Effect of 1-alkyl- or 1-alkenylazacycloalkanone derivatives on the penetration of drugs with different lipophilicities through guinea pig skin

Effect of hydrocarbon chain length in 1,2-alkanediols on percutaneous absorption of metronidazole: toward development of a general vehicle for controlled release

The objective of the present study is to investigate the effect of hydrocarbon chain length in 1,2-alkanediols on percutaneous absorption of metronidazole (MTZ). Twelve formulations (1,2-propanediol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol in 4% concentration, 1,2-hexanediol, and 1,2-heptanediol in 1% concentration, in the absence and presence of 1,4-cyclohexanediol, respectively) were studied in an in vitro hairless mouse skin model using Franz diffusion cell. Based on the flux values and retardation ratios (RR), a penetration retardation effect on percutaneous absorption of MTZ was observed for the formulations containing 1,2-diols having six- to seven-carbon chain in the presence of 1,4-cyclohexanediol (1,2-hexanediol with chain length of six hydrocarbons, RRs are 0.69 and 0.76 in the concentration of 4% and 1%, respectively; 1,2-heptanediol with chain length of seven hydrocarbons, RR is 0.78 in the concentration of 1%). On the other hand, no retardation effect was observed in formulations containing short alkyl chains (RRs of 1,2-propanediol, 1,2-butanediol, and 1,2-pentanediol are 0.99, 1.61, and 0.96, respectively). Instead, a penetration enhancement effect was observed for 1,2-diols having four and five carbons. In other words, effect of 1,2-alkanediols on percutaneous absorption of MTZ can be systematically modulated by simply varying number of -CH2 groups in the hydrocarbon chain-from being a penetration enhancer to retardant. These observations shed light on mechanism of the penetration enhancement and retardation effect and provide insight into rational design of penetration enhancers and retardants. Furthermore, the combination of 1,2-alkanediols and 1,4-cyclohexanediol could become a general vehicle for controlled release of pharmaceutical and cosmetic active ingredients.

Effects of solvent deposited enhancers on transdermal permeation and their relationship with Emax

Many topical pharmaceuticals such as aerosols, topical sprays, and hydro-alcoholic and polymer based gels contain chemical enhancers. The objectives of the present study were to (a) determine the enhancement effects induced by enhancers deposited from a volatile solvent on human epidermal membrane (HEM) upon transdermal permeation enhancement, (b) compare these enhancement factors with Emax, and (c) examine the relationship between enhancer-induced permeation enhancement and stratum corneum equilibrium uptake enhancement. In this study, HEM was treated with enhancer/ethanol (enhancer dissolved in ethanol). After the evaporation of ethanol, passive transport experiments were conducted using corticosterone (CS) as the model permeant. The uptake of another model corticosteroid, estradiol (E2beta), into the intercellular lipid domain of stratum corneum after enhancer/ethanol treatment was also determined. The results show a correlation between Emax and the enhancement effect of most enhancers when the enhancers were deposited on the skin using the volatile solvent ethanol. The data suggest that the CS transport rate limiting domain was likely the same as the intercellular lipid domain probed by E2beta uptake. The correlation between steady-state permeation enhancement and uptake enhancement into the intercellular lipid domain suggests that the permeation enhancement mechanism is primarily due to enhancement of permeant partitioning into the transport rate limiting domain.

Chemical Enhancers for the Absorption of Substances Through the Skin: Laurocapram and Its Derivatives

Absorption enhancers are substances used for temporarily increasing a membrane's permeability (e.g., the skin and mucosa), either by interacting with its components (lipids or proteins) or by increasing the membrane/vehicle partition coefficient. This article presents the results of biophysical and permeability studies performed with Laurocapram and its analogues. As shown, Laurocapram and its analogues present different enhancing efficacies, for most of both hydrophilic and lipophilic substances. The enhancing effect of Laurocapram (Azone) is attributed to different mechanisms, such as insertion of its dodecyl group into the intercellular lipidic bilayer, increase of the motion of the alkylic chains of lipids, and fluidization of the hydrophobic regions of the lamellate structure. Toxicological studies reveal a low toxicity for Laurocapram, and for some derivatives, a relationship exists between toxicity and the number of carbons in the alkylic chain. Very important, when applied to human skin, Laurocapram shows a minimal absorption, being quickly eliminated from circulation. However, although Laurocapram and its derivatives have been shown to provide enhancement, they have not been widely accepted because of their suspected pharmacological activity or questions about their safety.

