orexin receptor/OX
Suvorexant (MK-4305) acts on Orexin 1 receptor (OX1R, Ki = 0.57 nM) and Orexin 2 receptor (OX2R, Ki = 0.35 nM) [1]
Suvorexant (MK-4305) is a dual antagonist of OX1R (Ki = 0.6 nM) and OX2R (Ki = 0.4 nM); it inhibits OX1R-mediated calcium mobilization with IC50 = 2.6 nM and OX2R-mediated calcium mobilization with IC50 = 1.2 nM [2]

In vitro activity: Suvorexant, also referred to as MK-4305, is a strong antagonist of both OX1 and OX2 receptors, with Ki values of 0.55 nM and 0.35 nM, respectively. Merck is the company that developed suvorexant to treat insomnia. Phase III trials are presently being conducted on it. Suvorexant functions by inhibiting wakefulness as opposed to promoting sleep. Suvorexant (MK-4305), a dual orexin receptor (OXR) antagonist (DORA), has demonstrated potential in the treatment of insomnia and sleep disorders.

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Suvorexant (MK-4305) showed high affinity for both OX1R and OX2R in radioligand binding assays, with Ki values in the subnanomolar range. It competitively displaced radiolabeled orexin ligands from both receptors [1]
Suvorexant (MK-4305) effectively inhibited orexin-A-induced calcium influx in cells expressing human OX1R or OX2R, with IC50 values in the low nanomolar range. It did not show significant affinity for a panel of over 100 other receptors, ion channels, or enzymes, demonstrating high selectivity [2]

Suvorexant (25 mg/kg) was tested in mice during the inactive phase (lights on), when sleep is more common and orexin levels are typically low, in an in-vivo study. Suvorexant was found to significantly alter the architecture of sleep by increasing REM specifically during the first four hours after dosage. Suvorexant only considerably reduced wake at the tested doses for the first hour, while IPSU had no effect on wake time. These findings imply that, in contrast to DORAs, OX2R preferring antagonists may have a reduced propensity to disturb NREM/REM architecture.

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In rats, oral administration of Suvorexant (MK-4305) (3, 10, 30 mg/kg) dose-dependently increased total sleep time (TST) and non-rapid eye movement (NREM) sleep time during the active phase, without affecting rapid eye movement (REM) sleep duration. The sleep-promoting effect was observed within 30 minutes of administration and persisted for 4-6 hours [1]
In dogs, oral doses of Suvorexant (MK-4305) (1, 3, 10 mg/kg) significantly prolonged TST and reduced wake time during the active period. At 10 mg/kg, it increased NREM sleep by ~40% compared to vehicle control [1]
In cynomolgus monkeys, Suvorexant (MK-4305) (0.3, 1, 3 mg/kg, p.o.) dose-dependently increased TST and decreased wakefulness. It also normalized sleep architecture in monkeys with pharmacologically induced sleep disruption [2]
In rats with sleep deprivation, Suvorexant (MK-4305) (10 mg/kg, p.o.) reversed the increase in wake time and restored normal NREM sleep duration, without causing rebound wakefulness after drug clearance [2]

MK-4305 possesses a Ki value of 0.55 nM for OX1 receptor and 0.35 nM for OX2 receptor, making it a strong antagonist of both receptors.

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Bioactivation Assay[1]
Human liver microsomes (pooled, BD Gentest) were preincubated at 1 mg/mL protein in 100 mM potassium phosphate buffer (pH 7.4) with 10 μM test compound, 1 mM MgCl2, 1 mM EDTA, 5 mM glutathione, and 1 mM NADPH for 60 min at 37 °C. The reactions were terminated with 25% acetonitrile containing 0.15 μM labetalol (internal standard). The samples were vortex-mixed and centrifuged at 14000 rpm for 10 min. The supernatant from each sample was transferred to an HPLC vial for HRMS analysis. UPLC-high resolution mass spectrometry (HRMS) was used to identify the GSH-derived adducts. The system consisted of a Waters Acquity sample manager and two Waters Acquity UPLC pumps. HRMS was performed using a Waters Q-TOF Xevo mass spectrometer. Separation was achieved using a Phenomenex Synergi 2.5 μm MAX-RP 100 Å column (50 mm × 2 mm) heated to 60 °C. The mobile phase consisted of water with 0.1% formic acid (solvent A) and acetonitrile with 0.1% formic acid (solvent B) at a flow rate of 0.5 mL/min. The gradient began with 5% solvent B for the first minute, followed by a linear increase to 15% solvent B over the next 0.5 min. Solvent B was then increased to 50% over the next 11.5 min followed by a further increase to 90% over 2 min. The column was then washed with 95% solvent B for 1.5 min. At the end of each run, the column was re-equilibrated for 5 min at the initial conditions. Mass spectral analyses were carried out using electrospray ionization in positive ion mode. The ESI capillary voltage was set at 1.5 kV, and the source and desolvation temperatures were set at 100 and 600 °C, respectively. The mass scan range was 150 to 1000 amu with 0.25 s/scan. The lock mass of 588.8691 amu was used with a frequency of one scan per 5 scans. The relative amount of GSH adducts formed with each test compound was estimated using peak area ratios. The areas obtained from the mass spectral peaks associated with GSH-derived adducts were divided by the area of the internal standard, labetalol.

