Bisbibenzyl derivatives sensitize vincristine-resistant KB/VCR cells to chemotherapeutic agents by retarding P-gp activity.

P-glycoprotein (P-gp) is known to mediate multidrug resistance (MDR) by acting as an efflux pump to actively transport chemotherapeutic agents out of carcinoma cells. Inhibition of P-gp function may represent one of the strategies to reverse MDR. We have previously reported that marchantin C (MC), a macrocyclic bisbibenzyl compound from liverworts, exerts anti-tumor activity as an antimitotic agent. This study was designed to evaluate the possible modulatory effect of MC and its three synthetic derivatives (MC1, MC2 and MC3) on P-gp in VCR-resistant KB/VCR cells. Results of the cytotoxicity assay revealed that MC was the most potent inhibitor of cell proliferation in both KB and KB/VCR cells among these four compounds, while the three MC-derived chemicals had little anti-proliferative activity under the same condition. However, in P-gp-expressing MDR cells, analysis of potency of these compounds in enhancing cytotoxicity of VCR led to the identification of MC2 as a more effective chemical on reversal of resistance. Further study showed that MC2 was able to reduce efflux of rhodamine-123, and in turn, increase the accumulation of rhodamine-123 and adriamycin in KB/VCR cells, indicating that MC2 re-sensitized cells to VCR by inhibition of the P-gp transport activity. In addition, the combination of MC2 and VCR at a concentration that does not inhibit cell growth resulted in an induction of apoptosis in KB/VCR cells. These results suggest that MC2, as a novel and effective inhibitor of P-gp, may find potential application as an adjunctive agent with conventional chemotherapeutic drugs to reverse MDR in P-gp overexpressing cancer cells.