A high-throughput response to the SARS-CoV-2 pandemic.

Four years after the beginning of the COVID-19 pandemic, it is important to reflect on the events that have occurred during that time and the knowledge that has been gained. The response to the pandemic was rapid and highly resourced; it was also built upon a foundation of decades of federally funded basic and applied research. Laboratories in government, pharmaceutical, academic, and non-profit institutions all played roles in advancing pre-2020 discoveries to produce clinical treatments. This perspective provides a summary of how the development of high-throughput screening methods in a biosafety level 3 (BSL-3) environment at Southern Research Institute (SR) contributed to pandemic response efforts. The challenges encountered are described, including those of a technical nature as well as those of working under the pressures of an unpredictable virus and pandemic.

Discovery of a potent M5 antagonist with improved clearance profile. Part 2: Pyrrolidine amide-based antagonists.

This Letter describes our ongoing effort to improve the clearance of selective M5 antagonists. Herein, we report the replacement of the previously disclosed piperidine amide (4, disclosed in Part 1) with a pyrrolidine amide core. Several compounds within this series provided good potency, subtype selectivity, and low to moderate clearance profiles. Interestingly, the left-hand side SAR for this series diverged from our earlier efforts.

Discovery of a potent M5 antagonist with improved clearance profile. Part 1: Piperidine amide-based antagonists.

The lack of potent and selective tool compounds with pharmaceutically favorable properties limits the in vivo understanding of muscarinic acetylcholine receptor subtype 5 (M5) biology. Previously, we presented a highly potent and selective M5 antagonist VU6019650 with a suboptimal clearance profile as our second-generation tool compound. Herein, we disclose our ongoing efforts to generate next-generation M5 antagonists with improved clearance profiles. A mix and match approach between VU6019650 (lead) and VU0500325 (HTS hit) generated a piperidine amide-based novel M5 antagonist series. Several analogs within this series, including 29f, provided good on-target potency with improved clearance profiles, though room for improvement remains.

A Novel Approach To Identify Inhibitors of Iron Acquisition Systems of Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an opportunistic pathogen that has been declared by the World Health Organization as a "priority 1 critical pathogen" needing immediate new strategies for chemotherapy. During infection, P. aeruginosa uses redundant mechanisms to acquire ferric, heme (Hm), or ferrous iron from the host to survive and colonize. Significant efforts have been undertaken to develop siderophore blockers to inhibit ferric iron acquisition by P. aeruginosa, but there is a lack of inhibitors that can block Hm or ferrous iron acquisition by P. aeruginosa. We developed and employed a targeted high-throughput screen (HTS) and identified a molecule(s) that can specifically inhibit the Hm and ferrous iron acquisition systems of P. aeruginosa. Our targeted approach relies on screening a small-molecule library against P. aeruginosa under three growth conditions, where the only variable was the iron source (ferric, Hm, or ferrous iron). Each condition served as a counterscreen for the other, and we identified molecules that inhibit the growth of P. aeruginosa in the presence of only Hm or ferrous iron. Our data indicate that econazole, bithionate, and raloxifene inhibit the growth of P. aeruginosa in the presence of Hm and that oxyquinoline inhibits the growth of P. aeruginosa in the presence of ferrous iron. These iron-specific inhibitors do not interfere with the activity of meropenem, a commercial antipseudomonal, and can also increase meropenem activity. In conclusion, we present a proof of concept of a successful targeted conditional screening method by which we can identify specific iron acquisition inhibitors. This approach is highly adaptable and can easily be extended to any other pathogen. IMPORTANCE Since acquiring iron is paramount to P. aeruginosa's survival and colonization in the human host, developing novel strategies to block the access of P. aeruginosa to host iron will allow us to starve it of an essential nutrient. P. aeruginosa uses siderophore, heme, or ferrous iron uptake systems to acquire iron in the human host. We have developed a novel approach through which we can directly identify molecules that can prevent P. aeruginosa from utilizing heme or ferrous iron. This approach overcomes the need for the in silico design of molecules and identifies structurally diverse biologically active inhibitor molecules. This screening approach is adaptable and can be extended to any pathogen. Since Gram-negative pathogens share many similarities in iron acquisition at both the mechanistic and molecular levels, our screening approach presents a significant opportunity to develop novel broad-spectrum iron acquisition inhibitors of Gram-negative pathogens.

