Metabolic activation of dacarbazine by human cytochromes P450: The role ofCYP1A1, CYP1A2, and CYP2E1

Dacarbazine (DTIC), a widely used anticancer agent, is inactive until metab olized in the liver by cytochromes P450 to form the reactive N-demethylated species 5-[3-hydroxymethyl-3-methyl-triazen-1-yl]-imidazole- (HMMTIC) and 5-[3-methyl-triazen-1-yl]-imidazole-4-carboxamide (MTIC). The modest activi ty of DTIC in the treatment of cancer patients has been attributed in part to lower activity of cytochromes P450 (P450) in humans when compared with r odents. Importantly, the particular P450 isoforms involved in the activatio n pathway have not been reported. We now report that the DTIC N-demethylati on involved in MTIC formation by human liver microsomes is catalyzed by CYP 1A1, CYP1A2, and CYP2E1, The most potent inhibitors of DTIC N-demethylation were alpha-naphthoflavone (CYP1A1 and CYP1A2), quercetin (CYP1A2), chlorzo xazone (CYP1A2 and CYP2E1), and di-sulfiram (CYP2E1). Antihuman CYP1A2 anti serum also inhibited DTIC N-demethylation, DTIC N-demethylation in a panel of 10 human liver microsome preparations was correlated with the catalytic activities for CYP1A2 (ethoxyresorufin O-deethylation and caffeine N-3-deme thylation) in the absence of alpha-naphthoflavone and with the catalytic ac tivities for CYP2E1 (chlorzoxazone 6-hydroxylations) in the presence of cn- naphthoflavone. DTIC metabolism was catalyzed by recombinant human CYP1A1, CYP1A2, and CYP2E1. The K-m (V-max) values for metabolism of DTIC by recomb inant human CYP1A1 and CYP1A2 were 595 mu M (0.684 nmol/min/mg protein) and 659 mu M (1.74 nmol/min/mg protein), respectively. The CYP2E1 K(m)value ex ceeded 2.8 mM. Thus, we conclude that (a) CYP1A2 is the predominant P450 th at catalyzes DTIC hepatic metabolism; (b) CYP2E1 contributes to hepatic DTI C metabolism at higher substrate concentrations; and (c) CYP1A1 catalyzes e xtrahepatic metabolism of DTIC.