Formation of 7??-methyl-ethinyl estradiol during treatment with tibolone

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Abstract
Objective To investigate whether tibolone, which is orally used for hormone replacement therapy, is transformed to a derivative of ethinyl estradiol (EE). Design In 10 young women who received 2.5 mg orally administered tibolone daily between cycle day 19 and 25, blood was obtained before and 0.5, 1, 1.5, 2, 3, 4, 6, 8, and 24 h after the last tablet intake. The concentration of 7α-methyl-EE was determined by the gas chromatography with mass spectrometry method. Results The results demonstrate that, during daily treatment of women with 2.5 mg tibolone, a small proportion of tibolone is transformed to 7α-methyl-EE. The maximal serum concentrations of 125 ± 40 pg/mL were in the range of the levels of EE observed during treatment with oral contraceptives containing 30 μg EE. Conclusions Caution is advisable when considering treatment with tibolone of postmenopausal women with contraindications for estrogens.