Discovery of a First-in-Class Gut-Restricted RET Kinase Inhibitor as a Clinical Candidate for the Treatment of IBS

Meta-analysis of gut microbiota alterations in patients with irritable bowel syndrome

Introduction

Irritable bowel syndrome (IBS) is a common chronic disorder of gastrointestinal function with a high prevalence worldwide. Due to its complex pathogenesis and heterogeneity, there is urrently no consensus in IBS research.

Methods

We collected and uniformly reanalyzed 1167 fecal 16S rRNA gene sequencing samples (623 from IBS patients and 544 from healthy subjects) from 9 studies. Using both a random effects (RE) model and a fixed effects (FE) model, we calculated the odds ratios for metrics including bacterial alpha diversity, beta diversity, common genera and pathways between the IBS and control groups.

Results

Significantly lower alpha-diversity indexes were observed in IBS patients by random effects model. Twenty-six bacterial genera and twelve predicted pathways were identified with significant odds ratios and classification potentials for IBS patients. Based on these feature, we used transfer learning to enhance the predictive capabilities of our model, which improved model performance by approximately 10%. Moreover, through correlation network analysis, we found that Ruminococcaceae and Christensenellaceae were negatively correlated with vitamin B6 metabolism, which was decreased in the patients with IBS. Ruminococcaceae was also negatively correlated with tyrosine metabolism, which was decreased in the patients with IBS.

Discussion

This study revealed the dysbiosis of fecal bacterial diversity, composition, and predicted pathways of patients with IBS by meta-analysis and identified universal biomarkers for IBS prediction and therapeutic targets.

Discovery of novel, orally bioavailable phenylacetamide derivatives as multikinase inhibitors and in vivo efficacy study in hepatocellular carcinoma animal models

Hepatocellular carcinoma (HCC) is considered as one of the leading causes of death in liver disease patients. Several signal transduction pathways are involved in HCC pathogenesis. Multikinase inhibitors (MKIs) show beneficial effects for HCC and the FDA approved a few MKIs including sorafenib, lenvatinib for HCC treatments. Here, a novel series of phenylacetamide derivatives were designed, synthesized and evaluated as multikinase inhibitors. Several compounds showed nanomolar IC50 values against FLT1, FLT3, FLT4, KDR, PDGFRα, PDGFRβ. The compounds were tested against human hepatocellular carcinoma (HCC), human colon adenocarcinoma and human gastric carcinoma cell lines. With favorable pharmacokinetics profiles, compound 12 and compound 14 were selected for in vivo efficacy studies in Hep3B mice models and demonstrated efficacious than sorafenib.

FK962 protects retinal ganglion cell under hypoxia/reoxygenation: Possible involvement of glial cell line-derived neurotrophic factor signaling pathway

Loss of retinal ganglion cells (RGCs) is the cause of visual impairment and blindness in glaucoma. Previously, our studies showed that FK962 (N-[1-acetylpiperidin-4-yl]-4-fluorobenzamide) promoted neurite elongation in rat RGCs and trigeminal ganglion (TG) cells. In TG cells, glial cell line-derived neurotrophic factor (GDNF) is known to be involved in the mechanism. The purpose of the present study is to investigate whether, 1) FK962 shows an RGC-protective effect under hypoxia/reoxygenation (H/R) and 2) GDNF is involved in the neuroprotective mechanism of FK962. Rat primary retinal cells were cultured under 24-h hypoxia/24-h reoxygenation conditions, with or without FK962, recombinant GDNF, GDNF antibody and RET receptor tyrosine kinase inhibitor, GSK3179106. Cells were co-immunostained with RBPMS and Neurofilament 200 as a RGC marker, and the number of survived RGCs was counted. Results showed H/R treatment decreased the number of survived RGCs. FK962 promoted RGC survival under H/R by a bell-shaped dose response, with the highest RGC-protective effect of 10-8 M. The protective effect was the same level with 10-12 M exogenous GDNF. Addition of GDNF antibody or GSK3179106 counteracted the neuroprotective effect of FK962. From these results, it is suggested that FK962 ameliorates RGC death under H/R, possibly via a GDNF signaling pathway.

