Abstract
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 3α-hydroxy and 3β-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 3α-hydroxydesogestrel with small amounts of 3β-hydroxydesogestrel also being observed. TheKm value for 3α-hydroxylation by CYP2C9 cell line microsomes was 6.5 μM and the corresponding Vmaxvalue was 1269 pmole · mg−1 · min−1. Sulfaphenazole potently inhibited 3α-hydroxydesogestrel formation by CYP2C9 microsomes with aKi 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 3α-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 3α-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.
Footnotes
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Send reprint requests to: Dr. David J. Back, Department of Pharmacology and Therapeutics, New Medical Building, Ashton Street, Liverpool, L69 3GE, United Kingdom.
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↵1 Current address: Department of Toxicology and Drug Disposition, N.V. Organon, PO Box 20, 5340 BH OSS, The Netherlands.
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↵2 Current address: Osaka Prefectural Institute of Public Health, Nakamichi, Higashinari-Ku, Osaka 537, Japan.
- Abbreviations:
- DSG
- desogestrel
- 3-KDSG
- 3-ketodesogestrel
- 3α-OHDSG
- 3α-hydroxydesogestrel
- 3β-OHDSG
- 3β-hydroxydesogestrel
- CYP
- cytochrome P450
- EE2
- ethinylestradial
- HPLC
- high-performance liquid chromatography
- G6P
- glucose 6-phosphate
- NADP
- nicotinamide adenine dinucleotide phosphate
- NADPH
- β-reduced form
- TOL
- tolbutamide
- SULF
- sulfaphenazole
- S-MEPH
- S-mephenytoin
- DEX
- dexamethasone
- CHLOR
- chlorzoxazone
- HLM
- human liver microsomes
- Received March 26, 1998.
- Accepted June 23, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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