when exposed to temperatures above 60 "C.

3 . Skin Age and Regional Sites

Percutaneous absorption of topical steroids occurs more readily in children than in adults due to the greater water content of infant than adult skin.46 Moisture content of aged skin (>40 years) has lower water content than younger skin. However, Roy and F l ~ n n ~ ~ demonstrated that the age was not a flux determining variable for fentanyl and sufentanil.

There are wide variations in the absorption rate of a given substance through the same skin site of the different individuals. For example, Mar- zullilZ3 found that the absorption from the most permeable region (post au- ricular) in some persons was similar to the least permeable (planter) in others. Roberts et al.lZ4 showed that the skin permeability for methyl salicylates in vivo from different areas of human body was in the rank order: abdomen > forearm > instep > heel > planter for all subjects. Similar results were observed by other workers als0.76,125

4. Species Variation

The species difference in skin permeability (K,) of nicorandil was determined by using excised skin sample from hairless mouse, hairless rat, guinea pig, dog, pig, and human. The permeability of nicorandil in hairless mice was the greatest among the six species, and those in pigs and humans were in good agreement. This may be due to differences in skin surface lipids in each species. Pig and human skins had similar surface lipids, barrier thickness, and morphological aspects. Thus pig skin samples would be useful for the estima- tion of in vitro human skin permeation behavior.lZ9 Roberts and M ~ e l l e r ~ ~ " also concluded that Yucatan pig skin appears to be a suitable model of human skin for in vitro permeation testing of transdermal nitroglycerine systems than hairless mouse skin. Bartek et a1.lZ6 found that absorption through the skin of miniature swine closely resembled that of human skin. However, Wester and

have recommended Rhesus monkey as a model whose permeability approximates man. Roy and F l ~ n n ~ ~ found no trend in the permeation of fentanyl and sufentanil as a function of gender and race.

5. TraumaticlPathological Injuries to Skin

Injuries that disrupt the continuity of stratum corneum increase the permeability due to the removal of barrier layer as well as increased vasodila- tion."' The absorption of polar solutes through the skin that has a physically disrupted stratum corneum increases but that of hydrophobic solutes is little affected.17 Even when the skin is not "broken," the skin's permeability changes in response to irritation and mild trauma. Chemical burns caused by saturated aqueous phenol, corrosive acids, and strong alkali increase the permeability of the skin.ll*

The mechanical disruption of the stratum corneum, as in cuts and abra- sions, also induces a high-permeability state in the skin proportional to the


extent of damage. Eruption of skin in disease has a similar effect to the extent that the stratum corneum's integrity is lost.I7

6 . Cutuneous Drug Metabolism

Catabolic enzymes present in the viable epidermis may render a drug inactive by metabolism and thus affect the topical bioavailability of the drug.'I1

Choi et ul.130 investigated the effects of the nonionic surfactants, n-decyl- methyl sulfoxide, pH, and inhibitors on the metabolism and the permeation of amino acids, dipeptides, and the pentapeptide enkephalin through hairless mouse skins. The results clearly show that hairless mouse skin has significant proteolytic enzyme activity and that the coadministration of inhibitors, permeation enhancer, and pH adjustment can increase the transder- ma1 flux.

C. Miscellaneous Factors

Other factors, such as particle size of the suspended drug, viscosity, surface tension, volatility of the vehicles, and polymorphism of certain drugs, have also been reported to affect their percutaneous ab~orpt ion.~

V. PERMEATION ENHANCERS

The substances that increase the permeability of the skin without severe irritation or damage to its structure and reversibily remove the barrier resistance of the stratum corneum are known as permeation e n h a n ~ e r s . l ~ ~ , ~ 3 ~ They are also termed as penetration enhancers, sorption promoters or acceler- ants. The following properties should be elucidated by an ideal penetration enhance1-2~~~~:

(a) It should not elicit any pharmacological response. (b) It should be specific in its action. (c) It should act immediately with a predictable duration, and its action

(d) It should be chemically and physically stable and compatible with all the

(e) It should be odorless, colorless, and tasteless. ( f ) It should be nontoxic, nonallergenic, and a nonirritant. It is difficult to find a substance with all the above attributes and therefore

compromises have to be made. The most widespread and ideal permeation enhancer is water. In nearly all instances, the hydrated skin is more permeable than the dry skin, e.g., an enhanced penetration of corticosteroids through occluded than nonoccluded skin.54

Hori et ~ 1 . ~ 3 ~ proposed a classification of chemicals using a conceptional diagram to estimate their potential as enhancers. They calculated the organic and inorganic values for chemicals reported to enhance percutaneous penetration (Table I) and depicted the location of these cutaneous enhancers by plotting the organic value against the inorganic value. The enhancers were located in two different areas on the diagram. Area I includes ethanol, propylene glycol, N-methylpyrrolidone, and dimethylsulfoxide. Area I1 includes l-dodecylazacycloheptan-2-one, oleic acid, and lauryl alcohol (Fig. 7). The different locations suggest that the chemicals in the two groups may have different physicochemical properties.

should be reversible.

components of the formulation.

584 SINGH AND SINGH

TABLE I Organic and Inorganic Values of Percutaneous Penetration Enhancers

(Ref. 133)

Enhancer Organic Inorganic

1. Water 0 100 2. Ethanol 40 100 3. Propylene glycol 60 200 4. N, N-Dimethylacetamide 80 200 5. N, N-Dimethylforamide 60 200 6. 2-pyrrolidone 80 145 7 . N-Methyl pyrrolidone 100 145 8. 5-Methyl-2-pyrrolidone 100 145 9. 1,15-Dimethyl-2-pyrrolidone 120 145

10. I-Ethyl-2-pyrrolidone 120 145 11. 2-Pyrrolidone-5-carboxylic acid 100 295 12. Dimethylsulfoxide 80 140 13. Oleic acid 360 152 14. 1-Dodecylazacycloheptan-2-one 360 145 15. N,N-Dimethyl-m-toluamide 240 215 16. n-Decylmethylsulfoxide 260 140

18. Lauric acid 240 150 19. Isopropyl myristate 330 60

17. Lauryl alcohol 240 100

Dimethylsulfoxide (DMSO), N, N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), and azone are the organic solvents which have been reported to increase the permeability of a number of drugs by alteration of the barrier resistance of the stratum corneum. In some cases, these changes in the barrier resistance are found to be of short duration, with the stratum corneum returning to its normal barrier function within a few hours. 131,134-136

The permeation enhancers, e.g., DMSO, DMA, and azone, were found to be superior vehicles for increasing the penetration of steroids and nonsteroids in comparison with the other vehicles such as propylene glycol, polyethylene

600

5 00

z x L O O

300

P ," 200 I+

1 0 0

0 0 100 2 0 0 300 400 500 600

Organic a x i s

Figure 7. Location of percutaneous penetration enhancers on an organic diagram. Closed squares indicate two enhancers. Enhancers are shown in Table 1.133


glycol, ethanol, benzene, and cream base. Studies on the ability of selected agent to increase the transepidermal water loss showed DMSO to be most effective, followed by DMF and DMA.91,134,137,138 In addition to DMSO, DMA, DMF, and azone, other agents such as benzene, ether, isopropanol, xylene, urea, methanol, chloroform, propylene glycol, polyethylene glycol, and phenol have also been reported to increase percutaneous absorption of a variety of drugs. 139

A. Dimethylsulfoxide

This is a colorless liquid with excellent solvent properties. It is miscible with water and many organic liquids. The versatility of dimethylsulfoxide (DMSO) is exemplified by its ability to accelerate the penetration of a wide range of compounds such as griseofulvin and hydroc~rtisone,~~,~~,~~~ phenylbuta- zone,131 s ~ o p o l a m i n e , ~ ~ ~ , ~ ~ ~ salicylic acid and salicylates, 143,144 fluocinolone acetonide and steroids,90 antibiotic^,^^^,^^^ and other solutes.146,147

DMSO functions as a penetrant rather as vehicle. 147,148 Several mechanisms have been advanced for the enhancement of dimethylsulfoxide on skin permeability, including elution of stratum corneum lipids, 120,149 denaturation of stratum corneum structural proteins (keratin),150-152 and delamination of the horny layer by stress resulting from cross currents of highly water-interactive dimethylsulfoxide and water.141J53 DMSO replaces integral water within the stratum corneum to form a continuous network through the skin and sub- stitute a looser s t r u ~ t u r e . ~ ~ , ~ ~ ~ It also forms a reservoir of the penetrant in the stratum ~ o r n e u m . ~ ~ It functions as a swelling agent,150 which may induce the formation of channels within the matrix of stratum corneum and thus favors the passage of various compounds or lowers the diffusional resistance of the stratum ~ o r n e u m . ~ ~ ~

DMSO is able to extract soluble components from the stratum corneum, suggesting the possibility of ultrastructural modifications consistent with an increase in the ~ermeabi1ity.l~~ Treatment of the horny layer with DMSO solutions of 60% and above promote significantly the steady-state drug permeation through the skin and lowered the lipid transitions. However, DMSO interacted with stratum corneum protein even down to the 20% l e ~ e 1 . l ~ ~ In suitable circumstances, DMSO may operate via both mechanisms, that is, reducing the skin's resistance and aiding drug p a r t i t i ~ n i n g l ~ ~ (Fig. 8).

When the DMSO was placed in contact with the stratum corneum surface of the skin, accelerating effects were noted at its concentrations <50%. The analysis of extracts of the stratum corneum indicated that the barrier impair- ment is due in part to elution of DMSO soluble components from the horny structure. The ability of DMSO to elute material from the stratum corneum increases when its concentration is greater than 50%. The neat DMSO resulted in 18% weight loss in the stratum ~ 0 r n e u m . l ~ ~

Pfiborsky and M i i h l b a ~ h o v a ~ ~ ~ found that the DMSO (5%) was least potent and the maximum efficacy was observed with sodium lauryl sulphate in the rat experiments while in man the results were approximately equal when using any of the studied enhancers.

