Unraveling the synergistic mechanisms of pyrotinib in combination with trastuzumab: Insights from the PHILA trial.

Abstract

e13015 Background: There are many options for the treatment of advanced HER2-positive breast cancer. The results of PHILA study suggest that pyrotinib, trastuzumab and docetaxel may be considered as a potential first line treatment for patients with untreated HER2-positive metastatic breast cancer. We aimed to investigate the synergistic mechanisms of pyrotinib plus trastuzumab and compare them to trastuzumab plus pertuzumab, providing further insights for the PHILA trial. Methods: The efficacy of combination treatments was detected by cell vitality assay, Western blot and immunohistochemistry. The relevant mechanism was examined by flow cytometry and co-immunoprecipitation. Cell-derived xenografts and TUBO tumor model were utilized to evaluate the anti-tumor activity of rational combinations. Next-generation sequencing of circulating tumor DNA from a cohort of 18 patients in the PHILA trial was performed to evaluate molecular alterations associated with clinical response. Results: We demonstrated that pyrotinib plus trastuzumab superiorly inhibited cell growth than trastuzumab plus pertuzumab in models of HER2-dependent breast cancer. In addition to inhibiting HER family phosphorylation, pyrotinib plus trastuzumab synergistically reduced membrane HER2 levels by inducing receptor ubiquitination and internalization via HSP90 dissociation, which resulted in a comprehensive blockade of HER signaling pathway. Moreover, TUBO tumor model was utilized to further confirm the synergistic effects of pyrotinib and anti-neu antibody on HER2 signaling cascade. In both treatment groups of PHILA study, patients with aberrant mutations in HER2 signaling cascade showed a significantly shorter median progression-free survival compared to individuals with wild-type pathway. Correspondingly, in models of HCC1954 breast cancer cell harboring PIK3CA H1047R mutation, we did not observe a synergy between pyrotinib and trastuzumab. Conclusions: Pyrotinib and trastuzumab synergistically achieve the comprehensive blockade of HER signaling pathway in HER2-dependent breast cancer. Patients with advanced HER2-positive breast cancer, who are addicted to HER2 signaling cascade, could be considered for pyrotinib in combination with trastuzumab and docetaxel as an alternative first line treatment.