New insights into metabolism of desogestrel in humans using liver microsomes and an HPLC/UV method

Desogestrel metabolites obtained from fungi were used as standards and an HPLC/UV method was newly developed for investigation of DSG metabolism in human liver microsomes. Steady-state kinetic conditions were followed for measurement of the initial velocities of formation of the metabolites 3-keto-DSG (Etonogestrel, M1), 6β-hydroxy-ETN (M5), 11-epoxy-ETN (M8), and an unidentified metabolite (Mx). So far, M5 and M8 are here identified for the first time in humans. The Michaelis-Menten constants of ENT, M5, M8, and Mx varied from 7 to 19 μM, whereas the maximal velocities for ENT, M5, M8 were found to be 123, 198, and 353 nM/min.mg proteins, respectively. The use of specific inhibitors of cytochrome P450 enzymes showed that CYP3A4 is major catalyzer of ETN oxidation (IC 50 of ketoconazole at 0.1–0.4 μM). CYP3A4 also is involved in DSG oxidation, at the same order of magnitude as CYP2C9 and CYP2C19, which in turn are similarly involved in ETN oxidation (IC 50 of inhibitors at 9–30 μM). CYP2D6 and CYP1A2 are involved as well, although at a lesser extent (IC 50 of inhibitors at 26–73 μM), while CYP2E1 is most likely not involved in the metabolism of desogestrel.