ABSTRACT
In an ongoing effort to explore the use of orexin receptor antagonists for the treatment of insomnia, dual orexin receptor antagonists (DORAs) were structurally modified, resulting in compounds selective for the OX2R subtype and culminating in the discovery of 23, a highly potent, OX2R-selective molecule that exhibited a promising in vivo profile. Further structural modification led to an unexpected restoration of OX1R antagonism. Herein, these changes are discussed and a rationale for selectivity based on computational modeling is proposed.
Subject(s)
Orexin Receptor Antagonists/pharmacology , Orexins/antagonists & inhibitors , Animals , Electroencephalography , Electromyography , Molecular Structure , Orexin Receptor Antagonists/chemistry , RatsABSTRACT
Highly selective orexin receptor antagonists (SORAs) of the orexin 2 receptor (OX2R) have become attractive targets both as potential therapeutics for insomnia as well as biological tools to help further elucidate the underlying pharmacology of the orexin signaling pathway. Herein, we describe the discovery of a novel piperidine ether 2-SORA class identified by systematic lead optimization beginning with filorexant, a dual orexin receptor antagonist (DORA) that recently completed Phase 2 clinical trials. Changes to the ether linkage and pendant heterocycle of filorexant were found to impart significant selectivity for OX2R, culminating in lead compound PE-6. PE-6 displays sub-nanomolar binding affinity and functional potency on OX2R while maintaining >1600-fold binding selectivity and >200-fold functional selectivity versus the orexin 1 receptor (OX1R). PE-6 bears a clean off-target profile, a good overall preclinical pharmacokinetic (PK) profile, and reduces wakefulness with increased NREM and REM sleep when evaluated in vivo in a rat sleep study. Importantly, subtle structural changes to the piperidine ether class impart dramatic changes in receptor selectivity. To this end, our laboratories have identified multiple piperidine ether 2-SORAs, 1-SORAs, and DORAs, providing access to a number of important biological tool compounds from a single structural class.
Subject(s)
Ethers/chemistry , Orexin Receptor Antagonists , Piperidines/chemistry , Pyrimidines/chemistry , Animals , Dogs , Drug Evaluation, Preclinical , Ethers/chemical synthesis , Ethers/pharmacokinetics , Half-Life , Humans , Orexin Receptors/metabolism , Piperidines/metabolism , Protein Binding , Pyrimidines/metabolism , Rats , Sleep/drug effects , Structure-Activity RelationshipABSTRACT
Dual orexin receptor antagonists (DORAs), or orexin 1 (OX1) and orexin 2 (OX2) receptor antagonists, have demonstrated clinical utility for the treatment of insomnia. Medicinal chemistry efforts focused on the reduction of bioactivation potential of diazepane amide 1 through the modification of the Western heterocycle resulted in the discovery of suvorexant, a DORA recently approved by the FDA for the treatment of insomnia. A second strategy towards reducing bioactivation risk is presented herein through the exploration of monocyclic quinazoline isosteres, namely substituted pyrimidines. These studies afforded potent DORAs with significantly reduced bioactivation risk and efficacy in rodent sleep models. Surprisingly, side products from the chemistry used to produce these DORAs yielded isomeric pyrimidine-containing diazepane amides possessing selective OX2R antagonist (2-SORA) profiles. Additional exploration of these isomeric pyrimidines uncovered potent 2-SORA diazepane amides with sleep efficacy in mouse EEG studies.
Subject(s)
Drug Discovery , Orexin Receptor Antagonists/pharmacology , Orexin Receptors/metabolism , Pyrimidines/pharmacology , Quinazolines/pharmacology , Sleep Initiation and Maintenance Disorders/drug therapy , Animals , Disease Models, Animal , Dogs , Dose-Response Relationship, Drug , Humans , Mice , Microsomes, Liver/drug effects , Models, Molecular , Molecular Structure , Orexin Receptor Antagonists/chemical synthesis , Orexin Receptor Antagonists/chemistry , Pyrimidines/chemical synthesis , Pyrimidines/chemistry , Quinazolines/chemical synthesis , Quinazolines/chemistry , Rats , Structure-Activity RelationshipABSTRACT
Antagonism of orexin receptors has shown clinical efficacy as a novel paradigm for the treatment of insomnia and related disorders. Herein, molecules related to the dual orexin receptor antagonist filorexant were transformed into compounds that were selective for the OX2R subtype. Judicious selection of the substituents on the pyridine ring and benzamide groups led to 6b; which was highly potent, OX2R selective, and exhibited excellent development properties.