Silicone Elastomer Uptake Method for Determination of Free 1‐Alkyl‐2‐Pyrrolidone Concentration in Micelle and Hydroxypropyl‐β‐Cyclodextrin Systems Used in Skin Transport Studies

Previous investigations in our laboratory demonstrated how the polar head group and alkyl chain of amphiphilic chemical skin permeation enhancers contribute to enhancer potency. In those studies enhancers with n-alkyl chain lengths of eight or less were investigated. In order to investigate enhancers with longer n-alkyl chain lengths, enhancer-solubilizing agents should be considered. Corticosterone (CS) flux enhancement along the lipoidal pathway of hairless mouse skin (HMS) was determined with the enhancers 1-hexyl- (HP), 1-octyl- (OP), 1-decyl- (DP), and 1-dodecyl-2-pyrrolidone (DoP) solubilized in 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine-N-[methoxy(polyethylene glycol-2000] (DSPE) micelles or in hydroxypropyl-beta-cyclodextrin (HPbetaCD). The free CS, HP, OP, DP, and DoP aqueous concentrations in the DSPE micelle and HPbetaCD systems were determined using a partitioning method. Comparisons of the enhancer potencies based on the free concentration of the enhancers revealed a nearly semi-logarithmic linear relationship between enhancer potency and the carbon number of the alkyl chain length with a slope of approximately 0.55. The observed n-alkyl chain length dependency in the aqueous phase is consistent with the hydrophobic effect. This study shows that longer chain enhancers may be studied by employing a solubilizing system, and free enhancer concentration in these systems can be determined with the aid of the silicone elastomer uptake method.

Model analysis of flux enhancement across hairless mouse skin induced by chemical permeation enhancers

Previous permeant partitioning studies with hairless mouse skin (HMS) in the presence of several chemical skin permeation enhancers have revealed that, when such enhancers induce significant skin permeability coefficient enhancement, it is accompanied by significant enhancement in the equilibrium uptake (partitioning) of the permeant into the intercellular lipid component of the stratum corneum (SC). Particularly, it was found that the 1-alkyl-2-pyrrolidones and the 1-alkyl-2-azacycloheptanones, at aqueous solution concentrations that gave skin permeation enhancement (E) of 10 for corticosterone (CS, the permeant), enhanced the equilibrium uptake of beta-estradiol (E2beta, a surrogate permeant) from the aqueous phase into the intercellular lipids of HMS SC by a factor of 5-7. This finding raised the question of whether this uptake enhancement induced by the permeation enhancer under equilibrium conditions would be essentially the same as that determined kinetically from time-dependent permeation experiments utilizing appropriate SC membrane models and Fick's laws of diffusion to treat the data. HMS transport experiments were conducted with CS as the permeant and 1-octyl-2-pyrrolidone (OP) and 1-hexyl-2-azacyloheptanone (HAZ) as the enhancers. In treating the experimental data, a one-layer skin transport model (SC only) and a two-layer model (SC layer and the epidermis/dermis layer) were both investigated. Both the partition coefficient enhancement (E(K)) and the diffusion coefficient enhancement (E(D)) were deduced from the data treatment. The results showed that when the total transport enhancement of CS was around 11, E(K) was in the range of 6-8 and E(D) was in the range of 1.5-1.9 using both the one-layer and the two-layer models. This E(K) value was found to be in good agreement with the E2beta partition enhancement obtained directly under equilibrium conditions in previous studies. This indicates that (a) the rate-limiting domain for the transport of the lipophilic permeants across HMS and the HMS SC intercellular lipid domain probed in the equilibrium partitioning experiments are essentially the same, and (b) the total flux enhancement (E) of lipophilic permeants across HMS was driven mainly by enhancing the partitioning of the permeant into the rate-limiting domain (E(K)) and secondarily by enhancing the diffusion coefficients (E(D)) of the permeant in the domain. Comparison of the one-layer and two-layer skin model results revealed that non-steady-state transport of lipophilic compounds across HMS was better described by the two-layer model because the dermis/viable epidermis played a significant role in lipophilic permeant binding.

