In vitro comparative analysis of metabolic capabilities and inhibitory profiles of selected CYP2D6 alleles on tramadol metabolism

Tramadol, the 41st most prescribed drug in the United States in 2021 is a prodrug activated by CYP2D6, which is highly polymorphic. Previous studies showed enzyme-inhibitor affinity varied between different CYP2D6 allelic variants with dextromethorphan and atomoxetine metabolism. However, no study has compared tramadol metabolism in different CYP2D6 alleles with different CYP2D6 inhibitors. We hypothesize that the inhibitory effects of CYP2D6 inhibitors on CYP2D6-mediated tramadol metabolism are inhibitor- and CYP2D6-allele-specific. We performed comparative analyses of CYP2D6*1, CYP2D6*2, CYP2D6*10, and CYP2D6*17 using recombinant enzymes to metabolize tramadol to O-desmethyltramadol, measured via UPLC-MS/MS. The Michaelis constant (K_m) and maximum velocity (V_max) for each CYP2D6 allele, and IC₅₀ values for different inhibitors were determined by nonlinear regression analysis. Intrinsic clearance was calculated as V_max/K_m. The intrinsic clearance of tramadol was almost double for CYP2D6*2 (180%) but was much lower for CYP2D6*10 and *17 (20% and 10%, respectively) compared to CYP2D6*1. The inhibitor potencies (defined by Ki) for the various inhibitors for the CYP2D6*1 allele were quinidine > terbinafine > paroxetine ≈ duloxetine >>bupropion. CYP2D6*2 showed the next greatest inhibition, with Ki ratios compared to CYP2D6*1 ranging from 0.96 to 3.87. For each inhibitor tested, CYP2D6*10 and CYP2D6*17 were more resistant to inhibition than CYP2D6*1 or CYP2D6*2, with most Ki ratios in the 3-9 range. Three common CYP2D6 allelic variants showed different metabolic capacities toward tramadol and genotype-dependent inhibition compared to CYP2D6*1. Further studies are warranted to understand the clinical consequences of inhibitor and CYP2D6 genotype-dependent drug-drug interactions on tramadol bioactivation.