CYP1A2 and ADH1C contribute to abacavir aldehyde intermediate formation in the human liver

Abacavir, an anti-human immunodeficiency virus agent, is metabolized to the pharmacologically inactive abacavir carboxylate via an aldehyde intermediate, abacavir aldehyde (abacavir-CHO). Because aldehyde formation is generally rate-limiting, the enzymes responsible for abacavir-CHO formation may significantly influence drug efficacy and toxicity. In this study, we aimed to identify the enzymes involved in abacavir-CHO formation in the human liver. Using 5 recombinant human NAD⁺-dependent alcohol dehydrogenase (ADH) isoforms, which are well known to catalyze aldehyde formation from alcohols, ADH1C was identified as the primary catalyst of abacavir-CHO formation. In human liver S9 fractions, abacavir-CHO formation was observed not only in the presence of NAD⁺ but also in the presence of NADPH, with intrinsic clearance comparable between the 2 conditions. Notably, NADPH-dependent activity was higher in human liver microsomes than in cytosolic fractions. This activity was markedly inhibited by 1-aminobenzotriazole, a general cytochrome P450 (P450) inhibitor, indicating the involvement of P450 enzymes. Among recombinant human P450 isoforms, CYP1A2 exhibited the highest catalytic activity. Quantitative analysis using the relative activity factor approach revealed that CYP1A2 accounted for 89.8% of abacavir-CHO formation under NADPH-dependent conditions in human liver microsomes. In HepaSH cells, hepatocytes from chimeric mice with humanized livers, abacavir carboxylate formation was decreased by 50% by α-naphthoflavone, a CYP1A2 inhibitor, and increased approximately 7-fold by omeprazole, a CYP1A2 inducer. Collectively, these findings demonstrate that CYP1A2 plays a significant role in abacavir-CHO formation in the human liver, comparable to ADH1C, and may have implications for variability in abacavir response. SIGNIFICANCE STATEMENT: This study demonstrates that CYP1A2, in addition to ADH1C, significantly contributes to the formation of the reactive aldehyde metabolite abacavir-CHO in the human liver. This finding suggests a need to reconsider the conventional view that alcohol dehydrogenases are the primary enzymes responsible for this pathway and indicates that CYP1A2-mediated metabolism may influence interindividual variability, drug-drug interactions, and the risk of abacavir-associated adverse reactions.