Hindawi Publishing CorporationISRN PharmaceuticsVolume 2013, Article ID 826798, 11 pageshttp://dx.doi.org/10.1155/2013/826798

Review ArticleMicroemulsion: New Insights into the Ocular Drug Delivery

Rahul Rama Hegde,1 Anurag Verma,1 and Amitava Ghosh2

1 School of Pharmaceutical Sciences, IFTM University, Lodhipur Rajput, Moradabad 244102, India2 Bengal College of Pharmaceutical Sciences & Research, West Bengal, Durgapur 713 212, India

Correspondence should be addressed to Rahul Rama Hegde; rahulhpharma@yahoo.co.in

Received 30 April 2013; Accepted 2 June 2013

Academic Editors: A. Al-Achi, K. Cal, Y. Murata, M. Y. Rios, and J. Torrado

Copyright © 2013 Rahul Rama Hegde et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Delivery of drugs into eyes using conventional drug delivery systems, such as solutions, is a considerable challenge to the treatmentof ocular diseases. Drug loss from the ocular surface by lachrymal fluid secretion, lachrymal fluid-eye barriers, and blood-ocularbarriers are main obstacles. A number of ophthalmic drug delivery carriers have been made to improve the bioavailability and toprolong the residence time of drugs applied topically onto the eye. The potential use of microemulsions as an ocular drug deliverycarrier offers several favorable pharmaceutical and biopharmaceutical properties such as their excellent thermodynamic stability,phase transition to liquid-crystal state, very low surface tension, and small droplet size, which may result in improved ocular drugretention, extended duration of action, high ocular absorption, and permeation of loaded drugs. Further, both lipophilic andhydrophilic characteristics are present in microemulsions, so that the loaded drugs can diffuse passively as well get significantlypartitioned in the variable lipophilic-hydrophilic corneal barrier. This review will provide an insight into previous studies onmicroemulsions for ocular delivery of drugs using various nonionic surfactants, cosurfactants, and associated irritation potentialon the ocular surface. The reported in vivo experiments have shown a delayed effect of drug incorporated in microemulsion andan increase in the corneal permeation of the drug.

1. Introduction

The human eye is a complex structure designed in such away that its anatomy, physiology, and biochemistry renderit almost impervious to foreign agents, including drugs.The human eye has two segments, that is, anterior segment(cornea, conjunctiva, etc.) and posterior segment (vitreoushumor, retina, etc.) as shown in detail in Figure 1. Thehuman corneal epithelium represents one of the major rate-limiting barriers which hinders permeation of hydrophilicdrugs and macromolecules. Another rate-limiting barrier isstroma which prevents diffusion of highly lipophilic drugsdue to abundant hydrated collagen contents [1]. Other signif-icant barriers include lacrimal fluid secretion and lachrymalfluid-eye barriers. Considering these barriers, it is verychallenging to develop ocular drug delivery systems whichcan circumvent these protective barriers and deliver thedrug to the posterior segment of the eye without causingpermanent tissue damage [2]. Conventional dosage forms

like ophthalmic solutions, suspensions, and so forth, are nowprimordial as they can only deliver the drug to the anteriorsegment of the eye but not to the posterior segment. To reducethe frequency of instillations per day, several gel formulationswere developed consisting of water soluble polymers thatincrease the viscosity of the solution, thereby improving theresidence time of drug in cul de sac. However, they were notmuch popular as they tend to blur the vision [3]. Semisolidpreparations such as petrolatum-based ointments presentedproblems for years because they could not be filtered toeliminate particulate matter and could not be made trulysterile, and no adequate tests had been devised to indicate thesuitability of added preservatives. These preparations, how-ever, occupy a position of minor importance since they areill-accepted on account of their greasiness, vision-blurringeffects, and so forth, and are generally used as night-timemedications. Novel drug-delivery systems like intraoculardevices require intravitreal procedures and often suturingand, hence, can cause significant discomfort with chance

