While osimertinib has resulted in striking improvements in outcomes for patients with NSCLC harboring classical EGFR mutations (Ex19del, L858R, and/or T790M), patients with atypical EGFR mutations have shown heterogeneous and in some cases inferior responses to EGFR inhibitors. The frequency, structural and clinical implications of atypical EGFR mutations are less understood.

While EGFR mutant NSCLC patients are initially responsive to EGFR targeted therapies, resistant disease inevitably emerges. In nearly half of resistance cases, tumors lack secondary EGFR mutations such as T790M and are refractory to 2nd- and 3rd-generation EGFR tyrosine kinase inhibitors (TKI). We and others have also observed that EGFR-independent resistant tumor cells may undergo a histologic and functional transformation through epithelial-to-mesenchymal transition (EMT) (Byers et al., 2013; Chung et al., 2011; Uramoto et al., 2010; Zhang et al., 2012), which can occur concurrently with other genomic alterations. The lack of treatment regimens with efficacy against EGFR-independent EGFR TKI resistance remains a major clinical challenge. We investigated transcriptomic and proteomic alterations that occur in NSCLC cells with acquired resistance to EGFR TKIs that occurs independent of EGFR and c-Met and screened >1,300 compounds to identify targetable vulnerabilities. T790M-negative EGFR TKI resistance was associated with evidence of a mesenchymal transition along with increased activation of the YAP/FOXM1 transcriptional program and a broad-spectrum multidrug resistance phenotype. EGFR TKI resistant cells displayed increased expression of spindle assembly checkpoint (SAC) proteins PLK1, Aurora kinases, survivin, and KSP, and expression of these proteins was dependent on the YAP/FOXM1 axis. Consistent with recent reports (Bertran-Alamillo et al., 2019; Shah et al., 2019), EGFR TKI resistant cells were found to be sensitive to aurora kinase inhibitors. We further determined that EGFR TKI resistant cells were likewise highly sensitive to inhibitors of components of the spindle assembly checkpoint (SAC) pathway including PLK1, KSP, and survivin, and treatment with these agents resulted in the accumulation of cells in the G2/M phase of the cell cycle and mitotic catastrophe. Using a patient-derived model of T790M negative EGFR TKI resistance, we observed that treatment with SAC component inhibitors, alisertib, ispinesib, or volasertib significantly inhibited tumor growth compared with vehicle-treated tumors. Analysis of NSCLC clinical data revealed that FOXM1 expression correlated with expression of SAC components including PLK1, Aurora kinases, KSP, and survivin. Moreover, in EGFR mutant NSCLC patients, high FOXM1 expression was associated with a worse clinical outcome compared to EGFR mutant NSCLC patients with low expression of FOXM1. In resistant models, targeting of YAP reduced FOXM1 expression and expression of SAC components. In conclusion, we provide novel insights into the molecular alterations associated with EGFR TKI resistance and demonstrate that upregulation of SAC components in EGFR TKI resistant cells occurs through the activation of the YAP/FOXM1 pathway. These results support the future targeting of these pathways in NSCLC patients with EGFR-independent resistance to EGFR-targeted agents.

