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Introduction: Novel rearranged in transfection (RET)-specific tyrosine kinase inhibitors (TKIs) such as selpercatinib (LOXO-292) have shown unprecedented efficacy in tumors positive for RET fusions or mutations, notably RET fusion-positive NSCLC and RET-mutated medullary thyroid cancer (MTC). However, the mechanisms of resistance to these agents have not yet been described. Methods: Analysis was performed of circulating tumor DNA and tissue in patients with RET fusion-positive NSCLC and RET-mutation positive MTC who developed disease progression after an initial response to selpercatinib. Acquired resistance was modeled preclinically using a CCDC6-RET fusion-positive NSCLC patient-derived xenograft. The inhibitory activity of anti-RET multikinase inhibitors and selective RET TKIs was evaluated in enzyme and cell-based assays. Results: After a dramatic initial response to selpercatinib in a patient with KIF5B-RET NSCLC, analysis of circulating tumor DNA revealed emergence of RET G810R, G810S, and G810C mutations in the RET solvent front before the emergence of clinical resistance. Postmortem biopsy studies reported intratumor and intertumor heterogeneity with distinct disease subclones containing G810S, G810R, and G810C mutations in multiple disease sites indicative of convergent evolution on the G810 residue resulting in a common mechanism of resistance. Acquired mutations in RET G810 were identified in tumor tissue from a second patient with CCDC6-RET fusion-positive NSCLC and in plasma from patients with additional RET fusion-positive NSCLC and RET-mutant MTC progressing on an ongoing phase 1 and 2 trial of selpercatinib. Preclinical studies reported the presence of RET G810R mutations in a CCDC6-RET patient-derived xenograft (from a patient with NSCLC) model of acquired resistance to selpercatinib. Structural modeling predicted that these mutations sterically hinder the binding of selpercatinib, and in vitro assays confirmed loss of activity for both anti-RET multikinase inhibitors and selective RET TKIs. Conclusions: RET G810 solvent front mutations represent the first described recurrent mechanism of resistance to selective RET inhibition with selpercatinib. Development of potent inhibitor of these mutations and maintaining activity against RET gatekeeper mutations could be an effective strategy to target resistance to selective RET inhibitors.

BackgroundBrain metastasis mainly originates from lung cancer. Napsin A and TTF-1 factors have frequently been detected in lung adenocarcinoma cases. Brain metastasis tumors with napsin A and TTF-1 positive are easily classified as lung adenocarcinoma origin. However, some thyroid cancers also exhibit these clinical features. Besides, lung is the most common metastasis of undifferential thyroid cancer. Therefore, it requires development of novel diagnostic tools to aid in distinguishing between pulmonary and thyroid origin.Patient FindingsWe reported a case that was initially diagnosed as brain metastatic lung cancer based on immunohistochemistry results. Analysis of next-generation sequencing (NGS) data from the brain lesion revealed that the cancer may have originated from the thyroid. We detected combo mutations in TERT promoter mutation, RET fusion and TP53, which are common in undifferential thyroid cancer (UTC), but rare for lung cancer. These results, coupled with identification of PAX8, indicated that this patient had UTC. Additionally, her three sons, despite being asymptomatic, were all diagnosed with papillary thyroid carcinoma.SummaryThe patient received anlotinib treatment and showed good clinical outcomes. One month after anlotinib treatment, the pulmonary nodules were found to be controlled, and the thyroid tumor drastically reduced, and tracheal compression relieved. She continued anlotinib treatment for the following two months, but died one month later because the treatment stopped owing to financial reasons. All her sons underwent total thyroidectomy with lymph node dissection.ConclusionsAlthough NGS has been reported to assist in diagnosis of the origin of some tumors, this is the first evidence of NGS for the determination of the origin of thyroid tumors. To our knowledge, this is the first time that a combination of multiple mutations has been used to help determine the origin of a tumor, compared with the previous single mutant gene. Moreover, this is the first evidence on the use of anlotinib for treatment of UTC with distant metastasis. Besides, all three sons of the patient had thyroid carcinoma in subsequent examinations, indicating high-risk for familial non-medullary thyroid cancer in UTC patients and necessity for performing thyroid ultrasound testing in other family members.

Since the discovery of RET fusion–positive (RET+) NSCLC around late 2011 to early 2012, clinical trials of multikinase inhibitors and highly potent and selective RET tyrosine kinase inhibitors have indicated that RET fusion is an actionable oncogenic driver in NSCLC. There seems to be a differential response to multikinase inhibitors depending on the fusion partner (KIF5B-RET versus non–KIF5B-RET); thus, knowledge of the fusion partners in RET+ NSCLC is important. To date, we identified 48 unique fusion partners in RET from published literature and congress proceedings. Two of the novel fusion partners (CCNYL2 and TRIM24) were identified in RET fusions that emerged as resistant to EGFR tyrosine kinase inhibitors. In addition, multiple intergenic rearrangements were identified.

