Iranian Journal of Pharmaceutical Sciences Summer 2011: 7(3): 139-150ijps.sums.ac.ir

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Review Article

Pharmaceutical Nanoemulsions and Their Potential Topical andTransdermal Applications

Samira Sadat Abolmaalia, Ali Mohammad Tamaddona,*, Fakhr Sadat Farvadia, Saeed Daneshamuza, Hamidreza Moghimib

aDepartment of Pharmaceutics, Faculty of Pharmacy, and Pharmaceutical Sciences ResearchCenter, Shiraz University of Medical Sciences, Shiraz, Iran

bDepartment of Pharmaceutics, Faculty of Pharmacy, Shaheed Beheshti University of MedicalSciences, Tehran, Iran

AbstractTopical and transdermal drug delivery systems are noninvasive and can be self-

administered with the minimization of side-effects, have received increased attentionduring the past few years. Nanoemulsions, emulsions sized between 20-200 nm withnarrow distributions, offer several advantages for topical and transdermal deliveryof pharmaceutical agents including controlled droplet size, the ability to efficientlydissolve lipophilic drugs, enhanced skin permeation and extended release oflipophilic and hydrophilic drugs. Moreover, they exert good sensorial and physicalproperties such as complete dispersion on skin and skin hydration in cosmeticproducts. The review deals with nanoemulsion applications in topical and transdermaldrug and gene delivery.

Keywords: Drug Delivery; Nanoemulsion; Nanotechnology; Skin, Topical;Transdermal.

Received: December 22, 2010; Accepted: March 18, 2011.

1. IntroductionNanoemulsions: definition, composition,

preparation and characterizationAn emulsion is generally described as a

heterogonous system composed of twoimmiscible liquids. Emulsions like otherdisperse systems are thermodynamicallyunstable as a result of excess free energyassociated with the surface of the internalphase. Nanoemulsion is a type of emulsion

sized between 20-200 nm with narrowdistributions. They are transparent ortranslucent with a bluish coloration. So, thedefinition is different from that of sub-micronemulsions. It is worth saying that, while thedistinction between a nanoemulsion and anemulsion, in terms of their size, is ratherarbitrary, nanoemulsions because of their smalldroplet size, cause a large reduction in gravityforce; therefore, Brownian motion may besufficient to possess a higher stability againstsedimentation or creaming than an emulsion [1].

The distinction between a microemulsionand a nanoemulsion is blurred because the

*Corresponding author: Dr. Ali Mohammad Tamaddon,Pharmaceutical Nanotechnology and Cellular Delivery Lab,Department of Pharmaceutics, Shiraz University of MedicalSciences, Shiraz, 71345, Iran.Tel. (+98)711-2426070; Fax: (+98)711-2424126

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description of a nanoemulsion is very similarto that of a microemulsion, though thephysical appearance of a nanoemulsionresembles that of a microemulsion, in thatboth systems are transparent (or translucent)and of low viscosity, there is an essentialdifference between the two systems; namelythat a nanoemulsion (i.e. an emulsion) is, atbest, kinetically stable or metastable, while amicroemulsion is thermodynamically stable[2]. Besides, nanoemulsions are two-phasesystems where the dispersed phase droplet sizehas been made in the nanometer size range,the microemulsions, and micellar systemsare single-phase systems. As a consequence,many of the nanoemulsions reported in theliterature do not possess long-term stability.They may experience Ostwald ripening orcoalescence instabilities that could becontrolled by modifying oily phase solubility,surfactant quantity and molecular weight [1,3]. Some nanoemulsions have, however,exhibited sufficiently high levels of stabilityto be proposed as vehicles for drug delivery.

