polymeric nanoparticulate system: a potential approach for ocular drug delivery

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Review Polymeric nanoparticulate system: A potential approach for ocular drug delivery Ramesh C. Nagarwal a , Shri Kant b , P.N. Singh a , P. Maiti c , J.K. Pandit a, a Department of Pharmaceutics, Institute of Technology, Banaras Hindu University, Varanasi-221005, India b Department of Ophthalmology, Institute of Medical Science, Banaras Hindu University, Varanasi-221005, India c School of Material Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221005, India abstract article info Article history: Received 27 August 2008 Accepted 10 December 2008 Available online 3 February 2009 Keywords: Ocular delivery Nanoparticles Polymers Nanocompacts Nanoaggregates Various efforts in ocular drug delivery have been made to improve the bioavailability and to prolong the residence time of drugs applied topically onto the eye. The potential use of polymeric nanoparticles as drug carriers has led to the development of many different colloidal delivery vehicles. Drug loaded polymeric nanoparticles (DNPs) offer several favorable biological properties, such as biodegradability, nontoxicity, biocompatibility and mucoadhesiveness. These submicron particles are better than conventional ophthalmic dosage forms to enhance bioavailability without blurring the vision. DNPs have been shown to be amenable to targeting of the drug to the site of action, leading to a decrease in the dose required and a decrease in side effects. Additionally, DNPs can be fabricated by simple techniques with better physical stability than liposomes. This unique combination of properties makes DNPs a novel polymeric drug delivery device, which fulls the requirements for ophthalmic application. This review discusses the polymeric nanoparticles, physiochemical characterization, fabrication techniques, therapeutic signicances, patented technology of nanoparticles and future possibility in the eld of ocular drug delivery. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Limitations with eye drops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Colloidal system as modied eye drop formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Penetration enhancers to improve ocular bioavailability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Fabrication techniques of DNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Precipitation/coacervation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Modied coacervation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. Ionotropic gelation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Emulsication-solvent diffusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.5. Quasi emulsion solvent-diffusion (QESD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Physicochemical properties of NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Therapeutic signicance in ocular delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Ocular colloidal carriers: in-vitro stability, in-vivo fate, and cellular toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Patented technology in ocular delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Hydrogel nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.2. Polymeric micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. Future developments of modied DNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. NP compact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.2. Hydrogel nanoparticle aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.3. Nanoparticulate vitreous and scleral implants of GCV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Journal of Controlled Release 136 (2009) 213 Corresponding author. Tel.: +91 9451570863, +91 9935640101; fax: +91 542 2368428. E-mail addresses: [email protected] (R.C. Nagarwal), [email protected] (J.K. Pandit). 0168-3659/$ see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jconrel.2008.12.018 Contents lists available at ScienceDirect Journal of Controlled Release journal homepage: www.elsevier.com/locate/jconrel

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Page 1: Polymeric nanoparticulate system: A potential approach for ocular drug delivery

Journal of Controlled Release 136 (2009) 2–13

Contents lists available at ScienceDirect

Journal of Controlled Release

j ourna l homepage: www.e lsev ie r.com/ locate / jconre l

Review

Polymeric nanoparticulate system: A potential approach for ocular drug delivery

Ramesh C. Nagarwal a, Shri Kant b, P.N. Singh a, P. Maiti c, J.K. Pandit a,⁎a Department of Pharmaceutics, Institute of Technology, Banaras Hindu University, Varanasi-221005, Indiab Department of Ophthalmology, Institute of Medical Science, Banaras Hindu University, Varanasi-221005, Indiac School of Material Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221005, India

⁎ Corresponding author. Tel.: +91 9451570863, +91E-mail addresses: [email protected] (R.C

0168-3659/$ – see front matter. Crown Copyright © 20doi:10.1016/j.jconrel.2008.12.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 August 2008Accepted 10 December 2008Available online 3 February 2009

Keywords:Ocular deliveryNanoparticlesPolymersNanocompactsNanoaggregates

Various efforts in ocular drug delivery have been made to improve the bioavailability and to prolong theresidence time of drugs applied topically onto the eye. The potential use of polymeric nanoparticles as drugcarriers has led to the development of many different colloidal delivery vehicles. Drug loaded polymericnanoparticles (DNPs) offer several favorable biological properties, such as biodegradability, nontoxicity,biocompatibility and mucoadhesiveness. These submicron particles are better than conventional ophthalmicdosage forms to enhance bioavailability without blurring the vision. DNPs have been shown to be amenableto targeting of the drug to the site of action, leading to a decrease in the dose required and a decrease in sideeffects. Additionally, DNPs can be fabricated by simple techniques with better physical stability thanliposomes. This unique combination of properties makes DNPs a novel polymeric drug delivery device, whichfulfils the requirements for ophthalmic application. This review discusses the polymeric nanoparticles,physiochemical characterization, fabrication techniques, therapeutic significances, patented technology ofnanoparticles and future possibility in the field of ocular drug delivery.

Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1. Limitations with eye drops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2. Colloidal system as modified eye drop formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3. Penetration enhancers to improve ocular bioavailability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. Fabrication techniques of DNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1. Precipitation/coacervation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2. Modified coacervation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3. Ionotropic gelation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4. Emulsification-solvent diffusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.5. Quasi emulsion solvent-diffusion (QESD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. Physicochemical properties of NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64. Therapeutic significance in ocular delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65. Ocular colloidal carriers: in-vitro stability, in-vivo fate, and cellular toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86. Patented technology in ocular delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6.1. Hydrogel nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86.2. Polymeric micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7. Future developments of modified DNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107.1. NP compact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107.2. Hydrogel nanoparticle aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107.3. Nanoparticulate vitreous and scleral implants of GCV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

9935640101; fax: +91 542 2368428.. Nagarwal), [email protected] (J.K. Pandit).

09 Published by Elsevier B.V. All rights reserved.

Page 2: Polymeric nanoparticulate system: A potential approach for ocular drug delivery

3R.C. Nagarwal et al. / Journal of Controlled Release 136 (2009) 2–13

1. Introduction

The eye is characterized by its complex structure and highresistance to foreign substances including drugs. The anterior andposterior segments of the eye, although in juxtaposition to each other,and very different in their anatomical and physiological aspects,function both independently and in tandem upon application of anocular preparation. While it has been known since long thatconventional topical formulations are amenable to application to theanterior portion, most of the applied dose is lost due to the defensivemechanism of the eye. Consequently, much concerted effort has beendirected towards increased retention of the applied dose on the eyesurface, with the premise that such increased retention will result inbetter therapeutic effect and lowered local and/or systemic effects.Since most drugs poorly penetrate the cornea, fulminating diseases ofthe posterior segment viz. vitreous, retina and choroid are required tobe treated with either systemic administration or through intravitrealinjections and vitreal implants. While therapy with systemic admin-istration requires large doses due to strong blood-ocular tissue barrier,the other two routes are very invasive requiring skilled administration,and are associated with a high degree of risk, such as development ofretinal detachment and endophthalmitis. Clearly there is a strong casein favor of formulating ocular delivery systems by focusing onimproved ocular bioavailability and extended drug effect in targetedtissues. Prolonging pre-corneal residence time through viscosityenhancers and gels has only a limited value, because such liquidformulations are eliminated by the usual routes in the ocular domain.The highly sensitive corneal/conjuctival tissues towards penetrationenhancers to maximize drug transport requires great caution in theselection of the enhancer. An alternative approach is to develop a drugdelivery system that would circumvent the problems associated withthe conventional systems, and provide the advantages of targeteddelivery of drugs for extended periods of time and be patient-friendly.The latter requisite becomes more crucial in cases where the patienthas to use the drug preparation throughout his life, e.g. in glaucoma.These advantages have been reported in the literature through the useof nanoparticles [1].

