2-Cyclohexylcarbonylbenzimidazoles as potent,orally available and brain-penetrable opioid receptor-like 1 (ORL1) antagonists

The synthesis and biological evaluation of new potent opioid receptor-like 1 (ORL1) antagonists are presented. Conversion of the thioether linkage of the prototype [It is reported prior to this communication as a consecutive series.: Kobayashi, K.; Kato, T.; Yamamoto, I.; Shimizu, A.; Mizutani, S.; Asai, M.; Kawamoto, H.; Ito, S.; Yoshizumi, T.; Hirayama, M.; Ozaki, S.; Ohta, H.; Okamoto, O. Bioorg. Med. Chem. Lett., in press] to the carbonyl linker effectively reduces susceptibility to P-glycoprotein (P-gp) efflux. This finding led to the identification of 2-cyclohexylcarbonylbenzimizole analogue 7c, which exhibited potent ORL1 activity, excellent selectivity over other receptors and ion channels, and poor susceptibility to P-gp. Compound 7c also showed satisfactory pharmacokinetic profiles and brain penetrability in laboratory animals. Furthermore, 7c showed good in vivo antagonism. Hence, 7c was selected as a clinical candidate for a brain-penetrable ORL1 antagonist.     

Identification of MK-1925: A selective,orally active and brain-penetrable opioid receptor-like 1 (ORL1) antagonist

Structure–activity relationship studies directed toward improving the metabolic stability of compound 1 resulted in the identification of 3-[5-(3,5-difluorophenyl)-3-({[(1S,3R)-3-fluorocyclopentyl]amino}methyl)-4-methyl-1H-pyrazol-1-yl]propanenitrile 39 (MK-1925) as a selective, orally available and brain-penetrable opioid receptor-like 1 (ORL1) antagonist. The compound also showed in vivo efficacy after oral dosing. Therefore, compound 39 was selected to undergo further studies as a clinical candidate.     

Discovery of novel arylpyrazole series as potent and selective opioid receptor-like 1 (ORL1) antagonists

The synthesis and biological evaluation of new potent opioid receptor-like 1 antagonists are presented. A structure–activity relationship (SAR) study of arylpyrazole lead compound 1 obtained from library screening identified compound 31, (1S,3R)-N-{[1-(3-chloropyridin-2-yl)-5-(5-fluoro-6-methylpyridin-3-yl)-4-methyl-1H-pyrazol-3-yl]methyl}-3-fluorocyclopentanamine, which exhibits high intrinsic potency and selectivity against other opioid receptors and hERG potassium channel.     

Pharmacological characterization of the nociceptin receptor, ORL1

A novel opioid receptor-like orphan receptor (ORL1) was cloned and identified to be homologous to classical opioid receptors but insensitive to traditional opioids. A heptadecapeptide, termed orphanin FQ or nociceptin (OFQ/N), was identified as its endogenous ligand. OFQ/N shares overlapping distribution sites in pain-processing areas and common cellular mechanisms with opioids but exerts diverse effects on nociceptive responses. Of the two reported ORL1 antagonists, [Phe(1)psi(CH(2)-NH)- Gly(2)] nociceptin-(1-13)-NH(2) (Phepsi) and naloxone benzoylhydrazone (NBZ), antagonisms were validated in the activation of inward rectifying K channels induced by OFQ/N, using the patch clamp technique in ventrolateral periaqueductal gray slices. Results showed that Phepsi acted as a partial agonist and NBZ was a weak nonselective antagonist of ORL1. It is comparable with most but not all of the findings from other tissues. Comparing all the reports supports the above inference for these two antagonists. The possible causes for the discrepancy were discussed. A brief review on the putative ORL1 antagonists, acetyl-RYYRIK-NH2, some sigma-ligands and the functional antagonist, nocistatin, is also included. It indicates that a potent and selective ORL1 antagonist is expecting to elucidate the physiological role of OFQ/N.     

