Analysis of Skin Disposition and Metabolism of Ethyl Nicotinate after Topical Application Using Dual Agar Gel Disc-Inserted Rats

Abstract

Following application of a topical formulation to the skin surface, the drug penetrates the stratum corneum, the uppermost layer of skin, and passes through the viable epidermis and dermis to reach the cutaneous vessels. Most drugs are taken up by these vessels to migrate to the systemic circulation, whereas a portion is further diffused into deeper tissues such as the subcutis and muscle. 1,2) If the applied drug is a prodrug, a peptide or an oligonucleotide that can be readily metabolized in skin, its metabolism makes the skin disposition of the drug very difficult.Such dermato-pharmacokinetics have been analyzed by several in vitro and in vivo studies.3,4) Figure 1 schematically illustrates and compares the in vitro and in vivo skin dispositions of a drug and its metabolite after topical application. Overall skin permeations of the drug and its metabolites can be evaluated from the results of in vitro skin permeation experiments (Fig. 1a). Information on enzyme distribution may be obtained by in vitro experiments using split or sliced layers of excised skin.5) It is difficult, however, to determine the effects of the bloodstream on the drug disposition and metabolism in skin from in vitro methods. In contrast, body fluids and tissues are sampled for in vivo percutaneous absorption experiments (Fig. 1b). It is possible to evaluate each fraction of the drug amount from the formulation to the systemic circulation or deeper skin tissues against the initial amount applied to skin. On the other hand, the time course of changes in the drug level in the skin tissues is not easy to follow due to difficulty in consecutive sampling, although some studies had achieved to give in-depth consideration for using multi-compartment model. 6,7) In addition, the metabolic rate of a prodrug in the systemic circulation and the liver is generally so rapid that the metabolic rate in skin cannot be followed from the systemic pharmacokinetics. When such a prodrug is topically applied with the expectation of therapeutic effects after systemic absorption of its parent drug, the total (prodrugϩparent drug) amount permeated through the The skin disposition and metabolism of topically applied ethyl nicotinate (EN) were evaluated in dual agar gel disc-inserted hairless rats, which have two agar gel discs subcutaneously inserted into the abdominal region as drug receptors, and a topical formulation containing EN placed on either side of the gel disc through the skin. Plasma and agar levels of EN and its metabolite, nicotinic acid (NA), were followed every 2 h over 6 h. EN permeated through the skin barrier and partly metabolized to NA with 89.4% of the metabolite ratio [NA/(EN؉NA)] at 6 h. Some EN and NA in the skin moved to the systemic circulation, and the remainder migrated into the agar gel below the formulation. The total amount (EN؉NA) in the skin that distributed from the formulation directly to the systemic circulation and the application side of the gel corresponded to 95.2% and 4.8% of the total skin permeation at 6 h, respectively...