Nanoparticles (NPs), as the very name implies, are particlesvarying in size from 10 to 1000 nm and depending on the end use,may or may not contain a drug molecule. The drug may be attached toa nanoparticle matrix, or dissolved, encapsulated and entrapped,giving rise to different terminologies as nanoparticles, nanospheres ornanocapsules (Fig. 1). All these terms signify their most generalcharacteristic, i.e. they are nanosize particles. Drug loaded nanopar-ticles (DNPs) constitute an almost versatile drug delivery system, withtheir ability to overcome physiological barriers and guide the drug tospecific cells or intracellular compartments either by passive orligand-mediated targeting mechanisms [1–4].

For ophthalmic applications, properly formulated DNPs arereported to provide ease of application just like eye drop solutions,with the added advantage of being patient friendly, due to less frequentapplication and extended duration of retention in the extraocularportion.

Although the size of the nanoparticles are in the colloidal rangewhich is more precisely accepted to fall between 1 nm and 0.5 µm forophthalmic formulations, such a preparation may contain largerparticles albeit within the colloidal range stated earlier. Terminologieslike lyophilic and lyophobic have been used to characterize thedispersion medium, lyophilic systems are usually easier to prepareand have the greater stability. Thus, DNPs being organic moleculesdisperse readily upon addition to the dispersion medium to formcolloidal dispersions. Most of the advantages of DNPs accrue due totheir colloidal characteristics.

Dispersion of DNPs in a suitable ophthalmic vehicle have theadvantage of application in liquid form just like eye drop solutions. Theyavoid the discomfort associatedwith the application of viscous or sticky

preparations such as ointments, which lead to a total blurring of vision ifproperly administered. To overcome the problem of blurred vision andpoor bio-availability of drugs, it has been suggested that colloidalcarriers would have better effect. Colloidal carriers which have beenstudied for ocular delivery are mainly liposomes and nanoparticlesbecause of their extremely small size. The main limitation of liposomesas ocular drug delivery system is its surface charge. Positively chargedliposomes seem to be preferentially captured at the negatively chargedcorneal surface compared to neutral and negatively charged liposomes.Another limitation of liposomes is the instability of the lipid aggregatedon the mucin surface. The vesicular aggregates of positively chargedlipids are completely disintegrated on the negatively charged mucinmembrane surface.

The formulation of biodegradable polymers as colloidal systemholds significant promise for ophthalmic drug delivery. A colloidalsystem is suitable for poorly water soluble drugs, and would allowdrop-wise administration while maintaining the drug activity at thesite of action [5]. To achieve sustained drug release and prolongedtherapeutic activity, the particles need to be retained in the ocular cul-de-sac after topical administration, and the consequent release of thedrug from the particles at an appropriate rate. If the drug leaches outof the particles too fast or too slow, then there will be either littlesustained drug release, or the concentration of the drug in the tearsmay be too low to allow adequate drug penetration into ocular tissues[6]. It is important that the particle size for ophthalmic applications bewithin the nano range because with larger sizes a scratching feeling offoreign body sensation might occur [7]. For effective retention inocular cul-de-sac, it is essential to fabricate the particles withbioadhesive materials. Without bioadhesion, nanoparticles are elimi-nated from the precorneal site almost as quickly as aqueous solution.

DNPs of various sizes based on polymers and biomaterials such aspoly(lactide-co-glycolide; PLGA), poly(lactide; PLA), poly ε-caprolac-tone, albumin, and chitosan have been developed and tested in variouscell culture and animal models [5–12] in order to improve ocular drugdelivery. DNPs can also be employed to achieve multiple purposes,including enhanced cellular uptake of poorly permeable drugs, reducedcellular and tissue clearance of drugs, and to sustain drug delivery.Precorneal residence as well as uptake of poorly permeable drugs byocular epithelia can potentially be enhanced by topical delivery of DNPs.

In this context, chitosan has been investigated as a superiormucoadhesive cationic polymer due to its ability to developmolecularattraction forces by electrostatic interactions with the negativecharges of mucin, which is determined by the formation of eitherhydrogen bonds, or ionic interactions between the positively chargedamino groups of chitosan and negatively charged sialic acid residuesof mucin, depending on environmental pH [8].

The potential of polymeric nanoparticles as an ocular drug deliverysystem has been explored by a colloidal system consisting of anaqueous suspension of nanoparticles. These nanoparticles can berapidly fabricated under extremely mild conditions with their abilityto incorporate bioactive compounds [9]. The stability of colloidalparticles in biological fluids containing proteins and enzymes is acrucial issue, because the size of the nanoparticles plays an importantrole in its ability to interact with mucosal surfaces and, in particular,with the ocular mucosa (Fig. 2).

Nanoparticles have been fabricated by using various techniqueslike solvent evaporation, spontaneous emulsification/solvent diffu-sion, salting out/emulsification–diffusion, ionotropic gelation, anddesolvation. The ionotropic gelation method offers simplicity ofprocedure, less need for purification, high scaling up facility andhigh entrapment efficiency, and safety of the DNPs.

Nanoparticulate can be targeted to organs such as liver, spleen, lungand lymph, andbecause of their very small volume, canpass through thenarrowest capillaries. Theycan remain in theblood streamfor prolongedtimes because of their ability to avoid the phagocytes, and as aconsequence, they are amenable to controlled release properties.

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4 R.C. Nagarwal et al. / Journal of Controlled Release 136 (2009) 2–13

Nanoparticles have been intensively investigated to deliver a variety ofsmall and large chemical entities like drugs, polypeptides, proteins,vaccines and genes for improved utilization and reduced toxic sideeffects. Since DNPs reside in the target tissues and in circulation forextended period of time, the biocompatibility and biodegradability ofthe polymers used in their preparation are the two basic prerequisites.A variety of polymers fulfilling these requirements have been utilized,and their utility vis a vis the characteristics of the DNPs fabricated withthem make an extremely interesting study in the development of analmost ideal therapeutics tool for the treatment of a variety of oculardisorders.

1.1. Limitations with eye drops

Typically less than 5% of the topically applied drug penetrates thecornea and reaches intraocular tissues, while a major fraction of theinstilled dose is often absorbed systemically via the conjunctiva andnasolacrimal duct [10]. Ophthalmic solutions are popular becauseof costadvantage, simplicity of formulation development and production, andgood acceptance by patients, but a rapid and short-input entry of somedrugs into the systemic circulation is unavoidable. Systemic adverseeffects such as hypertension, tachycardia, and bronchial asthma canoccur. Timolol eye drop is reported to cause such dangerous side effects.

