A series of novel N-arylalkyl-3,4-diaryl-substituted pyrrole-2,5-diones were synthesized. They exhibited promising P-gp modulating activity in a P-gp overexpressing breast cancer cell line (LCC6MDR). Compound 6 (with three methoxy groups at D-ring) displayed the highest P-gp modulating activity. 6 at 1 microM can sensitize LCC6MDR cells toward paclitaxel by 18.2-fold. Interestingly, a synergy on modulating P-gp was noted when 6 and 25 were used together (fractional inhibitory concentration index FICI = 0.42). Combination of 6 (0.5 microM) and 25 (0.5 microM) resulted in a 66-fold sensitization of LCC6MDR cells toward paclitaxel. They also reversed P-gp mediated doxorubicin (DOX) and vincristine resistance. Kinetic characterization suggests that permethyl ningalin B analogues likely act as a noncompetitive inhibitor of P-gp-mediated DOX transport (K(i) = 5.4-5.8 microM). The present study demonstrates that synthetic analogues of permethyl ningalin B can be employed as effective and safe modulators of P-gp-mediated drug resistance in cancer cells.