Abstract

The metabolism of cis-tramadol has been studied in human liver microsomes and in cDNA-expressed human cytochrome P-450 (CYP) isoforms. Human liver microsomes catalyzed the NADPH-dependent metabolism of tramadol to the two primary tramadol metabolites, namely,O-desmethyl-tramadol (metabolite M1) andN-desmethyl-tramadol (metabolite M2). In addition, tramadol was also metabolized to two minor secondary metabolites (each comprising ≤3.0% of total tramadol metabolism), namely,N,N-didesmethyl-tramadol (metabolite M3) and N,O-didesmethyl-tramadol (metabolite M5). Kinetic analysis revealed that multiple CYP enzymes were involved in the metabolism of tramadol to both M1 and M2. For the high-affinity enzymes involved in M1 and M2 formation, K_mvalues were 116 and 1021 μM, respectively. Subsequent reaction phenotyping studies were performed with a tramadol substrate concentration of 250 μM. In studies with characterized human liver microsomal preparations, good correlations were observed between tramadol metabolism to M1 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 CYP3A4. Tramadol metabolism in human liver microsomes to M1 and M2 was markedly inhibited by the CYP2D6 inhibitor quinidine and the CYP3A4 inhibitor troleandomycin, respectively. In summary, this study demonstrates thatcis-tramadol 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 catalyzed by multiple CYP isoforms. Hepatic CYP2D6 appears to be primarily responsible for M1 formation, whereas M2 formation is catalyzed by CYP2B6 and CYP3A4.