At high concentrations, DMSO can produce erythma and wheals131 and 100% DMSO caused severe skin irritation in nude mouse skin.155 Another side effect of DMSO is caused by its metabolite dimethylsulfide, which produces a

586 SINGH AND SINGH

Kerutin in c e l l : hydrogen Expanded tell containing bondinq groups hinder O M S O competes with drug mot ion drug f o r bonding sites

more-permeable environment

P A R T I A L L Y H Y D R A T E D OMSO- T R E A T E O T ISSUE T ISSUE

Figure 8. Effect of dimethylsulfoxide on the permeability of human stratum corneum-interaction with intercellular lipids and keratin.132

characteristic foul taste and bad breath.l3I DMSO also showed in vitro the platelet aggregation activity. 156

DMSO is thus not widely used, although it is employed as a solvent for idoxuridine in the treatment of herpes zoster and herpes simplex skin infec- t ion~. '~ '

B. Dimethylformamide and Dimethylacetamide

Dimethylacetamide (DMA) and dimethylformamide (DMF) are chemically related to DMSO. They also enhance cutaneous penetrati0n~ls~3~ and retain agents in the stratum corneum reservoir57 but to a lesser degree than DMS0.91 Like DMSO, these enhancers are polar solvents which mix exothermally with water in all proportions. At low concentrations, they appeared to partition preferentially into the keratin regions. At high concentrations, their mode of action is to provide a substantial solvation shell around the polar head groups of the lipid and thus loosen lipid packing. The drug mobility within this region would then increase.132 These penetration enhancers have been shown to be effective for both griseofulvin and hydrocortisone. 13'

C. Azone

Stoughton and M ~ C l u r e l ~ ~ discussed azone (l-dodecylazacycloheptan-2- one) as a new nontoxic enhancer for percutaneous absorption. Penetration of


both hydrophobic and hydrophilic molecules is enhanced although more enhancements are usually seen with hydrophilic drug. They reported the structural properties and toxicity of azone and compared its effect with other percutaneous absorption enhancers.

Azone is a colorless, odorless, and chemically inert oily liquid insoluble in water but freely soluble in many organic solvents. The toxicity is very low and comparable to that observed with nutritional compounds. 136 Ten-percent azone caused severe skin irritation in nude mouse skin.155

Azone enhanced the permeability of 5 - f l ~ o r o u r a c i l . ~ ~ ~ - ~ ~ ~ Banerjee and RitschePO found that the vasopressin permeability in skin was enhanced in vifro and in vivo when azone was used up to 3% v/v. Sheth e f a1.l6' reported the accelerating influence of azone, propylene glycol, and polyethylene glycol on the in vitro skin permeation of trifluorothimidine. Azone increased the flux of naloxone-HCL approximately 15-f0ld.~O~ Azone (5%) in propylene glycol increased 43 times the flux of diazepam and 86 times of midazolam maleate.162 It is also found to enhance the permeability of triamcinolone acetonid 2-5 times.163 Okamoto et ~ 1 . l ~ ~ investigated the penetration enhancing effect of azone and two other azacycloheptan-2-one derivatives on seven penetrants having a wide range of n-octanol-water partition coefficients. The penetration of the drugs from water vehicle (aqueous system) and ethanol vehicle (eth- anolic system) through excised guinea pig skin was increased by pretreatment with the enhancers. The influence of the penetration enhancers azone (5% v/v) and lauryl alcohol (5% v/v) on the transport of acetaminophen and ibuprofen through shed snake (Elaphe obsoleta obsolefa) skin has been investigated i n vifro. 165 The addition of either enhancer increased the transport of acetaminophen. In contrast, only lauryl alcohol increased the transport of ibuprofen. Azone provided no significant enhancement. Infinite dose studies conducted in side-by-side diffusion cells showed that the mechanism of enhancement was an increase in partition coefficient with no significant increase in the diffusion coefficient. Several other studies on the penetration enhancing effects of azone were performed giving promising results for a number of therapeutic

Azone probably increases the skin permeability by a different mechanism to that proposed for molecules such as DMSO. Azone showed no protein interaction, suggesting that it does not enter the cells in significant amounts, at least at low concentrations, and enhances intercellular drug diffusion only. This is understandable as azone is a nonpolar material.132 The possible mechanism of action is illustrated in Fig. 9.

The percutaneous absorption and elimination of azone were investigated in humans. These studies reveal that the pure azone is poorly absorbed and rapidly cleared from the circulation by the kidneys. No accumulation of azone in the skin was observed.168

The influence of azone, 2-n-octylcyclohexanone, 2-tert-butylcyclohexanone, 1-n-dodecylpiperidone, and 1-n-cetylpiperidone on the histopathology of rat skins was investigated. All of the enhancers at each concentration caused epidermal liquefaction and collagen fiber swelling to some extent. The skin damage by the enhancers increased with increasing concentration of enhancer. The recoverability of the damage was significant in case of l-n-dodecylpiperidone and a ~ 0 n e . l ~ ~

agents. 95,166,167

588 SINGH AND SINGH

AZONE

CLOSELVPACKED HYDROCARBON 1 llg 11 CHAIN

IIkliT PARTIALLY HY ORATED

TISSUE AZONE-TREATED

TISSUE

A Z O Y INSERTS DLTWECN THE LIPIDS PREVENTING CHAIN CRYSTALLISATION

Figure 9. Azone molecules disrupting the lipid structure of the intercellular region of the stratum corneum.132

D. Miscellaneous Enhancers

Fatty acids such as lauric a ~ i d , ~ ~ , ~ ~ ~ oleic acid,*04 linoleic a ~ i d , ' ~ ~ , ' ~ ~ and fatty alcohols such as lauryl alcohol and cetyl alcohol107 comprise one class of permeation enhancers. Aungst et a l . 62 evaluated capric acid, neodecanoic acid, and dodecylamine by assessing the permeation rates of model diffusants (naloxone, testosterone, benzoic acid, indomethacin, fluorouracil, and methotrexate) with diverse physicochemical properties. Oleic acid and lauric acid enhanced the diffusion of metoprolol, oxprenolol and propranolol across an artificial lipid membrane.86 Attenuated total reflectance IR spectroscopy indicates that the oleic acid causes enhanced transport of 4-cyanophenol across the stratum corneum by increasing lipid chain d i ~ 0 r d e r . l ~ ~

Skin penetration enhancers such as sodium oleate, sodium lauryl sulfate, N-decylmethylsulfoxide, and N,N-dimethyldodecylamine-N-oxide substantially enhanced the permeability coefficient of the pyridostigmine bromide through human cadaver skin.17' Okamoto et aL106 used four enhancers- 1-farnesylazacycloheptan-2-one, 1,geranylazacycloheptan-2-one, l-geranyl- azacyclopentan-2-one, and 1-dodecylazacycloheptan-2-one-in combination with four vehicles to enhance the penetration of acyclovir through excised hairless mouse and rat skin. N-decylmethylsulfoxide was also used to enhance the permeation of a~idothymidine l~~ and 5 - f l ~ o r o u r a c i l . ~ ~ ~ Essential oils were evaluated as penetration enhancers towards 5-fluorouracil using excised human skin. Eucalyptus and chenopodium were found to be very effective causing nearly 30-fold increase in the drug-permeability coefficient. Ylang ylang was mildly effective and anise had little effect on the permeability coefficient.173

Surfactant(s) also appears to be an important contributor in promoting percutaneous absorption. Several mechanisms have been suggested for the increase in the percutaneous penetration of a number of substances in presence of


VI. MATHEMATICS OF SKIN PERMEATION

The mathematical relationship describing permeation of a drug through the stratum corneum is an important consideration in the optimal design of topical dosage forms and drug-delivery systems.

Crankls* defined passive diffusion as a process by which solute is transported from one part of the system to the another as a result of random molecular motions. A distinction between permeation and diffusion has been made by Nakano and Patel.Is2 Permeation can be regarded as the movement of a drug from a solution on the surface of the stratum corneum to the receptor side or into the blood stream.

(amount/area/time) is the amount of material Q flowing through a unit section A of a barrier in time t . It can be expressed as

According to Fick's law of diffusion the flux

= d Q I A d t . (1)

The flux in turn is proportional to the concentration gradient dc / d x ,

dc d x '

I = - D -

where D is the diffusion coefficient or diffusivity (cm2/sec). Flux of the penetrant is equal to the slope of the steady-state diffusion curve. Therefore Fick's first law of diffusion can be defined as the flux ( I ) of a substance which permeates a membrane with the rate of penetration being directly proportional to the concentration gradient (dc / d x ) where D is a proportionality constant.18

Fick's first law can also be written in the form18,19,45

or

and also,

K , = K , D I h, (5)

where K p is permeability coefficient or constant (cmltime), C, the drug concentration in the vehicle when sink conditions apply and the depletion in the donor compartment is around lo%, I,, the steady-state flux of the solute, D the average diffusion coefficient (cm2/time), K , partition coefficient or distribution coefficient, and h the skin thickness.

The diffusion coefficient of the drug through skin can be estimated from the lag time (t,),31

D = h2 1 6 t,. . (6)

590 SINGH AND SINGH

t, is defined as the time required for the solute to pass transepidermally (mainly intracellularly) through a membrane (or skin). It also represents the time required to attain steady state. Banerjee and RitscheP calculated the diffusion coefficient of the vasopressin by this equation. The use of Eq. (6) is restricted to the case where binding between the drug and the skin does not occur. 17,18,183

VII. OTHER APPROACH TO DESIGN TRANSDERMAL THERAPEUTIC SYSTEMS

The limitations of conventional transdermal delivery systems can be overcome by physical, chemical, and biochemical modifications in the transdermal therapeutic system (TTS). These approaches have been shown to potentially decrease skin's barrier properties and enhance the transdermal permeation of drugs for TTS.Is4

Several approaches have been investigated with the aim of enhancing ionic or hydrophilic (HPL) drug permeation through hydrophobic (HPB) membranes including iontophoresis, phonophoresis, prodrug design, and ion-pair formation, as shown in Fig.

