Discovery and development of orexin receptor antagonists as therapeutics for insomnia

Formulation development for the orexin receptor antagonist almorexant: assessment in two clinical studies

Almorexant, a dual orexin receptor antagonist, was investigated for the treatment of insomnia. The following observations initiated further formulation development: the active pharmaceutical ingredient (API) was sticking to the apparatus used during tablet compression; almorexant has an absolute bioavailability of 11.2%; and almorexant modestly decreased the latency to persistent sleep by 10.4 minutes in patients. Two randomized crossover studies were performed to investigate the pharmacokinetics of several new formulations in healthy subjects. In study I, the old "sticky" tablet was compared to two new formulations developed to prevent sticking: a qualitatively similar tablet but with a larger API crystal size and a tablet with 30% more excipients as well as a larger API crystal size. This latter formulation was available in two strengths. The geometric mean ratios and 90% confidence interval of the area under the curve (AUC) were within the bioequivalence range of 0.80-1.25 for the different comparisons between formulations. In study II, 100 mg of the reference tablet was compared to 25 and 50 mg of a liquid-filled hard gelatin capsule developed to increase the bioavailability of almorexant. The geometric mean ratios of the maximum concentration and AUC comparing the new 25 and 50 mg capsule formulations to the reference tablet did not exceed 0.25 and 0.50, respectively, indicating that the new capsule formulation did not increase the maximum concentration of or the total exposure to almorexant. In conclusion, a new tablet was developed but formulation development aimed at increasing the bioavailability of almorexant failed.

Function and dysfunction of hypocretin/orexin: an energetics point of view

The basic elements of animal behavior that are critical to survival include energy, arousal, and motivation: Energy intake and expenditure are fundamental to all organisms for the performance of any type of function; according to the Yerkes-Dodson law, an optimal level of arousal is required for animals to perform normal functions; and motivation is critical to goal-oriented behaviors in higher animals. The brain is the primary organ that controls these elements and, through evolution, has developed specialized structures to accomplish this task. The orexin/hypocretin system in the perifornical/lateral hypothalamus, which was discovered 15 years ago, is one such specialized area. This review summarizes a fast-growing body of evidence discerning how the orexin/hypocretin system integrates internal and external cues to regulate energy intake that can then be used to generate sufficient arousal for animals to perform innate and goal-oriented behaviors.

Relaxin-3/RXFP3 networks: an emerging target for the treatment of depression and other neuropsychiatric diseases?

Animal and clinical studies of gene-environment interactions have helped elucidate the mechanisms involved in the pathophysiology of several mental illnesses including anxiety, depression, and schizophrenia; and have led to the discovery of improved treatments. The study of neuropeptides and their receptors is a parallel frontier of neuropsychopharmacology research and has revealed the involvement of several peptide systems in mental illnesses and identified novel targets for their treatment. Relaxin-3 is a newly discovered neuropeptide that binds, and activates the G-protein coupled receptor, RXFP3. Existing anatomical and functional evidence suggests relaxin-3 is an arousal transmitter which is highly responsive to environmental stimuli, particularly neurogenic stressors, and in turn modulates behavioral responses to these stressors and alters key neural processes, including hippocampal theta rhythm and associated learning and memory. Here, we review published experimental data on relaxin-3/RXFP3 systems in rodents, and attempt to highlight aspects that are relevant and/or potentially translatable to the etiology and treatment of major depression and anxiety. Evidence pertinent to autism spectrum and metabolism/eating disorders, or related psychiatric conditions, is also discussed. We also nominate some key experimental studies required to better establish the therapeutic potential of this intriguing neuromodulatory signaling system, including an examination of the impact of RXFP3 agonists and antagonists on the overall activity of distinct or common neural substrates and circuitry that are identified as dysfunctional in these debilitating brain diseases.

Pharmacokinetic interactions between the orexin receptor antagonist almorexant and the CYP3A4 inhibitors ketoconazole and diltiazem

Almorexant, a tetrahydroisoquinoline orexin receptor antagonist and first representative of a new class of compounds for the treatment of insomnia, is a substrate of the cytochrome P450 3A4 isoenzyme (CYP3A4). Two randomized two-way crossover studies were performed in healthy subjects investigating the pharmacokinetic interaction between almorexant and the CYP3A4 inhibitors ketoconazole and diltiazem. When administered as a single dose of 100 mg almorexant during steady state of ketoconazole (400 mg once daily for 14 days) or diltiazem treatment (300 mg once daily for 11 days), the exposure to almorexant was 10.5- and 3.5-fold, respectively, greater when compared with almorexant alone. Exposure to the phenol metabolites M3 and M8 increased in the presence of the CYP3A4 inhibitors, whereas that to M6 (dealkylated metabolite) decreased. Concomitant ketoconazole decreased formation of the dehydrogenated metabolite M5 and diltiazem increased concentrations of this metabolite. Higher almorexant exposure was associated with an increased incidence of typical almorexant-related adverse events such as fatigue (both studies) and somnolence (ketoconazole study only). The present results indicate that dose adaptation must be considered when almorexant would be coadministered with inhibitors of CYP3A4.