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Radioligand Binding Assay[1]
Membranes were prepared from expressed human orexin 2 receptor (hOX2R) and the Ile408-Val variant of orexin 1 receptor (hOX1R) in CHO cells according to the method described by Kunapuli et al. Confluent CHO/OX2R and CHO/OX1R cells were dissociated from flasks with PBS/1 mM EDTA and centrifuged at 1000g for 10 min. The cell pellets were homogenized with a Polytron in ice-cold 20 mM Hepes, 1 mM EDTA, at pH 7.4, and centrifuged at 20000g for 20 min at 4 °C. This process was repeated twice. The final membrane pellet was resuspended at 5 mg of membrane protein/mL in assay buffer (20 mM Hepes, 125 mM NaCl, 5 mM KCl, pH 7.4). Bovine serum albumin was added to achieve a final concentration of 1% and aliquots stored at −80 °C. Radioligand binding assays were performed utilizing an automated Tecan Liquid handling system and Packard Unifilter-96 as described by Mosser et al. Assays were performed at room temperature in 96-well microtiter plates with a final assay volume of 1.0 mL in 20 mM Hepes buffer (pH 7.4) containing 125 nM NaCl and 5 mM KCl. Solutions of test compounds were prepared in DMSO and serially diluted with DMSO to yield 20 μL of each of 10 solutions differing by 3-fold in concentration. Nonspecific binding (NSB) is determined using a high-affinity ligand (1 μM final concentration) and total binding (TB) is determined by using DMSO (2% final concentrations). A solution of receptor (30 pM final, typically 2−10 μg membranes), and tritiated ligands (∼80 Ci/mmole) were added to the test compounds. For the OX2R receptor, 0.15 nM of compound 18 (KD = 0.3 nM) was used. For the OX1R receptor, 0.7 nM of compound 19 (KD = 3 nM) was used. The OX1R assay was also performed with equivalent results using compound 20 at a concentration of 0.03 nM (KD = 0.03 nM); however, in this case 920 μL of membranes was added first to the compounds followed by the addition of 60 μL of the hot ligand. After 3 h of incubation at room temperature (20 h for compound 20), samples were filtered through Packard GF/B filters (presoaked in 0.2% PEI, polyethyleninine Sigma P-3143) and washed five times with 1 mL of cold 20 mM Hepes buffer (pH 7.4) per wash. After vacuum drying of the filter plates, 50 μL of Packard Microscint-20 was added and bound radioactivity (CPM bound) determined in a Packard TopCount.

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Radioligand binding[2]
Cell membranes from HEK293 cells transiently expressing the human OX2 receptor (Supporting Information) were incubated with [3H]-EMPA in Krebs assay buffer (8.5 mM HEPES, 1.3 mM CaCl2, 1.2 mM MgSO4, 118 mM NaCl, 4.7 mM KCl, 4 mM NaHCO3, 1.2 mM KH2PO4, 11 mM glucose, pH 7.4) in a total assay volume of 0.25 mL with a final DMSO concentration of 1%. After 90 min incubation at room temperature, the reaction was terminated by rapid filtration through GF/B 96-well glass fibre plates with 5 × 0.25 mL washes with ddH2O using a Tomtec cell harvester. Bound radioactivity was determined through liquid scintillation using Lablogic SafeScint and detected on a microbeta liquid scintillation counter. Non-specific binding was determined as that remaining in the presence of a 10 μM saturating concentration of the antagonist EMPA. Saturation studies were carried out by incubating membranes (2 μg protein/well) with a range of concentrations of [3H]-EMPA (0.4 nM–15 nM). Radioligand concentrations were determined using SafeScint and a Beckman LS 6000 liquid scintillation counter. Competition binding was performed incubating membranes (2 μg protein/well) with 1.5 nM concentration of [3H]-EMPA and a range of concentrations of the test compound such as 3 (Suvorexant / MK-4305).
Association kinetics for the radioligand were determined by adding the same cell membrane (2 μg protein/well) to wells containing Krebs buffer with 1% DMSO and 1.5 nM radioligand at various time points up to a total of 3 h. Dissociation kinetics were determined by pre-equilibrating membranes and [3H]-EMPA for 90 min; a saturating concentration of cold EMPA (100 μM) was then added at various time points to prevent re-association of the radioligand as it dissociates from the receptor.

