Inhibition of rh123 efflux was measured in the absence and presence of eight serial dilutions of GPV005. (A) Open diamonds, wild-type; filled circles, Y950F/Y953F; triangles, Y950F; squares, Y953F. IC50 values for GPV005 (B), GPV031 (C), and GPV366 (D) were determined by fitting hyperbolic concentration response curves to data points by nonlinear regression analysis and calculation of 50% occupancy values. Mean ± S.D. values were calculated from at least three independent experiments. *P < 0.05; **P < 0.01; ***P < 0.001, compared with wild-type P-gp.