Pyrotinib Sensitizes 5-Fluorouracil-resistant HER2+ Breast Cancer Cells to 5-Fluorouracil.

Fluorouracil (5-FU) is a widely used chemotherapeutic agent for breast cancer. However,acquired chemoresistance leads to a loss of its efficacy; methods to reverse are urgentlyneeded. Some studies have shown that pyrotinib, an ErbB receptor tyrosine kinaseinhibitor, is effective against HER2+ breast cancer. However, whether pyrotinibsensitizes 5-FU-resistant breast cancer cells to 5-FU is unknown. We hypothesized thatthe combination of pyrotinib and 5-FU would show synergistic antitumor activity andpyrotinib could reverse 5-FU resistance in HER2+ breast cancer cells in vitro and in vivo.Our data showed that pyrotinib inhibited the growth of 5-FU-resistant SKBR-3/FU andMDA-MB-453/FU cell lines and the parental cell lines. 5-FU remarkably suppressed thegrowth of SKBR-3 and MAD-MB-453 cells. However, SKBR-3/FU and MAD-MB-453/FU cells showed resistance to 5-FU. A combination of pyrotinib and 5-FU resultedin the synergistic inhibition of the growth of the 5-FU-resistant SKBR-3/FU and MDA36MB-453/FU cell lines and the parental cell lines. Pyrotinib decreased significantly theIC50 values of 5-FU and the TS mRNA expression levels in the 5-FU-resistant SKBR-3/FU and MDA-MB-453/FU cell lines and the parental cell lines and increasedsignificantly the intracellular concentration of 5-FU in SKBR-3/FU and MDA-MB-453/FU cells. In addition, pyrotinib reduced the ABCG2 mRNA and protein expressionlevels in SKBR-3/FU and MDA-MB-453/FU cells and downregulated the proteinexpression levels of pAKT, pHER-2, and pHER-4 in all four cell lines. After TS orABCG2 in 5-FU-resistant breast cancer cells were knocked down, the sensitivity ofSKBR-3/FU and MDA-MB-453/FU cells 44 to 5-FU was restored. Moreover, in vivoexperiments demonstrated that pyrotinib in combination with 5-FU more effectivelyinhibited SKBR-3/FU tumor growth than either pyrotinib or 5-FU alone. In conclusion,our findings suggest that pyrotinib could restore sensitivity of 5-FU-resistant HER2+breast cancer cells to 5-FU through downregulating the expression levels of TS andABCG2.