MDR chemosensitization. In the upper part of the figure the dose-response growth curves of drug sensitive parental cell lines (CCRF-CEM and HL60) and their derivative MDR variants (CEM-VBL100 and HL60/DNR) are shown (Panel A). The cells were cultured for 72 h in medium containing increasing concentrations of VBL alone (open circles), VBL plus 12.5 μg/mL of the IIN RALT (open triangles), and VBL plus 2.5 μg/mL of the MDR1-Pgp blocker Vrp (open squares). In the lower part of the figure (Panel B), the IC50 values (concentrations of the compound that inhibits cell growth by 50%) for each cell culture condition are reported. Values are means of three independent experiments, each done in triplicate.