Synthesis and evaluation of carbon-linked analogs of dual orexin receptor antagonist filorexant

Analogs of the dual orexin receptor antagonist filorexant were prepared. Replacement of the ether linkage proved highly sensitive toward modification with an acetylene linkage providing compounds with the best in vitro and in vivo potency profiles.     

Discovery of dual orexin receptor antagonists with rat sleep efficacy enabled by expansion of the acetonitrile-assisted/diphosgene-mediated 2,4-dichloropyrimidine synthesis

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.     

Discovery of 2,5-diarylnicotinamides as selective orexin-2 receptor antagonists (2-SORAs)

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.     

Diaryl urea analogues of SB-334867 as orexin-1 receptor antagonists

As a part of our program to develop OX1-CB1 bivalent ligands, we required a better understanding of the basic structure-activity relationships (SARs) of orexin antagonists. A series of SB-334867 analogues were synthesized and evaluated in calcium mobilization assays. SAR results suggest that the 2-methylbenzoxazole moiety may be replaced with a disubstituted 4-aminophenyl group without loss of activity and an electron-deficient system is generally preferred at the 1,5-naphthyridine moiety for OX1 antagonist activity. In particular, substitution of larger potential linkers such as n-hexyl provided compound 33 with equivalent activity at the OX1 receptor compared to the lead compound SB-334867. These compounds should be of value in the development of ligands targeting the orexin-1 receptor and its potential heterodimers.     

Discovery of novel substituted octahydropyrrolo[3,4-c]pyrroles as dual orexin receptor antagonists for insomnia treatment

A series of octahydropyrrolo[3,4-c]pyrroles were synthesized and evaluated by orexin 1 and 2 receptor (OX_{1 & 2} R) antagonists assays. Compound 14l with potent OXR antagonist activity and suitable pharmacokinetic behavior was chosen to be investigated in an EEG study, which demonstrated effects of sleep promotion comparable to Suvorexant. Furthermore, the di-fluro substituted analogs exhibited reduced hERG inhibition while maintaining moderate potency.     

Discovery of MK-3697: A selective orexin 2 receptor antagonist (2-SORA) for the treatment of insomnia

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.     

Discovery of spiropiperidine-based potent and selective Orexin-2 receptor antagonists

To generate novel human Orexin-2 Receptor (OX2R) antagonists, a spiropiperidine based scaffold was designed and a SAR study was carried out. Compound 4f possessed the highest OX2R antagonistic activity with an IC(50) value of 3nM with 450-fold selectivity against Orexin-1 Receptor (OX1R).     

Discovery process and pharmacological characterization of a novel dual orexin 1 and orexin 2 receptor antagonist useful for treatment of sleep disorders

The hypothalamic peptides orexin-A and orexin-B are potent agonists of two G-protein coupled receptors, namely the OX(1) and the OX(2) receptor. These receptors are widely distributed, though differentially, in the rat brain. In particular, the OX(1) receptor is highly expressed throughout the hypothalamus, whilst the OX(2) receptor is mainly located in the ventral posterior nucleus. A large body of compelling evidence, both pre-clinical and clinical, suggests that the orexin system is profoundly implicated in sleep disorders. In particular, modulation of the orexin receptors activation by appropriate antagonists was proven to be an efficacious strategy for the treatment of insomnia in man. A novel, drug-like bis-amido piperidine derivative was identified as potent dual OX(1) and OX(2) receptor antagonists, highly effective in a pre-clinical model of sleep.     

Discovery of potent, selective, orally active benzoxazepine-based Orexin-2 receptor antagonists

During our efforts to identify a series of potent, selective, orally active human Orexin-2 Receptor (OX2R) antagonists, we elucidated structure-activity relationship (SAR) on the 7-position of a benzoxazepine scaffold by utilizing Hammett σ(p) and Hansch-Fujita π value as aromatic substituent constants. The attempts led to the discovery of compound 1m, possessing good in vitro potency with over 100-fold selectivity against OX1R, good metabolic stability in human and rat liver microsome, good oral bioavailability in rats, and in vivo antagonistic activity in rats by oral administration.     

Replacement of amide with bioisosteres led to a new series of potent adenosine A2A receptor antagonists

We have previously reported a series of 2,4,6-trisubstituted pyrimidines as potent A_2A receptor antagonists. The leading compounds often feature a potentially labile acetamide functional group which tends to hydrolyze under acidic conditions. Here we report the replacement of the acetamide functional group with bioisosteres. This effort led us to a new series of adenosine A_2A receptor antagonists with improved potency and chemical stability.     

