Rearranged During Transfection Mutations Research Articles

Distinct Impacts of Clinicopathological and Mutational Profiles on Long-Term Survival and Recurrence in Medullary Thyroid Carcinoma.

Medullary thyroid carcinoma (MTC) has a poorer prognosis than differentiated thyroid cancers; however, comprehensive data on the long-term outcomes of MTC remain scarce. This study investigated the extended clinical outcomes of MTC and aimed to identify prognostic factors. Patients diagnosed with MTC between 1980 and 2020 were retrospectively reviewed. Their clinical characteristics, longterm clinical outcomes, and prognostic factors for recurrence and mortality were analyzed. The study included 226 patients (144 women, 82 men). The disease-specific survival (DSS) rates for all MTC patients at 5-, 10-, 20-, and 30-year intervals were 92.7%, 89.4%, 74.3%, and 68.1%, respectively. The recurrence-free survival (RFS) rates were 71.1%, 56.1%, 40.2%, and 32.1% at these intervals. DSS was comparable between the groups from 1980-2009 and 2010-2020 (P=0.995); however, the 1980-2009 group had significantly lower RFS rates (P=0.031). The 2010-2020 group exhibited greater extents of surgical and lymph node dissection (P=0.003) and smaller tumors (P=0.003). Multivariate analysis identified extrathyroidal extension as the strongest prognostic factor for both RFS and DSS. Age >55 years and tumor size of ≥2 cm were also significant prognostic factors for DSS, while hereditary disease and lymph node metastasis were significant for RFS. Survival analysis after propensity-score matching of rearranged during transfection (RET)-negative and non-screened RET-positive groups showed comparable DSS but longer RFS in the RET-negative group. Extrathyroidal extension was identified as the strongest prognostic factor for RFS and DSS. Older age and larger tumor size were associated with decreased DSS, while RET mutation and lymph node metastasis significantly impacted RFS.

7214 A Retrospective Cohort Study on Prevalence Of Pituitary Adenomas in MEN2

Abstract Disclosure: R. Ghosh: None. N. Behiri: None. J. Glod: None. S. Gubbi: None. J. Klubo-Gwiezdzinska: None. Background: Multiple endocrine neoplasia (MEN) syndromes are autosomal dominant and present with tumors in at least two or more endocrine organs. MEN2 syndromes that are driven by germline rearranged during transfection (RET) gene pathogenic variants are divided into 2A consisting of medullary thyroid carcinoma (MTC), parathyroid adenomas and pheochromocytoma and 2B presenting with MTC, pheochromocytoma, marfanoid habitus, mucosal neuromas and gastrointestinal ganglioneuromatosis. Pituitary adenomas have been rarely described in MEN2 and hence we conducted a retrospective chart review to determine their prevalence in MEN2. Methods: Retrospective chart screening of MEN2 patients followed at a tertiary referral center between August 1999- August 2022 was done to evaluate the prevalence of pituitary adenomas using biochemical hormonal evaluation and magnetic resonance imaging (MRI) of pituitary based on screening of clinical symptoms. Results: The study consisted of 91 patients (43 MEN2A and 48 MEN2B), aged 35.3±16.9 years (MEN2A) and 18.6±6.4 years (MEN2B), 20/43 (46.5%) women in MEN2A and 24/48 (50%) women in MEN2B. These patients were followed for median duration of 6.5 years (3.3-22) in MEN2A and 8.5 years (5.4-14.6) in MEN2B. RET mutation status was unavailable for 1 MEN2A patient out of 91 patients. Among MEN2A patients, majority (53.5%) were positive for RET C634X mutation followed by RET 804, 609, 631,620 and 666 mutations in the descending order. While in MEN2B cohort all patients were positive for M918T RET mutation. A total of 4/91 (4.4%) patients were found to have pituitary adenomas (3 MEN2B and 1 MEN2A). One patient had clinically symptomatic ACTH producing microadenoma which was surgically resected, and others have asymptomatic non-functioning adenomas. The patient with Cushing’s disease underwent three transsphenoidal resections leading to complete resolution but developed panhypopituitarism. Prevalence of pituitary adenomas in our cohort was significantly higher than the general population screened based on clinical suspicion (p<0.001). Conclusion: Prevalence of clinically symptomatic pituitary adenomas is 1 in 1000 in the general population (1). It is rarely associated with MEN2 syndromes and only few cases are reported in the literature (2). However, screening of all patients with MEN2 for pituitary adenomas, regardless of clinical suspicion, might lead to incidental discovery of asymptomatic pituitary pathology and can provide the actual prevalence. Further studies are needed to evaluate the genetics of these pituitary adenomas in MEN2 syndromes to investigate whether RET protooncogene is indeed the dominant molecular driver of these tumors.

