Microemulsions as transdermal drug delivery vehicles

Advances in Colloid and Interface Science 123–126 (2006) 369 – 385 www.elsevier.com/locate/cis

Microemulsions as transdermal drug delivery vehicles Anna Kogan, Nissim Garti ⁎ Casali Institute of Applied Chemistry, The Institute of Chemistry, Givat Ram Campus, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Available online 14 July 2006

Abstract Microemulsions are clear, stable, isotropic mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. Microemulsions have been intensively studied during the last decades by many scientists and technologists because of their great potential in many food and pharmaceutical applications. The use of microemulsions is advantageous not only due to the facile and low cost preparation, but also because of the improved bioavailability. The increased absorption of drugs in topical applications is attributed to enhancement of penetration through the skin by the carrier. Saturated and unsaturated fatty acids serving as an oil phase are frequently used as penetration enhancers. The most popular enhancer is oleic acid. Other permeation enhancers commonly used in transdermal formulations are isopropyl myristate, isopropyl palmitate, triacetin, isostearylic isostearate, R(+)-limonene and medium chain triglycerides. The most popular among the enhancing permeability surfactants are phospholipids that have been shown to enhance drug permeation in a different mode. L-α-phosphatidylcholine from egg yolk, L-α-phosphatidylcholine 60%, from soybean and dioleylphosphatidyl ethanolamine which are in a fluid state may diffuse into the stratum corneum and enhance dermal and transdermal drug penetration, while distearoylphosphatidyl choline which is in a gel-state has no such capability. Other very commonly used surfactants are Tween 20®, Tween 80®, Span 20®, Azone®, Plurol Isostearique® and Plurol Oleique®. As cosurfactants commonly serve short-chain alkanols such as ethanol and propylene glycol. Long-chain alcohols, especially 1-butanol, are known for their enhancing activity as well. Decanol was found to be an optimum enhancer among other saturated fatty alcohols that were examined (from octanol to myristyl alcohol). Many enhancers are concentrationdependent; therefore, optimal concentration for effective promotion should be determined. The delivery rate is dependent on the type of the drug, the structure and ingredients of the carrier, and on the character of the membrane in use. Each formulation should be examined very carefully, because every membrane alters the mechanism of penetration and can turn an enhancer to a retarder. Various potential mechanisms to enhance drug penetration through the skin include directly affecting the skin and modifying the formulation so the partition, diffusion, or solubility is altered. The combination of several enhancement techniques such as the use of iontophoresis with fatty acids leads to synergetic drug penetration and to decrease in skin toxicity. Selected studies of various microemulsions containing certain drugs including retinoic acid, 5-fluorouracil, triptolide, ascorbic acid, diclofenac, lidocaine, and prilocaine hydrochloride in transdermal formulations are presented in this review. In conclusion, microemulsions were found as an effective vehicle of the solubilization of certain drugs and as protecting medium for the entrapped of drugs from degradation, hydrolysis, and oxidation. It can also provide prolonged release of the drug and prevent irritation despite the toxicity of the drug. Yet, in spite of all the advantages the present formulations lack several key important characteristics such as cosmeticpermitted surfactants, free dilution in water capabilities, stability in the digestive tracts and sufficient solubilization capacity. © 2006 Elsevier B.V. All rights reserved. Keywords: Microemulsions; Drug delivery system; Transdermal

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . Microemulsions as transdermal drug delivery vehicles . 2.1. Mechanisms of skin permeation . . . . . . . . . 2.2. Components of the transdermal formulations . . 2.2.1. Oil phase . . . . . . . . . . . . . . . . 2.2.2. Surfactants . . . . . . . . . . . . . . .

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⁎ Corresponding author. Tel.: +972 2 658 6574/5; fax: +972 2 652 0262. E-mail address: [email protected] (N. Garti). 0001-8686/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2006.05.014

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A. Kogan, N. Garti / Advances in Colloid and Interface Science 123–126 (2006) 369–385

2.2.3. Cosurfactants . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Aqueous phase . . . . . . . . . . . . . . . . . . . . . 2.3. Selected studies on microemulsions as transdermal drug vehicles 2.3.1. Ascorbic acid . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Diclofenac. . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Lidocaine and prilocaine hydrochloride . . . . . . . . . 2.3.4. Triptolide . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5. 5-Fluorouracil . . . . . . . . . . . . . . . . . . . . . . 2.3.6. Retinoic acid . . . . . . . . . . . . . . . . . . . . . . 3. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Pharmaceutical formulators aim to deliver the active molecule to the target organ at therapeutically relevant levels, with negligible discomfort and side effects to the patient. This delivery is significantly influenced by the physical and chemical properties of the drug. The biopharmaceutical classification system of drugs was defined by Amidon et al. in 1995 [1]. Class I and Class II drugs demonstrate high gastrointestinal permeability. In contrast to Class II, Class I covers drugs with high water solubility. Class I drugs are well absorbed, but their bioavailability can still be low because of the first pass metabolism [1]. The rate-limiting step to drug absorption is the drug dissolution. In cases of very rapid dissolution, the rate-controlling step will be gastric emptying. Compounds of Class II with solubility below 10 mg/ml present difficulties related to solubilization during formulation. In these cases, the rate controlling step to drug absorption is drug dissolution. The drug dissolution profile can be affected by many formulation and in vivo variables; therefore, drugs in this class can be expected to have variable absorption. The absorption of these drugs is usually slower than for Class I. Class III and IV drugs are compounds featuring high solubility and low permeability, and low solubility and low permeability, respectively. Class III drugs feature variability in both the rate and extent of absorption. However, if the dissolution is rather fast (i.e., 85% dissolved in less than 15 min) the variability will be due to the variations in luminal content, gastrointestinal transfer, and membrane permeability which is the rate-determining step, and not due to the dosage form factors. Due to their characteristics, Class IV drugs exhibit many problems in their successful delivery [1]. In the conventional pharmaceutical industry and in modern medicine, it is well known that many promising drugs that are discovered never make it to the market because of difficulties in delivery. This means that such drugs need to be formulated with smart drug delivery systems and/or delivery technology to make them acceptable for the treatment of patients. Some of these drugs are insoluble while others are eliminated by the acidity of the stomach, or are cleared from the blood too rapidly to be effective. Intravenous delivery may serve as an alternative in such cases, but may require either very frequent administration or large a volume of drug injected each time. The problems that occur with many drugs can be related to

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the physical or chemical properties of the drug, administrative matters such as approval for use, excipients, and engineering issues [2]. Some of the major challenges of drug delivery are [2] poor solubility, short in vitro (shelf-life) and in vivo (half-life) stability, low bioavailability, strong side effects (therefore, targeted delivery is needed) and regulatory issues. In order to increase the probability of a drug delivery formulation entering the pharmaceutical market, it should fulfill (as much as possible) the main requirements [2]: ease of production, applicability to as many drugs as possible, physical stability, excipients that are well tolerated and accepted by regulatory authorities and availability of large scale production allowable by regulatory authorities. By many estimates, up to 40% of the new chemical entities discovered by the pharmaceutical industry today are poorly soluble or lipophilic compounds. The solubility issue, complicating the delivery of these new drugs, also affects the delivery of many existing drugs. Researchers are making great efforts to discover new methods for delivery of poorly soluble drugs that will be efficient and economically acceptable for drug manufacturers. The most common approach to improving the solubility of drugs possessing a net negative electrical charge is to form salts (e.g., hydrochlorides, sulfates, nitrates, maleates, citrates, tartarates) of the basic drugs. Yet, other drugs do not form such salts and this method of improving solubility is not possible. Another route is to reduce particle size of powdered drug by new milling technologies or by applying new crystallization processes to improve dissolution kinetics. Researchers are constantly searching for new and more efficient vehicles to carry active molecules into the blood stream. One approach to overcome these problems is to package the drugs into a particulate carrier system. Microcapsules, micro- and nanospheres, nanopowders, nanocrystals, and nanodispersions are only some of the options. Other options include delivery of the nutraceuticals or drugs via liquid vehicles such as liposomes, emulsions, double emulsions, microemulsions, micellar solutions and lyotropic liquid crystals such as cubosomes and hexosomes. These last two vehicles are still in an experimental stage. The incorporation of the drug into a carrier-system can be envisioned to protect it against degradation in vitro as well as in vivo; the release can be controlled, and targeting can also be achieved.


