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

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.

Figure 1.. Summary of anatomic and molecular pathology findings from osimertinib-resistant cohort.

This heat map summarizes the findings of tissue (top) and ctDNA (bottom) analysis obtained at the time of clinical progression on osimertinib. Key resistance mechanisms are highlighted (see legend). Note that for patients with multiple tissue biopsies (4A/B, 5A/B, 14A/B), the same plasma results are shown below each tissue biopsy result.

Figure 2.. The CCDC6-RET fusion is sufficient for conferring resistance to EGFR-TKIs and can be overcome by combined EGFR and RET inhibition.

A, PC9 and MGH134 cells expressing the CCDC6-RET gene fusion or empty vector (EV) were treated with 1 μM osimertinib (OSI) or vehicle (VEH) and cell proliferation determined over the course of five days (ratio compared to the beginning of treatment). Data shown are the mean ± s.e.m. of three independent biological replicates. B, PC9^EV and PC9^CCDC6-RET cells were treated with 100 nM afatinib, 1 μM osimertinib, BLU-667 or combinations for 6 hours and harvested for western blotting with the indicated antibodies. The arrow indicates the phospho-RET band. C, PC9^EV and PC9^CCDC6-RET cells were treated with BLU-667, or afatinib or osimertinib in the absence or presence of 1μM BLU-667 and cell viability was determined after 72 hours. The same BLU-667 data is replotted in both panels for comparison purposes. Data are shown as a percentage of vehicle treated control and are the mean ± s.e.m of three independent biological replicates.

Figure 3.. Responses observed in the two patients treated with osimertinib and BLU-667.

A. Treatment response of patient 1 to Osimertinib and BLU-667. Serial coronal contrast-enhanced computed-tomography images of the thorax demonstrate a right lower lobe lung mass and pleural nodularity (red arrows) seen at baseline (left) with partial response after 8 weeks of treatment with BLU-667 and osimertinib (right). B. Treatment response of patient 44 to osimertinib and BLU-667, with significant improvement in left upper and left lower lobe pulmonary opacities (right; circled) compared to baseline (left.)