A Patient-derived Xenograft Model of Pancreatic Neuroendocrine Tumors Identifies Sapanisertib as a Possible New Treatment for Everolimus-resistant Tumors

Targeting the RAS upstream and downstream signaling pathway for Cancer treatment

Cancer often involves the overactivation of RAS/RAF/MEK/ERK (MAPK) and PI3K-Akt-mTOR pathways due to mutations in genes like RAS, RAF, PTEN, and PIK3CA. Various strategies are employed to address the overactivation of these pathways, among which targeted therapy emerges as a promising approach. Directly targeting specific proteins, leads to encouraging results in cancer treatment. For instance, RTK inhibitors such as imatinib and afatinib selectively target these receptors, hindering ligand binding and reducing signaling initiation. These inhibitors have shown potent efficacy against Non-Small Cell Lung Cancer. Other inhibitors, like lonafarnib targeting Farnesyltransferase and GGTI 2418 targeting geranylgeranyl Transferase, disrupt post-translational modifications of proteins. Additionally, inhibition of proteins like SOS, SH2 domain, and Ras demonstrate promising anti-tumor activity both in vivo and in vitro. Targeting downstream components with RAF inhibitors such as vemurafenib, dabrafenib, and sorafenib, along with MEK inhibitors like trametinib and binimetinib, has shown promising outcomes in treating cancers with BRAF-V600E mutations, including myeloma, colorectal, and thyroid cancers. Furthermore, inhibitors of PI3K (e.g., apitolisib, copanlisib), AKT (e.g., ipatasertib, perifosine), and mTOR (e.g., sirolimus, temsirolimus) exhibit promising efficacy against various cancers such as Invasive Breast Cancer, Lymphoma, Neoplasms, and hematological malignancies. This review offers an overview of small molecule inhibitors targeting specific proteins within the RAS upstream and downstream signaling pathways in cancer.

Multifaceted role of mTOR (mammalian target of rapamycin) signaling pathway in human health and disease

The mammalian target of rapamycin (mTOR) is a protein kinase that controls cellular metabolism, catabolism, immune responses, autophagy, survival, proliferation, and migration, to maintain cellular homeostasis. The mTOR signaling cascade consists of two distinct multi-subunit complexes named mTOR complex 1/2 (mTORC1/2). mTOR catalyzes the phosphorylation of several critical proteins like AKT, protein kinase C, insulin growth factor receptor (IGF-1R), 4E binding protein 1 (4E-BP1), ribosomal protein S6 kinase (S6K), transcription factor EB (TFEB), sterol-responsive element-binding proteins (SREBPs), Lipin-1, and Unc-51-like autophagy-activating kinases. mTOR signaling plays a central role in regulating translation, lipid synthesis, nucleotide synthesis, biogenesis of lysosomes, nutrient sensing, and growth factor signaling. The emerging pieces of evidence have revealed that the constitutive activation of the mTOR pathway due to mutations/amplification/deletion in either mTOR and its complexes (mTORC1 and mTORC2) or upstream targets is responsible for aging, neurological diseases, and human malignancies. Here, we provide the detailed structure of mTOR, its complexes, and the comprehensive role of upstream regulators, as well as downstream effectors of mTOR signaling cascades in the metabolism, biogenesis of biomolecules, immune responses, and autophagy. Additionally, we summarize the potential of long noncoding RNAs (lncRNAs) as an important modulator of mTOR signaling. Importantly, we have highlighted the potential of mTOR signaling in aging, neurological disorders, human cancers, cancer stem cells, and drug resistance. Here, we discuss the developments for the therapeutic targeting of mTOR signaling with improved anticancer efficacy for the benefit of cancer patients in clinics.

Models in Pancreatic Neuroendocrine Neoplasms: Current Perspectives and Future Directions

Pancreatic neuroendocrine neoplasms (pNENs) are a heterogeneous group of tumors derived from multiple neuroendocrine origin cell subtypes. Incidence rates for pNENs have steadily risen over the last decade, and outcomes continue to vary widely due to inability to properly screen. These tumors encompass a wide range of functional and non-functional subtypes, with their rarity and slow growth making therapeutic development difficult as most clinically used therapeutics are derived from retrospective analyses. Improved molecular understanding of these cancers has increased our knowledge of the tumor biology for pNENs. Despite these advances in our understanding of pNENs, there remains a dearth of models for further investigation. In this review, we will cover the current field of pNEN models, which include established cell lines, animal models such as mice and zebrafish, and three-dimensional (3D) cell models, and compare their uses in modeling various disease aspects. While no study model is a complete representation of pNEN biology, each has advantages which allow for new scientific understanding of these rare tumors. Future efforts and advancements in technology will continue to create new options in modeling these cancers.

