Abstract: AimTo study the effect of lomerizine (Lom) on the reversal of multidrug resistance (MDR) in K562/ADM cells and its mechanism. MethodsMTT assay was used to determine the influence of Lom on the cytotoxicity of adriamycin (ADM). The effect of Lom on the apoptosis induced by ADM and vincristine (VCR) in K562/ADM cells was detected using flow cytometry. Intracellular accumulation of ADM was measured by fluorescence spectrophotometry. Flow cytometry was used to investigate the efflux of rhodamine 123 (Rh123) and the expression of P-glycoprotein (P-gp) in K562/ADM cells. ResultsLom increased the cytotoxicity of ADM and the apoptosis induced by ADM or VCR in K562/ADM cells. At the concentration of 3, 10 and 30 μmol·L^-1, Lom reduced the IC₅₀ value of ADM from 79.03 μmol·L^-1 to 28.14, 8.16 and 3.16 μmol·L^-1, respectively. Lom increased the intracellular accumulation of ADM and inhibited the efflux of Rh123 in K562/ADM cells. No change in P-gp expression was observed after the treatment of Lom for 72 h. ConclusionLom had strong reversal effect on MDR in K562/ADM cells by inhibiting P-gp function.