PT - JOURNAL ARTICLE AU - Jun Lu AU - Nuala Helsby AU - Brian D. Palmer AU - Malcolm Tingle AU - Bruce C. Baguley AU - Philip Kestell AU - Lai-Ming Ching TI - Metabolism of Thalidomide in Liver Microsomes of Mice, Rabbits, and Humans AID - 10.1124/jpet.104.067793 DP - 2004 Aug 01 TA - Journal of Pharmacology and Experimental Therapeutics PG - 571--577 VI - 310 IP - 2 4099 - http://jpet.aspetjournals.org/content/310/2/571.short 4100 - http://jpet.aspetjournals.org/content/310/2/571.full SO - J Pharmacol Exp Ther2004 Aug 01; 310 AB - Thalidomide is increasingly important in clinical treatment, not only of various inflammatory conditions but also in multiple myeloma and other malignancies. Moreover, the metabolism of thalidomide varies considerably among different species, indicating a need to understand its mechanistic basis. Our previous in vivo studies showed the plasma half-life of thalidomide to be much shorter in mice than in humans, with rabbits showing intermediate values. We were unable to detect hydroxylated thalidomide metabolites in humans and suggested that interspecies differences in thalidomide hydroxylation might account for the differences in plasma half-life. We sought here to establish whether these species differences in the formation of hydroxylated thalidomide metabolites could be discerned from in vitro studies. Liver microsomes of mice, rabbit, and human donors were incubated with thalidomide and analyzed using liquid chromatography-mass spectrometry. Hydrolysis products were detected for all three species, and the rates of formation were similar to those for spontaneous hydrolysis, except in rabbits where phthaloylisoglutamine formation increased linearly with microsomal enzyme concentration. Multiple hydroxylation products were detected, including three dihydroxylated metabolites not observed in vivo. Thalidomide-5-O-glucuronide, detected in vivo, was absent in vitro. The amount of 5-hydroxythalidomide formed was high in mice, lower in rabbits, and barely detectable in humans. We conclude that major interspecies differences in hepatic metabolism of thalidomide relate closely to the rate of in vivo metabolite formation. The very low rate of in vitro and in vivo hydroxylation in humans strongly suggests that thalidomide hydroxylation is not a requirement for clinical anticancer activity. The American Society for Pharmacology and Experimental Therapeutics