Mechanistic study of chemical skin permeation enhancers with different polar and lipophilic functional groups

In a previous study, the enhancement effects on the transport of a steroidal permeant along the hairless mouse skin (HMS) stratum corneum (SC) lipoidal pathway were investigated for two homologous series of chemical enhancers: the 1-alkyl-2-pyrrolidones and the 1-alkyl-2-azacycloheptanones. The objective of the present study was to extend this investigation to a broader range of enhancers in order that generalizations with regard to the mechanistic aspects of enhancer function might be established. Specific questions to be addressed included: (a) what is the nature of the microenvironment of the enhancer site of action? (b) what is the extent of the equilibrium uptake of the enhancer from its E = 10 aqueous enhancer solution (the aqueous concentration for which the enhancer induces a tenfold transport enhancement) into the HMS SC intercellular lipid "phase"? and (c) are the microenvironment of the enhancer site of action and that for the equilibrium enhancer uptake at E = 10 relatively independent of the molecular characteristics of the enhancers (as suggested by the earlier study)? Enhancers selected for this study included: a wide range of polar head group size and polarity; n-alkyl group chain lengths from C(4) to C(12); and enhancers in which a double bond is substituted for a single bond in the hydrocarbon chain (3-alkenols) from C(5) to C(9). In addition to the main study, an ancillary set of experiments were to be conducted on the partitioning of a surrogate permeant (estradiol) into the intercellular lipid "phase" under E = 10 isoenhancement conditions to assess the extent to which the permeant partition coefficient may contribute to the permeation enhancement. The following were the principal findings of this research. First, there was very good correlation between the E = 10 isoenhancement aqueous enhancer concentrations and K(octanol/water) for all the studied enhancers. Second, the partitioning of the enhancer from the E = 10 aqueous enhancer solution into the HMS SC intercellular lipid "phase" was found to be relatively independent of the molecular characteristics for all studied enhancers, and the partition coefficients also correlated well with K(octanol/water). These results may have the following meanings: both the microenvironment of the enhancer site of action and the SC intercellular lipid "phase" involved in the enhancer partitioning experiments are well mimicked by liquid n-octanol, and the "intrinsic" potencies (as assessed by the equilibrium enhancer concentration in the microenvironment at the site of action) of the enhancers are relatively independent of the molecular characteristics of the studied enhancers. Finally, the estradiol partitioning experiments suggest the permeant partitioning into the HMS SC intercellular lipid "phase" is enhanced around five- to seven-fold when permeation is enhanced ten-fold for most of the studied enhancers; therefore, the enhancement of the permeant partition coefficient rather than the permeant diffusion coefficient seems to be more important in permeation enhancement of the SC barrier lipoidal pathway.

Effect of Sucrose Fatty Acid Esters on Transdermal Permeation of Lidocaine and Ketoprofen

The effect of sucrose fatty acid esters on transdermal permeation of lidocaine (LC) and ketoprofen was examined. A drug solution was applied to excised hairless mouse skin pretreated with a sugar ester solution to examine the direct effects of the sugar esters on skin permeability. LC was applied with a pH 6 buffer solution (98.8% ionized), pH 10 buffer solution (99.2% unionized), or propylene glycol, while KP was applied with a pH 6 buffer solution (99.1% ionized), pH 2 buffer solution (98.9% unionized), or propylene glycol. Pretreatment with J-1216 (sucrose laurate, HLB = 16) or J-1205 (sucrose laurate, HLB = 5) significantly increased the permeation of LC from the pH 6 solution and KP from propylene glycol, respectively. The permeability coefficients of the ionized and unionized LC and KP were calculated from the permeability data. The ionized LC and KP permeated even through skin not pretreated with sugar esters, although the permeability coefficients were 24 times and 38 times less than those of the unionized LC and KP, respectively. J-1216 pretreatment increased the permeability of ionized LC from aqueous vehicle 2.7 fold. In the next series of experiments, we formulated 1.5% of J-1205 and J-1216 in various vehicles to examine their effect on the permeation of LC applied on the excised hairless mouse skin with no pretreatment. The results coincided with the results of the pretreatment experiment, and the effect of J-1205 in propylene glycol was more remarkable than that observed in the pretreatment study. When these sugar esters were dissolved in propylene glycol at 1.5%, J-1205 increased significantly the KP permeation rate as well as LC permeation rate, suggesting that the combination of J-1205 and propylene glycol would be a potent vehicle for transdermal formulations.