VIII. IONTOPHORESIS A. Historical Development

The idea of applying electric current to increase the penetration of charged drugs into surface tissues was probably originated by Pivati.'s6 Later, Mor-

wrote a book on cataphoresis of ions into the tissues. The first well-

HPL M E M I R A W

PERMEABLE

PERMEABLE SOLUTES

PB MEmRANE

J IMPERMEABLE

PERMEABLE

RECENT APPROACHES TO ACHIEVE TRANSPORT

1. 10NlOPHORLSIS 2. L i P o P n n r

PROMUGS 3 AOUIOUSION

PAIR ALL SUBJtCl TO Llnl fAl10NS

NONAOUEOUS ION PAIR FORMATION FOR LIP0PHILRATH)N OF IONIC DRUGS

Figure 10. Schematic representation of solute permeation through membranes.185


documented experiments were done at the beginning of the twentieth cen- tury by L e d u ~ . * , ~ ~ ~ Leduc was able to introduce strychnine and cyanide ions into the rabbits placed in series with a direct-current generator when the correct polarity was applied. The first rabbit was seized by tetanic convul- sions, due to the introduction of strychnine ion, while the second rabbit died with symptoms of cyanide poisoning. Later on, AlbrechP9 tried for local anesthetization of tympanic membrane by this technique. The treatment of sepsis by zinc ions was also reported.190 Gibson and Cooke5 used iontophoretic application of pilocarpine to induce sweating. The procedure was found painless and required only 5 min; rapid sweating was induced and continued for 30 min. Many workers have reviewed the different aspects of iontophoresis such as historical d e v e l ~ p m e n t s , ~ ~ ~ biomedical applications,6,192 delivery of drugs and pep tide^,'^^,^^^ and iontophoretic devices. I95

B. Definition, Advantages, and Limitations

Iontophoresis is a process which induces an increased migration of ions or charged molecules through the skin when an electrical potential gradient is applied.6

Iontophoresis has some advantage over passive drug delivery. It provides larger and time-varying delivery rates for ionic and higher-molecular-weight entities. The interpatient variability and side effects are reduced. It also avoids the risk of localized infection, inflammation and fibrosis associated with injection and infusion and improves the patient compliance.

The main disadvantage of the technique is iontophoretic burn produced by high-current densities and often by low voltages. This may limit the use of the technique. Burns are caused without the sensation of pain and tend to heal slowly.196 Molitor and FernandezI9’ conducted experimental studies on the causes and prevention of iontophoretic burns. Tapper198,199 has described a method for constructing an iontophoretic electrode that will offer burn protection. The limitations of iontophoresis also depend on the properties of drug species. The drug should be ionized to carry a measurable current (in excess of A) in biological systems.200

Iontophoresis increases the penetration of ionic drugs into surface tissues by repulsion of ions at the active electrode. Negative ions are delivered by cathode [cathodal (-) iontophoresis] and positive ions by anode [anodal (+) iontophore~is]~ (Fig. 11).

Figure 12 illustrates the difference between iontophoretic and passive diffusion process. The current strength/applied voltage in iontophoresis is equivalent to the concentration/activity of solute in passive diffusion as the applied driving force for absorption. The movement of solutes in iontophoresis appears to be mainly through pores such as hair follicles and sweat It is also possible that some transport may occur through aqueous channels in the intercellular regions of the stratum corneum. Passive diffusion of most solutes on the other hand, occurs predominantly through the intact stratum corneum rather than through the pores.247 In the iontophoretic transport of selected solutes, the transport number (defined as the fraction of the current carried by the solute) is of particular importance and is analogous to the importance of permeability coefficient in passive diffusion.2

592 SINGH AND SINGH

ANODAL ( * ) IONTOPHORESIS

+ - c I-

C ATH O D A L ( 4 IONTOPHORESIS

DONOR- RECEPTOR- E P l D E R M I S D E R M I S

Figure 11. Anodal (+) and cathodal (-) iontophoresis?

The iontophoretic electrodes are applied at two sites on the skin and that the dermal blood supply carry away the drug before it is transported back to the receiving electrode. Certain solutes (morphine, clonidine) may migrate back to this electrode, as evidenced by in vitro studies of Scheuplein and Blank.ll

C. Theoretical Considerations

At steady-state flux during iontophoresis, the total flux of the drug ( Iti) can be given as the sum of passive diffusion flux (I,,), electrical flux (I,,), and the convective flux ( Jc,)*O1:

PASSIVE DIFFUSION IONTOPHORESIS

I I ’

CURRENT ST RE NGHT 1

A P P L I E D f - CONCENTRATION

TRANSPORT

ION B WATER - DERMIS - DERMAL BLOOD SUPPLY AN0 OlFFUSlON

Figure 12. Diagrammatic representation of iontophoresis through the epidermis and a comparison with passive diffusion.2


The passive flux is202

d dC d In A, = -u C 3 = -U,RT 2 - U,RTC, -

P' ' I dx dx dx ' I

where IPi is the passive diffusional flux of the ith solute molecule, U the mobility, C the solute concentration of the membrane, IJ. the molar free energy in the membrane, R the universal gas constant, T the temperature, A the activity coefficient in the membrane, and x is the spatial coordinate. The subscript i refers to the ith component, taken here to be the drug.

The electrical flux is defined as f o l l o ~ s ~ ~ ~ ~ ~ ~ ~ :

where I,, is the electrical flux of the ith solute component, Z is the valence of the solute molecule, F is Faraday's constant, R is the resistance, I, is the current density, A is the membrane surface area, and h is the proportionality constant.

The convective flux is given by204-207:

Jci = [ K ( = -dP - pel 2 I D ) Ah + K' IDA] Ci,

where Jci is the convective flow of the solute component, K is the water permeability, P is the hydrostatic pressure, pel is the electrical space and K' is the proportionality constant.

Thus, by neglecting the pressure gradient and letting qei = - Ui Zi Fi and qci = -Kp,,, the relationship for the total flux of the ith component is obtained by substituting the respective term in Eq. (7):

If the pH changes and/or the transport numbers change then the relative magnitude of the electrical and convective terms for each charged species in the system will change in such a way that total current flow and the charge neutrality conditions are satisfied.201

Abramson and Gorin208 also defined an equation which includes contributions due to electrical mobility, electroosmosis, and simple diffusion.

Masada et ~ 1 . ~ ~ 9 used the following equations for describing the in vitro studies using a four-compartment diffusion cell electrode:

E = Y / Yo = (FZAV) [RT {exp -(FZAV / RT -1)}]-I, (12)

where, E is the flux enhancement ratio, Y the flux with an electric field, Yo the flux with out electric field, V the potential drop, Z the molecular charge, F the Faraday's constant (96,500 coulomb mol-l), R the gas constant (8.314

594 SINGH AND SINGH

JmolklK-l and J = 1 volt coulomb), and T is absolute temperature (310 K). Experimental values were in good agreement with theoretical values at low voltage (0-0.25 V).

Lelawongs et a[. 210 described the arginine-vasopressin (AVP) flux J during the iontophoretic process by the following equation:

where I p is the passive diffusion flux of AVP and I, is the electrically facilitated flux of AVP. It can be also written as

where y is activity coefficient of AVP in the donor solution, a the activity of AVP, dvldx the electrical potential gradient across the skin, D the diffusivity of AVP, Z the charge of AVP, and C is concentration in solution or skin.

Kasting et ~ 1 . ~ " and Masada et ~ 1 . ~ ~ ~ used the Nernst-Planck equation to describe the iontophoretic transport of some model drugs. These authors found that the enhancement of drug flux by iontophoresis is a nonlinear function of the membrane potential for voltages greater than 0.25 V. The Nernst-Planck equation describes the flux of an ion under the influence of both a concentration gradient and an electric field. When the potential gradient becomes zero, the expression reduces to Fick's law for passive diffusion. This suggests that after an iontophoresis treatment is terminated, the transdermal permeation rate of the ionic drug would return to that which characterizes its passive diffusion.213 However, the movements of ions into a membrane under an applied field establishes a potential gradient across a membrane and causes the flux to remain greater than that of passive diffusion even after the termination of iontophoresis. This is evident from the following derivation of the Nernst-Planck equation214:

A$nl = I J A dx / F2 (U + V) + RT / F J k d(U - V) / (U + V), (15)

where U = Xi+ Ui+ Ci+, V = X - v k - c k - , and A$, = $1 - $2. I is the current flow, is the potential difference across the membrane, and subscripts 1 and 2 refer to the donor and receptor sides, respectively. The term F 2 (U + V) is the conductivity of the solution at any point x in the membrane and the reciprocal of this term is the specific resistivity at that point. This equation shows that the membrane potential is made up of an IR drop (the first term on the right) and diffusion potential (the second term on the right). When I = zero the diffusion potential still remains as a driving force in the flux equation for an ion. It is evident from the above equation that the membrane potential is dependent on the concentration and mobility of all the ions in the membrane. Immediately after an iontophoresis treatment, the resulting potential difference may not be the same as that observed during passive diffusion, because the iontophoresis treatment results in different concentrations of each ion on either side of the membrane than would result from passive diffusion alone. Gradually, the diffusion potential will be reestablished and the poten-


tial difference will become the same as that due to passive diffusion alone (unless the membrane has been damaged).

