P-Glycoprotein (P-gp) overexpression is a major mechanism by which cancer cells acquire the multidrug resistance (MDR) phenotype, and is associated with poor clinical outcome in patients. In an effort to develop MDR-reversal agents, we synthesized and evaluated a series of thiophenylbenzofuran derivatives (4-31) as P-gp inhibitors, among which compounds 4, 10, and 14 represented the optimal agent in reversing the MDR phenotype. These P-gp inhibitors were dramatically effective than verapamil in sensitizing the human ABCB1-overexpressing ABCB1/Flp-In™-293 cells and MDR KBvin cells to a series of chemotherapeutic agents, including vincristine and paclitaxel, as manifested by multi-fold decreases in the respective IC values to therapeutically attainable levels.