Sipholenol A (84), sipholenol I (91), sipholenol L (94), sipholenone A (95), sipholenone E (96), siphonellinol C (97), and siphonellinol D (98) have found to show potent reversal of multidrug resistance in cancer cells that over expressed P-glycoprotein. The gene discussed is ABCB1; the disease is cancer.