The RET fusion is considered as the potential novel target in solid tumors. However, RET fusion is not well yet identified in colorectal cancer (CRC), and the effect of RET kinase inhibitor is also not evaluated in CRC with RET fusion. We established patient-derived tumor cells (PDCs) with RET fusion from recurrent brain metastatic lesion that newly appeared during the surveillance for stage III CRC patient. To investigate therapeutic options to CRC patient with a RET fusion, we performed cell viability assays using the PDCs. NCOA4-RET fusion was detected by FusionPlex using the resected brain metastatic tissue of CRC patient with solitary brain metastasis and then reconfirmed by fluorescence in situ hybridization (FISH) test. We also confirmed the RET fusions by a qPCR in matched PDCs. We tested whether the PDCs from RET fusion colon cancer were sensitive to carbozantinib, sorafenib, vandetanib, and PD0331992. Cell viability assays showed that carbozantinib, sorafenib, and PD0332991 did not suppress cell viability. Only, vandetanib revealed the significant inhibitory effect in MTT proliferation assay. Next, we analyzed regulation of targeted downstream pathways upon exposure to vandetanib by immunoblot assay. In colon cancer PDCs with NCOA4-RET fusion, vandetanib potently inhibited AKT and ERK phosphorylation. This study shows that vandetanib might be one of useful treatment strategies for CRC patient with NCOA4-RET fusion. Therefore, inhibition of the RET kinase is a promising targeted therapy for cancer patients whose tumors harbor a RET rearrangement.

// Sachiko Arai 1, * , Kenji Kita 1, * , Azusa Tanimoto 1 , Shinji Takeuchi 1 , Koji Fukuda 1 , Hiroshi Sato 2 and Seiji Yano 1 1 Division of Medical Oncology, Cancer Research Institute, Kanazawa University, Kanazawa, Japan 2 Division of Molecular Virology and Oncology, Cancer Research Institute, Kanazawa University, Kanazawa, Japan * These authors contributed equally to this work Correspondence to: Seiji Yano, email: syano@staff.kanazawa-u.ac.jp Keywords: RET fusion, lung cancer, alectinib, pleural carcinomatosis, NCOA4-RET Received: February 20, 2017 Accepted: March 19, 2017 Published: May 16, 2017 ABSTRACT Rearranged during transfection (RET) fusion-positive non-small cell lung cancer (NSCLC) accounts for approximately 1–2% of all NSCLCs. To date, RET fusions that involve at least six fusion partners in NSCLC, such as KIF5B, CCDC6, NCOA4, TRIM33, CLIP1, and ERC1, have been identified. Recent clinical trials for RET fusion-positive NSCLC using vandetanib or cabozantinib demonstrated positive clinical response and considerable differential activities for RET inhibitors among fusion partners. Alectinib, an approved ALK inhibitor, is reported to inhibit KIF5B-RET and CCDC6-RET. However, the activity of alectinib with respect to RET with other fusion partners is unknown. In the present study, we investigated the effects of alectinib on NCOA4-RET fusion-positive tumor cells in vitro and in vivo . Alectinib inhibited the viability of NCOA4-RET-positive EHMES-10 cells, as well as CCDC6-RET-positive LC-2/ad and TPC-1 cells. This was achieved via inhibition of the phosphorylation of RET and induction of apoptosis. Moreover, alectinib suppressed the production of thoracic tumors and pleural effusions in an orthotopic intrathoracic inoculation model of EHMES-10 cells. In vivo imaging of an orthotopically inoculated EHMES-10 cell model also revealed that alectinib could rescue pleural carcinomatosis. These results suggest that alectinib may be a promising RET inhibitor against tumors positive for not only KIF5B-RET and CCDC6-RET, but also NCOA4-RET.

Lung cancer in nonsmokers tends to be driven by a single somatic mutation or a gene fusion. KIF5B-RET fusion is an oncogene identified in non-small cell lung cancers. In this study, we verified the oncogenic activity of KIF5B-RET fusion and investigated how KIF5B-RET activates the specific signaling pathways for cellular transformation. We aimed to provide a basis for the further development of the therapy for KIF5B-RET positive lung cancer patients.RT-PCR was used to screen for KIF5B-RET fusions in Chinese lung cancer patients. To verify the oncogenic activity of KIF5B-RET kinase in lung cancer cells, we manipulated its expression genetically followed by colony formation and tumor formation assays. The mechanism by which KIF5B-RET kinase induces proliferation was investigated by western blot, coimmunoprecipitation, and administration of RET, MAPK and STAT3 inhibitors.Our study identified a KIF5B-RET fusion in Chinese NSCLC patients and demonstrated that KIF5B-RET transfected cells showed a significantly increased proliferation rate and colony-forming ability. Furthermore, we found that KIF5B-RET fusion kinase induced multilevel activation of STAT3 at both Tyr705 and Ser727, and KIF5B-RET-STAT3 signaling related inhibitors repressed the proliferation and tumorigenicity of lung cancer cells significantly.Our data suggest that KIF5B-RET promotes the cell growth and tumorigenicity of non-small cell lung cancers through multilevel activation of STAT3 signaling, providing possible strategies for the treatment of KIF5B-RET positive lung cancers.

