Lack of aromatisation of the 3-keto-4-ene metabolite of tibolone to an estrogenic derivative

Tibolone is used for the treatment of climacteric symptoms in postmenopausal women. It is metabolised in a tissue-specific manner so that while some metabolites exert estrogenic effects on bone and the CNS, others are thought to protect the breast and endometrium from estrogenic stimulation. Tibolone is a 7alpha-methyl derivative of 19-norethynodrel. Since the introduction of synthetic progestagens for therapeutic use there has been considerable controversy as to whether they can undergo aromatisation to give rise to the potent estrogen, ethinylestradiol. In this study, we examined whether the delta-4-ene (7alpha-methyl norethisterone) metabolite of tibolone, which has a similar delta-4-ene A-ring structure to that of the estrone precursor, androstenedione, could undergo aromatisation to the potent estrogen, 7alpha-methyl ethinylestradiol. For these studies, JEG-3 choriocarcinoma cells were employed as they have a very high level of aromatase activity. TLC and HPLC procedures were developed to separate phenolic from non-phenolic compounds and were initially used to confirm that JEG-3 cells readily aromatised androstenedione to estrogens (up to 74%). The aromatisation of androstenedione to estrogens by these cells could be completely blocked with the potent aromatase inhibitor letrozole. When [(3)H] 7alpha-methyl norethisterone was incubated with JEG-3 cells no evidence for its conversion to [(3)H] 7alpha-ethinylestradiol was obtained. Radioactivity detected on the TLC plate or HPLC fractions where standard 7alpha-methyl ethinylestradiol was located, revealed that similar levels were present when 7alpha-methyl norethisterone was incubated with culture medium alone or with JEG-3 cells in the absence or presence of letrozole. From these investigations, it is concluded that 7alpha-methyl norethisterone does not undergo aromatisation to an estrogenic derivative.