In EGFR, exon 18 encodes for the P-loop (L718-V726), and mutations in this region (G719S/A, L718Q/V, G724S) are known to reduce sensitivity to osimertinib and first-generation EGFR TKIs. Osimertinib resistance is associated with a number of acquired mutations in exons 19 and 20 (S784F, L747S, C797S and L792H). We investigated the frequency and drug sensitivity of these and other osimertinib-resistant EGFR mutations We generated ∼50 different Ba/F3 cell line models expressing classical and/or atypical EGFR mutations (exons 18-21) and evaluated the transforming ability and sensitivity to 14 EGFR TKIs including non-covalent (first-generation), afatinib, dacomitinib, and poziotinib (quinazoline and covalent, second-generation), and covalent T790M-specific (third-generation) inhibitors. Impact of atypical mutations was analyzed by in silico modeling. We found 3.6% (N=32/895) of EGFR-mutant patients had atypical, exon 18, P-loop mutations in the MD Anderson GEMINI database. Modeling of classical EGFR mutations revealed osimertinib has distinct interactions between the solvent front of osimertinib and residues within the P-loop of EGFR, whereas second-generation quinazoline TKIs, such as poziotinib, extend into the pocket, near T790, lacking these interactions. Mutations in the P-loop were predicted to shift osimertinib out of alignment with V726 and F723, causing resistance to osimertinib but not quinazoline-based TKIs. Atypical exon 18 mutations (G719S/A, L718Q/V, G724S) had IC50 values of 113.6nM, 1.6nM, and 137.5nM for first-, second-, and third-generation TKIs, respectively. Second-generation TKIs inhibited G719S/A-T790M mutations at concentrations 2-fold lower than third-generation TKIs (IC50 = 23.4nM and 46nM). Osimertinib-resistance mutations (L747S, S784F, C797S, L792H) co-occurring with classical sensitizing mutations (L858R or ex19del) had IC50 values of 56.8nM, 1.4nM, and 996nM to first, second and third-generation inhibitors. Of the second-generation TKIs tested, poziotinib was the most potent for atypical exon 18 P-loop mutations; G719S/A-T790M mutations; and classical mutants with acquired osimertinib-resistance mutations (IC50= 0.4nM, 3.2nM, 0.8nM). Exon 18 atypical P-loop mutations and osimertinib-resistance mutations demonstrated high sensitivity to second-generation quinazoline TKIs, compared to first- and third-generation inhibitors. Mutations in the P-loop of EGFR confer resistance to third-generation TKIs by destabilizing solvent front interactions of the molecule, and osimertinib-resistance mutations interfere with covalent binding at C797. Second-generation TKIs, especially poziotinib, are potent inhibitors of these mutations because they have increased hydrophobic interactions at the back of the drug binding cleft that are retained without covalent binding. Together, these data indicate that poziotinib and other second-generation TKIs may be useful for the treatment of NSCLC patients with atypical P-loop and selected osimertinib-resistant EGFR mutations.

Insertions/mutations in exon 20 of EGFR occur in ∼2% Insertions/mutations in exon 20 of EGFR occur in ∼2% of all lung adenocarcinomas. These alterations are characterized by primary resistance to approved tyrosine kinase inhibitors (TKIs) with response rates of <12%. We have shown that exon 20 insertions restrict the size of the drug-binding pocket, limiting binding of large inhibitors. However, poziotinib can circumvent these steric changes and is a potent inhibitor of EGFR exon 20 mutants. In our investigator-initiated phase 2 trial of EGFR exon 20 mutant NSCLC, poziotinib was associated with a best objective response rate of 55% (Heymach et al, 19th WCLC). Herein, we use preclinical models and clinical samples from our phase 2 study to identify mechanisms of acquired poziotinib resistance (NCT03066206). EGFR exon 20 insertion (D770insNPG) genetically engineered mice (GEM) were treated with poziotinib until progression. Upon progression, tumor DNA and protein were analyzed using whole exome sequencing (WES) and reverse phase protein assay (RPPA). Mandatory and optional biopsies were obtained at baseline and progression, respectively, from patients treated in our phase 2 trial of poziotinib in EGFR exon 20 mutant NSCLC. Serial cfDNA was collected at baseline, 8 weeks of therapy, and on progression. Patient samples were analyzed using targeted next generation sequencing or WES. Poziotinib acquired-resistance GEM tumors acquired mutations in ErbB4, KRAS, and other genes which represent potential targetable bypass pathways. Resistant GEM tumors displayed increased activation of MAPK, AKT, ERK and MEK compared to sensitive tumors, suggesting that poziotinib acquired resistance is associated with reactivation of the MAPK/PI3K pathways. We enrolled 50 EGFR exon 20 mutant patients in our phase 2 trial. Analysis of matched pre-poziotinib and on-progression samples from 20 responding patients revealed acquired EGFR tyrosine kinase domain point mutations in 4 patients (T790M (2), V774A (1), D770A, (1)). Ba/F3 cells co-expressing EGFR exon 20 insertion (S768supSVD) and T790M were resistant to poziotinib, suggesting that T790M is a poziotinib resistance driver. Potential acquired EGFR-independent resistance mechanisms identified in patients to date include PIK3CA E545K (1), MAP2K2 S94L (1), MET amplification (1), EGFR amplification (2), and CDK6 amplification (2). Parallel to acquired resistance mechanisms seen in classical EGFR mutation, acquired resistance to poziotinib can be mediated through EGFR-dependent mechanisms, notably T790M and other EGFR tyrosine kinase domain point mutations. EGFR-independent resistance mechanisms include activation of bypass pathways. Preclinical validation of resistance mechanisms and additional analysis of patient samples will be presented at the meeting.