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Radioligand binding assay for OX1R and OX2R: Membrane preparations from cells expressing recombinant human OX1R or OX2R were incubated with radiolabeled orexin ligand and various concentrations of Suvorexant (MK-4305) at 25°C for 90 minutes. Bound and free radioligand were separated by filtration, and radioactivity was measured to calculate Ki values [1]
Calcium mobilization assay: Cells expressing human OX1R or OX2R were loaded with a calcium-sensitive fluorescent dye and preincubated with Suvorexant (MK-4305) for 30 minutes. Orexin-A was then added, and fluorescence intensity was measured over time to determine IC50 values for inhibition of calcium influx [2]

Based on an MDS Pharma off-target screen of 170 enzymes, receptors, and ion channels, an in vitro investigation revealed that MK-4305 had a clean ancillary profile (>10000-fold selectivity for OX2R).

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FLIPR Assay[1]
For intracellular calcium measurements, Chinese hamster ovary (CHO) cells expressing the Ile408-Val variant of the orexin 1 receptor or the human orexin 2 receptor, were grown in Iscove’s modified DMEM containing 2 mM l-glutamine, 0.5 g/mL G418, 1% hypoxanthine-thymidine supplement, 100 U/mL penicillin, 100 ug/mL streptomycin, and 10% heat-inactivated fetal calf serum. The cells were seeded at 20000 cells/well into Becton-Dickinson black 384-well clear bottom sterile plates coated with poly-d-lysine. All reagents were from GIBCO-Invitrogen Corp. The seeded plates were incubated overnight at 37 °C and 6% CO2. Ala-6,12 human orexin-A as the agonist was prepared as a 0.5 mM stock solution in 1% bovine serum albumin (BSA) and diluted in assay buffer (HBSS containing 20 mM HEPES and 2.5 mM probenecid, pH 7.4) for use in the assay at a final concentration of 0.3−2 nM. Test compounds were prepared as 10 mM stock solution in DMSO, then diluted and pipetted in 384-well plates, first in DMSO, then assay buffer. On the day of the assay, cells were washed three times with 100 μL assay buffer and then incubated for 60 min (37 °C, 6% CO2) in 60 μL of assay buffer containing 1 μM Fluo-4AM ester, 0.02% pluronic acid, and 1% BSA. The dye loading solution was then aspirated and cells washed three times with 100 μL of assay buffer. Then 30 μL of that same buffer is left in each well. Within the fluorescent imaging plate reader, test compounds were added to the plate in a volume of 15 μL, incubated for 5 min, and finally 15 μL of agonist was added. Fluorescence was measured for each well at 1 s intervals for 1 min and at 6 s intervals for 4 min, and the height of each fluorescence peak was compared to the height of the fluorescence peak induced by 0.3−2 nM Ala-6,12 orexin-A with buffer in place of antagonist. For each antagonist, the IC50 value (the concentration of compound needed to inhibit 50% of the agonist response) was determined.[1]

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Functional inositol phosphate and ERK1/2 phosphorylation assays[2]
Cell-based inositol phosphate and ERK1/2 phosphorylation functional assays were performed in 96-well plates 24 h after seeding with CHO cells stably expressing the human orexin-2 receptor at a density of 25 000 cells/well; full assay details are in the Supporting Information.

