Aromatase Inhibition by an Enzyme-activated Irreversible Inhibitor in Human Carcinoma Cell Cultures1

7or-(4'-Amino)phenylthio-l,4-androstadiene-3,17-dione(7a-APTADD), a potent enzyme-activated irreversible inhibitor, was examined in two different human cell culture lines, MCF-7 human mammary carcinoma cells and JAr choriocarcinoma cells. Both the MCF-7 and JAr cell culture systems exhibit aromatase activity, and 7a-APTADD was evaluated for its aromatase-inhibitory activity, for its ability to inactivate the enzyme com plex, and for the time course of recovery of enzymatic activity. This inhibitor produced a dose-dependent inhibition of aromatase activity in MCF-7 cells and in JAr cells, with EC»values of 91 and 7.3 mi. respectively. Two other steroidal inhibitors, 7a-(4'-amino)phenylthio-4-androstene-3,17dione and 4-hydroxyandrostenedione, produced similar dose-response curves and EC«values, while the nonsteroidal aminoglutethimide was less effective. Both cell culture systems exhibited prolonged inhibition of aro matase activity following exposure to 7a-APTADD, suggesting enzyme inactivation by this inhibitor. Thus, 7a-APTADD is an effective inhibitor of aromatase in MCF-7 mammary carcinoma cells and in JAr choriocar cinoma cells. These studies encourage further development of this group of medicinal agents for the treatment of estrogen-dependent mammary carcinoma. inhibitor produced to date (13). In microsomal enzyme prepa rations from human term placenta, this compound exhibited an apparent K¡ of 10 HM. 7a-APTADD produced inactivation of microsomal aromatase only in the presence of NADPH, with an apparent rate of inactivation of 8.4 x 10~3 s~'. Evaluation of aromatase inhibition and inactivation by 7<*-APTADD in cell culture systems will provide further information critical for de velopment of these compounds as potential therapeutic agents. The MCF-7 human mammary cancer cell line has been utilized extensively as a model system for studying the regulation of breast cancer cell growth by steroids (17-21). Aromatase activity has been demonstrated in these cell cultures (15, 22), and studies on several aromatase inhibitors have been performed (15, 23). The JAr choriocarcinoma cell line has high levels of aromatase and also has been utilized to evaluate aromatase inhibition (2427). This report describes the evaluation of 7a-APTADD in the MCF-7 and JAr cell culture systems for its aromatase-inhibitory activity, for its ability to inactivate the enzyme complex, and for the time course of recovery of enzymatic activity.