Reversal of P-glycoprotein is greatly reduced by the presence of plasma but can be monitored by an ex vivo clinical assay.

The effects of nine reversers of P-glycoprotein on the uptake of daunomycin into MDR1-transfected P388 cells were quantitatively determined in undiluted human or mouse plasma and compared with their effects when measurements are made in a conventional cell culture medium (RPMI 1640) containing only 10% serum. Plasma diminished or greatly diminished the effectiveness of the reversers, reductions of up to 20-fold being found for reversers (cyclosporin A, prochlorperazine and amiodarone) that have been used in clinical trials, although quinidine was almost as effective in plasma as in cell culture medium containing 10% fetal calf serum. Human or bovine serum albumin could mimic the effect of whole plasma. When measurements of the effectiveness of the reverser cyclosporin A were made in an ex vivo assay, using these P388 cells, complete accord was found between such ex vivo determinations and cyclosporin A's effectiveness in vivo, as monitored by its ability to increase the accumulation of vinblastine in mouse kidney tissue. The ex vivo assay was shown to be suitable to monitor the effectivity of reversers present in plasma taken from patients receiving quinidine and cyclosporine A in routine clinical treatment.