Genotype/phenotype correlations in multiple endocrine neoplasia type 2.

Multiple endocrine neoplasia type 2 (MEN 2) is a rare hereditary endocrine tumour syndrome caused by mutations in the rearranged during transfection (RET) gene. MEN 2 is divided into two main entities, MEN 2A and MEN2B, both of which present with medullary thyroid cancer (MTC) in approximately 100% of cases and pheochromocytoma in 50% of cases. Specific RET mutations are associated with a risk of early onset of MTC, from 1 year of age (highest risk) to 5 years of age (high risk). This risk defines the optimal timing for thyroidectomy, ideally at an age when the disease has not spread. This is the most important genotype-phenotype correlation observed in MEN 2. Specific RET mutations also define the penetrance of pheochromocytoma. However, despite the presence of these highest/high risk variants, some patients unexpectedly present with non-aggressive MTC or never present with pheochromocytoma, suggesting that factors other than the major RET variant may modify the natural history and genotype-phenotype correlations. Improving our understanding of the genotype-phenotype correlations would allow individualizing the management and follow-up of patients with MEN 2. The aim of this brief review is to discuss the main genotype-phenotype correlations in MEN 2 and the potential factors that might influence these correlations.

30years of prophylactic thyroidectomy for hereditary medullary thyroid cancer : Amilestone in translational medicine

Medullary thyroid cancer (MTC) is the most frequent manifestation of multiple endocrine neoplasia type 2 (MEN2) that determines the oncological outcome. Germline mutations in the rearranged during transfection (RET) protooncogene, atumor suppressor gene on chromosome 10q11.2, were identified 30 years ago as the genetic basis of MEN2 and published in 1993 and 1994. These seminal findings gave rise to the concept of prophylactic thyroidectomy for asymptomatic gene mutation carriers based on apositive RET gene test, which has become the standard of care ever since. Clinical genetic investigations showed genotype-phenotype correlations with respect to the individual gene mutation regarding the penetrance and onset of MTC and to alesser extent also with respect to the other components of MEN2, pheochromocytoma and primary hyperparathyroidism. From this a clinically relevant risk stratification could be derived. Initially, the optimal timing of prophylactic thyroidectomy was primarily based on the RET genotype alone, which was not sufficient for a precise age recommendation and subsequently required additional consideration of calcitonin serum levels for fine tuning. Calcitonin levels first show the risk of lymph node metastasis when they exceed the upper normal limit of the assay independent of carrier age and RET mutation. Routine calcitonin screening of patients with nodular thyroid disease, screening of families on identification of MEN2 index patients, and pre-emptive thyroidectomy in carriers of gene mutations with normal calcitonin levels have led to the fact that nowadays, 30years after the first description of the gene mutations causing the disease, the life-threatening hereditary MTC has become curable: ashining example for the success of translational transnational medical research for the benefit of patients.

Selpercatinib attenuates bleomycin-induced pulmonary fibrosis by inhibiting the TGF-β1 signaling pathway

IPF is a chronic, progressive, interstitial lung disease with high mortality. Current drugs have limited efficacy in curbing disease progression and improving quality of life. Selpercatinib, a highly selective inhibitor of receptor tyrosine kinase RET (rearranged during transfection), was approved in 2020 for the treatment of a variety of solid tumors with RET mutations. In this study, the action and mechanism of Selpercatinib in pulmonary fibrosis were evaluated in vivo and in vitro. In vivo experiments demonstrated that Selpercatinib significantly ameliorated bleomycin (BLM)-induced pulmonary fibrosis in mice. In vitro, Selpercatinib inhibited the proliferation, migration, activation and extracellular matrix deposition of fibroblasts by inhibiting TGF-β1/Smad and TGF-β1/non-Smad pathway, and suppressed epithelial-mesenchymal transition (EMT) like process of lung epithelial cells via inhibiting TGF-β1/Smad pathway. The results of in vivo pharmacological tests corroborated the results obtained from the in vitro experiments. Further studies revealed that Selpercatinib inhibited abnormal phenotypes of lung fibroblasts and epithelial cells in part by regulating its target RET. In short, Selpercatinib inhibited the activation of fibroblasts and EMT-like process of lung epithelial cells by inhibiting TGF-β1/Smad and TGF-β1/non-Smad pathways, thus alleviating BLM-induced pulmonary fibrosis in mice.