Patient-derived xenograft models in cancer therapy: technologies and applications

Patient-derived xenograft (PDX) models, in which tumor tissues from patients are implanted into immunocompromised or humanized mice, have shown superiority in recapitulating the characteristics of cancer, such as the spatial structure of cancer and the intratumor heterogeneity of cancer. Moreover, PDX models retain the genomic features of patients across different stages, subtypes, and diversified treatment backgrounds. Optimized PDX engraftment procedures and modern technologies such as multi-omics and deep learning have enabled a more comprehensive depiction of the PDX molecular landscape and boosted the utilization of PDX models. These irreplaceable advantages make PDX models an ideal choice in cancer treatment studies, such as preclinical trials of novel drugs, validating novel drug combinations, screening drug-sensitive patients, and exploring drug resistance mechanisms. In this review, we gave an overview of the history of PDX models and the process of PDX model establishment. Subsequently, the review presents the strengths and weaknesses of PDX models and highlights the integration of novel technologies in PDX model research. Finally, we delineated the broad application of PDX models in chemotherapy, targeted therapy, immunotherapy, and other novel therapies.

Neuroendocrine neoplasms in the context of inherited tumor syndromes: a reappraisal focused on targeted therapies

PURPOSE

Neuroendocrine neoplasms can occur as part of inherited disorders, usually in the form of well-differentiated, slow-growing tumors (NET). The main predisposing syndromes include: multiple endocrine neoplasias type 1 (MEN1), associated with a large spectrum of gastroenteropancreatic and thoracic NETs, and type 4 (MEN4), associated with a wide tumour spectrum similar to that of MEN1; von Hippel-Lindau syndrome (VHL), tuberous sclerosis (TSC), and neurofibromatosis 1 (NF-1), associated with pancreatic NETs. In the present review, we propose a reappraisal of the genetic basis and clinical features of gastroenteropancreatic and thoracic NETs in the setting of inherited syndromes with a special focus on molecularly targeted therapies for these lesions.

METHODS

Literature search was systematically performed through online databases, including MEDLINE (via PubMed), and Scopus using multiple keywords' combinations up to June 2022.

RESULTS

Somatostatin analogues (SSAs) remain the mainstay of systemic treatment for NETs, and radiolabelled SSAs can be used for peptide-receptor radionuclide therapy for somatostatin receptor (SSTR)-positive NETs. Apart of these SSTR-targeted therapies, other targeted agents have been approved for NETs: the mTOR inhibitor everolimus for lung, gastroenteropatic and unknown origin NET, and sunitinib, an antiangiogenic tyrosine kinase inhibitor, for pancreatic NET. Novel targeted therapies with other antiangiogenic agents and immunotherapies have been also under evaluation.

CONCLUSIONS

Major advances in the understanding of genetic and epigenetic mechanisms of NET development in the context of inherited endocrine disorders have led to the recognition of molecular targetable alterations, providing a rationale for the implementation of treatments and development of novel targeted therapies.

Recent progress of experimental model in pancreatic neuroendocrine tumors: drawbacks and challenges

The neuroendocrine neoplasm, in general, refers to a heterogeneous group of all tumors originating from peptidergic neurons and neuroendocrine cells. Neuroendocrine neoplasms are divided into two histopathological subtypes: well-differentiated neuroendocrine tumors and poorly differentiated neuroendocrine carcinomas. Pancreatic neuroendocrine tumors account for more than 80% of pancreatic neuroendocrine neoplasms. Due to the greater proportion of pancreatic neuroendocrine tumors compared to pancreatic neuroendocrine carcinoma, this review will only focus on them. The worldwide incidence of pancreatic neuroendocrine tumors is rising year by year due to sensitive detection with an emphasis on medical examinations and the improvement of testing technology. Although the biological behavior of pancreatic neuroendocrine tumors tends to be inert, distant metastasis is common, often occurring very early. Because of the paucity of basic research on pancreatic neuroendocrine tumors, the mechanism of tumor development, metastasis, and recurrence are still unclear. In this context, the representative preclinical models simulating the tumor development process are becoming ever more widely appreciated to address the clinical problems of pancreatic neuroendocrine tumors. So far, there is no comprehensive report on the experimental model of pancreatic neuroendocrine tumors. This article systematically summarizes the characteristics of preclinical models, such as patient-derived cell lines, patient-derived xenografts, genetically engineered mouse models, and patient-derived organoids, and their advantages and disadvantages, to provide a reference for further studies of neuroendocrine tumors. We also highlight the method of establishment of liver metastasis mouse models.

A phase II study of sapanisertib (TAK-228) a mTORC1/2 inhibitor in rapalog-resistant advanced pancreatic neuroendocrine tumors (PNET): ECOG-ACRIN EA2161

