RET fusion genes are associated with chronic myelomonocytic leukemia and enhance monocytic differentiation

Targeting Solvent-Front Mutations for Kinase Drug Discovery: From Structural Basis to Design Strategies

Solvent-front mutations have emerged as a common mechanism leading to acquired resistance to kinase inhibitors, representing a major challenge in the clinic. Several new-generation kinase inhibitors targeting solvent-front mutations have either been approved or advanced to clinical trials. However, there remains a need to discover effective, new-generation inhibitors. In this Perspective, we systematically summarize the general types of solvent-front mutations across the kinome and describe the development of inhibitors targeting some key solvent-front mutations. Additionally, we highlight the challenges and opportunities for the next generation of kinase inhibitors directed toward overcoming solvent-front mutations.

Targeting FGFR for cancer therapy

The FGFR signaling pathway is integral to cellular activities, including proliferation, differentiation, and survival. Dysregulation of this pathway is implicated in numerous human cancers, positioning FGFR as a prominent therapeutic target. Here, we conduct a comprehensive review of the function, signaling pathways and abnormal alterations of FGFR, as well as its role in tumorigenesis and development. Additionally, we provide an in-depth analysis of pivotal phase 2 and 3 clinical trials evaluating the performance and safety of FGFR inhibitors in oncology, thereby shedding light on the current state of clinical research in this field. Then, we highlight four drugs that have been approved for marketing by the FDA, offering insights into their molecular mechanisms and clinical achievements. Our discussion encompasses the intricate landscape of FGFR-driven tumorigenesis, current techniques for pinpointing FGFR anomalies, and clinical experiences with FGFR inhibitor regimens. Furthermore, we discuss the inherent challenges of targeting the FGFR pathway, encompassing resistance mechanisms such as activation by gatekeeper mutations, alternative pathways, and potential adverse reactions. By synthesizing the current evidence, we underscore the potential of FGFR-centric therapies to enhance patient prognosis, while emphasizing the imperative need for continued research to surmount resistance and optimize treatment modalities.

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 ≤300 mg 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 400 mg starting dose of pralsetinib, allowing for dose reduction in the event of AEs.

The Eμ-Ret mouse is a novel model of hyperdiploid B-cell acute lymphoblastic leukemia

The presence of supernumerary chromosomes is the only abnormality shared by all patients diagnosed with high-hyperdiploid B cell acute lymphoblastic leukemia (HD-ALL). Despite being the most frequently diagnosed pediatric leukemia, the lack of clonal molecular lesions and complete absence of appropriate experimental models have impeded the elucidation of HD-ALL leukemogenesis. Here, we report that for 23 leukemia samples isolated from moribund Eμ-Ret mice, all were characterized by non-random chromosomal gains, involving combinations of trisomy 9, 12, 14, 15, and 17. With a median gain of three chromosomes, leukemia emerged after a prolonged latency from a preleukemic B cell precursor cell population displaying more diverse aneuploidy. Transition from preleukemia to overt disease in Eμ-Ret mice is associated with acquisition of heterogeneous genomic abnormalities affecting the expression of genes implicated in pediatric B-ALL. The development of abnormal centrosomes in parallel with aneuploidy renders both preleukemic and leukemic cells sensitive to inhibitors of centrosome clustering, enabling targeted in vivo depletion of leukemia-propagating cells. This study reveals the Eμ-Ret mouse to be a novel tool for investigating HD-ALL leukemogenesis, including supervision and selection of preleukemic aneuploid clones by the immune system and identification of vulnerabilities that could be targeted to prevent relapse.

Clinical and Therapeutic Intervention of Hypereosinophilia in the Era of Molecular Diagnosis

Hypereosinophilia (HE) presents with an elevated peripheral eosinophilic count of >1.5 × 109/L and is composed of a broad spectrum of secondary non-hematologic disorders and a minority of primary hematologic processes with heterogenous clinical presentations, ranging from mild symptoms to potentially lethal outcome secondary to end-organ damage. Following the introduction of advanced molecular diagnostics (genomic studies, RNA sequencing, and targeted gene mutation profile, etc.) in the last 1-2 decades, there have been deep insights into the etiology and molecular mechanisms involved in the development of HE. The classification of HE has been updated and refined following to the discovery of clinically novel markers and targets in the 2022 WHO classification and ICOG-EO 2021 Working Conference on Eosinophil Disorder and Syndromes. However, the diagnosis and management of HE is challenging given its heterogeneity and variable clinical outcome. It is critical to have a diagnostic algorithm for accurate subclassification of HE and hypereosinophilic syndrome (HES) (e.g., reactive, familial, idiopathic, myeloid/lymphoid neoplasm, organ restricted, or with unknown significance) and to follow established treatment guidelines for patients based on its clinical findings and risk stratification.

Enhancer-activated RET confers protection against oxidative stress to KMT2A-rearranged acute myeloid leukemia

Ectopic activation of rearranged during transfection (RET) has been reported to facilitate lineage differentiation and cell proliferation in different cytogenetic subtypes of acute myeloid leukemia (AML). Herein, we demonstrate that RET is significantly (p < 0.01) upregulated in AML subtypes containing rearrangements of the lysine methyltransferase 2A gene (KMT2A), commonly referred to as KMT2A-rearranged (KMT2A-r) AML. Integrating multi-epigenomics data, we show that the KMT2A-MLLT3 fusion induces the development of CCCTC-binding (CTCF)-guided de novo extrusion enhancer loop to upregulate RET expression in KMT2A-r AML. Based on the finding that RET expression is tightly correlated with the selective chromatin remodeler and mediator (MED) proteins, we used a small-molecule inhibitor having dual inhibition against RET and MED12-associated cyclin-dependent kinase 8 (CDK8) in KMT2A-r AML cells. Dual inhibition of RET and CDK8 restricted cell proliferation by producing multimodal oxidative stress responses in treated cells. Our data suggest that epigenetically enhanced RET protects KMT2A-r AML cells from oxidative stresses, which could be exploited as a potential therapeutic strategy.