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Conjunctiva

Iris

Cornea

Tear film

I

Aqueoushumor

Cornealepithelium

Conjunctivalepithelium

Blood-aqueousbarrier

Ciliary body

ZonuleRetina

Rectus lateralis

Blood-retinalbarrier

Macula

Sclera

Optic nerve

Choroid

Rectus medialisIII

1

2 3 4

II

Vitreous humor

Figure 1: Schematic illustration of ocular structures and barriers. The primary physiologic obstacle against topically instilled drugs is thetear film. The cornea is the main route for drug transport into the anterior chamber (I). The retinal pigment epithelium and the retinalcapillary endothelium are main barriers against systemically administered drugs (II). Intravitreal injection is an invasive strategy to reach thevitreous (III). Administered drugs can be carried out of the anterior chamber by venous blood flow after diffusion across the iris surface (1)or by aqueous humor outflow (2). Drugs may be removed from the vitreous cavity through diffusion into the anterior chamber (3) or by theblood-retinal barrier (4). Figure 1 is taken from Barar et al., 2009 [6].

of infection in the patients. Noninvasive ocular inserts alsosuffer from the disadvantages of foreign body sensation inthe eye, membrane rupture; unnoticed expulsion from theeye [4]. In situ gelling systems undergo a viscosity changewhen administered into the eye, thereby favouring precornealretention andwhen ladenwith nanoparticles, they can deliverthe drug to the posterior segment of eye. But these systems areoften difficult to develop and scale up [5]. Hence, there is astrong need to formulate ocular drug delivery systems whichnot only provide improved ocular bioavailability but alsoextended drug effect in targeted tissues. The latter requisiteis very important since patients suffering from the disease ofposterior segment have to take the drug throughout his/herlife. These prerequisites have been appropriately reported inthe literature through the use of microemulsions (MEs).

2. Microemulsion Science

MEs are thermodynamically stable-phase transition systems,which possess low surface tension and small droplet size(5–200 nm), which may result in high drug absorption andpermeation, and hence, strong possibility of drug deliveryto the posterior segment of the eye. The term ME was firstcoined by Hoar and Schulman in 1943 [7]. Scientifically, aME is a system of water, oil, and an amphiphile, frequentlyin combination with a cosurfactant, which is a single opti-cally isotropic and thermodynamically stable liquid solu-tion. Pharmaceutically, MEs are colloidal nanodispersions

of o/w or w/o types stabilized by a surfactant film. Theformation of ME along with various colloidal phases isdiagrammatically explained in Figure 2. MEs are generallyformed spontaneously, without any significant energy input,by mixing an oil phase with an aqueous phase containinga primary surfactant and a cosurfactant, which is usually amedium-chain-length alcohol [8]. During the mixing, theprimary surfactant is adsorbed at the oil/water interface anddetermines the initial curvature of the dispersed phase. Therequired curvature for the surfactant film to attain the mini-mal interfacial tension is assisted by the presence of a cosur-factant.This mixed monolayer of surfactant and cosurfactantat oil/water interface can exert a two-dimensional surfacepressure.The crowding of themixed surfactants system at theinterface produces stress in the system and releases it. Theinterface bends with the expansion of the film on one sideto maintain a balance with the other side until the surfacepressure at both sides of the interface becomes constant [9].At this point, the system can be thermodynamically stableswollen mixed micelle (o/w) or inverse mixed micelle (w/o)system. The size and shape of the dispersed nanodropletsare mainly governed by the curvature free energy and aredetermined by the bending elastic constant and curvature ofthe surfactant film.The elasticity of the film depends not onlyon the surfactant type and the thermodynamic conditions,but also on the presence of additives like alcohols, electrolytes,block copolymers, and polyelectrolytes.

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Micelles

o/w microemulsiondroplets w/o microemulsiondroplets

Water

Surfactant

Lamellar

Oil

Reverse micelles1Φ

2Φ

Figure 2: A model pseudoternary phase diagram, with the regionof existence of o/w ME, w/o ME micelles, reverse micelles, andbicontinuous two-phase system with three corners representing oil,water, and surfactant. Figure 2 is taken from Lawrence and Rees,2000 [10].

3. Theories of Microemulsion Formation

Thethreemain theories ofME formation are briefly discussedhere: interfacial or mixed film theory [11], solubilizationtheory [12], and thermodynamic theory [13]. Accordingto the thermodynamic theory of stabilization, ME formsspontaneously because of the low value of interfacial tensionon account of the diffusion of surfactant in the interfaciallayer and to the major entropy contribution that depends onthe mixing of one phase in the other in the form of numeroussmall droplets. In the mixed film theory, the interfacial filmis considered to demonstrate dissimilar behavior towards theaqueous and oily segment of the interface. The solubilizationtheory considers ME as swollen micellar systems, in whichwater or oil is solubilized the reverse micelle structures toform one-phase system (Figure 2). However, despite of all thetheories ofME formation, the reduction of the interfacial freeenergy to a very low value is of prime importance in the MEformation.

