Introduction: Although KRAS mutations in NSCLC have been considered mutually exclusive driver mutations for a long time, there is now growing evidence that KRASmutated NSCLC represents a genetically heterogeneous subgroup. We sought to determine genetic heterogeneity with respect to cancer-related co-mutations and their correlation with different KRAS mutation subtypes.

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Activation of MAPK signaling via BRAF mutations may limit the activity of EGFR inhibitors in EGFR-mutant lung cancer patients. However, the impact of BRAF mutations on the selection and fitness of emerging resistant clones during anti-EGFR therapy remains elusive. We tracked the evolution of subclonal mutations by whole-exome sequencing and performed clonal analyses of individual metastases during therapy. Complementary functional analyses of polyclonal EGFR-mutant cell pools showed a dose-dependent enrichment of BRAFV600E and a loss of EGFR inhibitor susceptibility. The clones remain stable and become vulnerable to combined EGFR, RAF, and MEK inhibition. Moreover, only osimertinib/trametinib combination treatment, but not monotherapy with either of these drugs, leads to robust tumor shrinkage in EGFR-driven xenograft models harboring BRAFV600E mutations. These data provide insights into the dynamics of clonal evolution of EGFR-mutant tumors and the therapeutic implications of BRAF co-mutations that may facilitate the development of treatment strategies to improve the prognosis of these patients.

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e20682 Background: There is growing insight in the mechanisms underlying resistance to the 3rdgeneration EGFR inhibitor osimertinib. Unlike resistance to 1stgeneration inhibitors, these mechanisms not necessarily lead to sequential targeted therapy approaches. Here we report on the treatment of two patients with acquired resistance to osimertinib with a new detected BRAF V600E mutation as resistance mechanism. Methods: We identified two patients with EGFR-T790M-mutant advanced NSCLC with progression on osimertinib and detection of a new BRAFV600E mutation in a tumor rebiopsy by next-generation sequencing (NGS). No other known resistance mechanism beside T790M loss in one patient was found. Osimertinib was discontinued and BRAF-targeted combination therapy with dabrafenib and trametinib at standard dose was initiated. We monitored the clinical course with sequential 18F-2-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) / computed tomography (CT) assessing maximum standard uptake value (SUVmax), sequencing based liquid biopsies and tumor marker assessment. Results: Patient (1) with EGFR del19 (E746_A750del), preserved T790M mutation and acquired BRAF V600E mutation showed reduction in FDG uptake of 18% after 2 weeks of dabrafenib/trametinib that demonstrated a slight increase of 12% in a FDG-PET/CT scan 4 weeks thereafter and combination treatment has been continued. Patient (2) with EGFR del19 (E746_A750del), T790M loss and new BRAF V600E mutation showed continuous metabolic (+8% and + 39%, respectively) and morphologic progression after 2 and 4 weeks of dabrafenib and trametinib. A tumor rebiopsy showed no additional molecular changes. We changed the treatment to osimertinib and dabrafenib combination and observed an impressive metabolic response (-33%) after 2 weeks by FDG-PET/CT. Conclusions: BRAF V600E mutation has recently been described as a novel molecular resistance mechanism in osimertinib-resistant EGFR-mutant NSCLC. We describe one patient where combined BRAF/MEK inhibition with no additional EGFR-inhibition resulted in a preliminary feasible tumor control, but confirmatory CT staging is pending. In a second patient, co-inhibition of EGFR and BRAF pathway with osimertinib and dabrafenib was needed to overcome BRAF-mediated osimertinib resistance resulting in an impressive early tumor response that was not observed to either single-target inhibition of EGFR or BRAF. FDG-PET/CT was able to monitor tumor dynamics. Updated data will be presented.

Background: MAP2K1 mutations are rare in non-small cell lung cancer (NSCLC) and considered to be mutually exclusive from known driver mutations. Activation of the MEK1-cascade is considered pivotal in resistance to targeted therapy approaches, and MAP2K1 K57 N mutation could be linked to resistance in preclinical models. We set out this study to detect MAP2K1 mutations and potentially targetable co-mutations using a molecular multiplex approach. Methods: Between 2012 and 2018, we routinely analyzed 14.512 NSCLC patients with two next-generation sequencing (NGS) panels. In a subset of patients, fluorescence in-situ hybridization was performed to detect rearrangements or amplifications. We assessed clinical parameters and co-occurring mutations and compared treatment outcomes of different forms of systemic therapy. Results: We identified 66 (0.5%) patients with MAP2K1 mutations. Both adenocarcinoma (n = 62) and squamous cell carcinoma (n = 4) histology. The presence of the mutations was linked to smoking, and transversions were more common than transitions. K57 N was the most frequent MAP2K1 mutation (n = 25). Additional mutations were found in 57 patients (86.4%). Mutations of TP53 were detected in 33 patients, followed by KEAP1 mutations in 28.1%. 24 patients (36.4%) had either MAP2K1-only or a co-occurring aberration considered targetable, including EGFR mutations, a BRAF V600E mutation and ROS1 rearrangements. Outcome analyses revealed a trend toward benefit from pemetrexed treatment. Conclusion: Our analysis shows that MAP2K1-mutated NSCLC patients might frequently present with potentially targetable aberrations. Their role in providing resistance in these subtypes and the possible therapeutic opportunities justify further analyses of this rare NSCLC subgroup.

9030 Background: MET mutations ( MET∆ex14), amplifications or translocations can activate oncogenic signaling in lung cancer and are sensitive to MET inhibition. Acquired resistance to therapy with MET tyrosine kinase inhibitors (TKI) occurs inevitably. Methods: Between 2015 and 2018, eighteen patients with MET-driven NSCLC were treated with capmatinib or crizotinib as single agent at our site. Rebiopsy samples from five patients were analyzed by NGS and fluoreszenz-in-situ hybridization (FISH) at time of progression. Results: Of the five patients with rebiopsy samples at time of progression, two had initially a MET amplification (one patient with low-level and one patient with high-level amplification), two patients had a MET∆ex14 and one patient had a KIF5B-MET fusion. Patient 1 (low-level MET amplification) showed a partial response to crizotinib. The rebiopsy revealed an acquired KRAS mutation as a potential mechanism of resistance. Patient 2 (high-level MET amplification) showed stable disease as best response to capmatinib and patient 3 (MET∆ex14) showed a partial response to capmatinib. Both patients developed acquired HER2 amplifications. Patient 4 ( MET∆ex14) showed initially a partial response to crizotinib. The rebiopsy sample revealed an acquired MET kinase domain mutation (p.D1246N). As preclinical findings suggested that D1246N confers resistance to type I MET inhibitors but remains sensitive to type II inhibitors, cabozantinib was started. A CT six weeks after therapy initiation showed progressive disease. Patient 5 ( KIF5B-MET) had a partial response to crizotinib. An acquired MET p.Y1248H mutation was found at time of progression. Therapy was changed to cabozantinib. A new CT scan is pending. Conclusions: Resistance to MET inhibition is heterogeneous with on- and off-target-mechanisms occurring. We found HER2 amplification as a potential new bypass mechanism. The MET mutation D1246N conferred resistance to type I and type II inhibitors. We describe the first case of an acquired mutation of the MET tyrosine kinase domain in a patient with an oncogenic MET fusion. Further investigations are needed to collect comprehensive data to understand resistance mechanisms in MET inhibition and to develop novel therapeutic strategies.

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