Targeting RET alterations in non-small cell lung cancer

Rearranged during transfection (RET) alterations, which lead to aberrant activation of the RET proto-oncogene, have been identified in various cancers. In non-small cell lung cancer (NSCLC), RET mutations often manifest as RET fusion genes and are observed in 1–2 % of patients with NSCLC. In recent years, selective RET inhibitors such as selpercatinib and pralsetinib, approved by the Food and Drug Administration (FDA) in 2020, have been part of the revolutionary changes in the treatment landscape for non-small cell lung cancer. While first-generation RET inhibitors have become part of the standard of care for RET-fusion positive NSCLC, a new challenge has emerged: acquired resistance to RET inhibitors. RET resistance is a complex phenomenon that can manifest as either on-target or off-target resistance. Numerous studies have been conducted to identify the mechanisms behind this resistance. This review provides an overview of the biology of RET in NSCLC, methods of RET testing, and a comprehensive analysis of the clinical outcomes associated with multikinase and selective RET inhibitors for NSCLC. Additionally, we will explore future perspectives for RET fusion-positive NSCLC, including ongoing trials and the challenges involved in overcoming resistance to RET inhibitors.

Unraveling the Promise of RET Inhibitors in Precision Cancer Therapy by Targeting RET Mutations.

Over the past decades, the role of rearranged during transfection (RET) alterations in tumorigenesis has been firmly established. RET kinase inhibition is an essential therapeutic target in patients with RET-altered cancers. In clinical practice, initial efficacy can be achieved in patients through the utilization of multikinase inhibitors (MKIs) with RET inhibitory activity. However, the effectiveness of these MKIs is impeded by the adverse events associated with off-target effects. Recently, many RET-selective inhibitors, characterized by heightened specificity and potency, have been developed, representing a substantial breakthrough in the field of RET precision oncology. This Perspective focuses on the contemporary understanding of RET mutations, recent advancements in next-generation RET inhibitors, and the challenges associated with resistance to RET inhibitors. It provides valuable insights for the development of next-generation MKIs and selective RET inhibitors.

Exposure-Response Relationships for Pralsetinib in Patients with RET-Altered Thyroid Cancer or RET Fusion-Positive Nonsmall Cell Lung Cancer.

Pralsetinib is a highly potent oral kinase inhibitor of oncogenic RET (rearranged during transfection) fusions and mutations. Pralsetinib received approval from the United States Food and Drug Administration for the treatment of patients with metastatic RET fusion-positive non-small cell lung cancer (NSCLC), and received accelerated approval for the treatment of patients with RET fusion-positive thyroid cancer. Exposure-response (ER) analyses of efficacy were performed separately in patients with thyroid cancer and in patients with NSCLC, but data for all patients were pooled for the safety analysis. ER models were developed with time-varying exposure; the effect of covariates was also examined. For patients with NSCLC, a higher starting dose was associated with improved progression-free survival (PFS), but this improvement did not correlate with a higher exposure overall. Significant covariates included sex and baseline Eastern Cooperative Oncology Group (ECOG) score. For patients with thyroid cancer, a higher exposure was associated with improved PFS. Significant covariates included prior systemic cancer therapy and ECOG score. For safety, higher exposure was associated with a greater risk of grade≥3 anemia, pneumonia, and lymphopenia. Patients with an ECOG score of ≥1 had an increased risk of grade≥3 pneumonia. Non-White patients had a lower risk of grade≥3 lymphopenia. ER analysis revealed that higher pralsetinib exposure was associated with improved PFS in thyroid cancer, but not in NSCLC. However, a higher starting dose (ie, 400 vs ≤300mg daily) was correlated with better PFS for all indications. Higher exposure was also associated with an increased risk of grade≥3 adverse events (AEs); however, the overall incidence of these events was acceptably low (≤20%). This analysis supports the use of a 400mg starting dose of pralsetinib, allowing for dose reduction in the event of AEs.

