The characterization of idelalisib binding to PI 3 K δ 1 Structural , biochemical and biophysical characterization of idelalisib binding to phosphoinositide 3-kinase delta *

Idelalisib (also known as GS-1101, CAL101, IC489666 and Zydelig) is a phosphoinositide 3-kinase delta (PI3Kδ) inhibitor that has recently been approved for the treatment of several hematological malignancies. Given its use in human diseases, we needed a clear picture of how idelalisib binds to and inhibits PI3Kδ . Our data show that idelalisib is a potent and selective inhibitor of the kinase activity of PI3Kδ . A kinetic characterization clearly demonstrated ATP-competitive inhibition, and several additional biochemical and biophysical assays showed that the compound binds reversibly and non-covalently to the kinase. A crystal structure of idelalisib bound to the p110δ subunit of PI3Kδ furthers our understanding of the binding interactions that confer the potency and selectivity of idelalisib. Class I phosphoinositide-3-kinases (PI3Ks) catalyze the intracellular conversion of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-trisphosphate (PIP3), which functions as a second messenger, recruiting and activating signaling proteins such as Akt. Signals are relayed from receptor tyrosine kinases and G protein-coupled receptors to pathways regulating metabolism, cell growth and proliferation, motility, and differentiation. PI3Kδ is a class I PI3K that is formed by a catalytic subunit (p110δ) and a regulatory subunit (p85). The observation that PI3Kδ is selectively expressed in leukocytes suggested that this isoform might be a therapeutic target for diseases in which there is pathological activation of the Akt pathway in hematopoietic cells. Hematological malignancies of B cells, including indolent nonHodgkin lymphoma (iNHL), chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), have constitutively active PI3K/Akt signaling pathways and respond to pathway inhibition (1-4). Aberrant signaling in B cells is also found in a number of inflammatory diseases. PI3Kδ inhibition interferes with B cell activation, survival and migration (5). In rheumatoid arthritis http://www.jbc.org/cgi/doi/10.1074/jbc.M114.634683 The latest version is at JBC Papers in Press. Published on January 28, 2015 as Manuscript M114.634683 Copyright 2015 by The American Society for Biochemistry and Molecular Biology, Inc. by gest on Jauary 0, 2015 hp://w w w .jb.org/ D ow nladed from The characterization of idelalisib binding to PI3Kδ 2 (RA), PI3Kδ is highly expressed in the RA synovium (6) and in fibroblast-like synoviocytes, macrophages, and Th1 and Th17 cells (7). PI3Kδ inhibition can modulate both B cell and T cell production of inflammatory cytokines. In models of RA, PI3Kδ inhibitors have reduced inflammation and decreased bone and cartilage