Current Strategies to Overcome Resistance to ALK-Inhibitor Agents

The rearrangements of the anaplastic lymphoma kinase (ALK) gene are key drivers in the carcinogenesis of a portion of anaplastic large cell lymphomas (ALCL) and non-small cell lung cancers (NSCLC). Crizotinib, an orally available small molecule, has been the first ALK inhibitor to demonstrate a significant clinical activity in patients with ALK-positive tumors and, thus, to achieve the US food and drug administration approval for the treatment of advanced NSCLC harboring ALK-rearrangements. However, despite initially dramatic and quite durable responses in most cases, acquired resistance to crizotinib arises unavoidably often within the first year of treatment. Three main mechanisms of resistance to crizotinib have been identified to date: mutations in the ALK kinase domain, amplifications of ALK gene, and activation of escape signaling pathways. As ALK signaling dependence is retained in most cases become refractory to crizotinib, newer and more potent ALK-inhibitors have been developed and tested in clinical trials with encouraging activity results. Ceritinib has been recently approved by FDA for the treatment of locally advanced and metastatic NSCLC, and several more agents, including alectinib, ASP3026, and X396, are in active clinical development, demonstrating to be safe, selective and potent. Dual inhibition approaches targeting both ALK and the escape pathways bypassing ALK are currently under investigation. Moreover, being ALK a partner of the heat shock protein Hsp90, inhibitors of this chaperone have been proposed as potential alternative therapeutic strategies for ALKdriven tumors.