Lapatinib is active at the ATP-binding site of tyrosine kinases that are associated with the human epidermal growth factor receptor (Her-1 or ErbB1) and Her-2. It is conceivable that lapatinib may inhibit the function of ATP-binding cassette (ABC) transporters by binding to their ATP-binding sites. The aim of this study was to investigate the ability of lapatinib to reverse tumor multidrug resistance (MDR) due to overexpression of ABC subfamily B member 1 (ABCB1) and ABC subfamily G member 2 (ABCG2) transporters. Our results showed that lapatinib significantly enhanced the sensitivity to ABCB1 or ABCG2 substrates in cells expressing these transporters, although a small synergetic effect was observed in combining lapatinib and conventional chemotherapeutic agents in parental sensitive MCF-7 or S1 cells. Lapatinib alone, however, did not significantly alter the sensitivity of non-ABCB1 or non-ABCG2 substrates in sensitive and resistant cells. Additionally, lapatinib significantly increased the accumulation of doxorubicin or mitoxantrone in ABCB1-or ABCG2-overexpressing cells and inhibited the transport of methotrexate and E 2 17BG by ABCG2. Furthermore, lapatinib stimulated the ATPase activity of both ABCB1 and ABCG2 and inhibited the photolabeling of ABCB1 or ABCG2 with [ 125 I]iodoarylazidoprazosin in a concentration-dependent manner. However, lapatinib did not affect the expression of these transporters at mRNA or protein levels. Importantly, lapatinib also strongly enhanced the effect of paclitaxel on the inhibition of growth of the ABCB1-overexpressing KBv200 cell xenografts in nude mice. Overall, we conclude that lapatinib reverses ABCB1-and ABCG2-mediated MDR by directly inhibiting their transport function. These findings may be useful for cancer combinational therapy with lapatinib in the clinic.

Apatinib, a small-molecule multitargeted tyrosine kinase inhibitor, is in phase III clinical trial for the treatment of patients with non-small-cell lung cancer and gastric cancer in China. In this study, we determined the effect of apatinib on the interaction of specific antineoplastic compounds with P-glycoprotein (ABCB1), multidrug resistance protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2). Our results showed that apatinib significantly enhanced the cytotoxicity of ABCB1 or ABCG2 substrate drugs in KBv200, MCF-7/adr, and HEK293/ABCB1 cells overexpressing ABCB1 and in S1-M1-80, MCF-7/FLV1000, and HEK293/ ABCG2-R2 cells overexpressing ABCG2 (wild-type). In contrast, apatinib did not alter the cytotoxicity of specific substrates in the parental cells and cells overexpressing ABCC1. Apatinib significantly increased the intracellular accumulation of rhodamine 123 and doxorubicin in the multidrug resistance (MDR) cells. Furthermore, apatinib significantly inhibited the photoaffinity labeling of both ABCB1 and ABCG2 with [ 125 I] iodoarylazidoprazosin in a concentration-dependent manner. The ATPase activity of both ABCB1 and ABCG2 was significantly increased by apatinib. However, apatinib, at a concentration that produced a reversal of MDR, did not significantly alter the ABCB1 or ABCG2 protein or mRNA expression levels or the phosphorylation of AKT and extracellular signal-regulated kinase 1/2 (ERK1/2). Importantly, apatinib significantly enhanced the effect of paclitaxel against the ABCB1-resistant KBv200 cancer cell xenografts in nude mice. In conclusion, apatinib reverses ABCB1-and ABCG2-mediated MDR by inhibiting their transport function, but not by blocking the AKT or ERK1/2 pathway or downregulating ABCB1 or ABCG2 expression. Apatinib may be useful in circumventing MDR to other conventional antineoplastic drugs.