Hematological Neoplasms with Eosinophilia

Eosinophils in peripheral blood account for 0.3-5% of leukocytes, which is equivalent to 0.05-0.5 × 109/L. A count above 0.5 × 109/L is considered to indicate eosinophilia, while a count equal to or above 1.5 × 109/L is defined as hypereosinophilia. In bone marrow aspirate, eosinophilia is considered when eosinophils make up more than 6% of the total nuclear cells. In daily clinical practice, the most common causes of reactive eosinophilia are non-hematologic, whether they are non-neoplastic (allergic diseases, drugs, infections, or immunological diseases) or neoplastic (solid tumors). Eosinophilia that is associated with a hematological malignancy may be reactive or secondary to the production of eosinophilopoietic cytokines, and this is mainly seen in lymphoid neoplasms (Hodgkin lymphoma, mature T-cell neoplasms, lymphocytic variant of hypereosinophilic syndrome, and B-acute lymphoblastic leukemia/lymphoma). Eosinophilia that is associated with a hematological malignancy may also be neoplastic or primary, derived from the malignant clone, usually in myeloid neoplasms or with its origin in stem cells (myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions, acute myeloid leukemia with core binding factor translocations, mastocytosis, myeloproliferative neoplasms, myelodysplastic/myeloproliferative neoplasms, and myelodysplastic neoplasms). There are no concrete data in standardized cytological and cytometric procedures that could predict whether eosinophilia is reactive or clonal. The verification is usually indirect, based on the categorization of the accompanying hematologic malignancy. This review focuses on the broad differential diagnosis of hematological malignancies with eosinophilia.

Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase fusion genes: A workshop report with focus on novel entities and a literature review including paediatric cases

Myeloid/lymphoid neoplasms with eosinophilia (M/LN-eo) and tyrosine kinase (TK) gene fusions are a rare group of haematopoietic neoplasms with a broad range of clinical and morphological presentations. Paediatric cases have increasingly been recognised. Importantly, not all appear as a chronic myeloid neoplasm and eosinophilia is not always present. In addition, standard cytogenetic and molecular methods may not be sufficient to diagnose M/LN-eo due to cytogenetically cryptic aberrations. Therefore, additional evaluation with fluorescence in-situ hybridisation and other molecular genetic techniques (array-based comparative genomic hybridisation, RNA sequencing) are recommended for the identification of specific TK gene fusions. M/LN-eo with JAK2 and FLT3-rearrangements and ETV6::ABL1 fusion were recently added as a formal member to this category in the International Consensus Classification (ICC) and the 5th edition of the WHO classification (WHO-HAEM5). In addition, other less common defined genetic alterations involving TK genes have been described. This study is an update on M/LN-eo with TK gene fusions with focus on novel entities, as illustrated by cases submitted to the Bone Marrow Workshop, organised by the European Bone Marrow Working Group (EBMWG) within the frame of the 21st European Association for Haematopathology congress (EAHP-SH) in Florence 2022. A literature review was performed including paediatric cases of M/LN-eo with TK gene fusions.

Targeting the RET tyrosine kinase in neuroblastoma: A review and application of a novel selective drug design strategy

The RET (REarranged during Transfection) gene, which encodes for a transmembrane receptor tyrosine kinase, is an established oncogene associated with the etiology and progression of multiple types of cancer. Oncogenic RET mutations and rearrangements resulting in gene fusions have been identified in many adult cancers, including medullary and papillary thyroid cancers, lung adenocarcinomas, colon and breast cancers, and many others. While genetic RET aberrations are much less common in pediatric solid tumors, increased RET expression has been shown to be associated with poor prognosis in children with solid tumors such as neuroblastoma, prompting an interest in RET inhibition as a form of therapy for these children. A number of kinase inhibitors currently in use for patients with cancer have RET inhibitory activity, but these inhibitors also display activity against other kinases, resulting in unwanted side effects and limiting their safety and efficacy. Recent efforts have been focused on developing more specific RET inhibitors, but due to high levels of conservation between kinase binding pockets, specificity remains a drug design challenge. Here, we review the background of RET as a potential therapeutic target in neuroblastoma tumors and the results of recent preclinical studies and clinical trials evaluating the safety and efficacy of RET inhibition in adults and children. We also present a novel approach to drug discovery leveraging the chemical phenomenon of atropisomerism to develop specific RET inhibitors and present preliminary data demonstrating the efficacy of a novel RET inhibitor against neuroblastoma tumor cells.

Medullary thyroid carcinoma

INTRODUCTION

Medullary thyroid carcinoma (MTC) constitutes approximately 5-10% of all thyroid cancers. Although the tumor forms in the thyroid, it doesn't originate from thyroid cells, but from the C cells or parafollicular cells which produce and release a hormone called calcitonin (CT). Starting from the second half of the 1900s, MTC was progressively studied and defined.

AREAS COVERED

This study aims to analyze the history, clinical presentation and biological behavior of MTC, bio-humoral and instrumental diagnosis, molecular profiling, genetic screening, preoperative staging and instrumental procedures, indispensable in expert and dedicated hands, such as high-resolution ultrasonography, CT-scan, MRI and PET/TC. We examine recommended and controversial surgical indications and procedures, prophylactic early surgery and multiple endocrine neoplasia surgery. Also, we discuss pathological anatomy classification and targeted therapies. The role of serum CT is valued both as undisputed and constant preoperative diagnostic marker, obscuring cytology and as early postoperative marker that predicts disease persistence.

EXPERT OPINION

With a complete preoperative study, unnecessary or useless, late and extended interventions can be reduced in favor of tailored surgery that also considers quality of life. Finally, great progress has been made in targeted therapy, with favorable impact on survival.

Myeloid and lymphoid neoplasm with novel complex translocation: unusual case report with T-lymphoblastic lymphoma, myeloid hyperplasia, eosinophilia, basophilia, and t(1;8;10)( (p31;q24;q11.2)

Myeloid and lymphoid neoplasms with eosinophilia (M/Ls-Eo) encompass heterogeneous but aggressive hematopoietic disorders triggered by fusion genes or mutations that typically lead to constitutive overexpression of tyrosine kinase. The occurrence of T-lymphoblastic lymphoma in the setting of M/Ls-Eo has been reported rarely in the literature. Herein, we present an unusual case of a 28-year-old male patient who presented with massive lymphadenopathy and T-lymphoblastic lymphoma in the lymph node occurring concurrently with myeloid hyperplasia, eosinophilia and basophilia in peripheral blood and bone marrow biopsy. The syndrome was associated with a novel complex karyotype involving der(8)t(1;8;10)(p31;q24;q11.2). The FISH study was negative for BCR::ABL1, JAK2, PDGFRA, PDGFRB, and FGFR1 rearrangements. The patient's clinical course was aggressive and resistant to multiple lines of intensive chemotherapy regimens. Therefore, he underwent allogenic stem cell transplantation with a fully matched donor. A brief review of the occurrence of T-LBL in conjunction with M/Ls-Eo neoplasm was made with a special focus on molecular aspects.