Sims and Higuchi71 presented a physical model of the skin and developed the following equation for the permeability coefficient for a weak electrolyte solute in the presence of an electric field:

P , = X, P, + X, P, + Eion (1 - X,) P,,

where P, is the total (or effective) permeability coefficient of a weak electrolyte, P, the permeability coefficient for the lipoidal pathway, PI, the permeability coefficient for the aqueous pore pathway and X, is the fraction of undissociated species. Eion is the enhancement factor for the ionized species due to the applied electric potential drop across the skin. It is defined as215:

where Ii and J! are the fluxes of the ionized species with and without the applied voltage, respectively. It can be shown from the Nernst-Planck equation with the constant-field assumption that212r215:

Eion = -K I [l-exp ( K ) ] , (18)

where K is a dimensionless constant defined as

K = -Zi FA+ I RT. (19)

Zi is the charge on species i, F the Faraday's constant, A+ the electric potential drop, R the gas constant, and T the absolute temperature. Sims and Higuchi7' used these equations to predict total permeability coefficient for butyric acid and found a semiquantitative agreement between experiment and theory.

The skin is a permselective membrane and exists with an apparant net negative charge at neutral and alkaline pHs and positively charged at acidic pHs. During iontophoresis, this permselectivity leads to current-induced volume This current-induced convective flow is due to electroosmosis and a transport number effect.201 Electroosmosis is the net water flow induced by the momentum transfer between the ions as they are transported across the membrane and the surrounding solvent.217 Electro-osmosis is the highest in solutions with a low conductivity; iontophoresis, on the other hand, is the greatest in fluids with a high electrolyte c~ncentration.~ Some theoretical models have been developed which also include the effect of convective solvent flow (electro-osmotic flow) on the steady-state flux during transdermal iontophoresis.

Wearley et a1.218 estimated the effect of convective solvent flow by two methods: (i) from the flux of tritiated water and (ii) from the enhancement in the passive permeation of the amino acids at its isoelectric point. The convective flow estimated from the flux of tritiated water was calculated as follows:

596 SINGH AND SINGH

where J T , Z , F, R, T, E , and h are the total flux of tritiated water, charge, Faraday’s constant, gas constant, absolute temperature, electric potential, and thickness of the stratum corneum, respectively. v is the linear velocity resulting from convective solvent flow, and C,, and D,, are the concentration of tritiated water inside the stratum corneum on the donor side and the diffusivity, respectively. By rearranging the above equation and substituting the respective values, the linear velocity can be calculated. The linear velocity was used to calculate Peclet number (Pe) by the following equationz1?

where D,,, in this case, is the diffusivity of the amino acid in the stratum corneum. The Peclet number is a dimensionless expression which allows one to determine the relative contribution from convective flow compared with that from diffusion alone. If the permeant diffusivity is very low, the Peclet number will be large relative to the diffusion term and thus the concentration from the convective flow would be more significant than the contribution from pure molecular diffusion of the permeant.

Srinivasan and Higuchi2I9 also proposed a theoretical model which includes the effect of convective solvent flow on iontophoretic flux. They defined the iontophoretic flux by the following equation:

J, = -D, ( dCn dx + (C,Z,F/RT) - d r n hIdA) * vc, dx

where J,, D,, C,,, and Z,, are the steady-state flux, diffusivity, concentration, and charge of nth solute, respectively. IJJ, F, R, and T are the electric potential, Faraday’s constant, gas constant, and absolute temperature, respectively. v is the average velocity of the solvent and vc a measure of the transport of permeant resulting from the convective solvent flow. +vc is the term to account for the effect of convective solvent flow on the flux of a positively charged or neutral permeant in a negatively charged pore during anodal iontophoresis. The term -uc is the appropriate one for a negatively charged permeant in a negatively charged pore.

They also used Peclet number to calculate iontophoretic flux enhancements due to the cumulative effect of the applied voltage across the membrane and the convective solvent flow. The enhancement factor for a positively charged permeant is given by

E = -K [l - (Pe/k)] / [l - exp {k (1 - Pe/k)}]. (23)

For a negatively charged permeant, the enhancement factor is

E = -K [l + (Pe/K)] / [l - exp {K (1 + Pe/K)}], (24)

where K = (ZFA$, I RT) and Pe = vhlD,,. The Peclet number characterizes the effect of convective solvent flow on the flux of the permeant while K involves the direct electric-field effects. Equation (23) for cation enhancement reflects convective solvent flow from donor to receiver. Similarly, Eq. (24) for anion enhancement reflects flow from receiver to donor.220


Assuming the direct electric-field effect applies only for the charged permeants, any enhancement in the flux of uncharged solutes can be assumed to be due to convective flow only. The enhancement factor (due only to the solvent flow) can be obtained by taking the limit as K approaches 0 (i.e., Z = 0) in Eqs. (23) and (24)219:

anode in the donor : E = P, / [l - exp (-Pe)], (25)

cathode in the donor : E = - P, / [l - exp (P,)]. (26)

Equation (25) predicts enhancement factors greater than 1 for the anode in the donor and cathode in the receiver while Eq. (26) predicts enhancement factor less than 1 for the opposite polarity.220 The enhancement factor from the Nernst-Planck equation without the convective transport term can be obtained from Eqs. (23) and (24) by setting P, = 0 and is independent of the sign of the permeant's charge.219

Pika1221 developed a detailed theoretical model which allowed the evaluation of the effect of volume flow on flux enhancement. The model assumes that transport occurs in three types of aqueous pores: positively charged, neutral, and negatively charged. The flux enhancement ratio is then calculated as follows :

J1 / = XAi ai / [ 1 - exp (- ai) 1, (27)

where i is pore type, and the summation runs over the three pore types. Ai is the area fraction of pore type i effective for transport, JI and JFare flux of species 1 with and without the electric field, respectively, and ai is given by

ai = F (-A$ / RT) [ Z , + (-ZL) Bar? Ck (Gi + F ) ] .

F, R, T, and Z , are Faraday's constant, gas constant, absolute temperature, and charge on species, respectively. -A$ is voltage drop, Zt, is charge of pore i, Ct, is charge concentration in membrane pore of radius ri, B is a known collection of constants, a is the stokes radius of the transported solute, Gi is a function of membrane change and pore radius coming from the electrical volume force effect, and F is a function of membrane charge and ion mobility arising from the induced osmotic pressure effect. For transdermal iontophoresis, F 4 G, and the induced osmotic pressure effect is not significant. Pika1221 proposed that the electroosmotic flow transport coefficient was proportional to the product of the pore charge concentration and the square of the pore radius and would always increase the flux enhancement ratio for anodic delivery of a positively charged ion (Zi > 0) or a neutral species ( Z , = 0) in a negatively charged pore. It was found that the theoretical results on the basis of these models were consistent with data in the literature.

The coupling between ionic flux and volume flow is neglected in the Nernst-Planck equation. However, the phenomenological equations of On- sager express the linear dependence of all mechanical flows (solute flow, current flow, and solvent flow) on all the mechanical forces (potential difference, induced osmotic pressure, and the chemical potential gradient across the membrane) operating in a system.222 Each mechanical flow is related to

598 SINGH AND SINGH

these three forces. Therefore the solute flow li of the ith component in an n- component system is the product of a coefficient and the forces conjugate to that flow and can be written as follows:

where Lik is the coefficient, Z is the charge, A+ is the potential difference across the membrane and is assumed to be constant (therefore A+ = E/h, where h is the membrane thickness), V is the partial molar volume, AP is the pressure gradient, and A p is the chemical potential gradient across the membrane. li is expressed as mol/cm2 sec. For neutral molecule the term containing Z would drop out.

The other two flows, i.e., current flow and solvent flow can be written in terms of solute flow as follows:

n

I = C ZiFJi, i=l

where I is the current flow and J i is the solvent flow. It is obvious from the above equations that the flow of any one of the solute

components will affect the flow of other components and the flow of uncharged solute can be affected by an electric field because of its coupling to the charged solute components. If two of the flows are held constant, the following useful expression may be obtained and related to the familiar elec- trokinetic terms of Helmholtz-Smoluchowski:223

The electric permeability may be calculated when AP = 0 and Ap = 0.

n n iz

(32) I , = (1/A+) Z i F J i = C LIkZiZk = KC,, I = 1 i = l k=l

when A+ = 0, and Ak = 0, the mechanical permeability may be determined as follows:

and the electro-osmotic permeability may be calculated when A p = 0.

n n

L,, = L, C V,FJ , / I = L, C Vitir = E < C , M ~ (34) i=l i = 1

where K , q, and E are the conductivity, viscosity, and dielectric constant of the liquid in the pores of the membrane, respectively. C, is the cross-section/


length ratio of the pores, 5 the zeta potential, and Y the mean radius of the pores.

The Helmholtz-Smoluchowski expressions from Eq. (32)-(34) shows the importance of the conductivity and viscosity of the liquid and the zeta potential of the membrane to convective flow.

Sims et al. 220 presented an expression for the electro-osmotic velocity using the Boltzmann distribution and Poisson's equation :

V = (a / kq) (A+/Ax), (35)

where V is the electro-osmotic velocity, k is the Debye-Huckel reciprocal length and is a measure of electrical double-layer thickness, q the viscosity of the bulk phase, u is the surface charge density, and + the electrical potential.

Transport number ( t i ) of a solute can be defined as the fraction of the current carried by the s o l ~ t e . ~ , ~ ~ ~ , ~ ~ ~ Hence,

where t + is the transport number for a cation and t ~ is the transport number for an anion. The transport number of a given ion t i can also be defined by the steady-state molar flux of the ion Ji , the charge of the ion Zi, the total current density I , and fraction of the current carried by the ion Ii as follows :

t i = Ii / I , = JiZiF I I, (37)

The remaining fraction of the total current includes contributions from the other ionic species present in the system (e.g., Na+, C1-, H + , OH-) as well as ionic species which may be present in the membrane itself. So, the above equation can also be written aszz4:

Alternatively, t i may also be calculated as213:

ti = DiZiC, I (DiZiCi + DjZjCj +...+ D,Z,C,), (39)

where Di is the diffusitivity of the ith ion. The membrane transference or transport number t,, is the fraction of cur-

rent carried by a particular ion in a membrane. Since the donor solution, membrane, and receptor solution are in series, the total current flowing in each of these segments must be equal. Therefore t , may be calculated by measuring the flux of ions i (in terms of molls) and the total current as follows2":

t , = ZFJi I I , (40)

Lelawongs et (AVP) by the following equation:

calculated the transport number of arginine-vasopressin

600 SINGH AND SINGH

'AVP = 'AVP IT - - CAv, ZAVp UAvp 1 (CAvp ZAVP UAVP + Ci Zi Ui + CjZj'j). (41)

where U is the ionic mobility of protonated AVP, i represents competitive ions present in the donor solution, and j represents competitive ions in the receptor solution.

