Camptothecin exhibits selective cytotoxicity towards human breast carcinoma as compared to normal bovine endothelial cells in vitro.

We have reported earlier that camptothecin (CPT) incorporated into multilamelar liposomes of appropriate lipid composition displayed effective anti-tumor activity with minimal host toxicity in a nude mouse model xenographed with the human breast carcinoma Clouser nut 1. To investigate this observation further, we have determined the differential effects of CPT on the Clouser tumor cells as well as normal vascular (BVEC) endothelial cells in culture. We report here that Clouser cells are approximately 200-fold more sensitive to CPT (IC50 = 4.0 nM) than the normal endothelial cells (IC50 approximately 1 microM) as assayed by MTT; however, CPT demonstrates a potent anti-proliferative activity on both cell lines at low drug concentrations as measured by [3H]thymidine uptake. At higher concentrations (> 25.0 nM), however, the Clouser cells maintained a higher percentage of cells capable of incorporating [3H]thymidine. No significant differences in the levels of topoisomerase 1 protein and in vitro enzymatic activity were seen; although, the Clouser cells showed a 2-fold greater incidence of cleavable complex formation by CPT in vivo. Based on the data presented here, we propose that the selective cytotoxic activity of CPT towards tumor cells may be a function of the tumor cells' reduced ability to prevent cleavable complex formation. We also propose that the antitumor effect of CPT may be enhanced in vivo by its anti-proliferative effect on vascular endothelial cells which are normally solicited to promote tumor growth.