Although the drug-induced side effect of gynecomastia is, in many cases, considered to be a result of drug-estrogen interaction mediated by cytochrome P450 (CYP), the prediction of this side effect for inclusion in package inserts is difficult. This is because in addition to the phase I reaction, the effects of the drug on the entire enzyme system involved in the estrogen pool must be taken into consideration.Thus far, we have attempted to construct a system for predicting drug-induced gynecomastia by not only focusing on the phase I reaction, but also extending the sites of interaction to include estrogen production and regeneration systems. By confirming drug inhibition in those systems, we intend to develop a model unit reaction system for the determination of the percentage inhibition under identical conditions, and to determine its potential for clinical application.2-4) The results obtained are discussed below.The main route of metabolism of estradiol (E2) in humans is the phase I reaction of 2-hydroxylation, and although there are a lot of human hepatic CYP molecules that catalyze this reaction, their individual contribution ratios are unknown. The previous studies have shown that CYP3A4 has the highest contribution ratio at roughly 71%, 2) based on which a model unit reaction system using as reagent the recombinant CYP3A4 of an Escherichia coli expression system has been established. Next, the inhibitory effects of 29 drugs reported to induce gynecomastia on the 2-hydroxylation of E2 were investigated. Thirteen drugs of them showed high inhibition on the 2-hydroxylation, 3) but the rest 16 showed slight or no inhibition. Thus, gynecomastia by these drugs could not be explained in terms of the inhibition of the 2-hydroxylation alone.The possible candidates for the inhibition mechanism include drug inhibition of the estrogen production system involved in the estrogen pool (aromatase pathway), the estrogen regeneration system (sulfatase pathway), and/or 17b-hydroxysteroid dehydrogenase (17b-HSD) and its related systems. As a result of studies on these possibilities, it was found that drugs that induce gynecomastia may also inhibit the 17b-HSD system as well as the phase I reaction of E2. 3,4)The above results indicate that the phase II reaction (conjugation) must also be taken into consideration as one of the reaction systems affecting the estrogen pool (Fig. 1). The phase II reaction of E2 in humans consists primarily of the 3-or 17-glucuronidation by UDP-glucuronosyltransferase (UGT). UGT is divided into two subfamilies, UGT1 and UGT2, and each subfamily has some isoforms.5) UGT1A1 and UGT2B7 catalyze the 3-and 17-glucuronidation of E2, respectively, yielding estradiol 3-glucuronide (E2-3G) and estradiol 17-glucuronide (E2-17G).6) Because the two UGTs have low substrate specificity and are involved in the glucuronidation of drugs and other xenobiotic substances, [7][8][9][10][11] the interactions between E2 and drugs are predicted.This study was conducted for the purpose of investigating the inhibitory ef...