The equation indicates that the transport number of AVP is related to its mobility and to the mobility of other competitive ions. The moving velocity of ions depends on their size and charge density as well as the degree of hydration. The concentration of competitive ions in both donor and receptor solutions also affect the transport number of AVP.

In the set of experiments where only total current ( I T ) was varied and the donor and receiver concentration were same and steady-state conditions were in effect, ti may be viewed as a constant. The magnitude of t i under these circumstances may be obtained from the slope of the plot of steady state flux vs current density.225

In addition to ionic conduction and electro-osmosis, phenomenon such as solute-solvent and solute-solute coupling may account for observed enhancement of drug absorption when an electric field is pre~ent.~~6,227

These theoretical considerations are important from the point of studying the mechanisms involved in iontophoresis. The practical implication of these phenomenon would be increasing the efficiency of the drug delivery and reducing the size and cost of an iontophoretic system.

The efficiency of drug delivery ( E d ) can be defined as the moles of drug ion which cross the skin for each mole of electrons flowing through the external

It can be calculated as follows:

F E - f - - l

d - i M w

where f i is the slope of the plot of steady-state flux versus current density, F is the Faraday constant (96487 c/mol) [A current of one ampere (A) equals one coulomb per mole] and M , is the molecular weight of the ion. The efficiency of drug delivery was found to be independent of the type of skin employed but was affected by the electrode material and drug counterion employed on the iontophoretic system.

D. Factors Affecting Iontophoresis

The technique of iontophoresis depends upon several physicochemical var- iables apart from the factors which affect the skin uptake of drugs during passive diffusion. Current density, vehicle pH, ionic strength, transport number of ions and water, drug conductivity, solute concentration, and skin impedence affect the transport of drugs by iontophoresis.

1. Current Density

The steady-state flux of the drug is proportional to the current density. By rearranging Eq. (37), we can say that the drug-ion transport is function of total current and the fluxes of other ions present.


1, = til, I Z,F. (43)

B e l l a n t ~ n e ~ ~ ~ reported that the increase in the applied current produced a linear increase in benzoate flux. The flux of thyrotropin releasing hormone,201 erap pa mil,^^^ and morphine hydrochloridezz8 was found to be directly proportional to applied current density. Pika1 and Shah229,230 showed that the volume flow due to electro-osmosis and steady-state flux across hairless mouse skin increased with current density. The skin permeation rate of arginine-vasopressin was found to be a linear function of the density and duration of pulse current.231

Similar relationship was observed for dexamethasone sodium phosphate, hydrocortisone sodium succinate and prednisolone sodium s ~ c c i n a t e , ~ ~ ~ model inorganic and drug ions,233,234 sodium benzoate and sodium salicy- 1ate,235,236 and tryptophanamide h y d r ~ c h l o r i d e , ~ ~ ~ and propranolol hydro- ~ h l o r i d e . ~ ~ ~ , ~ ~ ~

Studies utilizing a constant-voltage approach have also been reported. Utili- ty of an equation for predicting the flux enhancement of ions across membranes (relative to passive diffusion) due to an applied voltage drop across the membrane has been illustrated and usefulness of a newly developed four- electrode system for carrying out constant-voltage iontophoresis has been demonstrated.212 The experimental results of other workers also indicate rea- sonable agreement with the theoretical predictions at low applied volt-

Srinivasan ef ~ 1 . ~ 3 ~ assessed the relative contributions of the skin permeability increase and water flow effects to the iontophoretic flux of a non- electrolyte across hairless mouse skin using the constant-voltage approach. Sims and Higuchi7I studied the transport of butyric acid and glucose by controlling the voltage drop across the hairless mouse skin.

age. 21 L238

2. Vehicle pH

Changes in the pH of the fluid at the driving electrode can also influence the transport of the drug. Weak electrolytes which show poor percutaneous penetration may be administered topically using iontophoresis provided the drug is kept in a highly ionized form. The effect of pH of aqueous vehicles on the rate and extent of permeation of lignocaine, salicylic acid, ephedrine hydrochloride, pilocarpine hydrochloride, chlorpromazine, chlorpheniramine maleate, and methotrexate by iontophoresis through human stratum corneum was i n v e ~ t i g a t e d . ~ ~ , ~ ~ ~ The amount of ionized drug species present in the drugs solution is an important factor in delivery of drugs by this route.

SinghZ4' studied the effect of pH on the iontophoretic and passive transport of an amphoteric compound, p-amino benzoic acid, through the full thickness rat skin. The maximum transport was observed at pH 7 during iontophoresis where PABA was present as anion in the bulk solution. The results implied that the steady-state flux of PABA was primarily due to a direct electrically induced ion motion and convection.

Roberts et ~ 1 . ~ studied the passive and iontophoretic transport of ampho- tericin through human epidermis at pHs 3.5, 7.5, and 12. The maximum transport and flux was obtained at pH 12 during passive diffusion and iontophoresis.

602 SINGH AND SINGH

Several other workers also demonstrated the dependence of pH at donor

For peptide and protein drugs, the pH of the solution can control the charge of the peptide and protein molecule based on their isoelectric point. It was found that insulin, a protein drug, has a greater skin permeability above its isoelectric pH.88,242 Sanderson et suggested methods to minimize pH drifts during iontophoresis.

site on the flux of ~01~te~.71,197,210,224,229,234.290,338

3 . lonic Strength

At a constant current strength, the addition of ions competing with the solute to carry the current will result in a decrease in the transport number for that solute, whereas addition of more solute may increase the transport number. Roberts et ~ 1 . ~ proposed a theoretical model to predict the effect of ionic composition on the transport number of the solute and also compared this with the experimental values obtained by others.225,234 Wearley et showed that the increasing concentration of a competing ion, like Na+, in the donor solution was noted to first reduce the permeation rate of verapamil and then cause some increase in drug flux. Pika1 and Shah22y,230 reported a decrease in volume flow and anodic flux of glucose with increasing NaCl concentration. Masada et ~ 1 . ~ ~ ~ reported that the ionic strength has no significant effect on iontophoretic transport up to about 0.5 V. At higher voltage drops, increasing the ionic strength seems to cause a greater positive deviation from the theoretical predictions. Wearley and Chein172 reported that the addition of varying amounts of sodium chloride to the donor solution of azidothymidine (a neutral compound) enhanced the iontophoretic permeation rate 2-3- fold, possibly due to convective flow. The flux of arginine-vasopressin at pH 5.0 and 7.4 was similar at constant ionic strength but increased substantially as the ionic strength decreased.210 At a constant current of 2.4 mA, the delivery rate of propranolol hydrochloride was found to decrease with an increase in ionic strength of 0.039 to 0.2, tending toward a value for delivery rate which was produced by passive diffusion alone.261

4. Concentration

Bellantone et showed a linear increase in benzoate ion flux with do- nor concentrations. Similar results were also reported by other workers. 210,224,228,234

5. Conductivity of Drugs

An electrical conductivity is an important criteria for successful transdermal transport of drug by iontophoresis. Gangarosa et a l . 244 measured electrical conductivities of drug in vitro using a conductivity MHO meter. They reported the specific conductivity for a number of drugs with practical or theoretical importance in the treatment of disorders or diseases in humans. The contribution to condutivity of buffers and nonspecific ions in the same solution with the drug was also defined. On the basis of the above experiment, many agents could be suggested for i o n t o p h o r e s i ~ . ~ ~ ~


6 . Resistance of Skin

The skin resistance decreases as current or voltage increases.245 For current densities less than 15 yA/cm2 and exposure times of 10-20 min, the decrease in the resistance was almost completely reversible; at higher current densities, both reversible and irreversible effects were observed.245 The dc resistance of frozen skin was found to be lower than that reported for human skin in vivo, yet the sodium ion permeabilities were similar to in vivo values.

The electrical and sodium ion transport properties of fresh (without freez- ing) and frozen (stored frozen) human allograft skin were compared.246 Over- all, the electrical behavior of the two tissues was similar enough to support the use of frozen tissue in iontophoresis studies. However, the fresh skin samples were less permeable to sodium ions during passive diffusion and less conductive than frozen skin at low current levels.246

E. In Vitro and In Situ Studies

1 . In Vitro Apparatus

Almost all of the in vitro apparatus used for iontophoresis consists of simple two compartment diffusion cell. Bellantone et al. 225 designed three different type of diffusion cells changing the electrode configuration and utilized them for iontophoresis. One of the design involved three compartments separated by two pieces of skin, with both stratum corneum sides oriented toward the compartment containing an electrode. In another design, the location of return electrode was changed. It was observed that the alteration in diffusion cell configuration and/or return electrode placement relative to the membrane had little effect on the transport of benzoate ions, thus permitting the use of a simple experimental design (Fig. 13).

Burnette and Marrero201 used side-by-side diffusion cells in all transport studies. The cells were water jacketed and had volumes of 3.5 ml. A 0.64-cm2 area of tissue membrane was exposed to the donor and receptor compartment of these diffusion cells. The skin pore transport studies during iontophoresis were performed in modified diffusion cell by Burnett and Ongpipat- t a n a k ~ l . ~ ~ ~ The diffusion cell was machined from plexiglass; it had an overall diameter of 3 cm and a height of 1.7 cm. The exposed surface area of the inner chamber was 0.7 cm2.