Tritium-labelled isovaleryl-RYYRIK-NH2 as potential antagonist probe for ORL1 nociceptin receptor

IsoVa-RYYRIK-NH₂ is a highly specific antagonist ligand of the opioid receptor-like 1 (ORL1) receptor, an endogenous ligand of which is 17-mer peptide nociceptin. ORL1 antagonists have potential for clinical use as analgesic and antineuropathic drugs, and thus information on the receptor-binding characteristics of antagonists is very important for rational drug design. In the present study, we prepared tritium-labelled isova-RYYRIK-NH₂ from its precursor with the 3-methylcrotonyl (CH₃)₂CCHCO group by a catalytic reduction using tritium gas. The resulting [³H]isoVa-RYYRIK-NH₂ was evaluated in a saturation binding assay using the COS-7 cell membrane preparations of transiently expressed ORL1. It exhibited more than 90% specific binding with a dissociation constant of 1.21 ± 0.03 nM. From the mutual heterologous binding assays using [³H]isoVa-RYYRIK-NH₂ and [³H]nociceptin, isoVa-RYYRIK-NH₂ and nociceptin were found to share the receptor-binding site, but each also had a separate specific binding site of its own. They differentiated the two different binding states or conformations of ORL1, which might represent the agonist-active and antagonist-inactive conformations of ORL1. [³H]isoVa-RYYRIK-NH₂ is thus a key tracer to uncover the amino acid residues important for receptor inactivation.     

Synthesis and biological evaluation of imidazole derivatives as novel NOP/ORL1 receptor antagonists: Exploration and optimization of alternative pyrazole structure

Nonpeptidic small-molecule NOP/ORL1 receptor antagonists with an imidazole scaffold were designed and synthesized to investigate alternatives to the pyrazole analog. Systematic modification of the original pyrazole lead [Kobayashi et al., Bioorg. Med. Chem. Lett. 2009, 19, 3627; Kobayashi et al., Bioorg. Med. Chem. Lett., in press] to change the heterocyclic core, substituted side chain, and pendant functional group demonstrated that examining the structure–activity relationship for novel templates allowed the identification of potent, fully substituted 4-aminomethyl-1H-imidazole and 2-aminomethyl-1H-imidazole. These compounds exhibited excellent potency for ORL1 receptor with minimal P-gp efflux and/or reduced hERG affinity.     

孤啡肽受体的配体研究进展

孤啡肽(nociceptin或orphanin FQ)发现于1995年底, 它是阿片受体样受体(ORL1或LC 132) 的内源性配体,在痛觉调节、心血管系统、离子通道、依赖和耐受、学习和记忆等方面具有广泛的生物学活性. 最近几年, 对孤啡肽受体与相关配体构效关系的研究成为一个新的热点.对在研究构效关系过程中所发现的孤啡肽受体相关配体（片段、拮抗剂、激动剂、部分激动剂和阻断剂）的研究情况进行了介绍.     

Discovery and structure–activity relationships of 4-aminoquinazoline derivatives,a novel class of opioid receptor like-1 (ORL1) antagonists

Synthesis and structure–activity relationship studies of a series of 4-aminoquinazoline derivatives led to the identification of (1R,2S)-17, N-[(1R,2S)-2-({2-[(4-chlorophenyl)carbonyl]amino-6-methylquinazolin-4-yl}amino)cyclohexyl]guanidine dihydrochloride, as a highly potent ORL1 antagonist with up to 3000-fold selectivity over the μ, δ, and κ opioid receptors. Molecular modeling clarified the structural factors contributing to the high affinity and selectivity of (1R,2S)-17.     

Heterodimerization of ORL1 and Opioid Receptors and Its Consequences for N-type Calcium Channel Regulation

We have investigated the heterodimerization of ORL1 receptors and classical members of the opioid receptor family. All three classes of opioid receptors could be co-immunoprecipitated with ORL1 receptors from both transfected tsA-201 cell lysate and rat dorsal root ganglia lysate, suggesting that these receptors can form heterodimers. Consistent with this hypothesis, in cells expressing either one of the opioid receptors together with ORL1, prolonged ORL1 receptor activation via nociceptin application resulted in internalization of the opioid receptors. Conversely, μ-, δ-, and κ-opioid receptor activation with the appropriate ligands triggered the internalization of ORL1. The μ-opioid receptor/ORL1 receptor heterodimers were shown to associate with N-type calcium channels, with activation of μ-opioid receptors triggering N-type channel internalization, but only in the presence of ORL1. Furthermore, the formation of opioid receptor/ORL1 receptor heterodimers attenuated the ORL1 receptor-mediated inhibition of N-type channels, in part because of constitutive opioid receptor activity. Collectively, our data support the existence of heterodimers between ORL1 and classical opioid receptors, with profound implications for effectors such as N-type calcium channels.     

