Strategies to enhance permeability via stratum corneum lipid pathways.
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Animals↗
Biomedical subjects
R H Guy
Publications and source records attributed to R H Guy.
In vivo percutaneous penetration/absorption, Washington, D.C., May 1989.
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Animals↗
Iontophoretic delivery of a series of tripeptides across the skin in vitro.
The iontophoresis of eight tripeptides, of the general structure alanine-X-alanine, has been measured across hairless mouse skin in vitro. The peptides were blocked (a) at the carboxyl terminus using the mixed anhydride reaction with t-butylamine and (b) at the amino terminus by acetylation with 14C-acetic anhydride. The nature of the central residue (X) was varied by selecting one of five neutral amino acids, two negatively chargeable moieties (aspartic and glutamic acids), and a positively chargeable species (histidine). Constant current iontophoresis at 0.36 mA/cm2, using Ag/AgCl electrodes, was performed for 24 hr in diffusion cells, which allowed both anode and cathode to be situated on the same (epidermal) side of a single piece of skin. Due to a combination of osmotic and electroosmotic forces, the anodal iontophoretic flux of neutral peptides was significantly greater than passive transport. Steady-state fluxes were not achieved, however, suggesting that time-dependent changes in the properties of the skin barrier may be occurring. Limited, further experiments confirmed that, on a 24-hr time scale, these changes were not fully reversible. The cathodal delivery of anionic permeants was well controlled at a steady and highly enhanced rate by the current flow. This behavior closely paralleled earlier work using simple negatively charged amino acids and N-acetylated amino acid derivatives. It appears that the normalized iontophoretic flux of these anionic species is independent of lipophilicity but may be inversely related to molecular weight. The positively charged peptide, Ac-Ala-His-Ala-NH(But), showed greater anodal iontophoretic enhancement when delivered from a donor solution at pH 4.0 than from a solution at pH 7.4.(ABSTRACT TRUNCATED AT 250 WORDS)
Administration, Cutaneous↗
Does hydration affect intercellular lipid organization in the stratum corneum?
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Animals↗
Percutaneous penetration kinetics of nitroglycerin and its dinitrate metabolites across hairless mouse skin in vitro.
The percutaneous penetration kinetics of the antianginal, nitroglycerin (GTN), and its primary metabolites, 1,2- and 1,3-glyceryl dinitrate (1,2- and 1,3-GDN), were evaluated in vitro, using full-thickness hairless mouse skin. GTN and the 1,2- and 1,3-GDNs were applied (a) in aqueous solution as pH 7.4 phosphate-buffered saline (PBS) and (b) incorporated into lipophilic ointment formulations. The cutaneous transformation of GTN to its dinitrate metabolites was detected, but no interconversion between 1,2-GDN and 1,3-GDN was observed. Following application of the nitrates in PBS solution, all three compounds exhibited steady-state transport kinetics. The steady-state flux of GTN (8.9 +/- 1.5 nmol cm-2 hr-1) was significantly greater (P less than 0.05) than those of 1,2-GDN (0.81 +/- 0.54 nmol cm-2 hr-1) and 1,3-GDN (0.72 +/- 0.20 nmol cm-2 hr-1). The corresponding permeability coefficient (rho) for GTN (20 +/- 3 x 10(-3) cm hr-1) was significantly larger than the corresponding values for 1,2-GDN (1.4 +/- 0.9 x 10(-3) cm hr-1) and 1,3-GDN (1.2 +/- 0.4 x 10(-3) cm hr-1), which were statistically indistinguishable (P greater than 0.05). Further analysis of the transport data showed that the differences between GTN and the GDNs could be explained by the relative stratum corneum/water partition coefficient (Ks) values of the compounds. The apparent partition parameters, defined as kappa = Ks.h [where h is the diffusion path length through stratum corneum (SC)] were 19.8 +/- 2.5 x 10(-2) cm for GTN and 1.91 +/- 1.07 x 10(-2) and 1.81 +/- 0.91 x 10(-2) cm for 1,2- and 1,3-GDN, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Administration, Cutaneous↗
Iontophoretic delivery of amino acids and amino acid derivatives across the skin in vitro.