One supposed advantage of ananoemulsion over a microemulsion is that itrequires a lower surfactant concentration forits formation. When comparing this surfactantconcentration with the 20% surfactanttypically needed to prepare a microemulsioncontaining a comparable amount of oil, oneshould realize that the droplet size of amicroemulsion thus produced would typicallybe ~10 nm. Consequently, in order to producenanoemulsion droplets of the comparablesize, the amount of surfactant required wouldincrease (the surface area of the droplet varieswith the square of the droplet radius) to acomparable value. Moreover unlikemicroemulsions, they can be diluted withwater without changing the droplet sizedistribution [4]. There are several reports inliterature which seems wrongly addressednanoemulsions prepared by titration methodbased on pseudoternary phase diagram atrelatively high levels of surfactant and co-

surfactant [5-16]. Nanoemulsions, as a consequence of their

relatively high kinetic stability, low viscosity,and transparency/translucency, are veryattractive for a range of industrial applications,including the pharmaceutical field where theyhave been explored as drug delivery systems.They offer several advantages for the deliveryof drugs, biological, or diagnostic agents.Traditionally, nanoemulsions have been usedin clinics for more than four decades as totalparenteral nutrition fluids. Several otherproducts for drug delivery applications suchas Diprivan, Liple, and Ropion have beenmarketed [17] (Table1).

Although nanoemulsions are chiefly seenas vehicles for administering insoluble drugs,they have more recently received increasingattention as colloidal carriers for targeteddelivery of various anticancer drugs, genes,photosensitizers, or diagnostic agents.Research with perflurochemicalnanoemulsions has shown promising resultsfor enhanced sonography imaging and thetreatment of cancer in conjugation with othertreatment modalities or by targeted deliveryto the neovasculature [17].

Nanoemulsions contain oil phase,surfactants or emulsifiers, activepharmaceutical ingredients (drugs ordiagnostic agents), and additives. The oilphases are mainly natural or synthetic lipids,fatty acids, oils such as medium or long chaintriglycerides, or perflurochemicals. The mostcommonly used emulsifiers and co-emulsifiers include either natural or modifiedlecithins, poly (ethylene oxide) (PEO)-containing block copolymers,PEG-conjugated castor oil derivatives(Cremophore EL), glycerides, and positivelycharged lipids. Other pharmaceutical additivessuch as pH adjustment agents, antioxidants,flavors, and preservatives may also beincluded in the final formulation, if requested[17].

Nanoemulsions are thermodynamically

Pharmaceutical nanoemulsions potential applications

unstable and require considerable mechanicalenergy for their preparation. The mechanicalenergy can be supplied in the form of highpressure homogenizer, Microfluidizer, or anultrasonic generator.

High energy methods cannot be used forsome cases, especially for labile moleculesand if there is a limited access to theirrespective expensive equipments. In thesecases, low energy emulsification methods,such as spontaneous emulsification or phaseinversion are employed. Bouchemal et al.prepared nanoemulsions by injecting oil phasesolution in a water miscible organic solvent,e.g. alcohols, into aqueous phase undermagnetic stirring [18]. Diffusion of organicsolvent into the external aqueous phase leadsto the formation of nanodroplets. Fernandezet al. proposed the method of phase inversiontemperature for polyoxyethylene type non-ionic surfactants [4]. Increasing emulsiontemperature over phase inversion point causesoil swollen droplets (o/w emulsions) transforminto water swollen droplets (w/o emulsions).The system crosses a point of zero curvatureand minimal interfacial tension promotingthe formation of finely dispersednanoemulsions.

Nanoemulsion systems are routinelycharacterized for their particle size and surfaceproperties (surface electrostatic charge andmorphology). Size of nanoemulsion dropletsdetermines their behavior both in vitro and invivo. This can be measured using an ensemble(e.g., spectroscopic methods such as lightscattering), counting (e.g., microscopy suchas freeze fracture electron microscopy) orseparation method (e.g., analytical ultracen-trifugation). Similar to particle size, surfacecharge of the nanoemulsion droplets hasmarked effect on the stability of the emulsionsystem and the droplets’ in vivo dispositionand clearance. Conventionally, the surfacecharge on the emulsion droplets has beenexpressed in terms of zeta potential, which isroutinely measured using a Zetasizer or the

