Antiviral Drug Discovery for the Treatment of COVID-19 Infections.

The coronavirus disease 2019 (COVID-19) pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a recently emerged human coronavirus. COVID-19 vaccines have proven to be successful in protecting the vaccinated from infection, reducing the severity of disease, and deterring the transmission of infection. However, COVID-19 vaccination faces many challenges, such as the decline in vaccine-induced immunity over time, and the decrease in potency against some SARS-CoV-2 variants including the recently emerged Omicron variant, resulting in breakthrough infections. The challenges that COVID-19 vaccination is facing highlight the importance of the discovery of antivirals to serve as another means to tackle the pandemic. To date, neutralizing antibodies that block viral entry by targeting the viral spike protein make up the largest class of antivirals that has received US FDA emergency use authorization (EUA) for COVID-19 treatment. In addition to the spike protein, other key targets for the discovery of direct-acting antivirals include viral enzymes that are essential for SARS-CoV-2 replication, such as RNA-dependent RNA polymerase and proteases, as judged by US FDA approval for remdesivir, and EUA for Paxlovid (nirmatrelvir + ritonavir) for treating COVID-19 infections. This review presents an overview of the current status and future direction of antiviral drug discovery for treating SARS-CoV-2 infections, covering important antiviral targets such as the viral spike protein, non-structural protein (nsp) 3 papain-like protease, nsp5 main protease, and the nsp12/nsp7/nsp8 RNA-dependent RNA polymerase complex.

Metabolism and Disposition of a Novel Selective α7 Neuronal Acetylcholine Receptor Agonist ABT-126 in Humans: Characterization of the Major Roles for Flavin-Containing Monooxygenases and UDP-Glucuronosyl Transferase 1A4 and 2B10 in Catalysis.

Mass balance, metabolism, and excretion of ABT-126, an α7 neuronal acetylcholine receptor agonist, were characterized in healthy male subjects (n = 4) after a single 100-mg (100 µCi) oral dose. The total recovery of the administered radioactivity was 94.0% (±2.09%), with 81.5% (±10.2%) in urine and 12.4% (±9.3%) in feces. Metabolite profiling indicated that ABT-126 had been extensively metabolized, with 6.6% of the dose remaining as unchanged parent drug in urine. Parent drug accounted for 12.2% of the administered radioactivity in feces. The primary metabolic transformations of ABT-126 involved aza-adamantane N-oxidation (M1, 50.3% in urine) and aza-adamantane N-glucuronidation (M11, 19.9% in urine). M1 and M11 were also major circulating metabolites, accounting for 32.6% and 36.6% of the drug-related material in plasma, respectively. These results demonstrated that ABT-126 is eliminated primarily by hepatic metabolism, followed by urinary excretion. Enzymatic studies suggested that M1 formation is mediated primarily by human liver flavin-containing monooxygenase (FMO)3 and, to a lesser extent, by human kidney FMO1; M11 is generated mainly by human uridine 5'-diphospho-glucuronosyltransferase (UGT) 1A4, whereas UGT 2B10 also contributes to ABT-126 glucuronidation. Species-dependent formation of M11 was observed in hepatocytes; M11 was formed in human and monkey hepatocytes, but not in rat and dog hepatocytes, suggesting that monkeys constitute an appropriate model for predicting the fate of compounds undergoing significant N-glucuronidation. M1 and M11 are not expected to have clinically relevant on- or off-target pharmacologic activities. In summary, this study characterized ABT-126 metabolites in the circulation and excreta and the primary elimination pathways of ABT-126 in humans.

Synthesis and SAR of 4-aminocyclopentapyrrolidines as orally active N-type calcium channel inhibitors for inflammatory and neuropathic pain.

A novel series of N-type calcium channel inhibitors have been discovered. Optimization of potency and HT-ADME properties provides 4-aminocyclopentapyrrolidines with analgesic efficacy after oral dosing.

Synthesis and evaluation of new radioligands [(11)C]A-833834 and [(11)C]A-752274 for positron-emission tomography of α7-nicotinic acetylcholine receptors.

