Activation of anticancer prodrugs by human carboxylesterases

Ester prodrugs are commonly used to improve pharmacokinetic, pharmacodynamic, or pharmaceutical properties of drugs. Two anticancer prodrugs, irinotecan and capecitabine, require in vivo activation by carboxylesterase enzymes. One of the dose-limiting side effect of these agents is diarrhea. Local activation in the gastrointestinal tract by carboxylesterases may influence efficacy and toxicity of these drugs. Colorectal cell lines, tumor and matched normal tissue were examined for variability in expression and activity of carboxylesterases and topoisomerase I (TOPOI). The kinetics of hydrolysis of irinotecan\u27s oxidative metabolites, APC and NPC, and of capecitabine were examined using recombinant CES1A1, CES2, and CES3. For irinotecan, APC, and NPC, SN-38 was detected via HPLC. 5′DFCR formation from capecitabine was analyzed using LC/MS. Inhibition of CES1A1 and CES2 by various compounds commonly co-administered with irinotecan and capecitabine were examined. Variability in expression of carboxylesterases along the gastrointestinal tract was examined by multi-tissue Northern blot analysis. Carboxylesterase expression and activity varied 56-fold across the tumor tissues studied. Irinotecan-hydrolyse activity correlated with hydrolysis of 4-methylumbelliferyl acetate, CES2 band activity, and CES2 expression by quantitative real-time PCR. Topoisomerase activity correlated with TOPOI expression by real-time PCR. In colorectal cell lines, TOPOI activity best predicted sensitivity to irinotecan and SN-38. Irinotecan, APC, and NPC are hydrolyzed by all three isozymes with catalytic efficiencies of CES2 \u3e CES1A1 \u3e CES3. CES2 hydrolyzes irinotecan with catalytic efficiency 2.4-fold greater than NPC and 800-fold higher than APC. Capecitabine is hydrolyzed by CES1A1 and CES2 to a similar extent. CES3 has very low activity for capecitabine. Loperamide inhibited CES2 with Ki of 1.51 ± 0.14 μM. Dolasetron mesylate, docetaxel, capecitabine, and atropine were weak inhibitors of CES1A1 and CES2. All three enzymes are expressed in the liver with CES1A1 \u3e\u3e CES2 \u3e CES3. Expression in other segments of the gastrointestinal tract varies with highest expression of CES2 in the duodenum, jejunum, and descending colon. CES2 in colon tumors and the gastrointestinal tract is important in activation of both irinotecan and capecitabine. Capecitabine is also activated by CES1A1 in liver. Interindividual variation in carboxylesterase activity and drug-drug interactions may contribute to the variability in toxicity and efficacy of irinotecan and capecitabine