nanoparticle technology drug delivery

[Ide@s CONCYTEG, 6 (72): Junio, 2011] ISSN: 2007-2716

Cómo citar: Ober. C. A. and R. B. Gupta (2011), “Nanoparticle Technology for Drug Delivery”, Ide@s CONCYTEG, 6 (72), pp. 714-726.

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Nanoparticle Technology for Drug Delivery Courtney A. Ober1 Ram B. Gupta2

Resumen La tecnología de nanopartículas se espera que revolucione la manera en que se lleva a cabo la administración de fármacos. Las tecnologías de nanopartículas tienen la capacidad de mejorar la eficacia de los medicamentos, reducir al mínimo los efectos secundarios, y proporcionar una entrega específica, sólo para nombrar algunos. Con el fin de aprovechar las aplicaciones y ventajas de las nanopartículas, una comprensión fundamental de sus propiedades, producción, y caracterización es necesaria. En esta revisión se tratará de explorar estos temas en relación con la administración de fármacos. Palabras clave: administración de fármacos, caracterización de nanopartículas, producción de nanopartículas. Summary Nanoparticle technology is expected to revolutionize the way in which drug delivery is conducted. Nanoparticle technologies have the capacity to improve drug efficacy, minimize side-effects, and provide targeted delivery, just to name a few. In order to exploit the applications and advantages of nanoparticles, a fundamental understanding of their properties, production, and characterization is necessary. This review will seek to explore these topics as they relate to drug delivery. Keywords: drug delivery, nanoparticle characterization, nanoparticle production.

1 Ms. Courtney A. Ober obtained a B.S. in chemical engineering from the University of Virginia in Charlottesville, VA. She is currently pursuing a Ph.D. in chemical engineering at Auburn University in Auburn, AL. Her current research interests are nanoparticles for drug delivery, nanomixing, and pharmaceutical cocrystals. Email: [email protected] 2 Dr. Ram B. Gupta is WVW chair professor in chemical engineering at Auburn University in Auburn, AL. He obtained his Ph.D. from the University of Texas in Austin, TX. His current research interests are supercritical carbon dioxide technology, nanomedicine, and liquid fuels from biomass. Email: [email protected]

[Ide@s CONCYTEG 6(72): Junio, 2011]

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Nanoparticles: Small Size, Big Advantages

he most defining property of

nanoparticles, their small size, offers

a number of unique advantages for

drug delivery. Table 1 compares the size of

various biological entities, illustrating that the

nanometer scale is found frequently in

biological systems (Gupta and Kompella,

2006). By matching the treatment scale with

the biological entities to be treated,

nanoparticles offer a number of treatment

strategies unachievable with conventional

medicine. For example, 100 nanometer (nm)

particles can diffuse into the submuscosal

layer of the gastrointestinal tract while larger

microparticles are excluded, persisting

predominantly in the epithelial lining (Desai

et al., 1996).

Table 1. Size comparison of various biological

entities Object Size

(nm)

DNA double helix

(diameter)

3

Ribosome 10

Virus 100

Bacterium 1,000

Red blood cell 5,000

Human hair (diameter) 50,000

Source: self-elaboration.

Decreasing particle size to the nanoscale

dramatically increases the surface area for a

given quantity of material. In addition to

increased surface area, the percentage of

molecules on the surface also increases.

These effects are shown in Table 2 for

spherical particles of a 1 nm drug molecule

(Gupta and Kompella, 2006).

Table 2. Surface area and percentage of surface molecules for different particle sizes Particle

diameter

(nm)

Surface

Area

(nm2)

Surface

molecules

(%)

1 12.6 100.00

10 1260 27.10

100 1.26 × 105 2.97

1,000 1.26 × 107 0.30

10,000 1.26 × 109 0.03


The increased surface area of nanoparticles

can significantly increase the dissolution of

poorly water-soluble drugs, which are

estimated to comprise 40% of drugs under

development (Lipinski, 2001, 2002). The

relationship between surface area and drug

dissolution is governed by the Noyes-

Whitney equation,

Dissolution Rate (1)

T

Nanoparticle Technology for Drug Delivery Courtney A. Ober y Ram B. Gupta

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where A is surface area, D is diffusivity, h is

boundary layer thickness, Cs is saturation

solubility, and Cb is bulk concentration

(Dressman et al., 1998). Nanoization is an

advantageous method for increasing the

dissolution of poorly water-soluble drugs

since it is a technique that can be applied to

virtually all pharmaceutical compounds.

