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Much effort has been spent on searching for better P-glycoprotein- (P-gp-) based multidrug resistance (MDR) modulators. Our approach was to target the binding sites of P-gp using dimers of dietary flavonoids. A series of apigenin-based flavonoid dimers, linked by poly(ethylene glycol) chains of various lengths, have been synthesized. These flavonoid dimers modulate drug chemosensitivity and retention in breast and leukemic MDR cells with the optimal number of ethylene glycol units equal to 2-4. Compound 9d bearing four ethylene glycol units increased drug accumulation in drug-resistant cells and enhanced cytotoxicity of paclitaxel, doxorubicin, daunomycin, vincristine, and vinblastine in drug-resistant breast cancer and leukemia cells in vitro, resulting in reduction of IC50 by 5-50 times. This compound also stimulated P-gp's ATPase activity by 3.3-fold. Its modulating activity was presumably by binding to the substrate binding sites of P-gp and disrupting drug efflux.

Here we showed that bivalency approach is effective in modulating multidrug resistance protein 1 (MRP1/ABCC1)-mediated doxorubicin (DOX) and etoposide (VP16) resistance in human 2008/MRP1 ovarian carcinoma cells. Flavonoid dimers bearing five or six ethylene glycol (EG) units with 6-methyl (4e, 4f) or 7-methyl (5e, 5f) substitution on the ring A of flavonoid dimers have the highest modulating activity for DOX against MRP1 with an EC(50) ranging from 73 to 133 nM. At 0.5 microM, the flavonoid dimer 4e was sufficient to restore DOX accumulation in 2008/MRP1 to parental 2008/P level. Lineweaver-Burk and Dixon plot suggested that it is likely a competitive inhibitor of DOX transport with a K(i) = 0.2 microM. Our data suggest that flavonoid dimers have a high affinity toward binding to DOX recognition site of MRP1. This results in inhibiting DOX transport, increasing intracellular DOX retention, and finally resensitizing 2008/MRP1 to DOX. The present study demonstrates that flavonoid dimers can be employed as an effective modulator of MRP1-mediated drug resistance in cancer cells.

Here we report a great improvement in reversal potency of cancer drug resistance when flavonoid dimers possess a functionally substituted aminopolyethylene glycol linker. The most potent compound, 18, contains a N-benzyl group at the linker. It has many advantages including (1) high potencies in reversing P-glycoprotein (P-gp) mediated resistance in LCC6MDR cells to various anticancer drugs with EC(50) in the nanomolar range, (2) low toxicity and high therapeutic index, and (3) preferential inhibition of P-gp over multidrug resistance protein 1 and breast cancer resistance protein. Compound 18 stimulates P-gp-ATPase activity by 2.7-fold and mediates a dose-dependent inhibition of doxorubicin (DOX) transport activity. Lineweaver-Burk and Dixon plots suggest that 18 is a competitive inhibitor to DOX in binding to P-gp with a K(i) of 0.28-0.34 μM and a Hill coefficient of 1.17. Moreover, the LCC6MDR cell displays about 2.1-fold lower intracellular accumulation of 18 compared to the wild type, suggesting that 18 is a P-gp substrate as well.