Next-generation sequencing methodologies to identify patients for targeted therapy: focus on HR+/HER2- metastatic breast cancer

Alterations in the phosphoinositide 3-kinase (PI3K)/AKT/PTEN signaling pathway are a well-recognized mechanism of resistance in hormone receptor-positive, HER2-negative metastatic breast cancer (HR+/HER2-mBC). These alterations are present in approximately half of patients with HR+/HER2-mBC. The major alterations in the pathway are somatic mutations in the PIK3CA (40-45%) and AKT1 (5%) genes, and loss-of-function alterations in PTEN (5-10%). New targeted agents that act against these alterations have been developed. Therefore, it is important to determine the mutational status of genes in this pathway to potentially offer a therapeutic alternative for these patients. In this review, we discuss the clinical and biological significance of PI3K pathway alterations in HR+/HER2-mBC, focusing on tumors that progress following endocrine therapy and CDK4/6 inhibitor treatment. We then highlight how different diagnostic strategies, including sample type, testing methodology, and timing, can improve the identification of patients who are eligible for targeted therapies and promote the effective integration of molecular diagnostics into routine clinical care.