RET aberrant cancers and RET inhibitor therapies: Current state-of-the-art and future perspectives

Precision oncology informed by genomic information has evolved in leaps and bounds over the last decade. Although non-small cell lung cancer (NSCLC) has moved to center-stage as the poster child of precision oncology, multiple targetable genomic alterations have been identified in various cancer types. RET alterations occur in roughly 2% of all human cancers. The role of RET as oncogenic driver was initially identified in 1985 after the discovery that transfection with human lymphoma DNA transforms NIH-3T3 fibroblasts. Germline RET mutations are causative of multiple endocrine neoplasia type 2 syndrome, and RET fusions are found in 10-20% of papillary thyroid cases and are detected in most patients with advanced sporadic medullary thyroid cancer. RET fusions are oncogenic drivers in 2% of Non-small cell lung cancer. Rapid translation and regulatory approval of selective RET inhibitors, selpercatinib and pralsetinib, have opened up the field of RET precision oncology. This review provides an update on RET precision oncology from bench to bedside and back. We explore the impact of selective RET inhibitor in patients with advanced NSCLC, thyroid cancer, and other cancers in a tissue-agnostic fashion, resistance mechanisms, and future directions.

RET Proto-Oncogene-Not Such an Obvious Starting Point in Cancer Therapy

Mutations and fusions of RET (rearranged during transfection) gene are detected in a few common types of tumors including thyroid or non-small cells lung cancers. Multiple kinase inhibitors (MKIs) do not show spectacular effectiveness in patients with RET-altered tumors. Hence, recently, two novel RET-specific inhibitors were registered in the US and in Europe. Selpercatinib and pralsetinib showed high efficacy in clinical trials, with fewer adverse effects, in comparison to previously used MKIs. However, the effectiveness of these new drugs may be reduced by the emergence of resistance mutations in RET gene and activation of different activating signaling pathways. This review presents the function of the normal RET receptor, types of molecular disturbances of the RET gene in patients with various cancers, methods of detecting these abnormalities, and the effectiveness of modern anticancer therapies (ranging from immunotherapies, through MKIs, to RET-specific inhibitors).

RNA-seq and ChIP-seq Identification of Unique and Overlapping Targets of GLI Transcription Factors in Melanoma Cell Lines

BACKGROUND

Despite significant progress in therapy, melanoma still has a rising incidence worldwide, and novel treatment strategies are needed. Recently, researchers have recognized the involvement of the Hedgehog-GLI (HH-GLI) signaling pathway in melanoma and its consistent crosstalk with the MAPK pathway. In order to further investigate the link between the two pathways and to find new target genes that could be considered for combination therapy, we set out to find transcriptional targets of all three GLI proteins in melanoma.

METHODS

We performed RNA sequencing on three melanoma cell lines (CHL-1, A375, and MEL224) with overexpressed GLI1, GLI2, and GLI3 and combined them with the results of ChIP-sequencing on endogenous GLI1, GLI2, and GLI3 proteins. After combining these results, 21 targets were selected for validation by qPCR.

RESULTS

RNA-seq revealed a total of 808 differentially expressed genes (DEGs) for GLI1, 941 DEGs for GLI2, and 58 DEGs for GLI3. ChIP-seq identified 527 genes that contained GLI1 binding sites in their promoters, 1103 for GLI2 and 553 for GLI3. A total of 15 of these targets were validated in the tested cell lines, 6 of which were detected by both RNA-seq and ChIP-seq.

CONCLUSIONS

Our study provides insight into the unique and overlapping transcriptional output of the GLI proteins in melanoma. We suggest that our findings could provide new potential targets to consider while designing melanoma-targeted therapy.

Molecular Testing and Treatment Strategies in RET-Rearranged NSCLC Patients: Stay on Target to Look Forward

RET alterations are recognized as key oncogenic drivers in different cancer types, including non-small cell lung cancer (NSCLC). Multikinase inhibitors (MKIs) with anti-RET activities resulted in variable efficacy with significant toxicities because of low target specificity. Selective RET kinase inhibitors, such as pralsetinib and selepercatinib, demonstrated high efficacy and favorable tolerability in advanced RET-rearranged NSCLC patients, leading to their introduction in the clinical setting. Among the different approaches available for the identification of RET rearrangements, next-generation sequencing (NGS) assays present substantial advantages in terms of turnaround time and diagnostic accuracy, even if potentially limited by accessibility issues. The recent advent of novel effective targeted therapies raises several questions regarding the emergence of resistance mechanisms and the potential ways to prevent/overcome them. In this review, we discuss molecular testing and treatment strategies to manage RET fusion positive NSCLC patients with a focus on resistance mechanisms and future perspectives in this rapidly evolving scenario.

OLFM4-RET fusion is an oncogenic driver in small intestine adenocarcinoma

Small intestine adenocarcinoma is a rare intestinal malignancy with distinct clinical, pathological, and molecular characteristics. Recently, a fusion of the intestinal stem-cell marker olfactomedin 4 (OLFM4) and the proto-oncogene RET has been identified in a small intestine adenocarcinoma patient. Here we investigated the biological effects of OLFM4-RET fusion and whether it can initiate tumorigenesis in small intestine. OLFM4 expression was found to be frequently lost or reduced in human small intestine adenocarcinoma, and its downregulation correlated with high tumor grade and advanced tumor stage. Expression of OLFM4-RET fusion-induced cellular transformation in HEK293 cells and blocked RET-induced inhibition of colony growth in HuTu 80 small intestine adenocarcinoma cells. Further, expression of OLFM4-RET activated the RAS-RAF-MAPK and STAT3 cell signaling pathways in both HEK293 cells and HuTu 80 cells. OLFM4-RET expression in HEK293 cells upregulated multiple families of genes related to carcinogenesis, cancer progression, and metastasis. Targeted expression of OLFM4-RET in the small intestine led to the development of hyperplasia, adenoma, or adenocarcinoma in transgenic mice. Our study suggests that OLFM4-RET is an oncogenic driver of small intestine tumorigenesis. Therefore, the small intestine adenocarcinoma patients with OLFM4-RET fusion may benefit from treatment with RET kinase inhibitor.

RET receptor signaling: Function in development, metabolic disease, and cancer

The RET proto-oncogene encodes a receptor tyrosine kinase whose alterations are responsible for various human cancers and developmental disorders, including thyroid cancer, non-small cell lung cancer, multiple endocrine neoplasia type 2, and Hirschsprung's disease. RET receptors are physiologically activated by glial cell line-derived neurotrophic factor (GDNF) family ligands that bind to the coreceptor GDNF family receptor α (GFRα). Signaling via the GDNF/GFRα1/RET ternary complex plays crucial roles in the development of the enteric nervous system, kidneys, and urinary tract, as well as in the self-renewal of spermatogonial stem cells. In addition, another ligand, growth differentiation factor-15 (GDF15), has been shown to bind to GFRα-like and activate RET, regulating body weight. GDF15 is a stress response cytokine, and its elevated serum levels affect metabolism and anorexia-cachexia syndrome. Moreover, recent development of RET-specific kinase inhibitors contributed significantly to progress in the treatment of patients with RET-altered cancer. This review focuses on the broad roles of RET in development, metabolic diseases, and cancer.