Orexin-1 receptor blockade dysregulates REM sleep in the presence of orexin-2 receptor antagonism

In accordance with the prominent role of orexins in the maintenance of wakefulness via activation of orexin-1 (OX1R) and orexin-2 (OX2R) receptors, various dual OX1/2R antagonists have been shown to promote sleep in animals and humans. While selective blockade of OX2R seems to be sufficient to initiate and prolong sleep, the beneficial effect of additional inhibition of OX1R remains controversial. The relative contribution of OX1R and OX2R to the sleep effects induced by a dual OX1/2R antagonist was further investigated in the rat, and specifically on rapid eye movement (REM) sleep since a deficiency of the orexin system is associated with narcolepsy/cataplexy based on clinical and pre-clinical data. As expected, the dual OX1/2R antagonist SB-649868 was effective in promoting non-REM (NREM) and REM sleep following oral dosing (10 and 30 mg/kg) at the onset of the dark phase. However, a disruption of REM sleep was evidenced by a more pronounced reduction in the onset of REM as compared to NREM sleep, a marked enhancement of the REM/total sleep ratio, and the occurrence of a few episodes of direct wake to REM sleep transitions (REM intrusion). When administered subcutaneously, the OX2R antagonist JNJ-10397049 (10 mg/kg) increased NREM duration whereas the OX1R antagonist GSK-1059865 (10 mg/kg) did not alter sleep. REM sleep was not affected either by OX2R or OX1R blockade alone, but administration of the OX1R antagonist in combination with the OX2R antagonist induced a significant reduction in REM sleep latency and an increase in REM sleep duration at the expense of the time spent in NREM sleep. These results indicate that additional blockade of OX1R to OX2R antagonism elicits a dysregulation of REM sleep by shifting the balance in favor of REM sleep at the expense of NREM sleep that may increase the risk of adverse events. Translation of this hypothesis remains to be tested in the clinic.

Human orexin/hypocretin receptors form constitutive homo- and heteromeric complexes with each other and with human CB1 cannabinoid receptors

Human OX1 orexin receptors have been shown to homodimerize and they have also been suggested to heterodimerize with CB1 cannabinoid receptors. The latter has been suggested to be important for orexin receptor responses and trafficking. In this study, we wanted to assess the ability of the other combinations of receptors to also form similar complexes. Vectors for expression of human OX1, OX2 and CB1 receptors, C-terminally fused with either Renilla luciferase or GFP(2) green fluorescent protein variant, were generated. The constructs were transiently expressed in Chinese hamster ovary cells, and constitutive dimerization between the receptors was assessed by bioluminescence energy transfer (BRET). Orexin receptor subtypes readily formed homo- and hetero(di)mers, as suggested by significant BRET signals. CB1 receptors formed homodimers, and they also heterodimerized with both orexin receptors. Interestingly, BRET efficiency was higher for homodimers than for almost all heterodimers. This is likely to be due to the geometry of the interaction; the putatively symmetric dimers may place the C-termini in a more suitable orientation in homomers. Fusion of luciferase to an orexin receptor and GFP(2) to CB1 produced more effective BRET than the opposite fusions, also suggesting differences in geometry. Similar was seen for the OX1-OX2 interaction. In conclusion, orexin receptors have a significant propensity to make homo- and heterodi-/oligomeric complexes. However, it is unclear whether this affects their signaling. As orexin receptors efficiently signal via endocannabinoid production to CB1 receptors, dimerization could be an effective way of forming signal complexes with optimal cannabinoid concentrations available for cannabinoid receptors.

