Inhibition of rhodamine efflux from CD56⁺cells after initial exposure to PSC 833. Whole blood was obtained 0, 15, 30, 45, 60, and 120 minutes after an initial dose of PSC 833. Rhodamine 123 was added to whole blood and incubated for 30 minutes and the mononuclear cells were separated and incubated for an additional 60 minutes in rhodamine-free media before staining with PE-labeled CD56 antibody to generate the Efflux histogram (dotted line), or rhodamine 123 and PSC 833 were both added to the blood during the 30-minute incubation period and the mononuclear cells were separated and incubated in rhodamine-free media with PSC 833 before PE-labeled CD56 staining to yield the PSC/Efflux histogram (solid line). A small difference between the Efflux and PSC/Efflux peak signifies greater blocking of Pgp by PSC 833 in patient blood; a larger difference indicates poor reversal. (A) Patient T7 shows near complete reversal of Pgp-mediated rhodamine efflux in CD56⁺ cells. Reversal can be seen 45 minutes after a single oral dose of PSC 833. (B) Patient T12 shows incomplete reversal of Pgp in CD56⁺ cells. Virtually no effect is seen 2 hours after administration of PSC 833. The two patients shown received identical doses of PSC 833. The PSC 833 levels in the two patients after two hours are 3130 ng/mL (T7) and 249 ng/mL (T12).