1.2. Colloidal system as modified eye drop formulation

The conventional eye drop solution suffers from three mainimpediments: low bioavailability, non-specificity of drug action towarda specific target and enzyme inactivation. As a result of intensiveinvestigations for ameliorating these constraints, colloidal carriers likebiodegradable polymeric nanoparticles, solid nanoparticles and nano-liposomes have been studied in the last two decades.

However, in addition to the three main impediments stated abovethe other modified ocular delivery systems like ocular inserts and in-situ gelling systems, though providing some advantages in term ofextended drug delivery, could not overcome the problems of blurredvision, sticking of the eye lids, undesirable systemic absorption andlow patient acceptance. Eye drops in the form of suspensions havemany problems such as non-homogeneity, settling, cake formationand clumping of the suspended particles, resuspendibility, and ocularirritation due to the presence of particles larger than 1000 nm [13].

Fig. 1. Different types of drug loaded Nanoparticles (DNPs).

For disorders like glaucoma, diabetic retinopathy, macular degen-eration and squamous cell carcinoma, continuous treatment is required.With simple eye drop it is not possible to maintain therapeuticconcentration for prolonged time. The common alternative, ophthalmicinsert, provides sustained drug delivery but suffers from otherlimitations: they are difficult to insert (especially for the elderly andothers with visual impairment) and easy to misapply; they are easilyexpulsed from the eye; patient compliance is low (discomfort andblurring of vision, difficulty of insertion, need for removal at the end oftheir useful life); and, they are costly to manufacture.

Presently, pharmaceutical scientists are concentrating on investiga-tionsonnanoparticulate-loadeddrugs, gene andprotein asmodified eyedrops. Nanoparticles have the potential to target ocular tissues atminimum cost and high therapeutic value. Biodegradable polymers canbe combined with drugs in such away that the drug is released into theeye in a very careful and controlled manner. The formulation ofbiodegradable or bioerodible polymers as water-based colloidal nano-systems holds significant promise for ophthalmic drug delivery. Forretinal drug delivery biodegradable polymers are preferable and inmostcases required. Both lactic acid and glycolic acid are biodegradable andthey are produced by the body and eliminated as carbon dioxide andwater. Polymeric nanoparticles can be administered by two routes,topical and injection. Injectable route is an invasive method foradministration of therapeutic agents to ocular tissues. Nanosuspensionsare similar to eye drops because they have nano size particles loadedwith drug in colloidal form in a liquid vehicle and viscosity similar to eyedrops. These colloidal topical preparations can be convenientlyadministered to the ocular surface like aqueous solution by incessantfrequency of dose. A colloidal system for poorly water-soluble drugswould allow drop-wise administration while maintaining drug activityat the site of action. It provides the additional advantages of higherpenetration and enhancement of residence time at ocular surface forbetter drug absorption through ocular barriers. Mucoadhesive proper-ties of biodegradable polymers minimize the drainage from the ocularsurface by interacting with the mucin present in the ocular surface toenhance contact time and increase the bioavailability of the drug.

1.3. Penetration enhancers to improve ocular bioavailability

Penetration enhancers can improve corneal barrier restrictions.From a safety point of view, it is important to maintain the viability ofthe living epithelial cells after application of the enhancer. Among thepenetration enhancers studied previously, cationic polymers such aschitosan, aminated gelatin, and poly-L-arginine are reported toincrease the transepithelial absorption of peptide drugs by dissocia-tion of tight junction assemblies which restrict the paracellularpermeation in intestinal and nasal epithelia without producingsignificant epithelial damage. Thus poly-cationic polymers may beuseful penetration enhancers for ocular drug delivery [12].

On the other hand, cyclodextrins are well-known cyclic oligosac-charide with lipophilic central cavity and a hydrophilic outer surface,and form inclusion complexes with hydrophobic molecules. Cyclo-dextrin complexation has been thoroughly investigated for over-coming the unfavorable biopharmaceutical properties of drugs, suchas poor solubility and/or stability. In practice, cyclodextrin derivativessuch as 2-hydroxypropyl-β-cyclodextrin (HPβCD) and randomlymethylated-β-cyclodextrin (RMβCD) are preferred to natural cyclo-dextrins for drug formulation because they have higher water-solubility and a better biocompatibility profile [14].

The uses of cyclodextrin in ophthalmic formulation have beenreviewed [15–18]. Cyclodextrins may be useful for the formulation of avariety of lipophilic drugs that have not been available as eye drops oronly in suboptimal formulations. Steroid drugs, including corticoster-oids, are good examples of such drugs. They are lipophilic and have onlybeen available in eye drops as prodrugs or suspensions with limitedconcentration and bioavailability. Likewise, carbonic anhydrase

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Fig. 3. Schematic illustration of the different ganciclovir-loaded albumin nanoparticleformulations [24].

Fig. 2. Interaction of DNPs to ocular surface and subconjuctival space of human eye.

5R.C. Nagarwal et al. / Journal of Controlled Release 136 (2009) 2–13

inhibitors have only been available as oral or aqueous eye dropformulations where the pH has to be adjusted to non-physiologicalvalues. Cyclodextrins make it possible to enhance ocular drug bioavail-ability for more effective and less frequent dosing schedules.

2. Fabrication techniques of DNPs

Biodegradable nanoparticles for pharmaceutical use are preparedfrom a variety of synthetic and natural polymers. Synthetic polymerssuch as polyacrylates, polycaprolactones, polylactides and its copoly-mers with polyglycolides arewidely used and discussed [19,20]. Naturalpolymers include albumin, alginate [21], gelatin, and probably the moststudied natural polymer for pharmaceutical nanoparticle applications:chitosan [22]. Synthetic polymers are typically dissolved in a convenientsolvent followed by precipitation in a liquid environment leading tonanoparticle formation. The drug intended to be encapsulated in theparticles is usually incorporated during the polymer solvation andprecipitation. Emulsification/solvent diffusion, emulsification/solventevaporation, nanoprecipitation, reversemicellarmethod and salting-outmethods are widely applied techniques and they have been discussedelsewhere [23]. However, a brief discussion of these techniques willbring out their wide array of possible and flexible applications.

2.1. Precipitation/coacervation

Bovine serum albumin nanoparticles containing ganciclovir (GCV)were preparedbya coacervationmethod, followedbycross-linkingwithglutaraldehyde. Three different processes of albumin nanoparticlespreparations are shown in Fig. 3. In model A, ganciclovir was directlybound to the surface of hardened and unloaded albumin nanoparticles.While inmodel B, ganciclovirwas incubatedwith theproteinprior to thenanoparticle formation and crosslinkage. For model C, both ganciclovirand glutaraldehyde were added to the albumin aqueous solution at thebeginningof theprocess. Although a substantial burst release of thedrugtookplace in1h (higher formodelANBNC), sustained release continuedfor 5 days and was almost constant for 30 days. Models B and C showedbetter controlled release profiles [24].

Preparation of chitosan nanoparticles by precipitation/coacervationmethod has been described [25]. Chitosan (0.25% w/w) is dissolved in2% (v/v) of acetic acid containing 1%w/v of Tween 80. The formation ofthe particles is obtained after the addition of 10% w/w sodium sulfatesolution at a rate of 1 ml/min to chitosan solution under mild agitationand continuous sonication. The suspension was centrifuged for 30 minat 4000 rpm and the supernatant was discarded. The particles wereresuspended inwater, centrifuged again for 30min and supernatantwasdiscarded. The nanoparticles were freeze dried overnight and their size

ranged between 100 and 1000 nm. Chitosan particles showed a positivezeta potential of about 37mV,which explains the stability of the particlesuspensions in water and in several buffer systems [26] (Table 1).