Due to their small size, nanoparticles are less

prone to gravitational settling and can be

easily suspended in liquid formulations. The

settling velocity, v, of a particle is given by

Stokes’ law,

(2)

where d is particle diameter, g is gravitational

acceleration, ρs is solid density, ρl is liquid

density, and μl is liquid viscosity. Resistance

to settling results from random thermal

motion, Brownian motion, for which the

Brownian displacement, x, can be calculated

by,

(3)

where kB is the Boltzmann constant, T is

absolute temperature, t is time, μ is liquid

viscosity, and d is particle diameter. As

particle size decreases to the nanoscale, their

settling velocity becomes less than their

Brownian motion, allowing the particles to

remain suspended in solution. Using

nanoparticles in liquid drug suspensions gives

a more homogeneous product with longer

shelf life and negates the need for shaking

before use.

Nanoparticles can also offer unique magnetic

and optical properties with relevance in

targeted treatment, diagnostics, and imaging.

For example, ferromagnetic materials lose

there magnetization at particle sizes less than

20 nm due to loss of magnetic domains, but

still respond to a magnetic field. Such

particles can be directed to tumors and locally

heated by pulsed electromagnetic radiation,

resulting in perforation of the tumor cell

membranes and enhanced drug delivery

(Gupta and Kompella, 2006). Due to surface

plasmon resonance, the color of nanoparticles

changes with particle size which can be

useful in diagnostic and imaging applications.

Nanoparticle Production: Top-Down or Bottom-Up?

The unique properties of nanoparticles just

mentioned can only be exploited if the

particles can be commercially produced

through safe and economically viable

technologies. There exist two categories of

nanoparticle production technologies: top-


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down and bottom-up (Reverchon and Adami,

2006). These two categories are illustrated in

Figure 1. Top-down technologies utilize

mechanical forces to break-down

macroscopic particles to nanoscale size.

Bottom-up technologies build-up nanoscale

particles from molecular solutions. Examples

of top-down processes include pearl/ball

milling and high pressure homogenization.

Examples of bottom-up technologies include

supercritical fluid precipitation and

emulsification-diffusion.

Figure 1. Comparison of top-down (top) and bottom-up (bottom) nanoparticle production technologies

Source: Gupta and Kompella, 2006

Breaking It Down

The use of mechanical forces to break down

macroscopic materials into nanoscale

particles are categorized into two groups:

pearl/ball milling and high pressure

homogenization.

Pearl/Ball Milling

Traditional micronization equipment, such as

jet mills and rotor-stator colloid mills, are

ineffective at creating sufficient quantities of

nanoparticles. Pearl mills, however, have

been found effective at creating

nanosuspensions when run for sufficient

times (Liversidge et al., 1992; Merisko-

Liversidge et al., 2003; Merisko-Liversidge

et al., 1996). Pearl mills generally consist of a

stainless steel vessel filled with steel, glass,

or hard polystyrene balls. Operation can

include moving the balls with an impeller

while keeping the vessel static or moving the

entire vessel such that the balls inside also

move. A schematic of a rotating pearl/ball

mill apparatus is shown in Figure 2.

Figure 2. Rotating pearl/ball mill vessel with drug nanoparticles


A macromolecular suspension is made of

drug particles in a stabilizer or surfactant

containing solution. The suspension is then

introduced to the pearl/ball mill vessel and

the mill is operated until drug particle size is

sufficiently reduced, providing a

Drug nanoparticles Pearls/Balls

Milling

Precipitation


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nanosuspension. The stabilizer or surfactant

prevents particle agglomeration and promotes

nanosuspension stability. Care must be taken

to ensure that the balls are not eroded over

extended operation and thus contaminate the

drug suspension. Also, since milling times

can range from hours to days, the drug must

be stable at the operational conditions (i.e.

temperature) for it to be processed using

pearl/ball milling.

High Pressure Homogenization

High pressure homogenization uses forces of

impaction to produce nanoparticle

suspensions from microparticle suspensions.

The two most common homogenization

configurations are piston-gap, shown in

Figure 3, and jet-stream, shown in Figure 4.

The piston-gap configuration forces a

macrosuspension through a small gap (~10

μm) causing particle diminution by shear,

impaction, and cavitation (Gupta and

Kompella, 2006). The jet-stream

configuration collides two high-velocity

streams of macrosuspension causing particle

diminution by impaction. The pressures

typically required to obtain nanosuspensions

are 1000-1500 bar and the number of

homogenization cycles can vary from 10-20,

depending on the drug (Gupta and Kompella,

2006).

