Methyltrienolone (R1881) is not aromatized by placental microsomes or rat hypothalamic homogenates.

The in vitro conversion of the synthetic steroidal androgen methyltrienolone (17 beta-hydroxy-17 alpha- methylestra -4,9,11-trien-3-one) to an estrogen was investigated. First, with a placental microsome preparation a 10-fold molar excess of methyltrienolone did not affect the aromatization of testosterone. Therefore, methyltrienolone did not compete with testosterone in this very active aromatase system, nor did it inhibit the aromatization. Second, while the placental aromatase very efficiently converted testosterone to estrogens, it did not convert methyltrienolone [3H] to estrogenic (phenolic) products. Third, homogenates of rat hypothalamic tissue were also unable to aromatize methyltrienolone, although they converted small quantities of testosterone to estradiol and 19-hydroxyandrostenedione to estrone. This lack of aromatization of methyltrienolone is consistent with observations of additional behavioral and in vivo biochemical experiments.