3-Oxo-2-piperazinyl acetamides as potent bradykinin B1 receptor antagonists for the treatment of pain and inflammation

The discovery of novel and highly potent oxopiperazine based B1 receptor antagonists is described. Compared to the previously described arylsulfonylated (R)-3-amino-3-phenylpropionic acid series, the current compounds showed improved in vitro potency and metabolic stability. Compound 17, 2-((2R)-1-((4-methylphenyl)sulfonyl)-3-oxo-2-piperazinyl)-N-((1R)-6-(1-piperidinylmethyl)-1,2,3,4-tetrahydro-1-naphthalenyl)acetamide, showed EC₅₀ of 10.3 nM in a rabbit biochemical challenge model. The practical syntheses of chiral arylsulfonylated oxopiperazine acetic acids are also described.     

C-terminal modifications of an endothelin antagonist RES-701-1: Production of ETA/ETB dual selective analogs

Summary RES-701-1 is an endothelin B receptor (ET_B) selective peptidic antagonist, which has a novel cyclic structure of microbial origin. Modification at the C-terminal free carboxyl group of RES-701-1 by a methyl ester results in an ET_A/ET_B dual selective analog, which showed relatively high affinity for ET_A receptor subtype, while retaining the affinity for ET_B. The carboxyl-group-deleted analog with tryptamine as the C-terminal residue also showed relatively weak affinity for ET_A; however, benzyl ester or amide analogs did not show remarkable affinity for ET_A. It is suggested that the binding mode of RES-701-1 and its analogs is different from those of known ligands for ET receptors.     

Non-peptidic antagonists of the CGRP receptor, BIBN4096BS and MK-0974, interact with the calcitonin receptor-like receptor via methionine-42 and RAMP1 via tryptophan-74

The receptor for calcitonin gene-related peptide (CGRP) has been the target for the development of novel small molecule antagonists for the treatment of migraine. Two such antagonists, BIBN4096BS and MK-0974, have shown great promise in clinical trials and hence a deeper understanding of the mechanism of their interaction with the receptor is now required. The structure of the CGRP receptor is unusual since it is comprised of a hetero-oligomeric complex between the calcitonin receptor-like receptor (CRL) and an accessory protein (RAMP1). Both the CLR and RAMP1 components have extracellular domains which interact with each other and together form part of the peptide-binding site. It seems likely that the antagonist binding site will also be located on the extracellular domains and indeed Trp-74 of RAMP1 has been shown to form part of the binding site for BIBN4096BS. However, despite a chimeric study demonstrating the role of the N-terminal domain of CLR in antagonist binding, no specific residues have been identified. Here we carry out a mutagenic screen of the extreme N-terminal domain of CLR (residues 23-63) and identify a mutant, Met-42-Ala, which displays 48-fold lower affinity for BIBN4096BS and almost 900-fold lower affinity for MK-0974. In addition, we confirm that the Trp-74-Lys mutation at human RAMP1 reduces BIBN4096BS affinity by over 300-fold and show for the first time a similar effect for MK-0974 affinity. The data suggest that the non-peptide antagonists occupy a binding site close to the interface of the N-terminal domains of CLR and RAMP1.     

Pharmacological characterization of DPTN and other selective A3 adenosine receptor antagonists

The A₃ adenosine receptor (AR) is emerging as an attractive drug target. Antagonists are proposed for the potential treatment of glaucoma and asthma. However, currently available A₃AR antagonists are potent in human and some large animals, but weak or inactive in mouse and rat. In this study, we re-synthesized a previously reported A₃AR antagonist, DPTN, and evaluated its affinity and selectivity at human, mouse, and rat ARs. We showed that DPTN, indeed, is a potent A₃AR antagonist for all three species tested, albeit a little less selective for mouse and rat A₃AR in comparison to the human A₃AR. DPTN’s K_i values at respective A₁, A_2A, A_2B, and A₃ receptors were (nM) 162, 121, 230, and 1.65 (human); 411, 830, 189, and 9.61 (mouse); and 333, 1147, 163, and 8.53 (rat). Its antagonist activity at both human and mouse A₃ARs was confirmed in a cyclic AMP functional assay. Considering controversial use of currently commercially available A₃AR antagonists in rats and mice, we also re-examined other commonly used and selective A₃AR antagonists under the same experimental conditions. The K_i values of MRS1523 were shown to be 43.9, 349, and 216 nM at human, mouse, and rat A₃ARs, respectively. MRS1191 and MRS1334 showed incomplete inhibition of [¹²⁵I]I-AB-MECA binding to mouse and rat A₃ARs, while potent human A₃AR antagonists, MRS1220, MRE3008F20, PSB10, PSB-11, and VUF5574 were largely inactive. Thus, we demonstrated that DPTN and MRS1523 are among the only validated A₃AR antagonists that can be possibly used (at an appropriate concentration) in mouse or rat to confirm an A₃AR-related mechanism or function.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11302-021-09823-5.     

