Effect of multidrug resistance reversal agents on GO-induced cytotoxicity in Pgp-positive/MRP-positive TF1 cells. Exposure of TF1 cells to GO for 2 hours did not change the frequency of apoptotic (annexin V⁺/PI^–) cells (A) or necrotic (PI⁺) cells (B) after 72 hours. CSA alone (1.25 μg/mL) enhanced GO-induced apoptosis and cell death, whereas MK-571 (10 μM) alone was ineffective. The combination of CSA and MK-571, however, significantly increased the percentage of necrotic cells compared with the effects of CSA alone. Results are shown as mean ± SEM from 3 individual experiments performed in triplicate wells.