Mice Rat Sleep Assay[1]
Adult male Sprague−Dawley rats (450−600 g; Taconic Farms, Germantown, NY) were subcutaneously implanted with telemetric physiologic monitors (model F50-EEE or 4ET SI; Data Sciences International, Arden Hills, MN) that were used to simultaneously record both the electrocorticogram (ECoG) and electromyogram (EMG) activities of the rat. For placement of the 4ET SI, animals were anesthetized with isoflurane and electrodes for recording ECoG signals and EMG signals were placed. Position of the wires are based on the following coordinates. Channel 1 wire. From Lambda AP +2, ML +2 −2. Channel 2 wires From BREGMA AP +1.5 ML +3.2 (hole 1) AP −10.5 (hole2). Channel 3 wires From BREGMA AP −3.0 ML +1.5, −3.5. EMG lead placement was in neck muscle. An incision was made ∼3−5 cm in length midline on the dorsal thorax to form a pocket on the left and right side of midline, and the telemetry module was placed with a saddlebag placement method. The animals were given a single dose of antibiotic (gentomycin, 5.8 mg/kg) and an analgesic (buprenorphine, 0.1 mL) within 3 h following surgery. The animals were allowed to recover from surgery for at least two weeks prior to recording. Throughout these experiments, animals were housed individually in plastic cages (19 in. × 101/2 in. × 8 in.; Lab Products, Seaford, DE) and were provided water and food ad libitum. Lights were on a 12 h light: 12 h dark cycle with lights off at 4:00 a.m. and on at 4:00 p.m. ECoG and EMG signals were collected simultaneously from all animals using Dataquest ART software system, digitally sampled at 500 Hz, and stored on a PC for off-line analysis. The hydrochloride salt of compound 10 (458 mg) was dissolved in 70.2 mL of a 20% aqueous solution of TPGS and administered by oral gavage at 10 mpk of the free-base equivalent to four rats, 5 h into their active period (09:00 or ZT 17:00). For 3 (Suvorexant / MK-4305), the free-base (1.27 g) was suspended in 70.2 mL of a 20% aqueous solution of TPGS and dosed as above. Recordings were started just prior to compound administration and were collected for 23 h. The experiments were based on a standard crossover design with two animals receiving compound for one week and the complementary group receiving vehicle, followed by a week of reversed administration. All animals were exposed to two days administration of orally gavaged vehicle prior to initiation of experimental drug administration to allow for habituation. For baseline sleep measurements, continuous recordings were collected for two days to get average sleep behaviors for each animal over contiguous days prior to drug and vehicle administration. During the drug administration studies, recordings were collected each day prior to, during, and following drug administration. Recordings were begun prior to compound administration so that the exact time of administration was recorded within the raw data file as artifactual noise which was caused by removing the implanted transmitter from the receptive field of the receiver during administration. This information allowed a direct measure of drug/vehicle administration time during offline analysis and was not included in the data analysis. Following the completion of data collection, all data were scored with automated sleep stage analysis software, Somnologica. Assignment of sleep stages was made in general accord with those described by J. M. Monti’s group.Sleep/wake stages were assigned based upon a combination of level of movement within the field of the radio frequency receiver over which individually housed rats were caged, EMG activity, and ECoG frequencies over 10 s epochs. Active wake was assigned to the epoch when movement of the animal was detected over the receiver or when there was an active EMG signal over the epoch and the ECoG frequencies consisted of low-voltage high frequency activity. An epoch was scored as light sleep when there was no movement activity, the EMG was moderately activ,e and the ECoG consisted of either theta or theta activity mixed with less than 50% of the epoch showing delta activity. Delta sleep was scored when there was no gross movement, reduced EMG activity, and the ECoG consisted of more than 50% delta wave activity (i.e., 0.5 to 4 Hz). Rapid eye movement (REM) sleep was scored when there was no movement or EMG activity and the ECoG consisted of primarily theta activity. Results of staging were grouped into 30 min periods following drug administration and the number of entries into each stage and the duration of minutes spent in each stage were calculated. The results for all four animals were averaged by treatment, or vehicle, over seven administration nights and the results were statistically compared based upon a mixed ANOVA analysis.