Large-Scale Identification of Multiple Classes of Host Defense Peptide-Inducing Compounds for Antimicrobial Therapy.

The rapid emergence of antibiotic resistance demands new antimicrobial strategies that are less likely to develop resistance. Augmenting the synthesis of endogenous host defense peptides (HDPs) has been proven to be an effective host-directed therapeutic approach. This study aimed to identify small-molecule compounds with a strong ability to induce endogenous HDP synthesis for further development as novel antimicrobial agents. By employing a stable HDP promoter-driven luciferase reporter cell line known as HTC/AvBD9-luc, we performed high-throughput screening of 5002 natural and synthetic compounds and identified 110 hits with a minimum Z-score of 2.0. Although they were structurally and functionally diverse, half of these hits were inhibitors of class I histone deacetylases, the phosphoinositide 3-kinase pathway, ion channels, and dopamine and serotonin receptors. Further validations revealed mocetinostat, a benzamide histone deacetylase inhibitor, to be highly potent in enhancing the expression of multiple HDP genes in chicken macrophage cell lines and jejunal explants. Importantly, mocetinostat was more efficient than entinostat and tucidinostat, two structural analogs, in promoting HDP gene expression and the antibacterial activity of chicken macrophages. Taken together, mocetinostat, with its ability to enhance HDP synthesis and the antibacterial activity of host cells, could be potentially developed as a novel antimicrobial for disease control and prevention.

Development of VU6019650: A Potent, Highly Selective, and Systemically Active Orthosteric Antagonist of the M5 Muscarinic Acetylcholine Receptor for the Treatment of Opioid Use Disorder.

The muscarinic acetylcholine receptor (mAChR) subtype 5 (M5) represents a novel potential target for the treatment of multiple addictive disorders, including opioid use disorder. Through chemical optimization of several functional high-throughput screening hits, VU6019650 (27b) was identified as a novel M5 orthosteric antagonist with high potency (human M5 IC50 = 36 nM), M5 subtype selectivity (>100-fold selectivity against human M1-4) and favorable physicochemical properties for systemic dosing in preclinical addiction models. In acute brain slice electrophysiology studies, 27b blocked the nonselective muscarinic agonist oxotremorine-M-induced increases in neuronal firing rates of midbrain dopamine neurons in the ventral tegmental area, a part of the mesolimbic dopaminergic reward circuitry. Moreover, 27b also inhibited oxycodone self-administration in male Sprague-Dawley rats within a dose range that did not impair general motor output.

Structure based virtual screening identifies small molecule effectors for the sialoglycan binding protein Hsa.

Infective endocarditis (IE) is a cardiovascular disease often caused by bacteria of the viridans group of streptococci, which includes Streptococcus gordonii and Streptococcus sanguinis. Previous research has found that serine-rich repeat (SRR) proteins on the S. gordonii bacterial surface play a critical role in pathogenesis by facilitating bacterial attachment to sialylated glycans displayed on human platelets. Despite their important role in disease progression, there are currently no anti-adhesive drugs available on the market. Here, we performed structure-based virtual screening using an ensemble docking approach followed by consensus scoring to identify novel small molecule effectors against the sialoglycan binding domain of the SRR adhesin protein Hsa from the S. gordonii strain DL1. The screening successfully predicted nine compounds which were able to displace the native ligand (sialyl-T antigen) in an in vitro assay and bind competitively to Hsa. Furthermore, hierarchical clustering based on the MACCS fingerprints showed that eight of these small molecules do not share a common scaffold with the native ligand. This study indicates that SRR family of adhesin proteins can be inhibited by diverse small molecules and thus prevent the interaction of the protein with the sialoglycans. This opens new avenues for discovering potential drugs against IE.

Screening for AMPA receptor auxiliary subunit specific modulators.