The effects of penetrant properties (lipophilicity and charge) and of vehicle pH on the iontophoretically enhanced delivery of amino acids and their N-acetylated derivatives have been examined in vitro. The penetrants were nine amino acids (five were zwitterionic, two positively charged, and two negatively charged) and four N-acetylated amino acids, which carry a net negative charge at pH 7.4. Iontophoresis at constant current (0.36 mA/cm2), using Ag/AgCl electrodes, was conducted across freshly excised hairless mouse skin. Iontophoretic flux of the zwitterions was significantly greater than passive transport. Delivery from the anode was greater than from the cathode for all zwitterions. The level of enhancement was inversely proportional to permeant octanol/pH 7.4 buffer distribution coefficient. Cathodal iontophoresis of the negatively charged amino acids and of the N-acetylated derivatives produced degrees of enhancement which were significantly greater than those measured for the "neutral" zwitterions. Furthermore, the enhanced flux reached a steady-state level within a few hours for the negatively charged species, whereas the transport of the zwitterions continued to increase with time. Anodal iontophoresis of histidine and lysine, the two positively charged amino acids studied, induced substantial enhancement which was sensitive to the pH of the delivery vehicle. For example, the flux of histidine from an applied solution at pH 4 (where the amino acid carries a net positive charge) was significantly greater than that from a vehicle at pH 7.4 (where histidine is essentially neutral). The behavior of lysine was more complex and suggested a certain degree of neutralization of the skin's net negative charge.
Administration, Cutaneous↗
Barrier function of human keratinocyte cultures grown at the air-liquid interface.
Stratum corneum (SC), the outermost and least permeable layer of skin, is the major barrier to passive transepidermal water loss. In the research described in this paper, we have used human keratinocyte cultures, grown at the air-liquid (A/L) interface, to examine the relationship between epidermal differentiation (including SC formation) and barrier function. Histologically, the A/L culture showed several markers of complete differentiation, including the presence of well-organized and defined epidermal cell layers, keratohyalin granules, and a multilayered SC. The permeability of tritiated water through epidermal cultures, which had grown for 3 weeks at the A/L interface, was measured with a microdiffusion apparatus. The results of these experiments demonstrated that: a) the human keratinocyte cultures developed a substantial barrier (i.e., a multilayered SC) to water diffusion across the entire surface. If the relative humidity of the culturing environment was lowered from 100% to around 75%, the barrier was significantly improved; b) the differentiation promoter, 1.25-dihydroxy-vitamin-D3, increased the number of SC layers and reduced water permeation through the culture; c) the nature of the keratinocyte support matrix could be altered to improve the morphology as well as the barrier function of the epidermal cultures. Overall, the observations are consistent with the relationship that is believed to exist between SC intercellular lipid content and percutaneous penetration. Confirmation of this hypothesis will further the considerable potential of human keratinocyte A/L cultures as a valuable and relevant model in which to study drug absorption and metabolism.
Air↗
Sites of iontophoretic current flow into the skin: identification and characterization with the vibrating probe electrode.
The routes taken by charged substances (e.g., peptides) through the skin during iontophoretic drug delivery are not well characterized. We have used a vibrating probe electrode to reproducibly identify and vectorize site-specific (spatial resolution = 20 microns) ionic flows as they were occurring in hairless mouse skin clamped at clinically relevant current densities. These iontophoretic currents were primarily appendageal, and certain appendages (e.g., small hairs) appeared to carry most of the current. This finding may have important ramifications with respect to irritation, allergic reaction, and electrical current damage in iontophoretic drug delivery. The size and direction of the current vectors could change under certain conditions (e.g., in an unbuffered preparation, where pH changes occurred during the experiment). The vibrating probe can operate in (and is not adversely affected by) the ranges of pH, tonicity, and current required for the study of iontophoretic currents.
Administration, Cutaneous↗
Workshop report on in vivo percutaneous penetration/absorption. Washington D.C., May 1-3, 1989.
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Administration, Topical↗
Percutaneous penetration of methyl nicotinate at three anatomic sites: evidence for an appendageal contribution to transport?
Percutaneous absorption of the vasodilator methyl nicotinate (MN) was evaluated in human volunteers at three anatomic sites (forehead, forearm and palm) using the technique of laser Doppler flowmetry (LDF). The experiments were designed to test the hypothesis that the kinetics and extent of skin penetration are dependent upon the appendageal density at the site of application. The LDF technique measured the increase in skin blood flow elicited by topically applied MN once the chemical had penetrated to the microvasculature. Significant differences in the measured LDF responses at the three sites were found, and further analysis of the data suggested that MN penetration was greatest through forehead skin, least through the palm and intermediate across the skin of the forearm. A correlation therefore existed between apparent MN absorption and appendageal density.
Administration, Cutaneous↗
Partitioning of chemicals into human stratum corneum: implications for risk assessment following dermal exposure.