In 1959, Schulman et al. visualized the existence of small emulsion-like structures by electron microscopy and subsequently coined the term “microemulsions” [3]. The term has been defined and redefined by many authors. In this review, however, we will use the most general definition provided by Danielsson and Lindman in 1981 [4]: a microemulsion is a single, optically isotropic structured solution of surfactant, oil, and water. Microemulsions seem to be ideal liquid vehicles for drug delivery since they provide all the possible requirements of a liquid system including thermodynamic stability (long shelflife), easy formation (zero interfacial tension and almost spontaneous formation), low viscosity with Newtonian behavior, high surface area (high solubilization capacity), and very small droplet size. The small droplets have better chance to adhere to membranes and to transport bioactive molecules in a more controlled fashion. Using the microemulsion vehicles, water-insoluble and oil-soluble components from different plant extracts can be co-solubilized in order to attain synergistic effect for a specific therapeutic goal. Microemulsions can be introduced into the body orally, topically on the skin, or nasally, as an aerosol for direct entry into the lungs. Microemulsions have been subjected to numerous studies during the last decades because of their great potential in many applications. Due to their rather complicated phase behavior and the fascinating microstructures encountered in microemulsion forming systems, many researchers have made significant efforts to obtain a better understanding of these microstructures. Comprehensive reviews discussing the microstructures encountered in microemulsion phases were written [5–8]. In order to investigate the potential of microemulsions as delivery vehicles, it is necessary to characterize their microstructures as well as the locus of the drug in the loaded microemulsion. In some cases, due to molecular interactions between the loaded drug and the microemulsion, the microstructure of the system may be altered. Due to the complexity of the microemulsions and the variety of the structures and components involved in construction of the microstructure, as well as the limitation associated with each technique, the characterization of such structures is a rather difficult task. Some of the major methods relevant to the characterization of the microemulsions include viscosity [9–14] and conductivity [9,10,15–25] measurements as well as more advanced methods such as cryo-TEM [26–34], pulsed gradient spin echo (self-diffusion) NMR [32,35–42], dynamic light scattering (DLS) [43,44], small angle X-ray scattering (SAXS) [45–55] and small angle neutron scattering (SANS) [56–59]. However, microemulsions suffer from high surfactant concentrations and in most cases from high alcohol, solvent, and co-solvent contents. High levels of non-active compounds are always a hazard. Patent search shows that microemulsions have been heavily patented by researchers, as well as by companies, as delivery systems, but a close examination of formulations that are available in the market place reveals that the technology is far from being exhausted and applied. The question is always why?

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The aim of this chapter is to review the current literature with respect to the use of microemulsions for cutaneous drug delivery and to discuss the influence of microemulsion composition, components, and structure on the drug delivery potential as well as the tolerability of these vehicles both in vitro as well as in vivo. Many other potential pharmaceutical applications of microemulsions have been studied such as pulmonary [60–68], intravaginal or intrarectal administration delivery vehicles for lipophilic drugs such as microcides, steroids, and hormones [69–77], and intramuscular formulations of peptide or celltargeting systems [78], other drugs have been also evaluated. The review recently written by Garti and Aserin includes among others the recent progress in microemulsions for oral and intravenous delivery [79]. The space limitations that are imposed on this review prevent us from further elaborating on these applications. 2. Microemulsions as transdermal drug delivery vehicles Transdermal drug delivery has many advantages over the oral route of administration: it avoids hepatic metabolism, the administration is easier and more convenient for the patient, and there is the possibility of immediate withdrawal of the treatment if necessary. Despite the great potential of transdermal delivery of drugs, only a few drug formulations are available commercially. The main reason is the barrier function of human skin that is considered to be the most impermeable epithelium to exogenous substances [80]. 2.1. Mechanisms of skin permeation The skin itself has two main layers: the epidermis, which is the outermost layer of the skin, covering the dermis that is the active part of the skin, holding the hair muscles, blood supply, sebaceous glands, and nerve receptors. There is a fat layer underneath the dermis. The skin is a very heterogeneous membrane and has a variety of cell types, but the layer that controls the penetration of drugs is called the stratum corneum and, despite its thickness of only 15–20 μm, it provides a very effective barrier to penetration. The permeation of the drug through the skin has several routes: transcellular, intercellular, and appendageal (through eccrine (sweat) glands or hair follicles) (Fig. 1). Since the appendages occupy a very low surface area, this means of permeation is less significant under

Fig. 1. Schematic representation of the different possible routes of penetration through the skin. Reprinted with permission from Hadgraft [81].

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normal conditions [81]. Nevertheless, in iontophoretic delivery this route is more significant [82]. The intercellular spaces consist of a mixture of lipids–ceramides, free fatty acids and their esters, and cholesterol and its sulphates that are structured in bilayers. Recent developments in spectroscopic techniques give interesting insights at the molecular level that may explain the impermeability of the skin by repeated partition and diffusion across structured bilayers [81]. Transdermal drug permeability is influenced mainly by three factors: the mobility of the drug in the vehicle, the release of the drug from the vehicle, and drug permeation through skin [83]. Therefore, the researchers are challenged to come up with formulations that increase the permeability of the drug without irreversibly changing the skin barrier function. Various potential mechanisms to enhance drug penetration through the skin include directly affecting the skin and modifying the formulation so the partition, diffusion, or solubility are altered [81,84]. Here we will present briefly these potential mechanisms that are interconnected with each other. 1. Direct effect on the skin [84] a. Denaturation of intracellular keratin or modification of its conformation causes swelling and increased hydration b. Affection of desmosomes (known as macula adherenscell structures specialized for cell to cell adhesion) that maintain cohesion between corneocytes (dead cells of the stratum corneum) c. Modification of lipid bilayers reduces resistance to penetration d. Altering the solvent properties of the stratum corneum to modify drug partitioning e. Use of solvent that can extract the lipids in the stratum corneum and decrease its resistance to penetration 2. Modification of the formulation [84] a. Supersaturation state produced by volatile solvent that leaves the active substance in a more thermodynamically active state b. Choosing the enhancer molecules in the vehicle that are good solvents for the active ingredient and which enhance permeation through the skin; this way the partition of the drug into the stratum corneum will be improved c. The diffusion of the active ingredient through the skin may be facilitated by using enhancers that create liquid pools within the bilayers like oleic acid, or disturb the bilayers uniformly as do the Azone® molecules (Azone® (1-dodecylazacycloheptan-2-one or lauro capram) is the first molecule specifically designed as a skin permeation enhancer. Azone® serves as surfactant and enhances the skin transport of a wide variety of drugs including steroids, antibiotics and antiviral agents [84]). Sometimes the synergy between several enhancement effects causes a greater enhancement in permeability of the desired substance. More detailed explanations and various examples of the enhancement mechanisms can be found in very comprehensive reviews recently written by Handgraft [81] and Williams and Barry [84].