In Silico Study of the Structurally Similar ORL1 Receptor Agonist and Antagonist Pairs Reveal Possible Mechanism of Receptor Activation

An opioid receptor like (ORL1) receptor is a member of a family of G-protein coupled receptors. It is a new pharmaceutical target with broad therapeutic potential in the regulation of important biological functions such as nociception, mood disorders, drug abuse, learning or cardiovascular control. The crystal structure of this receptor in complex with an antagonist was determined recently (PDB ID: 4EA3). By removing the ligand and subjecting the empty receptor to molecular dynamics simulation in a solvated lipid membrane we obtained an optimized ORL1 receptor structure which could be used in a subsequent docking study of two structurally similar agonist–antagonist ligand pairs. Ligands were docked to the empty ORL1 receptor (with and without the third intracellular loop, IC3) in different orientations, and the resulting complexes were monitored during molecular dynamics simulation in order to see how the subtle differences in structure of agonists and antagonists might affect ligand–receptor interactions and trigger receptor activation. It was established that agonists and antagonists bound to the same, relatively large, binding site in the receptor, created by residues from transmembrane helices TM2, TM3, TM5, TM6 and TM7 and close to the extra cellular end of the receptor bundle. The key difference between these two types of ligands is interaction with residue Val283^6.55 and a flexibility of ligand molecules. Ligands that cannot easily avoid this interaction will initiate movement of the intracellular end of TM6 (by a mechanism which involves Met134^3.36 and several aminoacids of TM5) and possibly activate the receptor when assisted by G-protein.     

Ligands for kappa-opioid and ORL1 receptors identified from a conformationally constrained peptide combinatorial library.

We have screened a synthetic peptide combinatorial library composed of 2 x 10(7) beta-turn-constrained peptides in binding assays on four structurally related receptors, the human opioid receptors mu, delta, and kappa and the opioid receptor-like ORL1. Sixty-six individual peptides were synthesized from the primary screening and tested in the four receptor binding assays. Three peptides composed essentially of unnatural amino acids were found to show high affinity for human kappa-opioid receptor. Investigation of their activity in agonist-promoted stimulation of [(35)S]guanosine 5'-3-O-(thio)triphosphate binding assay revealed that we have identified the first inverse agonist as well as peptidic antagonists for kappa-receptors. To fine-tune the potency and selectivity of these kappa-peptides we replaced their turn-forming template by other turn mimetic molecules. This "turn-scan" process allowed the discovery of compounds with modified selectivity and activity profiles. One peptide displayed comparable affinity and partial agonist activity toward all four receptors. Interestingly, another peptide showed selectivity for the ORL1 receptor and displayed antagonist activity at ORL1 and agonist activity at opioid receptors. In conclusion, we have identified peptides that represent an entirely new class of ligands for opioid and ORL1 receptors and exhibit novel pharmacological activity. This study demonstrates that conformationally constrained peptide combinatorial libraries are a rich source of ligands that are more suitable for the design of nonpeptidal drugs.     

Design,synthesis and biological evaluation of N-hydroxy-aminobenzyloxyarylamide analogues as novel selective κ opioid receptor antagonists

Aminobenzyloxyarylamide derivatives 1a-i and 2a-t were designed and synthesized as novel selective κ opioid receptor (KOR) antagonists. The benzoyl amide moiety of LY2456302 was changed into N-hydroxybenzamide and benzisoxazole-3(2H)-one to investigate whether it could increase the binding affinity or selectivity for KOR. All target compounds were evaluated in radioligand binding assays for opioid receptor binding affinity. These efforts led to the identification of compound 1c (κ K_i = 179.9 nM), which exhibited high affinity for KOR. Moreover, the selectivity of KOR over MOR and DOR increased nearly 2-fold and 7-fold, respectively, compared with (±)LY2456302.     

