PIK3CA in Cancer: Structure, Biology, Alterations, and Actionability.

PIK3CA, which encodes the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K), is one of the most frequently altered oncogenes in human cancer and a major driver of tumor initiation, progression, metastasis, and therapeutic resistance. Over the past two decades, advances in structural biology, cancer genomics, and translational research have substantially expanded our understanding of PIK3CA function and established the PI3K pathway as a clinically actionable therapeutic target. This review provides an overview of the structural organization and physiological functions of the PI3Kα complex, the molecular mechanisms underlying oncogenic activation, and the diverse spectrum of PIK3CA alterations across human malignancies. We also summarize the current landscape of PI3K-targeted therapies, highlighting both approved agents and emerging therapeutic strategies. Clinical evidence supports the rational integration of PI3K inhibitors with endocrine therapy, CDK4/6 inhibitors, MAPK pathway inhibitors, dual PI3K/mTOR inhibition, and immune checkpoint blockade. In addition, accumulating evidence indicates that PIK3CA plays a pivotal role in shaping the tumor immune microenvironment, providing a biological rationale for combining PI3K inhibition with immunotherapy. Finally, we discuss future directions in precision oncology, emphasizing integrated molecular profiling, liquid biopsy, single-cell and spatial technologies, functional genomics, and evolutionary approaches as complementary strategies to refine patient selection, overcome therapeutic resistance, and optimize clinical outcomes.