Skin Metabolism in Transdermal Therapeutic Systems

Skin has at least two barriers with protective functions: the stratum corneum physical barrier and a biochemical barrier in the epidermis and dermis. Numerous chemical and physical enhancers exist for transdermal therapeutic systems; some cause irritation, and possibly influence enzyme deactivation. Knowledge of enzymatic skin reactions is important for developing safe and efficacious transdermal systems for treatment not only of skin diseases but also for systemic application. This paper overviews the effects of (a) chemical enhancers and additives, (b) drug structure, and (c) physical enhancement on skin metabolism.

Mechanistic and empirical modeling of skin permeation of drugs

The skin forms a barrier to the external environment, maintaining body fluids within our system and excluding harmful substances, while the skin is a site of administration of drugs for topical and systemic chemotherapy. It is an important issue to predict the rate at which drugs or other xenobiotics penetrate the skin. In this article, we review modeling approaches for predicting skin permeation of compounds, including both mechanistic and empirical approaches. Mechanistic approaches can give us much information on understanding of skin permeation of the compounds, such as structure-permeability relationship, contribution of each barrier step, mechanism of penetration enhancers, and in vivo-in vitro relationship. On the other hand, empirical modeling can overcome any inaccuracies of mechanistic models caused by the existence of uncertainties and, therefore, give us better predictions from the practical point of view. Artificial neural networks are being available for empirical modeling of complex skin transport phenomenon.

Structure-activity relationship for chemical skin permeation enhancers: probing the chemical microenvironment of the site of action

Studies were previously conducted in our laboratory on the influence of n-alkanols, 1-alkyl-2-pyrrolidones, N,N-dimethlyalkanamides, and 1,2-alkanediols as skin permeation enhancers on the transport of a model permeant, corticosterone (CS). The experiments were conducted with hairless mouse skin (HMS) in a side-by-side, two-chamber diffusion cell, with enhancer present in an aqueous buffer in both chambers. The purpose of the present study was to extend these studies and investigate in greater detail the hypothesis that a suitable semipolar organic phase may mimic the microenvironment of the site of enhancer action, and that the enhancer partitioning tendency into this organic phase may be used to predict the enhancer potency. CS flux enhancement along the lipoidal pathway of HMS stratum corneum was determined with the 1-alkyl-2-azacycloheptanones, 1-alkyl-2-piperidinones, 1,2-dihydroxypropyl decanoate, 1,2-dihydroxypropyl octanoate, n-alkyl-beta-D-glucopyranosides, 2-(1-alkyl)-2-methyl-1,3-dioxolanes, 1,2,3-nonanetriol, and trans-hydroxyproline-N-decanamide-C-ethylamide as enhancers. Enhancement factors (E values) were calculated from the permeability coefficient and solubility data over a range of E values. Comparisons of the enhancer potencies for all studied homologous series and the carbon number of the n-alkyl group revealed a nearly semilogarithmic linear relationship with a slope of approximately 0.55, which is consistent with the hydrophobic effect. Moreover, comparisons of the enhancer potencies of all the enhancers with the n-hexanol-phosphate buffered saline (PBS), n-octanol-PBS, n-decanol-PBS, and n-hexane-PBS partition coefficients showed very good correlations for the n-alkanol solvents but not for n-hexane. This result supports the interpretation that the enhancer potency is directly related to the ability of the enhancer molecule to translocate to a site of action via its free energy of transfer from the bulk aqueous phase to a semipolar microenvironment in the stratum corneum lipid lamella that is well mimicked by water-saturated n-alkanols.

Mechanistic study of alkyl azacycloheptanones as skin permeation enhancers by permeation and partition experiments with hairless mouse skin

