Small-molecule antagonists of the orexin receptors

An Update on Assessment, Therapeutic Management, and Patents on Insomnia

Insomnia is an ordinary situation related to noticeable disability in function and quality of life, mental and actual sickness, and mishappenings. It represents more than 5.5 million appointments to family doctors every year. Nonetheless, the ratio of insomniacs who are treated keeps on being low, demonstrating the requirement for proceeding with advancement and dispersal of effective treatments. Accordingly, it becomes significant to provide a compelling treatment for clinical practice. It indicates a need for the determination of various critical viewpoints for the evaluation of insomnia along with various accessible alternatives for treatment. These alternatives incorporate both nonpharmacological therapy, specifically cognitive behavioural therapy for insomnia, and a number of pharmacological treatments like orexin antagonists, "z-drugs," benzodiazepines, selective histamine H1 antagonists, nonselective antihistamines, melatonin receptor agonists, antipsychotics, antidepressants, and anticonvulsants. Besides in individuals whose insomnia is due to restless leg syndrome, depression/mood disorder, or/and circadian disturbance, there is insignificant proof favouring the effectiveness of different prescriptions for the treatment of insomnia though they are widely used. Other pharmacological agents producing sedation should be prescribed with care for insomnia therapy because of greater risk of next-day sleepiness along with known adverse effects and toxicities. This review is also aimed at providing an update on various patents on dosage forms containing drugs for insomnia therapy.

Suvorexant, a Dual Orexin Receptor Antagonist, Protected Seizure through Interaction with GABAA and Glutamate Receptors

Orexin can increase neuronal excitability and induce epileptic activity. In this study, the effects of suvorexant (orexin receptor antagonist) on pentylenetetrazol (PTZ) and maximal electroshock (MES)-induced seizure were investigated. Mice were divided into 5 groups of six animals each including normal saline (10 mL/kg), diazepam (2 mg/kg), and suvorexant (50, 100 and 200 mg/kg) groups. In PTZ test, the latency to first minimal clonic seizure (MCS), latency to the first generalized tonic–clonic seizures (GTCS), total duration of seizure and also protection against mortality were evaluated. In MES, the hind limb tonic extension (HLTE) and the protection against mortality were recorded. In order to evaluate the role of GABA_A in anticonvulsant effect of suvorexant, flumazenil was used and to investigate the role of glutamate, the protein levels of AMPAR and NMDAR were measured in hippocampus by western blotting.  In PTZ model, suvorexant (200mg/kg) increased MCS and GTCS latencies. Suvorexant (100 and 200 mg/kg) decreased total duration of seizure compared to control group. In PTZ model, flumazenil inhibited the prolongation of seizure latency induced by suvorexant. In MES, the HLTE was decreased by suvorexant (100 and 200 mg/kg) and suvorexant was protected against mortality by 83.3%. Moreover, the protein levels of NMDAR and AMPAR were decreased by suvorexant.

Suvorexant exerted anticonvulsant activity and in addition to its inhibitory effect on orexin receptors, this effect may be mediated, at least partly, through interaction with GABA_A and glutamate receptors.

Functional characterization of an orexin neuropeptide in amphioxus reveals an ancient origin of orexin/orexin receptor system in chordate

Amphioxus belongs to the subphylum cephalochordata, an extant representative of the most basal chordates, whose regulation of endocrine system remains ambiguous. Here we clearly demonstrated the existence of a functional orexin neuropeptide in amphioxus, which is able to interact with orexin receptor, activate both PKC and PKA pathways, decrease leptin expression, and stimulate lipogenesis. We also showed the transcription level of amphioxus orexin was affected by fasting or temperature, indicating a role of this gene in the regulation of energy balance. In addition, the expression of the amphioxus orexin was detected at cerebral vesicle, which has been proposed to be a homolog of the vertebrate brain. These data collectively suggest that a functional orexin neuropeptide has already emerged in amphioxus, which provide insights into the evolutionary origin of orexin in chordate and the functional homology between the cerebral vesicle and vertebrate brain.

Arcuate Nucleus Orexin-A Signaling Alleviates Cisplatin-Induced Nausea and Vomiting Through the Paraventricular Nucleus of the Hypothalamus in Rats

The most common side effects of cisplatin chemotherapy are nausea and vomiting, and the overwhelming majority of research studies on the mechanism of cisplatin-induced nausea have been focused on the “vomiting center.” As a modulatory center of gastric motility, the roles of the hypothalamus in nausea and vomiting remain unclear. In the present study, we investigated the effects of exogenous orexin-A injected into the arcuate nucleus (ARC) on cisplatin-induced nausea and vomiting, and the possible underlying mechanism. Kaolin intake was calculated daily in cisplatin-treated and saline-treated rats. Gastric motility recording, injections into the ARC, and lesions of the paraventricular nucleus (PVN) were used to study the effects of orexin-A and the hypothalamic nucleus on disorders of gastrointestinal function in cisplatin-treated rats. The pathway from the ARC to the PVN was observed through Fluoro-Gold retrograde tracing. Furthermore, an NPY Y1 receptor antagonist was administered to explore the possible mechanisms involved in the effects of orexin-A in the ARC. We illustrated that exogenous orexin-A injected into the ARC reduced kaolin intake and promoted gastric motility in cisplatin-treated rats, and these effects could have been blocked by an ipsilateral PVN lesion or co-injected antagonist of orexin-A-SB334867. Additional results showed that orexin-A-activated neurons in the ARC communicated directly with other neurons in the PVN that express neuropeptide Y (NPY). Furthermore, activation of the downstream NPY pathway was required for the observed effects of orexin in the ARC on cisplatin-induced nausea and vomiting. These findings reveal a novel neurobiological circuit from the ARC to the PVN that might provide a potential target for the prevention and treatment of cisplatin-induced nausea and vomiting.