Subject(s)
Orexin Receptor Antagonists/chemistry , Orexin Receptors/chemistry , Piperidines/chemistry , Triazoles/chemistry , Animals , Dogs , Half-Life , Mice , Orexin Receptor Antagonists/pharmacokinetics , Orexin Receptor Antagonists/therapeutic use , Orexin Receptors/metabolism , Piperidines/pharmacokinetics , Piperidines/therapeutic use , Protein Binding , Pyrimidines/chemistry , Rats , Sleep Initiation and Maintenance Disorders/drug therapy , Sleep Initiation and Maintenance Disorders/veterinary , Structure-Activity Relationship , Triazoles/pharmacokinetics , Triazoles/therapeutic useABSTRACT
BACKGROUND: The current standard of care for insomnia includes gamma-aminobutyric acid receptor A (GABAA) activators, which promote sleep as well as general central nervous system depression. Dual orexin receptor antagonists (DORAs) represent an alternative mechanism for insomnia treatment that induces somnolence by blocking the wake-promoting effects of orexin neuropeptides. The current study compares the role and interdependence of these two mechanisms on their ability to influence sleep architecture and quantitative electroencephalography (qEEG) spectral profiles across preclinical species. RESULTS: Active-phase dosing of DORA-22 induced consistent effects on sleep architecture in mice, rats, dogs, and rhesus monkeys; attenuation of active wake was accompanied by increases in both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Eszopiclone, a representative GABAA receptor modulator, promoted sleep in rats and rhesus monkeys that was marked by REM sleep suppression, but had inconsistent effects in mice and paradoxically promoted wakefulness in dogs. Active-phase treatment of rats with DORA-12 similarly promoted NREM and REM sleep to magnitudes nearly identical to those seen during normal resting-phase sleep following vehicle treatment, whereas eszopiclone suppressed REM even to levels below those seen during the active phase. The qEEG changes induced by DORA-12 in rats also resembled normal resting-phase patterns, whereas eszopiclone induced changes distinct from normal active- or inactive-phase spectra. Co-dosing experiments, as well as studies in transgenic rats lacking orexin neurons, indicated partial overlap in the mechanism of sleep promotion by orexin and GABA modulation with the exception of the REM suppression exclusive to GABAA receptor modulation. Following REM deprivation in mice, eszopiclone further suppressed REM sleep while DORA-22 facilitated recovery including increased REM sleep. CONCLUSION: DORAs promote NREM and importantly REM sleep that is similar in proportion and magnitude to that seen during the normal resting phase across mammalian animal models. While limited overlap exists between therapeutic mechanisms, orexin signaling does not appear involved in the REM suppression exhibited by GABAA receptor modulators. The ability of DORAs to promote proportional NREM and REM sleep following sleep deprivation suggests that this mechanism may be effective in alleviating recovery from sleep disturbance.