AMPA receptors (AMPAR) are ligand gated ion channels critical for synaptic transmission and plasticity. Their dysfunction is implicated in a variety of psychiatric and neurological diseases ranging from major depressive disorder to amyotrophic lateral sclerosis. Attempting to potentiate or depress AMPAR activity is an inherently difficult balancing act between effective treatments and debilitating side effects. A newly explored strategy to target subsets of AMPARs in the central nervous system is to identify compounds that affect specific AMPAR-auxiliary subunit complexes. This exploits diverse spatio-temporal expression patterns of known AMPAR auxiliary subunits, providing means for designing brain region-selective compounds. Here we report a high-throughput screening-based pipeline that can identify compounds that are selective for GluA2-CNIH3 and GluA2-stargazin complexes. These compounds will help us build upon the growing library of AMPAR-auxiliary subunit specific inhibitors, which have thus far all been targeted to TARP γ-8. We used a cell-based assay combined with a voltage-sensitive dye (VSD) to identify changes in glutamate-gated cation flow across the membranes of HEK cells co-expressing GluA2 and an auxiliary subunit. We then used a calcium flux assay to further validate hits picked from the VSD assay. VU0612951 and VU0627849 are candidate compounds from the initial screen that were identified as negative and positive allosteric modulators (NAM and PAM), respectively. They both have lower IC50/EC50s on complexes containing stargazin and CNIH3 than GSG1L or the AMPAR alone. We have also identified a candidate compound, VU0539491, that has NAM activity in GluA2(R)-CNIH3 and GluA2(Q) complexes and PAM activity in GluA2(Q)-GSG1L complexes.

Selective Small Molecule Activators of TREK-2 Channels Stimulate Dorsal Root Ganglion c-Fiber Nociceptor Two-Pore-Domain Potassium Channel Currents and Limit Calcium Influx.

The two-pore-domain potassium (K2P) channel TREK-2 serves to modulate plasma membrane potential in dorsal root ganglia c-fiber nociceptors, which tunes electrical excitability and nociception. Thus, TREK-2 channels are considered a potential therapeutic target for treating pain; however, there are currently no selective pharmacological tools for TREK-2 channels. Here we report the identification of the first TREK-2 selective activators using a high-throughput fluorescence-based thallium (Tl+) flux screen (HTS). An initial pilot screen with a bioactive lipid library identified 11-deoxy prostaglandin F2α as a potent activator of TREK-2 channels (EC50 ≈ 0.294 µM), which was utilized to optimize the TREK-2 Tl+ flux assay (Z' = 0.752). A HTS was then performed with 76 575 structurally diverse small molecules. Many small molecules that selectively activate TREK-2 were discovered. As these molecules were able to activate single TREK-2 channels in excised membrane patches, they are likely direct TREK-2 activators. Furthermore, TREK-2 activators reduced primary dorsal root ganglion (DRG) c-fiber Ca2+ influx. Interestingly, some of the selective TREK-2 activators such as 11-deoxy prostaglandin F2α were found to inhibit the K2P channel TREK-1. Utilizing chimeric channels containing portions of TREK-1 and TREK-2, the region of the TREK channels that allows for either small molecule activation or inhibition was identified. This region lies within the second pore domain containing extracellular loop and is predicted to play an important role in modulating TREK channel activity. Moreover, the selective TREK-2 activators identified in this HTS provide important tools for assessing human TREK-2 channel function and investigating their therapeutic potential for treating chronic pain.

Delayed treatment with PTBA analogs reduces postinjury renal fibrosis after kidney injury.

No therapies have been shown to accelerate recovery or prevent fibrosis after acute kidney injury (AKI). In part, this is because most therapeutic candidates have to be given at the time of injury and the diagnosis of AKI is usually made too late for drugs to be efficacious. Strategies to enhance post-AKI repair represent an attractive approach to address this. Using a phenotypic screen in zebrafish, we identified 4-(phenylthio)butanoic acid (PTBA), which promotes proliferation of embryonic kidney progenitor cells (EKPCs), and the PTBA methyl ester UPHD25, which also increases postinjury repair in ischemia-reperfusion and aristolochic acid-induced AKI in mice. In these studies, a new panel of PTBA analogs was evaluated. Initial screening was performed in zebrafish EKPC assays followed by survival assays in a gentamicin-induced AKI larvae zebrafish model. Using this approach, we identified UPHD186, which in contrast to UPHD25, accelerates recovery and reduces fibrosis when administered several days after ischemia-reperfusion AKI and reduces fibrosis after unilateral ureteric obstruction in mice. UPHD25 and 186 are efficiently metabolized to the active analog PTBA in liver and kidney microsome assays, indicating both compounds may act as PTBA prodrugs in vivo. UPHD186 persists longer in the circulation than UPHD25, suggesting that sustained levels of UPHD186 may increase efficacy by acting as a reservoir for renal metabolism to PTBA. These findings validate use of zebrafish EKPC and AKI assays as a drug discovery strategy for molecules that reduce fibrosis in multiple AKI models and can be administered days after initiation of injury.