2.2. Modified coacervation

A modified coacervation technique is described by Calvo et al. [27]for the preparation of chitosan–ALG) nanoparticles. Briefly, anaqueous solution of sodium alginate is sprayed onto the chitosansolution under continuous magnetic stirring at 1000 rpm for 30 min.Nanoparticles formed due to the interaction between the negativegroups of ALG and the positively charged amino groups of CS(ionotropic gelation), were collected by centrifugation. For particlesize and size distribution study these nanoparticles were re-dispersedin 5 ml of ultrapure water and studied by standard methods [28].

2.3. Ionotropic gelation

Chitosan NP prepared by ionotropic gelation technique was firstreported [27] and has been widely studied [29,30]. The mechanism ofchitosan NP formation is based on electrostatic interaction betweenamine group of chitosan and negative charge group of a polyanionsuch as tripolyphosphate [31,32]. This is a simple process with mildpreparation condition. First, chitosan can be dissolved in acetic acid inthe absence or presence of stabilizing agent, such as poloxamer, whichcan be added in the chitosan solution before or after the addition ofpolyanion. Polyanion or anionic polymers are then added. Nanopar-ticles are formed upon mechanical stirring at room temperature. Thesize and surface charge of particles can be modified by varying theratio of chitosan and stabilizer [32].

2.4. Emulsification-solvent diffusion

El-Shabouri [33] reported chitosan NP preparation by emulsionsolvent diffusion method, by a modification of the method reportedearlier employing PLGA [34]. An o/w emulsion was obtained uponinjection of an organic phase into chitosan solution containing astabilizing agent (poloxamer) under mechanical stirring, followed byhighpressure homogenization. The emulsion is thendilutedwith a largeamount of water, whereupon polymer precipitation occurs as a result ofthe diffusion of organic solvent into water, leading to the formation ofnanoparticles. This method is suitable for high drug entrapment ofhydrophobic drugs. The major drawbacks of this method are harshprocessing conditions (e.g., the use of organic solvents) and the highshear forces used during nanoparticle preparation.

2.5. Quasi emulsion solvent-diffusion (QESD)

This method was first described by Kawashima et al. [47] for thepreparation of microspheres of ibuprofen with acrylic polymers.

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Table 1Fabrication techniques of DNPs: advantages and disadvantages.

Method Pros Cons

Precipitation/coacervation Size of particle can be controlled by varying air pressureor spray-nozzle diameter.

Not suitable for water soluble drugs.

Hardening of particles can be done by using cross linkingto that control drug release.

Cross linking agent is required such as glutaraldehyde/alcohol.

Involves aqueous solvents and mild processing conditionsand therefore ideal for maintaining the stability of proteinsand peptides.

Emulsification/solvent evaporation Water used as nonsolvent simplifies and improves processeconomics

Applicable to only liposoluble drugs.

Minimized agglomeration.High energy requirements in homogenization.

Ionotropic gelation Process is very simple and mild. Particle separation is difficult, high rpm is requiredSuitable for water soluble drugs and polymers. Redispersion is difficult; ultrasonication is requiredOrganic solvent is not required. Efficiency of the method is dependent upon the deacetylation of CS.

Sometimes aggregation on storage occurs.Size is dependent on ratio of polymer, drug and polyanion.

Emulsification solvent diffusion High encapsulation efficiency. High volumes of water to be eliminated from the suspension; leakageof water-soluble drug into the saturated aqueous external phase duringemulsification, Reduced encapsulating efficiency.

High batch to batch reproducibility.

Toxicity due to organic solvent.Narrow size distribution.Efficient in encapsulating lipophilic drugs.

Quasi emulsion solvent diffusion Avoidance of toxic organic solvents commonly used inmicro and nanoparticle solvent evaporation techniques.

Particle morphology and size is dependent on agitation speed, polymerconcentration in the initial ethanol solution as well as volume and injectionrate of the latter.

6 R.C. Nagarwal et al. / Journal of Controlled Release 136 (2009) 2–13

Pignatello et al. [35–37]modified this method to develop ibuprofen andflurbiprofen-Turrax. The solution immediately turns into a pseudo-emulsion.

3. Physicochemical properties of NPs

Colloidal systems like nanoparticles differ from macroscopicobjects because of sub-micron properties such as high surface areaand energy, and movement of the particles in liquid media (Brownianmotion). Size, morphology and physical state of the encapsulated drugas well as molecular weight and crystallinity of the polymer influencedrug release and degradation of the nanoparticles. Surface charge ofnanoparticles determines the performance of the nanoparticle systemin the body, e.g. interactions with cell membranes. Zeta (ζ) potentialmeasurements provide information about the particle surface charge[38].

Particle size of externally applied colloidal carriers influencesabsorption or permeation through the ocular barriers. Drug loadingefficiency of particles is dependent on size and shape of carriers.Uptake of PLGA particles in rabbit conjunctival epithelial cells wasfound to be dependent on the particle size. Smaller (100-nm) particlesexhibited the highest uptake compared to larger (800-nm and1000 nm) particles and particles of 100 nm were able to penetrateacross the corneal barrier. Delivery of drugs to the posterior site of eyeby application of drug solution is very difficult. The possibility of DNPsto reach the posterior site of ocular tissues and deliver drugs attargeted sites at effective therapeutic concentration in variousdisorders like age related macular degeneration, retinitis, diabeticretinopathy, and corneal/conjunctival squamous cell carcinoma isvery high.

Calvo et al. [39] observed a similar size-dependency pattern withthe in-vivo corneal uptake of indomethacin-loaded poly(epsilonca-prolactone) colloidal particles. The observed uptake was higher thanmicroparticles after topical application on the albino rabbit eye. Thesize dependency and efficiency of uptake along with the facilitateduptake of bovine serum albumin indicated that PLGA nanoparticlescan be used for enhancement of ocular drug absorption and thecontrolled release of proteins and drugs [40].

Surface modification or coating by biocompatible (hydrophilic)polymers improve uptake of nanoparticles and enhance stability. PEG,poloxamers andpoloxamines are examples [41–43]. The presence of PEG

on the surface of nanospheres can modulate the interfacial properties ofthe carrier, thereby, can positively influence the mucoadhesion andimproved drug permeation. Fresta et al. [44] investigated the effect ofcoating the surface of acyclovir-loaded polyalkyl-2-cyanoacrylate nano-sphere with PEG moieties by a simple adsorption process. Giannavolaet al. [45] studied the effect of PEG coating on PLA nanosphere andreported that coated nanosphereweremuchmore efficient in improvingthe ocular bioavailability of acyclovir.

Coatings by mucoadhesive polymers like chitosan, poly(acrylicacid), sodium alginate and poloxamers improve bioavailability of theencapsulated drug by prolonging the residence time of nanoparticlesat the mucus site [46–49] due to ionic interaction between thepositively charged amino groups of chitosan and the negativelycharged sialic acid residues in mucus [50]. Additionally, surfacemodifiers can be attached on the nanoparticles by adsorption (e.g.hydrophobic insertion) [51–55], grafting via covalent bonds [56–57]or by electrostatic interactions [58–60] with the aim of modulation ofdrug release profile (Fig. 4).