Subject(s)
Azabicyclo Compounds/pharmacology , Azepines/pharmacology , Benzimidazoles/pharmacology , GABA Modulators/pharmacology , Hypnotics and Sedatives/pharmacology , Piperazines/pharmacology , Piperidines/pharmacology , Sleep/drug effects , Triazoles/pharmacology , Animals , Brain/drug effects , Brain/physiology , Cross-Over Studies , Dogs , Electroencephalography , Eszopiclone , Intracellular Signaling Peptides and Proteins/deficiency , Intracellular Signaling Peptides and Proteins/genetics , Macaca mulatta , Male , Mice, Inbred C57BL , Neurons/drug effects , Neurons/physiology , Neuropeptides/deficiency , Neuropeptides/genetics , Orexin Receptor Antagonists , Orexins , Rats, Sprague-Dawley , Rats, Transgenic , Sleep/physiology , Sleep Deprivation/drug therapy , Sleep Deprivation/physiopathology , Sleep Stages/drug effects , Sleep Stages/physiology , Species Specificity , Wakefulness/drug effects , Wakefulness/physiologyABSTRACT
Recent clinical studies have demonstrated that dual orexin receptor antagonists (OX1R and OX2R antagonists or DORAs) represent a novel treatment option for insomnia patients. Previously we have disclosed several compounds in the diazepane amide DORA series with excellent potency and both preclinical and clinical sleep efficacy. Additional SAR studies in this series were enabled by the expansion of the acetonitrile-assisted, diphosgene-mediated 2,4-dichloropyrimidine synthesis to novel substrates providing an array of Western heterocycles. These heterocycles were utilized to synthesize analogs in short order with high levels of potency on orexin 1 and orexin 2 receptors as well as in vivo sleep efficacy in the rat.
Subject(s)
Orexin Receptor Antagonists , Pyrimidines/chemistry , Pyrimidines/pharmacology , Sleep/drug effects , Animals , Drug Discovery , Humans , Pyrimidines/chemical synthesis , Rats , Sleep Initiation and Maintenance Disorders/drug therapyABSTRACT
Orexin receptor antagonists have demonstrated clinical utility for the treatment of insomnia. The majority of clinical efforts to date have focused on the development of dual orexin receptor antagonists (DORAs), small molecules that antagonize both the orexin 1 and orexin 2 receptors. Our group has recently disclosed medicinal chemistry efforts to identify highly potent, orally bioavailable selective orexin 2 receptor antagonists (2-SORAs) that possess acceptable profiles for clinical development. Herein we report additional SAR studies within the 'triaryl' amide 2-SORA series focused on improvements in compound stability in acidic media and time-dependent inhibition of CYP3A4. These studies resulted in the discovery of 2,5-disubstituted isonicotinamide 2-SORAs such as compound 24 that demonstrated improved stability and TDI profiles as well as excellent sleep efficacy across species.
Subject(s)
Drug Discovery , Orexin Receptor Antagonists , Pyridines/pharmacology , Sleep Initiation and Maintenance Disorders/drug therapy , Thiazoles/pharmacology , Animals , Dogs , Dose-Response Relationship, Drug , Humans , Mice , Molecular Structure , Pyridines/chemical synthesis , Pyridines/chemistry , Rats , Structure-Activity Relationship , Thiazoles/chemical synthesis , Thiazoles/chemistryABSTRACT
BACKGROUND: Drugs targeting insomnia ideally promote sleep throughout the night, maintain normal sleep architecture, and are devoid of residual effects associated with morning sedation. These features of an ideal compound are not only dependent upon pharmacokinetics, receptor binding kinetics, potency and pharmacodynamic activity, but also upon a compound's mechanism of action. RESULTS: Dual orexin receptor antagonists (DORAs) block the arousal-promoting activity of orexin peptides and, as demonstrated in the current work, exhibit an efficacy signal window dependent upon oscillating levels of endogenous orexin neuropeptide. Sleep efficacy of structurally diverse DORAs in rat and dog was achieved at plasma exposures corresponding to orexin 2 receptor (OX2R) occupancies in the range of 65 to 80%. In rats, the time course of OX2R occupancy was dependent upon receptor binding kinetics and was tightly correlated with the timing of active wake reduction. In rhesus monkeys, direct comparison of DORA-22 with GABA-A modulators at similar sleep-inducing doses revealed that diazepam produced next-day residual sleep and both diazepam and eszopiclone induced next-day cognitive deficits. In stark contrast, DORA-22 did not produce residual effects. Furthermore, DORA-22 evoked only minimal changes in quantitative electroencephalogram (qEEG) activity during the normal resting phase in contrast to GABA-A modulators which induced substantial qEEG changes. CONCLUSION: The higher levels of receptor occupancy necessary for DORA efficacy require a plasma concentration profile sufficient to maintain sleep for the duration of the resting period. DORAs, with a half-life exceeding 8 h in humans, are expected to fulfill this requirement as exposures drop to sub-threshold receptor occupancy levels prior to the wake period, potentially avoiding next-day residual effects at therapeutic doses.
