Prevention of Acetaminophen (APAP)-induced Hepatotoxicity by Leflunomide Via Inhibition of APAP Biotransformation to N-acetyl-p-benzoquinone Imine

Acetaminophen (APAP) is safe at therapeutic levels but causes liver injury via N-acetyl-p-benzoquinone imine (NAPQI)-induced oxidative stress when overdose. Recent studies indicated that mitochondrial permeability transition (mPT) plays a key role in APAP-induced toxicity and leflunomide (LEF) protects against the toxicity through inhibition of c-jun NH2-terminal protein kinase (JNK)-mediated pathway of mPT. It is not clearly understood if LEF also exerts its protective effect through inhibition of APAP bioactivation to the toxic NAPQI. The present work was undertaken to study the effect of LEF on the bioactivation of APAP to NAPQI. Mechanism-based inhibition incubations performed in mouse and human liver microsomes (MLM and HLM) indicated that inhibition of APAP bioactivation to NAPQI was observed in MLM but not in HLM. Furthermore, LEF but not its active metabolite, A77-1726, was shown to be the main inhibitor. When APAP and LEF were incubated with human recombinant P450 enzymes, CYP1A2 was found to be the isozyme responsible for the inhibition of APAP bioactivation. Species variation in CYP1A2 enzymes probably accounted for the different observations in our MLM and HLM studies. We concluded that inhibition of NAPQI formation is not a probable pathway that LEF protects APAP-induced hepatotoxicity in human.