GSK3179106 ameliorates lipopolysaccharide-induced inflammation and acute lung injury by targeting P38 MAPK

Acute lung injury (ALI) is a serious acute respiratory disease that can cause alveolar-capillary barrier disruption and pulmonary edema, respiratory failure and multiple organ dysfunction syndrome. However, there is no effective drugs in clinic until now. GSK3179106 has been reported can alleviate intestinal stress syndrome, but the protective effect of GSK3179106 on ALI has not been elucidated. The present study will evaluate the pharmacological activity of GSK3179106 on lipopolysaccharide (LPS)-induced inflammation and lung injury and clarify its underlying mechanism. We found that GSK3179106 significantly attenuated LPS-induced lung injury in vivo, accompanied by inhibited infiltration of inflammatory cells and reduced expression of inflammatory cytokines. Meanwhile, GSK3179106 dose-dependently reduced the LPS-induced IL-6 expression both in protein and gene levels in macrophages. Mechanistically, GSK3179106 could inhibited the phosphorylation of P38 MAPK induced by LPS. Importantly, results showed that there is a direct combination between GSK3179106 and P38 MAPK. Together, our findings not only clarified the anti-inflammatory activity of GSK3179106 but also discovered its new clinical indications. Therefore, compound GSK3179106 may be a potential candidate for the treatment of acute inflammatory diseases.

RET Signaling Persists in the Adult Intestine and Stimulates Motility by Limiting PYY Release From Enteroendocrine Cells

BACKGROUND & AIMS

RET tyrosine kinase is necessary for enteric nervous system development. Loss-of-function RET mutations cause Hirschsprung disease (HSCR), in which infants are born with aganglionic bowel. Despite surgical correction, patients with HSCR often experience chronic defecatory dysfunction and enterocolitis, suggesting that RET is important after development. To test this hypothesis, we determined the location of postnatal RET and its significance in gastrointestinal (GI) motility.

METHODS

RetCFP/+ mice and human transcriptional profiling data were studied to identify the enteric neuronal and epithelial cells that express RET. To determine whether RET regulates gut motility in vivo, genetic, and pharmacologic approaches were used to disrupt RET in all RET-expressing cells, a subset of enteric neurons, or intestinal epithelial cells.

RESULTS

Distinct subsets of enteric neurons and enteroendocrine cells expressed RET in the adult intestine. RET disruption in the epithelium, rather than in enteric neurons, slowed GI motility selectively in male mice. RET kinase inhibition phenocopied this effect. Most RET+ epithelial cells were either enterochromaffin cells that release serotonin or L-cells that release peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), both of which can alter motility. RET kinase inhibition exaggerated PYY and GLP-1 release in a nutrient-dependent manner without altering serotonin secretion in mice and human organoids. PYY receptor blockade rescued dysmotility in mice lacking epithelial RET.

CONCLUSIONS

RET signaling normally limits nutrient-dependent peptide release from L-cells and this activity is necessary for normal intestinal motility in male mice. These effects could contribute to dysmotility in HSCR, which predominantly affects males, and uncovers a mechanism that could be targeted to treat post-prandial GI dysfunction.

Design and Synthesis of Novel Thieno[3,2-c]quinoline Compounds with Antiproliferative Activity on RET-Dependent Medullary Thyroid Cancer Cells

RET kinase gain-of-function mutations represent the main cause of the high aggressiveness and invasiveness of medullary thyroid cancer (MTC). The selective inhibition of the RET kinase is a suitable strategy for the treatment of this endocrine neoplasia. Herein, we performed an innovative ligand-based virtual screening protocol using the DRUDITonline web service, focusing on the RET kinase as a biological target. In this process, thieno[3,2-c]quinolines 6a-e and 7a-e were proposed as new potential RET inhibitors. The selected compounds were synthetized by appropriate synthetic strategies, and in vitro evaluation of antiproliferative properties conducted on the particularly aggressive MTC cell line TT(C634R) identified compounds 6a-d as promising anticancer agents, with IC50 values in the micromolar range. Further structure-based computational studies revealed a significant capability of the most active compounds to the complex RET tyrosine kinase domain. The interesting antiproliferative results supported by in silico predictions suggest that these compounds may represent a starting point for the development of a new series of small heterocyclic molecules for the treatment of MTC.