4. Formulation Aspect of Microemulsion

4.1. Selection of Lipid Phase. Many types of lipids are avail-able, which include vegetable oils, glycerides, partial glyc-erides of medium-chain and unsaturated long-chain fattyacids, and polyalcohol esters of medium-chain fatty acids[14]. The physicochemical properties of the lipids must beknown and understood to use them in the developmentof ocular MEs. Certain lipids, especially triglycerides, arecompletely lipophilic with HLB values of zero or close to zerobecause of the absence of any hydrophilic moiety. On theother hand, in case of lipids with hydrophilic moieties, therecan be difference in the degree of hydrophilicity. Further,it should be also considered that most of the commerciallyavailable lipids are not pure species, rather they are mixturesof lipids with differing hydrophilic-lipophilic properties and

Table 1: List of lipid phase.

Esters of fatty acidsEthyl oleate,isopropyl myristate, andisopropyl palmitate

Monounsaturated fatty acids Oleic acidSaturated fattyacids/low-molecular-weighttriglycerides

Capric-caprylictriglyceride (miglyol 80),octanoic acid

fatty acid chain lengths.Thiswill create difficultywhen a com-bination of lipids with different hydrophilic-lipophilic prop-erties is selected as lipid phase. Formation of ME with high-molecular-weight oils such as triglycerides is difficult as theycontain long-chain fatty acids which are difficult to penetratethe interfacial film formed by surfactants/cosurfactant toassist in the formation of an optimal curvature [15, 16]. Forthis reason, smaller-molecular-weight oils (e.g., medium-chain length triglycerides are more preferred). Hydrocarbonesters of medium-chain fatty acids play an optimal role in theformation of ME and are most frequently used as an organiccomponent of ophthalmic ME, enlisted in Table 1.

4.2. Selection of Surfactant(s). The selection of surfactantsystem is one of the most critical steps in the design of aME system. In ME, solubilization of oils is much greaterthan most micellar solutions. For one surfactant molecule,it may be possible to dissolve 10–30 oil molecules (o/wME) or 10–300 water molecules (w/o ME). The surfactant(s)must solubilize and reduce the interfacial tension to ultralow level (

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Table 2: List of surfactants commonly used in ophthalmic microe-mulsion.

General class Examples

Lecithin and lecithinderivatives

Pure phospholipids (e.g., soyaphosphatidyl choline) and mixedphospholipids, sodium cholateHydroxylated phospholipids/lecithin

Glycerol fatty acid esters

Polyglycerol fatty acid estersPolyglycerol polyricinoleatePropylene glycol fatty acid esters(e.g., polyoxyethyleneglyceroltriricinoleate,cremophor EL (macrogol-1500-glyceroltriricinoleate)monobutyl glycerol)

Sorbitan fatty acid esters Span 20 (sorbitan monolaurate)Span 80 (sorbitan monooleate)

Polyoxyethylene sorbitanfatty acid esters

Tween 20 (polyethylene glycolsorbitan monolaurate)Tween 80 (polyethylene glycolsorbitan monooleate)

Others (potentialcosurfactants)

Propylene glycolPEG 200

birefringence, flow properties, and stability which is alsoexplained later in the method of microemulsion preparationsection. After the approximate determination of ME region,a more detailed study of this region of the phase diagram isrequired to assess long-term stability of the ME.

Other desirable characteristics for surfactants include noor very low ocular toxicity, and the ability to biodegradeneither too quickly nor too slowly. Surfactants which maybe employed include both ionic agents such as cationic,anionic, or zwitterionic and nonionic agents or mixturesthereof. Among the various classes of surfactants nonionicsurfactants are more versatile functional agents because oftheir improved solubilization characteristics: nonirritancy,ability to prolong precorneal retentionwith enhanced perme-ability. In general, all the surfactants to be used in ophthalmicME must be subjected to extensive ocular irritation/toxicitystudies because large amount of surfactant is required for theME formation [19–21]. Nevertheless, very little research hasbeen carried out on the toxicity of surfactants in ME form.One has to be very careful and make sure that the ocularirritation does not persist. A tabulated list of surfactants asper the available literature is provided, which can be used inthe formation of ophthalmic ME in Table 2.