Precision therapy for RET-altered cancers with RET inhibitors

Rearranged during transfection (RET) is involved in the physiological development of some organ systems. Activating RET alterations via either gene fusions or point mutations are potent oncogenic drivers in non-small cell lung cancer, thyroid cancer, and in multiple diverse cancers. RET-altered cancers were initially treated with multikinase inhibitors (MKIs). The efficacy of MKIs was modest at the expense of notable toxicities from their off-target activity. Recently, highly potent and RET-specific inhibitors selpercatinib and pralsetinib were successfully translated to the clinic and FDA approved. We summarize the current state-of-the-art therapeutics with preclinical and clinical insights of these novel RET inhibitors, acquired resistance mechanisms, and future outlooks.

Higher RET Gene Expression Levels Do Not Represent anAlternative RET Activation Mechanism in Medullary Thyroid Carcinoma

This study was designed to investigate whether RET (rearranged during transfection) mRNA over-expression could be considered an alternative driver event for the development of medullary thyroid carcinoma (MTC), and if different RET isoforms could play a role in MTC tumorigenesis. Eighty-three MTC patients, whose mutational profile was previously identified by next-generation sequencing (NGS) IONS5, were included in this study. Expression analysis was performed by the quantitative reverse transcription-polymerase chain reaction technique. RET expression levels were found to be significantly higher in cases with RET somatic mutations than in cases that were negative for RET somatic mutations (p = 0.003) as well as in cases with a somatic mutation, either in RET or RAS than in cases negative for both these mutations (p = 0.01). All cases were positive for the RET51 isoform expression while only 72/83 (86.7%) were positive for RET9 isoform expression. A statistically significant higher expression of the RET51 isoform was found in cases positive for RET somatic mutation than in cases either positive for RAS mutation (p = 0.0006) or negative for both mutations (p = 0.001). According to our data, RET gene over-expression does not play a role in MTC tumorigenesis, neither as an entire gene or as an isoform. At variance, the RET gene, and in particular the RET51 isoform, is expressed higher in RET mutated cases. On the basis of these results we can hypothesize that the overexpression of RET, and in particular of RET51, could potentiate the transforming activity of mutated RET, making these cases more aggressive.

Targeting Rearranged during Transfection in Cancer: A Perspective on Small-Molecule Inhibitors and Their Clinical Development

Rearranged during transfection (RET) is a receptor tyrosine kinase essential for the normal development and maturation of a diverse range of tissues. Aberrant RET signaling in cancers, due to RET mutations, gene fusions, and overexpression, results in the activation of downstream pathways promoting survival, growth, and metastasis. Pharmacological manipulation of RET is effective in treating RET-driven cancers, and efforts toward developing RET-specific therapies have increased over the last 5 years. In 2020, RET-selective inhibitors pralsetinib and selpercatinib achieved clinical approval, which marked the first approvals for kinase inhibitors specifically developed to target the RET oncoprotein. This Perspective discusses current development and clinical applications for RET precision medicine by providing an overview of the incremental improvement of kinase inhibitors for use in RET-driven malignancies.

Targeted activity of the small molecule kinase inhibitor Pz-1 towards RET and TRK kinases

We have recently described Pz-1, a benzimidazole-based type-2 RET and VEGFR2 inhibitor. Based on a kinome scan, here we show that Pz-1 is also a potent (IC50 < 1 nM) TRKA/B/C inhibitor. Pz-1 potently inhibited proliferation of human cancer cells carrying either RET- or TRKA oncoproteins (IC50 ~ 1 nM), with a negligible effect against RET- and TRKA-negative cells. By testing mutations, known to mediate resistance to other compounds, RET G810R/S, but not L730I/V, E732K, V738A and Y806N, showed some degree of resistance to Pz-1. In the case of TRKA, G595R and F589L, but not G667C, showed some degree of resistance. In xenograft models, orally administered Pz-1 almost completely inhibited RET- and TRKA-mutant tumours at 1-3 mg/kg/day but showed a reduced effect on RET/TRKA-negative cancer models. The activity, albeit reduced, on RET/TRKA-negative tumours may be justified by VEGFR2 inhibition. Tumours induced by NIH3T3 cells transfected by RET G810R and TRKA G595R featured resistance to Pz-1, demonstrating that RET or TRKA inhibition is critical for its anti-tumourigenic effect. In conclusion, Pz-1 represents a new powerful kinase inhibitor with distinct activity towards cancers induced by oncogenic RET and TRKA variants, including some mutants displaying resistance to other drugs.

The importance of the RET gene in thyroid cancer and therapeutic implications

Since the discovery of the RET receptor tyrosine kinase in 1985, alterations of this protein have been found in diverse thyroid cancer subtypes. RET gene rearrangements are observed in papillary thyroid carcinoma, which result in RET fusion products. By contrast, single amino acid substitutions and small insertions and/or deletions are typical of hereditary and sporadic medullary thyroid carcinoma. RET rearrangements and mutations of extracellular cysteines facilitate dimerization and kinase activation, whereas mutations in the RET kinase coding domain drive dimerization-independent kinase activation. Thus, RET kinase inhibition is an attractive therapeutic target in patients with RET alterations. This approach was initially achieved using multikinase inhibitors, which affect multiple deregulated pathways that include RET kinase. In clinical practice, use of multikinase inhibitors in patients with advanced thyroid cancer resulted in therapeutic efficacy, which was associated with frequent and sometimes severe adverse effects. However, remarkable progress has been achieved with the identification of novel potent and selective RET kinase inhibitors for the treatment of advanced thyroid cancer. Although expanded clinical validation in future trials is needed, the sustained antitumoural activity and the improved safety profile of these novel compounds is opening a new exciting era in precision oncology for RET-driven cancers.

Mutations of Intrinsically Disordered Protein Regions Can Drive Cancer but Lack Therapeutic Strategies

Many proteins contain intrinsically disordered regions (IDRs) which carry out important functions without relying on a single well-defined conformation. IDRs are increasingly recognized as critical elements of regulatory networks and have been also associated with cancer. However, it is unknown whether mutations targeting IDRs represent a distinct class of driver events associated with specific molecular and system-level properties, cancer types and treatment options. Here, we used an integrative computational approach to explore the direct role of intrinsically disordered protein regions driving cancer. We showed that around 20% of cancer drivers are primarily targeted through a disordered region. These IDRs can function in multiple ways which are distinct from the functional mechanisms of ordered drivers. Disordered drivers play a central role in context-dependent interaction networks and are enriched in specific biological processes such as transcription, gene expression regulation and protein degradation. Furthermore, their modulation represents an alternative mechanism for the emergence of all known cancer hallmarks. Importantly, in certain cancer patients, mutations of disordered drivers represent key driving events. However, treatment options for such patients are currently severely limited. The presented study highlights a largely overlooked class of cancer drivers associated with specific cancer types that need novel therapeutic options.

