Leniolisib (CDZ 173) is primarily used in the treatment of activated phosphoinositide 3-kinase delta syndrome (APDS), a rare genetic disorder. It works by selectively inhibiting the PI3Kδ enzyme, which is overactive in APDS patients, helping to regulate immune system function and reduce symptoms such as recurrent infections, lymphoproliferation, and immune dysregulation. This targeted therapy offers a promising approach to managing the condition and improving the quality of life for affected individuals.