A branching model of lineage differentiation underpinning the neurogenic potential of enteric glia

Glial cells have been proposed as a source of neural progenitors, but the mechanisms underpinning the neurogenic potential of adult glia are not known. Using single cell transcriptomic profiling, we show that enteric glial cells represent a cell state attained by autonomic neural crest cells as they transition along a linear differentiation trajectory that allows them to retain neurogenic potential while acquiring mature glial functions. Key neurogenic loci in early enteric nervous system progenitors remain in open chromatin configuration in mature enteric glia, thus facilitating neuronal differentiation under appropriate conditions. Molecular profiling and gene targeting of enteric glial cells in a cell culture model of enteric neurogenesis and a gut injury model demonstrate that neuronal differentiation of glia is driven by transcriptional programs employed in vivo by early progenitors. Our work provides mechanistic insight into the regulatory landscape underpinning the development of intestinal neural circuits and generates a platform for advancing glial cells as therapeutic agents for the treatment of neural deficits.

Trust Your Gut: Strategies and Tactics for Intestinally Restricted Drugs

Non-absorbable small-molecule drugs targeted to the gut represent an alternative approach to safe, non-systemic therapeutics. Such drugs remain confined to the gastrointestinal tract upon oral dosing by virtue of their limited passive permeability, increasing the local concentration at the site of action while minimizing exposure elsewhere in the body. Herein we review the latest advances in the field of gut-restricted therapeutics, highlighting the different strategies and tactics that medicinal chemists have employed in pursuit of drugs with minimal intestinal absorption.

Discovery of 3,5-diaryl-1H-pyrazol-based ureas as potent RET inhibitors

Rearranged during transfection (RET) is a promising target for antitumor drug development. Multikinase inhibitors (MKI) have been developed for RET-driven cancers but displayed limited efficacy in disease control. Two selective RET inhibitors were approved by FDA in 2020 and proved potent clinical efficacy. However, the discovery of novel RET inhibitors with high target selectivity and improved safety is still highly desirable. Herein, we reported a class of 3,5-diaryl-1H-pyrazol-based ureas as new RET inhibitors. The representative compounds 17a/b displayed high selectivity to other kinases, and potently inhibited isogenic BaF3-CCDC6-RET cells harboring wild-type, or gatekeeper mutation (V804M). They also displayed moderate potency against BaF3-CCDC6-RET-G810C with solvent-front mutation. Compound 17b showed better pharmacokinetics properties and demonstrated promising oral in vivo antitumor efficacy in a BaF3-CCDC6-RET-V804M xenograft model. It may be utilized as a new lead compound for further development.

Improving the performance of single-cell RNA-seq data mining based on relative expression orderings

The advent of single-cell RNA-sequencing (scRNA-seq) provides an unprecedented opportunity to explore gene expression profiles at the single-cell level. However, gene expression values vary over time and under different conditions even within the same cell. There is an urgent need for more stable and reliable feature variables at the single-cell level to depict cell heterogeneity. Thus, we construct a new feature matrix called the delta rank matrix (DRM) from scRNA-seq data by integrating an a priori gene interaction network, which transforms the unreliable gene expression value into a stable gene interaction/edge value on a single-cell basis. This is the first time that a gene-level feature has been transformed into an interaction/edge-level for scRNA-seq data analysis based on relative expression orderings. Experiments on various scRNA-seq datasets have demonstrated that DRM performs better than the original gene expression matrix in cell clustering, cell identification and pseudo-trajectory reconstruction. More importantly, the DRM really achieves the fusion of gene expressions and gene interactions and provides a method of measuring gene interactions at the single-cell level. Thus, the DRM can be used to find changes in gene interactions among different cell types, which may open up a new way to analyze scRNA-seq data from an interaction perspective. In addition, DRM provides a new method to construct a cell-specific network for each single cell instead of a group of cells as in traditional network construction methods. DRM's exceptional performance is due to its extraction of rich gene-association information on biological systems and stable characterization of cells.

