Targeting PIK3CA Alterations in Hormone Receptor-Positive,Human Epidermal Growth Factor Receptor-2–Negative Advanced Breast Cancer: New Therapeutic Approaches and Practical Considerations

The phosphatidylinositol-3-kinase (PI3K) pathway is frequently dysregulated in human breast cancer. Approximately 30% of all patients with breast cancer will carry mutations of the PIK3CA gene, which encodes the PI3K catalytic subunit isoform p110α. Mutations in PIK3CA have been associated with resistance to endocrine therapy, HER2-directed therapy, and cytotoxic therapy. Early trials of pan-PI3K inhibitors showed little treatment benefit as monotherapy owing to disease resistance arising through enhanced estrogen receptor pathway signaling. Combining PI3K inhibition with endocrine therapy can help overcome resistance. Clinical trials of pan-PI3K inhibitors combined with endocrine therapy demonstrated modest clinical benefits but challenging toxicity profiles, facilitating the development of more selective PI3K-targeting agents. More recent trials of isoform-specific PI3K inhibitors in patients with PIK3CA mutations have shown promising clinical efficacy with a predictable, manageable safety profile. In the present review, we discuss the clinical relevance of mutations of PIK3CA and their potential use as a biomarker to guide treatment choices in patients with HR⁺ HER2^? advanced breast cancer.