Method for determination of first-pass metabolism in human skin

Pharmacotherapy of ectoparasitic infections

Epizoonoses such as scabies, lice and cimicosis are common, vexing disorders that occur worldwide. Historically, many treatment modalities have been employed in the management of these disorders, and most of the drugs described in this review are of historical interest and no longer recommended or in widespread use because of their wide spectrum of adverse effects. More recently, reports documenting resistance against various antiectoparasite drugs, complicated and severe courses of the diseases, and adverse effects of drug therapy have prompted the development of new treatment strategies and drugs for optimal disease management. Because the strategies currently recommended for the treatment of ectoparasites differ worldwide, this review proposes a rational approach to selecting the best therapeutic agent by comparing the pharmacokinetics, pharmacodynamics, drug efficacy and adverse effects. A literature search of the currently Internet accessible libraries PubMed, Medline and Ideal library, of citations of articles found there, and from communications with the Federal Institute for Drugs and Medical Devices, Germany, was conducted based on this approach. One major observation of this literature search is that permethrin is the treatment of choice for lice and scabies in the US and in Great Britain, whereas lindane is still recommended for scabies in most other European countries because of its longer-standing record of effectiveness. Although permethrin has not yet been proven to be more effective than lindane in treating infections with these ectoparasites, it currently appears to have the best efficacy versus safety profile of topical treatments for scabies and lice. Ivermectin is a newer oral drug for the treatment of ectoparasites, which has been used with great success in the treatment of onchocercosis and other endoparasites. Although ivermectin appears to be a promising drug, its role in the treatment of ectoparasitic infections will be clarified as more study data become available. Finally, it is important to emphasise the clinical aspects of ectoparasite therapy and that providing the patient with optimal instructions on the use of topical therapeutics is of great importance in avoiding adverse effects and assuring complete removal of the ectoparasite, thereby avoiding the development of drug-resistance.

Pre-systemic metabolism of viprostol in the monkey following oral and topical administration

1. 14C-Viprostol, (I), a synthetic PGE2 analogue, was administered to 6 monkeys, orally, topically and intravenously in a three way crossover study. Total radioactivity and the pharmacologically active acid formed by rapid hydrolysis of viprostol in vivo, (II), were measured in plasma to determine absorption and absolute bioavailability. 2. After oral dosing approx. 31% of drug-related radioactivity was absorbed. Systemic bioavailability of unchanged active acid was only 7.3%. This indicates significant first-pass metabolism after oral administration, since only 23% of the absorbed radioactivity was available as 'unchanged' active drug (II). 3. After topical dosing, transdermal absorption of total radioactivity by 48 h averaged only 5% of dose. Absolute bioavailability of II averaged 3.8% of dose. This indicates that after transdermal absorption 74% of the absorbed radioactivity was available systemically as the active acid II, with the remainder being subject to pre-systemic metabolism.

Deposition of viprostol (a synthetic PGE2 vasodilator) in the skin following topical administration to laboratory animals

1. Topical application of 14C-viprostol, a synthetic prostaglandin E2 analogue, to laboratory animals resulted in a significant depot of radioactivity in the skin at the application site in all species studied: mouse, rat, guinea pig, rabbit and monkey, with longer residence times in the larger species. 2. The location of the 14C-label in the skin in mice and monkeys was determined by microscopic autoradiography. Evaluation of the autoradiograms show rapid penetration of the drug into the skin via the hair follicles. 3. In mouse distribution of radioactivity was evident in the stratum corneum and down the hair shafts by 30 min. after dosing. By 2 h radioactivity was also observed throughout the viable epidermis; in the dermis only the hair shafts contained significant radioactivity. At 72 h after dose removal, radioactivity was evident only in the hair follicles and hair shaft. 4. In monkey the residence time of radioactivity in the skin was significantly longer than in mouse, but the general distribution pattern was similar in both species. 5. The presence of viprostol in the hair follicles and epidermal layer after topical administration is consistent with its extensive skin metabolism previously reported.

Metabolism of viprostol--a synthetic vasodilator PGE2 analog

Metabolic studies were done with 14C-Viprostol (I) administered by various routes (I.V., oral and topical) to six animal species and to man. Total radioactivity and metabolic profiles were analyzed in plasma, tissues and excreta. The main metabolites were isolated and identified by capillary GC/MS. Plasma and urinary metabolic profiles were qualitatively similar across species, with two major metabolic reactions being predominant: rapid hydrolysis to the pharmacologically active free acid (II) and oxidation of the alpha-chain to dinor and tetranor acids (III, IV). In the monkey and man, reduction of the 9-keto group lead to PGF2 type metabolites (VI-VIII). In the rat, omega oxidation of the beta-chain occurred as well, resulting in the formation of dicarboxylic acids (V).