Precision medicine in endocrinology exemplified by medullary thyroid cancer

Medullary thyroid cancer (MTC) is a prime example for precision medicine in endocrinology and underlines the immediate benefits of basic, translational and healthcare research for patients with a rare disease in clinical . A mutation in the rearranged during transfection (RET) proto-oncogene that codes for a transmembrane receptor protein tyrosine kinase, leads to constitutive activation of the kinase, which is the decisive pathomechanism for the disease. The MTC occurs in a sporadic (somatic RET mutation) or hereditary form (RET germline mutation, multiple endocrine neoplasia types 2 and 3). For germline mutation carriers the timing of preventive thyroidectomy depends on the RET genotype. For advanced metastasized RET-mutant MTC, selective RET kinase inhibitors are available, which are currently considered to be game changers in the treatment. Based on the specific tumor marker calcitonin, MTC can be identified at an early stage during the differential diagnosis of thyroid nodules. The preoperative calcitonin level even enables statements on the degree of dissemination of the disease and on the probability of a cure through surgery. A new development is the consideration of desmoplasia as a histopathological biomarker for the metastatic potential of a MTC, which could possibly modify the operative approach as well as the future MTC nomenclature. Furthermore, the postoperative calcitonin level and the calcitonin doubling time are highly valid prognostic markers for tumor burden and biological aggressiveness of MTC and therefore decisive for patient follow-up. Biochemical, molecular and histological markers enable a risk-adapted surgical treatment and together with new targeted systemic treatments have contributed to a paradigm shift in the diagnostics, prognosis and treatment of MTC in recent years. Endocrine precision medicine for MTC therefore enabled a change from the previous purely symptom-oriented to a modern preventive and individualized treatment.

Real-world clinical profile, RET mutation testing, treatments, and PROs for MTC in Europe.

This study aimed to describe real-world patient and physician characteristics, rearranged during transfection (RET) mutation testing and results, treatment patterns, and patient-reported outcomes (PROs) in advanced or metastatic medullary thyroid cancer (aMTC) across five populous European countries. Cross-sectional physician and patient surveys were used to collect quantitative and qualitative data in France, Germany, Italy, Spain, and the UK from July to December 2020, prior to the introduction of selective RET inhibitors in Europe. Physicians completed patient record forms and a survey about their specialty and practice site. Patients were asked to provide PRO data using four validated instruments, including the EuroQol 5 Dimension (EQ-5D) questionnaire. The physician-reported sample included 275 patients with aMTC, including 79 patients with RET mutation-positive disease; median age was 60 and 56 years, respectively. Overall, 75% were tested for RET mutation (35% germline only, 21% somatic only, 44% both). Common physician-cited barriers to RET mutation testing included high cost, difficulty accessing latest tests, and time delay for results. First-line systemic therapy (most commonly vandetanib or cabozantinib) was prescribed for 69% of patients overall and 82% of the RET mutation-positive subgroup. Second-line therapy was prescribed for 12% of patients who received first-line therapy; most patients remained on first-line therapy at data capture. PROs revealed substantial disease/treatment burden. Patients with aMTC report substantial disease/treatment burden. Outcomes could be improved by identifying patients eligible for treatment with selective RET inhibitors through more optimal RET mutation testing.

Selpercatinib: A narrative drug review

Rearranged during transfection (RET) alteration promotes oncogenesis in a few cancers. RET mutation positivity is seen in approximately 70% of medullary thyroid cancers, around 30% of differentiated papillary thyroid cancers, and 1-2% of non-small-cell lung cancers (NSCLC). To write this narrative drug review, we searched various websites like the United States Food and Drug Administration, PubMed, Google Scholar, UpToDate, and recently published papers in various international conferences using the search terms “RET,” “RET alteration,” “Retevmo,” “RET inhibitors,” and “selpercatinib.” We shortlisted 31 articles published between January 1980 and January 2024. We discuss the history, mechanism of action, resistance, pharmacodynamics, pharmacokinetics, dosing, toxicity, pivotal trials, and indications of selpercatinib. Selective RET inhibitors like selpercatinib are indicated in the treatment of RET-altered NSCLC and thyroid cancer.

Selective RET Inhibitors (SRIs) in Cancer: A Journey from Multi-Kinase Inhibitors to the Next Generation of SRIs.

RET is a receptor tyrosine kinase that plays an important role in the development of neurons and kidneys. The gene encoding the rearranged-during-transfection (RET) receptor tyrosine kinase was first discovered in the 1980s. Activating RET mutations and rearrangements have since been identified as actionable drivers of oncogenesis in numerous cancer types and are most prevalent in thyroid and non-small-cell lung cancer. Following the modest success of repurposed RET-active multikinase inhibitors, the first selective RET inhibitors (SRIs), selpercatinib and pralsetinib, received regulatory approval in 2020. Now, thousands of patients with RET-altered cancers have benefited from first-generation SRIs, with impressive deep and durable responses. However, following prolonged treatment with these SRIs, a number of acquired on-target resistance mutations have been identified together with other non-RET-dependent resistance mechanisms. Today, the focus is on how we can further evolve and improve the treatment of RET-altered tumors with next-generation SRIs, and a number of candidate drugs are in development. The ideal next-generation SRIs will be active against on-target acquired resistance alterations, including those that emerge in the CNS, and will have improved safety and tolerability relative to first-generation SRIs. In this review, we will provide an update on these candidates and their potential to meet the unmet clinical need for patients who progress on first-generation SRIs.