This was a two-stage phase II trial of a mTORC1/2 inhibitor (mTORC: mammalian target of rapamycin complex) Sapanisertib (TAK228) in patients with rapalog-resistant pancreatic neuroendocrine tumors (PNETs) (NCT02893930). Approved rapalogs such as everolimus inhibit mTORC1 and have limited clinical activity, possibly due to compensatory feedback loops. Sapanisertib addresses the potential for incomplete inhibition of the mTOR pathway through targeting of both mTORC1 and mTORC2, and thus to reverse resistance to earlier rapamycin analogues. In stage 1, patients received sapanisertib 3 mg by mouth once daily on a continuous dosing schedule in 28-day cycle. This trial adopted a two-stage design with the primary objective of evaluating objective tumor response. The first stage would recruit 13 patients in order to accrue 12 eligible and treated patients. If among the 12 eligible patients at least 1 patient had an objective response to therapy, the study would move to the second stage of accrual where 25 eligible and treated patients would be enrolled. This study activated on February 1, 2017, the required pre-determined number of patients (n = 13) had entered by November 5, 2018 for the first stage response evaluation. The accrual of this trial was formally terminated on December 27, 2019 as no response had been observed after the first stage accrual. Treatment-related grade 3 adverse events were reported in eight (61%) patients with hyperglycemia being the most frequent, in three patients (23%). Other toxicities noted in the trial included fatigue, rash diarrhea, nausea, and vomiting. The median PFS was 5.19 months (95% CI [3.84, 9.30]) and the median OS was 20.44 months (95% CI [5.65, 22.54]). Due to the lack of responses in Stage 1 of the study, the study did not proceed to stage 2. Thus the potential to reverse resistance was not evident.

Preclinical Models of Neuroendocrine Neoplasia

Neuroendocrine neoplasia (NENs) are a complex and heterogeneous group of cancers that can arise from neuroendocrine tissues throughout the body and differentiate them from other tumors. Their low incidence and high diversity make many of them orphan conditions characterized by a low incidence and few dedicated clinical trials. Study of the molecular and genetic nature of these diseases is limited in comparison to more common cancers and more dependent on preclinical models, including both in vitro models (such as cell lines and 3D models) and in vivo models (such as patient derived xenografts (PDXs) and genetically-engineered mouse models (GEMMs)). While preclinical models do not fully recapitulate the nature of these cancers in patients, they are useful tools in investigation of the basic biology and early-stage investigation for evaluation of treatments for these cancers. We review available preclinical models for each type of NEN and discuss their history as well as their current use and translation.

New Therapy Options for Neuroendocrine Carcinoma of the Pancreas—The Emergent Substance GP-2250 and Gemcitabine Prove to Be Highly Effective without the Development of Secondary Resistances In Vitro and In Vivo

Simple Summary

Neuroendocrine carcinoma of the pancreas is a highly aggressive form of neuroendocrine tumor associated with poor survival and increasing occurrence. GP-2250 is an emergent substance showing antineoplastic properties, especially in combination with Gemcitabine. This study was the first to evaluate the antineoplastic effects of GP-2250 on pancreatic neuroendocrine carcinoma. The combination of GP-2250 and Gemcitabine showed highly synergistic effects in a cell culture model, as well as in mice, without the development of secondary resistances. These findings form the basis for further clinical evaluation of a highly promising combination therapy.

Abstract

Neuroendocrine carcinoma of the pancreas (pNEC) is an aggressive form of neuroendocrine tumor characterized by a rising incidence without an increase in survival rates. GP-2250 is an oxathiazinane derivate possessing antineoplastic effects, especially in combination with Gemcitabine on the pancreatic adenocarcinoma. The cytotoxic effects of the monotherapy of GP-2250 (GP-2250_mono) and Gemcitabine (Gem_mono), as well as the combination therapy of both, were studied in vitro using an MTT-assay on the QGP-1 and BON-1 cell lines, along with in vivo studies on a murine xenograft model of QGP-1 and a patient-derived xenograft model (PDX) of Bo99. In vitro, Gem_mono and GP-2250_mono showed a dose-dependent cytotoxicity. The combination of GP-2250 and Gemcitabine exhibited highly synergistic effects. In vivo, the combination therapy obtained a partial response in QGP-1, while GP-2250_mono and Gem_mono showed progressive disease or stable disease, respectively. In Bo99 PDX, the combination therapy led to a partial response, while the monotherapy resulted in progressive disease. No development of secondary resistances was observed, as opposed to monotherapy. This study was the first to evaluate the effects of the emerging substance GP-2250 on pNEC. The substance showed synergism in combination with Gemcitabine. The combination therapy proved to be effective in vitro and in vivo, without the development of secondary resistances.

Establishment of Novel Neuroendocrine Carcinoma Patient-Derived Xenograft Models for Receptor Peptide-Targeted Therapy

Simple Summary

Gastroenteropancreatic neuroendocrine neoplasms (GEP NENs) are a family of rare cancers with rising incidence in recent years. GEP NEN tumor cells are difficult to propagate, and few cellular and patient-derived xenograft (PDX) models are available for testing new therapies and studying the heterogeneous nature of these cancers. Here, we described the establishment and characterization of two novel NEC cellular and PDX models (NEC913 and NEC1452). NEC913 PDX tumors express somatostatin receptor 2 (SSTR2), whereas NEC1452 PDX tumors are SSTR2 negative. As a proof-of-concept study, we demonstrated how these PDX models can be used for peptide imaging experiments targeting SSTR2 using fluorescently labelled octreotide. The NEC913 and NEC1452 PDX lines represent valuable new tools for accelerating the process of drug discovery for GEP NENs.