Pika1 and Shah229 modified the diffusion cell for volume flow measure- ments across hairless mouse skin during iontophoresis. The skin was sand- wiched between two stainless steel plates (0.75-mm thickness) perforated with numerous 1-mm-diameter holes. The skin area available for transport was 3.1 cm2. The steel plates and skin were clamped between the two large 0- ring joints of the half cells with a nylon clamp assembly. The O-ring and both joints were lightly coated with high-vacuum silicone grease to ensure a leak- tight seal. The desk-shaped electrodes were sealed into the glass of "inner" standard taper joints, which then fit into the "outer" standard taper joint attached to the body of each half cell. Two horizontal precision l-mm-diameter capillary tubes were attached via O-ring joints.

The cells similar to diffusion cells with the exception of the donor compartment volume were used for delivery of gonadotropin-releasing hormone and

604 SINGH AND SINGH

Constant c u r r e h t Source

I I I -ve' i v e 1

r 1 Q I cathode anode I - ve + ve

-0 F

0

Figure 13. Diagram of the apparatus used in permeation experiments and iontophoresis. (a) glass stoppers; (b) sampling ports; (c) magnetic fleas; (d) wire mesh; (e) stratum corneum; ( f ) platinum electrodes. Clamper is used to hold donor (1) and receptor (2) compartments together.

two analogues by iontophoresis. The donor volume was restricted to 1 ml and was not stirred. Receptor volume was 3 ml and was stirred. The effective skin surface area was 0.64 cm2.248

The passive and electrically assisted transport of salbutomol from a hydrogel matrix across a model membrane and human stratum corneum was stu-.- ied in specially modified glass diffusion The diffusion cells were modified to accommodate a platinum electrode approximately 3 cm below the membrane in the receptor compartment. The counter electrode was im- planted at the back of the transdermal disc. Glikfield249 used a new in vitro system during iontophoresis for studying the solvent flow during iontophoresis.

A new in vitro system was developed by Nyambi et ~ 1 . ~ ~ ~ that increased the efficiency by isolating the competing extraneous ions. The system was evaluated by carrying iontophoretic permeation studies of aqueous solution of sodium benzoate and sodium sulphate as donor and normal saline as the receptor solution across excised pig skin. No change in the pH was observed over 8 h of the experiment and there was a twofold increase in sodium benzoate flux over that reported by Bellantone et ~ 1 . ~ ~ ~ for similar levels of current density. The efficiency of the drug delivery can also be increased by the proper selection of anode or cathode material and an appropriate drug counterion. This will minimize the electrochemical generation of extraneous ions in the reservoir during i o n t o p h ~ r e s i s . ~ ~ ~


Most of the in vitro apparatus utilize a system of two electrodes, one in each compartment of the apparatus. Recently, a four-electrode system was developed by Masada et ~ 1 . ~ ~ ~ The four-electrode potentiostat was designed to maintain a constant voltage drop across a membrane in a two-chamber diffusion cell. In a two-electrode system the current is maintained constant by the driving system, while the membrane resistance determines the voltage drop across the membrane. This voltage drop is unknown. In a four-electrode potentiostat system, the voltage drop across the membrane is maintained constant and the current through the membrane is also simultaneously monitored. Thus the four-electrode potentiostat system provides additional information that permits a more rigorous test of the predictions of the Nernst- Planck equation. This is not possible with conventional two-electrode sys-

This system had also been used for studying the transport of glucose and butyric acid during i o n t o p h ~ r e s i s . ~ ~ , ~ ~ ~

Platinum wire was used as electrode in most in vitro studies.89~201~210~224~225~231~232~236~240 Platinum electrodes cause pH drift and gas bubbling by the decomposition of water and concomitant production of H + and OH- ions.210 To avoid this, many workers used Ag/AgCl electrodes in the in vitro s t ~ d i e s . ~ ~ ~ , ~ ~ ~ , ~ ~ ~ , ~ ~ ~ , ~ ~ ~ , ~ ~ ~ However, Ag/AgCl electrodes may precipitate peptidedprotein and so platinum electrodes are preferred for delivery of peptides.210

To prevent any possibility of skin burns, an electrode structure with iontophoretic-burn-protection features was designed by Tapper.198,199 Tapper251 also suggested a method of minimizing vesicle formation while applying iontophoretic treatment to a living body. These electrodes are so designed that a uniform current density is ensured.

Sanderson and de Rei1252 designed an electrode to minimize the changes in subcutaneous pH due to the migration of the hydronium and hydroxyl ions produced by the electrode reactions. The apparatus has a treatment electrode which contains a buffered electrolyte solution separated from the drug solution by an ion-exchange membrane of appropriate polarity to prevent the flow of ionic electrolysis products into the drug solution and then into the skin. This electrode was used in a study on the iontophoretic infusion of a novel inotropic c a t e ~ h o l a m i n e . ~ ~ ~

2. In Vitro Studies

Several workers have studied the in vitro delivery of drugs by the technique

and found an enhancement of the flux of the charged and uncharged species by varying and modifying the various physiochemical and electrochemical characteristics of the permeants.

Miller et ~ 7 1 . ~ ~ ~ iontophoretically delivered the gonadotropin-releasing hormone and two analogues across excised hairless mouse skin. The metabolism by the skin degrades GnRH to fragment peptides with a rate comparable to that of the delivery rate.

The in vitro transport of vasopressin and insulin across the excised hairless rat skin was studied by varying the different parameters of pulsed direct current and pH.I B ~ r n e t t e ~ ~ ~ compared sodium ion flux resulting from pulsed

of ~on~op~ores~s71,75,88,89,172,204,210-213,216,219,224,225,229-233,242,243,247,253-256

606 SINGH AND SINGH

constant current and voltage with the flux obtained from an equivalent continuous current using excised mouse skin. The pulsed iontophoretic sodium ion flux obtained without enhanced skin depolarization was shown to be equal to the flux obtained from an equivalent continuous current only at 10 KHz. The application of all constant pulsed current protocols resulted in a decrease in the skin's low-frequency impedance, a decrease which was greater than that obtained by skin hydration alone. Pika1 and Shah254 also suggested that pulsed current can yield lower resistance and enhanced drug delivery provided that (a) the steady-state current during the "on" phase of the pulse is very small and (b) the frequency is low enough to allow depolarization of the skin during the "off phase of the pulse.

A system was developed by Groning2s8 for the local application of di- phenhydramine hydrochloride (DPHC1). The release of the drug from a gel- type carrier is controlled by a weak electric field. In vifro studies reveals that through application of 9 V, the amount of DPHCl released from a gel carrier in 5 min can be increased by a factor of 2.5.

An in vitro study to elucidate the mechanisms controlling iontophoretic transport of a homologues series of ionized and nonionized model compounds was carried The data indicate that the iontophoretic facilitated transport is essentially pore mediated and the transport of ionized and nonionized molecules may be enhanced through the pore-type pathway.

Wearley and Chien260 found that the concentration gradient across the stratum corneum was significantly greater after an iontophoretic treatment than that obtained under passive diffusion; furthermore, the concentration of the drug (verapamil) in the viable skin was also significantly greater with iontophoresis. Verapamil was found to bind with both the stratum corneum and viable skin. The increased concentration gradient and binding of verapamil in the skin could be responsible for prolongation of enhanced permeation rate observed after iontophoresis treatment.

The effects of varying ionic strength, reservoir drug concentration and temperature on the changes in the drug-delivery rate produced by an electrophoretic current were examined using a model system. The changes in pH during electrophoresis were also examined. It was found that ionic strength had an inverse relationship with the change in delivery rate produced by a given current. Drug-reservoir concentration selection was critical in the design of an electrophoretic device and was based on achieving a balance between providing a suitable reservoir and allowing adequate electrophoretic control. Electrophoretic control was affected by temperature in a manner which can be predicted by using the Arrhenius relationship.261

3 . Iontophoretic Devices

A portable battery-powered iontophoretic device which safely supplies controlled current to an electrode pair may be conveniently attached to a patient.262 This device includes: (1) regulated current control with a meter readout, (2) controlled dosage time, (3) continuous high and low skin impedance monitor with automatic shutoff, (4) meter calibration check, (5) continu-


ous battery monitor, and (6) pulsewidth modulation with current feedback control assuring that a constant current is supplied to the patient irrespective of variations in skin impedance. This multipurpose iontophoretic device is called Phoreser (Motion Control Inc., Salt lake city, UT). The drug is placed in a reservoir consisting of a plastic chamber with a permeable polyvinylacetate copolymer membrane across the bottom. The chamber has a circle of adhesive plastic around its perimeter for the purpose of holding it on the skin. The top of the chamber is fitted with a rubber septum for introduction of drug and a small circular metal snap to serve as the electrode. Another electrode is a dispersive karaya electrode (Lec Tec, Minnetonka, MN). This electrode consists of a karaya pad having karaya, glycerin, and water as its major components. The karaya pad also provides adhesive properties to maintain skin contact and allows uniform passage of current densities to the skin. The two electrodes are placed at a distance of few centimeters from each other; however, its exact position is not important. The polarity of electrodes is selected depending upon the drug to be used. The direct current is generated by a 9-V power source.263 The instrument is used for delivering insulin (monomeric form) in pig262 and alloxan-diabetic male white rabbits.263

A device was constructed by Barner264 for introduction of pigments into the skin for artistic and cosmetic purposes. It consists of a transformer, rectifier, ammeter, and a 20-mA fuse for passing a direct current of 1-20 mA through a portion of the body by means of two copper sheet electrodes. A gauze pad lying in a plexiglass coat was saturated with a 6% solution of human serum albumin. A strip of pigment was placed on the gauze. The electrodes were aligned to produce an electric field at a right angle to the pigment strip. A current of 10 mA caused a portion of the pigment to migrate 2 mm every five minutes, increasing the current to 20 mA doubled the rate of migration. Each of the pigments used migrated at the same rate.

used a 45-V dry-cell battery for iontophoretic administration of antibiotic in burn patients. The current between 5 and 20 mA was regulated with a 25 000-ohm potentiometer. The current was maintained with a 0-25 milliameter. Large flat lead electrodes were employed. Gauge flats soaked in penicillin were applied adjacent to the burn schar under the negative electrode, and saline-soaked gauze was placed adjacent to intact skin under the positive electrode. The current and time of application varied from 5 to 50 mA and from 5 to 20 min, respectively.