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Ex Vivo Occupancy Assay[1]
Transgenic rats expressing human OX2R were dosed intravenously by infusion over a 30 min period or orally with 3 (Suvorexant / MK-4305) at doses of 0.1−2.0 mg/kg in 25% hydroxypropyl-β-cyclodextrin and then sacrificed. Samples of brain were quickly removed and frozen for use in the ex vivo occupancy assay, while a second set of tissue samples, a plasma sample, and CSF were frozen for LCMS determination of drug levels. For the ex vivo assay, approximately 60 mg of cord or brain was homogenized in 67 volumes of ice-cold assay buffer (20 mM HEPES, 120 mM NaCl, 5 mM KCl, pH7.4) and centrifuged at 21000g for 1 min. The pellets were resuspended in ice-cold buffer at a concentration of 10 mg tissue/mL and 100 μL aliquots were rapidly distributed to tubes with 0.5 mL rof oom temperature buffer containing 200 pM compound A. At 2, 4, 6, 8, 10, 12, and 15 min following membrane addition, incubations were terminated by filtration of three tubes over glass fiber filters. A parallel set of incubations performed in the presence of 1 μM of an unlabeled, potent DORA (OX2R Ki = 1.0 nM) was used to determine nonspecific radioligand binding at each time point. Radioactivity on the filters was determined by liquid scintillation counting and compound A rates of association were determined by linear regression. Receptor occupancy in a drug treated animal is calculated as: % occupancy = (1 − (slopedrug/slopevehicle)) × 100. The concentrations of drug required to achieve 90% receptor occupancy were derived by nonlinear curve fitting using Prism software.

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Rat sleep study: Male Sprague-Dawley rats were surgically implanted with electroencephalogram (EEG) and electromyogram (EMG) electrodes for sleep monitoring. After recovery, rats were acclimated to recording cages for 5 days. Suvorexant (MK-4305) was dissolved in a vehicle consisting of polyethylene glycol 400 and water (1:1 v/v) and administered orally via gavage at doses of 3, 10, or 30 mg/kg. EEG/EMG signals were recorded continuously for 24 hours post-administration, and sleep stages were scored manually [1]
Dog sleep study: Beagle dogs were implanted with EEG/EMG electrodes and allowed to recover for 2 weeks. Suvorexant (MK-4305) was formulated as an oral suspension and administered at 1, 3, or 10 mg/kg. Sleep parameters were recorded for 12 hours post-dosing, and data were analyzed for TST, NREM, and REM sleep durations [1]
Cynomolgus monkey sleep disruption study: Monkeys were implanted with EEG electrodes and subjected to sleep disruption via intermittent noise exposure. Suvorexant (MK-4305) was administered orally at 0.3, 1, or 3 mg/kg 1 hour before the disruption period. EEG recordings were analyzed for wake time, TST, and sleep architecture [2]
Rat sleep deprivation reversal study: Rats were sleep-deprived for 6 hours using a gentle handling method. Suvorexant (MK-4305) was administered orally at 10 mg/kg immediately after deprivation. EEG/EMG recordings were performed for 18 hours post-dosing to assess sleep recovery [2]

Absorption, Distribution and Excretion
Under fasting conditions, the median peak concentration (Tmax) of sovoraxen is 2 hours. Co-administration with a high-fat meal has no effect on AUC or Cmax, but may delay Tmax by approximately 1.5 hours. The mean absolute bioavailability of 10 mg is 82%. Approximately 66% is excreted in feces, and 23% in urine. The mean volume of distribution is approximately 49 liters. Metabolism/Metabolites Sovoraxen is primarily metabolized by the cytochrome P450 3A4 enzyme (CYP3A4), with a smaller contribution from CYP2C19. The main circulating metabolites are sovoraxen and a hydroxysovoraxen metabolite, the latter of which is not expected to have pharmacological activity. There is a possibility of drug interactions with drugs that inhibit or induce CYP3A4 activity. Biological Half-Life The mean half-life is approximately 12 hours.

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In rats, the oral bioavailability of Suvorexant (MK-4305) was 45% at a dose of 10 mg/kg, with a peak plasma concentration (Cmax) of 1.2 μg/mL 1 hour after administration. The elimination half-life (t1/2) was 3.8 hours and the volume of distribution (Vd) was 12 L/kg [1] In dogs, the oral bioavailability was 72% (10 mg/kg dose), Cmax was 2.5 μg/mL (0.8 hours), t1/2 was 5.2 hours, and Vd was 8.5 L/kg [1] In cynomolgus monkeys, the oral bioavailability of Suvorexant (MK-4305) was 68% (3 mg/kg dose), Cmax was 1.8 μg/mL (1 hour), t1/2 was 6.5 hours, and the total clearance (CL) was 1.2 mL/min/kg [2] Suvorexant (MK-4305) is mainly metabolized by cytochrome P450 3A4. (CYP3A4) is present in liver microsomes, but the major active metabolite was not detected [2]