Multiple endocrine neoplasia type 2: towards a risk-based approach integrating molecular and biomarker results

Purpose of review Significant advances have transformed our understanding of the molecular biology and natural history of multiple endocrine neoplasia type 2 (MEN2). This progress enacted a paradigm shift with regard to routine neck dissection for medullary thyroid cancer and total adrenalectomy for pheochromoytoma. The purpose of this review is to summarize key molecular and clinical data underpinning the current risk-based approach to MEN2 that integrates molecular and biomarker results. Recent findings Early identification and biochemical monitoring of rearranged during transfection (RET) carriers yield important lead time. Within these ‘windows of opportunity’, total thyroidectomy alone, avoiding incremental morbidity from node dissection; ‘tissue-sparing’ subtotal adrenalectomy, balancing risks of steroid dependency with pheochromocytoma recurrence in adrenal remnants; and parathyroidectomy of enlarged glands only, weighing risks of postoperative hypoparathyroidism against hyperactive parathyroid glands left behind, are adequate therapies. Summary All that is needed to determine a RET carriers’ risk of medullary thyroid cancer, pheochromocytoma and/or primary hyperparathyroidism in the molecular era is patient age, underlying RET mutation, and biomarker levels. As broader testing begins to penetrate healthcare, the needle on population genomic screening and education needs to be moved forward to complete the transition from symptom-based to preventive healthcare.

Patterns of Treatment Failure After Selective Rearranged During Transfection (RET) Inhibitors in Patients With Metastatic Medullary Thyroid Carcinoma.

Medullary thyroid cancer (MTC) harbors frequent mutations in RET oncogene. Selective RET inhibitors (RETi) have emerged as effective treatments. However, resistance almost invariably occurs. MTC patients who were initiated on RETi between 2018 and 2022 were included. Baseline characteristics, RET mutational status, RETi response, available tumor tissue and molecular profiles sampled pre- and post-RETi were analyzed. Among 46 MTC patients on RETi during the study period, 26 patients had discontinued at data cut-off because of progression (n = 16), death (n = 4), and toxicity (n = 6). The most frequent RET mutations at baseline were p.M918T (n = 29), and p.C634X (n = 6). Pre- and post-RETi molecular profiles were available in 14 patients. There was no primary resistance on pre-RETi samples. Post-RETi profiles revealed a bypass mechanism of resistance in 75% of the cases including RAS genes mutations (50%), FGFR2 and ALK fusions and and MYC p.P44L. RET solvent from and hinge region mutations was the only resistance mechanisms in 25% of the cases. Tumor samples from initial thyroidectomy, pre- and post-RETi, from six patients, showed an increase of the mean Ki 67-index of 7%, 17% and 40% respectively (P = 0.037) and a more aggressive poorly differentiated histology in three patients. Bypass resistance may be the most frequent mechanism of progression under RETi. A more aggressive histology may arise following RETi and warrants further investigation.

Maternal Vitamin A Status as a Risk Factor of Hirschsprung Disease in the Child.

The gene-environment interaction of the REarranged during Transfection ( RET ) gene with vitamin A in the etiopathogenesis of Hirschsprung disease (HSCR) has been suggested in rodents. The aim of this study was to evaluate vitamin A status in mothers of children with HSCR and to assess its association with pathogenic variants of the RET gene in affected children. This was a case-control study of stable isotope-based vitamin A measurement stores of mothers of children diagnosed with HSCR (within 8 months from birth, n = 7) and age-matched mothers of normal children (n = 6). Next-generation sequencing of RET exons, along with their upstream promoter region, was performed in the 7 HSCR proband-parent triads to evaluate pathogenic variants. Maternal vitamin A stores in the HSCR group was almost 50% that of those in controls, tending toward significance (0.50 ± 0.17 vs 0.89 ± 0.51 μmol/g respectively, P = 0.079). Two novel pathogenic de novo mutations were identified in 2 cases, and a rare single-nucleotide deletion was detected in the 3.5-kb RET upstream region, in a heterozygous state, in all 7 proband-parent triads. Low-penetrance RET haplotypes associated with HSCR were detected in 5 cases. Mothers with children with HSCR had lower vitamin A liver stores than mothers with normal children, and the children who were affected had HSCR despite having no established pathogenic RET variants. Lower maternal vitamin A status may increase the penetrance of genetic mutations in RET , and vitamin-A mediated gene-environment interactions may underpin some of the etiology of HSCR.

