Chemosensitisation and drug accumulation effects of cyclosporin A, PSC-833 and verapamil in human MDR large cell lung cancer cells expressing a 190k membrane protein distinct from P-glycoprotein.

The doxorubicin-selected multidrug resistant (MDR) human large cell lung cancer line COR-L23/R, lacks P-glycoprotein but shows a drug accumulation deficit. It does however overexpress a 190k membrane protein which shares an epitope with, but is otherwise distinct from, P-glycoprotein. The resistant cells show only a small sensitisation to vincristine and daunorubicin on treatment with cyclosporin A and its more potent analogue, PSC-833 despite an increase in drug accumulation. Verapamil, another effective resistance modifier in P-glycoprotein MDR cells, is slightly more effective. Fluorescent daunorubicin distributes in the cytoplasm and nucleus of sensitive parent COR-L23 cells but is confined to cytoplasmic perinuclear vesicles in resistant cells. Addition of cyclosporin A or PSC-833 slightly increases cytoplasmic fluorescence whereas verapamil also increases nuclear fluorescence. Resistance in this non-P-glycoprotein MDR line, COR-L23/R where these resistance modifiers have little effect may be associated with expression of the 190k protein.