An updated patent review of rearranged during transfection (RET) kinase inhibitors (2016-present)

INTRODUCTION

Rearranged during transfection (RET) is a transmembrane receptor tyrosine kinase. Aberrations in RET signaling due to mutations, gene fusions, or overexpression can lead to carcinomas. Six inhibitors have been approved for the treatment of RET-driven cancers: vandetanib, cabozantinib, lenvatinib, sorafenib, selpercatinib, and pralsetinib. Only selpercatinib and pralsetinib have been developed specifically for RET while the remaining are multikinase inhibitors. Several other RET targeted candidates are under clinical development.

AREAS COVERED

This review covers recent patent literature describing small molecules that are active against RET from 2016-present.

EXPERT OPINION

RET represents a major therapeutic target as its alterations occur in nearly 2% of all cancers. Recent approvals for RET targeted therapy have been developed specifically to target the RET oncogene. These approvals represent a paradigm shift from the last decade to now focus on development of selective RET inhibitors rather than multikinase inhibitors. These newly approved RET inhibitors still have clinical issues with drug resistance. It is imperative that the next iteration of RET inhibitors are developed to block common treatment resistant mutations. To accomplish this, RET inhibitors should be developed in concert with genomic profiling to ensure the most relevant clinical mutations are targeted.

Emerging therapies in the management of Irritable Bowel Syndrome (IBS)

INTRODUCTION

Irritable bowel syndrome (IBS) is a common, symptom-based disorder of chronic abdominal pain and altered bowel habits. The pathogenesis of IBS is multifactorial, leading to the potential for the development of multiple, diverse treatment strategies. This mechanistic heterogeneity also leads to the realization that available therapies are only effective in a subset of IBS suffers. Current US Food and Drug Administration (FDA) approved therapies for IBS with diarrhea (IBS-D) and IBS with constipation (IBS-C) are reviewed. Limited symptom responses and side effect experiences lead to considerable patient dissatisfaction with currently available IBS treatments. Only a small percentage of IBS patients are on prescription therapies underscoring the potential market and need for additional therapeutic options.

AREAS COVERED

: Expanding on currently available therapies, the serotonergic and endogenous opioid receptor systems continue to be a focus of future IBS treatment development. Additional novel emerging therapies include the endogenous cannabinoid system, bile acid secretion and sequestration, and exploit our enhanced understanding of visceral sensory signaling and intestinal secretomotor function.

EXPERT OPINION

While challenges remain for the future development of IBS therapies, the diverse etiologies underlying the disorder present an opportunity for novel therapies. Hence, great potential is anticipated for future IBS treatment options.

Recent Advances in Pain Management: Relevant Protein Kinases and Their Inhibitors

The purpose of this review is to underline the protein kinases that have been established, either in fundamental approach or clinical trials, as potential biological targets in pain management. Protein kinases are presented according to their group in the human kinome: TK (Trk, RET, EGFR, JAK, VEGFR, SFK, BCR-Abl), CMGC (p38 MAPK, MEK, ERK, JNK, ASK1, CDK, CLK2, DYRK1A, GSK3, CK2), AGC (PKA, PKB, PKC, PKMζ, PKG, ROCK), CAMK, CK1 and atypical/other protein kinases (IKK, mTOR). Examples of small molecule inhibitors of these biological targets, demonstrating an analgesic effect, are described. Altogether, this review demonstrates the fundamental role that protein kinase inhibitors could play in the development of new pain treatments.

First-in-Human Study for a Selective Rearranged During Transfection Tyrosine Kinase Inhibitor, GSK3352589, to Investigate the Safety, Tolerability, and Pharmacokinetics in Healthy Volunteers

