Diffusion and metabolism of prednisolone farnesylate in viable skin of the hairless mouse

Evaluation of simultaneous permeation and metabolism of methyl nicotinate in human, snake, and shed snake skin

The transdermal permeation and metabolic characteristics of methyl nicotinate (MN) in stratum corneum and split-thickness human skin and three species of shed snake and snake skin (Elaphae obsoleta, Naja kaouthia, and Python molurus bivittatus) were evaluated. In vitro skin transport using excised skin and hydrolysis experiments using skin homogenate were carried out. The flux of MN, a metabolite, nicotinic acid (NA), and the total (MN+NA), as well as kinetic parameters (V(max) and K(m)) for hydrolysis of MN were determined and compared among various skin types. The total flux from MN-saturated solution through human skin was not significantly different from that through snake and shed snake skin of Elaphae obsoleta, Naja kaouthia but was significantly higher than that through snake and shed snake skin of Naja kaouthia (p < 0.05). A great difference in skin esterase activity was observed between human and snake in both snake skin and shed snake skin of all species. In all skins except the stratum corneum of human skin, NA flux increased with an increase in MN donor concentration and reached a plateau, suggesting that metabolic saturation was taking place in the skin. NA flux at the plateau and MN donor concentrations at which the NA flux reached a plateau also varied by species. These findings indicated that the discrepancy in transdermal profiles of MN among skins tested was predominantly due to the difference in the esterase activity in the skin.

Skin penetration flux and lag-time of steroids across hydrated and dehydrated human skin in vitro

To study the effect of hydration on skin absorption, we investigated penetration across human skin of twelve model chemicals having steroidal structure but different molecular weight and compared the steady-state penetration rate (J) and lag-time (t) across hydration intact skin (Jh and th) with that across dehydrated intact skin (Jd and td). Stratum corneum (SC) thickness of hydrated (52 microm) is 3.3 times that of dehydrated skin (16 microm). Transepidermal water loss (TEWL) of hydrated (7.6+/-2.1 g/m2/h) is twice that of dehydrated skin (3.4+/-1.6 g/m2/h, p<0.05) which are similar to in vivo values, suggesting the SC barrier function was recovered. The ratio of Jh/Jd ranged between 0.7 and 3.6 (average of 1.9). On the other hand, the ratio of th/td was almost constant (average of 0.8). Ratios of Jh/Jd and th/td were independent of MW and Ko/w. In percutaneous absorption experiments in vitro, skin was preserved in culture medium until use and SC might swell during that time. Therefore, we consider the possibility that J and t varied between hydrated and dehydrated skin. We confirmed the difference of J and t between hydrated and dehydrated skin in vitro and now need to define these results under in vivo condition.

Analysis of in Vitro Skin Permeation of 22-Oxacalcitriol Having a Complicated Metabolic Pathway

PURPOSE

The purpose of this study is to analyze simultaneous skin permeation and metabolism of 22-oxacalcitriol (OCT) having several metabolites in skin by observing skin permeation of only unchanged OCT through excised rat skin.

METHODS

A diffusion model including metabolic processes was employed to express simultaneous skin permeation and metabolism of OCT. In vitro permeation experiments of OCT from Oxarol ointment through full-thickness and stripped rat skin were carried out using Franz-type diffusion cells. Time courses of unchanged OCT amounts in ointment, skin, and receptor fluid were determined and fitted to diffusion equations to obtain permeation parameters and a metabolic rate.

RESULTS

Fitting curves of the skin permeation profile obtained by the model were sufficiently close to observed data of unchanged OCT amounts in ointment, skin, and receptor fluid. The following parameters were obtained: metabolic rate of 1.37 x 10(-1) h(-1), and diffusion constants of OCT in stratum corneum (SC) (D(SC)) and viable epidermis and dermis (VED) (D(VED)) of 1.50 x 10(-7) and 2.96 x 10(-4) cm2/h, respectively. The partition coefficient of OCT for SC/ointment (K(SC/D)) was 7 times greater than that of VED/ointment (K(VED/D)).

CONCLUSIONS

The present analysis made it possible to calculate skin permeation parameters (partitioning, diffusivity, and metabolic rate) of OCT without requiring metabolic information, e.g., quantification of metabolites or identification of metabolic pathways. This would be widely applicable for drugs that are not suitable for conventional methods due to complicated metabolic pathways.

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.

