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`1~isi%:na
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`27 March 2004
`
`CONTENTS
`
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`b89078366598a
`uuue DELIVERY *
`Reviews
`
`www.elsevier,com/locate/addr
`
`Cited in: BIUSIS/Biological Abstracts; Chemical Abstracts Service;
`Current Contents (Life Sciences); EMBASE/Excerpta Medica;
`Elsevier BIOBASE/Currerzt/-itlvances in Biological Sciences; PUBMED/MEDLINE/Index Medicus;
`International Pharmaceutical Abstracts; Science Citation Index
`
`Theme Title: BREAKING THE SKIN BARRIER
`
`Preface
`Breaking the skin barrier
`S. Mitragotri (USA)
`
`Commentary
`Transdermal drug delivery: past progress, current status, and future prospects
`R. Langer (USA)
`
`Transdermal drug delivery with a pressure wave
`A.G. Doukas and N. Kollias (USA)
`
`Micronccdlcs for transdermal drug delivery
`M.R. Prausnitz (USA)
`
`Low-frequency sonophoresis: A review
`S. Mitragotri and J. Kost (USA, Israel)
`Penetration enhancers
`
`A.C. Williams and B.W. Barry (UK)
`
`lontophoretic drug delivery
`Y.N. Kalia, A. Naik, J. Garrison and RH. Guy (Switzerland, France, USA)
`
`Skin electroporation for transdennal and topical delivery
`A.—R. Denet, R. Vanbever and V. Préat (Belgium)
`
`Lipid vesicles and other colloids as drug carriers on the skin
`G. Cevc (Germany)
`
`Guide to Authors
`
`CO
`
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`ELSEVIER
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`Available online at www.sciencedirect.com
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`ScIENCE@DIREO‘I'°
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`Advanced Drug Delivery Reviews 56 (2004) 603—6l8
`
`Advanced
`
`Reviews
`
`www.elsevier.com/locate/addr
`
`Penetration enhancers
`
`Adrian C. Williams*, Brian W. Barry
`
`Drug Delivery Group, School of Pharmacy, University of Bradford, Richmond Road, Bradford, West Yorkshire, BD7 1 DP, UK
`
`Received 9 September 2003; accepted 13 October 2003
`
`Abstract
`
`One long-standing approach for improving transdermal drug delivery uses penetration enhancers (also called sorption
`promoters or accelerants) which penetrate into skin to reversibly decrease the barrier resistance. Numerous compounds have
`been evaluated for penetration enhancing activity, including sulphoxides (such as dimethylsulphoxide, DMSO), Azones (e.g.
`laurocapram), pyrrolidones (for example 2-pyrrolidone, 2P), alcohols and alkanols (ethanol, or decanol), glycols (for example
`propylene glycol, PG, a common excipient in topically applied dosage forms), surfactants (also common in dosage forms) and
`terpenes. Many potential sites and modes of action have been identified for skin penetration enhancers; the intercellular lipid
`matrix in which the accelerants may disrupt the packing motif, the intracellular keratin domains or through increasing drug
`partitioning into the tissue by acting as a solvent for the permeant within the membrane. Further potential mechanisms of action,
`for example with the enhancers acting on desmosomal connections between corneocytes or altering metabolic activity within
`the skin, or exerting an influence on the thermodynamic activity/solubility of the drug in its vehicle are also feasible, and are
`also considered in this review.
`© 2003 Elsevier B.V. All rights reserved.
`
`Keywords: Penetration enhancers; Accelerants; Skin; Lipids; Azone; Dimethylsulphoxide; Terpenes
`
`Contents
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`1.
`2.
`3.
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`Introduction .
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`General principles
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`Penetration enhancers .
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`3.1. Water .
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`3.2.
`Sulphoxides and similar chemicals .
`3.3.
`Azone .
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`3.4.
`Pyrrolidones
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`3.5.
`Fatty acids .
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`3.6.
`Alcohols, fatty alcohols and glycols .
`3.7.
`Surfactants .
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`3.8.
`Urea .
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`* Corresponding author. Tel.: +44-1274-23-4756; fax: +44-1274-23-3496.