4.3. Selection of Cosurfactant. One of the major consider-ations in the formulation of ME is the flexibility of theinterface to promote the formation of ME. For this purpose,surfactant(s) are often combined with a cosurfactant. Thepenetration of cosurfactant into the interfacial film producesa more fluid interface by allowing the hydrophobic tails ofthe surfactants to move freely at the interface. Sufficientlylow fluidity and low surface viscosity of the interfacial filmresults in the formation of nanodroplets with small radius

Table 3: List of cosurfactants.

Alkanol Ethanol, propanol, and 1-butanolAlkane-diols 1,2-Propane diol, 1,2-butane diolAlkane-polyols Glycerol, glucitol, and polyethylene glycol

of curvature (50–500 Å). Generally, low-molecular-weightalcohols and glycols with chain length ranging from C

2

toC10

are used as cosurfactants in preparing stable ME [16, 22].It is reported that the chain length of alcohols is inverselyproportional to the ocular irritation potential. Among var-ious alcohols used, aliphatic n-alcohols with carbon chainlength of 3–8 were ranked as strong irritants while ethanolwas ranked as a moderate irritant. 1,2-Alkanediols withcarbon chain length of 5–8, previously reported as nontoxicsubstitutes to n-alcohols, were found to be strong irritantswhile those with carbon chain length of 2-3 were observedto be only slightly irritating. It is also seen that ME preparedby incorporation of long carbon chain alcohols (pentanol,hexanol) as cosurfactant showed signs of ocular irritation,whereas that of short carbon chain behaved as mild irritants.Ruth and coworkers compared the efficacy of butanol andethanol as cosurfactants in ME constituted of isopropylmyristate (IPM), egg lecithin, and water. The quantity ofethanol required for the preparation of ME is seven timeshigher than the quantity of butanol.The difference in efficacybetween the cosurfactants is based on the length of thecarbon chain [23]. The distribution of the alcohol betweenthe interface and the continuous aqueous phase is based onits hydrophilic character. The ethanol has an interface/waterdistribution coefficient lower than the butanol. Therefore, itshigher solubility in water requires the use of higher quantitiesin order to obtain an interface with similar mechanicalproperties to that obtained by using butanol. It has also beenreported in some isolated studies that [23–25] clear stableMEcan be achieved without the use of cosurfactant; the MEs soprepared were found to be practically nonirritant. A list ofavailable cosurfactants is given in Table 3.

5. Charge Effect

Attempts have been made to prolong the time of residencein ocular tissues followed by topical instillation by means ofelectrostatic adhesion of droplets over the corneal surface.It was initially believed and now has become clearer frommany reports in the literature that an occurrence of electro-static attraction between the cationic emulsified droplets andanionic cellular moieties of the ocular tissues exists [26]. Asthe corneal area is negatively charged, the positively chargeddroplets might bind to the sites. The charge is provided by apositively charged lipid, for example, stearylamine or cationicpolysaccharide, for example, chitosan.

Beilin and coworkers reported that the presence of pos-itive charge on the surface of internal phase could influencedrug absorption through corneal penetration [27]. The sup-position was based on the presence of negative charge on thecorneal surface which would facilitate binding of positivelycharged droplets of the submicron emulsion [28]. Calvoand coworkers studied comparative behavior of drug release

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through colloidal systems, namely, nanocapsules, nanoparti-cles, and submicrons emulsion, the findings of which showedan increased corneal permeation of indomethacin due toincorporation of the drug in colloidal carriers instead ofthe electrostatic attraction between the negatively chargedcornea and positively charged drug carrier system [29]. Theyfound that the incorporation of the drug into a colloidalsystem facilitates the uptake of nanoglobules by the cornealepitheliumwithout causing any damage to the cell membrane[30].

6. Methods of Preparation

There are two methods of preparing microemulsion, viaphase inversion temperature and phase titration methods.However, no study has been reported yet on microemulsionfor ocular delivery prepared using phase inversion technique;thus, this technique is not discussed in the current review.

