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Basic Studies on Aminoglutethimide¹

Department of Population Sciences, Harvard School of Public Health, and Department of Obstetrics and Gynecology, Harvard Medical School, Boston, Massachusetts 02115

Aminoglutethimide (AG), a bicyclic substance with two optically active isomers, inhibits the cholesterol side-chain cleavage (SCC) and aromatase systems by blocking the terminal cytochrome P-450s. AG interacts with these cytochromes to induce typical type II, low-spin spectra; the amount of spectral change correlates directly with the amount of enzymatic inhibition. AG acts by preventing reduction of the cytochrome, an obligate step preceding oxygenation of substrate. The apparent K_i of D(+)-AG is about 2.5 times less than the K_i of L(–)-AG in inhibiting SCC and about 40 times less in inhibiting aromatase. The spectral K_s ratios of the D- and L-enantiomers for P-450_SCC and P-450_aromatase are 2.5 and 5, respectively, in the presence of substrate. The slope of the substrate concentration versus inhibition curves is less than 0.1 and extends over 2 logarithmic units of substrate concentration.

By in vivo rabbit and rat assays for SCC, D-AG is 5 and 25 times more potent than L-AG. In women, AG rapidly lowers estrogen and progesterone levels in the luteal phase of the menstrual cycle and in early pregnancy, eliciting a reflex release of luteinizing hormone in the luteal phase. In postmenopausal, adrenalectomized, ovariectomized women, AG prevents the aromatization of administered androstenedione.

In inhibition studies in women, plasma steroid concentrations were inversely proportional to AG levels, which were about 20 to 30 µM. This level of AG, which approximates the apparent K_is of the SCC and aromatase systems, caues a decrease in hormone levels of about 50%.