`E—mail address: A.C.Williams@bradford.ac.uk (A.C. Williams).
`
`0169-409X/$ - see front matter © 2003 Elsevier B.V. All rights reserved.
`doi:l0.1016/j.addr.2003.10.025
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`A.C. Williams, B. W. Barry /Advanced Drug Delivery Reviews 56 (2004) 603—6l8
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`Essential oils, terpenes and terpenoids .
`3.9.
`Phospholipids .
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`3.10.
`Solvents at high concentrations .
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`3.11.
`3.12. Metabolic interventions .
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`General comments on penetration enhancers .
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`4.
`References .
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`612
`614
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`616
`
`1. Introduction
`
`Human skin is a remarkably efficient barrier,
`designed to keep “our insides in and the outsides
`out”. This barrier property causes difficulties for
`transdermal delivery of therapeutic agents. One
`long-standing approach to increase the range of drugs
`that can be effectively delivered via this route has
`been to use penetration enhancers, chemicals that
`interact with skin constituents to promote drug flux.
`To-date, a vast array of chemicals has been evaluated
`as penetration enhancers (or absorption promoters),
`yet their inclusion into topical or transdermal formu-
`lations is limited since the underlying mechanisms of
`action of these agents are seldom clearly defined. In
`this article we review some uses of the more widely
`investigated chemical penetration enhancers and dis-
`cuss possible mechanisms of action.
`
`2. General principles
`
`Although many chemicals have been evaluated as
`penetration enhancers in human or animal skins, to-
`date none has proven to be ideal. Some of the more
`desirable properties for penetration enhancers acting
`within skin have been given as [1];
`
`They should be non-toxic, non-irritating and non-
`allergenic.
`
`They would ideally work rapidly, and the activity
`and duration of effect should be both predictable
`and reproducible.
`
`They should have no pharmacological activity within
`the body——i.e. should not bind to receptor sites.
`The penetration enhancers should work unidirec-
`tionally, i.e. should allow therapeutic agents into the
`body whilst preventing the loss of endogenous
`material from the body.
`
`When removed from the skin, barrier properties
`should return both rapidly and fully.
`The penetration enhancers should be appropriate for
`formulation into diverse topical preparations, thus
`should be compatible with both excipients and
`drugs.
`They should be cosmetically acceptable with an
`appropriate skin ‘feel’.
`
`Not surprisingly, no such material has yet been
`discovered that possesses the above ideal properties
`although some chemicals demonstrate several of the
`above attributes.
`
`Penetration enhancers may be incorporated into
`formulations in order to improve drug flux through
`diverse membranes including gastric epithelia or nasal
`membranes. Diffusion through skin, controlled by the
`outer most layer, the stratum comeum, can be regarded
`as diffusion through a passive membrane. The steady
`state flux (J) of a drug through skin can be approxi-
`mated by Fick’s second law of diffusion;
`2
`5c_D5c
`E — W
`
`W
`
`where C is the concentration of the diffusing substance,
`x the space co-ordinate measured normal to the section,
`D the diffusion coefficient, and t is time.
`
`investigators often
`With skin permeation studies,
`use an in vitro protocol with a membrane clamped
`between two compartments, one of which contains a
`drug formulation (the donor) and the other compart-
`ment holding a receptor solution which provides sink
`conditions (essentially zero concentration). With suf-
`ficient time, steady state diffusion across the mem-
`brane prevails. Under these conditions Eq. (1) may be
`simplified to;
`
`dm DCO_ : j
`
`dt
`h
`
`2
`
`(
`
`)
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`A.C. Williams, B.W Barry /Advanced Drug Delivery Reviews 56 (2004) 603—6I8
`
`605
`
`where m is the cumulative mass of perrneant that
`passes per unit area through the membrane in time 1,
`C0 is the concentration of diffusant in the first layer of
`the membrane at the skin surface contacting the source
`of the penetrant, and h is the membrane thickness.