3008 Background: RET (REarranged during Transfection) gene alterations (mutations and fusions) leading to constitutive kinase activity are identified as drivers of disease progression in multiple tumor types, including non-small cell lung cancer (NSCLC) and medullary thyroid cancer (MTC). BOS172738 is an investigational, potent, selective oral RET kinase inhibitor. This next-generation inhibitor was designed with nanomolar potency against RET and >300-fold selectivity against vascular endothelial growth factor receptor 2, to maximize the potential therapeutic window. Methods: NCT03780517 is a phase 1 study consisting of a dose-escalation and dose-expansion phase. During the escalation, 67 patients with RET-altered advanced solid tumors received once-daily oral doses of BOS172738 (10-150 mg). Intra-patient dose escalation was allowed as was over-accrual to dose levels deemed to be safe. Study endpoints were safety (CTCAE v. 4.03), tolerability and confirmed overall response rate (ORR; RECIST 1.1). The data cutoff was Dec. 11, 2020. Results: BOS172738 exhibited a favorable safety profile (n=67) for long-term administration with most treatment-emergent adverse events (TEAEs) classified as grade ≤2 and deemed unrelated to drug. The most common TEAEs were creatinine phosphokinase (CPK) increase (54%), dyspnea (34%), facial edema, aspartate aminotransferase elevation, anemia (25% each), neutropenia, diarrhea (22% each), fatigue (21%), and constipation (20%). BOS172738 was not associated with hypertension or significant hepatoxicity. BOS172738 demonstrated broad anti-tumor activity with an investigator-assessed ORR of 33% (n=18/54), a NSCLC ORR of 33% (n=10/30), MTC ORR of 44% (n=7/16, including 1 complete response) and one patient with RET fusion+ pancreatic cancer reported a partial response. Responders included patients with brain metastases with one patient whose brain lesion decreased by 43%. The median duration of response has not been reached. Many patients remain on study, including the longest of 659 days, at data cutoff. BOS172738 exhibited dose-dependent exposure (AUC, Cmax), rapid absorption (median Tmax 1 to 4.5 h), and an extended half-life (approximately 65 hours) maximizing target coverage. Conclusions: BOS172738 is a highly potent and selective RET inhibitor with a differentiated safety profile and clinical activity against RET-altered tumors, including patients with brain metastases. BOS172738 continues to be evaluated in patients with NSCLC, MTC, and in patients previously treated with other selective RET inhibitors. Clinical trial information: NCT03780517.

e21074 Background: Responses to EGFR-targeted therapy are generally temporary, due to inevitable drug resistance. Although RET fusions have been identified in resistant EGFR-mutant non–small cell lung cancer (NSCLC), their characteristics acquired resistance to EGFR tyrosine-kinase inhibitors (TKIs) are rarely investigated. Methods: We retrospectively reviewed next-generation sequencing data of EGFR+ lung cancer patients, and 8 patients were identified coexisting of EGFR mutations and RET fusion. Their treatment history was collected. Results: The co-occurrence of RET fusion with EGFR oncogenic variations was observed in eight patients, and all of the 8 patients have received previous EGFR-TKI treatment. EGFR mutations were including 4 L858R mutations, 4 exon 19 deletions, and 6 T790M mutations. And the partner genes of RET identified by NGS were including TRIM33 (2/8), GPRC6A (1/8), TLN1 (1/8), KIAA1598 (1/8), SPECC1 (1/8), TRIM24 (1/8) and CCDC6 (1/8). The allelic fractions (AFs) of first-generation EGFR-TKI sensitizing mutations were higher than AFs of EGFR T790M mutations as well as AFs of RET fusion. These RET fusions are fused with rare partner genes, rather than the most common KIF5B in lung cancer. Conclusions: This study extended the knowledge of RET fusion as resistance mechanism to EGFR TKIs in lung cancer. The detection of RET fusions may uncover potential resistance mechanisms of EGFR TKIs, which might inform therapeutic strategies, such as combination-therapy approaches.[Table: see text]