Subject(s)
Azepines/pharmacokinetics , Orexin Receptor Antagonists , Sleep/drug effects , Triazoles/pharmacokinetics , Animals , Dogs , Electroencephalography , Female , Humans , Immunoassay , Intracellular Signaling Peptides and Proteins/cerebrospinal fluid , Macaca mulatta , Male , Mice , Mice, Inbred C57BL , Neuropeptides/cerebrospinal fluid , Orexins , Rats , Rats, Sprague-Dawley , Rats, Transgenic , Sleep/physiologyABSTRACT
The orexin (or hypocretin) system has been identified as a novel target for the treatment of insomnia due to the wealth of biological and genetic data discovered over the past decade. Recently, clinical proof-of-concept was achieved for the treatment of primary insomnia using dual (OX1R/OX2R) orexin receptor antagonists. However, elucidation of the pharmacology associated with selective orexin-2 receptor antagonists (2-SORAs) has been hampered by the lack of orally bioavailable, highly selective small molecule probes. Herein, the discovery and optimization of a novel series of 2,5-diarylnicotinamides as potent and orally bioavailable orexin-2 receptor selective antagonists is described. A compound from this series demonstrated potent sleep promotion when dosed orally to EEG telemetrized rats.
Subject(s)
Nicotinic Acids/chemistry , Nicotinic Acids/pharmacology , Orexin Receptor Antagonists , Animals , Dogs , Drug Evaluation, Preclinical , Half-Life , Humans , Nicotinic Acids/chemical synthesis , Nicotinic Acids/pharmacokinetics , Orexin Receptors/metabolism , Protein Binding/drug effects , Rats , Structure-Activity RelationshipABSTRACT
The forebrain cholinergic system promotes higher brain function in part by signaling through the M(1) muscarinic acetylcholine receptor (mAChR). During Alzheimer's disease (AD), these cholinergic neurons degenerate, therefore selectively activating M(1) receptors could improve cognitive function in these patients while avoiding unwanted peripheral responses associated with non-selective muscarinic agonists. We describe here benzyl quinolone carboxylic acid (BQCA), a highly selective allosteric potentiator of the M(1) mAChR. BQCA reduces the concentration of ACh required to activate M(1) up to 129-fold with an inflection point value of 845 nM. No potentiation, agonism, or antagonism activity on other mAChRs is observed up to 100 microM. Furthermore studies in M(1)(-/-) mice demonstrates that BQCA requires M(1) to promote inositol phosphate turnover in primary neurons and to increase c-fos and arc RNA expression and ERK phosphorylation in the brain. Radioligand-binding assays, molecular modeling, and site-directed mutagenesis experiments indicate that BQCA acts at an allosteric site involving residues Y179 and W400. BQCA reverses scopolamine-induced memory deficits in contextual fear conditioning, increases blood flow to the cerebral cortex, and increases wakefulness while reducing delta sleep. In contrast to M(1) allosteric agonists, which do not improve memory in scopolamine-challenged mice in contextual fear conditioning, BQCA induces beta-arrestin recruitment to M(1), suggesting a role for this signal transduction mechanism in the cholinergic modulation of memory. In summary, BQCA exploits an allosteric potentiation mechanism to provide selectivity for the M(1) receptor and represents a promising therapeutic strategy for cognitive disorders.