Functional cardiac orexin receptors: role of orexin-B/orexin 2 receptor in myocardial protection

Orexins/hypocretins exert cardiovascular effects which are centrally mediated. In the present study, we tested whether orexins and their receptors may also act in an autocrine/paracrine manner in the heart exerting direct effects. Quantitative reverse transcription-PCR (RT-PCR), immunohistochemical and Western blot analyses revealed that the rat heart expresses orexins and orexin receptors (OXR). In isolated rat cardiomyocytes, only orexin-B (OR-B) caused an increase in contractile shortening, independent of diastolic or systolic calcium levels. A specific orexin receptor-2 (OX2R) agonist ([Ala¹¹, d-Leu¹⁵]-Orexin B) exerted similar effects as OR-B, whereas a specific orexin receptor-1 (OX1R) antagonist (SB-408124) did not alter the responsiveness of OR-B. Treatment of the same model with OR-B resulted in a dose-dependent increase in myosin light chain and troponin-I (TnI) phosphorylation. Following ischaemia/reperfusion in the isolated Langendorff perfused rat heart model, OR-B, but not OR-A, exerts a cardioprotective effect; mirrored in an in vivo model as well. Unlike OR-A, OR-B was also able to induce extracellular signal-regulated kinase (ERK) 1/2 (ERK1/2) and Akt phosphorylation in rat myocardial tissue and ERK1/2 phosphorylation in human heart samples. These findings were further corroborated in an in vivo rat model. In human subjects with heart failure, there is a significant negative correlation between the expression of OX2R and the severity of the disease clinical symptoms, as assessed by the New York Heart Association (NYHA) functional classification. Collectively, we provide evidence of a distinct orexin system in the heart that exerts a cardioprotective role via an OR-B/OX2R pathway.

Substituted cyclopentanes, tetrahydrofurans and pyrrolidines as orexin-1-receptor antagonists for treatment of various CNS disorders (WO2015/055994; WO2015/124932; WO2015/124934)

The three patent applications WO2015/055994, WO2015/124932 and WO2015/124934 from Takeda Pharmaceuticals describe antagonists for the orexin-1 receptor, based on saturated substituted five-membered carbo- or heterocycles. According to the patent applications, the compounds have utility in therapeutic areas such as schizophrenia and other psychotic disorders, anxiety or addiction disorders, among others. The patent applications together describe almost 300 examples, and for most of them activity data, determined by Fluorescence Imaging Plate Reader (FLIPR) technology on the orexin-1 as well as the orexin-2 receptor, are disclosed. Structurally, the building blocks used to prepare the compounds are reminiscent of other orexin antagonist programs recently disclosed in the literature. However, the templates used are novel in the orexin antagonist field and are probably the key feature for the selectivity of the derivatives towards the orexin-1 receptor.

Discovery of (1R,2S)-2-{[(2,4-Dimethylpyrimidin-5-yl)oxy]methyl}-2-(3-fluorophenyl)-N-(5-fluoropyridin-2-yl)cyclopropanecarboxamide (E2006): A Potent and Efficacious Oral Orexin Receptor Antagonist

The orexin/hypocretin receptors are a family of G protein-coupled receptors and consist of orexin-1 (OX1) and orexin-2 (OX2) receptor subtypes. Orexin receptors are expressed throughout the central nervous system and are involved in the regulation of the sleep/wake cycle. Because modulation of these receptors constitutes a promising target for novel treatments of disorders associated with the control of sleep and wakefulness, such as insomnia, the development of orexin receptor antagonists has emerged as an important focus in drug discovery research. Here, we report the design, synthesis, characterization, and structure-activity relationships (SARs) of novel orexin receptor antagonists. Various modifications made to the core structure of a previously developed compound (-)-5, the lead molecule, resulted in compounds with improved chemical and pharmacological profiles. The investigation afforded a potential therapeutic agent, (1R,2S)-2-{[(2,4-dimethylpyrimidin-5-yl)oxy]methyl}-2-(3-fluorophenyl)-N-(5-fluoropyridin-2-yl)cyclopropanecarboxamide (E2006), an orally active, potent orexin antagonist. The efficacy was demonstrated in mice in an in vivo study by using sleep parameter measurements.

Identification of MK-8133: An orexin-2 selective receptor antagonist with favorable development properties

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.

Discovery of HTL6641, a dual orexin receptor antagonist with differentiated pharmacodynamic properties

A novel series of potent, selective, and orally efficacious dual antagonists of the orexin receptors is described.