In Vivo Formation of Ethinylestradiol After Intramuscular Administration of Norethisterone Enantate

It is known that a small fraction of orally administered norethisterone is metabolically converted to ethinylestradiol. This exploratory, open‐label, nonrandomized study was conducted to investigate the systemic exposure to ethinylestradiol after intramuscular administration of norethisterone enantate in comparison with the exposure to ethinylestradiol after administration of a standard combined oral contraceptive. Sixteen healthy premenopausal women received an oral contraceptive (ethinylestradiol 30 μg/levonorgestrel 150 μg) once daily for 21 days and—after a 1‐week washout period—a single intramuscular dose of 200 mg norethisterone enantate. Blood samples to determine ethinylestradiol in serum were taken over 24 hours after the last dose of ethinylestradiol/levonorgestrel and over 8 weeks after administration of norethisterone enantate. Oral equivalent doses of ethinylestradiol were estimated based on area under the concentration–time curves. The ethinylestradiol serum concentrations observed after administration of norethisterone enantate were relatively low: The mean maximum concentration was only 32% of the maximum observed after ethinylestradiol/levonorgestrel (90% confidence interval, 22.5%–44.7%). The maximum oral equivalent dose of ethinylestradiol was markedly lower than 30 μg ethinylestradiol per day (20.3 μg/day; 90% confidence interval, 14.8–28.0 μg/day). The same applied to the average oral equivalent dose of ethinylestradiol for the 8‐week postdose interval (4.41 μg/day; 90% confidence interval, 3.57–5.46 μg/day). To conclude, the study results indicate that metabolic conversion of norethisterone to ethinylestradiol also occurs after intramuscular administration of 200 mg norethisterone enantate, but is associated with a lower exposure to ethinylestradiol than the use of a combined oral contraceptive containing 30 μg ethinylestradiol (plus 150 μg levonorgestrel).