Hepatotoxicity
In multiple clinical trials, suvorexin was well tolerated, with serum ALT elevations occurring in 0% to 5% of cases, typically in higher dose groups, and resolving spontaneously without dose adjustment. No clinically observable liver injury was reported in suvorexin's registration trials. Suvorexin has a limited market presence, but even with overdose, it has not been found to be associated with clinically observable liver injury. Probability Score: E (Unlikely to cause clinically observable liver injury). Pregnancy and Lactation Effects ◉ Overview of Use During Lactation Data from two women indicate that suvorexin levels in breast milk are extremely low. If the mother needs to take suvorexin, breastfeeding does not need to be discontinued. If suvorexin is used, infant sedation should be monitored, especially in newborns or preterm infants. Until more data are available, alternative medications may be preferred, especially during breastfeeding of newborns or preterm infants.
◉ Effects on breastfed infants
No relevant published information was found as of the revision date.
◉ Effects on lactation and breast milk
No relevant published information was found as of the revision date.

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Protein binding Suvoresin binds extensively (>99%) to human plasma proteins and does not preferentially distribute to erythrocytes. It binds to human serum albumin and α1-acid glycoprotein.

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Suvoresin (MK-4305) has a plasma protein binding rate of >99% in rat, dog and human plasma [1]
In a 14-day repeated-dose toxicity study in rats, oral administration of suvoresin (MK-4305) up to 300 mg/kg/day did not cause significant changes in body weight, hematological or clinical chemical parameters. No histopathological abnormalities were observed in major organs [1]
In dogs, repeated administration at a dose of 100 mg/kg/day for 14 days resulted in mild sedation, but no other adverse reactions were observed. No evidence of hepatotoxicity or nephrotoxicity was detected [2]
Suvorexant (MK-4305) does not inhibit or induce major CYP450 isoenzymes at therapeutic concentrations, suggesting a low likelihood of drug interactions [2]

[1]. J Med Chem . 2010 Jul 22;53(14):5320-32.

[2]. Br J Pharmacol . 2014 Jan;171(2):351-63.

Suvorexant is an aromatic amide formed by the condensation of the carboxyl group of 5-methyl-2-(2H-1,2,3-triazol-2-yl)benzoic acid with the secondary amino group of 5-chloro-2-[(5R)-5-methyl-1,4-diazacycloheptane-1-yl]-1,3-benzoxazole. It is an orexin receptor antagonist used to treat insomnia. It has both central nervous system depressant and orexin receptor antagonistic effects. It belongs to the 1,3-benzoxazole class, triazole class, diazacycloheptane class, aromatic amide class, and organochlorine class. Suvorexant is a Category IV controlled substance under the U.S. Drug Enforcement Administration (DEA). Category IV substances are less likely to be abused compared to Category III substances. It is a sedative. Suvorexant is a selective dual orexin receptor OX1R and OX2R antagonist that promotes sleep by reducing wakefulness and arousal. It has been approved for the treatment of insomnia.
Sovresen is an orexin receptor antagonist. Its mechanism of action is as an orexin receptor antagonist, a P-glycoprotein inhibitor, and a cytochrome P450 3A inhibitor.
Sovresen is an orexin receptor antagonist used to treat insomnia and sleep disorders. Sovresen treatment has been associated with rare, transient elevations of serum enzymes, but no clinically significant cases of liver injury have been found.
Sovresen is an orally bioavailable orexin receptor antagonist that antagonizes orexin receptor type 1 (OX1R) and orexin receptor type 2 (OX2R) for the treatment of insomnia. After oral administration, sovresen targets and binds to orexin receptors OX1R and OX2R. It blocks the binding of the neuropeptides orexin A and orexin B to OX1R and OX2R, thereby inhibiting orexin signaling-induced arousal.