Assessment of the health hazard associated with chemical contamination of the skin is a complex problem of occupational and environmental relevance. A particularly important question is to what extent can the skin permeability of a given compound be predicted from simple experiments. The literature on percutaneous absorption identifies two key observations: (i) the stratum corneum (SC), the skin's outermost layer, is the major barrier to chemical transport, and (ii) there are qualitative correlations between penetrant permeability and various oil/water partition coefficients (PCs). To obtain more quantitative predictions of permeation, we have evaluated SC/water and SC/isopropyl myristate (IPM, a model lipophilic vehicle) PCs of (a) para-substituted phenols of diverse physicochemical properties (4-acetamido-, 4-cyano-, 4-iodo-, and 4-pentyloxyphenol), (b) polychlorinated biphenyls (54%), and (c) 1,1,1,-trichloro-2,2-bis(p-chlorophenyl)ethane. Partition coefficients were determined as a function of the following variables: length of equilibration, initial drug concentration in the vehicle, SC delipidization, and SC source and preparation technique. The data demonstrate that reproducible partitioning can be obtained using the biological tissue of greatest relevance, and that the pattern of behavior observed, for the two different vehicles studied, is compatible with physicochemical expectations. We suggest that the PC values measured may be useful predictors of in vitro and in vivo skin transport and valuable assets, therefore, in the evaluation of risk following dermal exposure.
Animals↗
The relationship between pKa and skin irritation for a series of basic penetrants in man.
For a series of bases, which penetrate through human skin in vitro at similar rates (0.056-0.49 microM/cm2/hr), penetrant pKa is shown to correlate with erythema, edema, and color meter readings. As estimates of irritation, erythema, edema, and redness measurements are highly linearly correlated. For the selected series, irritation becomes significant for bases with a pKa greater than 8. The irritation potential of acids with pKa less than or equal to 4 has been previously reported; pKa appears highly predictive of acute skin irritation for acids and bases in man.
Adult↗
In vivo percutaneous absorption of chemicals: a multiple dose study in rhesus monkeys.
The effect of daily topical application on the in vivo percutaneous absorption of benzoic acid, parathion and salicylic acid in rhesus monkeys has been investigated. The study was designed to test further the hypothesis that topical bioavailability, or body burden, of a chemical following chronic exposure may be accurately predicted from the result of a single acute-dose experiment. No significant change in percutaneous absorption from that following the initial dose was observed following the eighth daily dose of a 14-day multidose regimen for each of the three penetrants considered. The results are consistent with those of recent experiments in humans with malathion and steroids, but not entirely consistent with the results of other animal studies.
Administration, Cutaneous↗
Cutaneous pharmacodynamics of transdermally delivered isosorbide dinitrate.
Laser doppler velocimetry (LDV) has been used to assess the cutaneous pharmacodynamics of isosorbide dinitrate (ISDN) following transdermal delivery of the drug from prototypal patches. The delivery systems, which were saturated with ISDN, (a) produced various degrees of skin occlusion and (b) spanned a six-fold range of adhesiveness. The patches were applied to the ventral forearm skin of 10 healthy volunteers and the local ISDN-induced increase in local skin blood flow was determined using LDV by locating the probe in a central hole in the delivery system. Measurements were made for 1.5 hr and the pharmacodynamics were quantified by (i) the maximum LDV response and (ii) the area under the LDV response versus time curve. These parameters were not sensitive to patch occlusivity. They were significantly (P less than 0.01) dependent on patch adhesiveness, though, and decreased with increasing adhesion. Although this observation suggested that ISDN diffusion through the adhesive could determine, at least in part, the rate of drug delivery, it was subsequently demonstrated that ISDN release (in vitro, into a perfect "sink") was unaffected by the level of cross-linking in the adhesive polymer. Because the drug was present in all systems at unit thermodynamic activity, these results cannot be explained on the basis of altered ISDN partitioning at the device-stratum corneum interface. We speculate that the in vivo-in vitro discrepancy may be due to the efficiency of skin contact achieved by different adhesives: that is, the more adhesive, less flexible systems make poorer contact with the skin surface, thereby decreasing the effective surface area of drug delivery.(ABSTRACT TRUNCATED AT 250 WORDS)
Adhesives↗
Optimization of topical therapy: partitioning of drugs into stratum corneum.