9065 Background: Selpercatinib, a first-in-class highly selective and potent, CNS-active RET kinase inhibitor, is approved in multiple countries for treatment of RET fusion+ lung or thyroid cancers. Here we report an update of efficacy and safety results which provide a longer follow up and increased number of patients (safety population: N = 345 vs N = 329). Methods: Pts with RET fusion+ NSCLC enrolled in the global, multicenter, ongoing LIBRETTO-001 trial (NCT03157128; 16 countries, 89 sites) were included in this analysis. Pts with the opportunity to be followed ≥6 months from their first dose were included in the efficacy-evaluable population for these analyses. Integrated analysis set (IAS) included 218 NSCLC pts with prior platinum-chemotherapy. Primary analysis set (PAS) was a subset of the IAS and included the first 105 consecutively enrolled pts. The treatment-naïve population included 48 efficacy-evaluable pts. Primary endpoint was objective response rate (ORR, RECIST v1.1) by independent review committee (IRC). Secondary endpoints included ORR by investigator, duration of response (DoR), progression-free survival (PFS), clinical benefit rate (CBR; CR+PR+SD ≥16 weeks), and safety. Safety population (N = 345) included all pts with NSCLC who received ≥1 selpercatinib dose by data cutoff (30 Mar 2020). Results: In pts with prior treatment (N = 218) and treatment-naïve (N = 48) pts, 56% and 60% were female, with a median pt age of 61 and 64 years, respectively. The ORR with selpercatinib was 57% in the IAS, 64% in the PAS, and 85% in the treatment-naïve population (Table). In both the IAS and PAS, the median DoR was 17.5 months, median PFS was 19.3 months at median follow-up of 12.0 and 15.7 months, respectively (Table). The most common treatment-emergent adverse events (TEAEs) reported in ≥25% of pts were dry mouth, diarrhea, hypertension, increased ALT/AST, edema peripheral, and fatigue. Twenty-five pts (7%) permanently discontinued due to TEAEs, with 10 pts (3%) discontinuing selpercatinib due to treatment-related AEs as per investigator. Conclusions: In this updated data set, selpercatinib continued to demonstrate durable antitumor activity in pts with RET-fusion+ NSCLC. Selpercatinib was well-tolerated with a safety profile consistent with previous reports. A global, randomized, phase 3 trial (LIBRETTO-431) evaluating selpercatinib compared with standard frontline therapy is ongoing. Clinical trial information: NCT03157128. [Table: see text]

9032 Background: Selpercatinib, a first-in-class highly selective, potent, CNS-active RET kinase inhibitor, is approved in multiple countries for treatment of RET fusion+ lung or thyroid cancers. Selpercatinib demonstrated durable antitumor activity in previously treated pts with RET fusion+ NSCLC in an ongoing Phase 1/2 trial, LIBRETTO-001 (Besse et al., ASCO 2021). Methods: Pts with RET fusion+ NSCLC enrolled in the global, multicenter, LIBRETTO-001 trial (NCT03157128; 16 countries, 89 sites). Primary endpoint was objective response rate (ORR). Secondary endpoints included progression-free survival, duration of response, and safety. This post-hoc intrapatient analysis was based on a 30 March 2020 data cutoff date. Historical physician-reported best overall response (BOR) from last systemic therapy received prior to enrollment was compared with selpercatinib BOR by independent review committee per RECIST v1.1, with each patient serving as his/her own control. Results: In efficacy-evaluable pts (N = 218) who previously received platinum-based chemotherapy (chemo), median pt age was 61 years, the majority with ECOG of 0/1 (37%/61%), with a median of 2 (range: 1-15) prior systemic therapies. Overall, 57% of patients responded to selpercatinib while 16% responded to the immediate prior therapy. ORR improvements with selpercatinib were observed regardless of prior therapy: chemotherapy + immune checkpoint inhibitor (ICI) (57% vs 14%), single-agent ICI (48% vs 3%), or chemotherapy (58% vs 15%). A total of 108 patients (49%) did not respond to immediate prior therapy but responded to selpercatinib. Fewer patients had progressive disease as their BOR with selpercatinib (2%) compared to the immediate prior therapy (28%). The median duration of therapy for selpercatinib was notably extended compared with that of the immediate prior therapy (11.8 vs. 3.4 months, respectively). Conclusions: In pts with RET fusion+ NSCLC treated on LIBRETTO-001, systemic therapies administered prior to enrollment achieved less meaningful clinical benefit than selpercatinib. Selpercatinib demonstrated consistent efficacy regardless of the type of prior therapy. Clinical trial information: NCT03157128.

