Hideko Isozaki

Abstract We present a cohort of 41 patients with osimertinib resistance biopsies, including 2 with an acquired CCDC6–RET fusion. Although RET fusions have been identified in resistant EGFR-mutant non–small cell lung cancer (NSCLC), their role in acquired resistance to EGFR inhibitors is not well described. To assess the biological implications of RET fusions in an EGFR-mutant cancer, we expressed CCDC6–RET in PC9 (EGFR del19) and MGH134 (EGFR L858R/T790M) cells and found that CCDC6–RET was sufficient to confer resistance to EGFR tyrosine kinase inhibitors (TKI). The selective RET inhibitors BLU-667 and cabozantinib resensitized CCDC6–RET-expressing cells to EGFR inhibition. Finally, we treated 2 patients with EGFR-mutant NSCLC and RET-mediated resistance with osimertinib and BLU-667. The combination was well tolerated and led to rapid radiographic response in both patients. This study provides proof of concept that RET fusions can mediate acquired resistance to EGFR TKIs and that combined EGFR and RET inhibition with osimertinib/BLU-667 may be a well-tolerated and effective treatment strategy for such patients. Significance: The role of RET fusions in resistant EGFR-mutant cancers is unknown. We report that RET fusions mediate resistance to EGFR inhibitors and demonstrate that this bypass track can be effectively targeted with a selective RET inhibitor (BLU-667) in the clinic. This article is highlighted in the In This Issue feature, p. 1494

Crizotinib is the standard of care for advanced non-small cell lung cancer (NSCLC) patients harboring the anaplastic lymphoma kinase (ALK) fusion gene, but resistance invariably develops. Unlike crizotinib, alectinib is a selective ALK tyrosine kinase inhibitor (TKI) with more potent antitumor effects and a favorable toxicity profile, even in crizotinib-resistant cases. However, acquired resistance to alectinib, as for other TKIs, remains a limitation of its efficacy. Therefore, we investigated the mechanisms by which human NSCLC cells acquire resistance to alectinib. We established two alectinib-resistant cell lines that did not harbor the secondary ALK mutations frequently occurring in crizotinib-resistant cells. One cell line lost the EML4-ALK fusion gene, but exhibited increased activation of insulin-like growth factor-1 receptor (IGF1R) and human epidermal growth factor receptor 3 (HER3), and overexpressed the HER3 ligand neuregulin 1. Accordingly, pharmacologic inhibition of IGF1R and HER3 signaling overcame resistance to alectinib in this cell line. The second alectinib-resistant cell line displayed stimulated HGF autocrine signaling that promoted MET activation and remained sensitive to crizotinib treatment. Taken together, our findings reveal two novel mechanisms underlying alectinib resistance that are caused by the activation of alternative tyrosine kinase receptors rather than by secondary ALK mutations. These studies may guide the development of comprehensive treatment strategies that take into consideration the various approaches ALK-positive lung tumors use to withstand therapeutic insult.

Overexpression of ING1, a candidate tumor suppressor gene, efficiently blocks cell growth or induces apoptosis in different experimental systems. ING1 maps to chromosome 13q33-34, and because loss of the terminal region of chromosome 13q has been implicated in esophageal squamous cell cancer (ESCC), we examined ESCC for genetic alterations of ING1. Among 31 informative cases of ESCC, 58.9% of the tumors showed allelic loss at chromosome 13q33-34, and we detected four tumor-specific missense nucleotide changes. These alterations were found within the PHD finger domain and nuclear localization motif of the ING1 and may be functionally involved in the development of ESCC. Because immunohistochemical study revealed that all of the ESCC samples showed loss of ING1 protein expression, genetic or epigenetic alterations that abrogate the normal function of ING1 may contribute to esophageal squamous cell carcinogenesis.

The discovery of an echinoderm microtubule-associated protein-like 4 (EML4)-anaplastic lymphoma kinase (ALK) fusion gene led to improved clinical outcomes in patients with lung cancer after the development of the first ALK-targeting agent, crizotinib. Some second-generation ALK tyrosine kinase inhibitors (TKIs), which might be more potent than crizotinib or effective on crizotinib-resistant patients, have been developed. Although these ALK-TKIs show an excellent response initially, most patients eventually acquire resistance. Therefore, careful consideration of the resistance mechanisms might lead to superior therapeutic strategies. Here, we summarize the history of ALK-TKIs and their underlying resistance mechanisms in both the preclinical and clinical settings. In addition, we discuss potential future treatment strategies in ALK-TKI-naïve and -resistant patients with lung cancer harboring the EML4-ALK fusion gene.