The role of CYP2C in the in vitro bioactivation of the contraceptive steroid desogestrel.

Desogestrel is a 3-deoxo progestogenic steroid that requires bioactivation to 3-ketodesogestrel. In these studies we have attempted to define the pathway of 3-ketodesogestrel formation and characterise the enzymes responsible for this biotransformation in vitro. Initial studies using deuterated desogestrel confirmed that desogestrel is metabolised by human liver microsomes via 3alpha-hydroxy and 3beta-hydroxydesogestrel to 3-ketodesogestrel. Metabolites were analysed by radiometric high-performance liquid chromatography and were identified by liquid chromatography-mass spectrometry and by cochromatography with authentic standards. Desogestrel was metabolised by microsomes from lymphoblasts containing cDNA-expressed CYP2C9 and CYP2C19 to 3alpha-hydroxydesogestrel with small amounts of 3beta-hydroxydesogestrel also being observed. The Km value for 3alpha-hydroxylation by CYP2C9 cell line microsomes was 6.5 microM and the corresponding Vmax value was 1269 pmole. mg-1. min-1. Sulfaphenazole potently inhibited 3alpha-hydroxydesogestrel formation by CYP2C9 microsomes with a Ki value of 0.91 microM. There was a significant negative correlation between 3-ketodesogestrel and CYP3A4 content/activity in a panel of human livers suggesting that the further metabolism of 3-ketodesogestrel is mediated by CYP3A4. Sulfaphenazole partially inhibited 3alpha-hydroxydesogestrel and 3-ketodesogestrel formation in human liver microsomes indicating a possible in vivo role for CYP2C9. In addition, when sulfaphenazole was combined with S-mephenytoin, further inhibition of 3alpha-hydroxydesogestrel formation was observed suggesting a possible role for CYP2C19. This was confirmed in incubations with inhibitory antibodies. Whereas an anti-CYP2C9/2C19 antibody completely abolished desogestrel metabolism, anti-CYP3A4 and anti-CYP2E1 were not inhibitory. We conclude that CYP2C9 and possibly CYP2C19 and important isoforms catalysing the initial hydroxylation of desogestrel.