Opioid receptor affinity and selectivity effects of second residue and carboxy terminal residue variation in a cyclic disulfide-containing opioid tetrapeptide.

The previously described cyclic, delta opioid receptor-selective tetrapeptide H-Tyr-D-Cys-Phe-D-Pen-OH, where Pen, penicillamine, is beta-beta-dimethylcysteine, was modified at residues 2 and 4 by varying combinations of D- and L-Cys and D- and L-Pen, and effects on mu and delta opioid receptor binding affinities and on potency in the mouse vas deferens (MVD) smooth muscle assay were evaluated. A comparison was drawn between consequences of alterations in this series of analogs and those of analogous modifications in the related cyclic pentapeptide series which includes the highly delta receptor-selective [D-Pen2,D-Pen5]enkephalin, DPDPE. Unlike effects observed in the cyclic pentapeptide series, the mu receptor binding affinities of the cyclic tetrapeptides are not dramatically influenced by substitution of Pen for Cys at residue 2. Conversely, while binding of the pentapeptides is only slightly affected by alteration of the chirality of the carboxy-terminal residue, modification of stereochemistry at the carboxy terminus in the tetrapeptides critically alters binding behavior at both mu and delta sites. In contrast with the pentapeptide series, the tetrapeptides appear to be highly dependent upon primary sequence for binding and activity, as only the lead compound binds with high affinity to the delta site. Results suggest that the less flexible cyclic tetrapeptides, lacking the Gly3 residue, display more stringent structural requirements for binding and activity than do the corresponding cyclic pentapeptides.     

Opioid receptor selectivity profile change via isosterism for 14-O-substituted naltrexone derivatives

Isosterism is commonly used in drug discovery and development to address stability, selectivity, toxicity, pharmacokinetics, and efficacy issues. A series of 14-O-substituted naltrexone derivatives were identified as potent mu opioid receptor (MOR) antagonists with improved selectivity over the kappa opioid receptor (KOR) and the delta opioid receptor (DOR), compared to naltrexone. Since esters are not metabolically very stable under typical physiological conditions, their corresponding amide analogs were thus synthesized and biologically evaluated. Unlike their isosteres, most of these novel ligands seem to be dually selective for the MOR and the KOR over the DOR. The restricted flexibility of the amide bond linkage might be responsible for their altered selectivity profile. However, the majority of the 14-N-substituted naltrexone derivatives produced marginal or no MOR stimulation in the ³⁵S-GTP[γS] assay, which resembled their ester analogs. The current study thus indicated that the 14-substituted naltrexone isosteres are not bioisosteres since they have distinctive pharmacological profile with the regard to their opioid receptor binding affinity and selectivity.     

Cyclic side-chain-linked opioid analogs utilizing cis- and trans-4-aminocyclohexyl-d-alanine

Cyclization of linear sequences is a well recognized tool in opioid peptide chemistry for generating analogs with improved bioactivities. Cyclization can be achieved through various bridging bonds between peptide ends or side-chains. In our earlier paper we have reported the synthesis and biological activity of a cyclic peptide, Tyr-c[d-Lys-Phe-Phe-Asp]NH₂ (1), which can be viewed as an analog of endomorphin-2 (EM-2, Tyr-Pro-Phe-Phe-NH₂). Cyclization was achieved through an amide bond between side-chains of d-Lys and Asp residues. Here, to increase rigidity of the cyclic structure, we replaced d-Lys with cis- or trans-4-aminocyclohexyl-d-alanine (d-ACAla). Two sets of analogs incorporating either Tyr or Dmt (2′,6′-dimethyltyrosine) residues in position 1 were synthesized. In the binding studies the analog incorporating Dmt and trans-d-ACAla showed high affinity for both, μ- and δ-opioid receptors (MOR and DOR, respectively) and moderate affinity for the κ-opioid receptor (KOR), while analog with Dmt and cis-d-ACAla was exceptionally MOR-selective. Conformational analyses by NMR and molecular docking studies have been performed to investigate the molecular structural features responsible for the noteworthy MOR selectivity.     

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.     