ZetaPlus instrument. As the emulsion dropletsare a result of interfacial phenomenon broughtout by surface active agents, their zetapotential is dependent on the extent ofionization of these surface active agents andcounter-ion concentration. According toDLVO electrostatic theory, the stability ofthe colloid is a balance between the attractivevan der Waals’ forces and the electricalrepulsion because of the net surface charge.If the zeta potential falls below a certain level,the emulsion droplets will aggregate as aresult of the attractive forces. Conversely, ahigh zeta potential (either positive ornegative), typically more than 30 mV,maintains a stable system [17].

2. Rationales for topical and transdermalapplications of nanoemulsions

Nanoemulsions have been formulated fora broad variety of topical and transdermalapplications in fields of cosmetic, drug andgene delivery. They are generallyadvantageous because of low skin irritation,high drug loading capacity and potential forskin hydration and drug permeationenhancement. The previously reportedpublications considering topical andtransdermal applications of nanoemulsionsare categorized into cosmetic, drug and genedelivery sections.

2.1. Cosmetic applicationsNanoemulsions have attracted considerable

attention for application in personal hairproducts. They were found useful for anoptimized dispersion on skin and controlleddelivery of cosmetics. They are easily valuedin skin care because of their good sensorialproperties and their biophysical propertiesespecially hydrating power [19].

Yilmaz and Borchert have shown the effectof lipid content and charge of nanoemulsionson skin hydration, elasticity and erythema[20, 21]. Positively charged nanoemulsionwith stratum corneum lipids (PNSC),

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positively charged nanoemulsion withoutstratum corneum lipids (PN) and negativelycharged nanoemulsion with stratum corneumlipids (NNSC) were compared in 14 healthyfemale subjects. The formulations wereprepared by high-pressure homogenizationfollowed by addition of Carbopol 940 asthickener to improve low viscosity of thenanoemulsions. The formulations were stable,indicated by no significant change of themean droplet size and the viscosity due to thepresence of the co-surfactants phytosphin-gosine (PS) and myristic acid, providing zetapotential values of +35±4 mV for PNSCcreams, +38±5 mV for PN creams and −43±5mV for NNSC creams. These high zeta-potentials lead to strong repulsion of thenanodroplets and prevent them fromaggregation, flocculation and coalescence.PNSC cream was compared to Physiogel®

cream with similar compositions regarding thecontent of ceramide. For both formulations,the levels of skin humidity and elasticity weredetermined similar using Corneometer® 825and Cutometer® SEM 575. If PNSC creamwas compared to PN, all values of PNSCcreams were significantly higher than thoseof PN creams, indicating the requirement ofSC lipids in order to prolong the effect on skinproperties and to improve the barrier functionof skin by leading to an increase of skinhumidity and thus an increase in skinelasticity. All values of PNSC creams weresignificantly higher than the ones of NNSCcreams, indicating that PS, inducing thepositive charge, was crucial for the enhancedefficacy on skin humidity and elasticity. Thecombined results suggested that, PNSC wassignificantly more effective in increasing skinhydration and elasticity than PN and NNSCindicating that phytosphingosine inducingthe positive charge, SC lipids and ceramide 3Bare crucial for the enhanced effect on skinhydration and viscoelasticity. In another study,Zhou and coworkers developed lecithin

nanoemulsion with droplet sizes below 100nm and improved skin hydration capacity ifincorporated into o/w cream by 2.5 times ofgeneral emulsion [22]. Besides, a significantimprovement in dry hair product is obtainedwith a prolonged effect after a cationicnanoemulsion use. Hair becomes more fluidand shiny, less brittle and non glassy [23].

2.2. Drug and gene deliveryIn the field of topical and transdermal drug

and gene delivery, there are several studiesbased on the enhancement of skin permeationand extended release for hydrophilic andlipophilic drugs, application of nanoemulsionsfor topical gene delivery, and photodynamictherapy.