2.2. Components of the transdermal formulations A large variety of chemicals were defined as penetration enhancers, but their use in transdermal formulations is limited because of toxicity issues or unclear mechanisms of action. Here, we will briefly discuss the most common penetration enhancers that are used as the components of topically applied microemulsions. In the very comprehensive review of Williams and Barry [84] the reader can find more widely used penetration enhancers and their possible mechanisms of action. 2.2.1. Oil phase Saturated and unsaturated fatty acids can be used as effective penetration enhancer for a variety of drugs (e.g., naloxone, hydrocortisone, estradiol, and peptides like LHRH, and CCK-8) [81,85–89]. Aungst et al. [90] studied various fatty acids, alcohols, sulphoxides, surfactants, and amides as enhancers for naloxone. It was suggested that enhancers with a saturated alkyl chain of C10–C12 and a polar head, and enhancers with an unsaturated C18 alkyl chain such as oleic acid, appeared to be the optimal ones. The unsaturated cis configuration perturbs the lipid packing more than does the trans configuration [90]. Since the tail of the stratum corneum bilayer is hydrophobic, the fatty acids can enter the bilayer, perturb it by creating separate domains, and in this way may induce highly permeable pathways in the stratum corneum [91]. The most popular enhancer is oleic acid. It increased the flux of salicylic acid 28-fold and of 5-flourouracil 56-fold through human skin membrane in vitro [92]. Nevertheless, in examination of hexyl nicotinate on human skin in vivo, the addition of oleic acid to propylene glycol (PG) did not improve the permeation flux achieved with PG alone [93]. It is important to note that formulators should use oleic acid in their formulations with great care, since its application may cause changes in the morphology of the Langerhans cells that are located in the superbasal layer of the epidermis and play a key role in the initiation and coordination of the T-cell mediated immune response. Their depletion from the epidermis can cause skin immunosuppression [94]. The combination of fatty acids and iontophoretic delivery resulted in enhanced permeation of many drugs. In iontophoretic delivery, a small electric current applied between two electrodes placed on the skin enhances the penetration of ionized, neutral, or polar molecules in a controlled manner [95,96]. Appendageal sites are suggested as important sites through which electrotransport occurs; ion and water transport have also been reported to be associated, at least in part, with stratum corneum lipid lamellae. Jadoul et al. [82] suggested that permeation enhancement by iontophoresis was related to lipid layer stacking disorganization. Since fatty acids enhance permeation by selective perturbation in the intercellular lipids of the stratum corneum, and ions are known to take the path with less resistance: so combining the use of iontophoresis and fatty acids is an alternative that researchers have used to increase the permeation of not only low-molecular-weight drugs, but also high-molecular-weight proteins such as insulin, and peptides like LHRH, CCK-8 and AVP [85,86,97–99]. Furthermore, the combination of these two


methods can decrease skin toxicity and irritation that are associated with use of high levels of either of them. Rastogi and Singh [99] investigated the influence of R(+)-limonene and fatty acids such palmitic (C16:0), palmitoleic (C16:1), stearic (C18:0), oleic (C18:1), linoleic (C18:2), and linolenic (C18:3) on the penetration enhancement of insulin. It was shown that iontophoresis in combination with chemical enhancers synergistically increased (p < 0.05) the in vitro permeability of insulin. Linolenic acid enhanced the permeability of insulin through the epidermis more than other fatty acids did during both passive (44.45 × 10− 4 cm/h) and iontophoretic (78.03 × 10− 4 cm/h) transport. The authors claim that by using an iontophoretic patch size of 10 cm2, they would be able to deliver 50 IU of insulin within 3 h [99]. Nair and Panchagnula [98] assessed in vitro the effect of oleic, linoleic, and lauric fatty acids alone and in combination with iontophoresis on the transdermal penetration of arginine vasopressin (AVP), a cyclic peptide that is used in treatment of diabetis insipidus, alzheimer, and other diseases. It was found that all fatty acids increased the AVP flux compared to rat skin that was not pretreated with enhancer, but their effectiveness in permeability enhancement was similar. This study provided direct evidence that oleic acid in mixture of ethanol and water causes disruption of the stratum cornea lipid lamellae. Nevertheless, this study showed that iontophoresis did not further increase the permeation of AVP through linoleic pretreated skin [98]. Other researchers detected similar effects with fatty acids that increased the permeation of midodrine hydrochloride and leuprolide acetate [100,101]. Iontophoresis was also effective for enhancing transdermal penetration from microemulsions as it was shown for methotrexate [102]. Another compound that is often used as an oil phase and as a permeation enhancer in transdermal formulations is isopropyl myristate (IPM), but the mechanism of its action is poorly understood [13,83,102–112]. In some cases IPM does not show a significant effect [113]. Other oils commonly used are isopropyl palmitate [114–116], triacetin [117], isostearylic isostearate [36,118], R(+)-limonene [118] and Mygliol 812® (medium chain length triglyceride) [119–122]. 2.2.2. Surfactants More than 15 years ago, Kato et al. [123] proposed the use of low toxicity substances, e.g., phospholipids, as penetration enhancers. The penetration of theophylline was enhanced by 1% phosphatidylcholine in propylene glycol through hairless mouse skin. Numerous studies have been conducted using various lecithin-based microemulsions as topical formulations for enhancing the penetration of methotrexate [124], ascorbic acid [115], diclofenac [116,125], indomethacin [116], ketoprofen [117,119], hematoporphyrin [106], etc. Absorption of phospholipids on skin can increase tissue hydration, consequently increasing drug permeation. When phospholipids are applied to skin as vehicles due to their physico-chemical properties and structures they can fuse with stratum corneum lipids, perturb its structure and facilitate drug delivery [84,126]. As early as 1990, Friberg and later Atwood et al. published works on drug delivery systems for skin applications [127,128].

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The phase properties of systems contained isopropyl myristate (IPM) as an oil, lecithin from egg or soy as a low toxicity surfactant, butanol [128] or ethanol as cosurfactant, and water. The influence of lecithin and butanol or ethanol on the area of the microemulsion was examined. In the system containing butanol oil-in-water microemulsion regions were detected at a surfactant/cosurfactant weight ratio between 1:0.6 and 1:0.33. In the systems consisting of ethanol, microemulsion area was obtained in only 60–80% ethanol in water w/w. It was shown that in order to obtain a microemulsion, the amount of lecithin required increased as the amount of the ethanol decreased. In comparison to the systems containing butanol it was found that at the same amount of IPM more ethanol was required. Larger amounts of ethanol are required to reduce the rigidity of the lecithin condensed film, creating the curvature that is needed for droplet formation. This is explained by the low interface/water partition coefficient of ethanol because of its hydrophilicity [128]. It is known that with an increase in the aqueous phase fraction, the droplet size increases. Atwood et al. [128] found that at constant aqueous phase content, droplet size increased as the amount of lecithin decreased. Other authors have investigated formulations based on lecithins [106,116,125]. Different phospholipids enhance drug permeation differently. Kirjavainen et al.'s [126] study showed that the reason for this difference was that different phospholipids had different effects on the partitioning of drugs into the lipid bilayers. The authors used stratum corneum lipid liposomes as a model for the lipid matrix of stratum corneum. Estradiol, progesterone, and propranolol were studied. They concluded that EPC (L-α-phosphatidylcholine from egg yolk), SPC (L-α-phosphatidylcholine 60%, from soybean), and DOPE (dioleylphosphatidyl ethanolamine) which are in a fluid state may diffuse into the stratum corneum and enhance dermal and transdermal drug penetration, while DSPC (distearoylphosphatidyl choline) which is in a gel-state is not able to do this [126]. There is wide use of nonionic surfactants in topical formulations as solubilizing agents. Among them, the polysorbates (ethoxylated sorbitan esters) are very common. Tween 80® was reported to accelerate hydrocortisone and lidocaine permeation, and Tween 20® improved the permeation of 5-flourouracil across hairless mouse skin [129–131]. Some recent results indicate that a nonionic surfactant may also affect the skin barrier function [132,133]. In the experiments conducted in vitro on rat skin it was found that Span 20®, Tween 20®, and Azone® have different mechanisms of enhancement. Span 20® and Azone® affect the intercellular lipids of the stratum corneum by making them more fluid. This way the diffusion of lipophilic compounds through the stratum corneum (lipophilic pathway) is enhanced. Tween 20® enhances penetration by allowing the polar molecule to partition across the barrier more easily. It is also possible that since the experiments were conducted in aqueous medium, large micelles were formed. These micelles have the potential to extract lipids from the skin. This modifies the composition of the membrane and favors permeation of the hydrophilic compounds [132,133]. Recently, new low-irritant surfactants based on caprylocaproyl macrogolglycerides for microemulsions as drug delivery vehicles for topical application

