Discovery of [11C]MK-8056: A Selective PET Imaging Agent for the Study of mGluR2 Negative Allosteric Modulators

Modification of potent, selective metabotropic glutamate receptor 2 negative allosteric modulator (mGluR₂ NAM) led to a series of analogues with excellent binding affinity, lipophilicity, and suitable physicochemical properties for a PET tracer with convenient chemical handles for incorporation of a ¹¹C or ¹⁸F radiolabel. [¹¹C]MK-8056 was synthesized and evaluated in vivo and demonstrated appropriate affinity, selectivity, and physicochemical properties to be used as a positron emission tomography tracer for mGluR₂.

Abstract 2455: Development of a bioluminescence reporter mouse model for tracking and quantifying Ly6G+ neutrophils in vivo

Immunotherapy has revolutionized the way cancer is treated; however, despite tremendous success, not all patients benefit from immunotherapy. Identifying biomarkers predictive of effective treatment response to current and novel immunotherapies is of utmost importance. Multiple studies have indicated neutrophil-lymphocyte ratio (NLR) may be a predictive biomarker of immunotherapy response. We analyzed hematological parameters across multiple preclinical murine tumors to determine if NLR predicts tumor growth and response to treatment. The hematological survey identified NLR to be strongly correlated to tumor weights, with larger tumors exhibiting higher NLRs. In addition, mouse tumors that have been characterized as resistant or partially responsive to immunotherapy had a higher NLR than tumors responsive to immunotherapy. To track neutrophils in vivo, we generated a BAC transgenic mouse wherein firefly luciferase (Luc2), diphtheria toxin receptor (DTR), and enhanced green fluorescence protein (eGFP) genes were expressed under the endogenous Ly6G promoter. The Ly6G promoter enables tracking of Ly6G+ neutrophils using luciferase-based bioluminescence imaging (BLI), depletion of Ly6G+ neutrophils by diphtheria toxin treatment, and the ability to perform flow analyses using the eGFP marker. In vivo and ex vivo BLI demonstrated that the Ly6G+ cells were primarily expressed in bone marrow, blood, spleen, and the digestive tract. To ensure the specificity of the bioluminescence signal, we isolated Ly6G+ and Ly6G- immune cells and performed in vitro BLI. In vitro BLI confirmed that bioluminescence was exclusive to Ly6G+ cells. To confirm that the Luc2-DTR-eGFP (LDG) reporter cassette expression did not affect neutrophil activity and function, we quantified the level of myeloperoxidase (MPO), a marker of neutrophil activation, in the phorbol myristate acetate (PMA)-induced ear edema mouse model. Neutrophil activity was measured using luminol enabled BLI of MPO. Luminol-BLI following PMA-induced ear edema demonstrated similar MPO activity between wildtype and transgenic mice. Finally, we investigated if the LDG reporter cassette impacted tumor growth. Measures of tumor burden in this transgenic line recapitulated our hematological findings. Tumors with higher NLR showed higher tumor BLI signal when compared to tumors with lower NLR ratio, demonstrating that the LDG reporter cassette did not impact tumor growth in the transgenic mice and importantly, providing a tool to track Ly6G+ cells in tumors. Our results demonstrate that NLR in tumor correlates with response to immunotherapy across multiple tumor types. Here we describe the development of a transgenic mouse to track intratumoral Ly6G+ cells. These transgenic mice provide a valuable tool for profiling oncology compounds, including those targeting immune cells, and for understanding the mechanism of action of such agents.

Citation Format: Milind D. Chalishazar, Nicolas Solban, Johnny Kopinja, Doug Linn, Razvan Cristescu, Heather Zhou, Thomas Rosahl, Brian Long, Weisheng Zhang, Eric Hostetler. Development of a bioluminescence reporter mouse model for tracking and quantifying Ly6G+ neutrophils in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2455.