Capturing of the free cysteine residue in the ligand-binding site by affinity labeling of the ORL1 nociceptin receptor

All of the δ, μ, and κ opioid receptors have a free thiol group of the Cys residue in the ligand-binding site, although its functional role is not yet known. In order to examine whether or not a similar Cys is also present in the ORL1 nociceptin receptor, we attempted to identify it by affinity labeling using a specific antagonist peptide. We first treated ORL1-expressing COS-7 cell membrane preparations with the thiol-alkylation reagent N-ethylmaleimide (NEM) to perform a binding assay using [³H]nociceptin as a tracer and nociceptin, an ORL1 agonist, or Ac-Arg-Tyr-Tyr-Arg-Ile-Lys-NH₂, a nociceptin/ORL1 antagonist, as a competitor. It was suggested that ORL1 has a free Cys in its ligand-binding site, since the NEM treatment reduced the population of ligand-binding sites. This was further confirmed by affinity labeling using Cys(Npys)-Arg-Tyr-Tyr-Arg-Ile-Lys-NH₂ with the SNpys group that can react with a free thiol group, resulting in the formation of a disulfide bond. This affinity labeling was approximately 23 times more specific than NEM alkylation. The results revealed that the ORL1 nociceptin receptor does contain a free Cys residue in the ligand-binding site.     

Opioid receptor-like 1 (ORL1) receptor binding and the biological properties of Ac-Arg-Tyr-Tyr-Arg-Ile-Arg-NH2 and its analogs.

Hexapeptides such as Ac-Arg-Tyr-Tyr-Arg-Ile-Lys-NH(2) and Ac-Arg-Tyr-Tyr-Arg-Trp-Arg-NH(2) have been isolated from a combinatorial peptide library as small peptide ligands for the opioid peptide-like 1 (ORL1) receptor. To investigate the detailed structural requirements of hexapeptides, 25 analogs of these hexapeptides, based on the novel analog Ac-Arg-Tyr-Tyr-Arg-Ile-Arg-NH(2) (1), were synthesized and tested for their ORL1 receptor affinity and agonist/antagonist activity on mouse vas deferens (MVD) tissues. Analog 1 and its Cit(6)-analog (10) were found to possess high affinity to the ORL1 receptor, comparable to that of nociceptin/orphanin FQ, and exhibited potent antagonist activity (pA(2) values of 7.77 for 1 and 7.51 for 10, which are higher than that of [NPhe(1)]nociceptin(1-13)-NH(2) (6.90) on MVD assay. It was also found that the amino acid residue in position 5 plays a key role in agonist/antagonist activity, i.e. an L-configuration aliphatic amino acid is required for potent antagonist activity, while a nonchiral or D-configuration residue produces potent agonist activity. These lines of evidence may provide insight into the mechanisms controlling agonist/antagonist switching in the ORL1 receptor, and may also serve to help developing more potent ORL1 agonists and antagonists.     

Discovery of N-substituted-endo-3-(8-aza-bicyclo[3.2.1]oct-3-yl)-phenol and -phenyl carboxamide series of μ-opioid receptor antagonists

Gastrointestinal dysfunction as a consequence of the use of opioid analgesics is of significant clinical concern. First generation drugs to treat these opioid-induced side-effects were limited by their negative impact on opioid receptor agonist-induced analgesia. Second generation therapies target a localized, peripherally-restricted, non-CNS penetrant drug distribution of opioid receptor antagonists. Herein we describe the discovery of the N-substituted-endo-3-(8-aza-bicyclo[3.2.1]oct-3-yl)-phenol and -phenyl carboxamide series of μ-opioid receptor antagonists. This report highlights the discovery of the key μ-opioid receptor antagonist pharmacophore and the optimization of in vitro metabolic stability through the application of a phenol bioisostere. The compounds 27a and 31a with the most attractive in vitro profile, formed the basis for the application of Theravance Biopharma’s multivalent approach to drug discovery to afford the clinical compound axelopran (TD-1211), targeted for the treatment of opioid-induced constipation.     

Synthesis and inhibitory activity on hepatitis C virus RNA replication of 4-(1,1,1,3,3,3-hexafluoro-2-hydroxy-2-propyl)aniline analogs

Using our recently developed assay system for full-genome-length hepatitis C virus (HCV) RNA replication in human hepatoma-derived Li23 cells (ORL8), we identified 4-(1,1,1,3,3,3-hexafluoro-2-hydroxy-2-propyl)aniline analog 1a as a novel HCV inhibitor. Structural modifications of 1a provided a series of sulfonamides 7 with much more potent HCV RNA replication-inhibitory activity than ribavirin. Compound 7a showed an additive anti-HCV effect in combination with standard anti-HCV therapy (IFN-α plus ribavirin). Since 7a generated reactive oxygen species (ROS) in the ORL8 system and its anti-HCV activity was blocked by vitamin E, its anti-HCV activity may be mediated at least in part by ROS.     

