Abstract The Anaplastic Lymphoma Kinase (ALK) is a receptor tyrosine kinase with a recognized role in the pathogenesis of different tumors. There is strong preclinical evidence that inhibition of ALK kinase activity results in anti-tumor efficacy and the approval of the ALK inihibitor Xalkori (crizotinib) for treatment of NSCLC patients bearing ALK positive tumor represents the most recent success of targeted therapy. Nevertheless, clinical data show that after initial response to Xalkori, patients experience relapse, the mechanisms of which are still under study, but which at least in some cases appear due to emergence of mutations that confer resistance to the drug. Thus, the development of next-generation ALK inihibitors, able to overcome ALK-dependent Xalkori resistance is needed. We have previously presented the identification of NMS-E628, a potent and selective small-molecule inhibitor of ALK kinase activity. The compound displays strong anti-tumor efficacy in several models of ALK dependent tumors after oral administration, with complete and durable regression observed in treated animals. NMS-E628 passes the blood brain barrier in all the species tested and was found able to effectively control the growth of intracranial tumors. Being structurally distinct from Xalkori, NMS-E628 might be able to overcome mutations in the ALK kinase domain which confer resistance by affecting Xalkori binding. To test this hypothesis, the activity of NMS-E628 on WT ALK and on two mutants identified in Xalkori-relapsed patients, L1196M and C1156Y, was investigated using different approaches. Ki determination revealed that at the biochemical level, NMS-E628 is ca. 7–8 fold more potent than Xalkori on both L1196M and C1156Y. In Ba/F3 cells made dependent upon mutated ALK forms, NMS-E628 was found superior than Xalkori in inhibiting the proliferation of both L1196M ALK and C1156Y ALK-driven cells in vitro. Mechanism of action studies confirmed that of the two drugs, NMS-E628 is better able to downmodulate ALK phosphorylation, having similar potency on wt ALK and mutated forms. To assess antitumor activity in vivo, Ba/F3 xenografts driven by different ALK mutants were generated. As expected from in vitro results, Xalkori showed poor efficacy on L1196M and C1156Y mutants when tested at 100 and 200 mg/kg, while NMS-E628 retains significant anti-tumor activity in both mutant models. Taken together, these data support the idea that NMS-E628 might represent a valid therapeutic opportunity for Xalkori-relapsed patients that experience acquired resistance to specific ALK mutations. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A232.