INTRODUCTION: α7-nicotinic acetylcholine receptor (α7-nAChR) is one of the major neuronal nAChR subtypes. α7-nAChR is involved in variety of neuronal processes and disorders including schizophrenia and Alzheimer's disease. A number of α7-nAChR PET radioligands have been developed, but a quality radiotracer remains to be discovered. METHODS: High binding affinity α7-nAChR ligands A-833834 and A-752274 were radiolabeled with (11)C. Baseline and blockade biodistribution studies in the mouse brain of [(11)C]A-833834 (5-(6-(5-[(11)C]methylhexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl)pyridazin-3-yl)-1H-indole) and [(11)C]A-752274 (2-(6-[(11)C]methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-7-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-9H-fluoren-9-one) were performed. [(11)C]A-752274 was evaluated in a baseline baboon PET study. RESULTS: [(11)C]A-833834 and [(11)C]A-752274 were synthesized by radiomethylation of corresponding des-methyl precursors. The radioligands were prepared with radiochemical yield of 12%-32%, high specific radioactivity (330-403GBq/µmol) and radiochemical purity>95%. Dissection studies with [(11)C]A-833834 demonstrated low specific α7-nAChR binding in the mouse brain. [(11)C]A-752274 specifically (~50%) labeled α7-nAChR in the mouse thalamus. However, [(11)CA-752274 exhibited low brain uptake in baboon (%SUV<100). CONCLUSION: Two novel α7-nAChR ligands radioligands were synthesized and studied in animals. Specific binding of [(11)C]A-833834 in the mouse brain is low due to the insufficient binding affinity of the radioligand. The very high binding affinity [(11)C]A-752274 exhibited good specific binding in the α7-nAChR-rich mouse brain regions. The low uptake of [(11)C]A-752274 in the baboon brain is due to its high hydrophilicity, rapid metabolism or other properties. Future development of α7-nAChR PET radioligands will be based on compounds with high binding affinities and good blood-brain barrier permeability.

An automated electrophysiological assay for differentiating Ca(v)2.2 inhibitors based on state dependence and kinetics.

Ca(V)2.2 (N-type) calcium channels are key regulators of neurotransmission. Evidence from knockout animals and localization studies suggest that Ca(V)2.2 channels play a critical role in nociceptive transmission. Additionally, ziconotide, a selective peptide inhibitor of Ca(V)2.2 channels, is clinically used to treat refractory pain. However, the use of ziconotide is limited by its low therapeutic index, which is believed, at least in part, to be a consequence of ziconotide inhibiting Ca(V)2.2 channels regardless of the channel state. Subsequent efforts have focused on the discovery of state-dependent inhibitors that preferentially bind to the inactivated state of Ca(V)2.2 channels in order to achieve an improved safety profile relative to ziconotide. Much less attention has been paid to understanding the binding kinetics of these state-dependent inhibitors. Here, we describe a novel electrophysiology-based assay on an automated patch platform designed to differentiate Ca(V)2.2 inhibitors based on their combined state dependence and kinetics. More specifically, this assay assesses inactivated state block, closed state block, and monitors the kinetics of recovery from block when channels move between states. Additionally, a use-dependent assay is described that uses a train of depolarizing pulses to drive channels to a similar level of inactivation for comparison. This use-dependent protocol also provides information on the kinetics of block development. Data are provided to show how these assays can be utilized to screen for kinetic diversity within and across chemical classes.

SAR of α7 nicotinic receptor agonists derived from tilorone: exploration of a novel nicotinic pharmacophore.

The well-known interferon-inducer tilorone was found to possess potent affinity for the agonist site of the α7 neuronal nicotinic receptor (K(i)=56 nM). SAR investigations determined that both basic sidechains are essential for potent activity, however active monosubstituted derivatives can also be prepared if the flexible sidechains are replaced with conformationally rigidified cyclic amines. Analogs in which the fluorenone core is replaced with either dibenzothiophene-5,5-dioxide or xanthenone also retain potent activity.

Syntheses and structure-activity relationship (SAR) studies of 2,5-diazabicyclo[2.2.1]heptanes as novel alpha7 neuronal nicotinic receptor (NNR) ligands.

Biaryl substituted 2,5-diazabicyclo[2.2.1]heptanes have been synthesized and tested for their affinity toward alpha7 neuronal nicotinic receptors (NNRs). SAR studies established that 5-N-methyl substituent, heteroaryl linker and the nature of terminal aryl group are critical for the ligand to achieve potent alpha7 NNR agonist activity.

Structure-activity relationships of N-substituted ligands for the alpha7 nicotinic acetylcholine receptor.

A series of alpha7 neuronal nicotinic acetylcholine receptor ligands were designed based on a structural combination of a potent, but non-selective ligand, epibatidine, with a selective lead structure, 2. Three series of compounds in which aryl moieties were attached via a linker to different positions on the core structure were studied. A potent and functionally efficacious analog, (3aR,6aS)-2-(6-phenylpyridazin-3-yl)-5-(pyridin-3-ylmethyl)octahydropyrrolo[3,4-c]pyrrole (3a), was identified.

