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Conversion of testosterone to dihydrotestosterone (DHT) has been demonstrated to be catalyzed by two isoforms of steroid 5 alpha-reductase, designated types I and II. Although several classes of steroid-based inhibitors of the type II isoform have been identified, these agents have not demonstrated highly selective pharmacological activity against human type I 5 alpha-reductase. LY191704 is representative of a series of nonsteroidal agents that have potent [apparent inhibitory constant (Ki) = 11.3 nM] inhibitory activity in human scalp skin homogenates (pH 7.5), a source of type I 5 alpha-reductase. [3H]-DHT production in the presence and absence of LY191704 is consistent with a noncompetitive mode of inhibition. In human prostatic homogenates (pH 5.5), a source of type II 5 alpha-reductase, LY191704 is virtually inactive as an inhibitor [concentration of inhibitor producing 50% inhibition of enzymatic activity (IC50) > 1,000 nM] of [3H]-DHT formation. LY191704 does not inhibit the type I or type II isoforms of rat 5 alpha-reductase, nor does the compound compete for binding to the murine androgen receptor expressed in SF9 cells using a baculo virus expression system. The benzoquinolinones, as exemplified by LY191704, possess exquisite pharmacological selectivity and provide a tool to understand the role of human type I 5 alpha-reductase in normal and pathophysiological states. These agents may also find clinical utility in treating androgen-dependent dermatological conditions.

The conversion of testosterone (T) to dihydrotestosterone (DHT) has been demonstrated to be catalysed by at least two isoforms of human steroid 5 alpha-reductase, designated types I and II. Type II 5 alpha-reductase expression predominates in human accessory sex tissues, localized to the fibromuscular stromal compartment. The type I isoform predominates in skin, prostatic epithelia and, to a lesser extent, in prostatic fibromuscular stroma. The significance of the type I isoform to prostatic cellular growth and function remains undefined. In cultured DU145 cells, we evaluated the metabolism of [14C]-T and demonstrated the time-dependent formation of [14C]-DHT. Oxidative metabolism (conversion of [14C]-T to [14C]-androstenedione) and the formation of conjugated androgen metabolites occurred at a relatively low rate in the DU145 cells. Using human type I 5 alpha-reductase cDNA, Northern blot analysis of DU145 cell mRNA revealed high levels of type I isoform expression. Analogous probing of the DU145 cells with a human 5 alpha-reductase II cDNA failed to reveal expression of the type II isoform. The expression of functional type I activity has been confirmed pharmacologically using isoform-selective 5 alpha-reductase inhibitors. Reductive metabolism of [3H]-T in the DU145 cells was inhibited in a concentration-dependent manner by LY306089, a potent non-steroidal type I-selective inhibitor (IC50 = 10.0 nM). SKF105657, a steroidal type II-specific inhibitor was distinctly less active at inhibiting [3H]-DHT formation. LY306089 was a non-competitive inhibitor of type I 5 alpha-reductase in DU145 cellular homogenates with an apparent Ki value of 4.0 nM. These studies have identified and pharmacologically defined type I 5 alpha-reductase activity in an androgen-insensitive prostatic cancer cell line and provide the basis for additional investigations into the significance of type I 5 alpha-reductase to human prostatic pathophysiology.