Abstract

Gastroenteropancreatic neuroendocrine neoplasms (GEP NENs) are rare cancers consisting of neuroendocrine carcinomas (NECs) and neuroendocrine tumors (NETs), which have been increasing in incidence in recent years. Few cell lines and pre-clinical models exist for studying GEP NECs and NETs, limiting the ability to discover novel imaging and treatment modalities. To address this gap, we isolated tumor cells from cryopreserved patient GEP NECs and NETs and injected them into the flanks of immunocompromised mice to establish patient-derived xenograft (PDX) models. Two of six mice developed tumors (NEC913 and NEC1452). Over 80% of NEC913 and NEC1452 tumor cells stained positive for Ki67. NEC913 PDX tumors expressed neuroendocrine markers such as chromogranin A (CgA), synaptophysin (SYP), and somatostatin receptor-2 (SSTR2), whereas NEC1452 PDX tumors did not express SSTR2. Exome sequencing revealed loss of TP53 and RB1 in both NEC tumors. To demonstrate an application of these novel NEC PDX models for SSTR2-targeted peptide imaging, the NEC913 and NEC1452 cells were bilaterally injected into mice. Near infrared-labelled octreotide was administered and the fluorescent signal was specifically observed for the NEC913 SSTR2 positive tumors. These 2 GEP NEC PDX models serve as a valuable resource for GEP NEN therapy testing.

Cdk5 drives formation of heterogeneous pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PanNETs) are a heterogeneous population of neoplasms that arise from hormone-secreting islet cells of the pancreas and have increased markedly in incidence over the past four decades. Non-functional PanNETs, which occur more frequently than hormone-secreting tumors, are often not diagnosed until later stages of tumor development and have poorer prognoses. Development of successful therapeutics for PanNETs has been slow, partially due to a lack of diverse animal models for pre-clinical testing. Here, we report development of an inducible, conditional mouse model of PanNETs by using a bi-transgenic system for regulated expression of the aberrant activator of Cdk5, p25, specifically in β-islet cells. This model produces a heterogeneous population of PanNETs that includes a subgroup of well-differentiated, non-functional tumors. Production of these tumors demonstrates the causative potential of aberrantly active Cdk5 for generation of PanNETs. Further, we show that human PanNETs express Cdk5 pathway components, are dependent on Cdk5 for growth, and share genetic and transcriptional overlap with the INS-p25OE model. The utility of this model is enhanced by the ability to form tumor-derived allografts. This new model of PanNETs will facilitate molecular delineation of Cdk5-dependent PanNETs and the development of new targeted therapeutics.

Pancreatic Neuroendocrine Tumors: Molecular Mechanisms and Therapeutic Targets

Pancreatic neuroendocrine tumors (pNETs) are unique, slow-growing malignancies whose molecular pathogenesis is incompletely understood. With rising incidence of pNETs over the last four decades, larger and more comprehensive 'omic' analyses of patient tumors have led to a clearer picture of the pNET genomic landscape and transcriptional profiles for both primary and metastatic lesions. In pNET patients with advanced disease, those insights have guided the use of targeted therapies that inhibit activated mTOR and receptor tyrosine kinase (RTK) pathways or stimulate somatostatin receptor signaling. Such treatments have significantly benefited patients, but intrinsic or acquired drug resistance in the tumors remains a major problem that leaves few to no effective treatment options for advanced cases. This demands a better understanding of essential molecular and biological events underlying pNET growth, metastasis, and drug resistance. This review examines the known molecular alterations associated with pNET pathogenesis, identifying which changes may be drivers of the disease and, as such, relevant therapeutic targets. We also highlight areas that warrant further investigation at the biological level and discuss available model systems for pNET research. The paucity of pNET models has hampered research efforts over the years, although recently developed cell line, animal, patient-derived xenograft, and patient-derived organoid models have significantly expanded the available platforms for pNET investigations. Advancements in pNET research and understanding are expected to guide improved patient treatments.

Current treatments and future potential of surufatinib in neuroendocrine tumors (NETs)

Neuroendocrine tumors (NETs) are rare, heterogeneous, often indolent tumors that predominantly originate in the lungs and gastrointestinal tract. An understanding of the biology and tumor microenvironment of NETs has led to the development of molecularly targeted treatment options including somatostatin analogs, tyrosine kinase inhibitors, mammalian target of rapamycin inhibitors and peptide receptor radionuclide therapy. Although increases in progression-free survival have been demonstrated, most currently approved NET therapies are limited by the development of tumor resistance. Surufatinib (HMPL-012, previously known as sulfatinib) is a new, oral, small-molecule tyrosine kinase inhibitor that potently inhibits vascular endothelial growth-factor receptor 1-3, fibroblast growth-factor receptor 1, and colony-stimulating-factor-1 receptor. This unique combination of molecular activities inhibits tumor angiogenesis, regulates tumor-immune evasion, and may decrease tumor resistance. Surufatinib demonstrated statistically significant, clinically meaningful antitumor activity, including tumor shrinkage, in two phase III studies recently completed in China in advanced pancreatic NETs and advanced extrapancreatic NETs. The safety profile of surufatinib in neuroendocrine tumors studies was consistent with previous surufatinib clinical studies. In an ongoing study in United States (US) patients with NETs of pancreatic origin and NETs of extrapancreatic origin previously treated with everolimus or sunitinib, surufatinib has also demonstrated promising efficacy. Furthermore, the pharmacokinetic and safety profile of surufatinib in US patients is similar to data collected in studies done in China. These positive phase III results support the efficacy of surufatinib in patients with advanced, progressive, well-differentiated NETs regardless of tumor origin.

