Evaluation of human P-glycoprotein (MDR1/ABCB1) ATPase activity assay method by comparing with in vitro transport measurements: Michaelis-Menten kinetic analysis to estimate the affinity of P-glycoprotein to drugs.

Human ABC transporter P-glycoprotein (P-gp/ABCB1) encoded by the multidrug resistance (MDR1) gene is recognized as one of the most important factors regulating pharmacokinetics of a number of clinically important drugs because of its function of extruding a wide range of structurally unrelated amphiphilic and hydrophobic drugs from the inside to the outside of cells in an ATP-driven mechanism. In the present study, we have evaluated the high-speed ATPase activity assay method by comparing with in vitro transport assay systems using MDR1-transfected MDR1-MDCK cells. Since substrate drugs were found to interfere with the photometric detection of inorganic phosphate (Pi) that was liberated according to the hydrolysis of ATP to ADP in ATPase activity assay, at first, a method in which the amount of Pi can be calculated correctly. Results demonstrate that the kinetic parameters obtained in ATPase activity assay are not necessarily correspond with those in in vitro transport assay, suggesting that these methods might detect the different processes of drug-P-gp interaction. The combining of the ATPase activity assay and in vitro transport technologies provides us the insight into mechanisms of the membrane transport of drugs by P-gp.