In vivo iontophoretic delivery of pyridostigmine was accompanied with a custom-built, battery-powered control module and two hydrogel-electrode patches. The power source was a one-channel constant-current device com- pliant to within 5% of the set point value that could sustain a current of up to 2 mA into a resistive load of 10 kfl. The hydrogel-electrode patches consisted of a conductive PVA (polyvinyl alcohol) hydrogel matrix that was in contact with a metallic electrode mesh and housed in a circular section of polyethylene foam tape. Active patch had silver electrode and other patch had silver

A steady direct current has been used in these iontophoretic systems. This usually causes skin irritation due to continuous electrical polarization and the high electrical barrier of the skin. Metroprolol, a P blocker, has been intro-

Rapperport et

608 SINGH AND SINGH

duced transdermally into the veins from a small, square electrode pad on the forearm by a newly developed iontophoretic device without causing any de- tectable skin damage.266

A pencil-shaped transdermal drug-application system was reported for the topical application of antihistamines to the skin for a short-term, localized treatment of acute skin irritations such as insect bite.258 The pencil-shaped application system contains an integrated voltage supply and an electronic indicator to show when the system is in operation.

A two-phase adhesive matrix for use in an electrically powered iontophoretic delivery device is developed.268 The adhesive can be used to ad- here an electrode assembly of an iontophoretic delivery device to a body surface, e.g., skin or mucosal membrane. This adhesive matrix was used for iontophoretic transport of metoclopramide across skin.

Many iontophoretic devices are commercially available, such as DrionicO (General Medical Company, Los Angeles, CA),7,267 Iontophor-PM@ model number 6110 PM (Life-Tech, Inc., Houston, TX),349 Phoresor II@ model number PM 700 (Iomed, Inc., Salt Lake City, UT),349 Macroduct(2) model 3700 (Wescor, Inc., Logan, UT),349 and Model IPS-25 (Farrall Instruments Inc., Grand Island, NB).349 They all consist of a battery-operated generator as the source of electric current. Other available models are Daganm model 6400 (Dagan Corporation, MN), Electro-Medicator@ model A1349 (Medtherm Cor- poration, Hunstrille, AL), and Desensitron II@ Stock No. D 6423D for treatment of hypersensitive teeth (Parkell, Farmingdale, NY).349

4. In Situ Studies

Glass et al.269 conducted experiments on two Rhesus monkeys to assess the iontophoretic technique by studying the quantity and distribution of radi- olabeled dexamethasone delivered to tissue. Salbutam01~~~ Metoprol01~~~ were iontophoretically delivered into systemic circulation. Local anesthesia was iontophoretically delivered in children and compared with subcutaneous injection.271 The effect of iontophoretic etorphine and naloxone, and elec- troacupunture on nociceptive responses from thalamic neurons in rabbits was

The in vitro transdermal iontophoretic delivery of an aqueous solution of hydromorphone hydrochloride (HMHCI) through pig and human skin was compared with in vivo delivery of a hydrogel formulations of the drug in the pig.273 Considering the diversity of the techniques used, the agreement between the in vitro and in vivo data was good.

Pulsed-mode constant-current iontophoretic enhancement was used for transdermal administration of the P-adrenoceptor-blocking agent, metoprolol tartrate, to spontaneously hypersensitive rabbits. Systolic pressure was reduced from a pretreatment pressure of 126+9 mmHg to a treatment pressure of 86211 mmHg (P < 0.05) within two hours. Diastolic pressure was lowered during this time from 99k7 mmHg to 72510 mmHg (P < 0.05).274

Riviere et ~ 1 . ~ ~ ~ investigated the effect of vasoactive drugs in transdermal delivery of lidocaine into the swine skin by iontophoresis. The lidocaine hydrochloride (I4C) was iontophoresed in vivo in anesthetized weanling pigs either alone or with the vasodilator tolazoline or the vasoconstrictor nor-


epinephrine. Tissue cores under the active electrode were then collected, quick frozen, and sectioned on a crystal, and then the radioactivity was determined in each 40-pm section. It was found that the coiontophoresis of vasoactive drugs modulates the transdermal delivery of lidocaine, in part by altering the cutaneous "depot."

F. Clinical Studies

1. Ear

Several local anaesthetics have been delivered successfully by iontophoresis for the anesthesia of the ear. 189,276-281 Iontophoresis has also been found useful in the treatment of chondritis of ear282,283 and burned ears.284

2 . Dental Area

In dental work, iontophoresis of local anaesthetic agents have been used for tooth extraction,285 treatment of infected tooth canals,286 and deposition of fluoride into the dentin of teeth.287-289 Iontophoresis may also be useful in tooth and alveolar bone m i n e r a l i ~ a t i o n . ~ ~ ~

3 . Eye

Iontophoresis of sulpha drugs such as salt of sulphapyradine and sul- phaacetamide for pyocyaneous infection resulted in 3-12 times greater concentration in the cornea and 3-15 times greater in the aqueous humor of the eye in comparison to passive diffusion alone.291 Similarly, iontophoresis of penicillin increases its concentration in the aqueous humor of rabbit.2'2 The concentration of iontophoretically administered streptomycin in human and rabbit eyes was higher than obtained by intramuscular and intravenous injection and drops.2y3,294

Iontophoretic method was found to be effective in administration of com- mon mydriatics such as atropin and s ~ o p o l a r n i n e . ~ ~ ~ , ~ ~ ~ This technique was also applied for the treatment of eye lesions in endocrine diseases.297

4. Muscles and Ioints

Iontophoresis was used for introduction of steroids for treatment of subcutaneous conditions such as synovitis and tendinitis.298 Acetic acid was used for the treatment of calcium deposits of muscles and joints by ionto- p h ~ r e s i s . ~ ~ ~ Rothfeld and Murray300 used iontophoresis for treatment of Peyroni's disease. Sokolowski et a1.301 applied this technique to treat arthritic pains. a-chymotrypsin was used iontophoretically to decrease articular and periarticular edema of joints and soft tissues.302

5. Skin

Antiviral chemotherapy in surface tissues was better accomplished by iontophoresis since the drug was delivered specifically at the desired site and the overall dose was small compared to systemic a d m i n i s t r a t i ~ n . ~ ~ ~ , ~ ~ ~

Cathodal iontophoresis of Ara-AMP, a negatively charged antiviral compound, has been successful in counteracting herpes simplex infections of the

610 SINGH AND SINGH

skin of mice305 and herpes keratitis in rabbits.306 By increasing the penetration of this drug, the mortality and morbidity of HSV-l-infected animals were significantly reduced in comparison with topical treatment. The herpes simplex labialis infection was successfully treated with idoxuridine ionto- phores i~ .~O~

Iontophoresis of lignocaine was carried out to topically aneasthetise the punctured spot before i n j e ~ t i o n . ~ ~ ~ , ~ ~ ~

The amount of penicillin deposited to burn eschar and into the avascular tissue was 200-fold greater with iontophoresis in comparison to passive diffu- s i ~ n . ~ ~ ~ Other workers have also tried to administer penicillin, streptomycin, and tetracycline into the skin by electrophoresis and phonophoresis.310-312

6. Miscellaneous

The chronic-pain syndrome in patients showed improvement in their condition by transcutaneous iontophoresis of vinblastine and ~ i n c r i s t i n e . ~ ~ ~ Vari- ous amines (noradrenaline, acetylcholine, methacoline, etc.) were microion- tophoretically delivered to examine the effect of these substances on the central nervous Buspirone, a nonbenzodiazepine anxiolytic drug, was administered iontophoretically in the rat to inhibit the firing of serotonergic dorsal raphe neurons. 316 Acetyl beta methyl choline chloride (Mecholyl) iontophoresis was successfully used clinically for the treatment of scleroderma317 and pelvic i n f l a m m a t i ~ n . ~ ~ ~ Sodium salicylate iontophoresis was used to resolve plantar warts.319

It has been conclusively shown that the vascularity of pedicled tissues can be augmented to a significant degree by histamine i o n t o p h o r e ~ i s . ~ ~ ~ These studies were limited almost exclusively to changes which occurred in the afferent (anterior) ~ i r c u l a t i o n . ~ ~ ’ , ~ ~ ~ Further, a study was carried out on the changes in efferent circulation of tubed pedicles and in the transplantability of large composite grafts by histamine iontophoresi~.~~3

Pilocarpine iontophoresis was used as a diagonistic test for cystic fibrosis and to eliminate hyperpyrexia in critically ill infants or small ~hildren.~,324,325

Iontophoresis has found application in allergy t e ~ t i n g . ~ ~ ~ - ~ ~ * Patients al- lergic to grasses were tested iontophoretically with extract of different grasses. The extraction of bioactive materials from the body is also feasible by using the technique of i o n t o p h o r e s i ~ . ~ ~ ~ Idiopathic hyperhydrosis was treated by glycopyronium bromide and tap water iontophore~is.~3~ Shelly et ~1.331

used iontophoresis for retention of sweat and created experimental miliaria in man.