Sapanisertib Plus Exemestane or Fulvestrant in Women with Hormone Receptor-Positive/HER2-Negative Advanced or Metastatic Breast Cancer

PURPOSE

This open-label, multicenter, phase IB/II study evaluated sapanisertib, a dual inhibitor of mTOR kinase complexes 1/2, plus exemestane or fulvestrant in postmenopausal women with hormone receptor-positive (HR⁺)/HER2-negative (HER2^-) advanced/metastatic breast cancer.

PATIENTS AND METHODS

Eligible patients had previously progressed on everolimus with exemestane/fulvestrant and received ≤3 (phase IB) or ≤1 (phase II) prior chemotherapy regimens. Patients received sapanisertib 3 to 5 mg every day (phase IB), or 4 mg every day (phase II) with exemestane 25 mg every day or fulvestrant 500 mg monthly in 28-day cycles. Phase II enrolled parallel cohorts based on prior response to everolimus. The primary objective of phase II was to evaluate antitumor activity by clinical benefit rate at 16 weeks (CBR-16).

RESULTS

Overall, 118 patients enrolled in phase IB (n = 24) and II (n = 94). Five patients in phase IB experienced dose-limiting toxicities, at sapanisertib doses of 5 mg every day (n = 4) and 4 mg every day (n = 1); sapanisertib 4 mg every day was the MTD in combination with exemestane or fulvestrant. In phase II, in everolimus-sensitive versus everolimus-resistant cohorts, CBR-16 was 45% versus 23%, and overall response rate was 8% versus 2%, respectively. The most common adverse events were nausea (52%), fatigue (47%), diarrhea (37%), and hyperglycemia (33%); rash occurred in 17% of patients. Molecular analysis suggested positive association between AKT1 mutation status and best treatment response (complete + partial response; P = 0.0262).

CONCLUSIONS

Sapanisertib plus exemestane or fulvestrant was well tolerated and exhibited clinical benefit in postmenopausal women with pretreated everolimus-sensitive or everolimus-resistant breast cancer.

Development and comparison of novel bioluminescent mouse models of pancreatic neuroendocrine neoplasm metastasis

Pancreatic neuroendocrine neoplasms (pNENs) are slow growing cancers of increasing incidence that lack effective treatments once they become metastatic. Unfortunately, nearly half of pNEN patients present with metastatic liver tumors at diagnosis and current therapies fail to improve overall survival. Pre-clinical models of pNEN metastasis are needed to advance our understanding of the mechanisms driving the metastatic process and for the development of novel, targeted therapeutic interventions. To model metastatic dissemination of tumor cells, human pNEN cell lines (BON1 and Qgp1) stably expressing firefly luciferase (luc) were generated and introduced into NSG immunodeficient mice by intracardiac (IC) or intravenous (IV) injection. The efficiency, kinetics and distribution of tumor growth was evaluated weekly by non-invasive bioluminescent imaging (BLI). Tumors formed in all animals in both the IC and IV models. Bioluminescent Qgp1.luc cells preferentially metastasized to the liver regardless of delivery route, mimicking the predominant site of pNEN metastasis in patients. By comparison, BON1.luc cells most commonly formed lung tumors following either IV or IC administration and colonized a wider variety of tissues than Qgp1.luc cells. These models provide a unique platform for testing candidate metastasis genes and anti-metastatic therapies for pNENs.

Targeted Cancer Therapy: What's New in the Field of Neuroendocrine Neoplasms?

Neuroendocrine tumors (NETs) are a heterogeneous family of neoplasms of increasing incidence and high prevalence due to their relatively indolent nature. Their wide anatomic distribution and their characteristic ability to secrete hormonally active substances pose unique challenges for clinical management. They are also characterized by the common expression of somatostatin receptors, a target that has been extremely useful for diagnosis and treatment (i.e., somatostatin analogues (SSAs) and peptide-receptor radionuclide therapy (PRRT)). Chemotherapy is of limited use for NETs of non-pancreatic origin, and the only approved targeted agents for advanced progressive NETs are sunitinib for those of pancreatic origin, and everolimus for lung, gastrointestinal and pancreatic primaries. Despite recent therapeutic achievements, thus, systemic treatment options remain limited. In this review we will discuss the state-of-the-art targeted therapies in the field of NETs, and also future perspectives of novel therapeutic drugs or strategies in clinical development, including recently presented results from randomized trials of yet unapproved antiangiogenic agents (i.e., pazopanib, surufatinib and axitinib), PRRT including both approved radiopharmaceuticals (177Lu-Oxodotreotide) and others in development (177Lu-Edotreotide, 177Lu-Satoreotide Tetraxetan), immunotherapy and other innovative targeted strategies (antibody-drug conjugates, bites,…) that shall soon improve the landscape of personalized treatment options in NET patients.