`In most experimental protocols it is difficult to
`measure C0 but C’, the concentration of diffusant in
`the donor phase, which bathes the membrane, is usually
`known. C0 and Cl, are related by;
`
`C0 = pa’,
`
`(3)
`
`where P is the partition coefficient of the diffirsant
`between the membrane and bathing solution. Substi-
`tuting Eq. (3) into Eq. (2) gives:
`
`d_m_DC()P
`dt— h
`
`(4)
`
`From Eq. (4), the classic equation used to analyse
`skin permeation data, it can be seen that the flux (dmf
`dt) is governed by the diffusion coefficient of the drug
`in the stratum comeum, the dissolved effective con-
`
`the partition
`centration of the drug in the vehicle,
`coefficient between the formulation and the stratum
`
`comeum and the membrane thickness. This equation
`thus illustrates some of the properties that may be
`manipulated on application of a penetration enhancer
`to the skin. Thus, an effective penetration enhancer
`may increase the diffusion coefficient of the drug in
`the stratum comeum (i.e. disrupt the barrier nature of
`the stratum comeum), may act to increase the effec-
`tive concentration of the drug in the vehicle (for
`example acting as an anti-solvent), could improve
`partitioning between the formulation and the stratum
`comeum (perhaps by altering the solvent nature of the
`skin membrane to improve partitioning into the tissue)
`or, less likely, by decreasing the skin thickness (per-
`haps by providing a penneation ‘shortcut’ as opposed
`to a tortuous pathway for a penneant).
`The modes of action of penetration enhancers in
`general are complex. As will be seen below, at
`clinically acceptable concentrations, most enhancers
`interact with the intercellular lipid domain of the
`stratum comeum. Fig.
`l is a modification of a scheme
`
`Action at Irztercellular Lipids
`
`lipid
`enhancer
`
`} lipid tails
`
`‘i’
`
`fluidisation
`
`polar
`
`; O
`
`polar
`}headgroups
`
`ii
`lipid extraction
`
`DO
`enhancer
`. / fluidisation
`
`.
`
`--v‘us-a
`
`-s.::4..
`
`{$l\ll'fi""
`
`polar
`
`\ <§°§§gz
`2 slat
`
`polarity alteration
`
`poo]
`
`l
`
`E
`§ E3
`
`phase separation
`
`vesicle
`
`
`/ Lipid Bilayer
`headgroups
`+
`liig
`
`%‘\3.}%f”
`
`phase separation
`
`Fig. 1. Actions of penetration enhancers within the intercellular lipid domain (modified from [2]).
`
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`proposed by Menon and Lee [2] for the action of
`solvents on this tissue. However, the diagram is useful
`for accelerants in general as a visual aid to much of
`the discussion following below. Further consideration
`of penetration enhancer mechanisms of action is given
`in Section 4.
`
`3. Penetration enhancers
`
`The literature contains reports describing various
`elegant formulations that may contain materials which
`have penetration enhancing activity. For example,
`vesicles are often prepared from phospholipids; phos-
`pholipids themselves have some penetration enhanc-
`ing activity (see below). Likewise, penetration
`enhancers have been formulated into eutectic systems
`or into slow or sustained release delivery systems. The
`focus of this chapter is not the formulations per se,
`rather the penetration enhancing activity of these
`materials.
`
`3.1. Water
`
`One long-standing approach to improve transder—
`mal and topical delivery of medicaments is to use
`water. The water content of human stratum comeum
`
`is typically around 15-20% of the tissue dry weight,
`although clearly this varies depending on the external
`environment such as humidity. Soaking the skin in
`water, exposing the membrane to high humidities or,
`as is more usual under clinical conditions, occluding
`the tissue so preventing transepiderrnal water loss can
`allow the stratum comeum to reach water contents in
`
`equilibrium with that of the underlying epidermal
`skin cells. Thus, on occlusion, the water content of
`this outer membrane can approach 400% of the tissue
`dry weight. Many clinically effective preparations
`and products such as ointments and patches are
`occlusive, which provides one mechanism of en-
`hanced drug delivery; numerous patch formulations
`deliver drugs at higher than would be expected rates
`due to modification of the stratum comeum water
`content.