G . Peptides

Iontophoresis has been used for in uitro and in vim delivery of some peptides. Thyrotropin-releasing hormone (TRH), a tripeptide with molecular weight of 362 and pK, of 6.2 has been used as a model peptide for in vitro passive and iontophoretic studies through excised dorsal nude mouse skin.201 The results indicate that both the charged and uncharged TRH fluxes across the excised tissue were greater than those obtained by passive diffusion alone. The steady-state flux of both the uncharged and charged TRH was

TRANSDERMAL DRUG DELIVERY 61 1

directly proportional to the applied current density, with flux being greater for the uncharged TRH. These results imply that the steady-state flux of TRH is primarily due to a direct electrically induced ion motion and convection. A practical implication of these results is that it may be possible to enhance and control the transdermal delivery of peptides.

Green et a l . 332 also determined the feasibility of iontophoretically delivering proteins and peptides. They studied the effects of penetrant properties (lipophilicity and charge) and of vehicle pH on the iontophoretically enhanced delivery of amino acids and their N-acetylated derivatives in vitro. The penetrants were nine amino acids (five were zwitterionic, two positively charged, and two negatively charged) and four N-acetylated amino acids, which carry a net negative charge at pH 7.4. Iontophoresis at constant current (0.36 mA/cm2), using Ag/AgCl electrodes, was conducted across freshly excised hairless mouse skin. Iontophoretic flux of zwitterions was significantly greater than passive transport. Delivery from the anode was greater than from the cathode for all zwitterions. The level of enhancement was inversely proportional to permeant octanol/pH 7.4 buffer distribution coefficient. Cath- odal iontophoresis of the negatively charged amino acids and of the N-acetylated derivatives produced degrees of enhancement which were significantly greater than those measured for the "neutral" zwitterions. Anodal iontophoresis of histidine and lysine, the two positively charged amino acids studied, induced substantial enhancement which was sensitive to the pH of the delivery vehicle. The flux of histidine from an applied solution at pH 4 (where the amino acid carries a net positive charge) was significantly greater than that from a vehicle at pH 7.4 (where histidine is essentially neutral).

investigated the iontophoretic transport of the peptide hormone, gonadotropin-releasing hormone (GnRH), and two of its analogues across excised hairless mouse skin. For all three peptides, passive transport was negligible and stability is evident when in contact with stratum corneum. Slow metabolism occurs when GnRH contacts the dermal side of hairless mouse skin.

The GnRH analogue leuprolide was iontophoresed transdermally using 13 normal men in a controlled study; serum-leutenizing hormone (LH) concentration were measured 12 times over an 8-h period. Significant elevation of LH levels were found when iontophoresis (0.2 mA) was Wearley et ~ 1 . 2 1 8 investigated the effect of binding on the iontophoretic transport of a series of amino acids (glycine, alanine, valine, and leucine). The overall effect of binding on the iontophoretic profile was found to dampen the effect of iontophoretic treatment; the profiles appears flatter and the transition to passive diffusion less distinct compared to profiles which do not include the binding parameters.

The iontophoresis of eight tripeptides, of the general structure a1anine-X- alanine, has been measured across hairless mouse skin in vitvo. The nature of the central residue (X) was varied by selecting one of the five neutral aminoacids, two negatively chargeable moieties (aspartic and glutamic acids), and a positively chargeable specie (histidine). Constant-current iontophoresis at 0.36 mA/cm2, using Ag/AgCl electrodes, was performed for 24 h in diffusion cells, which allowed both anode and cathode to be situated on the same

Miller et

612 SINGH AND SINGH

(epidermal) side of a single piece of skin. Due to a combination of osmotic and electro-osmotic forces, the anodal iontophoretic flux of neutral peptides was significantly greater than passive transport. The cathodal delivery of anionic permeants was well controlled at a steady and highly enhanced rate by the current flow. The positively charged peptide, Ac-Ala-His-Ala-NH (Bul), showed greater anodal iontophoretic enhancement when delivered from a donor solution at pH 4.0 than from solution at pH 7.4.334

The percutaneous absorption of insulin has received little attention in the past due to its large molecular weight (-6000 daltons), short biological half- life, and chemical instability. The therapeutically significant level of insulin could not be achieved by conventional transdermal systems. However, a few workers tried to deliver insulin by iontophoresis. Shapiro et ~ 1 . ~ ~ ~ observed that the chloride concentration in the sweat reduced when insulin was administered topically to patients suffering from cystic fibrosis. Stephen et ~ 1 . ~ ~ ~ failed in administering conventional, regular (soluble) insulin by iontophoresis to human volunteers, probably because such insulin is only weakly ionized and much of it is present in the polymeric form. However, they were able to demonstrate delivery of highly ionized and monomeric form of insulin by iontophoresis in pigs.

The iontophoresis was used to administer regular soluble insulin to alloxan- diabetic rabbits.263 Regardless of the level of current used, within 1 h of turning the current on, blood glucose levels decreased and serum insulin concentration increased. Moreover, in most cases, blood glucose levels continued to decrease and serum insulin concentration continued to increase after the current was turned off, suggesting that the iontophoresis could be used to accumulate insulin in the skin and subcutaneous tissues. More insulin can be delivered if the stratum corneum is disrupted or removed by gentle scraping. Siddiqui et a1.88 reduced the blood glucose levels in diabetic hairless rats by administration of insulin through the intact skin using simple dc with a current density of 0.67 mA/cm2 for 80 min. This ionotophoretic delivery of insulin was improved and made more efficient by Liu et ul.336 They used direct current with various pulse waveform modes to control blood glucose levels in diabetic rats. It was found that a considerable reduction in blood glucose levels can be achieved by a lower current density and shorter application time using a pulse current instead of simple direct current. The effect of dc pulse waveform parameters on the blood glucose control in diabetic rats was also investigated. Frequency and current density were found to be two important parameters. Blood glucose levels were observed to be better controlled when higher frequency with an on/off ratio of 1:l was used.

Chien et ~ 1 . ~ ~ ~ also concluded that the transdermal delivery of peptides and protein drugs can be more efficiently accomplished by using pulsed dc than conventional dc. Chien et a1.l successfully delivered insulin and vasopressin through intact skin by iontophoresis. Using diabetic animals, like hairless rats and hairy rabbits, the systemic bioavailability of insulin resulting from iontophoretic transdermal delivery was quantitatively assessed by radioim- munoassay and correlated well with the pharmacodynamic responses as determined by glucose level measurement. Comparative studies with parenteral administration demonstrated that the insulin delivered transdermally by the application of a specially designed transdermal periodic iontotherapeutic sys-


tem achieved the therapeutic concentration range required for the effective control of hyperglycemia in the diabetic animals.

H. Nonelectrolytes

Iontophoresis has also been shown to increase the penetration of non- electrolyte^.^,^^^,^^^,^^^ Iontohydrokinesis is a possible mechanism for greater transport of nonelectrolytes during buffer iontophoresis. The term iontohydrokinesis (IHK) describes transport of water into tissue as a result of iontophoresis irrespective of the mechanism of transport. The hypothesis that IHK is responsible for the penetration of nonelectrolytes was tested by per- forming experiments designed to determine if enhanced penetration of nonionic substances could be achieved during sodium chloride (NaCl) iontophoresis. Gangarosa et studied IHK after cathodal and anodal iontophoresis of dilute NaCl solutions containing various nonelectrolytes. Both anodal and cathodal iontophoresis resulted in the statistically significant increases in penetration compared to the topical application. Iontophoresis of NaCl resulted in increased transport of nonelectrolytes azidothymidine and tritiated water.

Iontohydrokinesis, i. e., increased water movement during iontophoresis, may be due to solvent and solute d r ~ g s , ~ ~ ~ , ~ ~ ~ electrophoretic e f f e ~ t , 3 ~ ~ electroosmotic effect,345 and transport of water by hydrated ion m0vement.3~~ The ionic strength of NaCl also affects IHK of nonelectrolytes. The flux of benzyl alcohol was found to increase with increasing ionic strength of NaCl up to 0.05. Further, increase in ionic strength decreases the flux of benzyl alcoh01.~47 The further increase in ionic strength of NaCl is likely to dehydrate the epidermis due to osmotic effects. This is evident from the mean water contents after equilibration of guinea pig footpad corneum in 0.2-M and 2.0-M NaCl solutions. These were reported to be 291.1 and 102.2 mg per 100 mg corneum, respectively.348 Assymetric and electrophoretic effects may also be responsible for decreasing the flux of nonelectrolytes across the skin at higher ionic strength of buffer or NaCI.

I. Iontophoresis in Conjunction with Permeation Enhancers

Few studies have reported the Combination of chemical permeation enhancer with iontophoresis as a potential means for controlling and enhancing the transdermal delivery of drugs. Srinivasan et ~ 1 . ~ ~ ~ examined a synergism of iontophoresis and pretreatment with a chemical permeation enhancer as a means for delivering a high-molecular-weight polypeptide. Iontophoresis in combination with permeation enhancer ethanol resulted in several-fold increase in the permeability coefficient of insulin over that obtained with iontophoresis alone. This combination enables therapeutic levels to be reached at much lower applied voltage drops. Srinivasan et ~ 1 . ~ ~ 8 also reported a successful combination of iontophoresis and ethanol for controlled transdermal delivery of leuprolide and cholecystokinen-8-analogue (CCK-8). However, Wearley and Chien172 found no additional enhancement in the flux of azidothymidine, a neutral compound, with the combination of Cl0MSO and either constant-current or constant-voltage iontophoresis across the male hairless rat skin.

614 SINGH AND SINGH

IX. FUTURE

Iontophoresis in combination with phonophoresis and various chemical penetration enhancers can enhance the transdermal delivery of drugs in a synergistic manner.193,312,339 The combination of these permit the use of lower quantities of either drug or enhancers within the delivery system, potentially circumventing adverse reactions, toxicity problems, and formulation difficulties.

It is clear from the above review that the iontophoresis has been used for the transport of a large number of compounds. However, several important issues are still to be resolved, such as mechanism and extent of current- induced changes in skin barrier function, technology of device, toxicities due to electrode and current after prolonged administration, and substances for which the technique is most applicable.

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