Figure 3. Piston gas homogenization


Figure 4. Jet-stream homogenization


High-pressure homogenization processes are

well-suited to scale-up, with such processes

already being used in the food industry for

homogenization of milk. The limitation of

such processes is for hard or tough drugs

which are resistance to crack and fracture,

and therefore cannot be broken down merely

by particle collision.


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Top-down technologies for the production of

nanoparticles are advantageous because of

process simplicity and applicability to a wide

range of materials. Although the products

obtained by pearl/ball milling and high

pressure homogenization were liquid

nanosuspensions, technologies such as spray

drying can be used to obtain a solid

formulation more amenable to the patient.

Examples of two drugs commercially

produced by top-down nanoparticle

production technologies are Rapamune® and

Emend® (Gupta and Kompella, 2006).

Supercritical Fluids for Production of Nanoparticles

The application of supercritical fluids for the

production of nanoparticles has found

widespread use due to a number of

advantageous properties. Supercritical fluids

exist at temperatures beyond their critical

temperature (Tc) and pressures above their

critical pressure (Pc), as shown in Figure 5.

Supercritical fluids have diffusivities higher

than those of traditional liquid solvents,

viscosities similar to gases, and densities that

can be tuned by small changes in pressure, all

of which make them unique reaction media.

Figure 5. Generic pressure versus temperature phase diagram highlighting the supercritical fluid region


Supercritical carbon dioxide (CO2) is the

most commonly used supercritical fluid for

pharmaceutical particle production because it

is nonflammable, nontoxic, inexpensive, and

has mild critical parameters (Tc = 31.3 °C, Pc

= 73.7 bar). Supercritical CO2 has been used

as both a solvent, in the rapid expansion of

supercritical solution (RESS), and as an

antisolvent, in the supercritical antisolvent

(SAS), particle production technologies.

Rapid Expansion of Supercritical Solution (RESS) In the RESS method, the pressure-dependent

solubilizing power of supercritical CO2 is

exploited. A bulk drug is dissolved in

supercritical CO2 in a high pressure vessel.

The solution is then depressurized through a

nozzle into a collection vessel at ambient

conditions. When depressurization occurs, the

supercritical CO2 becomes gaseous CO2, in


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which the drug is not soluble, and the drug

precipitates. The faster the rate of

depressurization, the smaller the particles will

precipitate. A schematic of the RESS process

is given in Figure 6 (Gupta and Kompella,

2006).

Figure 6. Schematic of RESS process


As with all particle production technologies,

the conditions under which the process is

carried out, such as solubilization

temperature, expansion temperature, pressure

drop across nozzle, and nozzle geometry, as

well as the molecular structure of the drug,

greatly effect particle morphology. A

disadvantage of the RESS process is the

limited solubility of many pharmaceutical

compounds in supercritical CO2, for which

the SAS process may be more suitable.

Supercritical Antisolvent (SAS)

Contrary to the RESS process, the SAS

process relies on the weak solubilizing power

of CO2 for many drugs by utilizing

supercritical CO2 as an antisolvent. In this

method, the drug is dissolved in a liquid

organic solvent and this solution is sprayed

through a fine nozzle into a high pressure

vessel filled with supercritical CO2. As the

CO2 dissolves into the liquid solvent, the

solubilizing power of the organic solvent is

reduced, inducing supersaturation and

causing particle precipitation. Excess CO2 is

flushed through the vessel to remove residual

solvent and the vessel is depressurized to

collect the particles. A schematic of the SAS

process is shown in Figure 7 (Gupta and

Kompella, 2006).

Figure 7. Schematic of SAS process


A number of variations on the RESS and SAS

processes have also been introduced in the

literature to control particle size, reduce

agglomeration, and further improve particle

properties. In general, supercritical CO2

processes reduce organic solvent use and

produce dry nanoparticle powders suitable for

Supercritical CO2

Solvent + CO2

Drug + Solvent

Particles

CO2 gas

Nozzle

Drug + CO2

Supercritical CO2 Particles

Bul

k D

rug


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direct capsule filling or table compression.

Furthermore, a comprehensive compilation of

solubility data in supercritical carbon dioxide

has been published by Gupta and Shim,

which can facilitate selection of an

appropriate supercritical CO2 process for the

compound under consideration (Gupta and

Shim, 2007). Nonetheless, companies have

been slow to adopt supercritical technology

as high pressure equipment requires

additional safety measures.