2.2.1. Enhancement of skin permeationWu et al. [24] described topical transport

of hydrophilic compounds using w/onanoemulsion containing sorbitan monooleate(Span®80), polyoxyethylene 20 sorbitanmonooleate (Tween®80), olive oil and water.Nanoemulsions were tested for their ability tofacilitate transport of a model hydrophilicsolute, inulin, across hairless and hairy mouseskin and hairy rat skin following topical invitro application. The similarity of permeationprofiles of inulin incorporated in water-in-oil nanoemulsions through hairy rat andhairless and hairy mouse skin strongly impliesthat transport of inulin from nanoemulsionshas been independent of animal skin charac-teristics such as stratum corneum thicknessand follicle-type. Besides, they have foundthat the rate and the extent of inulin transportacross hairy mouse skin were highlydependent on the hydrophile-lipophile balance(HLB) of the surfactant mixture in thenanoemulsion. Nanoemulsions prepared usingmixtures with lower HLB exhibitedsignificantly higher rate and extent oftransport. The authors concluded that water-in-oil nanoemulsions prepared with a lipid

Pharmaceutical nanoemulsions potential applications

phase whose HLB is compatible with normalsebum would efficiently facilitate skintransport of large hydrophilic moleculesdissolved in the aqueous core and suchtransport is expected via a transfollicularpathway.

Mou et al. [25] have developed a hydrogel-thickened nanoemulsion system for topicaldelivery of lipophilic drugs such as camphor,menthol and methyl salicylate. Thenanoemulsions had been prepared using highpressure homogenization followed bydispersion into carbomer 940-based gel matrixwhich had no significant influence on dropletsize. The formulation, containing 5% drug,soy lecithin, Tween 80, Poloxamer 407 andpropylene glycol, had spherical shape, smalldiameters (50-60 nm) and high permeationrate. The high permeation rates of theformulations had been attributed to severalfactors. The high concentration (5%) of drugsresulted in high concentration gradient, whichmight be the main permeation mechanism ofdrugs into the skin and could act as drugreservoirs where drug is released from theinner phase to the outer phase and then furtheronto the skin. In addition, due to the smalldroplet diameters, the oily droplets mightembed into the stratum corneum and the drugmolecules could directly be delivered fromoily droplets into the stratum corneum withouta transfer via hydrophilic phase ofnanoemulsions. Then drug moleculespermeate more easily into stratum corneum.

There are several classes of drugsconsidered for nanoemulsion formulationswith enhanced topical and transdermaldelivery such as steroids, non-steroidal anti-inflammatory and cytotoxic drugs.

A. Non-steroidal anti-inflammatory agentsKuo et al. [26] investigated bioavailabil-

ity and anti-inflammatory effect ofMicrofluidizer based nanoemulsionscontaining alpha, delta or gamma tocopherolcompared to their respective nanosuspen-

sions. The antioxidant nanoemulsionformulations, made of phosphatidyl cholineand soybean oil in Tween 80 and water, hadmean sizes in range of 42-56 nm. Theformulations exhibited a significant anti-inflammatory effect in croton oil inducedinflammation in CD-1 mouse that wereassociated with decreased auricular thicknessand production of IL-1α and TNF-α.Nanoemulsions raised blood concentrations ofdelta and gamma tocopherol 2.2-2.4 fold ofnanosuspension formulations, while the effectwas not significant for alpha tocopherol. Asimilar nanoemulsion formulation wasdeveloped for aspirin [27]. Thenanoemulsions, having mean particle size of90 nm, increased twofold anti-inflammatoryproperty of aspirin in a CD-1 mouse model ofinduced inflammation. This was associatedwith similar changes in the accumulation ofauricular levels of pro-inflammatorycytokines.

