Abstract

Medetomidine is a chiral imidazole derivate whose dextroenantiomer is pharmacologically active. The major metabolic pathway of dexmedetomidine [(+)-4-(S)-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole] in humans is N-glucuronidation at the imidazolate nitrogens. We have purified the N₃- and N₁-glucuronides of dexmedetomidine, termed DG1 and DG2, respectively, according to their elution order in liquid chromatography and determined their structure by ¹H nuclear magnetic resonance (NMR). Studying medetomidine glucuronidation by human liver microsomes (HLMs) and recombinant UDP glucuronosyltransferase (UGT) 1A4 indicated that another human UGT plays a major role in these activities. We now demonstrate that this enzyme is UGT2B10. HLMs catalyzed DG1 and DG2 formation, at a ratio of 3:1, with two-enzyme kinetics that contain both a high-affinity component, K_m1 values of 6.6 and 8.7 μM, and a low-affinity component, K_m2 values > 1 mM. The DG1/DG2 ratio in the case of UGT2B10 was lower, 1.4:1, whereas the substrate affinity for both reactions was high, K_m values of 11 and 16 μM. UGT1A4 produced mainly DG1 (DG1/DG2 ratio of 6.6:1) at low substrate affinities, K_m values above 0.6 mM, but superior expression-normalized V_max values. Levomedetomidine [(-)-4-(R)-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole] glucuronidation by HLMs yielded mostly the N₃-glucuronide (LG1, structure determined by NMR), with monophasic kinetics and a K_m value of 14 μM. The activity of UGT1A4 toward levomedetomide was low and generated both LG1 and LG2, whereas UGT2B10 exhibited relatively high activity and sharp regioselectivity, yielding only LG1, with a K_m value of 7.4 μM. The results highlight the contribution of UGT2B10 to medetomidine glucuronidation and its potential importance for other N-glucuronidation reactions within the human liver.