A total of forty novel glycyrrhetinic acid (GA) derivatives were designed and synthesized. The cytotoxic activity of the novel compounds was tested against two human breast cancer cell lines (MCF-7, MDA-MB-231) in vitro by the MTT method. The evaluation results revealed that, in comparison with GA, compound 42 shows the most promising anticancer activity (IC₅₀ 1.88 ± 0.20 and 1.37 ± 0.18 µM for MCF-7 and MDA-MB-231, respectively) and merits further exploration as a new anticancer agent.

Multidrug resistance (MDR) is a major obstacle for successful cancer chemotherapy, and the main cause of MDR has been attributed to overexpression of P-glycoprotein (P-gp). In this present study, four P-gp modulators (E,E)-4,6-bis(styryl)-2-(substituted amino)-pyrimidines were evaluated for their activity in a breast cancer cell line overexpressing P-gp (LCC6MDR). The four modulators displayed significantly better P-gp modulating activity compared with the positive control verapamil (RF = 5.4), with a relative fold (RF) increase in activity ranging from 33.3 to 86.0. In contrast to compounds a and c that exhibited lower cytotoxicity, compounds b and d were nontoxic towards both cancer cells and normal cells, with IC₅₀ values greater than 100 μmol/L. The qRT-PCR results demonstrated that after exposure to 2 μmol/L of compounds a, b, c, and d, the mRNA expression level of MDR1 in LCC6MDR cells decreased to 45%, 50%, 38%, and 51%, respectively. However, the Western-blot results indicated that compound c could reverse P-gp mediated MDR, but not via decreases in protein expression. DOX and Rh123 accumulation and efflux results further confirmed that the reversal of MDR activity happens via inhibition of P-gp efflux and increases in intracellular drug accumulation. These results demonstrated that compound c has low toxicity and is an efficient P-gp modulator, highlighting its potential as a promising candidate for P-gp-mediated reversal of MDR.