Daunomycin accumulation in resistant tumor cells as a screening model for resistance modifying drugs: Role of protein binding

A potential screening system for drugs that modify anthracycline resistance was investigated. The effect of the calcium antagonistic agents verapamil, diltiazem, bepridil and Ro 11-2933/001 on daunorubicin (DNR) accumulation in the doxorubicin resistant cell line 2780^AD was measured. For this purpose cells were suspended in medium with varying concentrations of bovine serum albumin (BSA) and in human plasma to establish the influence of protein binding on DNR accumulation. Increase of protein concentration did not affect DNR accumulation when no calcium antagonists were present but caused an increase in concentrations of calcium antagonists needed to promote DNR accumulation. It appeared that DNR accumulation in suspended cells correlated well with cytotoxicity assays, performed on cells growing in monolayer in 96-well microtiter plates, when accumulation was measured in medium with the same protein constitution as in growth medium (10% fetal calf serum). It is concluded that protein binding effects should be taken into account in the screening of compounds for the circumvention of multidrug resistance.