Acyl dihydropyrazolo[1,5-a]pyrimidinones as metabotropic glutamate receptor 5 positive allosteric modulators.

We report the optimization of a series of metabotropic glutamate receptor 5 (mGlu5) positive allosteric modulators (PAMs) from an acyl dihydropyrazolo[1,5-a]pyrimidinone class. Investigation of exocyclic amide transpositions with this unique 5,6-bicyclic core were conducted in attempt to modulate physicochemical properties and identify a suitable backup candidate with a reduced half-life. A potent and selective PAM, 1-(2-(phenoxymethyl)-6,7-dihydropyrazolo[1,5-a]pyrimidin-4(5H)-yl)ethanone (9a, VU0462807), was identified with superior solubility and efficacy in the acute amphetamine-induced hyperlocomotion (AHL) rat model with a minimum effective dose of 3mg/kg. Attempts to mitigate oxidative metabolism of the western phenoxy of 9a through extensive modification and profiling are described.

Discovery of VU0409551/JNJ-46778212: An mGlu5 Positive Allosteric Modulator Clinical Candidate Targeting Schizophrenia.

Herein, we report the structure-activity relationship of a novel series of (2(phenoxymethyl)-6,7-dihydrooxazolo[5,4-c]pyridine-5(4H)-yl(aryl)methanones as potent, selective, and orally bioavailable metabotropic glutamate receptor subtype 5 (mGlu5) positive allosteric modulators (PAMs). On the basis of its robust in vitro potency and in vivo efficacy in multiple preclinical models of multiple domains of schizophrenia, coupled with a good DMPK profile and an acceptable therapeutic window, 17a (VU0409551/JNJ-46778212) was selected as a candidate for further development.

Biased mGlu5-Positive Allosteric Modulators Provide In Vivo Efficacy without Potentiating mGlu5 Modulation of NMDAR Currents.

Schizophrenia is associated with disruptions in N-methyl-D-aspartate glutamate receptor subtype (NMDAR)-mediated excitatory synaptic signaling. The metabotropic glutamate receptor subtype 5 (mGlu5) is a closely associated signaling partner with NMDARs and regulates NMDAR function in forebrain regions implicated in the pathology of schizophrenia. Efficacy of mGlu5 positive allosteric modulators (PAMs) in animal models of psychosis and cognition was previously attributed to potentiation of NMDAR function. To directly test this hypothesis, we identified VU0409551 as a novel mGlu5 PAM that exhibits distinct stimulus bias and selectively potentiates mGlu5 coupling to Gαq-mediated signaling but not mGlu5 modulation of NMDAR currents or NMDAR-dependent synaptic plasticity in the rat hippocampus. Interestingly, VU0409551 produced robust antipsychotic-like and cognition-enhancing activity in animal models. These data provide surprising new mechanistic insights into the actions of mGlu5 PAMs and suggest that modulation of NMDAR currents is not critical for in vivo efficacy. VIDEO ABSTRACT.

Discovery and SAR of a novel series of metabotropic glutamate receptor 5 positive allosteric modulators with high ligand efficiency.

We report the optimization of a series of novel metabotropic glutamate receptor 5 (mGlu5) positive allosteric modulators (PAMs) from a 5,6-bicyclic class of dihydropyrazolo[1,5-a]pyridin-4(5H)-ones containing a phenoxymethyl linker. Studies focused on a survey of non-amide containing hydrogen bond accepting (HBA) pharmacophore replacements. A highly potent and selective PAM, 2-(phenoxymethyl)-6,7-dihydropyrazolo[1,5-a]pyridin-4(5H)-one (11, VU0462054), bearing a simple ketone moiety, was identified (LE=0.52, LELP=3.2). In addition, hydroxyl, difluoro, ether, and amino variations were examined. Despite promising lead properties and exploration of alternative core heterocycles, linkers, and ketone replacements, oxidative metabolism and in vivo clearance remained problematic for the series.