Rearranged during transfection (RET), a neuronal growth factor receptor tyrosine kinase, regulates the development of sympathetic, parasympathetic, motor, and sensory neurons in the enteric nervous system. The intended site of action for GSK3352589 is intestinal tissues. GSK3352589 is an RET kinase inhibitor that was administered in double-blind, randomized, placebo-controlled single-dose (SD) and repeat-dose (RD) studies in healthy subjects to investigate its safety/tolerability and pharmacokinetics. In the SD study (n = 28), GSK3352589 was dosed from 2 to 400 mg, including a food effect arm (25 mg). In the RD study (n = 40), GSK3352589 was dosed for 14 days with food twice daily from 5 to 200 mg. With single (fed and fasted) and repeat (fed) doses, bioavailability was low and less than dose-proportional. There was a food effect with 25 mg once daily but may not be clinically relevant. Elimination half-life was ≥17 hours at SD ≥ 15 mg. Accumulation ratios for Cmax , AUCt , AUCtau , and AUC24 after twice-daily dosing to steady state ranged from 1.2 to 1.8 for all doses except the 200-mg dose, which had ratios between 1.9 and 2.7. Administration of GSK3352589 after SD and RD was well tolerated with no safety concerns in healthy subjects.

Recent developments of RET protein kinase inhibitors with diverse scaffolds as hinge binders

RET is a proto-oncogene encoding a receptor tyrosine kinase. RET regulates key aspects of cellular proliferation, differentiation and survival. The activation of RET via gene fusions or point mutations is closely related to lung, thyroid and other cancers. This review summarizes the developments of a diversity of small molecule RET protein kinase inhibitors in the past 10 years. These RET inhibitors are classified according to their hinge binder chemotypes as: pyrimidines, including the pyrazolopyrimidines, pyrimidine oxazines, quinazolines, 4-aminopyrimidines and 4-aminopyridines; indolinones; 5-aminopyrazole-4-carboxamides; 3-trifluoromethylanilines; imidazopyridines, imidazopyridazines and pyrazopyridines; nicotinonitriles; pyridones and 1,2,4-triazoles. In each section, the biological activities of the inhibitors, their structure-activity relationships and possible binding modes with the RET kinase are introduced.

Acute stress disrupts intestinal homeostasis via GDNF-RET

Objectives

Enterochromaffin (EC) cells have been associated with functional gastrointestinal disorders such as IBS. Recently, we found that glial cell‐derived neurotrophic factor (GDNF)‐rearranged during transfection (RET) localized in EC cells in human colonic epithelia. Here, we examine the role of GDNF‐RET in the pathophysiology of diarrhoea‐predominant irritable bowel syndrome (IBS‐D).

Materials and Methods

GDNF was assessed by ELISA and immunohistochemistry in biopsies from IBS‐D patients and healthy controls. Stress was induced by using a wrap‐restraint stress (WRS) procedure to serve as an acute stress‐induced IBS model. The function of GDNF‐RET axis to intestinal stem cell (ISC) homeostasis, and EC cell numbers were assessed in vivo and in vitro.

Results

GDNF‐RET was expressed in EC cells in human colon. GDNF was significantly increased in IBS‐D patients. WRS mice showed increased GDNF‐RET levels in colon. WRS induced visceral hypersensitivity by expanding of ISC and differentiation of EC cell via GDNF‐RET. Furthermore, GDNF‐treated mice recapitulated the phenotype of WRS mice. In vitro, GDNF treatment amplified Wnt signal and increased serotonin levels in colonic organoids in a dose‐dependent manner.

Conclusions

We identified GDNF‐RET was presented in colonic epithelium of patients with IBS‐D. GDNF‐RET played important roles in regulating ISC and EC cell differentiation. Our findings, thus, provide RET inhibitor as new therapeutic targets for treatment of patients with IBS‐D.

Efficacy and Tolerability of Pyrazolo[1,5-a]pyrimidine RET Kinase Inhibitors for the Treatment of Lung Adenocarcinoma

RET (REarranged during Transfection) kinase gain-of-function aberrancies have been identified as potential oncogenic drivers in lung adenocarcinoma, along with several other cancer types, prompting the discovery and assessment of selective inhibitors. Internal mining and analysis of relevant kinase data informed the decision to investigate a pyrazolo[1,5-a]pyrimidine scaffold, where subsequent optimization led to the identification of compound WF-47-JS03 (1), a potent RET kinase inhibitor with >500-fold selectivity against KDR (Kinase insert Domain Receptor) in cellular assays. In subsequent mouse in vivo studies, compound 1 demonstrated effective brain penetration and was found to induce strong regression of RET-driven tumor xenografts at a well-tolerated dose (10 mg/kg, po, qd). Higher doses of 1, however, were poorly tolerated in mice, similar to other pyrazolo[1,5-a]pyrimidine compounds at or near the efficacious dose, and indicative of the narrow therapeutic windows seen with this scaffold.