Nanoparticles dispersed in aqueous solutions can be stabilizedeither by electrostatic stabilization or by steric stabilization or by acombination of both [61,62]. Adequately high ζ-potential valuesbeyond +/−30 mV indicate a stable colloidal dispersion [63]. Lossof stability leads to an increase in visual cloudiness, which is indicativeof decreased stability [64,65].

Lopez-Leon et al. [66] reported that Chitosan particles undergovolume phase transitions (swelling/shrinking processes) upon altera-tion of the physicochemical conditions of the medium. Alteration of pHfrom acidic to basic values caused a deswelling process. Additionally, ataround the isoelectric point of chitosan–TPP ionic interactions.

4. Therapeutic significance in ocular delivery

A colloidal carrier system may be applied in liquid form like eyedrop solutions, whereupon their interaction with the glycoprotein ofthe cornea and conjunctiva can form a precorneal depot resulting inprolonged release of the bound drug. Nanotechnology-based drugdelivery is also very efficient in crossing membrane barriers, such asthe blood-retinal barrier in the eye [35,67]. Drug delivery based onnanotechnology can function as excellent systems for chronic oculardiseases requiring frequent drug administration, for example inophthalmic diseases like chronic cytomegalovirus retinitis (CMV).

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Merodio et al. [24] developed GCV loaded albumin nanoparticle forthe treatment of cytomegalovirus retinitis. In-vitro studies indicated aburst release of the drug in 1 h, which continued in a sustainedmanner for 5 days and continued for almost 30 days.

Uptake of poly-ε-caprolactone NPs of cyclosporine by cornealepithelial cells achieved drug levels which were five fold higher thanoily drug solution [39]. Marchal-Heussler et al. [68] fabricatedcarteolol loaded poly-ε-caprolactone nanoparticles and tested oninduced intraocular hypertensive rabbits. The effect was therapeuti-cally much more pronounced with carteolol colloidal carrier than thecommercial eye drops. Therefore colloidal suspensions could be agood and safe alternative for effective ophthalmic delivery of drugs.

Gelatin nanoparticles encapsulating pilocarpine HCl (50%) orhydrocortisone (35% to 45%) as model drugs produced a sustainedrelease for both drugs. In case of pilocarpine HCl-loaded spheres, noinfluence of the gelatin type or the pH level was observed, which couldbe attributed to the shielding effect of ions present in the dispersionmedium. All particle sizes of pilocarpine HCl and hydrocortisoneloaded gelatin nanoparticles were obtained in the range 300–500 nmand 110–220 nm, respectively. The average zeta potential value forparticles prepared at pH 6 equals −6.95 mV, for spheres prepared atpH 4 it amounts to −6.10 mV. Consequently, there is no significanteffect of the preparation pH on zeta potential value of the particlesobtained. But in case of hydrocortisone zeta potential was found to bebetween −4 to −12 mV that showed significant influence of gelatintype on the zeta potential value. [69].

Nanoparticle suspensions, prepared fromEudragit RS 100 andRL 100are reported to prevent myosis induced during extracapsular cataractsurgery. Bucolo et al. [70] have investigated the ocular pharmacody-namic profile of nanoparticles loaded with sodium ibuprofen (IBU-RS)and compared with administration of an aqueous solution of ibuprofenlysinate (IBL) in the rabbit eye. The IBU-RS nanosuspension formulationsignificantly reduced the primary signs of ocular inflammation proteinlevel and the number of polymorphonuclear leukocytes in the aqueoushumor compared with the IBL formulation. Additionally, the nanocap-sules produced a lower degree of cardiovascular effect in comparison toaqueous eye drops because of reduced non-corneal absorption.

The in-vivo studies of nanoparticles of piroxicam:Eudragit RS100formulated using the solvent evaporation/extraction technique, andof methylprednisolone acetate (MPA) formulated by using a copoly-mer of poly(ethylacrylate, methyl-methacrylate and chlorotrimethyl-ammonioethyl methacrylate) revealed that inflammation was inhib-ited by the DNP suspension more significantly than the microsuspen-sion of drug alone in the rabbit eye with endotoxin-induced uveitis[71,72].

For increased retention in the precorneal pocket, the hydro-lipophilic properties of the polymer-drug system of DNPs need to beoptimized. Li et al. [73] studied polybutylcyanoacrylate nanoparticlesloaded with [3H] progesterone using a hydrophilic continuous phase.Tissue concentration of progesterone following topical administrationof nanoparticles was generally four to five times less than thatobserved with control solutions, due to the high affinity of progester-one for the nanoparticles, as the drug becomes less available forabsorption during its residence time in the precorneal area.

Fig. 4. Interaction of cationic polymer and polyanion (adsorption of polyelectrolytes ona charged particle).

Polymeric nanocapsules of metipranolol for ocular delivery wasdeveloped using polyisobutylcyanoacrylate (PIBCA) and polyepsilonca-prolactone (PECL) and evaluated for the abilityof this colloidal systemtoprevent the conjunctival absorption of metipranolol and subsequentsystemic side effects. The formation (PIBCA) or precipitation (PECL) ofthe polymer around the oily nanodroplets prevents their coalescenceduring the solvent evaporation process. Consequently, the final particlesize is smaller than the droplet size of the control emulsion. Theinfluence of the oil is explainable in terms of its hydrophobic character:the more hydrophilic oil (Labrafil) is dispersed to a greater extent, andhence, a smaller particle size was obtained. It was concluded that thepolymeric coating formed around the oily droplets is not a continuouspolymeric wall. Hence, the nature of the oil should be considered as themain factor determining the particle surface charge of the nanocapsules.The percentage of the drug encapsulated is related to the solubility ofmetipranolol base in the oil. The solubility ofmetipranolol is higherwithLabrafil than Miglilol. Consequently, formulations prepared with a highvolume of Labrafil 1944 CS have greater loading efficiency. Results fromthe drug release studies indicated insignificant contribution of thepolymer coating on the release of the drug from these colloidal systems[74].

The author observed that reduced bradycardia of one formulationwas due to reduced conjunctival absorption and greater cornealabsorption. The lower systemic toxicity of metipranolol was assignedto its encapsulation in PECL nanocapsules, although the pharmacologiceffect (lowering of intra ocular pressure) of the formulation wasstatistically not significantly different fromthecommercial eyedrop [70].

It is not desirable that release of the drug from the system beprolonged excessively with respect to the commercial eye drops, giventhe limited time the polymeric colloidal particles are in the eye.Although PIBCA and PECL nanocapsules are not able to control therelease of metipranolol their stability and efficiency in reducing thesystemic absorptionofmetipranolol their potential asnewdrugdeliverysystems for ophthalmic use is of great therapeutic significance.