Regulation of Beclin 1-Mediated Autophagy by Oncogenic Tyrosine Kinases

Beclin 1 is a major regulator of autophagy, and it is a core component of the class III PI3K complexes. Beclin 1 is a highly conserved protein and its function is regulated in a number of ways, including post-translational modifications. Several studies indicate that receptor and non-receptor tyrosine kinases regulate autophagy activity in cancer, and some suggest the importance of Beclin 1 tyrosine phosphorylation in this process. Here we summarize the current knowledge of the mechanism whereby some oncogenic tyrosine kinases regulate autophagy through Beclin 1.

REToma: a cancer subtype with a shared driver oncogene

RET (REarranged during Transfection), which encodes a receptor tyrosine kinase for members of the glial cell line-derived neurotrophic factor, plays a role as driver oncogene in a variety of human cancers. Fusion of RET with several partner genes has been detected in papillary thyroid, lung, colorectal, pancreatic and breast cancers, and tyrosine kinase inhibitors (TKIs) for RET (particularly RET-specific inhibitors) show promising therapeutic effects against such cancers. Oncogenic mutations within the extracellular cysteine-rich and intracellular kinase domains of RET drive medullary thyroid carcinogenesis; the same mutations are also observed in a small subset of diverse cancers such as lung, colorectal and breast cancers. Considering the oncogenic nature of RET mutants, lung, colorectal and breast cancers are predicted to respond to RET TKIs in a manner similar to medullary thyroid cancer. In summary, cancers carrying oncogenic RET alterations as a driver mutation could be collectively termed 'REToma' and treated with RET TKIs in a tissue-agnostic manner.

Progresses Toward Precision Medicine in RET-altered Solid Tumors

RET (rearranged during transfection) gene encodes a receptor tyrosine kinase essential for many physiologic functions, but RET aberrations are involved in many pathologies. While RET loss-of-function mutations are associated with congenital disorders like Hirschsprung disease and CAKUT, RET gain-of-function mutations and rearrangements are critical drivers of tumor growth and proliferation in many different cancers. RET-altered (RET⁺ ) tumors have been hitherto targeted with multikinase inhibitors (MKI) having anti-RET activities, but they inhibit other kinase targets more potently and show limited clinical activities. The lack of target specificity and consequently increased side effects, responsible for dose reduction and drug discontinuation, are critical limitations of MKIs in the clinics. New selective RET inhibitors, selpercatinib and pralsetinib, are showing promising activities, improved response rates, and more favorable toxicity profiles in early clinical trials. This review critically discusses the oncogenic activation of RET and its role in different kinds of tumors, clinical features of RET⁺ tumors, clinically actionable genetic RET alterations and their diagnosis, and the available data and results of nonselective and selective targeting of RET.

Roles of the RET Proto-oncogene in Cancer and Development

RET (REarranged during Transfection)is activated by DNA rearrangement of the 3' fragment of the receptor tyrosine kinase gene, namely, RET proto-oncogene, with the 5' fragment of various genes with putative dimerization domains, such as a coiled coil domain, that are necessary for constitutive activation. RET rearrangements have been detected in a variety of human cancers, including thyroid, lung, colorectal, breast, and salivary gland cancers. Moreover, point mutations in RET are responsible for multiple endocrine neoplasia types 2A and 2B, which can develop into medullary thyroid cancer and pheochromocytoma. Substantial effort is currently being exerted in developing RET kinase inhibitors. RET is also responsible for Hirschsprung's disease, a developmental abnormality in the enteric nervous system. Gene knockout studies have demonstrated that RET plays essential roles in the development of the enteric nervous system and kidney as well as in spermatogenesis. Studies regarding RET continue to provide fascinating challenges in the fields of cancer research, neuroscience, and developmental biology.

Bioisosteric Discovery of NPA101.3, a Second-Generation RET/VEGFR2 Inhibitor Optimized for Single-Agent Polypharmacology

RET receptor tyrosine kinase is a driver oncogene in human cancer. We recently identified the clinical drug candidate Pz-1, which targets RET and VEGFR2. A key in vivo metabolite of Pz-1 is its less active demethylated pyrazole analogue. Using bioisosteric substitution methods, here, we report the identification of NPA101.3, lacking the structural liability for demethylation. NPA101.3 showed a selective inhibitory profile and an inhibitory concentration 50 (IC₅₀) of <0.003 μM for both RET and VEGFR2. NPA101.3 inhibited phosphorylation of all tested RET oncoproteins as well as VEGFR2 and proliferation of cells transformed by RET. Oral administration of NPA101.3 (10 mg/kg/day) completely prevented formation of tumors induced by RET/C634Y-transformed cells, while it weakened, but did not abrogate, formation of tumors induced by a control oncogene (HRAS/G12V). The balanced synchronous inhibition of both RET and VEGFR2, as well the resistance to demethylation, renders NPA101.3 a potential clinical candidate for RET-driven cancers.

RET Gene Fusions in Malignancies of the Thyroid and Other Tissues

Following the identification of the BCR-ABL1 (Breakpoint Cluster Region-ABelson murine Leukemia) fusion in chronic myelogenous leukemia, gene fusions generating chimeric oncoproteins have been recognized as common genomic structural variations in human malignancies. This is, in particular, a frequent mechanism in the oncogenic conversion of protein kinases. Gene fusion was the first mechanism identified for the oncogenic activation of the receptor tyrosine kinase RET (REarranged during Transfection), initially discovered in papillary thyroid carcinoma (PTC). More recently, the advent of highly sensitive massive parallel (next generation sequencing, NGS) sequencing of tumor DNA or cell-free (cfDNA) circulating tumor DNA, allowed for the detection of RET fusions in many other solid and hematopoietic malignancies. This review summarizes the role of RET fusions in the pathogenesis of human cancer.

RET Solvent Front Mutations Mediate Acquired Resistance to Selective RET Inhibition in RET-Driven Malignancies

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.