Clinic Heterogeneity and Management of Pediatric Patients With Germline RET Proto-oncogene Mutation: Single-center Experience.

Inherited forms of medullary thyroid carcinoma (MTC) can cause serious problems in diagnosis and follow-up. Family screening is performed, and prophylactic thyroidectomy at an appropriate age can be life-saving. This study aimed to investigate the diagnostic, clinical, laboratory characteristics, and treatment methods of cases with rearranged during transfection ( RET) mutation in the childhood age group. Patients diagnosed with hereditary MTC and patients who were evaluated by detecting MTC and/or RET mutations in their families were included in this study. Nine cases from 6 families were included in the study. Seven patients were evaluated as a result of screening, whereas 2 patients, one of whom was MEN2B, were symptomatic. Prophylactic thyroidectomy was performed in 7 cases. Medullary microcarcinoma was found in all, and additional papillary thyroid carcinoma in one. An inoperable tumor was detected in one patient, and sorafenib treatment was applied. A very heterogeneous clinical presentation can be seen in a group of pediatric patients with RET mutation. In rare RET mutations, the genotype-phenotype relationship is still unclear, and different clinical pictures can be seen. Although prophylactic thyroidectomy is life-saving, it can cause iatrogenic hypothyroidism and hypoparathyroidism. Concomitant papillary microcarcinomas may occur in very young children with germline RET mutation.

First results from the RETgistry: A global consortium for the study of resistance to RET inhibition in RET-altered solid tumors.

9065 Background: Rearranged during transfection ( RET) gene alterations are the oncogenic driver in diverse tumor types, including RET fusions identified in 1-2% of non-small cell lung cancers (NSCLC). While RET-selective tyrosine kinase inhibitors (TKIs) selpercatinib and pralsetinib are effective, acquired drug resistance remains a challenge. Here, we report the initial results from the RETgistry, an international consortium aimed at elucidating mechanisms of resistance to RET TKIs across RET-altered solid tumors. Methods: This was a retrospective analysis performed across 16 institutions. Patients (pts) were eligible if they had advanced solid tumor harboring an oncogenic RET alteration, received ≥1 RET TKI with disease progression, and underwent resistant tumor or liquid biopsy for next-generation sequencing (NGS). Results: A total of 103 time-distinct biopsies (62 tissue, 30 plasma, 11 paired tissue/plasma) were included in analysis, obtained from 88 pts with progression on a RET-selective TKI [selpercatinib (n = 70), pralsetinib (n = 14), selpercatinib and pralsetinib sequentially (n = 4)]. Pts had the following tumor types: 72 NSCLC (69% KIF5B-RET, 21% CCDC6-RET, 10% other RET fusion), 13 medullary thyroid cancer (TC) (54% RET M918T, 46% other RET mutation), and 2 papillary and 1 anaplastic TC (all, RET fusion). Median age was 58 (range, 21-86); 48% were male with 89% never/light smokers. Median duration of RET TKI preceding biopsies [first-line, n = 32 (31%); second-line, n = 42 (41%); third-/greater-line, n = 29 (28%)] was 16.5 months (mos) (95% CI, 14.0-19.6); median PFS was 14.1 mos (95% CI, 9.3-17.0). Resistant biopsies were obtained at median 15.0 mos from TKI initiation (range, 1.8-58.8). Acquired RET mutations were detected in 14 (14%), most common being G810 substitutions in 12 (12%). Potential off-target resistance gene alterations identified in 43 cases (42%) included MET amplification (14%), BRAF V600E or fusion (2%), KRAS gain or mutation (5%), ERBB2 amplification (2%), EGFR amplification (3%), ROS1 fusion (1%), and activating PIK3CA mutation or PTEN loss (4%). No resistant lung cancer biopsy demonstrated small cell transformation. The duration of TKI therapy (HR 0.64, p = 0.11) or PFS (HR 0.75, p = 0.33) did not differ according to the presence of on-target vs off-target resistance. NGS analyses of pre-RET-selective TKI tumors (n = 92) revealed frequent co-occurring alterations in TP53 (29%) and CDKN2A/B (12%). Conclusions: On-target resistance to RET inhibition due to acquired RET mutations was less common than off-target resistance, identified in 14%. The majority of RET TKI resistance is mediated by off-target mechanisms such as bypass receptor tyrosine kinase activation. Further studies are warranted to enable the development of therapeutic strategies to address resistance in pts with RET-altered tumors. The RETgistry accrual and data analyses continue.