Comparison of skin transport and metabolism of ethyl nicotinate in various species

The skin transport and metabolism characteristics of ethyl nicotinate (EN) in rabbit, rat, guinea-pig, pig, shed snake skin and human were compared. In vitro skin transport using excised skin and hydrolysis experiments using skin homogenate were carried out. Flux of EN, a metabolite, nicotinic acid (NA), and the total (EN + NA), as well as kinetic parameters (V(max) and K(m)) for hydrolysis of EN were determined and compared among various species. The enzymatic conversion of EN to NA was observed for all skin permeation experiments. Total flux from EN-saturated solution between rabbit, rat, guinea-pig and human was significantly different (P < 0.05). A great difference between species was observed in skin esterase activity. The NA/total flux ratio of human was significantly lower than that of rabbit, rat or guinea-pig but lower than that of shed snake skin (P < 0.05). There is no significant difference in skin permeation and metabolism between human and pig (P > 0.05). Total flux increased linearly with an increase in EN donor concentration for all species. For pig, shed snake skin and human, NA flux increased with an increase in EN donor concentration and reached a plateau, suggesting the metabolic saturation was taking place in the skin. NA flux at plateau and EN donor concentration in which the NA flux reached a plateau were also affected by species difference. These findings indicated that the discrepancy in transdermal profiles of EN among species tested was predominantly due to the difference in the esterase activity in the skin.

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.

Comparison of skin distribution of hydrolytic activity for bioconversion of beta-estradiol 17-acetate between man and several animals in vitro

We have investigated the distribution of hydrolytic enzymes which metabolize beta-estradiol 17-acetate (EA) to beta-estradiol (E) in man and animal skins in vitro. The distribution of hydrolytic enzymes in human cadaver, hairless dog, rat and hairless mouse skin, was investigated by a skin-slicing technique. We performed histological studies with hematoxylin and eosin stain. The highest amount of metabolite (E) appeared in the layers of 80-120 microm from the skin surface, the basement layer in human skin, while the amount of metabolite was distributed evenly in the hairless dog skin from 0 to 180 microm. In the rat and hairless mouse skin, on the other hand, peak levels of metabolite were observed in the basement layer of dermis, the surrounding area of the cutaneous plexus. The total metabolic activities in the area of epidermis in human, hairless dog and hairless mouse skin were 2.59, 8.03 and 0.33 x 10(-4) microg/ml/microm/h, respectively. The values in whole skin layers in the hairless dog and hairless mouse skin were 3.35 and 1.85 x 10(-4)microg/ml/microm/h, respectively. EA transported across the human and hairless dog skin can be effectively metabolized before entering the capillary. Among animal models investigated, hairless dog skin might be the most facile model in simulating drug metabolism for human skin under the clinical (in vivo) conditions. Hairless mouse skin, on the other hand, was also an excellent model in excised human skin under in vitro conditions.

Simultaneous transport and metabolism of ethyl nicotinate in hairless rat skin after its topical application: the effect of enzyme distribution in skin

An in vitro permeation study of ethyl nicotinate (EN) was carried out using excised hairless rat skin, and simultaneous skin transport and metabolism of the drug were kinetically followed. Fairly good steady-state fluxes of EN and its metabolite nicotinic acid (NA) through the skin were obtained after a short lag time for all the concentrations of EN applied. These steady-state fluxes were not proportional to the initial donor concentration of EN: EN and NA curves were concave and convex, respectively, which suggests that metabolic saturation from EN to NA takes place in the viable skin at higher EN application. Further permeation studies of EN or NA were then carried out on full-thickness skin or stripped skin with an esterase inhibitor to measure their permeation parameters, such as partition coefficient of EN from the donor solution to the stratum corneum and diffusion coefficients of EN and NA in the stratum corneum and the viable epidermis and dermis. Separately, enzymatic parameters (Michaelis constant K(m) and maximum metabolism rate V(max)) were obtained from the production rate of NA from different concentrations of EN in the skin homogenate. The obtained permeation and enzymatic parameters were then introduced to differential equations showing Fick's second law of diffusion in the stratum corneum and the law with Michaelis-Menten metabolism in the viable epidermis and dermis. The calculated steady-state fluxes of EN and NA by the equations were very close to the obtained data. We then measured the esterase distribution in skin microphotographically using fluorescein-5-isothiocyanate diacetate. A higher enzyme concentration was observed in the epidermal cells and near hair follicles than in the dermis. Simulation studies using the even and the partial enzyme distribution models suggested that no significant difference between the models was observed in the skin permeations of EN and NA, whereas concentration-distance profiles of EN and NA were very different. This finding suggests that the total amount of enzyme in skin which converts EN to NA is a determinant of the metabolic rate of EN in skin. The present approach is a useful tool for analyzing simultaneous transport and metabolism of many drugs, especially those showing Michaelis-Menten type-metabolic saturation in skin.

