Cellular pharmacology of the combination of the DNA topoisomerase I inhibitor SN-38 and the diaminocyclohexane platinum derivative oxaliplatin

CPT-11, a DNA topoisomerase I inhibitor, and oxaliplatin, a diaminocyclohex ane platinum derivative, are cytotoxic agents that have demonstrated clinic al antitumor activity in colorectal cancer. Given the therapeutic potential of their combination, we studied the cellular pharmacology of SN-38, the a ctive metabolite of CPT-11, and oxaliplatin in the human colon cancer HT29 cell line. Growth inhibition was studied after a 1- or 24-h exposure to SN- 38 or oxaliplatin, given alone or in combination. The cytotoxicity analysis by the isobolograms method elicited synergy. SN-38 delayed the reversion o f oxaliplatin-induced DNA interstrand cross-links (ISCs), measured in cells by alkaline elution, The amount of detectable ISCs 15 h after a l-h exposu re to 10 mu M oxaliplatin was 27% of the ISC peak levels and increased to 6 8% in the presence of 0.1 mu M SN-38, The presence of oxaliplatin DNA adduc ts led to a 3.3-fold increase in the SN-38-induced DNA elongation inhibitio n, as measured by pulse-labeling alkaline elution, Inhibition of DNA and RN A synthesis was longer after exposure to the combination of oxaliplatin and SN-38 than after exposure to each agent done. Consistently, flow cytometry analyses revealed that preexposure to oxaliplatin enhanced SN-38-induced S -phase arrest. Filter binding assays indicated that the cells arrested in S -phase at 48 h were undergoing apoptosis, Hence, supra-additive cytotoxicit y appears related to major modifications in the cellular response to DNA da mage rather than to changes in DNA damage formation. The combination of CPT -11 and oxaliplatin induced a 2-fold higher tumor growth reduction irt vivo than did oxaliplatin alone at feasible nonlethal doses. This study provide s a rationale for the optimal use of CPT-11 and oxaliplatin in combination.