Cyclic biphalin analogues with a novel linker lead to potent agonist activities at mu,delta, and kappa opioid receptors

In an effort to improve biphalin’s potency and efficacy at the µ-(MOR) and δ-opioid receptors (DOR), a series of cyclic biphalin analogues 1–5 with a cystamine or piperazine linker at the C-terminus were designed and synthesized by solution phase synthesis using Boc-chemistry. Interestingly, all of the analogues showed balanced opioid agonist activities at all opioid receptor subtypes due to enhanced κ-opioid receptor (KOR) activity. Our results indicate that C-terminal flexible linkers play an important role in KOR activity compared to that of the other cyclic biphalin analogues with a hydrazine linker. Among them, analogue 5 is a potent (Ki?=?0.27, 0.46, and 0.87?nM; EC₅₀?=?3.47, 1.45, and 13.5?nM at MOR, DOR, and KOR, respectively) opioid agonist with high efficacy. Based on the high potency and efficacy at the three opioid receptor subtypes, the ligand is expected to have a potential synergistic effect on relieving pain and further studies including in vivo tests are worthwhile.     

Effects of antisense oligodeoxynucleotides against opioid receptors on the receptor mRNA contents

It was demonstrated in the previous study that the microinjection of antisense oligodeoxynucleotide (AS ODN) against mu-opioid receptor (MOR) into periaqueductal gray (PAG) of rat brain selectively decreased the MOR mRNA content in PAG, and the decrease in MOR mRNA content was enhanced by pretreatment of the PAG with MOR AS ODN. In the present investigation, effects of the pretreatment of PAG with AS ODN against kappa- or delta-opioid receptor (KOR or DOR) on the decrease in the MOR mRNA content induced by MOR AS ODN were examined. Both KOR and DOR AS ODNs significantly decreased the target mRNA contents, while they did not significantly change MOR mRNA content. The decrease in MOR mRNA content induced by MOR AS ODN, however, was significantly enhanced by the pretreatment of PAG with either KOR or DOR AS ODNs. Results show that the AS ODN has both the specific target mRNA decreasing action and the nonspecific enhancing action on the AS-induced decrease in the mRNA content.     

An LP1 analogue,selective MOR agonist with a peculiar pharmacological profile,used to scrutiny the ligand binding domain

The hypothesis that central analgesia with reduced side effects is obtainable by occupying an ‘allosteric’ site in the MOR ligand binding domain requires the development of new ligands with peculiar pharmacological profile to be used as tools. New benzomorphan derivatives, analogues of LP1, a multitarget MOR agonist/DOR antagonist, were designed to examine in depth MOR ligand binding domain. Compound 5, bearing a diphenylic N-substituent on the benzomorphan nucleus, showed an affinity (K_i^μ = 0.5 ± 0.2 nM) comparable to that of LP1 and a better selectivity versus DOR and KOR. It elicits antinociceptive effects in ex vivo (GPI) and in vivo. This new compound engages receptor amino acidic residues not reached by LP1 and by other established MOR ligands. Molecular modeling studies, conducted on 5 and on several reference compounds, allowed us to propose possible residues in the MOR ligand binding domain essential for their interactions with ‘orthosteric’ and ‘allosteric’ binding sites.     

Development of highly potent and selective dynorphin A analogues as new medicines.

Dynorphin A (Dyn A), a 17 amino acid peptide H-Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln-OH, is a potent opioid peptide which interacts preferentially with kappa-opioid receptors. Research in the development of selective and potent opioid peptide ligands for the kappa-receptor is important in mediating analgesia. Several cyclic disulphide bridge-containing peptide analogues of Dyn A, which were conformationally constrained in the putative message or address segment of the opioid ligand, were designed, synthesized and assayed. To further investigate the conformational and topographical requirements for the residues in positions 5 and 11 of these analogues, a systematic series of Dyn A(1-11)-NH2 cyclic analogues incorporating the sulphydryl-containing amino acids L- and D-Cys and L- and D-Pen in positions 5 and 11 were synthesized and assayed. Cyclic lactam peptide analogues were also synthesized and assayed. Several of these cyclic analogues, retained the same affinity and selectivity (vs. the mu- and delta-receptors) as the parent Dyn A(1-11)-NH2 peptide in the guinea-pig brain (GPB), but exhibited a much lower activity in the guinea-pig ileum (GPI), thus leading to centrally vs. peripherally selective peptides. Studies of the structure-activity relationship of Dyn A peptide provide new insights into the importance of each amino acid residue (and their configurations) in Dyn A analogues for high potency and good selectivity at kappa-opioid receptors. We report herein the progress towards the development of Dyn A peptide ligands, which can act as agonists or antagonists at cell surface receptors that modulate cell function and animal behaviour using various approaches to rational peptide ligand-based drug design.     