14-O-Heterocyclic-substituted naltrexone derivatives as non-peptide mu opioid receptor selective antagonists: Design,synthesis, and biological studies

Mu opioid receptor antagonists have clinical utility and are important research tools. To develop non-peptide and highly selective mu opioid receptor antagonist, a series of 14-O-heterocyclic-substituted naltrexone derivatives were designed, synthesized, and evaluated. These compounds showed subnanomolar-to-nanomolar binding affinity for the mu opioid receptor. Among them, compound 1 exhibited the highest selectivity for the mu opioid receptor over the delta and kappa receptors. These results implicated an alternative ‘address’ domain in the extracellular loops of the mu opioid receptor.     

1-Methyl-1H-pyrrole-2-carbonitrile containing tetrahydronaphthalene derivatives as non-steroidal progesterone receptor antagonists

Non-steroidal 1-methyl-1H-pyrrole-2-carbonitrile containing tetrahydronaphthalenes and acyclic derivatives were evaluated as novel series of progesterone receptor (PR) antagonists using the T47D cell alkaline phosphatase assay. Moderate to potent PR antagonists were achieved with these scaffolds. Several compounds (e.g., 15 and 20) demonstrated low nanomolar PR antagonist potency and good selectivity versus other steroid receptors.     

Identification of a hexapeptide binding region in the nociceptin (ORL1) receptor by photo-affinity labelling with Ac-Arg-Bpa-Tyr-Arg-Trp-Arg-NH2

The interaction of Ac-Arg-Tyr-Tyr-Arg-Trp-Arg-NH₂ (HP1), a high-affinity partial agonist of the opioid receptor like (ORL1) receptor, has been investigated using the photo-labile analogue [p-benzoyl-l-Phe (Bpa)²]-HP1. In recombinant CHO cells expressing the human ORL1 receptor, [Bpa²]-HP1 binds the receptor with high affinity (K; ∼3 nM) and is as potent as HP1 in stimulating GTPγS binding (50-60% of nociceptin maximal effect). UV irradiation at 365 nm of the complex formed by the ORL1 receptor and radio-iodinated [Bpa²]-HP1 results in the irreversible labelling of a glycoprotein of M_r∼66 kDa, as determined by SDS-PAGE. Cyanogen bromide (CNBr) and enzymatic footprints of the photo-labelled receptor and an engineered receptor mutant (L113M), containing an additional CNBR cleavage site, allowed the photoreactive region to be identified as ORL1[107-113] at the C-terminal of TM helix II. In addition the presence of a disulphide bridge between Cysl23 and Cys200 has been confirmed biochemically.     

Design,synthesis, and opioid activity of arodyn analogs cyclized by ring-closing metathesis involving Tyr(allyl)

Kappa (κ) opioid receptor selective antagonists are useful pharmacological tools in studying κ opioid receptors and have potential to be used as therapeutic agents for the treatment of a variety of diseases including mood disorders and drug addiction. Arodyn (Ac[Phe^1–3,Arg⁴,d-Ala⁸]Dyn A-(1–11)NH₂) is a linear acetylated dynorphin A (Dyn A) analog that is a potent and selective κ opioid receptor antagonist (Bennett et al. J Med Chem 2002;45:5617–5619) and prevents stress-induced reinstatement of cocaine-seeking behavior following central administration (Carey et al. Eur J Pharmacol 2007;569:84–89). To restrict its conformational mobility, explore possible bioactive conformations and potentially increase its metabolic stability we synthesized cyclic arodyn analogs on solid phase utilizing a novel ring-closing metathesis (RCM) reaction involving allyl-protected Tyr (Tyr(All)) residues. This approach preserves the aromatic functionality and directly constrains the side chains of one or more of the Phe residues. The novel cyclic arodyn analog 4 cyclized between Tyr(All) residues incorporated in positions 2 and 3 exhibited potent κ opioid receptor antagonism in the [³⁵S]GTPγS assay (K_B?=?3.2?nM) similar to arodyn. Analog 3 cyclized between Tyr(All) residues in positions 1 and 2 also exhibited nanomolar κ opioid receptor antagonist potency (K_B?=?27.5?nM) in this assay. These are the first opioid peptides cyclized via RCM involving aromatic residues, and given their promising pharmacological activity represent novel lead peptides for further exploration.