Application of nanoemulsions for topicaladministration of ketoprofen was reported insome publications. Sakeena et al. haveevaluated nanoemulsions of palm oil estersprepared by spontaneous emulsificationmethod for delivery of ketoprofen incarrageenan-induced rat hind paw edema.The nanoemulsions demonstrated a significantdrug release through methyl acetatemembrane in-vitro and a comparable efficacyas compared with Fastun® gel in-vivo [28-30].In another study, Kim et al. preparednanoemulsions of ketoprofen with anacceptable stability and a high skin permeationrate [31].

Baboota et al. investigated the potential ofnanoemulsions for transdermal delivery ofcelecoxib. It was revealed that the steadystate flux and permeability coefficientincreased significantly compared to the gelformulations. In addition, the anti-inflammatory effects were higher oncarrageenan-induced paw edema in rats [32].Wang et al. [33] developed different o/w

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nanoemulsions of 1% curcumin, a naturalpolyphenolic phytochemical. They aimed tocompare two methods of nanoemulsionpreparation, high-speed (24000 rpm) andhigh-pressure homogenization (1500 bar) o/wusing medium chain triacylglycerols as oil andTween 20 as emulsifier. Mean droplet sizes of618.6 nm for high speed homogenization and79.5 nm for high pressure homogenizationhave been achieved. Enhancing effect of o/wnanoemulsions for anti-inflammation activityof curcumin was studied. Approximately,twofold more inhibitory effect of 12-O-tetrade-canoylphorbol-13-acetate (TPA)-inducededema of mouse ear was resulted for highpressure homogenizer compared to high speedhomogenizer. Such anti-inflammation activitywas attributed to drug permeation enhancementwhen the emulsion droplet sizes were reducedto below 100 nm.

B. SteroidsHoller et al. [34] have described a

comparative study that shows the influence ofnegative (sucrose laureate), non-ionic(polysorbate 80) and the cationic surfactant(phytosphingosine) on physicochemicalbehavior of o/w nanoemulsion and skinpermeation of the model drugs (fludrocorti-sone acetate and flumethasone pivalate). Thenanoemulsions were prepared by mixing theseparately prepared aqueous and oily phases.The aqueous phase containing sucrose laureateor polysorbate 80 and distilled water, heatedup to 50 °C under slight mixing. But, phy-tosphingosine was dissolved in the oil phase,containing PCL-liquid, Lipoid S75, tocopheroland the model drug (1%). The two phaseswere merged and pre-homogenized at 2500rpm. Then, raw formulation was high pressurehomogenized for 12 cycles at 600 bars untilmean size of 100-200 nm was obtained. Thephytosphingosine-free nanoemulsions withsucrose laureate and polysorbate 80 showeda uniform particle size over the whole pH-range, whereas the phytosphingosine

nanoemulsions exhibited that particle sizesincrease dramatically up to 1200 nm at pH 8.0.The positively charged nanoemulsionscontaining phytosphingosine were able tocarry more efficiently fludrocortisone acetateand flumethasone pivalate into the skin thanthe negatively charged ones and subsequentlypromote the penetration of the drugs throughskin. It was hypothesized by the author thatthe degree of skin binding is probably moreimportant with the positively charged particlesthan with the negatively one as it is known thatthe skin is negatively charged at neutral pH.In other studies, Khang et al. [35, 36] weredemonstrated an enhanced stability and skinpermeation of lecithin and sucrose stearatebased nanoemulsions of progesterone bycyclodextrins. This enhancement was morepronounced for gamma cyclodextrin andlecithin based nanoemulsions. Stecova andcoworkers developed topical nanoparticulatesystems (NLC, SLN and nanoemulsion) toimprove skin permeation of cyproteroneacetate. The highest penetration enhancementwas observed for SLN formulation [37].

