Effect of PSC 833 in 12 patients studied. (A) Difference in rhodamine fluorescence in CD56⁺ cells with and without exogenous PSC 833 (PSC − Control) or following the efflux period with and without exogenous PSC 833 (PSC/Efflux − Efflux) were compared at the “pre” time point, before initial chemotherapy treatment; the “peak” time point, 2 hours following ingestion of PSC 833; and the “trough” time point, 6 hours following PSC 833 administration for 12 patients on study. The peak and trough samples were obtained after at least 6 days of PSC 833 administration. (B) PSC − Control and PSC/Efflux − Efflux values were compared at 0, 15, 30, 45, 60, and 120 minutes after a single dose of PSC 833.