Multiple Endocrine Neoplasia Type 1: Latest Insights

Multiple endocrine neoplasia type 1 (MEN1), a rare tumor syndrome that is inherited in an autosomal dominant pattern, is continuing to raise great interest for endocrinology, gastroenterology, surgery, radiology, genetics, and molecular biology specialists. There have been 2 major clinical practice guidance papers published in the past 2 decades, with the most recent published 8 years ago. Since then, several new insights on the basic biology and clinical features of MEN1 have appeared in the literature, and those data are discussed in this review. The genetic and molecular interactions of the MEN1-encoded protein menin with transcription factors and chromatin-modifying proteins in cell signaling pathways mediated by transforming growth factor β/bone morphogenetic protein, a few nuclear receptors, Wnt/β-catenin, and Hedgehog, and preclinical studies in mouse models have facilitated the understanding of the pathogenesis of MEN1-associated tumors and potential pharmacological interventions. The advancements in genetic diagnosis have offered a chance to recognize MEN1-related conditions in germline MEN1 mutation-negative patients. There is rapidly accumulating knowledge about clinical presentation in children, adolescents, and pregnancy that is translatable into the management of these very fragile patients. The discoveries about the genetic and molecular signatures of sporadic neuroendocrine tumors support the development of clinical trials with novel targeted therapies, along with advancements in diagnostic tools and surgical approaches. Finally, quality of life studies in patients affected by MEN1 and related conditions represent an effort necessary to develop a pharmacoeconomic interpretation of the problem. Because advances are being made both broadly and in focused areas, this timely review presents and discusses those studies collectively.

The Roles of Post-Translational Modifications on mTOR Signaling

The mechanistic target of rapamycin (mTOR) is a master regulator of cell growth, proliferation, and metabolism by integrating various environmental inputs including growth factors, nutrients, and energy, among others. mTOR signaling has been demonstrated to control almost all fundamental cellular processes, such as nucleotide, protein and lipid synthesis, autophagy, and apoptosis. Over the past fifteen years, mapping the network of the mTOR pathway has dramatically advanced our understanding of its upstream and downstream signaling. Dysregulation of the mTOR pathway is frequently associated with a variety of human diseases, such as cancers, metabolic diseases, and cardiovascular and neurodegenerative disorders. Besides genetic alterations, aberrancies in post-translational modifications (PTMs) of the mTOR components are the major causes of the aberrant mTOR signaling in a number of pathologies. In this review, we summarize current understanding of PTMs-mediated regulation of mTOR signaling, and also update the progress on targeting the mTOR pathway and PTM-related enzymes for treatment of human diseases.

Models of Gastroenteropancreatic Neuroendocrine Neoplasms: Current Status and Future Directions

Gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs) are a rare, heterogeneous group of tumors that originate from the endocrine system of the gastrointestinal tract and pancreas. GEP-NENs are subdivided according to their differentiation into well-differentiated neuroendocrine tumors (NETs) and poorly differentiated neuroendocrine carcinomas (NECs). Since GEP-NENs represent rare diseases, only limited data from large prospective, randomized clinical trials are available, and recommendations for treatment of GEP-NEN are in part based on data from retrospective analyses or case series. In this context, tractable disease models that reflect the situation in humans and that allow to recapitulate the different clinical aspects and disease stages of GEP-NET or GEP-NEC are urgently needed. In this review, we highlight available data on mouse models for GEP-NEN. We discuss how these models reflect tumor biology of human disease and whether these models could serve as a tool for understanding the pathogenesis of GEP-NEN and for disease modeling and pharmacosensitivity assays, facilitating prediction of treatment response in patients. In addition, open issues applicable for future developments will be discussed.

Modelling Pancreatic Neuroendocrine Cancer: From Bench Side to Clinic

Pancreatic neuroendocrine tumours (pNETs) are a heterogeneous group of epithelial tumours with neuroendocrine differentiation. Although rare (incidence of <1 in 100,000), they are the second most common group of pancreatic neoplasms after pancreatic ductal adenocarcinoma (PDAC). pNET incidence is however on the rise and patient outcomes, although variable, have been linked with 5-year survival rates as low as 40%. Improvement of diagnostic and treatment modalities strongly relies on disease models that reconstruct the disease ex vivo. A key constraint in pNET research, however, is the absence of human pNET models that accurately capture the original tumour phenotype. In attempts to more closely mimic the disease in its native environment, three-dimensional culture models as well as in vivo models, such as genetically engineered mouse models (GEMMs), have been developed. Despite adding significant contributions to our understanding of more complex biological processes associated with the development and progression of pNETs, factors such as ethical considerations and low rates of clinical translatability limit their use. Furthermore, a role for the site-specific extracellular matrix (ECM) in disease development and progression has become clear. Advances in tissue engineering have enabled the use of tissue constructs that are designed to establish disease ex vivo within a close to native ECM that can recapitulate tumour-associated tissue remodelling. Yet, such advanced models for studying pNETs remain underdeveloped. This review summarises the most clinically relevant disease models of pNETs currently used, as well as future directions for improved modelling of the disease.

