SATURATION REVERSAL OF THE MULTIDRUG PUMP USING MANY REVERSERS IN LOW-DOSE COMBINATIONS

Multidrug resistance in cancer cells, in cell culture and in the clini c, is often associated with a membrane protein (the multidrug resistan ce pump or P-glycoprotein) that pumps out anti-cancer drugs as fast as they enter the cell. This pump is blocked by a range of well-known ph armaceuticals that reverse drug resistance. We have investigated wheth er effective reversal of drug resistance could be achieved by using ma ny reversers together, each at a low dose relative to its maximal tole rated plasma level, We measured in cell culture, using resistant P388 cells in suspension, the extent of reversal of the accumulation of two labeled cytotoxins (vinblastine and daunomycin). We fitted the data t o a modified Michaelis-Menten equation and extracted the half-inhibiti on constants for 18 reversers acting on the pump. We measured also the reversal of resistance in a cell growth assay using incorporation of labeled thymidine, We showed that these drugs in groups of up to 18 to gether, each drug being at a low dose, in many cases well-tolerated in humans, had additive effects so that the combination was as effective as any of the drugs present singly. This was the case both for revers al of cell accumulation and for the effects of cytotoxins on cell grow th. Our data show that a low-dose multidrug approach to saturation rev ersal of the multidrug pump is feasible In cell culture and provide th e initial experimental basis for the development of an effective regim e of such combination reversal therapy.