Subject(s)
Receptor, Muscarinic M1/metabolism , Allosteric Regulation , Amino Acid Sequence , Animals , Brain/drug effects , Brain/metabolism , CHO Cells , Calcium Signaling/drug effects , Cerebrovascular Circulation/drug effects , Cerebrovascular Circulation/physiology , Conditioning, Psychological/drug effects , Conditioning, Psychological/physiology , Cricetinae , Cricetulus , Dogs , Fear/drug effects , Fear/physiology , Humans , In Vitro Techniques , Inositol Phosphates/metabolism , Macaca mulatta , Mice , Mice, Knockout , Models, Molecular , Protein Structure, Tertiary , Quinolones/pharmacology , Radioligand Assay , Rats , Receptor, Muscarinic M1/chemistry , Receptor, Muscarinic M1/deficiency , Receptor, Muscarinic M1/genetics , Recombinant Proteins/genetics , Recombinant Proteins/metabolism , Sleep/drug effects , Sleep/physiologyABSTRACT
Orexins/hypocretins are key neuropeptides responsible for regulating central arousal and reward circuits. Two receptors respond to orexin signaling, orexin 1 receptor (OX(1)R) and orexin 2 receptor (OX(2)R) with partially overlapping nervous system distributions. Genetic studies suggest orexin receptor antagonists could be therapeutic for insomnia and other disorders with disruptions of sleep and wake. Suvorexant (MK-4305) is a potent, selective, and orally bioavailable antagonist of OX(1)R and OX(2)R currently under clinical investigation as a novel therapy for insomnia. Examination of Suvorexant in radioligand binding assays using tissue from transgenic rats expressing the human OX(2)R found nearly full receptor occupancy (>90%) at plasma exposures of 1.1 µM. Dosed orally Suvorexant significantly and dose-dependently reduced locomotor activity and promoted sleep in rats (10, 30, and 100 mg/kg), dogs (1 and 3 mg/kg), and rhesus monkeys (10 mg/kg). Consistent cross-species sleep/wake architecture changes produced by Suvorexant highlight a unique opportunity to develop dual orexin antagonists as a novel therapy for insomnia.
Subject(s)
Azepines/pharmacology , Receptors, G-Protein-Coupled/antagonists & inhibitors , Receptors, Neuropeptide/antagonists & inhibitors , Sleep/drug effects , Triazoles/pharmacology , Animals , Area Under Curve , Azides , CHO Cells , Cricetinae , Cricetulus , Dose-Response Relationship, Drug , Electrocardiography , Electromyography , Humans , Macaca mulatta , Motor Activity/drug effects , Octreotide/analogs & derivatives , Orexin Receptors , Protein Binding/drug effects , Rats , Reaction Time/drug effects , Receptors, G-Protein-Coupled/genetics , Receptors, G-Protein-Coupled/metabolism , Receptors, Neuropeptide/genetics , Receptors, Neuropeptide/metabolism , TransfectionABSTRACT
Despite the substantial impact of sleep disturbances on human health and the many years of study dedicated to understanding sleep pathologies, the underlying genetic mechanisms that govern sleep and wake largely remain unknown. Recently, the authors completed large-scale genetic and gene expression analyses in a segregating inbred mouse cross and identified candidate causal genes that regulate the mammalian sleep-wake cycle, across multiple traits including total sleep time, amounts of rapid eye movement (REM), non-REM, sleep bout duration, and sleep fragmentation. Here the authors describe a novel approach toward validating candidate causal genes, while also identifying potential targets for sleep-related indications. Select small-molecule antagonists and agonists were used to interrogate candidate causal gene function in rodent sleep polysomnography assays to determine impact on overall sleep architecture and to evaluate alignment with associated sleep-wake traits. Significant effects on sleep architecture were observed in validation studies using compounds targeting the muscarinic acetylcholine receptor M3 subunit (Chrm3) (wake promotion), nicotinic acetylcholine receptor alpha4 subunit (Chrna4) (wake promotion), dopamine receptor D5 subunit (Drd5) (sleep induction), serotonin 1D receptor (Htr1d) (altered REM fragmentation), glucagon-like peptide-1 receptor (Glp1r) (light sleep promotion and reduction of deep sleep), and calcium channel, voltage-dependent, T type, alpha 1I subunit (Cacna1i) (increased bout duration of slow wave sleep). Taken together, these results show the complexity of genetic components that regulate sleep-wake traits and highlight the importance of evaluating this complex behavior at a systems level. Pharmacological validation of genetically identified putative targets provides a rapid alternative to generating knock out or transgenic animal models, and may ultimately lead towards new therapeutic opportunities.
