Abstract 1737: Overcoming trastuzumab resistance with the irreversible Pan-HER inhibitor neratinib

Abstract Overexpression of HER2 is seen in 15-20% of breast cancer and is a predictor of poor prognosis. Trastuzumab, a monoclonal antibody to the ECD of the HER2 receptor, has been shown to increase survival in patients especially in combination with chemotherapy. However, a significant number of patients do not respond to trastuzumab treatment and resistance develops in those given trastuzumab as a monotherapy. The purpose of this study was to investigate whether the small molecule irreversible tyrosine kinase inhibitor neratinib (HKI-272) could be used to overcome resistance in trastuzumab resistant BT474 and SKBR3 cell lines and to look at the effects of neratinib on trastuzumab naive cell lines. The effects of acute trastuzumab, neratinib, and trastuzumab and neratinib combination treatments were investigated in SKBR3 and BT474 cells. It has already been established that trastuzumab does not abolish pHER2 expression and this has been implicated in trastuzumab resistance. Western blotting showed an increase in pHER2 and pERK with acute trastuzumab treatment. Conversely, acute neratinib treatment alone and in combination with trastuzumab reduced pERK and pHER2 expression. Furthermore, although acute treatment with trastuzumab decreased the expression of pHER3 and pAKT, the combination of neratinib with trastuzumab gave a much greater decrease in expression. Fluorescence microscopy was also performed on both the resistant and native SKBR3 cells after acute trastuzumab, neratinib and combination treatments. Trastuzumab treatment increased pHER2 membrane localisation in both native and resistant cells in a dose dependent manner, whilst neratinib treatment or its combination with trastuzumab reduced membrane pHER2. Cell viability experiments on BT474 and SKBR3 cells showed that neratinib treatment was much more effective at reducing cell viability than trastuzumab(p=0.0078, p=0.0499) and that combined trastuzumab and neratinib treatment was more effective at reducing cell viability than neratinib alone (p=0.0028, p=0.0148). In the trastuzumab resistant SKBR3 and BT474 cells, withdrawal of trastuzumab lead to an increase in cell viability. However, neratinib treatment was able to reverse trastuzumab resistance and significantly reduce cell viability (p=0.0022, p=0.0087). Furthermore, combined treatment with trastuzumab and neratinib was more effective at reducing cell viability than neratinib alone(p=0.0087, p=0.0260) suggesting that trastuzumab treatment should be continued even when resistance develops. These results showed that the combined treatment with neratinib and trastuzumab was more effective in the inhibition of pHER2, pHER3, pAkt and pERK, and led to a greater decrease in cell viability than trastuzumab treatment alone. We would therefore recommend neratinib to be used in combination with trastuzumab in both trastuzumab naive and resistant HER2 over-expressing breast cancer patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1737. doi:10.1158/1538-7445.AM2011-1737