Differential cytotoxicity of clinically important camptothecin derivatives in P-glycoprotein overexpressing cell lines

Camptothecin and its derivatives are specific inhibitors of eukaryotic topoisomerase I (top1) and are active in cancer patients against a variety of refractory solid tumors and leukemia. Purpose: The present study further investigated the relationship between multidrug resistance (MDR) mediated by P-glycoprotein(MDR) and potential resistance to camptothecin derivatives using two experimental systems: (1) MDR KB-V1 cells selected for vinblastine resistance, and (2) NIH3T3 cells transfected with a plasmid expressing wildtype P-glycoprotein(MDR) multidrug transporter (NIH-MDR-G185). Results: We found that both KBV-1 and NIH-MDR-G185 cells were resistant to topotecan, and that topotecan-induced cleavable complexes were reduced in KB-V1 cells, consistent with a role of P-glycoprotein(MDR) in cellular resistance to topotecan. By contrast, no significant resistance to camptothecin, 9-aminocamptothecin, 10, 11-methylenedioxycamptothecin, or SN-38 (the active metabolite of CPT-I1) was observed in NIH-MDR-G185 cells, while KB-V1 cells were cross-resistant to these compounds but produced cleavable complexes similar to those produced by parental KB-3-1 cells. Conclusions: These results suggest that topotecan is the only camptothecin tested with significant susceptibility to MDR in cell culture, and that multidrug resistant cells such as KBV1 probably exhibit additional resistance mechanisms to camptothecins besides P-glycoprotein(MDR) overexpression.