Subject(s)
Crosses, Genetic , Sleep Wake Disorders/drug therapy , Sleep Wake Disorders/genetics , Sleep/drug effects , Sleep/genetics , Animals , Calcium Channels, N-Type , Calcium Channels, P-Type/genetics , Calcium Channels, Q-Type/genetics , Male , Mice , Mice, Inbred BALB C , Mice, Inbred C57BL , Rats , Rats, Sprague-Dawley , Receptor, Muscarinic M3/genetics , Receptors, Dopamine D5/genetics , Receptors, Nicotinic/genetics , Sleep Wake Disorders/metabolismABSTRACT
A novel series of amide T-type calcium channel antagonists were prepared and evaluated using in vitro and in vivo assays. Optimization of the screening hit 3 led to identification of the potent and selective T-type antagonist 37 that displayed in vivo efficacy in rodent models of epilepsy and sleep.
Subject(s)
Amides/pharmacology , Calcium Channel Blockers/pharmacology , Calcium Channels, T-Type/drug effects , Animals , Mice , Rats , Rats, WistarABSTRACT
The discovery and synthesis of 4,4-disubstituted quinazolinones as T-type calcium channel antagonists is reported. Based on lead compounds 2 and 3, a focused SAR campaign driven by the optimization of potency, metabolic stability, and pharmacokinetic profile identified 45 as a potent T-type Ca(2+) channel antagonist with minimized PXR activation. In vivo, 45 suppressed seizure frequency in a rat model of absence epilepsy and showed significant alterations of sleep architecture after oral dosing to rats as measured by EEG.
Subject(s)
Calcium Channel Blockers/pharmacology , Calcium Channels, T-Type/drug effects , Quinazolinones/chemistry , Quinazolinones/pharmacology , Animals , Biological Availability , Calcium Channel Blockers/chemistry , Calcium Channel Blockers/pharmacokinetics , Chromatography, High Pressure Liquid , Drug Discovery , Haplorhini , Humans , Quinazolinones/pharmacokinetics , Rats , Structure-Activity RelationshipABSTRACT
Orexins are excitatory neuropeptides that regulate arousal and sleep. Orexin receptor antagonists promote sleep and offer potential as a new therapy for the treatment of insomnia. In this Letter, we describe the synthesis of constrained diazepanes having a 3,9 diazabicyclo[4.2.1]nonane bicyclic core with good oral bioavailability and sleep-promoting activity in a rat EEG model.
Subject(s)
Alkanes/pharmacology , Drug Discovery , Hypnotics and Sedatives/pharmacology , Receptors, G-Protein-Coupled/antagonists & inhibitors , Receptors, Neuropeptide/antagonists & inhibitors , Alkanes/chemistry , Alkanes/pharmacokinetics , Animals , Aza Compounds/chemistry , Aza Compounds/pharmacokinetics , Aza Compounds/pharmacology , Biological Availability , Bridged Bicyclo Compounds/chemistry , Bridged Bicyclo Compounds/pharmacokinetics , Bridged Bicyclo Compounds/pharmacology , Electroencephalography , Hypnotics and Sedatives/chemistry , Hypnotics and Sedatives/pharmacokinetics , Orexin Receptors , Rats , Rats, Sprague-DawleyABSTRACT
Orexin neuropeptides regulate sleep/wake through orexin receptors (OX1R, OX2R); OX2R is the predominant mediator of arousal promotion. The potential for single OX2R antagonism to effectively promote sleep has yet to be demonstrated in humans. MK-1064 is an OX2R-single antagonist. Preclinically, MK-1064 promotes sleep and increases both rapid eye movement (REM) and non-REM (NREM) sleep in rats at OX2R occupancies higher than the range observed for dual orexin receptor antagonists. Similar to dual antagonists, MK-1064 increases NREM and REM sleep in dogs without inducing cataplexy. Two Phase I studies in healthy human subjects evaluated safety, tolerability, pharmacokinetics and sleep-promoting effects of MK-1064, and demonstrated dose-dependent increases in subjective somnolence (via Karolinska Sleepiness Scale and Visual Analogue Scale measures) and sleep (via polysomnography), including increased REM and NREM sleep. Thus, selective OX2R antagonism is sufficient to promote REM and NREM sleep across species, similarly to that seen with dual orexin receptor antagonism.