State-of-the-Art Strategies for Targeting RET-Dependent Cancers

Activating receptor tyrosine kinase RET (rarranged during transfection) gene alterations have been identified as oncogenic in multiple malignancies. RET gene rearrangements retaining the kinase domain are oncogenic drivers in papillary thyroid cancer, non-small-cell lung cancer, and multiple other cancers. Activating RET mutations are associated with different phenotypes of multiple endocrine neoplasia type 2 as well as sporadic medullary thyroid cancer. RET is thus an attractive therapeutic target in patients with oncogenic RET alterations. Multikinase inhibitors with RET inhibitor activity, such as cabozantinib and vandetanib, have been explored in the clinic for tumors with activating RET gene alterations with modest clinical efficacy. As a result of the nonselective nature of these multikinase inhibitors, patients had off-target adverse effects, such as hypertension, rash, and diarrhea. This resulted in a narrow therapeutic index of these drugs, limiting ability to dose for clinically effective RET inhibition. In contrast, the recent discovery and clinical validation of highly potent selective RET inhibitors (pralsetinib, selpercatinib) demonstrating improved efficacy and a more favorable toxicity profile are poised to alter the landscape of RET-dependent cancers. These drugs appear to have broad activity across tumors with activating RET alterations. The mechanisms of resistance to these next-generation highly selective RET inhibitors is an area of active research. This review summarizes the current understanding of RET alterations and the state-of-the-art treatment strategies in RET-dependent cancers.

Targeting the RNA-Binding Protein QKI in Myeloid Cells Ameliorates Macrophage-Induced Renal Interstitial Fibrosis

In the pathophysiologic setting of acute and chronic kidney injury, the excessive activation and recruitment of blood-borne monocytes prompts their differentiation into inflammatory macrophages, a process that leads to progressive glomerulosclerosis and interstitial fibrosis. Importantly, this differentiation of monocytes into macrophages requires the meticulous coordination of gene expression at both the transcriptional and post-transcriptional level. The transcriptomes of these cells are ultimately determined by RNA-binding proteins such as QUAKING (QKI), that define their pre-mRNA splicing and mRNA transcript patterns. Using two mouse models, namely (1) quaking viable mice (qk^v) and (2) the conditional deletion in the myeloid cell lineage using the lysozyme 2-Cre (QKI^{FL/FL;LysM-Cre} mice), we demonstrate that the abrogation of QKI expression in the myeloid cell lineage reduces macrophage infiltration following kidney injury induced by unilateral urethral obstruction (UUO). The qk^v and QKI^{FL/FL;LysM-Cre} mice both showed significant diminished interstitial collagen deposition and fibrosis in the UUO-damaged kidney, as compared to wild-type littermates. We show that macrophages isolated from QKI^{FL/FL;LysM-Cre} mice are associated with defects in pre-mRNA splicing. Our findings demonstrate that reduced expression of the alternative splice regulator QKI in the cells of myeloid lineage attenuates renal interstitial fibrosis, suggesting that inhibition of this splice regulator may be of therapeutic value for certain kidney diseases.

Oncogenic fusion protein BCR-FGFR1 requires the breakpoint cluster region-mediated oligomerization and chaperonin Hsp90 for activation

Mutation and translocation of fibroblast growth factor receptors often lead to aberrant signaling and cancer. This work focuses on the t(8;22)(p11;q11) chromosomal translocation which creates the breakpoint cluster region (BCR) fibroblast growth factor receptor1 (FGFR1) (BCR-FGFR1) fusion protein. This fusion occurs in stem cell leukemia/lymphoma, which can progress to atypical chronic myeloid leukemia, acute myeloid leukemia, or B-cell lymphoma. This work focuses on the biochemical characterization of BCR-FGFR1 and identification of novel therapeutic targets. The tyrosine kinase activity of FGFR1 is required for biological activity as shown using transformation assays, interleukin-3 independent cell proliferation, and liquid chromatography/mass spectroscopy analyses. Furthermore, BCR contributes a coiled-coil oligomerization domain, also essential for oncogenic transformation by BCR-FGFR1. The importance of salt bridge formation within the coiled-coil domain is demonstrated, as disruption of three salt bridges abrogates cellular transforming ability. Lastly, BCR-FGFR1 acts as a client of the chaperonin heat shock protein 90 (Hsp90), suggesting that BCR-FGFR1 relies on Hsp90 complex to evade proteasomal degradation. Transformed cells expressing BCR-FGFR1 are sensitive to the Hsp90 inhibitor Ganetespib, and also respond to combined treatment with Ganetespib plus the FGFR inhibitor BGJ398. Collectively, these data suggest novel therapeutic approaches for future stem cell leukemia/lymphoma treatment: inhibition of BCR oligomerization by disruption of required salt bridges; and inhibition of the chaperonin Hsp90 complex.

BCR: a promiscuous fusion partner in hematopoietic disorders

Considerable advances have been made in our understanding of the molecular basis of hematopoietic cancers. The discovery of the BCR-ABL fusion protein over 50 years ago has brought about a new era of therapeutic progress and overall improvement in patient care, mainly due to the development and use of personalized medicine and tyrosine kinase inhibitors (TKIs). However, since the detection of BCR-ABL, BCR has been identified as a commonly occurring fusion partner in hematopoietic disorders. BCR has been discovered fused to additional tyrosine kinases, including: Fibroblast Growth Factor Receptor 1 (FGFR1), Platelet-derived Growth Factor Receptor Alpha (PDGFRA), Ret Proto-Oncogene (RET), and Janus Kinase 2 (JAK2). While BCR translocations are infrequent in hematopoietic malignancies, clinical evidence suggests that patients who harbor these mutations benefit from TKIs and additional personalized therapies. The improvement of further methodologies for characterization of these fusions is crucial to determine a patient’s treatment regimen, and optimal outcome. However, potential relapse and drug resistance among patients’ highlights the need for additional treatment options and further understanding of these oncogenic fusion proteins. This review explores the mechanisms behind cancer progression of these BCR oncogenic fusion proteins, comparing their similarities and differences, examining the significance of BCR as a partner gene, and discussing current treatment options for these translocation-induced hematopoietic malignancies.

Receptor tyrosine kinases in PI3K signaling: The therapeutic targets in cancer

The phosphoinositide 3-kinase (PI3K) pathway, one of the most commonly activated signaling pathways in human cancers, plays a crucial role in the regulation of cell proliferation, differentiation, and survival. This pathway is usually activated by receptor tyrosine kinases (RTKs), whose constitutive and aberrant activation is via gain-of-function mutations, chromosomal rearrangement, gene amplification and autocrine. Blockage of PI3K pathway by targeted therapy on RTKs with tyrosine kinases inhibitors (TKIs) and monoclonal antibodies (mAbs) has achieved great progress in past decades; however, there still remain big challenges during their clinical application. In this review, we provide an overview about the most frequently encountered alterations in RTKs and focus on current therapeutic agents developed to counteract their aberrant functions, accompanied with discussions of two major challenges to the RTKs-targeted therapy in cancer - resistance and toxicity.