Insertions/mutations in exon 20 of EGFR or HER2 occur in ∼3% of all lung adenocarcinomas. These alterations are characterized by primary resistance to tyrosine kinase inhibitors (TKIs) with response rates of <12%. We previously showed that exon 20 insertions restrict the size of drug-binding pocket, limiting binding of most available TKIs. However, poziotinib can potentially circumvent these steric changes due to its smaller, flexible structure and is a potent inhibitor of EGFR and HER2 exon 20 mutants (Robichaux et al.

Immune checkpoint blockade (IO) has demonstrated durable clinical benefit in metastatic non-small cell lung cancer (NSCLC). Tumors with driver mutations such as EGFR exon 19 and 21 mutations and ALK translocation tend to have low response rates to IO. However, IO response in NSCLC patients with rare driver mutations, such as EGFR exon 20 (∼2%), HER-2 (∼2%) and BRAF (∼3%), representing approximately 7% of lung adenocarcinomas, has been poorly addressed. We queried GEMINI (MD Anderson Lung Cancer Moon Shot funded database for prospective collection of clinical information on NSCLC) for patients with mutations in EGFR exon 19, 20, 21, HER-2 and BRAF treated with PD-1/PD-L1 checkpoint inhibitors. We assessed progression-free survival (PFS), overall response rate (ORR) and overall survival (OS) in each molecular group. Between 2014-2018, 108 patients with classic EGFR mutations (exon 19 del + exon 21 L858R, n=37), EGFR exon 20 mutations (n=36; no T790M included), HER-2 mutations (n=22) and BRAF mutations (n=13; V600E: 3pts; non-V600E: 10pts) had been treated with PD-1/PD-L1 inhibitors. EGFR exon 20 mutants and BRAF mutants demonstrated significantly higher PFS (EGFR exon 20: HR 0.4, p<0.001; BRAF: HR 0.2, p<0.001), higher disease control rate at 6 and 12 months as well as higher ORR when compared to classic EGFR mutants (Table). These differences remained significant in multivariate analysis after adjusting for age, smoking, PD-L1 status, radiation prior to treatment initiation, treatment with concurrent agents and prior treatment with TKIs. HER-2 mutants had similar PFS compared to EGFR classic mutants (HR 0.8, p=0.35) (Table). EGFR exon 20 and BRAF mutations are associated with superior outcome from PD-1/PD-L1 checkpoint inhibitors compared to classic EGFR and HER-2 mutations. Further studies on co-mutational status and tumor mutation burden in these molecularly-defined groups are ongoing to address potential underlying mechanisms associated with these findings.

While previous reports have established MET and HER2 amplification as two mechanisms of non-T790M driven EGFR TKI resistance in EGFR mutant NSCLC, resistance occurs in the absence of these modifications in a significant number of patients. Therefore, there exists an unmet need to define additional mechanisms of resistance to EGFR TKIs. We hypothesized that targeted next-generation sequencing could detect additional targetable activating mutations in paired tumor samples from patients with acquired resistance to first or second generation EGFR TKIs. We conducted an analysis of clinical and molecular data prospectively collected from 285 EGFR-mutant NSCLC patients enrolled into the MD Anderson Lung Cancer GEMINI database. Of 157 patients treated with first-line therapy (erlotinib, gefitinib, or afatinib), we identified 75 patients with TKI-acquired resistance with matched pre/post-TKI tumor samples. Matched tumor samples were analyzed with targeted gene sequencing. Recurrent alterations were defined as an alteration occurring more than 2 times. Recurrent acquired mutations were expressed in Ba/F3 and EGFR mutant (T790M+/-) NSCLC cells. Mutation expressing Ba/F3 cell lines were assayed for IL-3 independence, and mutation expressing NSCLC cell were screened against combination targeted TKIs. EGFR mutant NSCLC patients treated with first-line therapy had a median PFS of 14 months; and, of the patients with pre/post-TKI tumor molecular data, 47% of patients were T790M negative. There were 30 recurrent acquired alterations identified in 13 different genes. Genes included ARAF, BRAF, EGFR, FGFR, GNAS, JAK2, MCL1, PDGFRα, PIK3CA, RAF1, RB1, SMAD4, and TP53. Of the alterations identified, most occurred in 1 of 4 targetable genes: BRAF (N=3), FGFR (N=5), PDGFRα (N=3), or PIK3CA (N=2). Both previously reported and novel mutations were identified, and preliminary screening of mutant expressing Ba/F3 cell lines found that of the mutations tested (BRAF WT & G469H, FGFR2 A371G, PDGFRα WT & L682F, and PIK3Ca E545K) all grew independent of IL-3. HCC827 and H1975 cell lines expressing acquired mutations in BRAF, FGFR, PDGFRα, or PIK3CA were more sensitive to combination targeted therapy compared to EGFR TKIs or mutation specific TKIs alone unlike control cell lines, supporting the possibility that targeting these mutations would be of therapeutic benefit. Analysis of patient data identified 30 recurrent genomic alterations in 13 different genes including novel alterations in BRAF, EGFR, FGFR, PDGFRα, RB1, and SMAD4, many of which were found to be activating mutations. Our analysis identified potentially targetable mutations of BRAF, FGFR, PDGFRα, and PIK3CA which merits further pre-clinical and clinical investigation.