Among the mucoadhesive polymers investigated until now, thecationic polymer chitosan has attracted a great deal of attentionbecause of its unique properties, such as acceptable biocompatibility,biodegradability, and ability to enhance the paracellular transport ofdrugs. Hydrophilic nanoparticles based on chitosan received currentlyincreasing interest as they could control the rate of drug release,prolong the duration of the therapeutic effect, and deliver the drug tospecific sites in the body. De Campos et al. [46] explored the potentialof cyclosporin-A loaded nanoparticles for the management ofextraocular disorders, i.e. keratoconjunctivitis sicca or dry eye disease.They reported that the advantages of these systems in ocular drugdelivery include their ability to contact intimately with the cornealand conjunctival surface, thereby increasing delivery to externalocular tissues without compromising inner ocular structures andsystemic drug exposure, and to provide these target tissues with longterm drug level. De Salamanaca et al. [75] have reported that chitosannanoparticles readily penetrate conjunctival epithelial cells and arewell tolerated at the ocular surface of rabbits.

Papadimitriou et al. [76] prepared chitosan nanoparticles ofdorzolamidehydrochloride (DZ) by ionic gelationmethod. Theparticlesize of nanoparticles was affected by the CS/drug ratio but not by thetype of drug. By increasing the ratio of CS/TPP, nanoparticles withsmaller sizes are produced. DZ release followed a biphasic pattern, i.e.an initial rapid release followed by a period of slower release.

The potential of sodiumalginate and chitosan has been explored as anew vehicle for the prolonged topical ophthalmic delivery of gatiflox-acin. Gatifloxacin-loaded nanoparticles studied by Motwani et al. [28]revealed a fast release during the first hour followed by a more gradualdrug release during a 24-h period by a non-Fickian diffusion process.

The administration of biomolecules onto the eye involves numer-ous constraints, mainly because of their properties, but also due tophysiological limitations. Gene therapy offers a promising alternative

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for the treatment of ocular diseases but, the implementation of thistype of therapy is actually severely restricted by the lack of an efficientocular gene delivery carrier. De la Fuente et al. [77,78] investigated themechanism of action of a new type of nanoparticle made of twobioadhesive polysaccharides, hyaluronic acid (HA) and chitosan (CS),intended for the delivery of genes to the cornea and conjunctiva. Thetransfection studies showed that HA-CS nanoparticles provided hightransfection levels (up to 15% of cells transfected), without affectingcell viability. Confocal images indicated that HA-CS nanoparticles wereinternalized by fluid endocytosis and that this endocytic process wasmediated by the hyaluronan receptor CD44. Further experimentalresults demonstrated that HA-CS nanoparticles led to high levels ofexpression of secreted alkaline phosphatase in the human cornealepithelium model. Following topical administration to rabbits, thesenanoparticles entered the corneal and conjunctival epithelial cells and,then, became assimilated by the cells, providing an efficient delivery ofthe associated plasmid DNA inside the cells, reaching significanttransfection levels. They concluded that these nanoparticles mayrepresent a new strategy toward the gene therapy of several oculardiseases.

Diebold et al. [79] evaluated liposomal-chitosan nanoparticles(LCS-NP) for delivery of drugs to the ocular surface. The conjunctivalepithelial cell line IOBA-NHCwas exposed to several concentrations ofthree different LCS-NP complex to determine the cytotoxicity. The invitro toxicity of LCS-NP in the IOBA-NHC cells was very low.

Ocular disposition of DNPs of acyclovir, amikacin, betamethasone,betaxolol, ganciclovir, cloricromene, cyclosporin-A, flurbiprofen, indo-methacin, pilocarpine and tamoxifen has been studied in albino rabbitand Lewis rat [45,86–87]. All have been shown to be free of ocularinflammation or irritation and increase in aqueous humor AUC vsaqueous drug solution. Drug levels in ocular tissues were observed tobe higher. The polymers that have been studied include polycarboxylicacid [87], Eudragits [89], chitosan [88,89], poly (ε-caprolactone) [90],poly(isobutyl cyanoacrylate), and poly(lactic-co-glycolic acid) [91].

Chitosan is an excellent polymer for fabrication of drug, loadednanoparticles and nanocapsules, and enhances the paracellular trans-port of drugs [92], probably due to longer retention of nanoparticles tothe external ocular tissues. Chitosan nanoparticles represent aninteresting vehicle in order to circumvent the present limitations inthe management of external inflammatory/autoimmune ocular dis-eases, such as keratoconjunctivitis sicca or dryeye disease. DNPs are alsosuitable for retinal delivery, as it has no lymph system, the enhancedpermeability and retention (EPR) effect may be available making itsuitable for drug targeting by nanoparticles [93].

5. Ocular colloidal carriers: in-vitro stability, in-vivo fate, andcellular toxicity

The stability of colloidal particles in biological fluids containingimportant amounts of proteins and enzymes is a crucial issue. Atpresent, it is broadly accepted that the size of the particles plays animportant role in their ability to interact, in particular, with the ocularmucosa. It is reported that nanoparticles are able to slightly interactwith mucin. This interaction did not lead to a significant modificationof the viscosity of the mucin dispersion. Toxicity and stability ofnanoparticles in ocular delivery is still not much explored. De Camposet al. [46] studied the toxicity (survival and viability) of CSnanoparticles and observed that for the acetate buffer (pH 6) and forthe various concentrations of nanoparticles no inherent toxicity can beattributed to the nanoparticles at concentrations as high as 2 mg/ml.Further, the tolerance of the particles by the conjunctival cells, afterexposure to the nanoparticles in acetate buffers and correspondingcontrols showed a slight cell loss. Apparently, the suspension ofnanoparticles caused some deleterious effect due to the buffer used forresuspension. Small holes in the membrane and microvilli lossresulting in a plain surface were observed for cells exposed to acetate

buffer. These investigators opined that more experiments need to beperformed in order to differentiate the effect of the nanoparticles perse with that of the buffer used for their resuspension.

De Salamanca et al. [75] reported that chitosan nanoparticles(CSNPs) are well tolerated by ocular surface structures. A goodcorrelation between rabbit and human eye irritation data wasobserved when low volumes were instilled. The CSNPs seem to bevery promising, as no clinical or pathologic differences between theCSNPs and the control-treated eyes were seen. In in-vitro experi-ments, the IOBA-NHC cell line was used to determine whether theCSNP system was injurious to the conjunctival epithelium. Cellsurvival in cultures exposed to CSNPs was very high, especially after30 min of incubation. The significantly higher cell recovery levels at30 min compared with 15 and 60 min may be related to thenanoparticle aggregation observed after 1 h of incubation at 37 °C.Inherent toxicity was confirmed by the viability of recovered cells,which was approximately 90% when measured immediately afterCSNP exposure (92%) and after the 24-hour recovery period (86%).Even though PBS may cause transient damage to cells, it is the bestmedium for the CSNPs, because it maintains their physicochemicalcharacteristics and is readily adjusted for pH and osmolarity. Confocalmicroscopy demonstrated that CSNPs crossed the plasma membranewithout causing any apparent alteration in cytoskeleton (Table 2).

The mechanism of mucoadhesion of chitosan is due to an ionicinteraction between the positively charged amino groups of chitosanand the negatively charged sialic acid residues in mucus. IOBA-NHCcells express MUC1, -2, and -4 mucin RNA [73]. Both Muc1 and -4proteins are membrane-spanning mucins, whereas Muc2 protein is agel-forming type [81], and each contains many sialic residues.Therefore, electrostatic interaction of the CSNPs with the negativelycharged glycocalyx of the IOBA-NHC cells may be responsible for thehigh levels of uptake compared with other cell lines.