Tetrahydronaphthyridine and dihydronaphthyridinone ethers as positive allosteric modulators of the metabotropic glutamate receptor 5 (mGlu5).

Positive allosteric modulators (PAMs) of metabotropic glutamate receptor 5 (mGlu5) represent a promising therapeutic strategy for the treatment of schizophrenia. Starting from an acetylene-based lead from high throughput screening, an evolved bicyclic dihydronaphthyridinone was identified. We describe further refinements leading to both dihydronaphthyridinone and tetrahydronaphthyridine mGlu5 PAMs containing an alkoxy-based linkage as an acetylene replacement. Exploration of several structural features including western pyridine ring isomers, positional amides, linker connectivity/position, and combinations thereof, reveal that these bicyclic modulators generally exhibit steep SAR and within specific subseries display a propensity for pharmacological mode switching at mGlu5 as well as antagonist activity at mGlu3. Structure-activity relationships within a dihydronaphthyridinone subseries uncovered 12c (VU0405372), a selective mGlu5 PAM with good in vitro potency, low glutamate fold-shift, acceptable DMPK properties, and in vivo efficacy in an amphetamine-based model of psychosis.

Exploration of allosteric agonism structure-activity relationships within an acetylene series of metabotropic glutamate receptor 5 (mGlu5) positive allosteric modulators (PAMs): discovery of 5-((3-fluorophenyl)ethynyl)-N-(3-methyloxetan-3-yl)picolinamide (ML254).

Positive allosteric modulators (PAMs) of metabotropic glutamate receptor 5 (mGlu5) represent a promising therapeutic strategy for the treatment of schizophrenia. Both allosteric agonism and high glutamate fold-shift have been implicated in the neurotoxic profile of some mGlu5 PAMs; however, these hypotheses remain to be adequately addressed. To develop tool compounds to probe these hypotheses, the structure-activity relationship of allosteric agonism was examined within an acetylenic series of mGlu5 PAMs exhibiting allosteric agonism in addition to positive allosteric modulation (ago-PAMs). PAM 38t, a low glutamate fold-shift allosteric ligand (maximum fold-shift ~ 3.0), was selected as a potent PAM with no agonism in the in vitro system used for compound characterization and in two native electrophysiological systems using rat hippocampal slices. PAM 38t (ML254) will be useful to probe the relative contribution of cooperativity and allosteric agonism to the adverse effect liability and neurotoxicity associated with this class of mGlu5 PAMs.

Dihydrothiazolopyridone derivatives as a novel family of positive allosteric modulators of the metabotropic glutamate 5 (mGlu5) receptor.

Starting from a singleton chromanone high throughput screening (HTS) hit, we describe a focused medicinal chemistry optimization effort leading to the identification of a novel series of phenoxymethyl-dihydrothiazolopyridone derivatives as selective positive allosteric modulators (PAMs) of the metabotropic glutamate 5 (mGlu5) receptor. These dihydrothiazolopyridones potentiate receptor responses in recombinant systems. In vitro and in vivo drug metabolism and pharmacokinetic (DMPK) evaluation allowed us to select compound 16a for its assessment in a preclinical animal screen of possible antipsychotic activity. 16a was able to reverse amphetamine-induced hyperlocomotion in rats in a dose-dependent manner without showing any significant motor impairment or overt neurological side effects at comparable doses. Evolution of our medicinal chemistry program, structure activity, and properties relationships (SAR and SPR) analysis as well as a detailed profile for optimized mGlu5 receptor PAM 16a are described.

Biotransformation of a novel positive allosteric modulator of metabotropic glutamate receptor subtype 5 contributes to seizure-like adverse events in rats involving a receptor agonism-dependent mechanism.

