Highly selective N-glucuronidation of four piperazine-containing drugs by UDP-glucuronosyltransferase 2B10

N-glucuronidation is known to be an important metabolic pathway for detoxification and elimination of drugs containing aromatic amines. However, the metabolic pathways for piperazine-containing drugs are not fully established.

N-glucuronidation potential of four piperazine-containing drugs, namely two antihistamines (i.e. cyclizine and chlorcyclizine) and two tetracyclic antidepressants (i.e. mirtazapine and mianserin), was determined by using expressed UDP-glucuronosyltransferase (UGT) enzymes and liver microsomes from both human and animals.

Among 13 expressed UGT enzymes, only UGT1A4 and UGT2B10 showed conjugating activities toward these four drugs. Reaction phenotyping, chemical inhibition, and activity correlation analysis revealed that UGT2B10 was a high-affinity enzyme and mainly responsible for hepatic N-glucuronidation of all drugs except mianserin. Both UGT1A4 and UGT2B10 were important contributors to mianserin N-glucuronidation. Moreover, significant species differences were observed in N-glucuronidation of all test drugs. In particular, liver microsomes from four experimental animals (i.e. mouse, rat, dog, and monkey) showed none or negligible activity in catalyzing N-glucuronidation of four drugs.

UGT2B10 plays a critical role in N-glucuronidation of piperazine-containing drugs. Also, conventional experimental animals might be inappropriate models for studying human N-glucuronidation.

CL_int: intrinsic clearance; CL_max: maximal clearance; HLM: human liver microsomes; K_m: Michaelis-Menten constant; K_i: substrate inhibition constant; MS: mass spectroscopy; NNAL: 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol; QTOF: Quadrupole time-of-flight; S₅₀: the substrate concentration resulting in 50% of V_max; UDP-GlcA: uridine diphosphoglucuronic acid; UGT: UDP-glucuronosyltransferase; UPLC: ultra performance liquid chromatography; V_max: maximal velocity.