An orally bioavailable, small molecule inhibitor of the delta isoform of the 110 kDa catalytic subunit of class I phosphoinositide-3 kinase (PI3K) with potential immunomodulating and antineoplastic activities. Idelalisib inhibits the production of the second messenger phosphatidylinositol-3,4,5-trisphosphate (PIP3), preventing the activation of the PI3K signaling pathway and inhibiting tumor cell proliferation, motility, and survival. Unlike other isoforms of PI3K, PI3K-delta is expressed primarily in hematopoietic lineages. The targeted inhibition of PI3K-delta is designed to preserve PI3K signaling in normal, non-neoplastic cells. or A drug used with rituximab to treat chronic lymphocytic leukemia (CLL) that has come back. It is also used to treat follicular B-cell non-Hodgkin lymphoma (NHL) and small lymphocytic lymphoma (SLL) that have come back after treatment with other anticancer therapy. It is also being studied in the treatment of other types of cancer. Idelalisib blocks certain proteins, which may help keep cancer cells from growing and may kill them. It is a type of kinase inhibitor. Also called Zydelig.