To optimize a topical formulation for therapeutic effect generally implies that the flux of drug into the skin be maximized. This requirement means that the product of drug concentration in the vehicle (Cv) and drug partition coefficient (PC) between stratum corneum (SC) and vehicle be as large as possible. While Cv is a formulation variable which can be easily manipulated up to the drug's saturation solubility, PC is a parameter that is difficult to predict a priori. However, there is no question that an ability to evaluate PC would greatly facilitate the efficient screening of drugs and formulations. We have measured the SC/water and SC/isopropylmyristate (a model lipophilic vehicle) PCs of seven drugs; acitretin, progesterone, testosterone, diazepam, estradiol, hydrocortisone, and caffeine, SC/water PCs were determined as a function of the following variables: (i) initial drug concentration in the vehicle, (ii) length of equilibrium, (iii) SC source and preparation technique, and (iv) SC delipidization. The data obtained were reproducible and physicochemically consistent, and they show that useful partitioning information from both aqueous and nonaqueous vehicles can be obtained with the biological tissue of greatest relevance. The SC/water PCs of the steroids were in reasonable agreement with previous measurements. A facile approach to an integral determinant of formulation optimization is suggested, therefore, by these observations.
Administration, Cutaneous↗
Percutaneous penetration enhancement in vivo measured by attenuated total reflectance infrared spectroscopy.
A novel application of attenuated total reflectance IR spectroscopy (ATR-IR) was used to monitor the outer several microns of the stratum corneum (SC) and, thereby, demonstrate enhanced percutaneous absorption in vivo in man. 4-Cyanophenol (CP) as a model permeant yielded a unique IR signal, distinct from those of the stratum corneum and the vehicle components. CP was administered for 1, 2, or 3 hr as a 10% (w/v) solution either in propylene glycol or in propylene glycol containing 5% (v/v) oleic acid. The absorbance at 2230 cm-1, which corresponded to C identical to N bond stretching, diminished significantly faster when CP was codelivered with oleic acid. An IR absorbance due primarily to propylene glycol at 1040 cm-1 (C-O stretching) also disappeared more quickly following application of the enhancer-containing solution. In addition, only the formulations with oleic acid induced a higher wavenumber shift in the frequency of the asymmetric C-H bond stretching absorbance. This change indicates increased lipid-chain disorder, the mechanism by which oleic acid is believed to cause enhanced drug transport across the stratum corneum. Therefore, ATR-IR permits one to examine noninvasively the kinetics, extent, and mechanism of percutaneous penetration enhancement in vivo in human subjects.
Adult↗
Examination of stratum corneum barrier function in vivo by infrared spectroscopy.
It is generally accepted that the stratum corneum (SC) is the least permeable layer of the epidermis. Histologically, though, the SC is a non-uniform, inhomogeneous membrane, and the question "Is barrier function distributed uniformly across the SC thickness?" has been posed. To address this issue, human ventral forearm SC has been studied in vivo by attenuated-total-reflectance Fourier-transform infrared spectroscopy during the course of sequential tape-stripping. Because the intercellular lipids of the SC and the degree of hydration of the membrane have been shown to be crucial determinants of barrier function, attention has been focused on the spectral features, which report specifically on these parameters. The degree of disorder of the SC intercellular lipids has been found to decrease over the outer cell layers (up to three tape-strips) and then to remain essentially constant. The amount of lipids decreases similarly such that a 60% reduction (relative to the "no-strip" baseline) is observed after about four tape-strips. A plausible explanation for these measurements is that the lipids near the surface are a mixture of (a) "true" intercellular lipid (which is expected to be highly ordered), and (b) sebaceous lipid (which contains much greater amounts of low-melting components, such as fatty acids). The sequential infrared (IR) spectra provide at least circumstantial evidence to support this hypothesis. As expected, the IR spectra show that SC hydration increases from the surface towards the SC-stratum granulosum interface. Taken together, the results imply that the SC is indeed non-uniform. The properties of the outer layers (those removed by the first 3-4 tape-strips) change significantly with increasing depth.(ABSTRACT TRUNCATED AT 250 WORDS)
Epidermal Cells↗
Pharmacodynamic measurements of methyl nicotinate percutaneous absorption: the effect of aging on microcirculation.
The penetration of drugs through aged skin is important both in terms of transdermal delivery to elicit systemic pharmacological effects, and for topical treatment. Cutaneous microcirculation efficiency, an integral parameter in the overall process of percutaneous absorption, was studied in young (20-34 years) and old (64-86 years) individuals. Cutaneous erythema as induced by topical administration of methyl nicotinate to the ventral forearm, was monitored non-invasively using laser-Doppler flowmetry. Dose-response behaviour was characterized by five parameters: (i) the time of onset of action; (ii) the time to reach maximum response; (iii) the magnitude of the maximum response; (iv) the area under the response-time curve; and (v) the time to decay to 75% of the maximum response. Additionally, the sensitivity and efficiency of the cutaneous microcirculation in both age groups was evaluated using a pharmacokinetic-pharmacodynamic model. Statistical analysis of all data showed no significant differences between the age groups for the same concentrations. The results indicate that microvessel reactivity to the applied stimulus is comparable in the ventral forearm of both young and old populations.
Adult↗