`
`increased tissue hydration appears to
`In general,
`increase transdermal delivery of both hydrophilic and
`lipophilic permeants. However, Bucks and Maibach
`cautioned against such a generalisation, stating that
`
`occlusion does not necessarily increase percutaneous
`absorption, and that transdermal delivery of hydro-
`philic compounds may not be enhanced by occlusion
`[3]. Further,
`they warn that occlusion could cause
`some local skin irritation with clear implications for
`the design and manufacture of transdermal and topical
`preparations.
`Considering the heterogeneous nature of human
`stratum comeum it is not surprising that water within
`this membrane is found in several ‘states’. Typically,
`from thermal analysis and spectroscopic methodolo-
`gies, some 25-35% of the water present in stratum
`corneum can be assessed as ‘bound’,
`i.e.
`is associ-
`ated with some structural elements within the tissue
`
`[4]. The remaining water within the tissue is ‘free’
`and is available to act as a solvent within the
`
`membrane for polar permeants. Human skin also
`contains a hygroscopic humectant mixture of amino
`acids, amino acid derivatives and salts termed the
`natural moisturising factor (NMF). This material
`retains water within the stratum comeum and helps
`to maintain tissue pliability. Further, the keratin-filled
`comeocytes containing functional groups such as
`—OH and C—OOH are also expected to bind water
`molecules within the tissue. Considering such dispa-
`rate potential water binding sites, the absorption (and
`desorption) of water to and from stratum comeum is
`complex. However, it is notable that even maintain-
`
`ing a stratum comeum membrane over a strong
`dessicant such as phosphorous pentoxide, will not
`remove all the water from the tissue~there remains
`
`a strongly bound fraction of 5—10% water that
`cannot be removed under such conditions.
`
`Despite extensive research in the area, the mecha-
`nisms of action by which water increases transdermal
`drug delivery are unclear. Clearly free water within
`the tissue could alter the solubility of a perrneant in
`the stratum comeum and hence could modify parti-
`tioning from the permeant vehicle into the membrane.
`Such a mechanism could partially explain elevated
`hydrophilic drug fluxes under occlusive conditions
`but would fail to explain hydration-enhanced delivery
`for lipophilic permeants such as steroids. Since the
`principle barrier to transdermal drug delivery resides
`in the stratum comeum lipids it may be expected that
`high water contents, generated by occlusion or soak-
`ing, would cause some swelling and hence disruption
`to these domains possibly by swelling the polar head
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`607
`
`group regions of the bilayers. However, investigations
`by Bouwstra and co-workers using diffractometry
`methods have shown that water does not cause mod-
`
`ification to the lipid bilayer packing [5]. Such findings
`raise the question “Where does the water go?”.
`Clearly the corneocytes take up water and swell.
`One may expect that such swelling of cells would
`impact upon the lipid structure between the comeo-
`cytes causing some disruption to the bilayer packing.
`Again the experimental evidence contradicts this
`view. Data from freeze fracture electron microscopy
`of fully hydrated stratum comeum shows that
`the
`intercellular lipid bilayers contain water pools with
`vesicle-like structures occasionally found but no gross
`distortion to the lipid domains [6].
`Elias et al. [7] consider the presence of an aqueous
`pore pathway in the stratum comeum, consisting of
`lacunar domains (sites of comeodesmosome degrada-
`tion) embedded within the lipid bilayers. Although
`scattered and discontinuous under normal physiolog-
`ical conditions, they suggest that under high stress
`conditions (such as extensive hydration, iontophore—
`sis or ultrasound) the lacunae expand,
`interconnect
`and form a continuous “pore pathway”. The forma-
`tion of such a route would markedly enhance drug
`penetration.
`When examining the literature on the effects of
`water on transdermal permeation difficulties can arise
`from variable responses shown by different species.
`For example, Bond and Barry showed that hairless
`mouse skin is unsuitable as a model for human
`
`stratum comeum when examining hydration effects;
`the rodent skin permeability rose over 50-fold when
`hydrated for 24 h in contrast to results from human
`skin membranes [8]. Thus literature examining water
`effects on skin permeability using animal models
`should be viewed with some caution.
