Identification of cytochrome P-450 isoforms responsible for cis-tramadol metabolism in human liver microsomes

The metabolism of cis-tramadol has been studied in human liver microsomes a nd in cDNA-expressed human cytochrome P-450 (CYP) isoforms. Human liver mic rosomes catalyzed the NADPH-dependent metabolism of tramadol to the two pri mary tramadol metabolites, namely, O-desmethyl-tramadol (metabolite M1) and N-desmethyl-tramadol (metabolite M2). In addition, tramadol was also metab olized to two minor secondary metabolites (each comprising less than or equ al to3.0% of total tramadol metabolism), namely, N,N-didesmethyl-tramadol ( metabolite M3) and N,O-didesmethyl-tramadol (metabolite M5). Kinetic analys is revealed that multiple CYP enzymes were involved in the metabolism of tr amadol to both M1 and M2. For the high-affinity enzymes involved in M1 and M2 formation, K-m values were 116 and 1021 muM, respectively. Subsequent re action phenotyping studies were performed with a tramadol substrate concent ration of 250 muM. In studies with characterized human liver microsomal pre parations, good correlations were observed between tramadol metabolism to M 1 and M2 and enzymatic markers of CYP2D6 and CYP2B6, respectively. Tramadol was metabolized to M1 by cDNA-expressed CYP2D6 and to M2 by CYP2B6 and CYP 3A4. Tramadol metabolism in human liver microsomes to M1 and M2 was markedl y inhibited by the CYP2D6 inhibitor quinidine and the CYP3A4 inhibitor trol eandomycin, respectively. In summary, this study demonstrates that cis-tram adol can be metabolized to tramadol metabolites M1, M2, M3, and M5 in human liver microsomal preparations. By kinetic analysis and the results of the reaction phenotyping studies, tramadol metabolism in human liver is catalyz ed by multiple CYP isoforms. Hepatic CYP2D6 appears to be primarily respons ible for M1 formation, whereas M2 formation is catalyzed by CYP2B6 and CYP3 A4.