pBR expression is not necessary for efflux inhibition by PK11195 in MDR cells. (A) pBR-deficient Jurkat-cell derivatives were produced that overexpress Pgp (J-MDR) as demonstrated with MRK16 anti-Pgp antibody staining. Representative flow cytometry data are shown as histograms, and summary data from at least 3 assays are shown as mean arbitrary fluorescence units (a.f.u.). Both CSA (▨) and PK11195 (▪) blocked DiOC₂³ and MIT efflux in J-MDR cells but not in parental Jurkat cells or control J-Neo cells, demonstrating specific efflux capacity in J-MDR cells and demonstrating PK11195's ability to block Pgp-mediated efflux in pBR-deficient cells. (B) Derivative Jurkat cells that overexpress BCRP (J-BCRP) were produced and characterized with BXP-34 anti-BCRP antibody staining. CSA (▨) and Ko143 (KO-143) (□) both increased MIT dye retention in J-BCRP cells, and PK11195 (▪) significantly blocked both MIT and Hoechst efflux in J-BCRP cells. Neither Hoechst 33342 nor MIT efflux was measured in control transporter-deficient cells, demonstrating specific efflux capacity in J-BCRP cells and demonstrating PK11195's ability to block BCRP-mediated efflux in pBR-deficient cells. Efflux data from at least 3 assays are presented, as described for Figure 1.