Effects of a newly developed potent orexin-2 receptor-selective antagonist, compound 1 m, on sleep/wakefulness states in mice

Orexins (also known as hypocretins) play critical roles in the regulation of sleep/wakefulness states by activating two G-protein coupled receptors (GPCRs), orexin 1 (OX₁R) and orexin 2 receptors (OX₂R). In order to understand the differential contribution of both receptors in regulating sleep/wakefulness states we compared the pharmacological effects of a newly developed OX₂R antagonist (2-SORA), Compound 1 m (C1 m), with those of a dual orexin receptor antagonist (DORA), suvorexant, in C57BL/6J mice. After oral administration in the dark period, both C1m and suvorexant decreased wakefulness time with similar efficacy in a dose-dependent manner. While C1m primarily increased total non-rapid eye movement (NREM) sleep time without affecting episode durations and with minimal effects on REM sleep, suvorexant increased both total NREM and REM sleep time and episode durations with predominant effects on REM sleep. Fos-immunostaining showed that both compounds affected the activities of arousal-related neurons with different patterns. The number of Fos-IR noradrenergic neurons in the locus coeruleus was lower in the suvorexant group as compared with the control and C1m-treated groups. In contrast, the numbers of Fos-IR neurons in histaminergic neurons in the tuberomamillary nucleus and serotonergic neurons in the dorsal raphe were reduced to a similar extent in the suvorexant and C1m groups as compared with the vehicle-treated group. Together, these results suggest that an orexin-mediated suppression of REM sleep via potential activation of OX₁Rs in the locus coeruleus may possibly contribute to the differential effects on sleep/wakefulness exerted by a DORA as compared to a 2-SORA.

Orexin/hypocretin receptor signalling: a functional perspective

Multiple homeostatic systems are regulated by orexin (hypocretin) peptides and their two known GPCRs. Activation of orexin receptors promotes waking and is essential for expression of normal sleep and waking behaviour, with the sleep disorder narcolepsy resulting from the absence of orexin signalling. Orexin receptors also influence systems regulating appetite/metabolism, stress and reward, and are found in several peripheral tissues. Nevertheless, much remains unknown about the signalling pathways and targets engaged by native receptors. In this review, we integrate knowledge about the orexin receptor signalling capabilities obtained from studies in expression systems and various native cell types (as presented in Kukkonen and Leonard, this issue of British Journal of Pharmacology) with knowledge of orexin signalling in different tissues. The tissues reviewed include the CNS, the gastrointestinal tract, the pituitary gland, pancreas, adrenal gland, adipose tissue and the male reproductive system. We also summarize the findings in different native and recombinant cell lines, especially focusing on the different cascades in CHO cells, which is the most investigated cell line. This reveals that while a substantial gap exists between what is known about orexin receptor signalling and effectors in recombinant systems and native systems, mounting evidence suggests that orexin receptor signalling is more diverse than originally thought. Moreover, rather than being restricted to orexin receptor 'overexpressing' cells, this signalling diversity may be utilized by native receptors in a site-specific manner.

Dual orexin receptor antagonists show distinct effects on locomotor performance, ethanol interaction and sleep architecture relative to gamma-aminobutyric acid-A receptor modulators

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.

Differential actions of orexin receptors in brainstem cholinergic and monoaminergic neurons revealed by receptor knockouts: implications for orexinergic signaling in arousal and narcolepsy

Orexin neuropeptides influence multiple homeostatic functions and play an essential role in the expression of normal sleep-wake behavior. While their two known receptors (OX₁ and OX₂) are targets for novel pharmacotherapeutics, the actions mediated by each receptor remain largely unexplored. Using brain slices from mice constitutively lacking either receptor, we used whole-cell and Ca²⁺ imaging methods to delineate the cellular actions of each receptor within cholinergic [laterodorsal tegmental nucleus (LDT)] and monoaminergic [dorsal raphe (DR) and locus coeruleus (LC)] brainstem nuclei—where orexins promote arousal and suppress REM sleep. In slices from OX^−/−₂ mice, orexin-A (300 nM) elicited wild-type responses in LDT, DR, and LC neurons consisting of a depolarizing current and augmented voltage-dependent Ca²⁺ transients. In slices from OX^−/−₁ mice, the depolarizing current was absent in LDT and LC neurons and was attenuated in DR neurons, although Ca²⁺-transients were still augmented. Since orexin-A produced neither of these actions in slices lacking both receptors, our findings suggest that orexin-mediated depolarization is mediated by both receptors in DR, but is exclusively mediated by OX₁ in LDT and LC neurons, even though OX₂ is present and OX₂ mRNA appears elevated in brainstems from OX^−/−₁ mice. Considering published behavioral data, these findings support a model in which orexin-mediated excitation of mesopontine cholinergic and monoaminergic neurons contributes little to stabilizing spontaneous waking and sleep bouts, but functions in context-dependent arousal and helps restrict muscle atonia to REM sleep. The augmented Ca²⁺ transients produced by both receptors appeared mediated by influx via L-type Ca²⁺ channels, which is often linked to transcriptional signaling. This could provide an adaptive signal to compensate for receptor loss or prolonged antagonism and may contribute to the reduced severity of narcolepsy in single receptor knockout mice.