MK-8719, a Novel and Selective O-GlcNAcase Inhibitor That Reduces the Formation of Pathological Tau and Ameliorates Neurodegeneration in a Mouse Model of Tauopathy

Deposition of hyperphosphorylated and aggregated tau protein in the central nervous system is characteristic of Alzheimer disease and other tauopathies. Tau is subject to O-linked N-acetylglucosamine (O-GlcNAc) modification, and O-GlcNAcylation of tau has been shown to influence tau phosphorylation and aggregation. Inhibition of O-GlcNAcase (OGA), the enzyme that removes O-GlcNAc moieties, is a novel strategy to attenuate the formation of pathologic tau. Here we described the in vitro and in vivo pharmacological properties of a novel and selective OGA inhibitor, MK-8719. In vitro, this compound is a potent inhibitor of the human OGA enzyme with comparable activity against the corresponding enzymes from mouse, rat, and dog. In vivo, oral administration of MK-8719 elevates brain and peripheral blood mononuclear cell O-GlcNAc levels in a dose-dependent manner. In addition, positron emission tomography imaging studies demonstrate robust target engagement of MK-8719 in the brains of rats and rTg4510 mice. In the rTg4510 mouse model of human tauopathy, MK-8719 significantly increases brain O-GlcNAc levels and reduces pathologic tau. The reduction in tau pathology in rTg4510 mice is accompanied by attenuation of brain atrophy, including reduction of forebrain volume loss as revealed by volumetric magnetic resonance imaging analysis. These findings suggest that OGA inhibition may reduce tau pathology in tauopathies. However, since hundreds of O-GlcNAcylated proteins may be influenced by OGA inhibition, it will be critical to understand the physiologic and toxicological consequences of chronic O-GlcNAc elevation in vivo. SIGNIFICANCE STATEMENT: MK-8719 is a novel, selective, and potent O-linked N-acetylglucosamine (O-GlcNAc)-ase (OGA) inhibitor that inhibits OGA enzyme activity across multiple species with comparable in vitro potency. In vivo, MK-8719 elevates brain O-GlcNAc levels, reduces pathological tau, and ameliorates brain atrophy in the rTg4510 mouse model of tauopathy. These findings indicate that OGA inhibition may be a promising therapeutic strategy for the treatment of Alzheimer disease and other tauopathies.

Discovery of [11C]MK-6884: A Positron Emission Tomography (PET) Imaging Agent for the Study of M4Muscarinic Receptor Positive Allosteric Modulators (PAMs) in Neurodegenerative Diseases

The measurement of receptor occupancy (RO) using positron emission tomography (PET) has been instrumental in guiding discovery and development of CNS directed therapeutics. We and others have investigated muscarinic acetylcholine receptor 4 (M4) positive allosteric modulators (PAMs) for the treatment of symptoms associated with neuropsychiatric disorders. In this article, we describe the synthesis, in vitro, and in vivo characterization of a series of central pyridine-related M4 PAMs that can be conveniently radiolabeled with carbon-11 as PET tracers for the in vivo imaging of an allosteric binding site of the M4 receptor. We first demonstrated its feasibility by mapping the receptor distribution in mouse brain and confirming that a lead molecule 1 binds selectively to the receptor only in the presence of the orthosteric agonist carbachol. Through a competitive binding affinity assay and a number of physiochemical properties filters, several related compounds were identified as candidates for in vivo evaluation. These candidates were then radiolabeled with ¹¹C and studied in vivo in rhesus monkeys. This research eventually led to the discovery of the clinical radiotracer candidate [¹¹C]MK-6884.

