Vol. 287, Issue 3, 975-982, December 1998

The Role of CYP2C in the In Vitro Bioactivation of the Contraceptive Steroid Desogestrel

Daniela M. Gentile, Carole H. J. Verhoeven¹, Tsutomu Shimada² and David J. Back

Department of Pharmacology and Therapeutics, New Medical Building, Liverpool, L69 3GE, United Kingdom

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 3-hydroxy and 3-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 3-hydroxydesogestrel with small amounts of 3-hydroxydesogestrel also being observed. The K_m value for 3-hydroxylation by CYP2C9 cell line microsomes was 6.5 µM and the corresponding V_max value was 1269 pmole · mg¹ · min¹. Sulfaphenazole potently inhibited 3-hydroxydesogestrel formation by CYP2C9 microsomes with a K_i value of 0.91 µM. 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 3-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 3-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.