3046 Background: Osimertinib, a highly selective third generation EGFR tyrosine kinase inhibitor (TKI) became the standard front-line therapy for EGFR-mutant NSCLC. However, therapeutic options are limited for TKI resistance which commonly occurs. Therefore, overcoming acquired resistance to osimertinib remains an important high unmet need in the field of precision oncology. Herein, we present the first case of advanced adenocarcinoma of the lung that showed notable response with the addition of selpercatinib after acquired resistance to osimertinib monotherapy. Methods: Case presentation. Results: A 37-year-old woman with stage IVB adenocarcinoma of lung with osseous, hepatic and brain metastases initially received one cycle of carboplatin, pemetrexed and pembrolizumab. Based on the EGFR exon19 deletion detected from ctDNA NGS assay (Guardant 360) [variant allele frequency (VAF) 62.7%], the treatment regimen was changed to osimertinib monotherapy (80mg PO daily). Bevacizumab was empirically added given CNS involvement. She maintained overall stable disease for 10 months before subsequent CT showed disease progression. The treatment regimen was switched to atezolizumab, bevacizumab, paclitaxel and carboplatin combination therapy. She tolerated 6 cycles of the regimen in 4 month before a subsequent brain MRI revealed progression of the metastatic brain disease with new leptomeningeal disease. Whole brain radiotherapy was performed and decision was made to start combination TKI treatment of selpercatinib (120mg BID) added to the osimertinib (80mg daily) monotherapy based on her repeat ctDNA NGS assay result showing concurrent acquired CCDC6RET fusion (VAF 0.05%) and EGFR exon 19 deletion (VAF 10.0%). The 6 week follow-up CT demonstrated significant decrease in the largest lung mass (33.95*24.22mm->32.50*16.07mm). Repeat ctDNA NGS assay at one week after selpercatinib use showed disappearance of RET fusion and significant decrease in EGFR clone (VAF 10.0% to 0.05%). Conclusions: It has been reported that co-occurring RET fusions in NSCLC patients with EGFR mutations may contribute to acquired resistance to EGFR inhibitors. Several successful cases of cabozantinib, a non-selective RET inhibitor, or pralsetinib, a selective RET inhibitor combined with EGFR inhibitor, have been reported to aid in overcoming the acquired resistance to EGFR inhibitors. To date, there has been no report of clinical benefit in adding a RET inhibitor based on ctDNA detection of RET fusion with minute variant allele frequency. We for the first time report the case of overcoming acquired resistance to osimertinib by adding selpercatinib, a selective RET inhibitor in NSCLC patients with acquired RET fusion detected in ctDNA at VAF of 0.05%.

467 Background: Recent tumor-agnostic drug approvals have resulted in a paradigm shift in cancer treatment away from organ/histology specific indications to biomarker-guided tumor-agnostic approaches. Pralsetinib is a potent and selective RET inhibitor, which has recently been approved by the U.S. Food and Drug Administration (FDA) for the treatment of adults with metastatic RET fusion–positive non-small cell lung cancer (NSCLC) and is under New Drug Application review for RET mutant thyroid cancers by the FDA. RET fusions occur in up to approximately 7‒8% of patients with gastrointestinal malignancies, including pancreatic, liver, and colorectal cancers. There are currently no approved selective RET inhibitors for patients with RET fusion–positive solid tumors other than NSCLC and thyroid cancer. Here, we present data on the clinical activity of pralsetinib in patients with RET fusion–positive solid tumor types other than NSCLC enrolled in the Phase I/II ARROW study (NCT03037385). Methods: ARROW consists of a phase I dose escalation (30–600 mg once [QD] or twice daily) followed by a phase II expansion (400 mg QD) in patients with advanced RET-altered solid tumors. Primary objectives are overall response rate (ORR), per RECICT v1.1 and safety. Results: A total of 13 patients with RET fusion–positive thyroid cancer (12 papillary, 1 poorly differentiated; enrollment cutoff July 11, 2019) and 14 patients with RET fusion–positive solid tumors other than NSCLC and thyroid (3 pancreatic, 3 colon, 2 cholangiocarcinoma, 6 other; enrollment cutoff November 19, 2019) were enrolled in ARROW and received pralsetinib. At the February 13, 2020, data cutoff, the ORR (blinded central review) in response-evaluable patients with RET fusion–positive thyroid cancer was 91% (10/11; 95% CI: 59‒100) and disease control rate was 100% (95% CI: 72‒100). Treatment was ongoing in 7 of 11 patients. In RET fusion–positive solid tumors other than NSCLC and thyroid, ORR (investigator’s assessment) was 50% (6/12; 95% CI: 21‒79) and responses were observed in all patients with pancreatic cancer (3/3) and cholangiocarcinoma (2/2). Treatment was ongoing in 6 of 12 patients, including 2 of 3 patients with pancreatic cancer and 1 of 2 patients with cholangiocarcinoma. Responses were observed across multiple fusion genotypes. In the 27 patients with RET fusion–positive tumors other than NSCLC, most frequent treatment-related adverse events (TRAEs) were grade 1–2, and included anemia (33%), increased aspartate aminotransferase (33%), decreased white blood cell count (33%), hypertension (30%), increased alanine aminotransferase (26%), hyperphosphatemia (19%), and neutropenia (19%). No patients discontinued due to TRAEs. Conclusions: Pralsetinib demonstrated broad and durable antitumor activity across multiple advanced solid tumor types, regardless of RET fusion genotype, and was well tolerated. The study is ongoing. Clinical trial information: NCT03037385.