Discovery of N-aryl-2-acylindole human glucagon receptor antagonists

A novel class of N-aryl-2-acylindole human glucagon receptor (hGCGR) antagonists is reported. These compounds demonstrate good pharmacokinetic profiles in multiple preclinical species. One compound from this series, indole 33, is orally active in a transgenic murine pharmacodynamic model. Furthermore, a 1mg/kg oral dose of indole 33 lowers ambient glucose levels in an ob/ob/hGCGR transgenic murine diabetes model. This compound was deemed suitable for preclinical safety studies and was found to be well tolerated in an 8-day experimental rodent tolerability study. The combination of preclinical efficacy and safety observed with compound 33 highlights the potential of this class as a treatment for type 2 diabetes.     

Essential structure of orexin 1 receptor antagonist YNT-707, Part IV: The role of D-ring in 4,5-epoxymorphinan on the orexin 1 receptor antagonistic activity

The orexin 1 receptor (OX₁R) antagonists carrying a morphinan skeleton such as YNT-707 (2) and YNT-1310 (3) showed potent and extremely high selective antagonistic activity against OX₁R. In the course of our study of the essential structure of YNT-707 for high binding affinity against OX₁R, we prepared derivatives of 2 without the D- and 4,5-epoxy rings to clarify the roles of these structural determinants toward OX₁R antagonistic activity. The D- and 4,5-epoxy rings played important roles for the active orientation of the 17-sulfonamide and 6-amide side chains. Finally, we identified the simple structure required for selective OX₁R antagonistic activity in the complex morphinan skeleton, which is expected to be a useful scaffold for further design of OX₁R ligands.     

Essential structure of orexin 1 receptor antagonist YNT-707, Part I: Role of the 4,5-epoxy ring for binding with orexin 1 receptor

The essential structure of the orexin 1 receptor (OX₁R) antagonist YNT-707 (2) was clarified, particularly the roles to OX₁R antagonist activities of the 3-OMe, the 4,5-epoxy ring, the 14-hydroxy group, and the orientation of the 6-amide side chain.The 3-OMe and 17-sulfonamide group were shown to be essential for the OX₁R antagonistic activity. The 4,5-epoxy ring plays an important role for the active orientation of the 6-amide group. The 14-hydroxy group could lower the activity of the 6β-amide isomer by the interaction of the 14-hydroxy group with the 6-amide group, which could orient the 6-amide group toward the upper side of the C-ring.Finally, we proposed the difference in the active conformation between OX₁R and κ opioid receptor (KOR), especially in the orientation of the 6-amide group which is expected to be a useful guide for medicinal chemists to design OX₁R ligands.     

Essential structure of orexin 1 receptor antagonist YNT-707, part III: Role of the 14-hydroxy and the 3-methoxy groups in antagonistic activity toward the orexin 1 receptor in YNT-707 derivatives lacking the 4,5-epoxy ring

Morphinan derivatives lacking the 4,5-epoxy ring were synthesized to examine the participation of the 14-OH group, the 3-OMe group, and the aromaticity of the A-ring in the activity and selectivity for the orexin 1 receptor (OX₁R). The assay results and the conformational analyses of the 14-dehydrated and 14-H derivatives suggested that the orientations of the 6-amide side chain and the 17-benzenesulfonyl group would play important roles in the activity for OX₁R. In the 6β-derivatives, removal of the 3-OMe group and the reduction of the A-ring significantly decreased the activity toward the OX₁R, but these changes did not affect the 6α-derivatives. These results indicate that the 3-OMe group and the A-ring would be essential structural moieties for the 6β-derivatives.     

Discovery of novel diarylketoxime derivatives as selective and orally active melanin-concentrating hormone 1 receptor antagonists

Optimization of the lead 2a led to the identification of a novel diarylketoxime class of melanin-concentrating hormone 1 receptor (MCH-1R) antagonists. Our focus was directed toward improvement of hERG activity and metabolic stability. The representative derivative 4b showed potent and dose-dependent body weight reduction in diet-induced obese (DIO) C57BL/6J mice after oral administration. The synthesis and structure–activity relationships of the novel diarylketoxime MCH-1R antagonists are described.     

C-terminal modifications of an endothelin antagonist RES-701-1: Production of ETA/ETB dual selective analogs

RES-701-1 is an endothelin B receptor (ET_B) selective peptidic antagonist, which has a novel cyclic structure of microbial origin. Modification at the C-terminal free carboxyl group of RES-701-1 by a methyl ester results in an ET_A/ET_B dual selective analog, which showed relatively high affinity for ET_A receptor subtype, while retaining the affinity for ET_B. The carboxyl-group-deleted analog with tryptamine as the C-terminal residue also showed relatively weak affinity for ET_A; however, benzyl ester or amide analogs did not show remarkable affinity for ET_A. It is suggested that the binding mode of RES-701-1 and its analogs is different from those of known ligands for ET receptors.