6.1. Phase Titration Method. Phase titration is low-energyemulsification method. This utilizes the spontaneous diffu-sion of surfactant or solvent molecules into the continuousphase due to ultra low interfacial tension. Preparation of MEinvolves investigation of area of formation of single-phaseregion in the phase diagram which is composed of 4 corners,each of oil, water, surfactant, and cosurfactant, respectively.In this method, all the components of formulation are mixedin proportions varying from 0 to 100% representing inthe phase diagram, in anticipation to obtain a clear phase.Subsequently, optimization is appropriately done based onthemost clear region in the phase diagramand then to finalizethe composition of most stable ME [18, 22].

7. Characterization of Microemulsion

ME characterization can be divided into 3 major areas,physical evaluation, electrochemical evaluation, and micro-scopic evaluation. Appearance, viscosity, and optical clarityprovide useful information about the physical nature ofthe microemulsion. Osmolality is essential for physiolog-ical acceptance of the formulation by ocular tissues andis measured using osmometer whereas the surface tensionessential to ensure uniform spreading on corneal surfacewhich is determined by use of tensiometer. The presence ofcubic, rod-shaped, and elongated cylindrical micelles andthe transition between ME structures can be interpretedby changes in viscosity. The rheological properties of MEhave been extensively reviewed by Hellweg [31], Strey [32],and Ktistis [33]. Conductivity measurements can be usedto determine whether a ME is oil-continuous or water-continuous and may also be used to monitor percolation orphase inversion phenomena. Dielectric measurements havebeen used to investigate both the structural and dynamicfeatures of ME. The optical clarity of ME is due to thesmall droplet size and is evaluated by using microscopicmethods and light scattering methods which gives satisfac-tory results. However, the various structures arising due tointernal transition in the ME demand special measurementtechniques [34]. A variety of methods [35], such as freeze

fracture electron microscopy, and a range of light scatteringmethods, such as small-angle X-ray scattering, small-angleneutron scattering, total intensity light scattering, and photoncorrelation spectroscopy, may be used to determine theparticle size of a ME. SANS and SAXS are useful methods todetermine theMEmicrostructure and droplet size and shape.Pulsed gradient spin echo- (PGSE-) NMR technique can beutilized in the determination of the self-diffusion coefficientof the different components of the ME. DSC is utilized indetermination of the state of water in ME by distinguishingbetween bulk water and bound water.

8. Mechanism of Drug Release fromMicroemulsion

ME droplets exist in micelle form and various structures:droplets of oil or water, ordered or lamellar structures. Thedrug loaded in the ME exists mostly in the internal phase.However, at the equilibrium state, the drug can be distributedamong dispersed phase, continuous phase, and surfactantinterphases. The drug release from the ME can be explainedby using two models. One model explains the drug diffusionthroughout the droplet as rate-limiting step of drug release,whereas the other model considers the interfacial barrierbetween the droplet and surrounding as rate-determiningstep of drug release. The most acceptable model of drugrelease from ME described the combination of mass balanceand linear dependence of mass fluxes on concentrations.

Generally, the drug release from the ME is studiedby determining the mass transfer constants of the drugsthrough a biological membrane separating the ME from thereceiver phase. The mass transfer constant is directly relatedto the partition coefficient of the drug in oil-surfactant-water mixtures. Release of drug from ME mainly dependson oil-aqueous phase ratio, droplet size, and distribution ofdrug in the phases of ME. The release pattern is furthergoverned by the rate of transfer of drug from disperse phaseto continuous phase and thereby from continuous phasethrough the biological membrane. It is anticipated that thepermeation of hydrophilic drug through the biological mem-brane contacting the ME will depend on the concentrationof drug in the aqueous phase of ME and vice versa in case oflipophililc drug.

9. In Vitro Models for Drug Release Kineticsfrom Microemulsion

Although the available diffusion models for in vitro diffusionkinetics may not give the exact scene happening in vivo.In vivo sink conditions and continuous clearance of thereleased drug by tear from surface of cul de sac as well ofocular tissues are difficult to maintain. The artificial cellulosemembrane cannot mimic the barriers of corneal membrane.The constant volume of diffusion cell will not be able toeliminate the drug released by tear fluid turnover. So themethod of diffusion cell is not representative of the realsituation in vivo.

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Table 4: Brief summary of reported work on formulation development on ocular microemulsion.