Measurement of novel intestinal secretory and barrier pathways and effects of proteases

The epithelial lining of the gastrointestinal (GI) tract in conjunction with the enteric nervous system (ENS) plays an important role in mediating solute absorption and secretion. A dysregulated ionic movement across the epithelium can result in GI diseases that manifest as either watery diarrhea or constipation. Hirschsprung disease is an example of an ENS disorder characterized by absence of enteric ganglia in distal gut resulting in obstructive phenotype. Receptor rearranged during transfection (RET) gene variants are the most commonly recognized genetic associations with Hirschsprung disease. In this issue of Neurogastroenterology and Motility, Russell et al demonstrate that RET mediates colonic ion transport through modulation of cholinergic nerves. They go on to show inhibition of RET can attenuate accelerated transit in a rat model. Normalizing secretory and absorptive defects has been an attractive therapeutic strategy. In addition to the intrinsic regulation of secretory processes, luminal mediators like bile acids, short-chain fatty acids, and proteases can affect both secretion and barrier function of the intestinal epithelium. Elevated levels of proteases have been identified in a wide range of GI diseases including irritable bowel syndrome. Proteases are known to cause visceral hypersensitivity and barrier disruption in vitro and in animal models. The goals of this review are to describe fundamental concepts related to intestinal epithelial secretion, the utility of Ussing chambers to measure ionic mechanisms and to discuss examples of novel signaling pathways; namely the RET signaling cascade in secretomotor neurons and effects of luminal proteases on barrier and ionic secretion.

Enteric RET inhibition attenuates gastrointestinal secretion and motility via cholinergic signaling in rat colonic mucosal preparations

BACKGROUND

The expression of RET in the developing enteric nervous system (ENS) suggests that RET may contribute to adult intestinal function. ENS cholinergic nerves play a critical role in the control of colonic function through the release of acetylcholine (ACh). In the current study, we hypothesized that a RET-mediated mechanism may regulate colonic ion transport and motility through modulation of cholinergic nerves.

METHODS

The effect of RET inhibition on active ion transport was assessed electrophysiologically in rat colonic tissue mounted in Ussing chambers via measurements of short circuit current (Isc) upon electrical field stimulation (EFS) or pharmacologically with cholinergic agonists utilizing a gastrointestinal (GI)-restricted RET inhibitor. We assessed the effect of the RET inhibitor on propulsive motility via quantification of fecal pellet output (FPO) induced by the acetylcholinesterase inhibitor neostigmine.

KEY RESULTS

We found that enteric ganglia co-expressed RET and choline acetyltransferase (ChAT) transcripts. In vitro, the RET kinase inhibitor GSK3179106 attenuated the mean increase in Isc induced by either EFS or carbachol but not bethanechol. In vivo, GSK3179106 significantly reduced the prokinetic effect of neostigmine.

CONCLUSION AND INFERENCES

Our findings provide evidence that RET-mediated mechanisms regulate colonic function by maintaining cholinergic neuronal function and enabling ACh-evoked chloride secretion and motility. We suggest that modulating the cholinergic control of the colon via a RET inhibitor may represent a novel target for the treatment of intestinal disorders associated with increased secretion and accelerated GI transit such as irritable bowel syndrome with diarrhea (IBS-D).

