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Thalidomide Metabolism by the CYP2C Subfamily¹

Molecular Pharmacology Section, Cancer Therapeutic Branch, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland 20892

Purpose: This research investigated the biotransformation of thalidomide by cytochrome P-450 (CYP).

Experimental Design: We used liver microsomes from humans and/or animals and the recombinant specific CYP isozymes to investigate CYP-mediated metabolism of thalidomide.

Results: Thalidomide was biotransformed into 5-hydroxythalidomide (5-OH) and diastereomeric 5'-hydroxythalidomide (5'-OH) by liver microsomes. The human liver microsomes with higher CYP2C19 activity formed more metabolites than those with lower CYP2C19 activity and had less activity in metabolite formations than those from rats. Recombinant human CYP2C19 and rat CYP2C6 isozymes were primarily responsible for forming these metabolites, and the male rat-specific CYP2C11 formed only 5'-OH. 5-OH was subsequently hydroxylated to 5,6-dihydroxythalidomide by CYP2C19, CYP2C9, and CYP1A1 in humans and by CYP2C11, CYP1A1, CYP2C6, and CYP2C12 in rats. Incubations with S-mephenytoin and omeprazole, substrates of CYP2C19, inhibited metabolism by human liver microsomes, supporting the involvement of CYP2C19. -Naphthoflavone, an inhibitor of CYP1A, simultaneously stimulated the 5-OH formation and inhibited cis-5'-OH formation catalyzed by human liver microsomes. The contribution of the CYP2C subfamily was supported by the immunoinhibition study using human liver microsomes. When we used the microsomes from treated rats, the metabolite formations did not increase by inducers for CYP1A, CYP2B, CYP2E, CYP3A, or CYP4A, suggesting that these could not be involved in the main metabolic pathway in rats.

Conclusions: We discovered that the polymorphic enzyme CYP2C19 is responsible for 5- and 5'-hydroxylation of thalidomide in humans. In rats, thalidomide was hydroxylated extensively by CYP2C6 as well as the sex-specific enzyme CYP2C11.