The effect of inducers and inhibitors of urethane metabolism on its in vitro and in vivo metabolism in rats.

The activation of urethane (ethyl carbamate) is important in its exerting its carcinogenic effect. Rats were treated with inducers and inhibitors of urethane metabolism, and the conversion of [carbonyl-14C]urethane to 14CO2 in vivo was measured. The cytochrome P-450 inducers, phenobarbital and beta-naphthoflavone, and esterase inhibitor, paraoxon, were without effect while the CYP2E1 inhibitor, diethyldithiocarbamate, decreased metabolism to about 3% of control. Ethanol administered acutely inhibited urethane metabolism. Pyridine, shown previously to enhance this metabolism in microsomal preparations, greatly inhibited it in vivo. The discordant results between the in vitro and in vivo studies may be related to the presence of pyridine acting as an inhibitor in whole animals and suggest that caution is needed in extrapolating from in vitro results to in vivo implications.