Aureobasidins as new inhibitors of P-glycoprotein in multidrug resistant tumor cells.

Cyclic depsipeptide antibiotic aureobasidin A (AbA) and its analogs were tested for the inhibitory activity of P-glycoprotein in multidrug resistant cancer cells as well as for the antifungal activity. Some analogs with lower antifungal activity than AbA showed higher inhibition of P-glycoproteins indicating difference of the structure-activity relationships between the two activities. Among AbA analogs tested, [D-beta-hydroxy-methylvalyl9]-AbA newly prepared by chemical synthesis, which had much lower antifungal activity than AbA, showed 10-fold higher inhibitory activity of P-glycoprotein than AbA.