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were studied. In order to facilitate caprylocaproyl macrogolglyceride based microemulsion formation, the surfactants based on polyglycerol fatty acid esters were used as cosurfactants [13,134– 136]. Other surfactants such as dioctyl sodium sulphosuccinate [137,138], Plurol Isostearique® [36,102,135,139], and Plurol Oleique® [13,118,120,121,140] are used very often. 2.2.3. Cosurfactants Short-chain alkanols are widely used as permeation enhancers. Ethanol is very common among transdermal formulations and its addition is known to enhance the flux of several drugs. Levonorgestrel [141], estradiol [142,143], and an ionized form of diclofenac were shown to be better transported through rat skin when ethanol was added [144]. Sometimes when using ethanol-water-based vehicles, the effect of ethanol is concentration-dependent, and, therefore, under certain conditions it can even decrease the permeation [84,145–147]. Various mechanisms have been suggested for the enhancing activity of ethanol. It can increase the drug solubility in the vehicle [84,143] or it can alter the structure of the membrane and increase the permeability of the drug [84,146]. Another mechanism is based on the fact that ethanol is volatilized from the applied formulation and, consequently, increases the drug concentration to a supersaturated state with a greater driving force for permeation. In addition, ethanol may extract some of the lipid fraction from the stratum corneum and, thus, can improve the drug flux through it [84]. In some cases ethanol by itself does not serve as an enhancer, but needs a co-enhancer as demonstrated in the case of penetration of morphine hydrochloride [113]. In this study, a combination of 1-menthol with ethanol showed greater penetration enhancement than if ethanol alone was used as solvent. It was suggested that the high penetration enhancement was derived from the influence of 1menthol on the stratum corneum barrier and the effect of ethanol against the viable skin layer beneath it [84,113]. Long-chain alcohols are known for their enhancing activity as well. 1Butanol was reported to be an enhancer for levonorgesterol [141]. Decanol was found to be an optimum enhancer for melatonin among other saturated fatty alcohols that were examined (from octanol to myristyl alcohol) [141]. Propylene glycol (PG) has been found to act as an enhancer by a mechanism similar to that of ethanol [84], enhancing the permeability of hexyl nicotinate. The addition of oleic acid to PG did not show an improvement in permeation [93]. It is important to stress that each formulation should be examined very carefully, because every membrane alters the mechanism of penetration and can turn an enhancer to a retarder. In vitro studies on the transdermal absorption of flurbiprofen (a chiral non-steroidal anti-inflammatory drug that is used for treatment of gout, osteoarthritis, rheumatoid arthritis, and sunburn) from cellulose hydrogels were conducted by Fang et al. [148] using different kinds of additives. The permeation was studied on different types of skin, such as skin pre-treated by various additives, stratum corneum stripped skin, and delipidized skin. Propylene glycol and ethanol did not increase skin absorption of the drug, and phospholipids even markedly reduced it. Ethanol reduced the skin reservoir of the drug,

whereas oleic acid increased it. In this work ethanol, propylene glycol, and phospholipids that are known as enhancers turned out to be retarders [148]. 2.2.4. Aqueous phase In most of the studies water served as the aqueous phase [13,36,102–105,107,108,111,114–117,119–122,125, 137,138,140,149,150]. In some of the cases phosphate buffer of pH 7.4 was used [102,109,112,118,135]. 2.3. Selected studies on microemulsions as transdermal drug vehicles The application of microemulsion vehicles for cutaneous drug delivery is becoming increasingly popular due to their high solubilization potential for both lipophilic and hydrophilic drugs. It was demonstrated that permeation rates from microemulsions were significantly higher than from conventional emulsions. In emulsions (creams, lotions, etc.), the strong interactions between the surfactants that occur in the interfacial membrane film limit the mobility of the drug between the internal and external phases within the formulation. In addition, emulsions are not stable formulations and strong fluctuations in bioavailability were detected. In microemulsions, the cosurfactant assists in lowering the interfacial tension of the surfactant film, resulting in spontaneous formation of a microemulsion and promoting its thermodynamic stability [36,137,151]. The thermodynamic process of drug diffusion across the flexible interfacial surfactant film between the phases of the microemulsion can increase partitioning and diffusion into the stratum corneum [114]. The drug may also be retained in the droplets of the microemulsion formulation resulting in increased concentration of surfactant in dispersed systems that will decrease its permeation into the skin [107]. An excellent and comprehensive review on the role of microemulsions in percutaneous penetration of drugs was recently published by Kreilgaard [80] in which the author summarized the main studies published before 2002 in several tables. We have included the data gathered by this author as well as additional studies that have been conducted since then in Table 1 [13,36,83,102–112,114–122,124,125,135,137,138,140,149, 150,152–154]. Many studies have been conducted incorporating many drugs in microemulsions, we decided to describe only those studies conducted on certain drugs. 2.3.1. Ascorbic acid Ascorbic acid (vitamin C) is an essential nutrient protecting living tissues and cells from oxidation processes by free radicals and reactive oxygen derived species. Gallarate et al. [115] studied the stability of ascorbic acid in several o/w microemulsions, o/w and w/o emulsions, and a w/o/w multiple emulsion. Isopropyl palmitate or cetearyl octanoate were used as oils, dodecylglucoside and cocoamide propylbetaine were used as surfactants, and 2-ethyl-1,3-hexanediol was chosen as cosurfactant. These emulsifiers are non-ionic, non-ethoxylated, and skin compatible. The addition of only 1% w/w of phosphatidylcholine to formulations allowed a two-fold reduction in the


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Table 1 Overview of transdermal drug delivery systems containing microemulsions Drug

Microemulsion

Membrane/ skin

References

water water water water, Aerosil 200® water

human mouse pig mouse

[137,138] [149] [103] [104]

–

[115]

water

–

[121]

Lecithin Lecithin Labrasol®/Plurol Oleique® Tween 80®, Span 20®

water water water water

human human – human

[116] [125] [13] [108]

Tween 20®, Tween 80®, Span 80®/ethanol, isopropanol) Tween 20®, tauro-deoxycholate

water

human

[83]

water, Transcutol®, Carbopol® water water, PG

mouse

[105]

human mouse

[137] [106]

water water water water

human human rat mouse

[116] [119] [117] [114]

water water, PG Aq. 154 mM NaCl (pH 7.4) water water, Transcutol®, PG water

rat mouse pig pig mouse

[36] [124] [102] [2] [152]

human

[150]

Aqueous buffer (pH 5.5) water

rat rat

[109] [36]

Oil phase

Surfactants/cosurfactants

Aqueous phase

octanol isopropyl myristate isopropyl myristate isopropyl myristate, decanol isopropyl palmitate, cetearyl octanoate Mygliol 812®

dioctyl sodium sulphosuccinate AOT Tween 80®, Span 80®, 1,2-octanediol Epikuron 200®, 1, 2-propanediol benzyl alcohol dodecylglucoside cocoamide propylbetaine, phosphatidylcholine/2-ethyl-1,3-hexanediol Labrasol®/Plurol Oleique®

isopropyl palmitate Diclofenac diethylamine isopropyl myristate isopropyl myristate

Indomethacin Ketoprofen Ketoprofen Lidocaine

Epicuron 200®, oleic acid, isopropyl myristate isopropyl myristate, benzyl alcohol octanol isopropyl myristate, decanol, hexadecanol, oleic acid, monoolein isopropyl palmitate Miglyol 812 N® triacetin, oleic acid isopropyl palmitate

Lidocaine Methotrexate Methotrexate Methotrexate Nifedipine

iostearylic isostearate decanol ethyl oleate isopropyl myristate benzyl alcohol

Niflumic acid

octyl octanoate

Piroxicam Prilocaine hydrochloride Propranolol Prostaglandin E1 Prostaglandin E1 Retinoic acid

isopropyl myristate isostearylic isostearate

Sodium ascorbyl phosphate Sodium diclofenac Sodium fluorescein Sodium salicylate

Mygliol 812®

isopropyl myristate

Sucrose Triptolide Triptolide

[3H]H2O 5-Fluorouracil 8-Methoxsalen Apomorphine hydrochloride Ascorbic acid Ascorbyl palmitate Diclofenac Diclofenac Diclofenac Diphenhydramine hydrochloride Estradiol Felodipine Glucose Hematoporphyrin

isopropyl myristate oleic acid Gelucire® isopropyl myristate

dioctyl sodium sulphosuccinate lecithin, sodium monohexyl-phosphate, benzyl alcohol Lecithin lecithin/n-butanol Labrasol®, Cremophor RH® glyceryloleate, polyoxyl 40 fatty acid derivatives /tetraglycol Labrasol®, Plurol Isostearique® lecithin, benzyl alcohol Labrasol®/Plurol Isostearique® Tween 80®, Span 80®, 1,2-octanediol Tween 20®, tauro-deoxycholate sucrose monolaurate sucrose dilaurate/ medium chain alcohol hexadecyltrimethylammonium bromide Labrasol®, Plurol Isostearique® Tween 80® Labrasol®, Plurol Oleique® Labrafac®, Lauroglycol Epikuron 200®, Oramix® NS10/ethanol, 1,2 hexanediol Labrasol®/Plurol Oleique®

water water a Transcutol®, water phosphate buffer

artificial mouse mouse artificial, pig

[107] [140] [153] [112]

water

artificial

[120]

