1-Aminobenzotriazole, a Known Cytochrome P450 Inhibitor, Is a Substrate and Inhibitor of N-Acetyltransferase

1-Aminobenzotriazole (ABT) has been used widely as a nonselective in vitro and in vivo inhibitor of cytochrome P450 enzymes. To date, however, it has not been evaluated as an inhibitor of UDP-glucuronosyltransferase (UGT), sulfotransferase (SULT), and N-acetyltransferase (NAT). In the present study, ABT was shown not to inhibit UGT and SULT activity (acetaminophen and 7-hydroxycoumarin as substrates) in rat liver microsomes and rat liver 9000 g supernatant fraction (RLS9), respectively. However, it did inhibit the RLS9-catalyzed N-acetylation of procainamide (IC(50) ∼ 30 μM), and no preincubation time dependence was evident. In agreement, oral ABT (100 mg/kg, 2 h predose) decreased the clearance of intravenous procainamide (45%) in rats, accompanied by a decreased N-acetylprocainamide-to-procainamide ratio in urine (0.74 versus 0.21) and plasma (area under the curve ratio 0.59 versus 0.11). Additional studies with human forms of NAT (hNAT1 and hNAT2) revealed that ABT is a more potent inhibitor of hNAT2 compared with hNAT1 (IC(50) 158 μM versus > 1 mM). Consistent with the IC(50) estimate, formal inhibition studies revealed that inhibition of hNAT2 was competitive with an inhibition constant of 67 μM. In accordance with the competitive inhibition, ABT was shown to undergo N-acetylation in the presence of both human NAT forms, with hNAT1 exhibiting less activity under the same assay conditions (∼40% of hNAT2). In summary, the results described herein indicate that ABT is a substrate and inhibitor of NAT. Such an interaction should be considered when using ABT as a nonselective inhibitor of P450, especially when NAT-dependent metabolism is also involved.