Researchers Drugs used Surfactants Co-surfactantsOther

ingredients Description and outcome of the study

Gallarateet al., 1988;Gasco et al.,1989[42, 43]

Timolol Lecithin 1-Butanol

Isopropylmyristate,

octanoic acid(OA), and

distilled water

The topical administration of timolol as an ion-pairwith octanoate was achieved by the use of anoil-in-water ME. The areas under the curve fortimolol in aqueous humour after administration ofthe ME and the ion-pair solution were 3.5 and 4.2times higher, respectively, than that observed afterthe administration of timolol alone

Gallarateet al., 1993[44]

Levobunolol(LB) Lecithin 1-Butanol

Isopropylmyristate,

octanoic acid(OA), and

distilled water

Aqueous and aqueous-PEG 200 solutions and o/wME containing LB coupled to OA as lipophilicion-pair were prepared and investigated in vitro, inview of possible ophthalmic applications.Permeation studies in aqueous and inaqueous-PEG-200 solutions through the artificialmembrane indicated a higher apparentlipophilicity of LB-OA with respect to the drugalone. The ME, which was isotonic andnonirritating to rabbit eyes, appears as a potentiallyinteresting ophthalmic vehicle for LB

Haße andKiepert, 1997[45]

Pilocarpinenitrate

Macrogol-1500-glyceroltriricinoleate

and lecithin

PEG 200,Propyleneglycol,

Isopropylmyristate,

distilled water

The authors developed o/w ME for ocularapplication of pilocarpine. Prolonged in vitro drugrelease was observed fromME.The miotic activitywas measured on albino rabbits. Forophthalmological use, the miotic retarding effect ofpilocarpine in ME turns out to be advantageous

Fialho anddaSilva-Cunha,2004 [24]

Dexamethasone Cremophore EL Propyleneglycol

Isopropylmyristate,

benzalkoniumchloride, anddistilled water

Developed MEs showed acceptablephysicochemical properties and stability. Theocular irritation test suggested that the MEs didnot provide any significant alteration to the eyelids,conjunctiva, cornea, and iris. This formulationshowed greater penetration of dexamethasone inthe anterior segment of the eye and also release ofthe drug for a longer time when compared with aconventional preparation. The area under the curveobtained for the ME system was more than twofoldhigher than that of the conventional preparation

Alany et al.,2006 [23]

Pilocarpinehydrochloride

Sorbitan laurate,polysorbate 80

Alkanol oralkandiol

Ethyl oleate,water

w/o MEs capable of undergoing a phase-transitionto lamellar liquid crystals or bicontinuous MEsupon aqueous dilution were formulated. Resultsshowed only formulations having cosurfactants; allother ingredients were nonirritant to rabbit eyes. Itwas observed that cosurfactant irritation wasdependent on its carbon chain length. Precornealclearance studies revealed that the retention ofcolloidal and coarse dispersed systems wassignificantly greater than an aqueous solution withno significant difference between MEs

Lv et al.,2006; 2005[25, 46]

Chloramphenicol Tween 20 Span 20Isopropylmyristate,

distilled water

Chloramphenicol was trapped into oil core orpalisade layer of the o/w ME free of alcohols. Itsstability was investigated by the high-performanceliquid chromatography (HPLC) assays andH1-NMR in the accelerated experiments of 3months. The stability of the chloramphenicol in theME formulations was increased remarkably; thepseudoternary diagram of the ME is given inFigures 3 and 4

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Table 4: Continued.

Researchers Drugs used Surfactants Co-surfactantsOther

ingredients Description and outcome of the study

Chan et al.,2007 [47]

Pilocarpinehydrochloride

Polyoxyethylenesorbitan monooleate

Sorbitanmonolaurate

Ethyl oleate,water

ME-based phase transition systems were evaluatedfor ocular delivery of pilocarpine hydrochloride(model hydrophilic drug). These systems undergophase change fromME to liquid crystalline (LC)and to coarse emulsion (EM) with a change inviscosity depending on water content (Figure 5).The miotic response and duration of action weregreatest in case of ME and LC formulationsindicating high ocular bioavailability (Figure 5)

Baspinaret al., 2008[48]

Everolimus Poloxamer 184 Propyleneglycol

Triacetin,deionized andsterile water

In this study, ocular MEs bearing everolimus wereprepared for preventing corneal-graft rejection.The permeation rate of the model drug everolimusthrough a freshly isolated pig cornea wasdetermined ex vivo. Authors concluded thatprepared ME is a promising ocular formulation forpreventing corneal-graft rejection