C. Cytotoxic agentsTagne et al. [38] studied nanoemulsions of

dacarbazine, which is a highly lipid solublecytotoxic drug, and the topical applicationin xenograft nude mice model of a humanmelanoma cell line. The nanoemulsion,having mean particle size of 131 nm,demonstrated decreased negative chargeswhich associated with better skin bilayerpermeability. The formulation caused up totenfold greater reduction in tumor size ascompared with the drug suspension. Inaddition, during drug cessation period (12weeks), the nanoemulsion showed fivefoldgreater efficacy in preventing tumor growth.Similarly, Kakumenu et al. [39] demonstratedin-vivo efficacy of dacarbazine nanoemulsionin an epidermoid carcinoma xenograft mousemodel in comparison with dacarbazinesuspension after topical administration. This

Pharmaceutical nanoemulsions potential applications

could be attributed to the reduced particlesize (111 nm versus 6000 nm), the reducedzeta potential (-3.2 versus -89.1 mV), andthe greater drug dispersibility and stability.

3. Extended drug releaseAlves et al. [40, 41] reported human skin

penetration and distribution of a lipophilicdrug model, nimesulide, from hydrophilicgels containing nanocarriers. They comparednanocapsules (GNM-NC), nanospheres(GNM-NS) and nanoemulsion (GNM-NE)formulations in modulating the skinpenetration of nimesulide. Nanoprecipitation,interfacial deposition and spontaneousemulsification methods had been used toprepare the nanostructured dispersions. Theaverage hydrodynamic diameters weredetermined 252 nm for the nanoemulsion,277 nm for the nanocapsules and 202 nm forthe nanospheres containing nimesulide. Theencapsulation efficiencies were about 99%.The nanoemulsion demonstrated shear-thiningrheological characteristics with no thixotropicphenomenon. The different nanocarriersystems were incorporated into thehydrophilic gels and their ability for deliveryof the drug into the human skin wasinvestigated using stripping technique andFranz-type diffusion cells. The drug wasdetected in the stratum corneum for the gelcontaining nimesulide-loaded nanocapsulesand nanospheres, while no drug was detectedfor that of nanoemulsion (GNM-NE). Thehypothesized mechanism by the authors isthat, the formulations prepared with polycaprolactone (GNM-NC and GNM-NS) hadpresented a higher affinity for the horny layer.

So, the result is in agreement with otherstudies, which reported that particulated drugcarriers (microparticles and nanoparticles)improve the drug residence in the skin withoutincreasing transdermal transport. Additionally,the high specific area of the carrier facilitatesthe contact of the encapsulated moleculeswith the stratum corneum [25]. The functionof these particles is to deliver an ingredient tothe upper layer of the skin and to prolongthe time of their delivery.

Several publications concerned aboutapplication of nanoemulsions for extendedrelease topical administrations. Fontana etal. prepared hydrogels containing lipid corenanocapsules and nanoemulsions of clobetasolpropionate. They were compared according totheir controlled release behavior and activityusing in-vitro and in-vivo model of contactdermatitis. The authors concluded thatnanoencapsulation of clobetasol in thenanocapsules led to a better control of thedrug release and a better dermatologicalefficacy [42, 43]. Wang and coworkersprepared w/o nanoemulsion formulations withdroplet sizes in range of 50-200 nm for asustained release of morphine and an extendedanalgesic activity [44]. Silva et al. [45]described incorporation of genistein intotopical nanoemulsions composed of egglecithin, medium chain triglycerides oroctyldodecanol by spontaneous emulsification.Nanoemulsions had droplet sizes in range of230-280 nm. The loaded amount was close to100%. The nanoemulsions exhibited asustained permeation through pig ear skin inFranz cells.