Preparation and characterization of N-(3-pyridinyl) spirocyclic diamines as ligands for nicotinic acetylcholine receptors.

Several N-pyridin-3-yl spirobicyclic diamines, designed as conformationally restricted analogs of tebanicline (ABT-594), were synthesized as novel ligands for nicotinic acetylcholine receptors (nAChR). The spirocyclic compounds exhibited weaker binding affinity, than other constrained analogs in accord with a pharmacophore model. Nevertheless, some (1a, 1b) possessed (partial) agonist potencies comparable to nicotine at the alpha4beta2 subtype, but with greatly improved selectivity relative to the alpha3beta4* nAChR.

Synthesis and structure-activity relationship studies of 3,6-diazabicyclo[3.2.0]heptanes as novel alpha4beta2 nicotinic acetylcholine receptor selective agonists.

A series of novel, potent neuronal nicotinic acetylcholine receptor (nAChR) ligands derived from 3,6-diazabicyclo[3.2.0]heptane have been synthesized and evaluated for binding affinity and agonist activity at the alpha4beta2 nAChR subtype. Structure-activity relationship studies of these novel nAChR ligands focused on substitution effects on the pyridine ring, as well as stereo- and regiochemical influences of the 3,6-diazabicyclo[3.2.0]heptane core. Small 5-substituents on the pyridine ring had a modest impact on the binding affinities and functional activities. 6-Bromo, 6-chloro, and 6-methyl substituents on the pyridine ring led to increased binding affinities and improved functional activities. Most of the 6-N-pyridinyl-substituted 3,6-diazabicyclo[3.2.0]heptanes are selective for the alpha4beta2 nAChR subtype. Compounds (1R,5S)-25, (1R,5S)-55, and (1R,5S)-56 were virtually inactive as agonists at the halpha3beta4 nAChR but retained potency and efficacy at the halpha4beta2 nAChR subtype. 3-N-Pyridinyl-substituted series demonstrated more complex SAR. (1R,5R)-39, (1R,5R)-41, and (1R,5R)-42 were found to be much more potent at the halpha3beta4 nAChR subtype, whereas (1R,5R)-38 and (1R,5R)-40 were very selective at the halpha4beta2 nAChR subtype. The SAR studies of these novel ligands led to the discovery of several compounds with interesting in vitro pharmacological profiles.

Structure-activity studies and analgesic efficacy of N-(3-pyridinyl)-bridged bicyclic diamines, exceptionally potent agonists at nicotinic acetylcholine receptors.

A series of exceptionally potent agonists at neuronal nicotinic acetylcholine receptors (nAChRs) has been investigated. Several N-(3-pyridinyl) derivatives of bridged bicyclic diamines exhibit double-digit-picomolar binding affinities for the alpha 4 beta 2 subtype, placing them with epibatidine among the most potent nAChR ligands described to date. Structure-activity studies have revealed that substitutions, particularly hydrophilic groups in the pyridine 5-position, differentially modulate the agonist activity at ganglionic vs central nAChR subtypes, so that improved subtype selectivity can be demonstrated in vitro. Analgesic efficacy has been achieved across a broad range of pain states, including rodent models of acute thermal nociception, persistent pain, and neuropathic allodynia. Unfortunately, the hydrophilic pyridine substituents that were shown to enhance agonist selectivity for central nAChRs in vitro tend to limit CNS penetration in vivo, so that analgesic efficacy with an improved therapeutic window was not realized with those compounds.

Design of ligands for the nicotinic acetylcholine receptors: the quest for selectivity.

In the last decade, nicotinic acetylcholine receptors (nAChRs) have emerged as important targets for drug discovery. The therapeutic potential of nicotinic agonists depends substantially on the ability to selectively activate certain receptor subtypes that mediate beneficial effects. The design of such compounds has proceeded in spite of a general shortage of data pertaining to subtype selectivity. Medicinal chemistry efforts have been guided principally by binding affinities to the alpha4beta2 and/or alpha7 subtypes, even though these are not predictive of agonist activity at either subtype. Nevertheless, a diverse family of nAChR ligands has been developed, and several analogs with promising therapeutic potential have now advanced to human clinical trials. This paper provides an overview of the structure-affinity relationships that continue to drive development of new nAChR ligands.