NRG1 rearrangements are oncogenic drivers that are enriched in invasive mucinous adenocarcinomas (IMA) of the lung. The oncoprotein binds ERBB3–ERBB2 heterodimers and activates downstream signaling, supporting a therapeutic paradigm of ERBB3/ERBB2 inhibition. As proof of concept, a durable response was achieved with anti-ERBB3 mAb therapy (GSK2849330) in an exceptional responder with an NRG1-rearranged IMA on a phase I trial (NCT01966445). In contrast, response was not achieved with anti-ERBB2 therapy (afatinib) in four patients with NRG1-rearranged IMA (including the index patient post-GSK2849330). Although in vitro data supported the use of either ERBB3 or ERBB2 inhibition, these clinical results were consistent with more profound antitumor activity and downstream signaling inhibition with anti-ERBB3 versus anti-ERBB2 therapy in an NRG1-rearranged patient-derived xenograft model. Analysis of 8,984 and 17,485 tumors in The Cancer Genome Atlas and MSK-IMPACT datasets, respectively, identified NRG1 rearrangements with novel fusion partners in multiple histologies, including breast, head and neck, renal, lung, ovarian, pancreatic, prostate, and uterine cancers. Significance: This series highlights the utility of ERBB3 inhibition as a novel treatment paradigm for NRG1-rearranged cancers. In addition, it provides preliminary evidence that ERBB3 inhibition may be more optimal than ERBB2 inhibition. The identification of NRG1 rearrangements across various solid tumors supports a basket trial approach to drug development. Cancer Discov; 8(6); 686–95. ©2018 AACR. See related commentary by Wilson and Politi, p. 676. This article is highlighted in the In This Issue feature, p. 663

NRG1 rearrangements are recurrent oncogenic drivers in solid tumors. NRG1 binds to HER3, leading to heterodimerization with other HER/ERBB kinases, increased downstream signaling, and tumorigenesis. Targeting ERBBs, therefore, represents a therapeutic strategy for these cancers. We investigated zenocutuzumab (Zeno; MCLA-128), an antibody-dependent cellular cytotoxicity–enhanced anti-HER2xHER3 bispecific antibody, in NRG1 fusion–positive isogenic and patient-derived cell lines and xenograft models. Zeno inhibited HER3 and AKT phosphorylation, induced expression of apoptosis markers, and inhibited growth. Three patients with chemotherapy-resistant NRG1 fusion–positive metastatic cancer were treated with Zeno. Two patients with ATP1B1–NRG1–positive pancreatic cancer achieved rapid symptomatic, biomarker, and radiographic responses and remained on treatment for over 12 months. A patient with CD74–NRG1-positive non–small cell lung cancer who had progressed on six prior lines of systemic therapy, including afatinib, responded rapidly to treatment with a partial response. Targeting HER2 and HER3 simultaneously with Zeno is a novel therapeutic paradigm for patients with NRG1 fusion–positive cancers. Significance: NRG1 rearrangements encode chimeric ligands that activate the ERBB receptor tyrosine kinase family. Here we show that targeting HER2 and HER3 simultaneously with the bispecific antibody Zeno leads to durable clinical responses in patients with NRG1 fusion–positive cancers and is thus an effective therapeutic strategy. This article is highlighted in the In This Issue feature, p. 1171

Purpose: The RET proto-oncogene encodes a receptor tyrosine kinase that is activated by gene fusion in 1%–2% of non–small cell lung cancers (NSCLC) and rarely in other cancer types. Selpercatinib is a highly selective RET kinase inhibitor that has recently been approved by the FDA in lung and thyroid cancers with activating RET gene fusions and mutations. Molecular mechanisms of acquired resistance to selpercatinib are poorly understood. Patients and Methods: We studied patients treated on the first-in-human clinical trial of selpercatinib (NCT03157129) who were found to have MET amplification associated with resistance to selpercatinib. We validated MET activation as a targetable mediator of resistance to RET-directed therapy, and combined selpercatinib with the MET/ALK/ROS1 inhibitor crizotinib in a series of single patient protocols (SPP). Results: MET amplification was identified in posttreatment biopsies in 4 patients with RET fusion–positive NSCLC treated with selpercatinib. In at least one case, MET amplification was clearly evident prior to therapy with selpercatinib. We demonstrate that increased MET expression in RET fusion–positive tumor cells causes resistance to selpercatinib, and this can be overcome by combining selpercatinib with crizotinib. Using SPPs, selpercatinib with crizotinib were given together generating anecdotal evidence of clinical activity and tolerability, with one response lasting 10 months. Conclusions: Through the use of SPPs, we were able to offer combination therapy targeting MET-amplified resistance identified on the first-in-human study of selpercatinib. These data suggest that MET dependence is a recurring and potentially targetable mechanism of resistance to selective RET inhibition in advanced NSCLC.

Purpose: MET exon 14 splice site alterations that cause exon skipping at the mRNA level (METex14) are actionable oncogenic drivers amenable to therapy with MET tyrosine kinase inhibitors (TKI); however, secondary resistance eventually arises in most cases while other tumors display primary resistance. Beyond relatively uncommon on-target MET kinase domain mutations, mechanisms underlying primary and acquired resistance remain unclear. Experimental Design: We examined clinical and genomic data from 113 patients with lung cancer with METex14. MET TKI resistance due to KRAS mutation was functionally evaluated using in vivo and in vitro models. Results: Five of 113 patients (4.4%) with METex14 had concurrent KRAS G12 mutations, a rate of KRAS cooccurrence significantly higher than in other major driver-defined lung cancer subsets. In one patient, the KRAS mutation was acquired post-crizotinib, while the remaining 4 METex14 patients harbored the KRAS mutation prior to MET TKI therapy. Gene set enrichment analysis of transcriptomic data from lung cancers with METex14 revealed preferential activation of the KRAS pathway. Moreover, expression of oncogenic KRAS enhanced MET expression. Using isogenic and patient-derived models, we show that KRAS mutation results in constitutive activation of RAS/ERK signaling and resistance to MET inhibition. Dual inhibition of MET or EGFR/ERBB2 and MEK reduced growth of cell line and xenograft models. Conclusions: KRAS mutation is a recurrent mechanism of primary and secondary resistance to MET TKIs in METex14 lung cancers. Dual inhibition of MET or EGFR/ERBB2 and MEK may represent a potential therapeutic approach in this molecular cohort.

Tyrosine kinase domains dynamically fluctuate between two main structural forms that are referred to as type I (DFG-in) or type II (DFG-out) conformations. Comprehensive data comparing type I and type II inhibitors are currently lacking for NTRK fusion-driven cancers. Here we used a type II NTRK inhibitor, altiratinib, as a model compound to investigate its inhibitory potential for larotrectinib (type I inhibitor)-resistant mutations in NTRK. Our study shows that a subset of larotrectinib-resistant NTRK1 mutations (V573M, F589L and G667C) retains sensitivity to altiratinib, while the NTRK1^V573Mand xDFG motif NTRK1^G667Cmutations are highly sensitive to type II inhibitors, including altiratinib, cabozantinib and foretinib. Moreover, molecular modeling suggests that the introduction of a sulfur moiety in the binding pocket, via methionine or cysteine substitutions, specifically renders the mutant kinase hypersensitive to type II inhibitors. Future precision treatment strategies may benefit from selective targeting of these kinase mutants based on our findings.