Labrasol®/Plurol Oleique®

water, buffer pH 7.4

rat, rabbit

[118]

Brij 97®

water

pig

[122]

water, gelatin

pig

[110]

ethyl oleate

Tween 21/81/85®, bis-2-(ethylhexyl) sulphosuccinate Labrasol®, Plurol Isostearique®

mouse

[135]

isopropyl myristate oleic acid

Tween 80®/1,2-propylene glycol Tween 80®/propylene glycol

water, 154 mM NaCl water H2O/menthol

rat mouse

[111] [154]

isostearylic isostearate, oleic acid, R(+)-limonene tributyrine Mygliol 812®, soybean oil

Reprinted with permission from Kreilgaard et al. [80]. a A variety of seven different co-solvents was tested with the basic vehicle.

amount of surfactant. Ascorbic acid stability against oxidation was examined in the prepared systems and compared to the aqueous solutions at different pH values. It was found that all examined emulsified systems provided protection against

oxidation for ascorbic acid, and therefore its degradation rate was slower than in aqueous solutions. The degradation rate increased with increasing pH. The highest protection of the ascorbic acid was achieved by solubilizing it in a w/o/w

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multiple emulsion. From the kinetic studies performed, a pseudo first-order mechanism for ascorbic acid degradation was suggested in the experimental conditions where abundant dissolved oxygen was present (Fig. 2). Despite the fact that ascorbic acid has many favorable effects, its use in cosmetic and pharmaceutical products is limited due to its low stability. In order to solve this problem, derivatives of vitamin C, with action similar to that of ascorbic acid, but with improved chemical stability, have been synthesized. Two derivatives that are widely used in cosmetic products are lipophilic ascorbyl palmitate and hydrophilic sodium ascorbyl phosphate that differ in stability (Fig. 3) [121]. W/o microemulsions consisting of caprylic/capric triglyceride 24.75%, PEG-8 caprylic/capric glycerides 47.53%, polyglycerol-6 dioleate 11.88% and purified water 15.84%, were prepared by Špiclin et al. [121]. In parallel, o/w microemulsions containing caprylic/capric triglyceride 7.43%, PEG-8 caprylic/ capric glycerides 38.02%, polyglycerol-6 dioleate 9.50% and purified water 45.05% were also prepared. Ascorbic acid derivatives were incorporated into the two types of microemulsions at 0.25–2.00 wt.% concentrations. The influence of several parameters such as initial concentration, location in microemulsion, dissolved oxygen, and storage conditions on the stability of the less stable derivative ascorbyl palmitate was examined. It was found that incorporation of ascorbyl palmitate into microemulsions at the highest concentration (2.00 w/w%) reduced its degradation. In presence of oxygen, ascorbyl palmitate is more stable in w/o microemulsions; on the contrary if microemulsions are degassed, the compound is more stable in the o/w type. Therefore, the location of the active ingredient in the microemulsion may have a great influence on its stability against oxidation. In a w/o microemulsion the cyclic ring that is

Fig. 2. Pseudo first-order plot of ascorbic acid degradation at 45 ± 0.1 °C in emulsified systems at pH 5.0 (ascorbic acid = 1.00 wt.%) Reprinted with permission from Gallarate et al. [115].

sensitive to oxidation is located in the internal aqueous phase and, therefore, it is protected by the interface that serves as a barrier for oxygen diffusion. In o/w microemulsions, the cyclic ring is located in the external aqueous phase, where it is less protected, and therefore the degradation process is favored. In addition, it was found that light also accelerates the oxidative degradation of ascorbyl palmitate. In order to formulate an optimal carrier system for this ingredient, other factors influencing the stability have to be considered. The studies on sodium ascorbyl phosphate revealed that, compared to ascorbyl palmitate, it was equally stable towards oxidation in both types of microemulsions. Consequently, sodium ascorbyl phosphate has been found to be convenient for use as an active ingredient in topical formulations [121]. In subsequent studies different types of thickening agents, that increase viscosity, were added to the microemulsions described above, to optimize these topical formulations. The influence of thickening agents on the stability of the microemulsions and the release kinetics of the solubilized sodium ascorbyl phosphate were studied. Magnesium stearate, colloidal silica, or cetostearyl alcohol was used as thickening agent for w/o type microemulsions while for o/w microemulsions sodium alginate, methyl cellulose, HPMC K4M, HPMC E4M, and xanthan gum were incorporated. As a suitable thickening agent, colloidal silica was chosen for w/o microemulsions and xanthan gum for the o/w type. The presence of thickening agent and the location of sodium ascorbyl phosphate in the microemulsion influenced the in vitro drug release profiles. When incorporated in w/o microemulsions, sustained release profiles were observed from both thickened and non-thickened systems. This is due to the external oil phase which can act as a barrier for the diffusion of a hydrophilic compound. In the case of o/w systems, larger amounts were released due to the incorporation of the sodium ascorbyl phosphate in the external phase that increased the diffusion rate (Fig. 4). The type of thickening agent added to the examined systems influenced the rate of drug diffusion. The addition of xanthan gum (o/w system) increased the viscosity of the system and, therefore, decreased the amount of released compound. On the contrary, the amount of released sodium ascorbyl phosphate was higher in the case of thickened w/o microemulsion with colloidal silica. The authors assumed that colloidal silica modified the physicochemical characteristics of the external phase; therefore, the diffusion of sodium ascorbyl phosphate was faster. By evaluating the release profiles by fitting the experimental data to different orders kinetic equations, it was concluded that the rate-determining step for sodium ascorbyl phosphate release from microemulsions was its transport within the carrier system [120]. One can conclude that microemulsions are an effective solution to preserving ascorbic acid stability against oxidation and are effective vehicles for controlling the release kinetics of the active compound. 2.3.2. Diclofenac Several groups have selected diclofenac (a nonsteroidal antiinflammatory drug) as a model compound that was solubilized in a microemulsion medium [13,116,118,125]. Kriwet and Müller-Goymann [125] investigated vehicles containing phospholipids, diclofenac diethylamine, and water. Various colloidal


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Fig. 3. Chemical structures of ascorbyl palmitate (a) and sodium ascorbyl phosphate (b). Reprinted with permission from Špiclin et al. [121].

structures such as liposomal dispersions, microemulsions and lamellar liquid crystals can be formed by varying the ratios of the components forming the structures and method of preparation. In vitro studies, using a dialysis membrane impregnated with a silicone polymer, showed that the effective diffusion coefficient of diclofenac diethylamine changed rapidly with a phase transformation of the vehicle. By examination of the systems with constant concentration of diclofenac diethylamine (5% w/w) and variable amounts of phospholipids, it was found that the diffusion coefficient of the drug decreased with an increase of the phospholipids content [125] (Fig. 5). The systems with 6% phospholipids or less are microemulsions with low viscosity; therefore, the release is fast. Further addition of the phospholipids leads to isotropic and afterwards anisotropic gel formation, decreasing the release rate. The decrease in diffusion coefficient can be explained by the increase of the viscosity of the systems with the addition of phospholipids. The authors also investigated the influence of drug concentration in the vehicle on the effective diffusion coefficient [125] (Fig. 6). It was found that drug molecules participate in the microstructure of the resulting systems. At concentrations up to 4.5% diclofenac diethylamine, liposomal dispersion, dispersion of multilamellar vesicles in a microemulsion gel, or an isotropic gel occurred. The viscosity of these systems is high, therefore the drug release is slow. Further addition of diclofenac diethylamine leads to microemulsion formation with a low viscosity, and the rate of the drug release is increased. With the

Fig. 4. Release profiles of sodium ascorbyl phosphate from o/w and w/o microemulsions. Reprinted with permission from Špiclin et al. [120].

