Concentration profile in plasma after transdermal drug delivery

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 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.

Prediction of plasma concentration of GTS-21 in hairless rats following monolithic transdermal delivery

The transdermal therapeutic systems (TTS) usually achieve constant plasma concentration for an extended period of time. This is because a sufficient drug stored in the device can keep the constant concentration on the surface of the stratum corneum during the system application. When the drug molecules are not enough to provide the constant surface concentration, the rate of drug penetration decreases with time because of decreased supply of the drug molecules from the delivery device. This paper has proposed an empirical simple approach to predict the plasma concentration for such a TTS. A novel compound, GTS-21, for Alzheimers' disease currently under development was used as a model drug. In vivo and in vitro experiments were carried out in hairless rats. The in vivo plasma concentration-time profile in hairless rats following the application of TTS well agreed with the predicted profile based on the skin pharmacokinetic model together with the model parameters determined from the in vitro experiment.

Transdermal delivery of the potent analgesic dihydroetorphine: kinetic analysis of skin permeation and analgesic effect in the hairless rat

Dihydroetorphine is an extraordinarily strong opioid analgesic. To assess its effectiveness after topical application in hairless rats we have examined the kinetic analysis of skin permeation through excised skin and the in-vitro reservoir effect of skin, and have investigated the predictability of plasma concentration and analgesic effect following in-vivo transdermal application. Dihydroetorphine was moderately permeable from an aqueous suspension through excised hairless rat skin. Dihydroetorphine flux from drug-dispersed pressure-sensitive adhesive tape was threefold that from the applied aqueous suspension. The fluxes through the abdominal and the dorsal skin during tape application fitted the Fickian diffusion equation well after the tape was removed peeling off the outer layer of the stratum corneum. The relationship between the plasma concentration and the analgesic effect was examined for four different rates of infusion of dihydroetorphine. A non-linear pharmacokinetic disposition was observed. Following abdominal (0.28 cm2, 20 microg) and dorsal (0.50 cm2, 35 microg) applications of the dihydroetorphine tape, plasma concentration (0.2-0.8 ng mL(-1)) and analgesic effect were maintained at a suitable level, for more than 8 h, until removal of the tape. These profiles were predictable using the combined equation for percutaneous absorption, disposition and the analgesic effect, but the analgesic effect was slightly lower than the predicted value. The results show that it was possible to control the plasma concentration and the analgesic effect of dihydroetorphine by topical application of the analgesic using pressure-sensitive adhesive tape in the hairless rat. It was possible to predict the result using mathematical modelling.

Topical oleo-hydrogel preparation of ketoprofen with enhanced skin permeability

In an attempt to improve the skin penetration of ketoprofen, various transdermal formulations were prepared, and their in vitro skin permeability and in vivo percutaneous absorption were evaluated. In vitro permeation studies were performed using a modified Franz cell diffusion system in which permeation parameters such as cumulative amount at 8 hr Q8hr, steady-state flux Jss, or lag time tL were determined. In the in vivo percutaneous absorption study using the hairless mouse, maximum concentration Cmax and area under the curve at 24 hr AUC24h were measured. The optimal transdermal formulation (oleo-hydrogel formulation) of ketoprofen showed a Q8hr value of 227.20 micrograms/cm2, a Jss value of 29.61 micrograms/cm2/hr, and a tL value of 0.46 hr. The Q8hr and Jss values were about 10-fold (p < .01) higher than those (Q8hr = 19.61 micrograms/cm2; Jss = 2.66 micrograms/cm2/hr) from the K-gel and about 3.5-fold (p < .01) than those (Q8hr = 60.00 micrograms/cm2; Jss = 7.99 micrograms/cm2/hr) of the K-plaster. In the in vivo percutaneous absorption, the Cmax (6.82 micrograms/ml) and AUC24h (55.74 micrograms.hr/ml) values of the optimal formulation were significantly (p < .01) higher than those of K-gel and K-plaster. The relative bioavailability of the oleo-hydrogel following transdermal administration in reference to oral administration was about 37%, and the Cmax value (4.73 micrograms/cm2) in the hypodermis following topical administration was much higher than those from the conventional products (Cmax of K-gel and K-plaster were 0.92 +/- 0.19 microgram/cm2 and 1.27 +/- 0.37 microgram/cm2, respectively). These data demonstrate that the oleo-hydrogel formulation of ketoprofen was more beneficial than conventional products (K-gel and K-plaster) in enhancing transdermal permeation and skin absorption of ketoprofen. Furthermore, there was a good correlation between in vitro permeation parameters and in vivo percutaneous absorption parameters.