Emerging Strategies Targeting the PI3K/AKT/mTOR Pathway in HR+/HER2- Advanced Breast Cancer.
Hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) breast cancer accounts for approximately 70% of breast cancer cases. Despite recent advances with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i), resistance inevitably develops, often driven by activation of the phosphatidylinositol 3-kinase (PI3K)-AKT-mammalian target of rapamycin (mTOR) pathway. Genetic alterations such as PIK3CA mutations (present in ~ 45% of HR+/HER2- tumors), AKT1 mutations, and PTEN loss contribute to endocrine resistance and poor outcomes. This review summarizes emerging strategies targeting this pathway to overcome resistance in advanced disease. Isoform-specific PI3K inhibitors, including alpelisib and inavolisib, have demonstrated clinically meaningful progression-free survival benefits in PIK3CA-mutated populations, with inavolisib showing improved tolerability and efficacy. In contrast, pan-PI3K inhibitors such as buparlisib have been constrained by toxicity. Targeting downstream signaling, AKT inhibitors have also shown benefit: capivasertib has demonstrated clinical efficacy leading to US Food and Drug Administration approval, while ipatasertib has yielded encouraging results, particularly in tumors harboring PIK3CA, AKT1, or PTEN alterations. Mammalian target of rapamycin inhibitors, notably everolimus, have shown efficacy irrespective of mutation status. The dual PI3K-mTOR inhibitor (gedatolisib) has also shown promising progression-free survival benefit in a PIK3CA wild-type population. Next-generation agents, including mutant-selective PI3Kα inhibitors and bi-steric mTOR complex 1 inhibitors, are under active investigation. Optimal sequencing of these agents alongside endocrine therapy and CDK4/6i options remain a critical question, as does integration of genomic testing to guide therapy. Future directions include rational combination strategies, improved biomarker-driven selection, and novel modalities such as proteolysis-targeting chimeras (PROTACs). Collectively, these advances aim to enhance durability of response, minimize toxicity, and improve survival in HR+/HER2- metastatic breast cancer.