addition of diclofenac diethylamine, the number of vesicles with multilamellar layers decreases until the system transforms into a microemulsion. Since the release of the entrapped drug in the vesicle is hindered, the fewer are the vesicles, the easier is the drug release. Permeation through human stratum corneum was also examined. A linear relationship between the drug release out of the vehicle through the artificial membrane and the rate of drug permeability through the stratum corneum was observed. The rate-limiting step for vehicles with a high effective diffusion coefficient like microemulsions was the diffusional resistance in the stratum corneum. On the contrary, for vehicles with a low diffusion coefficient the rate-limiting step was the release from the vehicle. Only microemulsions showed enhanced permeability of diclofenac diethylamine through the stratum corneum, since the phospholipids were able to interact with the structures of the stratum corneum only if they were applied as microemulsion [125]. From these studies it can be concluded that phospholipid content can alter the structure of the vehicle, hence influences the drug release. Phospholipids can be used as a thickening agents since they increase the viscosity of the system. In addition, the drug itself may participate in the organization of the vehicle and consequently control its own release [125]. Other authors showed that diclofenac diethylamine participates in the vehicle structure due to its amphiphilic properties. Djordjevic et al. [13] investigated microemulsion systems composed of water, isopropyl myristate, PEG-8 caprylic/capric glycerides (Labrasol®), and polyglyceryl-6 dioleate (Plurol Oleique®), as potential diclofenac diethylamine delivery vehicles. It was found that in the experimental conditions, conductivity and rheological properties were influenced significantly by the incorporation of the drug. Conductivity values for drug loaded microemulsions were increased by a factor of 2–3 in comparison to microemulsions without the drug. On the contrary, the drug did not significantly influence either the pH value of the vehicles, or the stability and the optical texture of the examined systems [13]. Escribano et al. [118] investigated several ternary solvent systems and microemulsions in vitro using human skin in order to find the formulation that enhanced the percutaneous permeability of sodium diclofenac. In order to compare the effect of the vehicle on the percutaneous absorption, a fixed concentration of the drug was used in all examined systems. A 1 wt.% solution of sodium diclofenac and a commercially available semisolid preparation served as reference. Commonly known enhancers such as oleic and lauric acids, R(+)-limonene, and Transcutol® (diethylene glycol monoethyl ether) were used as the components of the vehicles. The highest values of permeability coefficient and drug

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Fig. 5. Effective diffusion coefficient of diclofenac diethylamine from vehicle with 5% diclofenac diethylamine vs. content of phospholipids. Reprinted with permission from Kriwet and Müller-Goymann [125].

flux, and the amount of drug that permeated and was retained in the skin at 24 h were observed for 1 wt.% drug formulations containing Transcutol® 59.2 wt.%, oleic acid 14.9 wt.%, and R (+)-limonene 5 wt.% as enhancers, and water 19.9 wt.%. This formulation had greater anti-inflammatory activity, both local and systemic, than the semisolid preparation. It was found that the synergistic enhancement effect with Transcutol®/oleic acid/Dlimonene was greater than that of Transcutol®/oleic acid or Transcutol®/lauric acid. In the comparison of permeation enhancement between oleic acid and lauric acid, it was shown that oleic acid provides more penetration capacity due to its cis double bond at C9, which causes a kink in the alkyl chain and disrupts the skin lipids more efficiently. Despite the expectation

that the microemulsion would alter the skin lipids and show enhanced permeation, it behaved like a solution. This may be attributed to the lower percentage of the enhancing agents (19%) as compared to 73% in other formulations. Therefore, the components of the vehicle are no less important than the structure of the vehicle [118]. In other studies, diclofenac and indomethacin in a microemulsion gel were used as model drugs. The enhancement of drug penetration was studied. The microemulsion gel consisted of soybean phosphatidylcholine (lecithin), isopropyl palmitate, and a small amount of water. Studies using Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), as well as low-temperature scanning electron microscopy, indicated

Fig. 6. Effective diffusion coefficient of diclofenac diethylamine in systems with 6% phospholipids vs. content of diclofenac diethylamine. Reprinted with permission from Kriwet and Müller-Goymann [125].

A. Kogan, N. Garti / Advances in Colloid and Interface Science 123–126 (2006) 369–385 Table 2 Microemulsion compositions System

Water (%)

Isostearylic isostearate (%)

Labrasol® (%)

Plurol Isostearique® (%)

A B C D E F G

20 20 20 7 11 55 65

10 10 10 70 26 8 3

35 a 47 b 53 c 11.5 a 42 b 25 b 24 c

35 a 23 b 17 c 11.5 a 21 b 12 b 8c

Reprinted with permission from Kreilgaard et al. [36]. a Labrasol®/Plurol Isostearique® 1:1 (w/w). b Labrasol®/Plurol Isostearique® 2:1 (w/w). c Labrasol®/Plurol Isostearique® 3:1 (w/w).

that isopropyl palmitate affected the stratum corneum lipid organization, but in vivo human skin irritation tests showed no significant irritancy [116]. 2.3.3. Lidocaine and prilocaine hydrochloride Lidocaine is a local anesthetic and cardiac depressant commonly used to restore a regular heartbeat in patients with arrhythmia. Prilocaine hydrochloride is also used as an anesthetic. Kreilgaard et al. [36] investigated the influence of the structure and composition of microemulsions on their transdermal delivery potential for a lipophilic (lidocaine) and a hydrophilic model drug (prilocaine hydrochloride), and compared the drug delivery potential of microemulsions to conventional vehicles. They used self-diffusion coefficients, as determined by the pulsed-gradient spin-echo NMR (PGSE-NMR), to study the transdermal flux using Franz-type diffusion cells. The authors prepared phase diagrams where w/o and o/w microemulsions were formed, and seven formulations for further studies were chosen (Table 2). The microemulsions were formulated with 4.8 wt.% of lidocaine and 2.4 wt.% of prilocaine hydrochloride. It was demonstrated clearly that microemulsions increased the

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transdermal flux of lidocaine up to four times, compared to a conventional oil-in-water emulsion, and increased the transdermal flux of prilocaine hydrochloride almost 10 times compared to hydrogels (Table 3). The flux seems to be dependent on drug solubility in the microemulsion as well as on drug mobility in the individual vehicle. It was found that the solubilities of the drugs in the microemulsions were greater in comparison to the solubilities obtained in the components of the microemulsion. Due to high solubilization property of microemulsions which leads to high concentration gradients, the transdermal flux increased. Lidocaine showed a 28–62% increase in solubility and prilocaine hydrochloride showed an increase of 24–40%. The large increase in solubility is due to surfactant film interface between the oil and water phases that leads to additional solubilization sites for drugs, compared to the molecular organization of bulk surfactants. A linear relationship between self-diffusion of the drugs in the vehicles and transdermal flux was found. It suggests that with given components, the percutaneous delivery potential of microemulsion vehicles may be decelerated by hindrance to diffusion due to the internal structure of the microemulsion [36]. Sintov and Shapiro [114] also chose lidocaine as a lipophilic drug model. To reduce irritation, special types of microemulsions were suggested. Glycerol oleate and polyoxyl [40 EO] fatty acid derivatives/tetraglycol (cosurfactant)/isopropyl palmitate/water were used to construct a pseudo-ternary phase diagram at fixed cosurfactant/surfactant ratios (Fig. 7). Lidocaine was solubilized and penetration studies using rat skin in vitro showed that the transdermal flux of lidocaine was significantly improved by microemulsion composed of glycerol oleate-PEG 400 stearate compared with that of glycerol oleatePEG 400 hydroxylated castor oil [114]. Fig. 8 shows a comparison of in vitro transdermal penetration between microemulsion, mixture of surfactants, micellar, and macroemulsion systems. Macroemulsion and micellar systems presented a lower flux of lidocaine relative to the microemulsion. The authors

Table 3 Steady-state flux (J; 20–28 h) and permeation coefficient (κp) of lidocaine and prilocaine hydrochloride (HCl) through rat skin from microemulsion systems A–G with near-maximum drug load and comparison with EMLA 5%, xylocain 5% o/w-cream and xylocain 2% hydrogel Formulation

A B C D E F G EMLAb EMLAb, c Xylocain 5% Xylocain 2%

Lipophilic model drug

Hydrophilic model drug

[Lidocaine] (%, w/w)

J (μg × h- 1 × cm− 2)

