Inhibitory Effects of Alkaloids on BCRP Implicated in Reversing Multidrug Resistance: A Case Example of Enhancing Temozolomide Cytotoxicity

Breast cancer resistance protein (BCRP), an important ATP-binding cassette transporter, is mainly responsible for drug efflux from cells, especially in high-expressing tumor cells, and is closely associated with multidrug resistance (MDR). Numerous studies have demonstrated that the inhibition of BCRP can reverse MDR, so inhibiting BCRP is considered to be a promising strategy for cancer treatment. Alkaloids are the primary bioactive ingredients in various traditional Chinese medicines (TCMs), some of which have been reported to reverse MDR by inhibiting BCRP. Our objective was to identify potential inhibitors of BCRP from 130 alkaloids, evaluate the reversion of MDR in TMZ-resistant U251T and T98G cells, and clarify the structure–activity relationships of alkaloids in BCRP inhibition. Among them, eight alkaloids, including sempervirine, reserpine, coptisine chloride, geissoschizine methyl ether, vincristine sulfate, tetrahydroberberine, cyclovirobuxine, and berberrubine, exhibited significant inhibition (>50%) of BCRP in BCRP-MDCK cells, with IC₅₀ ranging from 16.95–94.13 μM. Co-treatment with the inhibitor increased Temozolomide (TMZ) cytotoxicity in TMZ-resistant U251T and T98G cells, with IC₅₀ values declining by 2.1–97.3%. For sempervirine, coptisine chloride, and reserpine, the inhibition appeared to be even greater than the positive inhibitor KO143. Molecular docking analyses elucidated that the inhibitory effect of alkaloids on BCRP was related to π–π stacked, π–alkyl, and π–Sulfur interactions. The pharmacophore model illustrated that aromatic rings and hydrophobic groups may play a critical role in the potency of alkaloid inhibition on BCRP. Taken together, our findings provide valuable information for optimizing alkaloid structure and developing BCRP inhibitors with improved potency and specificity to reverse clinical MDR.