In previous studies (Yoneto et al., 1995. J Pharm Sci 84:312-317; Kim et al., 1992. Int J Pharm 80:17-31; and Warner et al., 2001. J Pharm Sci 90:1143-53), the transport enhancing effects of four homologous series of enhancers-the n-alkanols, 1-alkyl-2-pyrrolidones, 1,2-alkanediols, and N,N-dimethylalkanamides - on the transport of steroidal permeants across hairless mouse skin (HMS) were investigated. Isoenhancement concentrations are defined as the aqueous concentrations for which different enhancers induce the same extent of permeant transport enhancement, E, for the lipoidal pathway of the stratum corneum (SC). Our studies have shown that the E = 10 isoenhancement concentrations of these four homologous series were nearly the same when compared at the same n-alkyl group chain length and therefore that the contribution of the polar head group toward the enhancer potency was found to be essentially constant. In the present study, we have determined the isoenhancement concentrations (E = 10) for the 1-alkyl-2-azacycloheptanone series [1-butyl-2-azacycloheptanone (BAZ), 1-hexyl-2-azacycloheptanone (HAZ), and 1-octyl-2-azacycloheptanone (OAZ)] and compared the results with those of the previously studied four homologous series. We have found that the E = 10 isoenhancement concentrations (aqueous phase concentrations) of the 1-alkyl-2-azacycloheptanones (Azs) are around 10 times lower than those for the previously studied four homologous series when compared at the same alkyl group chain length. This indicates an approximately 10 times higher potency of Azs. This finding was a point of interest because the polar group of Azs is similar to that of 1-alkyl-2-pyrrolidones (Aps). To further probe the nature of the mechanism of action of the Azs and Aps and to better understand the lower E = 10 isoenhancement concentrations found with the Azs, it was decided (a) to determine the equilibrium partitioning (uptake) of the Azs and the Aps from the aqueous phase into the HMS SC at E = 10, and (b) to determine the equilibrium partitioning (uptake) of a surrogate permeant, estradiol (E2beta), into the SC in the absence of and in the presence of Azs and Aps at E = 10. The following were the outcomes from the two partitioning studies. Firstly, at the E = 10 isoenhancement concentrations, the extent of partitioning (uptake) of the Azs and Aps into the intercellular lipids of the HMS SC was found to be approximately the same, even though the E = 10 isoenhancement concentrations (aqueous phase concentrations) of the Aps were around 10 times greater than those of the Azs. We interpret this to mean (whereas the potencies of the Azs are around ten times greater than those of the Aps when related to their aqueous concentrations) that the potencies of the two enhancer series are about the same when expressed in terms of their concentrations in the intercellular lipid phase of the SC. Another outcome of the partitioning studies has been the finding that the extent of partitioning into the intercellular lipids of the SC at E = 10 isoenhancement conditions for both the Azs and Aps is essentially independent of the n-alkyl chain length (from butyl to octyl). A third result from these experiments has been that the partitioning of E2beta (the surrogate permeant) into the HMS SC under E = 10 isoenhancement concentration conditions is approximately the same with the Aps and Azs as enhancers. For both the Aps and Azs, the E2beta SC partitioning enhancement was found to be in the range of 5-6 at E = 10. This comparable partitioning enhancement for E2beta in the presence of Aps and Azs at E = 10 suggests that the same mechanism was involved and that these enhancers act, in part but to a significant extent, by inducing a higher partitioning tendency of the permeant into the transport rate-limiting lipoidal domains of the SC. (c) 2003 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 92:297-310, 2003

Chemical enhancers for transdermal drug transport

In its first part, this review paper discusses skin morphology and barrier function of the stratum corneum for drug permeation after its transdermal administration or topical application. Further, the paper presents the main methods for overcoming the skin permeation barrier, which plays an important role for transdermal drug administration. Focus is on the method of chemical permeation enhancement. The chemical enhancers are categorised by their chemical structure. Examples of the most effective enhancers are given for the chemical groups of alcohols, amines and amides, polyalcohols, terpenes, fatty acids and their esters, macro cyclic compounds, sulfoxides, tensides, and others, as e.g. soft enhancers.

Percutaneous penetration enhancers: local versus transdermal activity

The stratum corneum, poses a formidable challenge to formulators of drug delivery systems. Several approaches have been utilized to facilitate entry of drugs into the lower skin layers. Traditionally, permeation enhancers were designed to deliver high drug concentrations across the skin into the systemic circulation. The use of many of these agents resulted in unpleasant or toxic side effects. However, in recent years there has been a search for compounds that exhibit low toxicity, and maintain their enhancing activity. More recently, there has been interest in agents that may be used in topical formulations to prevent the passage of active ingredients or excipients into the lower skin layers. These so-called skin retardants have potential uses in many over-the-counter (OTC) skin formulations, such as sunscreens and pesticides, where the site of action is restricted to the skin surface or upper skin layers. Research in the area of permeation enhancement or retardation is yielding valuable insights into the structure-activity relationships of enhancers as well as retardants.