6. Patented technology in ocular delivery

6.1. Hydrogel nanocomposites

Nathan [94] investigated the hydrogel nanocomposites forophthalmic applications. The hydrogel is made up of copolymersolution containing nanoparticles that can form a hydrogel triggeredby changes in oxidation state, or light frequency and intensity, ormechanical stress.

The hydrogel can be reduced to form the solution; and the solutioncan be oxidized to form the hydrogel. The co-polymers were obtainedby copolymerization of a monomer with a cross linker. The crosslinkerprovided intermolecular crosslinkages to form hydrogel. The mono-mer can be acrylamide, N-ornithine acrylamide, N-(2-hydroxypropyl)acrylamide and a crosslinker such as N,N′-bis(acryloyl)cystamine(BAC). Polyacrylamide/BAC hydrogels were prepared and reduced toobtain water soluble copolymer with pendent thiol (–SH) groups. Thepolymer was used to prepare the hydrogel nanocomposites with threedifferent types of nanoparticle and regelled through the thiol-sulfideexchange reaction. The nanogels are made from reversible hydrogelsystems that are reversibly converted between a hydrogel state and asolution by oxidation/reduction or by different wavelengths of light.Polymer containing pendent –SH groups was used to prepare thenanoparticles through intermolecular crosslinking between –SHgroups at very dilute concentrations (Figs. 5 and 6).

The nanogel is made by copolymerization of a monomer with across linker to form crosslinked hydrogel. The hydrogel nanocompo-site is fabricated by combining a reversible hydrogel with nanopar-ticles. Nanocomposite comprises nanoparticles dispersed in a polymerhydrogel formulation, and is advantageous in that the refractive index

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Table 2Some polymeric nanoparticulates in ocular drug delivery.

Polymer Loaded drug/gene/protein Comments Refs.

Eudragit RS100, RL 100 Ibuprofen Drug levels was improved in the aqueous humor after application of thedrug-loaded nanosuspensions which did not show toxicity in ocular tissues.

[36,70–72,35,100,37,87,83].Flurbiprofen

Improved the stability of cloricromene in ophthalmic formulations and its drugavailability at the ocular level.

Cloricromene

The in vivo examinations revealed that the inflammation can be inhibited by thedrug:polymer nanosuspension more significantly than the microsuspension ofdrug alone in the rabbits with endotoxin-induced uveitis.

Diclofenac diethylammonium

Excellent encapsulation efficiency.

PiroxicamMethylprednisolone

Chitosan Cyclosporin A It may be a promising approach for management of external inflammatory/autoimmuneocular diseases. The particle size of NPs affected by CS/TPP ratio, whereas, it was notaffected by the type of drug. Nanoparticles may represent a new strategy toward the genetherapy of several ocular diseases.

[39,76,78,85]DorzolamidePilocarpineGene

Polyisobutylcyanoacylate/Polybutylcyanoacrylate

Metipranolol This polymer was not able to control the release of metipranolol but systemictoxicity was reduced.

[73,74,80,82].Amikacin sulphate

The increase of the amikacin concentration in cornea and aqueous humor was statisticallysignificant for this nanoparticle formulation with respect to the other formulations and thecontrol solution.

Betaxolol-chlorehydrate

The surface charge of the particles and the binding type of the drug with the nanoparticleswere much more important parameters than the drug adsorption percentage onto thenanoparticles.

Progesterone

Poly-ε-caprolectone Cyclosporin A Improvement of the ocular penetration. The therapeutic results (decrease in IOP) were muchmore pronounced with carteolol incorporated into the colloidal carriers than with thecommercial eye drops. Colloidal suspension made of poly(epsilon-caprolactone) couldoffer a good opportunity for ophthalmic delivery of drugs.

[68,90,74].Carteolol

The pharmacologic response was not affected by the encapsulated metipranolol comparedwith the commercial eye drops, a drastic reduction of the drug's systemic side effectswas observed.

Metipranolol

Albumin Pilocarpine The use of colloidal carriers for the intravitreal delivery of ganciclovir may prolong itsresidence in the eye, minimizing the opacification observed for macroscopic implants.

[7,101,102,84,24].HydrocortisoneGanciclovir

Chitosan–sodium alginate Gatifloxacin Mucoadhesive nanoparticles improved bioavailability by prolonging the contact timewith the ocular surface.

[27]

GelatinPilocarpine

The drug encapsulation is lower in case of pilocarpine HCl. [69,102]HydrocortisoneHydrocortisone release mechanism was anomalous, but close to zero order.

Polylactic acidAcyclovir

Potential ophthalmic drug delivery systems for the treatment of ocular viral infections. [45,81]Betamethasone phosphate

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and modulus of the material can be controlled using two variables,namely concentration of nanoparticles and the copolymer concentra-tion in the hydrogel. The nanoparticles preferably should have aparticle size less than 150 nm, and most preferably about 3–20 nm,most importantly it has to be non-scattering. It is critical that thenanoparticles be of such dimensions that they do not disperse orscatter vision light.

6.2. Polymeric micelles

The polymer micelle is basically a nanoparticle, formed with ahydrophilic polymer chain as a shell and a hydrophobic polymer chain asa core. Particle size of the micelle is between 10 nm and 100 nm. Guptaet al. [97] fabricated the polymeric micelles of copolymer of N-isopropylacrylamide (NIPAAM), vinyl pyrrolidone (VP) and acrylic acid(AA) having cross-linkage with N,N-methylene bis-acrylamide (MBA)loaded ketorolac. The temperature and pH dependent release of drug inaqueous buffer (pH7.2) from thepolymericmicelles at 25 °Cwere 20 and60% after 2 and 8 h respectively. In vitro corneal permeation studiesthrough excised rabbit cornea indicated two fold increases in ocular

Fig. 5. Schematic representation of the formation of nanoparticles from ABSH polymerthrough intramolecular crosslinking –SH groups [94].

availability with no corneal damage compared to an aqueous suspensioncontaining same amount of drug as in nanoparticles. The formulationshowed significant inhibition of lid closure up to 3 h and PMNmigrationup to 5 h compared to the suspension containing non-entrapped drug,which did not show any significant effect. Corneal penetrations ofketorolac and anti inflammatory activity from nanoparticles were muchhigher compared to aqueous suspension of drug of equivalent concen-tration. This couldbe attributed to the ultra small size (b50nmdiameter)of the polymeric micelles as well as their mucoadhesiveness [95–97].

Fig. 6. General process of making the reversible hydrogel system [94].

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Fig. 7. Diagrammatic representation of formation of nano-compactsFig. 8. Diagrammatic representation of fabrication of NPs dispersed in-situ gel and NPsloaded gel matrix.

Fig. 9. Formation of nanoaggregates.

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The drug from topically applied drops hardly reaches the posteriortissues such as choroid and retina. Thus there is a real problem for thedrug to be efficiently delivered to the posterior segment of the eye.Kataoka et al. [99] invented polymer micelle to resolve this problemand obtained U.S patent [98]. Another patent describes a solutioncontaining the drug loaded into a polymer micelle, and for adminis-tration as eye drop [95]. Polymer micelle was used as a carrier forphotosensitive substances like porphyrin derivatives and this can besubjected to therapy for age-related macular degeneration throughoccluding new choroidal vessels.

