Effect of MK-571 on GO-induced cytotoxicity in Pgp-negative/MRP-positive cell lines. Exposure of HL-60 and NB4 cells to GO for 2 hours resulted in dose-dependent cytotoxicity, as reflected by decreased viable cell numbers measured by the MTS assay (A), and increased percentages of apoptotic (annexin V⁺/PI^–) and necrotic (PI⁺) cells (B-C). The MRP inhibitor MK-571 (5 μM) increased GO-induced apoptosis and necrosis in both cell lines at 72 hours and further decreased HL-60 cell numbers compared with controls. Results are shown as mean ± SEM from 4 to 7 individual experiments performed in quadruplicate (A) or triplicate (B-C) wells.