PT - JOURNAL ARTICLE AU - Mingxing Xie AU - Dongfang Yang AU - Micheal Wu AU - Bob Xue AU - Bingfang Yan TI - Mouse Liver and Kidney Carboxylesterase (M-LK) Rapidly Hydrolyzes Antitumor Prodrug Irinotecan and the N-Terminal Three Quarter Sequence Determines Substrate Selectivity AID - 10.1124/dmd.31.1.21 DP - 2003 Jan 01 TA - Drug Metabolism and Disposition PG - 21--27 VI - 31 IP - 1 4099 - http://dmd.aspetjournals.org/content/31/1/21.short 4100 - http://dmd.aspetjournals.org/content/31/1/21.full SO - Drug Metab Dispos2003 Jan 01; 31 AB - Antitumor prodrug irinotecan is used for a variety of malignancies such as colorectal cancer. It is hydrolyzed to the metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), which exerts its antineoplastic effect. Several human and rodent carboxylesterases are shown to hydrolyze irinotecan, but the overall activity varies from enzyme to enzyme. This report describes a novel mouse liver and kidney carboxylesterase (M-LK) that is highly active toward this prodrug. Northern analyses demonstrated that M-LK was abundantly expressed in the liver and kidney and slightly in the intestine and lung. Lysates from M-LK transfected cells exhibited a markedly higher activity on irinotecan hydrolysis than lysates from the cells transfected with mouse triacylglycerol hydrolase (TGH) (6.9 versus 1.3 pmol/mg/min). Based on the immunostaining intensity with purified rat hydrolase A, M-LK had a specific activity of 173 pmol/mg/min, which ranked it as one of the most efficient esterases known to hydrolyze irinotecan. A chimeric carboxylesterase and its wild-type enzyme (e.g., M-LKn and M-LK), sharing three quarters of the entire sequence from the N-terminus, exhibited the same substrate preference toward irinotecan and two other substrates, suggesting that the N-terminal sequence determines substrate selectivity. M-LK transfected cells manifested more severe cytotoxicity than TGH transfected cells upon being exposed to irinotecan. Topoisomerase I inhibitors such as irinotecan represent a promising class of anticancer drugs. Identification of M-LK as an efficient carboxylesterase to activate irinotecan provides additional sequence information to locate residues involved in irinotecan hydrolysis and thus facilitates the design of new analogs. The American Society for Pharmacology and Experimental Therapeutics