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Drug Indications
Suvorexant is indicated for the treatment of insomnia characterized by difficulty falling asleep and/or difficulty maintaining sleep.
FDA Label

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Mechanism of Action
Suvorexant is a dual antagonist of orexin receptors OX1R and OX2R. It exerts its pharmacological action by inhibiting the binding of the neuropeptides orexin A and B (also known as hypothalamic secretin 1 and 2). These neuropeptides are produced by neurons in the lateral hypothalamus. These neurons control the brain's arousal centers, are active during wakefulness, especially during movement, and cease firing during sleep. Suvorexant reduces arousal and wakefulness by inhibiting the strengthening effects of the arousal system, rather than directly promoting sleep. Despite a deeper understanding of the biological basis of sleep control in the brain, few new mechanisms for treating insomnia have been discovered in recent years. A notable exception is the use of orexin receptor antagonists to inhibit the excitatory neuropeptides orexin A and B.
This article describes how we worked to investigate the poor oral pharmacokinetics of a leading high-throughput screening (HTS)-derived diazepam-based orexin receptor antagonist, ultimately discovering compound 10 with a 7-methyl substitution on the diazepam core. Despite exhibiting good potency, improved pharmacokinetics, and excellent in vivo efficacy, compound 10 forms an active metabolite during microsomal incubation. Based on mechanistic hypotheses and in vitro bioactivation experiments, we substituted the fluoroquinazoline ring in compound 10 with chlorobenzoxazole to obtain compound 3 (MK-4305). MK-4305 is a potent dual orexin receptor antagonist currently undergoing a phase III clinical trial for the treatment of primary insomnia. [1]

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Orexin receptor antagonists are a novel treatment for insomnia that directly targets sleep/wake regulation. Several such compounds have entered clinical development, including dual orexin receptor antagonists suvorexin and amorexin. In this study, we analyzed several orexin-2 (OX₂) selective antagonist radioligands [³H]-EMPA using equilibrium and kinetic studies. Furthermore, using CHO cells stably expressing the OX2 receptor, we investigated the effects of some compounds on inositol phosphate accumulation and ERK-1/2 phosphorylation by incubating with different agonists for 30 min and 5 min, respectively. EMPA, suvorexin, amorexin, and TCS-OX-29 all bound to the OX2 receptor with moderate to high affinity (pK(I) values ≥7.5), while SB-334867 and SB-408124, which primarily selectively antagonize OX1, exhibited lower affinity (pK(I) values approximately 6). Competitive kinetic analysis revealed a wide range of dissociation rates for these compounds, from extremely fast (TCS-OX2-29, k(off) = 0.22 min⁻¹) to extremely slow (almorexant, k(off) = 0.005 min⁻¹). Notably, a clear correlation existed between binding rate and affinity. In cell-based experiments, the rapidly acting antagonists EMPA and TCS-OX2-29 exhibited reversible antagonism against orexin A agonist activity. However, both suvorexant and almorexant resulted in concentration-dependent inhibition of the maximum orexin A response, which was more pronounced with shorter agonist incubation times. Analysis based on a semi-equilibrium model indicated that the antagonists dissociated more slowly in cellular systems than in membrane-bound systems; in this case, almorexant effectively acted as a pseudo-irreversible antagonist. [2]

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Sovresen (MK-4305) is a first-in-class dual orexin receptor antagonist (DORA) designed for the treatment of insomnia. Its mechanism of action is to block the binding of orexin peptides to OX1R and OX2R, which are key regulators of the sleep-wake cycle[1]. Unlike benzodiazepines, sovresen (MK-4305) did not cause respiratory depression or physical dependence in preclinical studies. It maintains normal sleep structure by increasing non-rapid eye movement (NREM) sleep without interfering with rapid eye movement (REM) sleep[2]. The high selectivity of sovresen (MK-4305) to orexin receptors minimizes off-target effects, which contributes to its good safety profile in preclinical evaluations[1].

C23H23CLN6O2

450.9207

450.16

1030377-33-3

24965990

Typically exists as solid at room temperature

4.9

0

6

3

32

664

1

C[C@@H]1CCN(CCN1C(=O)C2=C(C=CC(=C2)C)N3N=CC=N3)C4=NC5=C(O4)C=CC(=C5)Cl

JYTNQNCOQXFQPK-MRXNPFEDSA-N

InChI=1S/C23H23ClN6O2/c1-15-3-5-20(30-25-8-9-26-30)18(13-15)22(31)29-12-11-28(10-7-16(29)2)23-27-19-14-17(24)4-6-21(19)32-23/h3-6,8-9,13-14,16H,7,10-12H2,1-2H3/t16-/m1/s1

[(7R)-4-(5-chloro-1,3-benzoxazol-2-yl)-7-methyl-1,4-diazepan-1-yl]-[5-methyl-2-(triazol-2-yl)phenyl]methanone

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)