Approximately 10% of EGFR mutant NSCLCs have an insertion/mutation in exon 20 of EGFR resulting in primary resistance to currently available tyrosine kinase inhibitors (TKIs). We previously reported that the structural features of poziotinib could potentially enable it to circumvent the steric hindrance induced by exon 20 mutations. Here we further characterize the preclinical activity of poziotinib and report on initial clinical activity of poziotinib in patients with EGFR exon 20 mutations from an ongoing phase II study. We evaluated poziotinib activity in vitro using human NSCLC cell lines and the BAF3 model as well as several patient-derived xenograft (PDX) models and genetically engineered mouse models (GEMMs) of exon 20 insertion. We launched a phase 2 investigator-initiated trial of poziotinib in patients with metastatic NSCLC with EGFR exon 20 insertions (NCT03066206). In vitro poziotinib was approximately 100x more potent than osimertinib and 40x more potent than afatinib against a common panel of EGFR exon 20 insertions. Furthermore, it had ∼65-fold greater potency against common exon 20 insertions compared with EGFR T790M mutations; 3rd generation inhibitors osimertinib, EGF816, and rociletinib were all significantly less potent for exon 20 mutations/insertions compared with T790M. in vivo poziotinib led to >85% reduction in tumor burden in GEM models of EGFR exon 20 insertion (D770insNPG) NSCLC and the PDX model LU0387 (H773insNPH). To date, 8 platinum-refractory patients with EGFR exon 20 insertion mutation metastatic NSCLC have been enrolled in the clinical trial and treated with poziotinib at a dose of 16 mg PO daily. Two patients have reached the first interval-imaging time point (at 8 weeks of therapy per protocol). Both patients exhibited dramatic partial response, with one patient reporting improvement in dyspnea and cough at one week of therapy. In this early stage of the study, one case of grade 3 paronchycia was observed. One additional platinum- and erlotinib-refractory patient with EGFR exon 20 insertion was treated with poziotinib on compassionate basis. The patient achieved partial response after three weeks of treatment. Poziotinib has selective activity against EGFR exon 20 mutations and potent activity in cell lines, PDX, and GEM models. Three platinum-refractory patients with EGFR exon 20 mutations have been treated thus far and are evaluable for response; all three had partial responses at the time of the initial scan. Updated data from the ongoing phase 2 clinical trial of poziotinib will be presented at the meeting.

Insertions in Exon 20 of EGFR, which accounts for 5-10% of all EGFR mutations in lung adenocarcinoma, have been associated with primary resistance to EGFR tyrosine kinase inhibitors (TKI). We have previously reported that poziotinib fits into sterically hindered exon 20 binding pocket and therefore is a potent inhibitor of EGFR Exon 20 insertions. Recently, we have launched an investigator-initiated trial of poziotinib in EGFR Exon 20 mutated advanced NSCLC. Poziotinib is given orally at the established recommended phase II dose of 16 mg. Prior to the trial initiation, we treated a patient whose metastatic NSCLC progressed through chemotherapy and immunotherapy, with pozoiotinib on compassionate basis. The patient's NSCLC exhibited HER2 A771insAYVM mutation (aHER2 Exon 20 insertion that behaves like EGFR Exon 20 insertion). The patient had a pronounced clinical and radiological response to poziotinib. That corresponded to a dramatic decrease of HER2 mutant cfDNA. Adverse events were skin rash and diarrhea. Two patients have been enrolled in the clinical trial and started therapy at the time of abstract submission. Early follow up revealed that poziotinib is well tolerated with safety profile similar to other EGFR TKIs. Poziotinib has promising clinical activity in EGFR Exon 20 mutant NSCLC. An updated report of trial enrollment, poziotinib safety and activity will be presented at the meeting.