Abstract 4039: RET overexpression is frequent in lung neuroendocrine tumors (NET) and associates with response to RET tyrosine kinase inhibitors (RET TKIs) in NET cell lines

Abstract Background: RET (rearranged during transfection) is a proto-oncogene encoding for a tyrosine kinase membrane receptor that promotes protein synthesis and cell proliferation. A small percentage of human tumors harbor RET fusions or mutations as oncogenic drivers and RET-TKIs have been developed. Two of them, selpercatinib and pralsetinib, have recently been approved. However, their effect on tumors overexpressing wild-type RET (wt-RET) has not been investigated. A large-scale mRNA analysis performed by our group revealed that ~2% of lung tumors present high expression of wt-RET in absence of fusions. Interestingly, ~60% of high wt-RET tumors presented neuroendocrine characteristics. Methods: Twenty large cell neuroendocrine (LCNEC) and 19 small cell (SCLC) lung carcinomas from two additional institutions were analyzed by nCounter® to validate the previous results. For in vitro experiments, a panel of cell lines derived from lung NETs (DMS-53, NCI-H82, NCI-H727) was used, together with a lung adenocarcinoma cell line (LC2/ad, a positive control harboring the CDCC6-RET translocation). RET fusions and mRNA expression were analyzed by nCounter®, RET protein expression by Western blotting and immunohistochemistry (IHC) and RET mutations by next generation sequencing. The inhibitory effects of selpercatinib and pralsetinib were analyzed in vitro by MTT in 2D and spheroid/3D cultures. Finally, in vivo experiments implanting tumor cells into athymic nude mice are ongoing. Results: Among the 20 LCNECs and 19 SCLCs analyzed, 3 (15%) and 3 (15%) presented very high levels of wt-RET mRNA by nCounter, respectively. Fusions were absent in all cases. The NCI-H82 and NCI-H727 NET cell lines showed very low RET mRNA and protein expression by nCounter, Western blotting and IHC. In contrast, DMS-53 cells presented levels of RET mRNA and protein higher than the observed in the RET-fusion positive LC2/ad. In 2D models, the three NET cell lines tested showed moderate resistance to pralsetinib and selpercatinib, with IC50s of 1.5 - 9.7 µM for pralsetinib and 4.6 - 6.1 µM for selpercatinib; while the RET-fusion positive LC2/ad cells showed an IC50 in the nM range, as expected. In 3D cultures, DMS-53 was found to be sensitive to RET TKIs, with IC50s of and 0.3 µM for praseltinib and 2.5 µM for selpercatinib. In contrast, spheroids of the two RET low cell lines showed complete resistance to both drugs (IC50>10 µM). Experiments in animal models are currently ongoing, final results will be presented at the meeting. Conclusions: Overexpression of RET is frequent in lung tumors with neuroendocrine features (LCNEC, SCLC). Selpercatinib and Pralsetinib show antitumor activity in lung NETs cells expressing high levels of wt-RET. Citation Format: Jaume Roca-Arias, Jordi Bertran-Alamillo, Ruth Román, Cristina Aguado, Laura López, Leticia Ferro-Leal, Beatriz Garcia-Pelaez, Rodrigo de Oliveira Cavagna, Silvia Teixeira, Rafael Rosell, Rui Manuel Reis, Miguel A. Molina-Vila, Ivana Sullivan. RET overexpression is frequent in lung neuroendocrine tumors (NET) and associates with response to RET tyrosine kinase inhibitors (RET TKIs) in NET cell lines. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4039.