Kesavanet al., 2013[26]

Dexamethasone Tween 80 Propyleneglycol

Isopropylmyristate,

chitosan, anddistilled water

The mucoadhesive chitosan-coated cationic MEswere prepared for treatment in condition ofchronic uveitis. The average globule size was lessthan 200 nm with a positive surface charge. Thedeveloped microemulsion revealed stability for 3months. The in vivo studies evidenced markedimproved therapeutic effect of the incorporatedsteroid

In previous studies, the uptake of drugs across thecornea in vitro has been investigated using corneal perfusionchambers [36, 37] maintaining constant volume of buffer indonor side and the receiver side. Corneal permeability wasexpressed as the apparent permeability coefficient

𝑃app =𝑑𝑞

𝑑𝑡

1

𝐶𝑜

𝐴

, (1)

where 𝐶𝑜

is the initial donor side drug concentration and𝐴 is the corneal surface area [38]. The value of 𝑃app (cm/s)obtained describe how well the compounds penetrated thecornea from the buffer used [39]. Nevertheless, this param-eter was difficult to relate with in vivo bioavailability as itdid not take physiological and formulation variables intoaccount. In the in vitro experiments, the drug remains inintimate contact with the isolated cornea; certain drugs tendto swell cornea due to intrinsic corneal toxicity. Some poorpenetrants require very long incubation times to reach steadystate, thereby prolonging the corneal exposure time duringthe experiment as a result of which it becomes difficult tomaintain the corneal integrity throughout the study. Anotherlimitation is ocular tear flow dynamics which are differedfrom the design of in vitro chamber. When a drug is appliedto the eye in vivo, it is washed away with continuous tear flowand with the overfilling of cul de sac. The in vivo tear volumeis 7–12𝜇L with approx. 7% turnover per minute [40] which isdifficult to maintain in the laboratory in vitro conditions.

Recently, some authors [41] studied transcorneal diffu-sion of the drug by using novel modified Franz diffusionchamber with a mechanism to control tear fluid turnover.They performed diffusion of timolol maleate incorporated in

2Φ

M

M

HF

EB

H2O Tween 20 + 10% IPM

Span 20 + 10% IPM

Figure 3: The pseudoternary phase diagram of chloramphenicol(Free) microemulsion composed of Span 20 + Tween 20 + isopropylmyristate +water showing formation of single-phasemicroemulsionregion (M) and biphasic region (2Φ). Figure 3 is taken from Lv et al.,2005 [46].

in situ gel and aqueous solution. The in vitro assembly con-sisted of four cells, each having upper and lower chambers.The upper chamber served as a donor compartment in which100 𝜇L of drug solution/formulation was placed. The upperand lower chambers were separated by excising goat cornea.The lower chamber served as a receiver compartment thatwas infused continuously with simulated tear fluid at the rateof 20𝜇L/min with the help of peristaltic pump. The wholesystem was maintained at 37 ± 0.5∘C.

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Table 5: List of studied physicochemical parameters of various reported ocular MEs.

Authors DrugPhysicochemical properties (mean ± SD)

pH Averagediameter (nm)Refractiveindex

Surface tension(mN/m)

Viscosity(mPa⋅s)

Gasco et al., 1989 [43] Timolol maleate — 15 — — 24.8 ± 0.7

Haße and Keipert, 1997 [45] Pilocarpine nitrate 5.5–6.0 25–45 1.37–1.38 31-32 7.0–9.0

Fialho and da Silva-Cunha, 2004 [24] Dexamethasone 6.99 ± 0.02 50.85 ± 1.24(𝑛 = 6) 1.38 ± 0.01 27.79 ± 0.01 40.27 ± 0.98

Lv et al., 2006 [25] Chloramphenicol — 53–59.5 — — —

Chan et al., 2007 [47] Pilocarpine HCl —

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Table 6: Recent patents filed dealing with ocular MEs.

