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.
Department of Pharmacology and Therapeutics, New Medical Building, Liverpool, L69 3GE, United Kingdom., ,
MeSHAryl Hydrocarbon Hydroxylases
Contraceptives, Oral, Synthetic
Cytochrome P-450 CYP2C19
Cytochrome P-450 CYP2C9
Cytochrome P-450 CYP3A
Cytochrome P-450 Enzyme Inhibitors
Cytochrome P-450 Enzyme System
Mixed Function Oxygenases
Pub Type(s)Journal Article