The era of precision medicine has resulted in the identification of a number of genomic alterations that can be targeted with novel therapies. In lung adenocarcinomas, a histology structure that accounts for nearly 50% of all cases of lung cancer, and a number of genomic targets have been linked with effective targeted therapies. For patients with advanced-stage lung adenocarcinomas, molecular testing is now a standard part of diagnostic workup; for patients that have specific driver molecular events, targeted therapies have resulted in substantial improvement in efficacy without excessive toxicity. RET gene fusions are present in approximately 1% to 2% of NSCLC. It is emerging as a new targetable driver for this population. Despite sensitivity to platinum-based chemotherapy and conflicting small reports regarding the efficacy of immune checkpoint inhibitors, there have been limited treatment approaches for this subset of patients. Multiple nonselective RET tyrosine kinase inhibitors exhibited modest anti-RET activity with an increased off-target toxicity profile that often required dose interruption, reduction, or treatment cessation. Recently, novel selective RET inhibitors pralsetinib (BLU-667) and selpercatinib (LOXO-292) have exhibited promising clinical activity with low adverse effect profile in early clinical trials. These new agents are poised to represent a new hope for this special subgroup with unmet needs.

Introduction RET rearrangements define a distinct molecular subset of NSCLC. The multikinase inhibitor ponatinib reveals potent activity in preclinical models of RET-rearranged NSCLC. Methods In this single-arm, multicenter, phase II trial, we evaluated the clinical activity of ponatinib in patients with advanced, previously treated, RET-rearranged NSCLC (NCT01813734). RET rearrangements were identified through fluorescence in situ hybridization or next-generation sequencing. Ponatinib was administered at a dose of 30 mg once daily. Patients without a documented objective response were eligible to dose-escalate ponatinib to 45 mg daily. The primary end point was objective response rate. Results Between August 2014 and December 2017, nine patients were enrolled. The median age was 58 years (range 49–73 y). Eight patients (89%) had a history of brain metastases. The median number of previous lines of therapy was three (range 1–5). Of the nine evaluated patients, five (55%) experienced tumor shrinkage from baseline, but no confirmed responses were observed (objective response rate 0%). The disease control rate was 55%. With a median follow-up of 9.33 months, the median progression-free survival and overall survival were 3.80 months (95% CI: 1.83–5.30) and 17.47 months (95% CI: 6.57–19.20), respectively. The most common treatment-related adverse events were rash (n = 5; 56%), constipation (n = 4; 44%), and diarrhea (n = 4; 44%). No treatment-related thromboembolic or cardiac events were observed. The study was stopped prematurely owing to slow accrual and lack of clinical activity. Conclusions Ponatinib has limited clinical activity in patients with RET-rearranged NSCLC. Continued development of more potent and selective RET inhibitors is needed.

TPS9633 Background: RET gene fusions have been identified as oncogenic drivers in multiple tumor types, including 1-2% of NSCLC, but no selective RET inhibitors are approved for use. The investigational RET inhibitor, pralsetinib, potently and selectively targets oncogenic RET alterations, including those that confer resistance to multikinase inhibitors. In the registration-enabling phase 1/2 study (ARROW; NCT03037385), pts with RET-fusion+ NSCLC treated with 400 mg once daily (QD) of pralsetinib (N = 80) after platinum-based chemotherapy achieved an overall response rate (ORR) of 61% (95% CI 50, 72; 2 responses pending confirmation) per independent central review. In addition, a promising ORR of 73% (all centrally confirmed responses) was attained in the treatment naïve cohort (N = 26). Most treatment-related adverse events were grade 1-2 across the entire safety population treated at 400 mg QD (N = 354). AcceleRET Lung, an international, open-label, randomized, phase 3 study, will evaluate the efficacy and safety of pralsetinib versus standard of care (SOC) for first-line treatment of advanced/metastatic RET fusion+ NSCLC (NCT04222972). Methods: Approximately 250 pts with metastatic RET-fusion+ NSCLC will be randomized 1:1 to oral pralsetinib (400 mg QD) or SOC (non-squamous histology: platinum/pemetrexed ± pembrolizumab followed by maintenance pemetrexed ± pembrolizumab; squamous histology: platinum/gemcitabine). Stratification factors include intended use of pembrolizumab, history of brain metastases, and ECOG PS. Key eligibility criteria include no prior systemic treatment for metastatic disease; RET-fusion+ tumor by local or central assessment; no additional actionable oncogenic drivers; no prior selective RET inhibitor; measurable disease per RECIST v1.1. Pts randomized to SOC will be permitted to cross-over to receive pralsetinib upon disease progression. The primary endpoint is progression-free survival (blinded independent central review; RECIST v1.1). Secondary endpoints include ORR, overall survival, duration of response, disease control rate, clinical benefit rate, time to intracranial progression, intracranial ORR, safety/tolerability and quality of life evaluations. Recruitment has begun with sites (active or planned) in North America, Europe and Asia. Clinical trial information: NCT04222972 .