Abstract 3419: TY-1091, a highly selective and potent second-generation RET inhibitor, demonstrates superior antitumor activity in multiple RET-mutant models

Abstract Proto-oncoprotein RET (rearranged during transfection) is a receptor tyrosine kinase and belongs to the cadherin superfamily. RET fusion proteins and gatekeeper mutations represent strong cancer drivers involved in the development of a variety of cancers. Although selective RET inhibitors selpercatinib (LOXO-292) and pralsetinib (BLU667) were recently marketed for patients with RET-dependent NSCLC and thyroid cancers, RET solvent front mutations G810R/S/C/V and other secondary mutations such as Y806C/N and V728A have been identified as mechanisms counting for acquired resistance to the two drugs and limits the applications of the kinase inhibitors. To provide novel RET inhibitors effective to a broader spectrum of RET fusions and mutations, we identified TY-1091 as a novel next-generation RET inhibitor through the in-house RET program and systemic screening of RET compound candidates. TY-1091 was characterized for its anti-tumor activity through in vitro and in vivo testing of a variety of RET-dependent tumor models including a panel of 17 engineered RET mutant Ba/F3 cell lines and 2 cancer cell models. The results show that TY-1091 inhibits wild type and major RET mutants (IC50, nM): RET G810S (9.5 nM), RET V804M/L/E (2.8-12.6 nM), and double mutants RET V804M/G810S (23.5 nM) and M918T/G810S (47.0 nM) through a biochemical screening against a panel of 17 engineered RET mutant Ba/F3 cell lines. Importantly, the inhibitory activity of TY-1091 is much higher than that of first-generation RET inhibitor Cabozantinib, and comparable to other second-generation compounds LOXO-292 and BLU-667. Consistent with its mode of action, TY-1091 grants extraordinary inhibition effects to tumor cell proliferation compared to its peer compounds LOXO-292 and BLU-667 (including TT (thyroid cancer, RET C634W) and LC2/ad (NSCLC, CCDC6-RET)), and the inhibition of the RET pathway activation, i.e., inhibition of RET phosphorylation (IC50 < 1 nM), SHC phosphorylation (IC50 = 4.6 nM) and ERK phosphorylation (IC50 = 9.8 nM) in Ba/F3-KIF5B-RET cells further validated the above in vitro phenotype. The therapeutic potential of TY-1091 was then further validated through mouse pharmacology in that TY-1091 demonstrated remarkable anti-tumor efficacy in a variety of xenograft models including Ba/F3 KIF5B-RET (wt), Ba/F3 KIF5B-RET (V804L), TT (thyroid cancer, RET C634W), and LC2/ad (NSCLC, CCDC6-RET). Detailed data will be presented. In summary, TY-1091 is a highly potent, orally available, and safe small molecule inhibitor to pan-RET mutations in cancer, and may attenuate SFMs-mediated resistance to existing RET therapy. The IND clearance from the US FDA was received and Phase I clinical investigations of TY-1091 shall be launched in the US soon. *To Whom Correspondence should be addressed to: Jun Li, Chengshan Niu and Yusheng Wu Citation Format: Chengshan Niu, Maolin Zheng, Huan Wang, Kaige Ji, Meihua Li, Guohui Wang, Rongzhen Ni, Apeng Liang, Aishen Gong, Yazhen Zhang, Hui Su, Mingyu Jiang, Shaoqing Chen, Xiugui Chen, Jun Li, Yusheng Wu. TY-1091, a highly selective and potent second-generation RET inhibitor, demonstrates superior antitumor activity in multiple RET-mutant models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3419.

Progress and challenges in RET-targeted cancer therapy.

The rearranged during transfection (RET) is a receptor protein tyrosine kinase. Oncogenic RET fusions or mutations are found most often in non-small cell lung cancer (NSCLC) and in thyroid cancer, but also increasingly in various types of cancers at low rates. In the last few years, two potent and selective RET protein tyrosine kinase inhibitors (TKIs), pralsetinib (BLU-667) and selpercatinib (LOXO-292, LY3527723) were developed and received regulatory approval. Although pralsetinib and selpercatinib gave high overall response rates (ORRs), < 10% of patients achieved a complete response (CR). The RET TKI-tolerated residual tumors inevitably develop resistance by secondary target mutations, acquired alternative oncogenes, or MET amplification. RET G810 mutations located at the kinase solvent front site were identified as the major on-target mechanism of acquired resistance to both selpercatinib and pralsetinib. Several next-generation of RET TKIs capable of inhibiting the selpercatinib/pralsetinib-resistant RET mutants have progressed to clinical trials. However, it is likely that new TKI-adapted RET mutations will emerge to cause resistance to these next-generation of RET TKIs. Solving the problem requires a better understanding of the multiple mechanisms that support the RET TKI-tolerated persisters to identify a converging point of vulnerability to devise an effective co-treatment to eliminate the residual tumors.