Abstract 670: Bis-nitrophenylphosphate (BNPP) but not phenylmethylsulfonyl fluoride (PMSF) inhibits the activation of the novel anti-cancer pro-drug CZ48

Abstract CZ48, the 20-O-propionate ester of Camptothecin (CPT), is a non-toxic prodrug of CPT first described by Cao et al. (1998). The propionate side-chain is enzymatically cleaved in target tissues. This gives rise to CPT, an inhibitor of Topoisomerase I, which then blocks cell division causing selective tumor toxicity. We have previously established the preferential activation of CZ48 in tumor tissues compared to normal tissues and an ultra sensitive assay to detect it (2009). The present studies explore the specifics of that activation in vitro using both cells in culture and surgically removed xenografted tumors from nude mice. DOY lung adenocarcinoma was grown subcutaneously in nude mice and then surgically excised, homogenized and serially centrifuged to produce specific subcellular fractions. The bulk of esterase activity was found to be concentrated in the cytosol, while the microsomal fraction showed minimal activity. All other fractions tested were below the limit of detection. HT-29 colon carcinoma cells were grown in culture in the presence and absence of varying concentrations of bis-nitrophenylphosphate (BNPP), a potent inhibitor of carboxylesterase and butyrylcholinesterase (BChE), in an attempt to find the highest possible non-toxic concentration to the cells. This concentration was then included during a 7-day IC50 determination for CZ48. In the presence of BNPP, the IC50 of CZ48 increased 4-fold from 181.4 nM to 710.8 nM. In vitro conversion of CZ48 to CPT by cytosolic fraction of a HT-29 tumor xenograft grown subcutaneously in nude mice was decreased by BNPP in a dose dependent manner. In contrast, the serine protease and carboxylesterase inhibitor phenylmethylsulfonyl fluoride (PMSF) and other esterase inhibitors did not affect the activation of CZ48. Purified butyrylcholinesterase and acetylcholinesterase (SigmaAldrich, St. Louis MO) did not show any capacity to activate CZ48. In conclusion, the enzymatic activation of CZ48 is not mediated by butyrylcholinesterase but by another cytosolic enzyme inhibitable by BNPP. These data constitute a significant step forward in unraveling the exact mechanism for the activation of CZ48. This will help design a pre-treatment assay to establish tumor sensitivity, as well as give us direction for designing new, more potent prodrugs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 670. doi:10.1158/1538-7445.AM2011-670