REVERSAL OF ETOPOSIDE RESISTANCE IN NON-P-GLYCOPROTEIN EXPRESSING MULTIDRUG-RESISTANT TUMOR-CELL LINES BY NOVOBIOCIN

Previous reports from this laboratory have demonstrated that novobioci n produces supraadditive cytotoxicity and increases the formation of d rug-stabilized topoisomerase II-DNA covalent complexes in WEHI-3B myel omonocytic leukemia and A549 lung carcinoma cells when combined with e toposide (VP-16). Inhibition of the efflux of VP-16 by novobiocin is r esponsible for the increase in VP-16 accumulation, which in turn leads to increased formation of VP-16-stabilized topoisomerase II-DNA coval ent complexes and increased cytotoxicity. We now report that novobioci n synergistically enhanced the sensitivity of the multidrug resistant variants, WEHI-3B/NOVO and A549(VP)28, to VP-16, causing almost comple te reversal of the resistance to the epipodophyllotoxin. These two tum or cell variants are resistant to several topoisomerase II-targeted dr ugs, particularly VP-16, but not to Vinca alkaloids; this finding corr esponds to the fact that they do not overexpress the P-glycoprotein. T he effects of novobiocin in these resistant sublines are mediated thro ugh the intracellular accumulation of VP-16, resulting in an increase in the formation of lethal VP-16-induced topoisomerase II-DNA covalent complexes. In the P-glycoprotein expressing multidrug resistant HCT11 6(VM)34 colon carcinoma and L1210/VMDRC0.06 leukemia cell lines, the l atter being transfected with the human mdr-1 gene, novobiocin did not potentiate the cytotoxic activity of VP-16 nor increase the intracellu lar accumulation of VP-16 and the formation of covalent complexes, whe reas their normal counterparts were sensitive to the potentiating acti vity of novobiocin when used in combination with VP-16. These results indicate that the action of novobiocin on the intracellular transport of VP-16 is not directed at the level of the P-glycoprotein, but that the action of novobiocin is antagonized by the presence of the P-glyco protein. Since novobiocin is a clinically available antibiotic, has nu merous structural analogues available for comparative studies, and has a relatively low toxicity profile, this drug, as well as structurally related agents, would appear to have significant clinical potential i n combination with an epipodophyllotoxin for the treatment of non-P-gl ycoprotein expressing multidrug resistant tumors.