Emulsification for Polymer and Protein Stabilized Nanoparticles

An emulsion is a metastable dispersion of

two or more immiscible liquids in the

presence of surfactant. Emulsions can be used

to produce nanoparticles by dissolving a drug

and polymer in a water-immiscible solvent

and adding the mixture dropwise to an

aqueous solution containing surfactant. Shear

is applied through homogenization or

sonication to decrease droplet size to the

nanoscale. The droplets harden into

nanoparticles by evaporation of the solvent,

and can be separated from the aqueous phase

by lyophilization. The above described

process is termed emulsification solvent

evaporation and a schematic of the process is

given in Figure 8. Polymers commonly used

in this process are poly(d,l-lactide-co-

glycolide (PLGA), poly(lactic acid) (PLA),

and polymethacrylate (PMA) while

stabilizers include polysorbate, polyvinyl

alcohol, albumin, and poloxamer (Bala et al.,

2004; O'Donnell and McGinity, 1997).

Figure 8. Schematic of emulsification solvent evaporation process


Final particle properties are dictated by the

homogenization duration and intensity, type

and amount of surfactant, drug to polymer

loading, and rate of solvent removal. One

advantage of this process is the ability to

enhance drug delivery by selection of

appropriate surfactants. For example,

poloxamer stabilizers, in addition to

stabilizing the nanoparticles, also exhibit

mucoadhesive properties which can enhance

oral drug delivery (Gupta and Kompella,

2006). An example of a drug product

processed by emulsification solvent

evaporation is Abraxane™, a cancer therapy

treatment drug which consists of 130

nanometer paclitaxel nanoparticles stabilized

with albumin (Gupta and Kompella, 2006).

Hom

ogen

izer

Water + Stabilizer

Drug + Solvent + Polymer droplets

Drug + Polymer Nanoparticles

Solvent Evaporation


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Nanoparticle Characterization for Drug Delivery

While nanoparticle characterization is quite

similar across disciplines, there are a number

of clinically relevant parameters which must

be considered for drug delivery applications.

Such parameters include particle size, size

dispersity, structure, surface characteristics,

crystallinity, composition, and dissolution.

The size of a nanoparticle defines through

which biological routes the particle can travel

and through which it will be excluded, as will

be discussed in a subsequent section. A

number of methods can be used for

nanoparticle sizing. With counting methods,

such as single-particle optical sensing (SPOS)

or microscopy, that measure the size of

individual particles, a significant number of

particles must be measured to ensure an

accurate reflection of the sample. Separation

methods, such as filtration or field flow

fractionation (FFF), physically order a sample

according to particle size, taking all particles

into account. Often, two or more

complementary methods can be used for

verification. Knowing the particle size

dispersity of a sample is also clinically

important. Even through the average particle

size might be well below the required limit

for intravenous injection, the presence of a

few larger microparticles may increase risk of

an embolism. Scanning electron microscopy

(SEM) and transmission electron microscopy

(TEM) can be used for direct observation of

nanoparticles, with the former more suited to

observing particle morphology and the latter

more suited to observing the internal

structure.

Upon entering circulation in a biological

system, the surface of nanoparticles will

become coated with lipoproteins and other

species (Moghimi and Szebeni, 2003). The

type and surface coverage of such species are

important to predicting the persistence and

biodistribution of the nanoparticles.

Electrophoresis is one technique that can be

used to identify adsorbant surface proteins on

a nanoparticle. Surface hydrophobicity also

plays a role in determining the in vivo

behavior of nanoparticles and can be

characterized by hydrophobic interaction

chromatography (HIC).

Some analysis techniques, such as X-ray

diffraction (XRD) and differential scanning

calorimetry (DSC), are conducted the same as

they would be for macroscopic materials but

can provide clinically relevant information

for drug nanoparticles. Crystalline drug

formulations are most stable, and it is

generally desirable that the crystallinity of a


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drug compound be maintained when

converting it to a nanoparticle formulation

with better delivery properties. It should be

noted, however, that broadening of the XRD

peaks will occur for nanoparticles less than

100 nm in diameter (Gupta and Kompella,

2006). From the melting point and enthalpy

data obtained through DSC, the phases that

exist in a nanoparticle formulation and the

degree to which they interact can be

determined. For nanoparticles produced to

enhance the dissolution of poorly water-

soluble drugs, dissolution testing is

important. Standard protocols have been

developed and apparatuses are commercially

available which can measure the release of

drug into a physiologically simulated fluid

over time.

The characterization of drug nanoparticles is

heavily dependent upon their intended

application. For some properties,

macroscopic techniques are suitable while for

others characterization techniques more

tailored to the nanoscale are required. The

characterization techniques used for drug

nanoparticles are not unlike those used for

nanoparticles in other fields, but the findings

should be considered in view of their clinical

relevance and acceptability.

