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Received November 3, 1998; accepted February 5, 1999
KEY WORDS: cell culture; keratinocytes; androgen metabolism; testosterone; transdermal delivery; 5[alpha]-reductase.
INTRODUCTION
In recent years, the skin has been shown to contain a broad spectrum of enzymes capable of metabolizing a wide range of topically applied drugs and endogenous substrates. A major part of this metabolic activity is located within the epidermis (1).
The metabolic capacity of the skin may have consequences for the topical as well as the transdermal delivery of drugs, resulting in a probably reduced bioavailability for a transdermally delivered substance. For example, 16-21% of transdermally applied glyceryl trinitrate was reported to be metabolized in the skin (2). Cutaneous metabolism may also lead to reactive metabolites, having the potential for contact sensitization, an important problem in transdermal delivery.
For these reasons, there is a growing interest in methods for studying human skin metabolism. Since it is difficult to differentiate skin from systemic metabolism under in vivo conditions, there is need for suitable in vitro models. The spontaneously immortalized human keratinocyte cell line HaCaT represents a readily available in vitro model and has already been used as a model for skin toxicity studies (3). The full epidermal differentiation capacity of HaCaT cells was demonstrated by transplantation onto nude mouse skin (4).
The aim of our study was to investigate the metabolism of the androgen testosterone (T) in HaCaT cells. T is extensively used in TDS for the treatment of male hypogonadism. Furthermore, cutaneous metabolism of T is of general interest, because the skin has been recognized as a major site for endogenous androgen metabolism as well as a target organ for these steroids. T is reduced to 5ot-dihydrotestosterone (DHT) in the extranuclear compartment by two distinct isoforms of the membrane bound steroid 5[alpha]-reductase (5[alpha]-R) (EC 1.3.1.22) at the target cell site (5). DHT represents the most potent androgen and is thought to be involved in acne and other androgen-related disorders, such as male pattern baldness and hirsutism (6). We investigated the metabolism of T in HaCaT cells to determine the suitability of this in vitro model for steroid metabolism in human skin.
MATERIALS AND METHODS
Materials
Dulbecco's modified Eagle's medium (DMEM) with and without HEPES and fetal calf serum were obtained from Gibco BRL-Life...