Association between PIK3CA mutations and disease-free survival in patients with HR+, HER2- tumors treated with adjuvant hormonal therapy: A real-world study in Croatia.

e12512 Background: Disease recurrence of early hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) breast tumor subtype is particularly challenging to manage due to its complex and very heterogeneous biological nature. Namely, due to primary and secondary resistance, one-quarter of patients with early disease will present with the disease recurrence. This variability in the timing of recurrence highlights the need to better identify key biomarkers that could predict therapeutic outcomes and guide personalized treatment strategies for these patients. Mutations in the phosphatidylinositol 4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) gene have a high prevalence of 30% to 40% in HR+/HER2- advanced breast cancer. They lead to activation of the PI3K/AKT/mTOR pathway, promoting cell growth, and proliferation, and are associated with poor prognosis in advanced breast cancer. Our aim was to examine this association and impact of PIK3CA mutation status on disease free survival (DFS) in HR+/ HER2- early breast cancer patients. Methods: We conducted a multicentric, retrospective cohort study in five Croatian institutions from July 2020 to December 2021. Study included initially early HR+/HER2- breast cancer patients who have been diagnosed with disease recurrence during adjuvant hormonal treatment or within first six years of follow up. The Cobas PIK3CA Mutation Testing Kit was used for real-time PCR-based amplification and mutation detection on the Cobas z 480 analyzer. We have analyzed differences in DFS from initiation of adjuvant endocrine therapy, using Cox, proportional hazard regression. Results: There was total of 186 patients, out of which 40.9% tested positive for the PIK3CA mutation. Primary and adjuvant treatment and in particular adjuvant endocrine treatment was similar between the two groups. After adjustment for 14 relevant covariates, we found that patients with positive PIK3CA status and with H1047 PIK3CA mutation had significantly lower hazard for disease recurrence in comparison to patients with no PIK3CA mutation (HR 0.65; 95% CI 0.45; 0.95; p = 0.024; HR 6.52; 95% CI 1.15; 36.95; p = 0.034 respectively; false discovery rate, FDR < 10%) and higher odds for late recurrence (p = 0.001, FDR < 5%). Conclusions: This study highlights the potential impact of PIK3CA mutations on disease recurrence during/following adjuvant endocrine treatment and by that potentially opens the door for further investigation of possibly more personalized treatment strategies.