Recent patents Drugs used Surfactants Co-surfactantsOtheringredients Description and outcome of the study

Sergio et al.,WO 154985A1,2011 [49]

Steroids(difluprednate),prostaglandin(latanoprost)NSAID (diclofenac),antioxidant, andpegaptanib

d-𝛼-tocopheryl PEG1000 succinate Glycerol

Vitamin E,MCT, anddisodiumphosphate

The inventors developed o/w ME forencapsulation of water insoluble drugs fortopical ophthalmic application. Thedeveloped ME carrier remained stable fora period of 6 months displaying a particlesize of 15 nm without any signs ofinstability or separation

Gobel,EuropeanpatentEP-2485714A1,2012 [50]

Tacrolimus

Lecithin,decyl glucoside,span 80 (sorbitanmonooleate), andbrij 30(polyoxyethylene(4)lau-ryl ether)

Pentyleneglycol,propyleneglycol, andPEG-20

Dibutyladipate,isopropylmyristate, andtartaric acid

The transparent o/w ME for delivery ofimmunosuppressant agent tacrolimus issubjected to HET-CAM test and claimedto be free from signs of irritation. Theparticle size range varied from 5 to100 nm. Additionally, the tacrolimus MEwas found to penetrate efficiently thestratum corneum tissue and reach thedermis due to presence of lymphocyte,which is the target for the activeingredient

Carli et al.,US Patent US8414904B2,2013 [51]

Prostaglandinanalogue(latanoprost,travoprost, andbimatoprost)

Tween 80,brij 52, 56, 58 Tween 20

Ethyl oleate,miglyol 812,ricinus oilsorbitol,glycerol,chlorobutanol,andbuffer (pH 7.4)

o/w MEs composed of prostaglandinformulated with two nonionic surfactantsand one oily component displayed aparticle size not more than 700 nm and alow zeta potential of +2 to −2 due to theuse of nonionic surfactants as emulsifyingagents. The formulation was claimed tobe free from any signs of irritation onrabbit eyes. The ME remained stable for aperiod of 12 months

12. Conclusion andFuture Prospects

ME holds significant promise for topical ophthalmic appli-cation due to their eye-drop-like consistency, nano dropletsize range, and phase transition behavior.MEsmay constitutean effective system for the delivery of both water soluble andinsoluble drugs to the ocular tissues without compromisingthe convenience to the patients as well as ophthalmologistsfor adjustment of dose and dosing frequency according to thedisease therapy. Due to the phase transition behavior,ME canform in situ precorneal depots resulting in improved reten-tion and, thus, prolonged release of incorporated drug. Fromthe researched literature, it has been found that judiciouslyselected lipid phase (generally medium-chain triglycerides),surfactant phase (especially nonionic), and cosurfactant canbe combined with drugs in such a way that drug is releasedinto the eye in a precise and controlled manner. The MEsystems for ocular delivery have been reported to possessexcellent physicochemical properties and stability. Apartfrom this, they are easy to fabricate and characterize. Their

process of preparation is simple and inexpensive leading toeasy scale-up and reduced final cost of dosage form. As hasbeen discussed in this review, there is ample evidence thatMEs may constitute efficient future ocular drug delivery sys-tems that have the ability to penetrate different ocular tissuesby circumventing the anatomical and physiological barriers,thereby completely replacing rather primitive conventionalocular drug delivery systems like eye drop, eye ointments, andso forth. MEs are expected to deliver any drug to both theanterior and posterior segments of the eye, at the right timein a safe and reproducible manner at required level. However,a wider area of further studies, such as validation of drugrelease for ophthalmic application togetherwith developmentof new technologies, holds the future of clinical significanceof MEs in the effective treatment of ocular diseases.

Acknowledgment

The authors are grateful to Professor R. M. Dubey, ViceChancellor of IFTM University, for providing necessaryfacilities to carry out the work.

10 ISRN Pharmaceutics

O

ME 5%

S

ME 10%

LCB

C

A

o/w EM

SOL.W

100

100

10090

90

90

80

80

80

70

70

70

60

60

60

50

50 50

40

40

40

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30

30

20

20

20

10

10

10

0

0

0

Figure 5: Crillet 4 system. W: 100% water; O: 100% CrodamolEO; S: 100% surfactant blend of Crill 1 and Crillet 4 (ratio of2 : 3). (A) Systems formingwater-in-oilmicroemulsions; (B) systemscontaining liquid crystals; (C) systems forming coarse emulsions.ME 5%: water-in-oil microemulsion containing 5% (w/w) aqueousphase; ME 10%: water-in-oil microemulsion containing 10% (w/w)aqueous phase; LC: lamellar liquid crystalline systems; EM: oil-in-water coarse emulsion systems; SOL: aqueous solution. Figure 5 istaken from Chan et al., 2007 [47].

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