`
`3.2. Sulphoxides and similar chemicals
`
`Dimethylsulphoxide (DMSO) is one of the earli-
`est and most widely studied penetration enhancers
`(Fig. 2).
`It
`is a powerful aprotic solvent which
`hydrogen bonds with itself rather than with water;
`it is colourless, odourless and is hygroscopic and is
`often used in many areas of pharmaceutical sciences
`as a “universal solvent”. DMSO is used as a co-
`
`solvent in a vehicle for a commercial preparation of
`
`Aprotic Solvents
`
`
`
`Dimethyl-
`sulphoxide
`(DMSO)
`
`Dimethyl-
`acetamide
`(DMA)
`
`Dimethyl-
`formamide
`(DMF)
`
`Fig. 2. Aprotic solvents which act as potent penetration enhancers.
`
`idoxuridine, used to treat severe herpetic infections
`of the skin, particularly those caused by herpes
`simplex. DMSO alone has also been applied topi-
`cally to treat systemic inflammation, although cur-
`rently it is used only to treat animals.
`A vast array of literature describes the penetration
`enhancing activities of DMSO, and studies have
`shown it
`to be effective in promoting both hydro-
`philic and lipophilic permeants. Thus,
`it has been
`shown to promote the permeation of, for example,
`antiviral agents, steroids and antibiotics. DMSO
`works rapidly as a penetration enhancer—spillage
`of the material onto the skin can be tasted in the
`
`mouth within seconds. Although DMSO is an excel-
`lent accelerant it does create problems. The effects of
`the enhancer are concentration dependent and gener-
`ally co-solvents containing >60% DMSO are needed
`for optimum enhancement efficacy. However, at
`these relatively high concentrations DMSO can cause
`erythema and wheals of the stratum comeum and
`may denature some proteins. Studies performed over
`40 years ago on healthy volunteers painted with 90%
`DMSO twice daily for 3 weeks resulted in erythema,
`scaling, contact uticaria, stinging and burning sensa-
`tions and several volunteers developed systemic
`symptoms [9]. A further problem with DMSO use
`as a penetration enhancer is the metabolite .dimethyl-
`sulphide produced from the solvent; dimethylsul-
`phide produces a foul odour on the breath. When
`examining the wealth of literature reporting DMSO
`activity as a penetration enhancer it
`is essential
`to
`consider the membrane employed by the investiga-
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`
`tors since animal membranes and especially those
`from rodents tend to be considerably more fiagile
`than human skin membranes. Thus,
`the actions of
`this powerful aprotic solvent on animal tissue may
`be dramatically greater than the effects seen on a
`human skin membrane.
`
`Since DMSO is problematic for use as a penetra—
`tion enhancer, researchers have investigated similar,
`chemically related materials as accelerants. Dimethy—
`lacetamide (DMAC) and dimethylformamide (DMF)
`are similarly powerfiil aprotic solvents with struc-
`tures akin to that of DMSO. Also in common with
`
`DMSO, both solvents have a broad range of pene-
`tration enhancing activities, for example, promoting
`the flux of hydrocortisone,
`lidocaine and naloxone
`through skin membranes. However, Southwell and
`Barry, showing a 12-fold increase in the flux of
`caffeine permeating across the DMF treated human
`skin, concluded that the enhancer caused irreversible
`membrane damage [10]. Despite the evidence that
`DMF irreversibly damaged human skin membranes,
`this penetration enhancer has been used in vivo and
`promoted the bioavailability of betamethasone-17-
`benzoate as judged by the vasoconstrictor assay
`[11,12]. Further structural analogues have been pre-
`pared including alkylmethylsulphoxides such as
`decylmethylsulphoxide (DCMS). This analogue has
`been shown to act reversibly on human skin and,
`like its parent DMSO, also possesses a concentration
`dependent effect. The majority of available literature
`shows that DCMS is a potent enhancer for hydro-
`philic penneants but is less effective at promoting
`transderrnal delivery of lipophilic agents.
`The mechanisms of the sulphoxide penetration
`enhancers, and DMSO in particular, are complex.