GDNF and the RET Receptor in Cancer: New Insights and Therapeutic Potential

The Glial cell line-derived neurotrophic Family Ligands (GFL) are soluble neurotrophic factors that are required for development of multiple human tissues, but which are also important contributors to human cancers. GFL signaling occurs through the transmembrane RET receptor tyrosine kinase, a well-characterized oncogene. GFL-independent RET activation, through rearrangement or point mutations occurs in thyroid and lung cancers. However, GFL-mediated activation of wildtype RET is an increasingly recognized mechanism promoting tumor growth and dissemination of a much broader group of cancers. RET and GFL expression have been implicated in metastasis or invasion in diverse human cancers including breast, pancreatic, and prostate tumors, where they are linked to poorer patient prognosis. In addition to directly inducing tumor growth in these diseases, GFL-RET signaling promotes changes in the tumor microenvironment that alter the surrounding stroma and cellular composition to enhance tumor invasion and metastasis. As such, GFL RET signaling is an important target for novel therapeutic approaches to limit tumor growth and spread and improve disease outcomes.

Evolution of Our Understanding of the Hyperparathyroid Syndromes: A Historical Perspective

We review advancing and overlapping stages for our understanding of the expressions of six hyperparathyroid (HPT) syndromes: multiple endocrine neoplasia type 1 (MEN1) or type 4, multiple endocrine neoplasia type 2A (MEN2A), hyperparathyroidism-jaw tumor syndrome, familial hypocalciuric hypercalcemia, neonatal severe primary hyperparathyroidism, and familial isolated hyperparathyroidism. During stage 1 (1903 to 1967), the introduction of robust measurement of serum calcium was a milestone that uncovered hypercalcemia as the first sign of dysfunction in many HPT subjects, and inheritability was reported in each syndrome. The earliest reports of HPT syndromes were biased toward severe or striking manifestations. During stage 2 (1959 to 1985), the early formulations of a syndrome were improved. Radioimmunoassays (parathyroid hormone [PTH], gastrin, insulin, prolactin, calcitonin) were breakthroughs. They could identify a syndrome carrier, indicate an emerging tumor, characterize a tumor, or monitor a tumor. During stage 3 (1981 to 2006), the assembly of many cases enabled recognition of further details. For example, hormone non-secreting skin lesions were discovered in MEN1 and MEN2A. During stage 4 (1985 to the present), new genomic tools were a revolution for gene identification. Four principal genes ("principal" implies mutated or deleted in 50% or more probands for its syndrome) (MEN1, RET, CASR, CDC73) were identified for five syndromes. During stage 5 (1993 to the present), seven syndromal genes other than a principal gene were identified (CDKN1B, CDKN2B, CDKN2C, CDKN1A, GNA11, AP2S1, GCM2). Identification of AP2S1 and GCM2 became possible because of whole-exome sequencing. During stages 4 and 5, the newly identified genes enabled many studies, including robust assignment of the carriers and non-carriers of a mutation. Furthermore, molecular pathways of RET and the calcium-sensing receptor were elaborated, thereby facilitating developments in pharmacotherapy. Current findings hold the promise that more genes for HPT syndromes will be identified and studied in the near future. © 2018 American Society for Bone and Mineral Research.

65 YEARS OF THE DOUBLE HELIX: Exploiting insights on the RET receptor for personalized cancer medicine

The focus of precision cancer medicine is the use of patient genetic signatures to predict disease occurrence and course and tailor approaches to individualized treatment to improve patient outcomes. The rearranged during transfection (RET) receptor tyrosine kinase represents a paradigm for the power of personalized cancer management to change cancer impact and improve quality of life. Oncogenic activation of RET occurs through several mechanisms including activating mutations and increased or aberrant expression. Activating RET mutations found in the inherited cancer syndrome multiple endocrine neoplasia 2 permit early diagnosis, predict disease course and guide disease management to optimize patient survival. Rearrangements of RET found in thyroid and lung tumors provide insights on potential disease aggressiveness and offer opportunities for RET-targeted therapy. Aberrant RET expression in a subset of cases is associated with tumor dissemination, resistance to therapies and/or poorer prognosis in multiple cancers. The potential of RET targeting through repurposing of small-molecule multikinase inhibitors, selective RET inhibitors or other novel approaches provides exciting opportunities to individualize therapies across multiple pathologies where RET oncogenicity contributes to cancer outcomes.

Medullary Thyroid Cancer: Clinical Characteristics and New Insights into Therapeutic Strategies Targeting Tyrosine Kinases

Medullary thyroid carcinoma (MTC) is a hyperplasia of thyroid C-cells, accounting for 5-10% of all thyroid cancers. MTCs may appear as sporadic or hereditary forms, and several molecules and signaling pathways have been found to function defectively in MTC cells. Tyrosine kinases are the most well-studied molecules that have abnormal function in these tumor cells. Due to their limited response, chemotherapeutic agents and radiation therapy are not effective in treating patients with advanced metastatic MTC. In the past decade, significant attention has been given to the utilization of multikinase inhibitors as targeted therapeutic agents for treating MTC patients, with the most promising results arising from the study of tyrosine kinase inhibitors, which generally bind to the ATP binding sites of these kinases. Two drugs-vandetanib and cabozantinib-are approved for the treatment of aggressive advanced MTC; however, the potential for toxicities and adverse effects of these agents on patient quality of life need to be considered against any therapeutic gain. According to recent data, it appears that inhibition of only one receptor or molecule in a pathway is not as effective as simultaneous inhibition of different pathways, indicating the need to use combination therapy. The main purpose of this review is to describe the clinical characteristics, molecular mechanisms, and current molecular and targeted therapeutic strategies active in clinical trials for advanced MTC treatment.

RET-mediated autophagy suppression as targetable co-dependence in acute myeloid leukemia

Many cases of AML are associated with mutational activation of receptor tyrosine kinases (RTKs) such as FLT3. However, RTK inhibitors have limited clinical efficacy as single agents, indicating that AML is driven by concomitant activation of different signaling molecules. We used a functional genomic approach to identify RET, encoding an RTK, as an essential gene in multiple subtypes of AML, and observed that AML cells show activation of RET signaling via ARTN/GFRA3 and NRTN/GFRA2 ligand/co-receptor complexes. Interrogation of downstream pathways identified mTORC1-mediated suppression of autophagy and subsequent stabilization of leukemogenic drivers such as mutant FLT3 as important RET effectors. Accordingly, genetic or pharmacologic RET inhibition impaired the growth of FLT3-dependent AML cell lines and was accompanied by upregulation of autophagy and FLT3 depletion. RET dependence was also evident in mouse models of AML and primary AML patient samples, and transcriptome and immunohistochemistry analyses identified elevated RET mRNA levels and co-expression of RET and FLT3 proteins in a substantial proportion of AML patients. Our results indicate that RET-mTORC1 signaling promotes AML through autophagy suppression, suggesting that targeting RET or, more broadly, depletion of leukemogenic drivers via autophagy induction provides a therapeutic opportunity in a relevant subset of AML patients.