Targeted Delivery: Getting the Nanoparticles Where They are Needed

The size of nanoparticles makes them suitable

for a number of delivery strategies. Possible

strategies include injection, oral delivery,

ocular delivery, delivery to the brain, and

gene delivery.

Unlike microparticles, nanoparticles do not

pose risk of embolism when administered

intravenously due to their small size. Their

intravenous administration is also suitable for

targeting tumors, inflamed, and infected

vascular regions, all of which are

characterized by leaky vasculature. In order

to diffuse through these vascular pores, which

range in size from 300-700 nm, the

nanoparticles should be <250 nm for

maximum efficacy (Wu et al., 1993).

Injectable nanoparticulate systems could be

in the form of crystalline drug

nanosuspensions for immediate release or

polymeric drug nanoparticles for a more

sustained release (Gupta and Kompella,

2006).

Drugs are often formulated for oral delivery

due to ease of administration and patient

convenience. Nanoparticles are no exception,

with a number of nanoparticle drugs currently

being examined for oral formulation. Primary

goals of oral nanoparticle delivery are local


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colonic treatment for ulcerative colitis,

enhanced bioavailability for poorly water-

soluble drugs, and systemic targeting through

circulatory uptake from the intestine.

Two less understood but critically important

drug delivery strategies are ocular delivery

and delivery to the brain. Topical drug

delivery to the eye is challenging due to

natural tear flow, blinking, and multiple

tissue and vascular barriers. Furthermore,

treating ocular tissue through systemic

circulation requires high levels of dosing,

which can prove systemically toxic, in order

to compensate for the small amount of drug

that is actually delivered to the ocular tissue.

Nanoparticles may prove relevant for ocular

drug delivery due to their improved

accessibility to deep ocular tissues, and thus

their ability to maintain improved residence

time in the eye. Likewise to the challenges

encountered in ocular delivery, the blood

brain barrier (BBB) is a highly selective,

neuroprotective membrane which severely

impedes drug delivery to the brain.

Conventional methods of drug delivery to the

brain, including intraventricular drug

diffusion and intracerebral implants, have

been highly invasive. Preliminary results,

however, have shown that surface-modified

nanoparticles may be able to carry drugs

across the BBB.

Nanoparticles are also being considered as

delivery vehicles for DNA and ribonucleic

(RNA) in gene therapy applications. Gene

delivery of DNA and RNA to target cells in

order to manipulate protein expression has

the potential to combat a number of diseases

including cystic fibrosis, hemophilia, cancer

and AIDS.

Drug nanoparticles, due to their unique

properties, have been studied for a number of

delivery strategies. Their ability to permeate

biological membranes, accessibility to remote

tissues, and increased residence time in the

body offer a more diverse portfolio of

treatment options and improve drug efficacy

through both local and systemic targeting.

The Big Impact of Nanoparticles

As with any new technology, there still exist

quite a few unknowns concerning the use of

nanoparticles for drug delivery. In order for

drug nanoparticles to become a universally

used treatment option, their benefits must

significantly outweigh any possible side

effects. Due to nanoparticles’ penetration

ability, their accumulation and persistence in

the environment could be concerning.

Environmental exposure to nanoparticles

could cause adverse effects in biological and


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ecological systems. The diversity of

nanoparticles and the many possible

administration routes make regulating this

technology a challenge. Due to the

significant research and development costs

associated with drug nanoparticles and

ensuring their safety, nanoparticle treatment

options are likely to be expensive, especially

at their advent. Ensuring equal access to these

technologies for those who need them most

will be an additional regulatory challenge.

Despite a lack of long-term safety data and a

number of regulatory challenges, nanoparticle

technology has the potential to revolutionize

modern drug delivery. A fundamental

understanding of the properties of

nanoparticles show a number of unique

properties such as increased surface area, a

greater percentage of surface molecules, ease

of suspension, and size-dependent magnetic

and optical properties. A number of

technologies have been developed for

producing nanoparticles of virtually any type

of drug. Nanoparticle characterization for

drug delivery uses traditional analysis

techniques to determine physiologically

relevant parameters. Nanoparticles are

amenable to a number of administration

routes, which allows them to be delivered

both locally and systemically to biological

systems. As research continues and more

nanoparticle drugs advance to clinical testing

and commercial use, nanoparticle technology

is likely to become an integral part of future

drug delivery strategies.

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