Crystal structures of IL-2-inducible T cell kinase complexed with inhibitors: insights into rational drug design and activity regulation.

IL-2-inducible T cell kinase plays an essential role in T cell receptor signaling and is considered a drug target for the treatment of Th2-mediated inflammatory diseases. By applying high-throughput protein engineering and crystallization, we have determined the X-ray crystal structures of IL-2-inducible T cell kinase in complex with its selective inhibitor BMS-509744 and the broad-spectrum kinase inhibitors sunitinib and RO5191614. Sunitinib uniquely stabilizes IL-2-inducible T cell kinase in the helix C-in conformation by inducing side chain conformational changes in the ATP-binding site. This preference of sunitinib to bind to an active kinase conformation is reflective of its broad-spectrum kinase activity. BMS-509744 uniquely stabilizes the activation loop in a substrate-blocking inactive conformation, indicating that structural changes described for Src family kinases are also involved in the regulation of IL-2-inducible T cell kinase activity. The observed BMS-509744 binding mode allows rationalization of structure-activity relationships reported for this inhibitor class and facilitates further structure-based drug design. Sequence-based analysis of this binding mode provides guidance for the rational design of inhibitor selectivity.