Influence of serum protein binding on the uptake and retention of idarubicin by sensitive and multidrug resistant human leukemic cells

Objective: The objectives of the investigations were (1) to determine the binding characteristics of idarubicin (IDA) in human serum and cell culture solutions, (2) to determine the effect of protein binding on the uptake and retention of IDA by human leukemic cell lines in culture and the extent to which R-verapamil (R-VRP), an inhibitor of the P-glycoprotein (P-gp) transporter, can modulate these processes, and (3) to assess the importance of protein binding on cytostatic and chemosensitizer action in vivo. Methods: The protein binding of IDA was determined using equilibrium dialysis. Cell uptake of IDA was measured using sensitive and P-gp-containing resistant human leukemic cell lines (HL-60 and HL-60-Vinc) in vitro. IDA was assayed spectrophotofluorometrically. Results: In the incubation media examined, the free fraction of IDA varied more than seven-fold from approximately 60% in 15% fetal calf serum (FCS)/PBS to only 8% in human serum. Cellular uptake of IDA was approximately three times higher in medium containing low protein concentrations. R-VRP eliminated the difference in IDA uptake between resistant and sensitive cell lines and this was the case when the cells were incubated in solutions containing both high and low protein concentrations. However, R-VRP did not overcome the effect of high protein concentrations on IDA uptake. Conclusions: Plasma protein binding is an important determinant for cellular uptake of IDA in vitro. This should be taken into account when interpreting results of in vitro functional assays with patient material. Chemosensitizers such as R-VRP are effective in both high and low protein solutions. Investigations like these may be useful for evaluating cytostatic efficacy and chemosensitizer action in vivo.