Characterization of Ubrogepant: A Potent and Selective Antagonist of the Human Calcitonin Gene‒Related Peptide Receptor

A growing body of evidence has implicated the calcitonin gene-related peptide (CGRP) receptors in migraine pathophysiology. With the recent approval of monoclonal antibodies targeting CGRP or the CGRP receptor, the inhibition of CGRP-mediated signaling has emerged as a promising approach for preventive treatments of migraine in adults. However, there are no small-molecule anti-CGRP treatments available for treating migraine. The current studies aimed to characterize the pharmacologic properties of ubrogepant, an orally bioavailable, CGRP receptor antagonist for the acute treatment of migraine. In a series of ligand binding assays, ubrogepant exhibited a high binding affinity for native (K i=0.067 nM) and cloned human (K i=0.070 nM) and rhesus CGRP receptors (K i=0.079 nM), with relatively lower affinities for CGRP receptors from rat, mouse, rabbit and dog. In functional assays, ubrogepant potently blocked human α-CGRP stimulated cAMP response (IC50 of 0.08 nM) and exhibited highly selective antagonist activity for the CGRP receptor compared with other members of the human calcitonin receptor family. Furthermore, the in vivo CGRP receptor antagonist activity of ubrogepant was evaluated in a pharmacodynamic model of capsaicin-induced dermal vasodilation (CIDV) in rhesus monkeys and humans. Results demonstrated that ubrogepant produced concentration-dependent inhibition of CIDV with a mean EC50 of 3.2 and 2.6 nM in rhesus monkeys and humans, respectively. Brain penetration studies with ubrogepant in monkeys showed a CSF/plasma ratio of 0.03 and low CGRP receptor occupancy. In summary, ubrogepant is a competitive antagonist with high affinity, potency, and selectivity for the human CGRP receptor. SIGNIFICANCE STATEMENT: Ubrogepant is a potent, selective, orally delivered, small-molecule competitive antagonist of the human calcitonin generelated peptide receptor. In vivo studies using a pharmacodynamic model of capsaicin-induced dermal vasodilation (CIDV) in rhesus monkeys and humans demonstrated that ubrogepant produced concentration-dependent inhibition of CIDV, indicating a predictable pharmacokinetic-pharmacodynamic relationship.

Pathogenic FBN1 variants in familial thoracic aortic aneurysms and dissections

Marfan syndrome (MFS) due to mutations in FBN1 is a known cause of thoracic aortic aneurysms and acute aortic dissections (TAAD) associated with pleiotropic manifestations. Genetic predisposition to TAAD can also be inherited in families in the absence of syndromic features, termed familial TAAD (FTAAD), and several causative genes have been identified to date. FBN1 mutations can also be identified in FTAAD families, but the frequency of these mutations has not been established. We performed exome sequencing of 183 FTAAD families and identified pathogenic FBN1 variants in five (2.7%) of these families. We also identified eight additional FBN1 rare variants that could not be unequivocally classified as disease-causing in six families. FBN1 sequencing should be considered in individuals with FTAAD even without significant systemic features of MFS.

An In Vitro Assay for Predicting Successful Imaging Radiotracers

PURPOSE

To develop an in vitro binding assay able to predict whether a radiolabel is likely to be a useful clinical tracer for positron emission tomography (PET).

PROCEDURES

Rodent and rhesus brain sections were incubated with radioligands, most of which are tritiated or iodinated versions of known clinical PET radiotracers, and assayed for binding to brain receptors for a 20-minute period using a no-wash protocol (n=>/=3).

RESULTS

Radiolabeled flumazenil (RO-151788), WAY100635, N-methylscopolamine, N-methylspiperone, raclopride, citalopram, (1-)2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), paroxetine, and 4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-p-flurobenzamido]ethyl]piperazine (MPPF) were assessed for binding to either rhesus caudate putamen, and/or frontal cortex, or rat whole brain sections. Specific binding for these compounds ranged from 0 to 94% by 20 minutes. Those with %-specific binding less than 10% have also been shown to not be effective as in vivo PET radiotracers. In addition, successful PET radiotracers incubated in tissue sections with target receptor either absent or present in low density behaved poorly in this assay, as expected, as did radiolabels previously shown to possess high non-specific binding.

CONCLUSIONS

An in vitro binding assay using rodent and rhesus brain sections has been developed that, within the currently assayed radiotracers, is able to rapidly predict whether a radiolabeled compound is a useful clinical PET radiotracer. This method suggests significant potential for the rapid in vitro evaluation of potential in vivo PET radiotracers.