Subject(s)
Orexin Receptor Antagonists/administration & dosage , Orexin Receptors/metabolism , Sleep Aids, Pharmaceutical/administration & dosage , Sleep Stages/drug effects , Animals , Dogs , Dose-Response Relationship, Drug , Female , Humans , Male , Mice , Orexin Receptor Antagonists/pharmacology , Polysomnography , Rats , Sleep Aids, Pharmaceutical/pharmacology , Sleep, REM/drug effectsABSTRACT
The ability to awaken from sleep in response to important stimuli is a critical feature of normal sleep, as is maintaining sleep continuity in the presence of irrelevant background noise. Dual orexin receptor antagonists (DORAs) effectively promote sleep across species by targeting the evolutionarily conserved wake-promoting orexin signaling pathway. This study in dogs investigated whether DORA-induced sleep preserved the ability to awaken appropriately to salient acoustic stimuli but remain asleep when exposed to irrelevant stimuli. Sleep and wake in response to DORAs, vehicle, GABA-A receptor modulators (diazepam, eszopiclone and zolpidem) and antihistamine (diphenhydramine) administration were evaluated in telemetry-implanted adult dogs with continuous electrocorticogram, electromyogram (EMG), electrooculogram (EOG), and activity recordings. DORAs induced sleep, but GABA-A modulators and antihistamine induced paradoxical hyperarousal. Thus, salience gating studies were conducted during DORA-22 (0.3, 1, and 5 mg/kg; day and night) and vehicle nighttime sleep. The acoustic stimuli were either classically conditioned using food reward and positive attention (salient stimulus) or presented randomly (neutral stimulus). Once conditioned, the tones were presented at sleep times corresponding to maximal DORA-22 exposure. In response to the salient stimuli, dogs woke completely from vehicle and orexin-antagonized sleep across all sleep stages but rarely awoke to neutral stimuli. Notably, acute pharmacological antagonism of orexin receptors paired with emotionally salient anticipation produced wake, not cataplexy, in a species where genetic (chronic) loss of orexin receptor signaling leads to narcolepsy/cataplexy. DORA-induced sleep in the dog thereby retains the desired capacity to awaken to emotionally salient acoustic stimuli while preserving uninterrupted sleep in response to irrelevant stimuli.
ABSTRACT
The field of small-molecule orexin antagonist research has evolved rapidly in the last 15 years from the discovery of the orexin peptides to clinical proof-of-concept for the treatment of insomnia. Clinical programs have focused on the development of antagonists that reversibly block the action of endogenous peptides at both the orexin 1 and orexin 2 receptors (OX1 R and OX2 R), termed dual orexin receptor antagonists (DORAs), affording late-stage development candidates including Merck's suvorexant (new drug application filed 2012). Full characterization of the pharmacology associated with antagonism of either OX1 R or OX2 R alone has been hampered by the dearth of suitable subtype-selective, orally bioavailable ligands. Herein, we report the development of a selective orexin 2 antagonist (2-SORA) series to afford a potent, orally bioavailable 2-SORA ligand. Several challenging medicinal chemistry issues were identified and overcome during the development of these 2,5-disubstituted nicotinamides, including reversible CYP inhibition, physiochemical properties, P-glycoprotein efflux and bioactivation. This article highlights structural modifications the team utilized to drive compound design, as well as in vivo characterization of our 2-SORA clinical candidate, 5''-chloro-N-[(5,6-dimethoxypyridin-2-yl)methyl]-2,2':5',3''-terpyridine-3'-carboxamide (MK-1064), in mouse, rat, dog, and rhesus sleep models.
