Monitoring of PIK3CA and ESR1 mutations in circulating tumor DNA as predictive and prognostic biomarkers in patients with endocrine-resistant ER+/HER2- advanced breast cancer

e13045

Background: Combination therapeutic strategies including CDK4/6 inhibitors and an endocrine backbone are the standard of care of treatment for patients with estrogen receptor positive (ER+)/HER2- advanced breast cancer (ABC). Endocrine agents mainly include aromatase inhibitors, which target ER-driven transcription, and fulvestrant, which functions as ER antagonist. PI3K-AKT-mTOR is a key point of resistance to endocrine therapy, activated in 40% of these patients by mutations concentrated in critical regions of PIK3CA, coding for the p110 catalytic subunit α of PI3K. Additionally, 30% of patients previously exposed to non-steroidal aromatase inhibitors develop mutations in the ligand binding domain of ESR1, causing endocrine resistance by constitutive activation of the ER. Furthermore, metastasis and primary tumors may show a highly heterogenous mutational landscape. Monitoring the dynamic changes of these mutations in ctDNA may provide a non-invasive, real-time and accessible tool to convey predictive/prognostic information and guide decisions on sequential endocrine therapies. Methods: Pre-planned interim analysis results of an observational, prospective cohort pilot study to assess the predictive and prognostic value of monitoring PIK3CA and ESR1 mutations in ctDNA of patients with endocrine-resistant ER+/HER2- ABC. We have studied longitudinal liquid biopsies from 30 patients using digital PCR to interrogate PIK3CA mutations (H1047R / E545K) and ESR1 mutations (D538G / Y537S). Blood samples were drawn at the time of progression to endocrine therapy, at 8 weeks of subsequent endocrine line and at new progression. This exploratory analysis will provide preliminary data on clinical homogeneity, treatment regimens, median follow-up and progression-free survival, mutation incidence and intraindividual variation of mutant allele frequency and copy number. The percentage of progressors at 24 weeks of follow-up according to the mutational status will be evaluated by using the Fisher’s exact test. The predictive potential of ctDNA biomarkers will be characterized by ROC curve analysis. Tracking ctDNA mutations to predict endocrine resistance in a real-world setting represents a critical step towards precision medicine in oncology.