The efficacy of B4112 against multidrug resistant cancer cell lines was examined in vitro.The effects of B4112 (0.01-1 μg/mL) alone and in the presence of vinblastine (3 ng/mL) were examined on the proliferation of a P-glycoprotein-expressing erythroleukemic cell line.At 8 ng/mL, B4112 caused 50% restoration of chemosensitivity of this cancer cell line to vinblastine, while the corresponding concentration of B4112, which caused 50% direct cytotoxicity, was 200 ng/mL.B4112 inhibited growth of colon, liver, esophageal, and prostate cancer cell lines with varying patterns of intrinsic multidrug resistance by 50% at 0.29-0.98 μg/mL after exposure for 3-4 days.