Targeting ERK1/2 protein-serine/threonine kinases in human cancers

ERK1 and ERK2 are key protein kinases that contribute to the Ras-Raf-MEK-ERK MAP kinase signalling module. This pathway participates in the control of numerous processes including apoptosis, cell proliferation, the immune response, nervous system function, and RNA synthesis and processing. MEK1/2 activate human ERK1/2 by first catalyzing the phosphorylation of Y204/187 and then T202/185, both residues of which occur within the activation segment. The phosphorylation of both residues is required for enzyme activation. The only Raf substrates are MEK1/2 and the only MEK1/2 substrates are ERK1/2. In contrast, ERK1/2 catalyze the phosphorylation of many cytoplasmic and nuclear substrates including transcription factors and regulatory molecules. The linear MAP kinase pathway branches extensively at the ERK1/2 node. ERK1/2 are proline-directed kinases that preferentially catalyze the phosphorylation of substrates containing a PxS/TP sequence. The dephosphorylation and inactivation of ERK1/2 is catalyzed by dual specificity phosphatases, protein-tyrosine specific phosphatases, and protein-serine/threonine phosphatases. The combined functions of kinases and phosphatases make the overall process reversible. To provide an idea of the complexities involved in these reactions, somatic cell cycle progression involves the strict timing of more than 32,000 phosphorylation and dephosphorylation events as determined by mass spectrometry. The MAP kinase cascade is perhaps the most important oncogenic driver of human cancers and the blockade of this signalling module by targeted inhibitors is an important anti-tumor strategy. Although numerous cancers are driven by MAP kinase pathway activation, thus far the only orally effective approved drugs that target this signaling module are used for the treatment of BRAF-mutant melanomas. The best treatments include the combination of B-Raf and MEK inhibitors (dabrafenib and trametinib, encorafenib and binimetinib, vemurafenib and cobimetanib). However, resistance to these antagonists occurs within one year and additional treatment options are necessary. Owing to the large variety of malignancies that are driven by dysregulation of the MAP kinase pathway, additional tumor types should be amenable to MAP kinase pathway inhibitor therapy. In addition to new B-Raf and MEK inhibitors, the addition of ERK inhibitors should prove helpful. Ulixertinib, MK-8353, and GDC-0994 are orally effective, potent, and specific inhibitors of ERK1/2 that are in early clinical trials for the treatment of various advanced/metastatic solid tumors. These agents are effective against cell lines that are resistant to B-Raf and MEK1/2 inhibitor therapy. Although MK-8353 does not directly inhibit MEK1/2, it decreases the phosphorylation of ERK1/2 as well as the phosphorylation of RSK, an ERK1/2 substrate. The decrease in RSK phosphorylation appears to be a result of ERK inhibition and the decrease in ERK1/2 phosphorylation is related to the inability of MEK to catalyze the phosphorylation of the ERK-MK-8353 complex; these decreases characterize the ERK dual mechanism inhibition paradigm. Additional work will be required to determine whether ERK inhibitors will be successful in the clinic and are able to forestall the development of drug resistance of the MAP kinase pathway.

Exploring the Potential of RET Kinase Inhibition for Irritable Bowel Syndrome: A Preclinical Investigation in Rodent Models of Colonic Hypersensitivity

Abdominal pain represents a significant complaint in patients with irritable bowel syndrome (IBS). While the etiology of IBS is incompletely understood, prior exposure to gastrointestinal inflammation or psychologic stress is frequently associated with the development of symptoms. Inflammation or stress-induced expression of growth factors or cytokines may contribute to the pathophysiology of IBS. Here, we aimed to investigate the therapeutic potential of inhibiting the receptor of glial cell line-derived neurotrophic factor, rearranged during transfection (RET), in experimental models of inflammation and stress-induced visceral hypersensitivity resembling IBS sequelae. In RET-cyan fluorescent protein [(CFP) RetCFP/+] mice, thoracic and lumbosacral dorsal root ganglia were shown to express RET, which colocalized with calcitonin gene-related peptide. To understand the role of RET in visceral nociception, we employed GSK3179106 as a potent, selective, and gut-restricted RET kinase inhibitor. Colonic hyperalgesia, quantified as exaggerated visceromotor response to graded pressures (0-60 mm Hg) of isobaric colorectal distension (CRD), was produced in multiple rat models induced 1) by colonic irritation, 2) following acute colonic inflammation, 3) by adulthood stress, and 4) by early life stress. In all the rat models, RET inhibition with GSK3179106 attenuated the number of abdominal contractions induced by CRD. Our findings identify a role for RET in visceral nociception. Inhibition of RET kinase with a potent, selective, and gut-restricted small molecule may represent a novel therapeutic strategy for the treatment of IBS through the attenuation of post-inflammatory and stress-induced visceral hypersensitivity.