TTT-28 is a synthesized thiazole-valine peptidomimetic, a novel selective inhibitor of ABCB1 (P-gp/MDR1) with high efficacy and low toxicity, which reverses the ATP-binding cassette sub-family B member 1 (ABCB1)-mediated Multidrug resistance (MDR) by selectively blocking the efflux function of ABCB1.In Vitro: TTT-28 (0-100?μM; 72 hours) reverses ABCB1-mediated MDR in drug selected SW620/Ad300 cells and transfected HEK293/ABCB1 cells; the IC50s of TTT-28 in CCD-18Co, SW620 and SW620/Ad300 cells are 213.4±11.0 μM, 89.4±3.9 μM and 92.0±5.0?μM, respectively.TTT-28 (10 μM; 2 hours) raises the ABCB1-mediated MDR and increased the accumulation of [3H]-paclitaxel in ABCB1 overexpressing cells.TTT-28 (10 μM; 0-72 hours) does not interfer with the expression level and localization of ABCB1, it results from blocking the efflux function of ABCB1.TTT-28 (0-40?μM; 2 hours) interacts at the drug-substrate-binding site and actives the ATPase activity of ABCB1 in a concentration-dependent fashion.In Vivo: TTT-28 (deliver orally; 30 mg/kg; every 3 rd day; 18 days) potentiates the anticancer activity of paclitaxel due to its inhibitory effect on the efflux function of ABCB1, it enhances the inhibitory effect of paclitaxel on the growth of SW620/Ad300 tumor and promoted apoptosis.