RET-mediated modulation of tumor microenvironment and immune response in multiple endocrine neoplasia type 2 (MEN2)

Medullary thyroid carcinomas (MTC) arise from thyroid parafollicular, calcitonin-producing C-cells and can occur either as sporadic or as hereditary diseases in the context of familial syndromes, including multiple endocrine neoplasia 2A (MEN2A), multiple endocrine neoplasia 2B (MEN2B) and familial MTC (FMTC). In a large fraction of sporadic cases, and virtually in all inherited cases of MTC, activating point mutations of the RET proto-oncogene are found. RET encodes for a receptor tyrosine kinase protein endowed with transforming potential on thyroid parafollicular cells. As in other cancer types, microenvironmental factors play a critical role in MTC. Tumor-associated extracellular matrix, stromal cells and immune cells interact and influence the behavior of cancer cells both in a tumor-promoting and in a tumor-suppressing manner. Several studies have shown that, besides the neoplastic transformation of thyroid C-cells, a profound modification of tumor microenvironment has been associated to the RET FMTC/MEN2-associated oncoproteins. They influence the surrounding stroma, activating cancer-associated fibroblasts (CAFs), promoting cancer-associated inflammation and suppressing anti-cancer immune response. These mechanisms might be exploited to develop innovative anti-cancer therapies and novel prognostic tools in the context of familial, RET-associated MTC.

Targeting RET-driven cancers: lessons from evolving preclinical and clinical landscapes

The gene encoding the receptor-tyrosine kinase RET was first discovered more than three decades ago, and activating RET rearrangements and mutations have since been identified as actionable drivers of oncogenesis. Several multikinase inhibitors with activity against RET have been explored in the clinic, and confirmed responses to targeted therapy with these agents have been observed in patients with RET-rearranged lung cancers or RET-mutant thyroid cancers. Nevertheless, response rates to RET-directed therapy are modest compared with those achieved using targeted therapies matched to other oncogenic drivers of solid tumours, such as sensitizing EGFR or BRAF^V600E mutations, or ALK or ROS1 rearrangements. To date, no RET-directed targeted therapeutic has received regulatory approval for the treatment of molecularly defined populations of patients with RET-mutant or RET-rearranged solid tumours. In this Review, we discuss how emerging data have informed the debate over whether the limited success of multikinase inhibitors with activity against RET can be attributed to the tractability of RET as a drug target or to the lack, until 2017, of highly specific inhibitors of this oncoprotein in the clinic. We emphasize that novel approaches to targeting RET-dependent tumours are necessary to improve the clinical efficacy of single-agent multikinase inhibition and, thus, hasten approvals of RET-directed targeted therapies.

Drosophila Cancer Models Identify Functional Differences between Ret Fusions

We generated and compared Drosophila models of RET fusions CCDC6-RET and NCOA4-RET. Both RET fusions directed cells to migrate, delaminate, and undergo EMT, and both resulted in lethality when broadly expressed. In all phenotypes examined, NCOA4-RET was more severe than CCDC6-RET, mirroring their effects on patients. A functional screen against the Drosophila kinome and a library of cancer drugs found that CCDC6-RET and NCOA4-RET acted through different signaling networks and displayed distinct drug sensitivities. Combining data from the kinome and drug screens identified the WEE1 inhibitor AZD1775 plus the multi-kinase inhibitor sorafenib as a synergistic drug combination that is specific for NCOA4-RET. Our work emphasizes the importance of identifying and tailoring a patient's treatment to their specific RET fusion isoform and identifies a multi-targeted therapy that may prove effective against tumors containing the NCOA4-RET fusion.

Antitumor Activity of RXDX-105 in Multiple Cancer Types with RET Rearrangements or Mutations

Purpose: While multikinase inhibitors with RET activity are active in RET-rearranged thyroid and lung cancers, objective response rates are relatively low and toxicity can be substantial. The development of novel RET inhibitors with improved potency and/or reduced toxicity is thus an unmet need. RXDX-105 is a small molecule kinase inhibitor that potently inhibits RET. The purpose of the preclinical and clinical studies was to evaluate the potential of RXDX-105 as an effective therapy for cancers driven by RET alterations.Experimental design: The RET-inhibitory activity of RXDX-105 was assessed by biochemical and cellular assays, followed by in vivo tumor growth inhibition studies in cell line- and patient-derived xenograft models. Antitumor activity in patients was assessed by imaging and Response Evaluation Criteria in Solid Tumors (RECIST).Results: Biochemically, RXDX-105 inhibited wild-type RET, CCDC6-RET, NCOA4-RET, PRKAR1A-RET, and RET M918T with low to subnanomolar activity while sparing VEGFR2/KDR and VEGFR1/FLT. RXDX-105 treatment resulted in dose-dependent inhibition of proliferation of CCDC6-RET-rearranged and RET C634W-mutant cell lines and inhibition of downstream signaling pathways. Significant tumor growth inhibition in CCDC6-RET, NCOA4-RET, and KIF5B-RET-containing xenografts was observed, with the concomitant inhibition of p-ERK, p-AKT, and p-PLCγ. Additionally, a patient with advanced RET-rearranged lung cancer had a rapid and sustained response to RXDX-105 in both intracranial and extracranial disease.Conclusions: These data support the inclusion of patients bearing RET alterations in ongoing and future molecularly enriched clinical trials to explore RXDX-105 efficacy across a variety of tumor types. Clin Cancer Res; 23(12); 2981-90. ©2016 AACR.

The molecular basis for RET tyrosine-kinase inhibitors in thyroid cancer

RET receptor tyrosine kinase acts as a mutated oncogenic driver in several human malignancies and it is over-expressed in other cancers. Small molecule compounds with RET tyrosine kinase inhibitory activity are being investigated for the targeted treatment of these malignancies. Multi-targeted compounds with RET inhibitory concentration in the nanomolar range have entered clinical practice. This review summarizes mechanisms of RET oncogenic activity and properties of new compounds that, at the preclinical stage, have demonstrated promising anti-RET activity.