S-phase population analysis does not correlate with the cytotoxicity of camptothecin and 10,11-methylenedioxycamptothecin in human colon carcinoma HT-29 cells.

Previous studies in rapidly proliferating rodent cells have suggested that the lethal effect of the DNA topoisomerase I inhibitor, camptothecin (CPT) is dependent upon the active participation of DNA replication (Holm et al. Cancer Res. 49:6365-6368; 1989). The purpose of the current study was to determine if this relationship applies to more slowly growing human cells. In our present study, we employed the human colon carcinoma cell line, HT-29 (45 hr doubling time). Flow cytometric determination of S-phase cells either by S-phase fit model or rectangle fit model analysis predicted that 21% of exponentially growing HT-29 cells were undergoing DNA replication. These findings were confirmed by immunofluorescence microscopy of bromodeoxyuridine labeled cells. Based on these findings, we would have expected only 20-30% of the cells to be susceptible to brief treatment (30 min) with CPT. Instead, 90-95% of HT-29 cells were killed. This apparent disparity was not due to prolonged cellular retention of drug after treatment because protein-linked DNA strand breaks reversed within 15 min of drug removal. Moreover, the DNA replication inhibitor, aphidicolin, fully protected HT-29 cells against CPT-induced killing but did not affect the production of CPT-induced protein-linked DNA strand breaks. Similar results were also obtained using the CPT-analog, 10,11-methylenedioxy-camptothecin, which was 5- to 10-fold more potent than camptothecin (O'Connor et al. Cancer Commun. 2:395-400; 1990).(ABSTRACT TRUNCATED AT 250 WORDS)