Description: Leniolisib phosphate (formerly known as CDZ173 phosphate; Joenja) is a novel, potent and selective PI3K (phosphatidylinositol 3-kinase inhibitor) inhibitor with IC50 values of 0.244, 0.424, 2.23 and 0.011 μM for PI3Kα, PI3Kβ, PI3Kγ and PI3Kδ, respectively. It is currently in phase II/III clinical trials for the treatment of immunodeficiency disorders. Oral leniolisib led to a dose-dependent reduction in PI3K/AKT pathway activity assessed ex vivo and improved immune dysregulation. Leniolisib was well tolerated and improved laboratory and clinical parameters in APDS, supporting the specific inhibition of PI3Kδ as a promising new targeted therapy in APDS and other diseases characterized by overactivation of the PI3Kδ pathway. This trial was registered at www.clinicaltrials.gov as #NCT02435173. In Mar 2023, it has been approved by FDA for the treatment of activated phosphoinositide 3-kinase delta syndrome (APDS), a rare primary immunodeficiency.