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Table 1. Commercial formulations of therapeutic nanoemulsions in market.Drug Brand name Manufacturer Therapeutic indicationPropofol Diprivan Astrazeneca AnestheticPropofol Troypofol Troikaa AnestheticDexamethasone Limethason Mitsubishi Pharmaceutical SteroidAlprostadil palmitate Liple Mitsubishi Pharmaceutica l Vasodilator platelet inhibitorFlurbiprofen axetil Ropion Kaken Pharmaceuticals Nonsteroidal analgesicVitamin A, D, E, K Vitalipid Fresenius Kabi Parenteral nutrition

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4. Topical gene delivery Wu et al. [46] described the preparation of

water-in-oil nanoemulsions containingexpression plasmid DNA that appear tofacilitate follicular transfection followingtopical application in vivo. The nanoemulsionswithout DNA had mean particle sizes of 14.6-42.3 nm; while the nanoemulsion with DNAshowed sizes of 20.2-32.1 nm. Expressionplasmids encoding chloramphenicol acetyl-transferase (CAT) or human interferon-2cDNA had been formulated in water-in-oilnanoemulsions and applied to murine skin. Ithad been shown that the deposition site ofplasmid DNA was primarily in follicularkeratinocytes. The transgene expression wasoptimal at 24 h following topical applicationof a single dose of water-in-oil nanoemulsioncontaining plasmid DNA as quantified byRT-PCR and ELISA. The efficiency ofnanoemulsion mediated transfection was mosteffective in the context of normal versusatrophic hair follicles. The results of thisstudy have suggested that the efficiency oftransfection and the dynamics of transgeneexpression appear to be augmented by theuse of a nanoemulsion vehicle. This may bethe result of simple physical protection ofplasmid DNA from endogenous deoxyribonu-cleases present in the skin. It is also possiblethat this effect is a consequence of alterationsin cell membrane fluidity or membraneintegrity that results from the presence ofnon-ionic detergents in the oil phase of thenanoemulsion. Other possibilities includeundefined components of the organic plant oilthat may facilitate transfection.

5. Photodynamic therapyPhotodynamic therapy was ranked as first

line therapy for actinic keratosis and isacceptable for treatment of neoplastic skindiseases. The application of topicalnanoemulsions for photodynamic therapy ofskin conditions was reported for 5-aminolevulonic acid (ALA) and temoporfin

(Foscan®). Zhang et al. [47] prepared o/wand w/o nanoemulsions of ALA and themethyl ester by high pressure homogenizationand probe ultrasonication. The nanoemulsionswere negatively charged and sized below 256nm. It demonstrated higher loading of themethyl ester to about 68% in soybean oil o/wnanoemulsions. A higher skin permeation ratewas observed for o/w nanoemulsions ascompared with w/o nanoemulsions. The o/wnanoemulsion was advantageous overaqueous solution for successful topicalapplication of ALA which required anextended release for about 24-48 h. Besides,the o/w nanoemulsions were able to exert thehighest in-vitro flux without compromisingskin barrier function. Dirschka et al.demonstrated that nanoemulsion based gelformulation of ALA (BF-200 ALA) had abetter stability and skin permeation than ALAsolution. It was shown clinically that thenanoemulsion was superior over the methylester cream in ex-vivo skin model [48].Similarly, Primo et al. evaluated theapplication of o/w nanoemulsions for deliveryof Foscan in-vitro and demonstratedconfirming results [49].

6. Protection of natural antioxidantsThere are reports of using nanoemulsion

for stabilization of antioxidants. Junyapresertet al. prepared nanoemulsions of mediumchain triacylglycerols (MCT) andnanostructured lipid carriers (NLC) of cetylpalmiate and MCT to load coenzyme Q10.The particles remained in the nanosize rangeand preserved above 90% of their contentfor 12 months after storage at 4, 25 and 40 °C.Due to skin occlusiveness effect, the particlespromoted deep penetration of Q10 into skinin-vitro [50]. Mitri et al. have reported theapplication of lipid nanocarriers includingsolid lipid nanoparticle (SLN), NLC andnanoemulsion for ultraviolet protection oflutein, a lipid soluble antioxidant. Thenegatively charged nanocarriers prepared by

Pharmaceutical nanoemulsions potential applications

high pressure homogenization had sizes inrange of 150-350 nm. A better skin permeation(60% in 24 h), but a lower protection (14%after 10 minimal erythema dose) wasdemonstrated for nanoemulsions [51].

