Abstract 347: Identification of poziotinib alone or in combination with TDM1 as a pan-HER2 inhibitor (2019)

Clinical studies of HER2 targeting agents have shown mutational variant & cancer-specific differences in patient outcomes. Furthermore, the drug sensitivity profiles of HER2 mutational variants have not been fully characterized. To this end, we expressed the 16 most frequently detected HER2 mutations across exons 19-21 into Ba/F3 cells & determined activating potential & drug sensitivity to 13 HER1/2 targeting agents including afatinib, neratinib, dacomitinib, tarloxotinib-TKI, poziotinib, pyrotinib, TDM-1, & trastuzumab. Third generation EGFR TKIs, osimertinib, ibrutinib, & nazartinib were not effective at inhibiting cell viability in cells expressing exon 20 mutations; however, 3rd generation TKIs demonstrated activity against cells expressing D769 exon 19 variants & exon 21 variants. By comparison, covalent, quinazoline-based TKIs, afatinib, neratinib, dacomitinib, tarloxotinib-TKI, & poziotinib, differentially inhibited HER2 mutants across all three exons. Across all HER2 mutation variants & TKIs tested, poziotinib had the lowest average IC50 (1.73nM) & was significantly more effective in reducing cell viability than neratinib & tarloxotinib-TKI (p<0.001 & p=0.018). In addition, while poziotinib was more efficacious than either neratinib or tarloxotinib-TKI against HER2 exon 19 & 20 mutations, there was no significant difference in average IC50 for exon 21 mutants, suggesting that mutation location impacts drug binding. Furthermore, within exon 19, L755S & L755P variants had significant differences in drug sensitivity across all TKIs tested, & L755P & exon 20 insertion mutations were refractory to most inhibitors, but remained sensitive to poziotinib. We next tested the anti-tumor activity of HER2 targeting agents in human breast (MCF10A), colorectal (CW-2), & kidney (TUHR14TKB) cell lines with HER2 mutations in exons 19-20 & observed that poziotinib was the most potent inhibitor of the 13 TKIs tested. Furthermore, in a CW-2 xenograft model, poziotinib treatment resulted in a 58% reduction in tumor volume compared to the control group (p=0.011). In comparison, neratinib & afatinib treatment did not reduce tumor growth compared to control animals. Lastly, in a HER2 mutant NSCLC PDX model (HER2 Y772dupYVMA), the combination of low dose poziotinib (2.5mpk) & a single dose of TDM1 (10mpk) resulted in complete tumor regression in 9/9 mice, compared to 2/9 mice receiving TDM1 alone or 0/9 mice receiving low dose poziotinib (p<0.0001). In conclusion, HER2 exon 20 insertions & L755P mutations are resistant to the majority of HER2 TKIs, but remain sensitive to poziotinib. Combination of low dose poziotinib & TDM1 caused complete tumor regression in a HER2 exon 20 mutant PDX model. These data suggest that poziotinib alone & in combination with TDM1 has activity against the most common HER2 variants across diverse malignancies & that clinical studies testing these agents in HER2 mutant cancers are warranted.Citation Format: Jacqulyne P. Robichaux, Monique B. Nilsson, Fahao Zhang, Limei Hu, Junqin He, Marlese Pisegna, Alissa Poteete, John V. Heymach. Identification of poziotinib alone or in combination with TDM1 as a pan-HER2 inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 347.