Enhanced efficacy of JAK1 inhibitor with mTORC1/C2 targeting in smoldering/chronic adult T cell leukemia

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IL-2Rα is expressed in the leukemic cells of smoldering/chronic ATL patients, leading to activation of the JAK/STAT pathway.

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JAK1 inhibition with Upadacitinib inhibited cell proliferation and phosphorylation of STAT5 in cytokine-dependent ATL model.

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Dual mTORC1/C2 inhibitor was more effective than the single mTORC1 inhibitor in the cytokine-dependent ATL model.

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The combination of JAKi and mTORi showed synergistic effect in xenografts and leukemic cells from smoldering/chronic ATL.

Adult T-cell leukemia (ATL) is an aggressive T-cell lymphoproliferative malignancy of regulatory T lymphocytes (Tregs), caused by human T-cell lymphotropic virus 1 (HTLV-1). Interleukin 2 receptor alpha (IL-2Rα) is expressed in the leukemic cells of smoldering/chronic ATL patients, leading to constitutive activation of the JAK/STAT pathway and spontaneous proliferation. The PI3K/AKT/mTOR pathway also plays a critical role in ATL cell survival and proliferation. We previously performed a high-throughput screen that demonstrated additive/synergistic activity of Ruxolitinib, a JAK1/2 inhibitor, with AZD8055, an mTORC1/C2 inhibitor. However, effects of unintended JAK2 inhibition with Ruxolitinib limits it therapeutic potential for ATL patients, which lead us to evaluate a JAK1-specific inhibitor. Here, we demonstrated that Upadacitinib, a JAK-1 inhibitor, inhibited the proliferation of cytokine-dependent ATL cell lines and the expression of p-STAT5. Combinations of Upadacitinib with either AZD8055 or Sapanisertib, mTORC1/C2 inhibitors, showed anti-proliferative effects against cytokine-dependent ATL cell lines and synergistic effect with reducing tumor growth in NSG mice bearing IL-2 transgenic tumors. Importantly, the combination of these two agents inhibited ex vivo spontaneous proliferation of ATL cells from patients with smoldering/chronic ATL. Combined targeting of JAK/STAT and PI3K/AKT/mTOR pathways represents a promising therapeutic intervention for patients with smoldering/chronic ATL.

Pancreatic neuroendocrine tumors: Therapeutic challenges and research limitations

Pancreatic neuroendocrine tumors (PNETs) are known to be the second most common epithelial malignancy of the pancreas. PNETs can be listed among the slowest growing as well as the fastest growing human cancers. The prevalence of PNETs is deceptively low; however, its incidence has significantly increased over the past decades. According to the American Cancer Society’s estimate, about 4032 (> 7% of all pancreatic malignancies) individuals will be diagnosed with PNETs in 2020. PNETs often cause severe morbidity due to excessive secretion of hormones (such as serotonin) and/or overall tumor mass. Patients can live for many years (except for those patients with poorly differentiated G3 neuroendocrine tumors); thus, the prevalence of the tumors that is the number of patients actually dealing with the disease at any given time is fairly high because the survival is much longer than pancreatic ductal adenocarcinoma. Due to significant heterogeneity, the management of PNETs is very complex and remains an unmet clinical challenge. In terms of research studies, modest improvements have been made over the past decades in the identification of potential oncogenic drivers in order to enhance the quality of life and increase survival for this growing population of patients. Unfortunately, the majority of systematic therapies approved for the management of advanced stage PNETs lack objective response or at most result in modest benefits in survival. In this review, we aim to discuss the broad challenges associated with the management and the study of PNETs.

Enhancing the anti-tumour activity of 177Lu-DOTA-octreotate radionuclide therapy in somatostatin receptor-2 expressing tumour models by targeting PARP

Peptide receptor radionuclide therapy (PRRT) is an important treatment option for patients with somatostatin receptor-2 (SSTR2)-expressing neuroendocrine tumour (NET) though tumour regression occurs in only a minority of patients. Therefore, novel PRRT regimens with improved therapeutic activity are needed. Radiation induced DNA damage repair is an attractive therapeutic target to increase PRRT efficacy and consequently, we have characterised a panel of preclinical models for their SSTR2 expression, in vivo growth properties and response to 177Lu-DOTA-octreotate (LuTate) PRRT to identify models with features suitable for evaluating novel therapeutic combinations. In vitro studies using the SSTR2 expressing AR42J model demonstrate that the combination of LuTate and the small molecule Poly(ADP-ribose) polymerase-1 (PARP) inhibitor, talazoparib led to increased DNA double strand breaks, as assessed by γ-H2AX foci formation, as compared to LuTate alone. Furthermore, using the AR42J tumour model in vivo we demonstrate that the combination of LuTate and talazoparib significantly improved the anti-tumour efficacy of LuTate alone. These findings support the clinical evaluation of the combination of LuTate and PARP inhibition in SSTR2-expressing NET.

