Shrinking Spleens and Nodes, Expanding Horizons: Leniolisib and the Future of Activated Phosphoinositide 3-Kinase Delta Syndrome

Activated phosphoinositide 3-kinase delta syndrome (APDS) is an inborn error of immunity characterized by a combination of immunodeficiency with immune dysregulation and classified within autoimmune lymphoproliferative immunodeficiencies. Early therapeutic strategies extrapolated from autoimmune lymphoproliferative syndrome, including mechanistic target of rapamycin (mTOR) inhibition with sirolimus, provided variable control of lymphoproliferation. Recognition of constitutive phosphoinositide 3-kinase δ (PI3Kδ) signaling led to the development of leniolisib, approved by the US Food and Drug Administration in 2023, marking a transition toward pathway-specific precision therapy. In this Clinical Perspective, we review the evolution of targeted therapy in APDS, the biochemical structure and selectivity of leniolisib relative to other PI3K inhibitors, and the mechanistic distinction between proximal PI3Kδ inhibition and downstream mTOR blockade. We summarize current clinical evidence and practical considerations for dosing and monitoring in the absence of serum drug-level assays. Finally, we outline key unresolved questions, including long-term disease modification, biomarker validation, pediatric outcomes, treatment sequencing and combination strategies, while positioning leniolisib relative to hematopoietic stem cell transplantation, and desirability of its wider global access.