`DMSO is widely used to denature proteins and on
`application to human skin has been shown to change
`the intercellular keratin confirmation, from or helical
`to a B sheet [13,14]. As well as an effect on the
`proteins, DMSO has also been shown to interact with
`the intercellular lipid domains of human stratum
`comeum. Considering the small highly polar nature
`of this molecule it
`is feasible that DMSO interacts
`
`with the head groups of some bilayer lipids to distort
`to the packing geometry. Further, DMSO within skin
`membranes may facilitate drug partitioning from a
`formulation into this “universal solVent” within the
`tissue.
`
`3.3. Azone
`
`Azone (1-dodecylazacycloheptan-2—one or lauro-
`capram) was the first molecule specifically designed
`as a skin penetration enhancer (Fig. 3). Chemically it
`may be considered to be a hybrid of a cyclic amide,
`as with pyrrolidone structures (see Section 3.4 be-
`low) with an alkylsulphoxide but
`is missing the
`aprotic sulphoxide group that provides some of the
`disadvantages listed above for DMSO. Azone is a
`colourless, odourless liquid with a melting point of
`— 7 °C and it possesses a smooth, oily but yet non-
`greasy feel. As would be expected from its chemical
`structure, Azone is a highly lipophilic material with a
`logp0C,am,.,wate, around 6.2 and it is soluble in and is
`compatible with most organic solvents including
`alcohols and propylene glycol (PG). The chemical
`has low irritancy, very low toxicity (oral LD50 in rat
`of 9 g/kg) and little phannacological activity al-
`though some evidence exists for an antiviral effect.
`Thus, judging from the above, Azone appears to
`possess many of the desirable qualities listed for a
`penetration enhancer in Section 2.
`Azone enhances the skin transport of a wide
`variety of drugs including steroids, antibiotics and
`antiviral agents. The literature contains reports de-
`scribing activity in promoting flux of both hydro-
`philic and lipophilic permeants. As with many
`penetration enhancers, the efficacy of Azone appears
`strongly concentration dependent and is also influ-
`enced by the choice of vehicle from which it
`is
`applied. Surprisingly, Azone is most effective at low
`concentrations, being employed typically between
`
`Fig. 3. Azone, the first molecule to be synthesised so as to act as a
`skin penetration enhancer.
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`609
`
`0.1% and 5%, often between 1% and 3%. Although
`Azone has been in use for some 25 years, research-
`ers continue to investigate its mechanism of action.
`Azone probably exerts its penetration enhancing
`effects through interactions with the lipid domains
`of the stratum comeum. Considering the chemical
`structure of the molecule (possessing a large polar
`head group and lipid alkyl chain)
`it would be
`expected that the enhancer partitions into the bilayer
`lipids to disrupt their packing arrangement; integra-
`tion into the lipids is unlikely to be homogeneous
`considering the variety of compositional and packing
`domains within stratum comeum lipid bilayers. Thus,
`Azone molecules may exist dispersed within the
`barrier
`lipids or in separate domains within the
`bilayers. A ‘soup spoon’ model for Azone’s confir-
`mation within stratum comeum lipids supports the
`above mechanisms of action [15] and electron dif-
`fraction studies using lipids isolated from human
`stratum comeum provides good evidence that Azone
`exists (or partially exists) as a distinct phase within
`the stratum comeum lipids [16]. Extensive discus-
`sion concerning the metabolism and fate of Azone
`and on its use as a penetration enhancer has been
`reviewed and the molecule is still being investigated
`presently [l7—l9].
`
`3. 4. Pyrrolidones
`
`A range of pyrrolidones and. structurally related
`compounds have been investigated as potential pene-
`tration enhancers in human skin. As with Azone and
`
`they apparently
`many other penetration enhancers,
`have greater effects on hydrophilic permeants than
`for lipophilic materials, although this may be attrib-
`utable to the greater enhancement potential for the
`poorer hydrophilic permeants. N-methyl-2-pyrroli-
`done (NMP) and 2—pyrrolidone (2P) are the most
`widely studied enhancers of this group. NMP is a
`polar aprotic so