3584 Background: Selpercatinib (LOXO-292) is a highly selective and potent small molecule RET kinase inhibitor. Here we report an update on the efficacy and safety of selpercatinib in RET-fusion+ non-small-cell lung cancer (NSCLC). Methods: Patients with RET-fusion+ NSCLC were enrolled to the Phase 1/2 LIBRETTO-001 trial (NCT03157128), a global, multicenter trial (16 countries, 89 sites). Following the Phase 1 dose escalation portion of the trial, patients received the recommended dose of 160 mg orally twice daily. Each cycle was 28 days. The primary endpoint was objective response rate (ORR) per RECIST 1.1. Secondary endpoints included duration of response (DoR) and safety. Per health authority agreement, the primary analysis set was defined as the first 105 consecutively enrolled patients previously treated with platinum-based chemotherapy. Treatment-naïve patients were analyzed separately. All analyses were based on a 16-Dec-2019 data cutoff date. Results: In the primary analysis set of platinum-treated patients (median of 3 prior systemic regimens; range 1-15), the ORR by investigator assessment was 70% (95% CI 59.8–78.1, n = 73/105). Responses did not differ by fusion partner or number or type of prior therapies, including anti-PD-1/PD-L1 agents and off-label multikinase inhibitor use. The median DoR was 20.3 months (95% CI 15.6–24.0) with 45 of 73 (62%) responders censored at a median follow-up of 14.8 months. Among 39 treatment-naïve patients, the ORR by investigator assessment was 90% (95% CI 75.8–97.1, n = 35/39, including 2 responses pending confirmation). Median DoR was not reached with 27 of 33 (82%) confirmed responses ongoing at a median follow-up of 7.4 months. In the safety analysis set consisting of all selpercatinib dosed patients (N = 702), the most common treatment-related adverse events (TRAEs) that occurred in ≥15% of patients were dry mouth (33.3%), increased AST (24.5%), increased ALT (23.8%), hypertension (23.2%), diarrhea (19.7%), and fatigue (16.8%). Only 2% (14 of 702) of patients discontinued selpercatinib for TRAEs. Conclusions: Selpercatinib achieved marked and durable antitumor activity in patients with RET-fusion+ NSCLC. Selpercatinib was well tolerated. Efficacy data assessed by independent review committee based on the 16-Dec-2019 data cutoff date will be presented. Clinical trial information: NCT03157128 .

9515 Background: Pralsetinib is an investigational, highly potent, selective RET kinase inhibitor targeting oncogenic RET alterations. We provide the registrational dataset for pts with RET fusion+ NSCLC with and without prior treatment from the global ARROW study. Methods: ARROW (75 sites in 11 countries; NCT03037385) consists of a phase I dose escalation to establish recommended phase II dose (400 mg once daily [QD] orally) and phase II expansion cohorts defined by tumor type and/or RET alteration. Primary objectives were overall response rate (ORR; blinded independent central review per RECIST v1.1) and safety. Efficacy analyses are shown for response-evaluable pts (REP) with RET fusion+ NSCLC who initiated 400 mg QD pralsetinib by July 11 2019 and safety for all pts (regardless of diagnosis) treated with 400 mg QD. Results: As of November 18 2019, 354 pts with advanced solid tumors had received pralsetinib at starting dose of 400 mg QD with median follow-up 8.8 months. ORR, disease control rate (DCR), and % of pts with tumor size reduction are shown in the table for pts with metastatic RET fusion+ NSCLC (n=116; 72% KIF5B; 16% CCDC6; 12% other/fusion present but type unknown) and with prior platinum treatment (n=80) or without prior systemic treatment (n=26). ORR was similar regardless of RET fusion partner, prior therapies, or central nervous system involvement. Overall there were 7 (6%) complete responses, 4 (5%) in prior platinum pts and 3 (12%) in treatment naïve pts; median time to response overall was 1.8 months and median duration of response (DOR) was not reached (95% CI, 11.3–NR). In the safety population (n=354), most treatment-related adverse events (TRAEs) were grade 1-2, and included increased aspartate aminotransferase (31%), anemia (22%), increased alanine aminotransferase (21%), constipation (21%) and hypertension (20%). 4% of pts in the safety population (all tumor types) discontinued due to TRAEs. Conclusions: Updated, registrational, centrally reviewed data demonstrate that pralsetinib has rapid, potent, and durable clinical activity in pts with advanced RET fusion+ NSCLC regardless of RET fusion genotype or prior therapies, and QD oral dosing is well-tolerated. Clinical trial information: NCT03037385 . [Table: see text]

