Landscape of Acquired Resistance to Osimertinib in -Mutant NSCLC and Clinical Validation of Combined EGFR and RET Inhibition with Osimertinib and BLU-667 for Acquired Fusion.

We present a cohort of 41 patients with osimertinib resistance biopsies, including 2 with an acquired fusion. Although fusions have been identified in resistant -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 fusions in an -mutant cancer, we expressed CCDC6-RET in PC9 ( del19) and MGH134 ( 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 -mutant NSCLC and -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 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 fusions in resistant -mutant cancers is unknown. We report that 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..