Conversion of the CPT-11 metabolite APC to SN-38 by rabbit liver carboxylesterase

The anticancer drug CPT-11 (7-ethyl-[4(1-piperidino)-1-piperidino]carbonylo xycamptothecin) is a water-soluble derivative of camptothecin, We report he re the conversion of APC (7-ethyl-[4-N-(5-aminopentanoic acid)-1-piperidino ] carbonyloxycamptothecin), an inactive metabolite of CPT-11, to SN-38 (7-e thyl-10-hydroxycamptothecin), the active metabolite of CPT-11, by a rabbit liver carboxylesterase, This reaction is not catalyzed by any known human e nzyme. The formation of SN-38 from APC was characterized by an apparent K-m of 37.9 +/- 7.1 mu M and a V-max of 16.9 +/- 0.9 pmol/units/min, SN-38 was confirmed as a reaction product by high-performance liquid chromatography and mess spectrometry, A 24-h incubation of 10 mu M APC with 500 units/ml o f rabbit carboxylesterase produced 4 mu M SN-38, The product of this reacti on inhibited the growth of U373 MG human glioblastoma cells in vitro. The I C50 for a 24-h exposure of U373 MG cells to APC in the presence of 50 units /ml of rabbit carboxylesterase was 0.27 +/- 0.08 mu M, whereas APC alone de monstrated no inhibition of growth at concentrations up to 1 mu M. The IC50 of U373 MG cells transfected with the cDNA encoding the rabbit carboxylest erase (U373pIRESrabbit) and exposed to APC for 24 h was 0.8 +/- 0.1 mu M AP C, whereas the growth of cells transfected with vector control (U373pIRES) was unaffected by up to 1 mu M APC, Because APC is nontoxic to human cells, ,ve are investigating the possibility of using APC/rabbit carboxylesterase in a prodrug/enzyme therapeutic approach.