Uncaria alkaloids reverse ABCB1-mediated cancer multidrug resistance.

The overexpression of ATP-binding cassette (ABC) transporters is the main cause of cancer multidrug resistance (MDR), which leads to chemotherapy failure. Uncaria alkaloids are the major active components isolated from uncaria, which is a common Chinese herbal medicine. In this study, the MDR-reversal activities of uncaria alkaloids, including rhynchophylline, isorhynchophylline, corynoxeine, isocorynoxeine (Icory), hirsutine and hirsuteine, were screened; they all exhibited potent reversal efficacy when combined with doxorubicin. Among them, Icory significantly sensitized ABCB1-overexpressing HepG2/ADM and MCF-7/ADR cells to vincristine, doxorubicin and paclitaxel, but not to the non-ABCB1 substrate cisplatin. Noteworthy, Icory selectively reversed ABCB1-overexpressing MDR cancer cells but not ABCC1- or ABCG2-mediated MDR. Further mechanistic study revealed that Icory increased the intracellular accumulation of doxorubicin in ABCB1-overexpressing cells by blocking the efflux function of ABCB1. Instead of inhibiting ABCB1 expression and localization, Icory acts as a substrate of the ABCB1 transporter by competitively binding to substrate binding sites. Collectively, these results indicated that Icory reversed ABCB1-mediated MDR by suppressing its efflux function, and it would be beneficial to increase the efficacy of these types of uncaria alkaloids and develop them to be selective ABCB1-mediated MDR-reversal agents.