7. Future developments of modified DNPs

7.1. NP compact

NP loaded contact lenseshavebeen found toprovide longer residencetimeof the loadeddrug in the tearfilm, but a fractionof the loadeddrug isalso lost, as with the eye drops. Additionally, a contact lens is not quiteuser-friendly, as it is likely to produce ocular irritation and excesslacrimation in some sensitive individuals, as also corneal ulceration. Abetter and safer way would be to disperse the DNPs in a thin film of amucoadhesive polymer like chitosan or gellan for placing in the lowercul-de-sac. Depending on the ocular clearance kinetics of the drug, thesame may be incorporated in the polymer solution also to provide theinitial drug release, and the DNPs can then be incorporated in thepolymer solution. The film can be then cast, dried and cut into suitablesize. The films, after placement in the cul-de-sacwill imbibe the tear andslowly release the drug. As the process of biodegradation/dissolution ofthefilmproceeds, theDNPswill slowly release theirdrug content into thesurrounding tear film, thus providing a continuous release of the loadeddrug over a period of hours, which may extent up to 24 h. This conceptdiffers from the NP aggregate described in the next section.

NP compacts can be designed to release one or more pharmacolo-gically active agents over an extended period of time, such as for morethan one week, and up to one year or more. Particles entangled inpolymeric matrix would be remaining unaffected while the filmdegrades. It could be used as ocular inserts and implants. The NPcomposites may be placed in an eye to treat or reduce the occurrence ofone or more ocular conditions, such as retinal damage, includingglaucoma and proliferative vitreoretinopathy, among others (Fig. 7).

In a different system a modified NP-in-situ gel can also beenvisaged. Application of in-situ gel has been established in oculardelivery. Fabricated nanoparticles can be suspended in an in-situgelling vehicle, which has been characterized for in situ properties.DNPs dispersed in such a vehicle can be used as to form a gel at theexternal ocular surface. This concept requires studies to examine thepossibility of this conceptual NPs-loaded gel matrix (Fig. 8).

7.2. Hydrogel nanoparticle aggregates

This is a novel biomaterial which utilizes hydrogel nanoparticleswhich aggregate to form a material of varying strength and elasticitywhich has extensive application in ocular delivery. Nanoparticleaggregates have a chemical composition identical to bulk hydrogelpolymers. It may be useful as biodegradable ocular implant to takeadvantage of long lasting release and reduced host response. Biodegrad-

able polymers can be used to encapsulate particulate system asnanoparticle aggregates, which have the ability to incorporate activecompounds in the implant, and provide controlled release. Small andmacromolecule drugs are trapped within interstitial spaces betweenparticles during aggregate formation (Fig. 9).

The spaces between nanoparticles, or holes in the lattice, can betailored by varying the nanoparticle size. These spaces have been usedto encapsulate protein during aggregate formation. Aggregates areformed by injection of a suspension of hydrogel nanoparticles whichresult in a porous hydrogel implant. Hydrogel nanoparticle composedof pHEMA, has excellent biocompatibility. Nanoparticle aggregates arecomposed of a polymer that has FDA approval and used in implantabledevice and contact lenses [103–107].

7.3. Nanoparticulate vitreous and scleral implants of GCV

The high toxicity and danger of vitreal implants of GCV is reasonenough to search for safer and more convenient intraocular deliverydevice of GCV. Instead of the presently used vitreous implant,nanoaggregates could form the basis of a device meant for deliveryof GCV in the vitreous body in such a way that the biodegradablepolymer gradually releases the drug at zero order rate over a period ofN24weeks. Earlier vitreous implants have shown a triphasic release ina much shorter time frame. Similarly, a transscleral implant of ananocomposite of GCV could be a much less invasive device than thevitreous implant, though the transscleral implant would still require aminor surgical procedure for placement in the eye. It must be said thatat the moment these are concepts, but possess high potential forfuture successful development.

8. Conclusion

Micro and nanoparticles for topical ophthalmic application arepresently being researched based grossly on nanotechnology inwhichdrugs can be administered as an eye drop. Also poorly water soluble orinsoluble drugs can be successfully fabricated as effective systems toprovide easy administration to ocular tissues and convenience to thepatient as well as ophthalmologist to adjustment of dose and dosingfrequency according to disease therapy. It has been found thatbiodegradable polymers can be combined with drugs in such a waythat the drug is released into the eye in a very precise and controlledmanner. The formulation of biodegradable polymers as colloidalsystems holds significant promise for ophthalmic drug delivery, sinceit is suitable for poorly water-soluble drugs and would allow drop-

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wise administration while maintaining the drug activity at the site ofaction. By interaction with the glycoproteins of the cornea andconjunctiva they can form a precorneal depot resulting in a prolongedrelease of the bound drug. Nanoparticle formulations provide protec-tion for agents susceptible to degradation or denaturation in region ofharsh pH, and also prolong the duration of exposure of a new drug byincreasing retention of the formulation through bioadhesion. In thiscontext, more clinical studies are necessary to provide further infor-mation and insight into this new ophthalmic drug delivery system.

Polymeric nanoparticles have many advantages such as:

▪ Easy to fabricate and characterize▪ Process is inexpensive so the final cost of formulation is reduced.▪ Polymers are biocompatible, biodegradable, non toxic and non-immunogenic that are suitable for ocular application.

▪ Mostly polymers are water soluble such as chitosan and sodiumalginate.

▪ Applicable to a broad category of small molecules (drugs), genes,proteins and polynucleotides.

▪ Can be lyophilized and are stable after reconstitutionwithmediumfor application.

▪ Do not produce irritation to eye.▪ Mucoadhesive property of polymer (for example chitosan) increasesthe retention time at the site that is helpful for bioavailabilityimprovement.

The fabrication of nanoparticles is more complicated as compared toconventional dosage forms such as aqueous solution and suspensions.Several methods to prepare DNPs have been investigated such asprecipitation/coacervation, ionotropic gelation, desolvation, solventdiffusion, emulsion polymerization etc. Selection of manufacturingmethod depends on properties of polymer and drug. Particle size andzeta potential of DNPs play a key role in physical stability andbioavailability of the drug product.

Major developmental issues have to be resolved, includingformulation stability, particle size uniformity, and control of drugrelease rate and large-scale manufacture of sterile preparations.Clinically useful drug delivery systems need to deliver a certainamount of a drug that can be therapeutically effective, and often overan extended period of time. Such requirements can be met by themicro and nano scale drug delivery systems manufactured bynanotechnology. Nanotechnology will play a crucial role in the futureof ophthalmic medication. As has been discussed in this review, thereis ample evidence that future ocular drug delivery systems will havethe ability to penetrate different ocular tissues by circumventing thephysiological/anatomical barriers, and the rather primitive andwasteful eye drop may be a thing of the past. Nanomedicines willdeliver any drug at the right time in a safe and reproducible manner toa specific target (anterior and posterior segment of eye) at requiredlevel. These approaches await validation of drug release for ophthal-mic applications and most probably a combination of technologiesholds the key to success.

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