Activation of metabotropic glutamate receptor subtype 5 (mGlu5) represents a novel strategy for therapeutic intervention into multiple central nervous system disorders, including schizophrenia. Recently, a number of positive allosteric modulators (PAMs) of mGlu5 were discovered to exhibit in vivo efficacy in rodent models of psychosis, including PAMs possessing varying degrees of agonist activity (ago-PAMs), as well as PAMs devoid of agonist activity. However, previous studies revealed that ago-PAMs can induce seizure activity and behavioral convulsions, whereas pure mGlu5 PAMs do not induce these adverse effects. We recently identified a potent and selective mGlu5 PAM, VU0403602, that was efficacious in reversing amphetamine-induced hyperlocomotion in rats. The compound also induced time-dependent seizure activity that was blocked by coadministration of the mGlu5 antagonist, 2-methyl-6-(phenylethynyl) pyridine. Consistent with potential adverse effects induced by ago-PAMs, we found that VU0403602 had significant allosteric agonist activity. Interestingly, inhibition of VU0403602 metabolism in vivo by a pan cytochrome P450 (P450) inactivator completely protected rats from induction of seizures. P450-mediated biotransformation of VU0403602 was discovered to produce another potent ago-PAM metabolite-ligand (M1) of mGlu5. Electrophysiological studies in rat hippocampal slices confirmed agonist activity of both M1 and VU0403602 and revealed that M1 can induce epileptiform activity in a manner consistent with its proconvulsant behavioral effects. Furthermore, unbound brain exposure of M1 was similar to that of the parent compound, VU0403602. These findings indicate that biotransformation of mGlu5 PAMs to active metabolite-ligands may contribute to the epileptogenesis observed after in vivo administration of this class of allosteric receptor modulators.

Discovery of ML326: The first sub-micromolar, selective M5 PAM.

This Letter describes the further chemical optimization of the M5 PAM MLPCN probes ML129 and ML172. A multi-dimensional iterative parallel synthesis effort quickly explored isatin replacements and a number of southern heterobiaryl variations with no improvement over ML129 and ML172. An HTS campaign identified several weak M5 PAMs (M5 EC50 >10µM) with a structurally related isatin core that possessed a southern phenethyl ether linkage. While SAR within the HTS series was very shallow and unable to be optimized, grafting the phenethyl ether linkage onto the ML129/ML172 cores led to the first sub-micromolar M5 PAM, ML326 (VU0467903), (human and rat M5 EC50s of 409nM and 500nM, respectively) with excellent mAChR selectivity (M1-M4 EC50s >30µM) and a robust 20-fold leftward shift of the ACh CRC.

Approaches for probing allosteric interactions at 7 transmembrane spanning receptors.

In recent years, allosteric modulation of 7 transmembrane spanning receptors (7TMRs) has become a highly productive and exciting field of receptor pharmacology and drug discovery efforts. Positive and negative allosteric modulators (PAMs and NAMs, respectively) present a number of pharmacological and therapeutic advantages over conventional orthosteric ligands, including improved receptor-subtype selectivity, a lower propensity to induce receptor desensitization, the preservation of endogenous temporal and spatial activation of receptors, greater chemical flexibility for optimization of drug metabolism and pharmacokinetic parameters, and saturability of effect at target receptors, thus improving safety concerns and risk of overdose. Additionally, the relatively new concept of allosteric modulator-mediated receptor signal bias opens up a number of intriguing possibilities for PAMs, NAMs, and allosteric agonists, including the potential to selectively activate therapeutically beneficial signaling cascades, which could yield a superior tissue selectivity and side effect profile of allosteric modulators. However, there are a number of considerations and caveats that must be addressed when screening for and characterizing the properties of 7TMR allosteric modulators. Mode of pharmacology, methodology used to monitor receptor activity, detection of appropriate downstream analytes, selection of orthosteric probe, and assay time-course must all be considered when implementing any high-throughput screening campaign or when characterizing the properties of active compounds. Yet compared to conventional agonist/antagonist drug discovery programs, these elements of assay design are often a great deal more complicated when working with 7TMRs allosteric modulators. Moreover, for classical pharmacological methodologies and analyses, like radioligand binding and the assessment of compound affinity, the properties of allosteric modulators yield data that are more nuanced than orthosteric ligand-receptor interactions. In this review, we discuss the current methodologies being used to identify and characterize allosteric modulators, lending insight into the approaches that have been most successful in accurately and robustly identifying hit compounds. New label-free technologies capable of detecting phenotypic cellular changes in response to receptor activation are powerful tools well suited for assessing subtle or potentially masked cellular responses to allosteric modulation of 7TMRs. Allosteric modulator-induced receptor signal bias and the assay systems available to probe the various downstream signaling outcomes of receptor activation are also discussed.