Reversal of Pgp activity by PSC 833 in plasma. Ordinates, (PSC − Control). Abscissas, concentration of PSC 833 in plasma in ng/mL. (A) Data for 12 different patients. Open squares aberrant patient T5, open diamonds aberrant patient T6. (B) Data for patients T2 (left panel) and T4 (right panel) replotted from (A). Dashed lines, best-fit predictions using the simple binding isotherm model; solid lines, best fits for model that includes cooperative interactions between molecules of PSC 833. The lines drawn are obtained by using the Marquardt-Levenberg algorithm.