κp (μg × h− 1 × cm− 2)

[Prilocaine HCl] (%, w/w)

J (μg × h− 1 × cm− 2)

κp (μg × h− 1 × cm− 2)

23 27 24 17 25 12 9.1 2.5 2.5c 5 –

36.2 ± 4.2 41.1 ± 8.9 44.8 ± 4.3 56.5 ± 4.6 50.8 ± 8.6 44.6 ± 4.3 78.3 ± 3.9 52.3 ± 19 57.8 ± 18.8c 22.2 ± 6 –

1.6 ± 0.2 1.5 ± 0.3 1.9 ± 0.2 3.3 ± 0.3 2.0 ± 0.3 3.7 ± 0.4 8.7 ± 0.4 20.9 ± 7.6 23.1 ± 7.5c 4.4 ± 1.2 –

5 5 5 – 2.4a 13 14 – – – 2d

8.1 ± 2.6 6.3 ± 1.3 6.4 ± 2.1 – 6.2 ± 0.7 24.7 ± 4.3 29.7 ± 9.9 – – – 3.1 ± 1.0

1.6 ± 0.5 1.3 ± 0.3 1.3 ± 0.4 – 2.6 ± 0.3 1.9 ± 0.3 2.1 ± 0.7 – – – 1.6 ± 0.5

Reprinted with permission from Kreilgaard et al. [36]. a Near-maximum concentration equals 2.4%. b n = 5. c Prilocaine-free base. d Lidocainehydrochloride.

380


suggested that the presence of droplets of the microemulsion of only a few nanometers in size had a significant contribution to the percutaneous penetration of lidocaine. The use of a patch was also studied in this work. The results showed that patches based on a microemulsion had almost twofold higher fluxes than those obtained after applications of the corresponding liquid microemulsion and EMLA cream. (EMLA is a commercial product containing 1:1 eutectic mixture of 2.5 wt. % lidocaine and 2.5 wt.% prilocaine in a o/w macroemulsion.) It was concluded that patches were a very effective delivery device. It was also demonstrated that applying a liquid microemulsion or a patch based on a microemulsion significantly shortened the time

needed to reach steady-state drug penetration rates (lag time). In addition, there is an increase in the accumulation of drug quantities in skin layers. The decrease in lag time is an important issue, in particular for lidocaine and for topical analgesia in general, since commercial products require a relatively long application time to ease pains [114]. 2.3.4. Triptolide Triptolide is used as an immunosuppressive, anti-fertility, and anti-cancer drug. In order to decrease the adverse side effects caused by its toxicity, to prevent hydrolysis that may be caused by long-term storage of triptolide in an aqueous environment and be

Fig. 7. Pseudo-ternary phase diagrams of (a) microemulsion system made of isopropyl palmitate/water/tetraglycol/glyceryl oleate and PEG-40 hydrogenated castor oil at three different cosurfactant/surfactant ratios and (b) microemulsion system made of isopropyl palmitate/water/tetraglycol/glyceryl oleate and PEG-40 stearate, at three different cosurfactant/surfactant ratios. Reprinted with permission from Sintov and Shapiro [114].


Fig. 8. Percutaneous penetration of lidocaine from a microemulsion liquid vehicle of system made of isopropyl palmitate/water/tetraglycol/glyceryl oleate and PEG-40 stearate through rat skin in vitro (n = 5) during 3 h. The transport kinetics (squares) is compared to the penetration profiles of lidocaine from a surfactant mixture in tetraglycol (diamonds), oil-free micellar system (circles), and a macroemulsion (triangles). Reprinted with permission from Sintov and Shapiro [114].

promoted by pH values less than 3 or more than 8, a microemulsion system with controlled and prolonged percutaneous delivery of triptolide needs to be developed. The purpose of the studies by Mei et al. [111]and Chen et al. 154] was to investigate microemulsions for transdermal delivery of triptolide. Pseudoternary phase diagrams were developed and various microemulsion formulations were prepared using oleic acid as an oil, Tween 80® as a surfactant, and propylene glycol as a cosurfactant. The droplet size of the microemulsions was characterized by photocorrelation spectroscopy and it varied from 12 to 83 nm. The microemulsion formulations that were stored at 37 °C did not show any changes in size or phase separation for 6 months. No degradation of triptolide was detected either, since the drug was incorporated in the oily phase, preventing its contact with the external phase and pH values of the constructed microemulsions that were in the range of 4–6 inhibited hydrolysis. Permeation studies from microemulsion vehicles and aqueous solutions containing 20% propylene glycol using mouse skin were also performed. A steady increase of triptolide during 24 h was observed. On the contrary, the permeation flux from the aqueous solution decreased significantly after 10 h, which revealed the inability of this formulation to provide prolonged delivery of triptolide. In a microemulsion the drug can be released from the internal to the external phase and then from the external phase to the skin. The release from the internal phase supplements the depletion of the drug in the external phase, supplying sustained and controlled release of the triptolide into the skin. Zero-order release kinetics was obtained. Despite the great permeation enhancing properties of oleic acid, the formulation with higher oleic acid concentration did not show a significant enhancing effect. It was concluded that penetration of oleic acid was perturbed by the encapsulation of the surfactant and the cosurfactant. Menthol was found to be a suitable permeation enhancer for triptolide loaded microemulsions. It was also found

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that the microemulsions containing lower amounts of Tween 80® and propylene glycol provided higher permeation flux, which may be attributed to the increased thermodynamic activity of the drug in these microemulsions. A microemulsion containing 0.025% triptolide, 6% oleic acid, 20% Tween 80®, 10% propylene glycol, 62.975% water and 1% menthol was found to be the most suitable for triptolide delivery. The irritation studies on the skin of rabbits did not show any visible irritation for 7 days. In comparison, the aqueous solution induced significant erythema and edema. It may be concluded that a microemulsion is an effective method for triptolide delivery since it protects it from degradation, provides prolonged release, and despite the toxicity of the drug it does not cause irritation [154]. This formulation is better than the one previously suggested by Mei et al. [111] consisting of 40% isopropyl myristate, 50% Tween 80®/propylene glycol (5:1), and water, which was a w/o type of microemulsion. In the latest formulation, triptolide is solubilized in the outer oily phase and therefore might directly contact the surface of the skin. From the safety point of view this is not suitable for long-term use. 2.3.5. 5-Fluorouracil 5-Fluorouracil is an antineoplastic drug which, on intravenous administration, produces severe systemic gastrointestinal, hematological, neural, cardiac and dermatological effects. Therefore, transdermal delivery remains the choice route for administration. Very recently, Gupta et al. [149] investigated in vitro transdermal permeation through hairless mouse skin of this hydrophilic drug, which was encapsulated in AOT/water/isopropylmyristate waterin-oil microemulsions. Incorporation into microemulsions increased the flux of the drug 2–6 fold in comparison to the aqueous solution of the drug. The results revealed that microemulsions increased the average acyl lipid disorder as well as hydration of the stratum corneum, thus increasing the drug flux. The skin flux was dependent on the concentration of water and AOT. Skin toxicity studies indicate that the AOT/water/ isopropylmyristate ME is safe for the transdermal permeation of this drug [149]. 2.3.6. Retinoic acid Trotta et al. [112] examined the significance of ion pairing on the topical permeation of retinoic acid (RA) using microemulsions as delivery vehicles. Retinoic acid is used in treating acne vulgaris. Both oil-in-water (o/w) and water-in-oil (w/o) microemulsion formulations were prepared using water, isopropyl myristate, lecithin, caprylyl-capryl glucoside and ethanol or 1,2 hexanediol. The mean apparent diameters of these vehicles were 20–30 nm. Phenylalanine methyl ester, phenylalanine ethyl ester, histidine methyl ester, tryptophan methyl ester, and valine methyl ester were used as counter ions. The study revealed that the permeability of the RA from an ethanol–pH 6.4 buffer mixture through polydimethylsiloxane (PDMS) membrane was significantly increased in presence of the counter ions. These results were confirmed by examination of the permeability of the RA from the ethanol–pH 6.4 buffer mixture through pig skin. In order to clarify whether the enhancement was due to the ion pair formation or partly due to direct damage of the skin by

