Efflux of rhodamine from CD56(+) cells as a surrogate marker for reversal of P-glycoprotein-mediated drug efflux by PSC 833

The expression of high levels of P-glycoprotein (Pgp) in circulating mononu clear cells allowed us to use an ex vivo assay as a surrogate measure of Pg p antagonism. Efflux of rhodamine from CD56(+) cells was measured before th e start of PSC 833 and at varying times thereafter. Patients receiving PSC 833 had decreased rhodamine efflux from their circulating CD56(+) cells. Ti me course studies showed that following a single oral dose of PSC 833, decr eased rhodamine efflux was found in some patients within 15 minutes of trea tment. Maximal inhibition was observed at times ranging from 45 minutes to 60 minutes, A dose-response relationship was shown between the concentratio n of PSC 833 in the blood and the inhibition of rhodamine efflux, with an a pparent plateau of the inhibition of rhodamine efflux at approximately 1,00 0 ng/mL. The Ki, defined as the concentration required for half-maximal inh ibition of Pgp-mediated rhodamine efflux, was determined to be in the range of 29 to 181 ng/mL; although results in two patients were distinctly diffe rent, with Ki values of 914 and 916 ng/mL. MRK-16 staining was similar amon g all patients. We conclude that measurement of rhodamine efflux from CD56( +) cells provides a surrogate assay with the potential for monitoring Pgp a ntagonism in clinical trials. This is a US government work. There are no re strictions on its use.