Subject(s)
Drug Design , Intracellular Signaling Peptides and Proteins/antagonists & inhibitors , Neuropeptides/antagonists & inhibitors , Pyridines/chemistry , Pyridines/pharmacology , Sleep Initiation and Maintenance Disorders/drug therapy , Animals , Dogs , Humans , Intracellular Signaling Peptides and Proteins/metabolism , Macaca mulatta , Male , Mice , Mice, Inbred C57BL , Neuropeptides/metabolism , Orexins , Rats , Rats, Sprague-Dawley , Sleep Initiation and Maintenance Disorders/metabolismABSTRACT
Dual orexin receptor antagonists (DORAs) are a potential treatment for insomnia that function by blocking both the orexin 1 and orexin 2 receptors. The objective of the current study was to further confirm the impact of therapeutic mechanisms targeting insomnia on locomotor coordination and ethanol interaction using DORAs and gamma-aminobutyric acid (GABA)-A receptor modulators of distinct chemical structure and pharmacological properties in the context of sleep-promoting potential. The current study compared rat motor co-ordination after administration of DORAs, DORA-12 and almorexant, and GABA-A receptor modulators, zolpidem, eszopiclone, and diazepam, alone or each in combination with ethanol. Motor performance was assessed by measuring time spent walking on a rotarod apparatus. Zolpidem, eszopiclone and diazepam [0.3-30 mg/kg administered orally (PO)] impaired rotarod performance in a dose-dependent manner. Furthermore, all three GABA-A receptor modulators potentiated ethanol- (0.25-1.5 g/kg) induced impairment on the rotarod. By contrast, neither DORA-12 (10-100 mg/kg, PO) nor almorexant (30-300 mg/kg, PO) impaired motor performance alone or in combination with ethanol. In addition, distinct differences in sleep architecture were observed between ethanol, GABA-A receptor modulators (zolpidem, eszopiclone, and diazepam) and DORA-12 in electroencephalogram studies in rats. These findings provide further evidence that orexin receptor antagonists have an improved motor side-effect profile compared with currently available sleep-promoting agents based on preclinical data and strengthen the rationale for further evaluation of these agents in clinical development.
ABSTRACT
Dual orexin receptor antagonists (DORAs) induce sleep by blocking orexin 1 and orexin 2 receptor-mediated activities responsible for regulating wakefulness. DORAs represent a potential alternative mechanism to the current standard of care that includes the γ-aminobutyric acid (GABA)A receptor-positive allosteric modulators, eszopiclone and zolpidem. This work uses an innovative method to analyze electroencephalogram (EEG) spectral frequencies within sleep/wake states to differentiate the effects of GABAA modulators from DORA-22, an analog of the DORA MK-6096, in Sprague-Dawley rats. The effects of low, intermediate, and high doses of eszopiclone, zolpidem, and DORA-22 were examined after first defining each compound's ability to promote sleep during active-phase dosing. The EEG spectral frequency power within specific sleep stages was calculated in 1-Hz intervals from 1 to 100 Hz within each sleep/wake state for the first 4 h after the dose. Eszopiclone and zolpidem produced marked, dose-responsive disruptions in sleep stage-specific EEG spectral profiles compared with vehicle treatment. In marked contrast, DORA-22 exhibited marginal changes in the spectral profile, observed only during rapid eye movement sleep, and only at the highest dose tested. Moreover, while eszopiclone- and zolpidem-induced changes were evident in the inactive period, the EEG spectral responses to DORA-22 were absent during this phase. These results suggest that DORA-22 differs from eszopiclone and zolpidem whereby DORA-22 promotes somnolence without altering the neuronal network EEG activity observed during normal sleep.