9008 Background: RET fusions are targetable oncogenic drivers in up to 2% of NSCLC, yet no selective RET inhibitors are approved. BLU-667 is an investigational highly potent and selective RET inhibitor targeting oncogenic RET alterations, including those that confer resistance to multikinase inhibitors (MKIs). We provide an update on the registration-enabling ARROW study (NCT03037385) of BLU-667 in pts with RET-fusion+ NSCLC. Methods: The global ARROW study includes DE (30-600 mg daily [QD or BID]) and dose expansion (DX) at the recommended phase 2 dose (RP2D; 400 mg QD) in pts with advanced solid tumors. Primary objectives are overall response rate (ORR; RECIST 1.1) and safety. Results: As of 19Dec2018, 79 pts (21 DE, 58 DX) with advanced RET fusion+ NSCLC (44 KIF5B, 16 CCDC6, 19 other/pending) received BLU-667. Median number of prior therapies was 2 (range 0-8) and includes chemotherapy (76%), immunotherapy (41%), and MKI (27%). 39% had baseline brain metastases. ORR among 57 response-evaluable pts with measurable disease and at least one follow-up disease assessment was 56% (95% CI: 42, 69; 32 partial responses (PR), 9 PR pending confirmation, 20 stable disease, 5 progressive disease). 91% (29/32) of responding pts remain on treatment; 6 have achieved response duration ≥ 6 months. Disease control rate (DCR) was 91% (52/57). Among 30 pts at the RP2D previously treated with platinum chemotherapy, ORR was 60% (18 PRs; 7 pending confirmation). Responses occur regardless of prior treatment or RET fusion genotypes. Intracranial activity has been observed with shrinkage of brain metastases. 80% of NSCLC pts treated at RP2D remain on treatment and only 3% discontinued due to related adverse event. In NSCLC patients, treatment-related toxicity (TRT), generally low-grade and reversible (28% had ≥ grade 3 events), included increased AST (22%), hypertension (18%), increased ALT (17%), constipation (17%), fatigue (15%) and decreased neutrophils (15%). Conclusions: BLU-667 demonstrated potent, durable and broad antitumor activity and was well tolerated in pts with advanced RET-fusion+ NSCLC. Enrollment of the expansion is ongoing with registrational intent. Clinical trial information: NCT03037385.

e20634 Background: EGFR-TKIs is the standard first/second-line treatment for EGFR-mutant advanced non-small-cell lung cancer (NSCLC). The mechanisms of EGFR-TKIs resistance are still under exploration. Acquired fusion have been reported contribute to EGFR-TKIs resistance. Here we focus on RET fusion in first/third-generation EGFR-TKIs resistant NSCLC. Methods: We retrospectively reviewed 3600 cases of EGFR-TKIs resistant NSCLC samples from 2016 to 2018 in our institute. Tumor biopsy, ctDNA or pleural effusion samples were analyzed using hybridization capture-based NGS ER-seq method, which enables simultaneously assess single-nucleotide variants (SNV), insertions/deletions (indel), rearrangements and somatic copy-number(CNV) variation at least 59 genes (range 59-1021 genes). Results: Seven cases with RET fusion were identified (7/3600, 0.2%), with co-occurring EGFR mutations. All were adenocarcinoma, median diagnosis age was 55 years old (range 38-84), four male and three female. Most common RET fusion subtype was CCDC6-RET (5/7, 71%), the other was NCOA4-RET (2/7, 29%). The primary EGFR mutation include four EX19del and three L858R. Six patients received prior first and third-generation EGFR-TKIs treatment. The seventh patient had received only gefitinib treatment, EGFR L858R + T790M + NCOA4-RET were discovered in his plasma when disease progression. Interestingly, one patient had EGFR EX19del + T790M + C797S (cis) + CCDC6-RET in plasma after osimertinib resistance. One patient who had EGFR L858R and NCOA4-RET chose lenvatinib, a RET inhibitor, and had a progression free survival of seven months. Conclusions: Broad NGS panel test suggests that RET fusion could be a rare mechanism of EGFR-TKIs resistance, include first and third-generation. There is no currently target treatment strategy available for these patients, and further investigations, like change target therapy drug or combined target therapy are needed.