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the amino esters, the same permeability experiments were done with a Cortisone, a lipophilic drug that is unable to form an ion pair. Since its flux was not significantly increased in presence of phenylalanine ethyl ester hydrochloride, the authors concluded that RA flux increase might be attributed to the ion pair formation. Nevertheless, ion pairing in ethanolic solution could not help in optimizing RA targeting since drug accumulated/ drug delivered ratios of 0.6–0.9 and 0.4 μg h cm− 1 10− 3 were obtained for ion pair and RA, respectively. In the presence of counter ions, the permeabilities of RA from microemulsions using PDMS membranes decreased. These results were also confirmed using pig skin. w/o microemulsions exhibited the lowest accumulation of the drug in the skin. In comparison, o/w microemulsions containing counter ions (amino esters) showed drug accumulation four to five times higher than the corresponding counter ion-free microemulsions. These kinds of o/w microemulsions are very appealing vehicles for treatment of skin diseases and can be used to optimize drug targeting without a concomitant increase in systemic absorption. Despite the same components of these formulations, relative percentages of water, oil, and surfactant lead to different microstructures that provide different drug permeation and thus different skin accumulation [112]. 3. Summary and conclusions Microemulsions can be formed by numerous oil, surfactant, co-surfactant, and aqueous constituents. The main advantages of microemulsions as vehicles for drug delivery are their high solubilization capacities for both hydrophilic and hydrophobic drugs, their thermodynamic stability, the ease of formation, and the relatively low cost of formulation preparation. In some cases, the enhanced accumulation of the drug can significantly help optimize the targeting of the drug without increasing the systemic side effects. Microemulsions topically applied were shown to significantly increase the cutaneous absorption of the drugs. The vehicles frequently act as penetration enhancers, depending on the oil/surfactant constituents, which involves the risk of inducing local irritation. The process of skin permeation is very complex, but recent studies have improved the knowledge relative to the permeability process and its controllers. The understanding at the molecular level will enable the design of topical formulations with better rationale that will increase the bioavailability and decrease the side effects. One of the main problems is that after applying a topical formulation on the skin, the formulation composition may change. Accordingly, volatile compounds are promptly evaporated, small molecules can get absorbed by themselves, and others may interact with the skin in an irreversible manner causing irritation and possible permanent damage. Therefore, many drugs that penetrate the skin sufficiently to produce a therapeutic effect fail due to the incompatibility of the formulation in which they are entrapped. In addition, the choice of the formulation components is very complicated. Depending on the structure of the microemulsion, the character of the drug, and the membrane properties, the same substances can act as enhancers or as retarders. The

components that are well known as enhancers such as ethanol [141–144], propylene glycol [93] and phospholipids [106,115, 116,119,124,125], turned out to be retarders [145,148]. Thickening agents can serve as promoters or as retarders too [132]. In order to enhance the penetration ability of certain drugs one must modify its solubility, as well as diffusion and partition coefficients in each membrane. It is understood that in many cases the presence of a coenhancer to an enhancer optimizes the permeability, such as in the case of 1-menthol addition to ethanol that showed greater penetration enhancement of morphine hydrochloride [113]. It is important to note that the combination of several enhancing components leads to a synergistic effect and a greater enhancement effect than is observed for each component alone. In addition, the synergy between the iontophoresis and enhancers, such as fatty acids, has resulted in increase of both low and high molecular weight drug permeation as well as in decrease of skin toxicity [85,86,97–99]. Many enhancers are concentration-dependent; therefore optimal concentration for effective promotion should be determined. Sometimes when using an ethanol–water vehicle the effect of ethanol is concentration dependent, and therefore, can even decrease the permeation [145]. Ion pair creation provides prolonged activity due to the formation of a reservoir in topical formulations. This delay effect leads to increased bioavailability which promotes a decrease in daily intake of the drug and side effects. Many in vitro and in vivo studies have examined the relevance of the microemulsions as a transdermal route for administration. Nevertheless, more in vivo studies should be conducted to support the in vitro findings. In topical formulations, in many cases the irritation issue is not considered. In addition, chronic application consequences should be studied. For instance, for very common enhancers such as oleic acid, the benefit of penetration enhancement must be considered very carefully since immunosuppression of the skin might be affected [94]. Local and systemic toxicity should be further studied to determine therapeutic benefit/risk ratio and to evaluate the possible influence on the human body. Since some enhancers can harm the skin, researchers should examine not only the immediate influence of the formulations both in vitro and in vivo, but also examine the skin after chronic application. Despite all the complexity, it is possible to predict the appropriate enhancer based on similarity in physicochemical properties such as molecular weight, hydrophilic/lipophilic classification, and solubility. The microemulsion structure facilitates drug solubilization in greater amounts than in its individual components. A much better understanding of the relationship between the structure of the microemulsion droplets and the nature of the solubilized drug has been gained from the recent studies, but yet many of the results are strictly empirical and difficult to explain and even more difficult to predict or to forecast. Attention is being paid to the quantification of the maximum solubilization of a substance in the concentrate and on its behavior upon dilution. Attempts have been made to determine the locus of the quest molecule in the core of the microemulsion or at the interface. Understanding


the locus of solubilization can be useful in protecting the drug and for its sustained or immediate release. The potential in controlling the delivery rate has been demonstrated only to some extent and was demonstrated to be dependent on the structure and ingredients of the microemulsion. It should be noted that most of the studies conducted with topical formulations are done in vitro/in vivo on animal's skin showing greater permeability than that obtained with the same formulation in humans. It is obvious that both the type of examined membrane and the kind of vehicle play significant roles. The same components with varying concentration can form different vehicles affecting differently the rate of drug permeability, and the amount of drug accumulated into the skin. In most cases, the lower the viscosity of the vehicle the faster is the release [125,112]. There are some cases where the observations do not coincide with the expectations. Despite expectation of drug permeation enhancement due to the formation of a microemulsion vehicle, in some cases no enhancement occurs due to the lower percentage of the enhancing components in the vehicle in comparison to other formulations [118]. Therefore, the components, their concentrations and the form of the vehicle are of great interest to the pharmaceutical scientist. Phase transitions are known to occur in microemulsion microstructures as a result of changes in pH, temperature, dilution, etc., as well as due to the presence of the guest molecules, the solubilized drugs. Phase transitions were studied only by few scientists in past reports, and their effect on drug behavior in the microemulsion was not fully clarified. Phase equilibria and structural phase transformations within microemulsions in the presence of drugs are of high importance in drug delivery. Drugs tend to precipitate and crystallize as large crystals upon storage or dilution. These aspects were not considered in the past, but are relevant issues in relation to sustained or slow delivery. Therefore, stability examinations of drug-loaded microemulsions should be performed. In view of the fact that the entrapped drug may participate in the formation of the vehicle and, consequently, influence its release, it is very important to investigate the drug impact on the physicochemical properties of the vehicle. The dilution effects on the stability of the microemulsions were in most studies neglected and not considered. However, it is self-obvious that once the formulation is incorporated into the digestive tracts dilution occurs and the microemulsion in most cases decomposes and the drug is released in the uncontrolled patterns and has the tendency to precipitate. It was shown by very limited number of studies that microemulsions were efficient drugs protectors from environmental damages such as oxidative degradation [121], hydrolysis, inappropriate pH environment [154]. Many more such studies are required. Modeling the microemulsion phase transition behavior and the permeation process through the different membranes can be a very useful tool to study the change of physicochemical properties in case of drug upload. This way, hundreds of experiments will be eliminated automatically and will save researchers and the pharmaceutical industry a lot of money and precious time. In this area, there is a huge lack of reliable

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mathematical models to predict the behavior of real formulations. Currently, the use of in vitro models and in vivo animal models is the only way to predict in vivo human behavior. However, the design of such experiments should be done with care because human membranes either in vivo or extracted (in vitro) can behave differently from membranes taken from other mammals. During the last decade much progress has been made in exploring new types of microemulsion vehicles for drug delivery. More advanced analytical methods are becoming available; therefore, it becomes easier to better characterize the correlation between the nature of the microdroplets and the bioavailability of the embedded drug. Despite the enormous amount of information gathered in this field, there are still many questions left unanswered and good deal of work to be done. Acknowledgement We are very grateful to Dr A. Aserin and Ms. E. Oxman for their helpful remarks. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

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