7. Antimicrobial activityNanoemulsions have an enhanced

antibacterial activity due to discrete dropletsof oil that fuse with bacterial cell walls or viralenvelopes, destabilizing the organism’s lipidenvelope and initiate disruption of pathogens.

Hamouda et al. [52, 53] have presented anovel nanoemulsion formulation with aunique topical antimicrobial activity againstbacteria, enveloped viruses and fungi. Thenanoemulsion prepared by mixing of heatedoil phase (8 volumes tributyl phosphate, 64volumes soybean oil) and 8 volumes Triton X-100 at 82 °C for one h with 20 volumes ofdeionized water with a reciprocating syringepump. The final particle size of thenanoemulsion was reported in the range of400-800 nm using a laser light scatteringtechnique. The nanoemulsion has the potentialto be used as a topical antimicrobial agentagainst several pathogens. It has bactericidalactivity against vegetative forms of mostGram-positive bacteria and few Gram-negative bacteria, including H. influenzae,N. gonorrhoeae and V. cholera. P.aeruginosa.Enteric Gram-negative bacilli were resistantwhich may be attributable to their cell walllipopolysaccharide (LPS) and negative surfacecharge. It was reported the nanoemulsion hada negative charge that might result in repulsiveforces preventing the attachment to thebacterial cell wall.

All the tested enveloped viruses (Herpessimplex, influenza A and vaccinia) weresensitive to the nanoemulsion treatment todifferent extents. Moreover, it has beenreported that the nanoemulsion is virucidal tothe HIV virus. This consistent inactivation ofenveloped viruses suggests that thenanoemulsion may affect the viral lipid

membrane. This is further supported by thefact that nonenveloped viruses, such asadenovirus, are resistant to the treatment. Theresistance of yeast cells, e.g. Candidaalbicans, to the cidal effects of thenanoemulsions is most likely due to the rigidcell wall structure in yeast. The demonstratedfungistatic action may be due to disruption ofthe budding process by lysing newly formedyeast buds.

In other studies, Pannu et al. developednovel o/w nanoemulsions with broadantifungal activity against dermatophytes andactivity against propionibacterium acne fortreatment of skin, hair and nail infections[54, 55].

8. Conclusion and future perspectivesNanoemulsion formulations offer several

advantages for topical delivery of cosmeticand pharmaceutical agents includingcontrolled droplet size, lower concentration ofsurfactant and the ability to efficiently dissolveand stabilize lipophilic drugs. The trappeddrug molecules in the inner phase ofnanoemulsion may result in an extended drugrelease and a prolonged activity.

There are several mechanisms suggestedfor enhancement of skin permeation fromnanoemulsion formulations following topicaladministration. Surface charge-modifiednanoemulsion droplets had significantinfluence on the binding affinity of dropletsto the skins; nanoemulsion could act as drugreservoirs and the high concentration of drugsin nanoemulsion resulted in highconcentration gradient; due to the smalldroplet diameters of nanoemulsion, the oilydroplets might embed into the stratumcorneum and the drug molecules coulddirectly be delivered from oily droplets intothe stratum corneum without a transfer viahydrophilic phase of nanoemulsions. It hasbeen shown that the positively charged lipidsuch as phytosphingosine may play animportant role for droplet attachment to

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stratum corneum for successful drug delivery.Besides, nanoemulsions are easily valued incosmetics because of their good sensorialand physical properties such as completedispersion on skin and hydration action inskin, hair and nail care products.

Further basic researches are required tobe carried out for better understanding ofhow such system modifies the diffusion ofactives in to the skins, how they interact withstratum corneum and how they affectpenetration of active materials. Definitely,more human studies need to be carried out tohave a “real life” data.

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