Magnetic resonance imaging detects white adipose tissue beiging in mice following PDE10A inhibitor treatment.

Weight gain is a common harmful side effect of atypical antipsychotics used for schizophrenia treatment. Conversely, treatment with the novel phosphodiesterase-10A (PDE10A) inhibitor MK-8189 in clinical trials led to significant weight reduction, especially in patients with obesity. This study aimed to understand and describe the mechanism underlying this observation, which is essential to guide clinical decisions. We hypothesized that PDE10A inhibition causes beiging of white adipose tissue (WAT), leading to weight loss. Magnetic resonance imaging (MRI) methods were developed, validated, and applied in a diet-induced obesity mouse model treated with a PDE10A inhibitor THPP-6 or vehicle for measurement of fat content and vascularization of adipose tissue. Treated mice showed significantly lower fat fraction in white and brown adipose tissue, and increased perfusion and vascular density in WAT versus vehicle, confirming the hypothesis, and matching the effect of CL-316,243, a compound known to cause adipose tissue beiging. The in vivo findings were validated by qPCR revealing upregulation of Ucp1 and Pcg1-α genes, known markers of WAT beiging, and angiogenesis marker VegfA in the THPP-6 group. This work provides a detailed understanding of the mechanism of action of PDE10A inhibitor treatment on adipose tissue and body weight and will be valuable to guide both the use of MK-8189 in schizophrenia and the potential application of the target for weight loss indication.

Investigation of orexin-2 selective receptor antagonists: Structural modifications resulting in dual orexin receptor antagonists.

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.

Orexin 2 Receptor Antagonism is Sufficient to Promote NREM and REM Sleep from Mouse to Man.

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.

Structure and ligand-binding mechanism of the human OX1 and OX2 orexin receptors.

The orexin (also known as hypocretin) G protein-coupled receptors (GPCRs) regulate sleep and other behavioral functions in mammals, and are therapeutic targets for sleep and wake disorders. The human receptors hOX1R and hOX2R, which are 64% identical in sequence, have overlapping but distinct physiological functions and potential therapeutic profiles. We determined structures of hOX1R bound to the OX1R-selective antagonist SB-674042 and the dual antagonist suvorexant at 2.8-Å and 2.75-Å resolution, respectively, and used molecular modeling to illuminate mechanisms of antagonist subtype selectivity between hOX1R and hOX2R. The hOX1R structures also reveal a conserved amphipathic α-helix, in the extracellular N-terminal region, that interacts with orexin-A and is essential for high-potency neuropeptide activation at both receptors. The orexin-receptor crystal structures are valuable tools for the design and development of selective orexin-receptor antagonists and agonists.

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.

The duration of sleep promoting efficacy by dual orexin receptor antagonists is dependent upon receptor occupancy threshold.

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.

Pharmacological characterization of MK-6096 - a dual orexin receptor antagonist for insomnia.

Orexin (hypocretin) neuropeptides promote wakefulness by signaling through two G-protein coupled receptors, Orexin 1 Receptor (OX(1)R) and Orexin 2 Receptor (OX(2)R). MK-6096 is an orally bioavailable potent and selective reversible antagonist of OX(1)R and OX(2)R currently in clinical development for insomnia. In radioligand binding and functional cell based assays MK-6096 demonstrated potent binding and antagonism of both human OX(1)R and OX(2)R (<3 nM in binding, 11 nM in FLIPR), with no significant off-target activities against a panel of >170 receptors and enzymes. MK-6096 occupies 90% of human OX(2)Rs expressed in transgenic rats at a plasma concentration of 142 nM, and dose-dependently reduced locomotor activity and significantly increased sleep in rats (3-30 mg/kg) and dogs (0.25 and 0.5 mg/kg). DORA-22, an analog of MK-6096, exhibits similar sleep promoting properties that are absent OX(1/2)R double knockouts, demonstrating the mechanism of action and specificity of these effects. These findings with a novel, structurally distinct class of OxR antagonists provide further validation of the orexin pathway as an effective target to promote normal sleep. Comparative analysis of the biochemical and pharmacokinetic properties of these compounds relative to other OXR antagonists provides a basis for understanding the attributes critical for in vivo efficacy. This mechanism is distinct from current standard of care such that MK-6096 represents a novel and selective therapeutic for the treatment of insomnia. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Promotion of sleep by suvorexant-a novel dual orexin receptor antagonist.

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.

2,3-Diaminopyridine as a platform for designing structurally unique nonpeptide bradykinin B1 receptor antagonists.

A novel class of 2,3-diaminopyridine bradykinin B1 receptor antagonists is disclosed. Structure-activity relationship studies (SARs) that led to compounds with significantly improved potency and pharmacokinetic properties relative to the lead compound are described.