Chemical enhancement of percutaneous absorption in relation to stratum corneum structural alterations

The outermost layer of the skin, stratum corneum (SC), provides an outstanding barrier against the external environment and is also responsible for skin impermeability toward most solutes. The barrier function is related to the unique composition of the SC lipids and their complex structural arrangement. The lipoidal matrix of the SC, therefore, is a target of penetration enhancer action. The literature on the skin barrier structure and function and on the mechanisms of action of some well established permeation promoters, with a focus on their impact on SC structural alterations, is reviewed. Data obtained from infrared, thermal, and fluorescence spectroscopic examinations of the SC and its components imply enhancer improved permeation of solutes through the SC is associated with alterations involving the hydrocarbon chains of the SC lipid components. Data obtained from electron microscopy and X-ray diffraction reveals that the disordering of the lamellar packing is also an important mechanism for increased permeation of drugs induced by penetration enhancers.

A sequential algorithm for the non-linear dual-sorption model of percutaneous drug absorption

A sequential algorithm is developed for the non-linear dual-sorption model developed by Chandrasekaran et al. [1,2] which monitors pharmacokinetic profiles in percutaneous drug absorption. In the experimental study of percutaneous absorption, it is often observed that the lag-time decreases with the increase in the donor concentration when two or more donor concentrations of the same compound are used. The dual-sorption model has sometimes been employed to explain such experimental results. In this paper, it is shown that another feature observed after vehicle removal may also characterize the dual-sorption model. Soon after vehicle removal, the plots of the drug flux versus time become straight lines on a semilogarithmic scale as in the linear model, but the half-life is prolonged thereafter when the dual-sorption model prevails. The initial half-life after vehicle removal with a low donor concentration is longer than that with a higher donor concentration. These features, if observed in experiments, may be used as evidence to confirm that the dual-sorption model gives an explanation to the non-linear kinetic behaviour of a permeant.

Percutaneous penetration enhancement and its quantification

True penetration enhancing effects resulting from structural alterations of the barrier stratum corneum manifest themselves in an increase of the drug diffusion coefficient DB and/or of the drug solubility in the barrier csB. The quantification of enhancing effects on drug penetration is possible either by the direct determination of the drug fluxes or by an indirect determination through the measurement of the pharmacodynamic response. In both cases the thermodynamic drug activity has to be considered. In the case of pharmacodynamic measurements, enhancing effects may be determined from the horizontal distance of activity-response lines obtained without and with enhancer, respectively, i.e. the quotient of the drug concentrations that induce the same effect. The activity-standardized bioavailability factors fa obtained from the horizontal distances correspond to the enhancer-induced relative changes in the permeabilities PB, or more exactly in the product DB X csB. On the other hand, the vertical distance between the activity-response lines, i.e. the differences in the drug response after application of preparations with equal (even maximum) thermodynamic drug activities may be used to quantify penetration enhancing effects.

In vitro evaluation of a series of N-dodecanoyl-L-amino acid methyl esters as dermal penetration enhancers

A series of N-dodecanoyl-L-amino acid methyl esters (1-10) and n-pentyl N-acetylprolinate (11) were evaluated for dermal enhancement properties using an in vitro diffusion cell technique. Methods of synthesis of these compounds were described. Enhancers were applied 1 h prior to drug treatment. Hydrocortisone was used as the model drug and was applied to excised hairless mouse skin as a saturated suspension in propylene glycol. Enhancement ratios (ER) were determined for permeability coefficient, 24 h diffusion cell receptor concentration (Q24), and 24 h full-thickness skin steroid content. Controls received no enhancer pretreatment of the skin. N-Dodecanoyl-L-proline (10) showed the highest Q24 value for total steroid (ER 13.7) while N-dodecanoyl-L-phenylalanine (5) showed the highest total steroid skin retention (ER 16.5).