Molecular subtyping in pancreatic neuroendocrine neoplasms: New insights into clinical, pathological unmet needs and challenges

Pancreatic neuroendocrine neoplasms (PanNENs) contain two primary subtypes with distinct molecular features and associated clinical outcomes: well-differentiated neuroendocrine tumors (NETs) and poorly differentiated neuroendocrine carcinomas (NECs). PanNENs are a group of clinically heterogeneous tumors, whose diagnosis is based on tumor morphologic features and proliferation indices. However, these standards incompletely meet clinical needs by failing to adequately assess the likelihood of tumor recurrence and the potential for therapeutic response. We therefore focused on discussing molecular advances that facilitate the understanding of heterogeneity and exploration of reliable recurrence/treatment predictors. Taking advantage of high-throughput technologies, emerging methods of molecular subtyping in PanNETs include classifications based on co-existing multi-gene mutations, a large-scale loss of heterozygosity or copy number variation, and islet cell type-specific signatures. PanNEC molecular updates were discussed as well. This review aims to help the field classify PanNEN molecular subtypes, gain insights to aid in the solving of clinical, pathological unmet needs, and detect challenges and concerns of genetically-driven trials.

mTOR Pathway in Gastroenteropancreatic Neuroendocrine Tumor (GEP-NETs)

Gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs) originate from neuroendocrine cells in the gastrointestinal tract. They are heterogeneous, and though initially considered rare tumors, the incidence of GEP-NENs has increased in the last few decades. Therapeutic approaches for the metastatic disease include surgery, radiological intervention by chemoembolisation, radiofrequency ablation, biological therapy in addition to somatostatin analogs, and PRRT therapy (177Lu-DOTATATE). The PI3K-AKT-mTOR pathway is essential in the regulation of protein translation, cell growth, and metabolism. Evidence suggests that the mTOR pathway is involved in malignant progression and resistance to treatment through over-activation of several mechanisms. PI3K, one of the main downstream of the Akt-mTOR axis, is mainly involved in the neoplastic process. This pathway is frequently deregulated in human tumors, making it a central target in the development of new anti-cancer treatments. Recent molecular studies identify potential targets within the PI3K/Akt/mTOR pathway in GEP-NENs. However, the use of target therapy has been known to lead to resistance due to several mechanisms such as feedback activation of alternative pathways, inactivation of protein kinases, and deregulation of the downstream mTOR components. Therefore, the specific role of targeted drugs for the management of GEP-NENs is yet to be well-defined. The variable clinical presentation of advanced neuroendocrine tumors is a significant challenge for designing studies. This review aims to highlight the role of the PI3K/Akt/mTOR pathway in the development of neuroendocrine tumors and further specify its potential as a therapeutic target in advanced stages.

Targeting mTOR for cancer therapy

Mechanistic target of rapamycin (mTOR) is a protein kinase regulating cell growth, survival, metabolism, and immunity. mTOR is usually assembled into several complexes such as mTOR complex 1/2 (mTORC1/2). In cooperation with raptor, rictor, LST8, and mSin1, key components in mTORC1 or mTORC2, mTOR catalyzes the phosphorylation of multiple targets such as ribosomal protein S6 kinase β-1 (S6K1), eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1), Akt, protein kinase C (PKC), and type-I insulin-like growth factor receptor (IGF-IR), thereby regulating protein synthesis, nutrients metabolism, growth factor signaling, cell growth, and migration. Activation of mTOR promotes tumor growth and metastasis. Many mTOR inhibitors have been developed to treat cancer. While some of the mTOR inhibitors have been approved to treat human cancer, more mTOR inhibitors are being evaluated in clinical trials. Here, we update recent advances in exploring mTOR signaling and the development of mTOR inhibitors for cancer therapy. In addition, we discuss the mechanisms underlying the resistance to mTOR inhibitors in cancer cells.

Resistance to targeted treatment of gastroenteropancreatic neuroendocrine tumors

The mammalian target of rapamycin (mTOR) is part of the phosphoinositide-3-kinase (PI3K)/protein kinase B (Akt)/mTOR signaling. The PI3K/Akt/mTOR pathway has a pivotal role in the oncogenesis of neuroendocrine tumors (NETs). In addition, vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) drive angiogenesis in NETs and therefore contributes to neuroendocrine tumor development. Hence, mTOR and angiogenesis inhibitors have been developed. Everolimus, a first-generation mTOR inhibitor, has shown significant survival benefit in advanced gastroenteropancreatic NETs. Sunitinib, a pan-tyrosine kinase inhibitor that targets the VEGF receptor, has proven to increase progression-free survival in advanced pancreatic NETs. Nevertheless, primary and acquired resistance to rapalogs and sunitinib has limited the clinical benefit for NET patients. Despite the identification of multiple molecular mechanisms of resistance, no predictive biomarker has made it to the clinic. This review is focused on the mTOR signaling and angiogenesis in NET, the molecular mechanisms of primary and acquired resistance to everolimus and sunitinib and how to overcome this resistance by alternative drug compounds.