Enhancing effect of hydroxypropyl-β-cyclodextrin on cutaneous penetration and activation of ethyl 4-biphenylyl acetate in hairless mouse skin

The effect of hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) on the cutaneous penetration and activation of ethyl 4-biphenylyl acetate (EBA), a prodrug of non-steroidal anti-inflammatory drug 4-biphenylylacetic acid (BPAA), from hydrophilic ointment was investigated, using hairless mouse skin in vitro. When the hydrophilic ointment containing a complex of EBA with HP-beta-CyD was applied to the full-thickness skin, HP-beta-CyD facilitated the penetration of EBA into the skin, the conversion of EBA to BPAA in the epidermis and the transfer of BPAA to the receptor phase. Under the present condition, pre- and post-application of the ointment containing HP-beta-CyD onto the skin did not affect the cutaneous penetration of EBA and its activation. When the ointment containing the EBA:HP-beta-CyD complex was applied to the skin, the flux of BPAA through the tape-stripped skin was greater than that through the full-thickness skin, while the activation of the prodrug in the skin was slowed down by the tape-stripping. When propylene glycol was used as a vehicle, HP-beta-CyD no longer enhanced the cutaneous permeation of BPAA through the full-thickness skin. These results suggest that the enhancing effect of HP-beta-CyD on the cutaneous penetration of EBA would be ascribable largely to an increase in effective concentration of EBA in the ointment. Furthermore, the slow diffusion of EBA solubilized in HP-beta-CyD through the stratum corneum, together with the vehicle effect, could make the prodrug more susceptible to the metabolic process that is active in the epidermis, eventually leading to the facilitated activation of the prodrug.

Topical administration of racemic ibuprofen

In vitro experiments to investigate possible stereoselective aspects of the topical administration of ibuprofen have been conducted. Incubation of ibuprofen with rat skin homogenates in the presence of coenzyme A, ATP, and magnesium provided no evidence for the formation of ibuprofenyl coenzyme A (the initial intermediate in the metabolic inversion of [R]- to [S]-ibuprofen). Similar incubation studies gave no indication of a change in the enantiomeric ratios of ibuprofen over the time course of the experiments. Percutaneous penetration studies of ibuprofen gel through porcine skin indicated that the ibuprofen enantiomer levels in the reservoir solutions were consistent with racemic ibuprofen having traversed the skin with no metabolic inversion. These results suggest that, in the models studied, skin metabolism does not result in the chiral inversion of (R)- to (S)-ibuprofen and that the topical administration of ibuprofen will result in the delivery of 50% "isomeric ballast."

Analysis of simultaneous transport and metabolism of ethyl nicotinate in hairless rat skin

PURPOSE

Simultaneous skin transport and metabolism of ethyl nicotinate (EN), a model drug, were measured and theoretically analyzed.

METHODS

Several permeation studies of EN or its metabolite nicotinic acid (NA) were done on full-thickness skin or stripped skin with and without an esterase inhibitor. Permeation parameters such as partition coefficient of EN from the donor solution to the stratum corneum and diffusion coefficients of EN and NA in the stratum corneum and the viable epidermis and dermis were determined by these studies. Enzymatic parameters (Michaelis constant Km and maximum metabolism rate Vmax) were obtained from the production rate of NA from different concentrations of EN in the skin homogenate. Obtained permeation data were then analyzed by numerical method based on differential equations showing Fick's second law of diffusion in the stratum corneum and the law with Michaelis-Menten metabolism in the viable epidermis and dermis.

RESULTS

Fairly good steady-state fluxes of EN and NA through the skin were obtained after a short lag time for all the concentrations of EN applied. These steady-state fluxes were not proportional to the initial donor concentration of EN: EN and NA curves were concave and convex, respectively, which suggests that metabolic saturation from EN to NA takes place in the viable skin at higher EN application. The steady-state fluxes of EN and NA calculated by the differential equations with resulting permeation and enzymatic parameters were very close to the obtained data.

CONCLUSIONS

The present method is a useful tool to analyze simultaneous transport and metabolism of many drugs and prodrugs, especially those showing Michaelis-Menten type-metabolic saturation in skin.