Percutaneous absorption of 2',3'-dideoxyinosine in rats

This study explored the topical route for administering of 2',3'-dideoxyinosine (ddI), a nucleoside analog used for treating patients with acquired immunodeficiency syndrome. A dose of ddI (approximately 180 mg/kg) dispersed in approximately 1 g ointment base was applied, with or without occlusion, to the back of high follicular density (HFD) and low follicular density (LFD) rats. The systemic ddI clearance was determined using a concomitant administration of an intravenous tracer dose of [3H]ddI. At 24 hr, the experiment was terminated and skin sections at the application site were removed. After topical application, average plateau plasma levels of about 0.6 microgram/ml were achieved within 1 to 2 hr and maintained for 24 hr. Occlusion gave a more uniform plasma profile but did not increase the bioavailability. The systemic bioavailability in HFD and LFD rats was about the same at 33%. In addition, a depot of about 16% of the dose was recovered by rinsing the application area and extracting the drug from the excised application site. These data indicate that about 50% of the dermal dose penetrated the skin barrier in 24 hr. The similar bioavailability in the HFD and LFD rats further suggests an unimportant role for the transfollicular absorption route for ddI. The effect of a mixture of penetration enhancers, Azone and propylene glycol (5:95), was studied in HFD rats. Coadministration of ddI with the enhancers did not increase the ddI bioavailability. However pretreatment and coadministration with the enhancers significantly increased the bioavailability to 62%, which is a conservative estimate because the plasma drug level was still at a plateau when the experiment was terminated at 24 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative analysis of percutaneous absorption enhancement by d-limonene and oleic acid based on a skin diffusion model

Percutaneous absorption-enhancing effects of d-limonene and oleic acid were investigated using three model drugs with different lipophilicities in in vitro diffusion experiments with guinea pig skin. Pretreatment of the skin with d-limonene resulted in a large penetration enhancement for the lipophilic butylparaben (BP) and amphiphilic 6-mercaptopurine (6-MP) but had little effect on the hydrophilic mannitol (MT). Oleic acid caused a large effect only on 6-MP penetration. The penetration profiles were analyzed with a two-layer skin diffusion model consisting of stratum corneum with polar and nonpolar routes and viable epidermis plus dermis. Through curve-fitting, six parameters corresponding to drug diffusivity and partitioning in these three regions of the skin were obtained, and the mechanisms of enhancers were assessed in comparison with those of 1-geranylazacycloheptan-2-one (GACH) reported previously. Increased penetration was caused mainly by modification of the barrier property of the nonpolar route in the stratum corneum in all cases. In the nonpolar route, d-limonene increased mainly drug diffusivity, while GACH enhanced predominantly drug partitioning. On the other hand, oleic acid moderately increased both parameters.

In vivo and in vitro analysis of skin penetration enhancement based on a two-layer diffusion model with polar and nonpolar routes in the stratum corneum

In vitro and in vivo skin penetration of three drugs with different lipophilicities and the enhancing effects of 1-geranylazacycloheptan-2-one (GACH) were studied in rats. In vivo drug absorption profiles obtained by deconvolution of urinary excretion profiles were compared to the corresponding in vitro data obtained with a diffusion experiment. In vivo skin penetration of lipophilic butylparaben was considerably greater than that observed in vitro, while hydrophilic mannitol and acyclovir showed low penetration in both systems without GACH pretreatment. On the other hand, GACH enhanced mannitol and acyclovir penetration, especially in the in vivo system. Analysis of absorption profiles, using a two-layer skin model with polar and nonpolar routes in the stratum corneum, suggested that the diffusion length of a viable layer (viable epidermis and dermis) was shorter in vivo than in vitro and the effective area of the polar route in the stratum corneum was larger in vitro without GACH pretreatment. GACH increased the partitioning of acyclovir into the nonpolar route to the same extent in both systems. In addition, GACH increased the effective area of the polar route in vivo, probably because of enhanced water permeability; however, this effect was smaller in vitro since the stratum corneum was already hydrated even without GACH pretreatment.

Percutaneous absorption: a single-layer model

In vitro percutaneous permeation of betamethasone 17-valerate through excised human skin was studied. Pressure-sensitive silicone adhesive containing betamethasone 17-valerate in suspension was used as a vehicle. Steady-state flux through the split-thickness skin was similar to that through the isolated epidermis. However, the lag-time and half-life after removal of the vehicle were longer for the split-thickness skin than from epidermis. At steady state, 37% of the drug in the split-thickness skin was partitioned in dermis. When the kinetic parameters of a simple single-layer model are defined to specify the permeability coefficient and the drug amount in skin at steady state, this model can predict the longer half-life observed for the split-thickness skin sample compared with that for epidermis. The difference between the observed and theoretical values of the half-life after removal of the vehicle was within 23%. On the other hand, the lag-time had a large variation and the simple diffusion model failed to be predictive. A single-layer model described by two or three kinetic parameters may be able to describe percutaneous permeation kinetics even when the processes after the compound permeation through stratum corneum are not negligible. However, it is stressed that none of the kinetic parameters inherent in this simple model directly represents one of the single physicochemical parameters, such as diffusion and partition coefficients and path length of each skin layer.