2,3-diaminopyridine bradykinin B1 receptor antagonists.

Bradykinin B1 receptor antagonists embody a potentially novel approach for the treatment of chronic pain and inflammation. A series of 2,3-diaminopyridine B1 antagonists was optimized to have sub-nanomolar affinity and good pharmacokinetic properties. Lead compounds were shown to exhibit good efficacy in rabbit in vivo models of pain and inflammation.

Pharmacological characterization and radioligand binding properties of a high-affinity, nonpeptide, bradykinin B1 receptor antagonist.

Compound A (N-[2-[4-(4,5-dihydro-1H-imidazol-2-yl)phenyl]ethyl]-2-[(2R)-1-(2-napthylsulfonyl)-3-oxo-1,2,3,4-tetrahydroquinoxalin-2-yl]acetamide) is a member of a new class of aryl sulfonamide dihydroquinoxalinone bradykinin B1 receptor antagonists that should be useful pharmacological tools. Here we report on some of the pharmacological properties of compound A as well as the characterization of [35S]compound A as the first nonpeptide bradykinin B1 receptor radioligand. Compound A inhibited tritiated peptide ligand binding to the cloned human, rabbit, dog, and rat bradykinin B1 receptors expressed in CHO cells with Ki values of 0.016, 0.050, 0.56, and 29 nM, respectively. It was inactive at 10 microM in binding assays with the cloned human bradykinin B2 receptor. In functional antagonist assays with the cloned bradykinin B1 receptors, compound A inhibited agonist-induced signaling with activities consistent with the competition binding results, but had no antagonist activity at the bradykinin B2 receptor. Compound A was also found to be a potent antagonist in a rabbit aorta tissue bath preparation and to effectively block des-Arg9 bradykinin depressor responses in lipopolysaccharide-treated rabbit following intravenous administration. The binding of [35S]compound A was evaluated with the cloned bradykinin B1 receptors. In assays with human, rabbit, and dog receptors, [35S]compound A labeled a single site with Kd values of 0.012, 0.064, and 0.37 nM, respectively, and with binding site densities equivalent to those obtained using the conventional tritiated peptide ligands. Binding assays with the cloned rat bradykinin B1 receptor were not successful, presumably due to the low affinity of the ligand for this species receptor. There was no specific binding of the ligand detected in CHO cells expressing the human bradykinin B2 receptor. In assays with the cloned human bradykinin B1 receptor, the pharmacologies of the binding of [35S]compound A and [3H][Leu9]des-Arg10-kallidin were the same. The high signal-to-noise ratio obtained with [35S]compound A will allow this ligand to be a very useful tool for future investigations of the bradykinin B1 receptor.

Generation and characterization of a human bradykinin receptor B1 transgenic rat as a pharmacodynamic model.

Antagonists of the B1 bradykinin receptor (B1R) offer the promise of novel therapeutic agents for the treatment of inflammatory and neuropathic pain. However, the in vivo characterization of the pharmacodynamics of B1R antagonists is hindered by the low level of B1R expression in healthy tissue and the profound species selectivity exhibited by many compounds for the human B1R. To circumvent these issues, we generated a transgenic rat expressing the human B1R under the control of the neuron-specific enolase promoter. Membranes prepared from whole brain homogenates of heterozygous transgenic rats indicate a B1R expression level of 30 to 40 fmol/mg; there is no detectable B1R expression in control nontransgenic rats. The pharmacological profile of the B1R expressed in the transgenic rat matches that expected of the human, but not the rat receptor. The mapping of the transgene insertion site to rat chromosome 1 permitted the development of a reliable assay for the identification of homozygous transgenic rats. Significantly, homozygous transgenic rats express 2-fold more B1R than heterozygous animals. Autoradiographic analyses of tissue sections from transgenic rats reveal that the B1R is broadly expressed in both the brain and spinal cord. The human B1R expressed in the transgenic rat functions in an in vitro contractile assay and thus has the potential to elicit a functional response in vivo. Using the humanized B1R transgenic rat, an assay was developed that is suitable for the routine evaluation of a test compound's ability to occupy the human B1R in the central nervous system.

Benzodiazepines as potent and selective bradykinin B1 antagonists.

Antagonism of the bradykinin B(1) receptor was demonstrated to be a potential treatment for chronic pain and inflammation. Novel benzodiazepines were designed that display subnanomolar affinity for the bradykinin B(1) receptor (K(i) = 0.59 nM) and high selectivity against the bradykinin B(2) receptor (K(i) > 10 microM). In vivo efficacy, comparable to morphine, was demonstrated for lead compounds in a rodent hyperalgesia model.