Abstract
The marmoset monkey (Callithrix jacchus) has been used as a non-rodent species in non-clinical development studies for a number of years. However, there is only limited information available on the drug-metabolising enzymes in this species. This study determined the marmoset liver microsomal kinetic parameters (Km and Vmax) for the following activities: 7-ethoxyresorufin O-deethylase, S-mephenytoin N- demethylase, tolbutamide methylhydroxylase, debrisoquine hydroxylase, lauric acid 11-hydroxylase, testosterone 6B-hydroxylase and lauric acid 12-hydroxylase. These activities, which are indicative of the presence of specific cytochrome P450s expressed in the human liver, were evaluated in marmoset liver microsomes to investigate possible similarities between the two species. The metabolism of the human CYP2D6 probe substrate debrisoquine was investigated further, using marmoset liver microsomes and a combination of techniques including HPLC-RAM, LC-MS/MS and NMR. Debrisoquine was metabolised by marmoset liver microsomes to a number of hydroxylated products, including a previously unidentified 6,7-dihydroxydebrisoquine. Interestingly, the major marmoset hepatic metabolite appeared to be 7-hydroxydebrisoquine rather than 4-hydroxydebrisoquine, as is the case in human. The expression of CYP enzymes in marmoset liver was investigated by Western blot analysis of microsomal protein and compared by parallel analysis to liver microsomal protein of male human donors and male Sprague-Dawley rats, using anti-CYP antibodies primarily raised against the human enzyme. The analyses revealed extensive expression of: CYP2C19, CYP2E1, CYP3A4 and CYP3A5 in marmoset, CYP2C9 and CYP2E1 in human, and CYP2A6 and CYP2E1 in rat. There was little or no expression of proteins detected by antibodies raised against CYP2A6 and CYP2D6 in marmoset, CYP2B6 in human and CYP2B6, CYP2D6 and CYP3A5 in rat. Finally, Inter-species comparison of CYP expression between human and marmoset was examined at the genetic level. Primers and probes were selected to specifically match human CYP mRNA, and then used to determine sequence identity between the human hepatic CYPs and their marmoset counterparts. These analyses established a species comparison for CYP2B6, with the primer and probe targeted binding region within the marmoset enzyme being identical (or very similar) to that of the human protein.