Co-expressions of different opioid receptor types differentially modulate their signaling via G(16)

Combinations of two different types of opioid receptors - delta-, kappa-, mu-opioid receptors (DOR, KOR, and MOR) and opioid receptor-like receptor 1 (ORL(1)) - were co-expressed with the alpha subunit of G(16) in COS-7 cells, and the ability of various selective agonists to induce activation of phospholipase Cbeta was examined. Nociceptin/orphanin FQ-induced response was enhanced when ORL(1) was co-expressed with MOR or KOR but not DOR. The kappa-agonist U50,488H induced a modest inositol phosphate formation when KOR was expressed alone or with MOR, but the response was attenuated when co-expressing with either DOR or ORL(1). It is suggested that the co-expressions of two different opioid receptor types indeed modify their downstream signaling events.     

Synthesis and evaluation of 4-substituted piperidines and piperazines as balanced affinity μ opioid receptor (MOR) agonist/δ opioid receptor (DOR) antagonist ligands

In this letter, we describe a series of 4-substituted piperidine and piperazine compounds based on tetrahydroquinoline 1, a compound that shows balanced, low nanomolar binding affinity for the mu opioid receptor (MOR) and the delta opioid receptor (DOR). We have shown that by changing the length and flexibility profile of the side chain in this position, binding affinity is improved at both receptors by a significant degree. Furthermore, several of the compounds described herein display good efficacy at MOR, while simultaneously displaying DOR antagonism. The MOR agonist/DOR antagonist has shown promise in the reduction of negative side effects displayed by selective MOR agonists, namely the development of dependence and tolerance.     

mRNA display selection and solid‐phase synthesis of Fc‐binding cyclic peptide affinity ligands

Cyclic peptides are attractive candidates for synthetic affinity ligands due to their favorable properties, such as resistance to proteolysis, and higher affinity and specificity relative to linear peptides. Here we describe the discovery, synthesis and characterization of novel cyclic peptide affinity ligands that bind the Fc portion of human Immunoglobulin G (IgG; hFc). We generated an mRNA display library of cyclic pentapeptides wherein peptide cyclization was achieved with high yield and selectivity, using a solid‐phase crosslinking reaction between two primary amine groups, mediated by a homobifunctional linker. Subsequently, a pool of cyclic peptide binders to hFc was isolated from this library and chromatographic resins incorporating the selected cyclic peptides were prepared by on‐resin solid‐phase peptide synthesis and cyclization. Significantly, this approach results in resins that are resistant to harsh basic conditions of column cleaning and regeneration. Further studies identified a specific cyclic peptide—cyclo[Link‐M‐WFRHY‐K]—as a robust affinity ligand for purification of IgG from complex mixtures. The cyclo[Link‐M‐WFRHY‐K] resin bound selectively to the Fc fragment of IgG, with no binding to the Fab fragment, and also bound immunoglobulins from a variety of mammalian species. Notably, while the recovery of IgG using the cyclo[Link‐M‐WFRHY‐K] resin was comparable to a Protein A resin, elution of IgG could be achieved under milder conditions (pH 4 vs. pH 2.5). Thus, cyclo[Link‐M‐WFRHY‐K] is an attractive candidate for developing a cost‐effective and robust chromatographic resin to purify monoclonal antibodies (mAbs). Finally, our approach can be extended to efficiently generate and evaluate cyclic peptide affinity ligands for other targets of interest. Biotechnol. Bioeng. 2013; 110: 857–870. © 2012 Wiley Periodicals, Inc.     

mu-Opioid receptors desensitize less rapidly than delta-opioid receptors due to less efficient activation of arrestin

Receptor desensitization by G-protein receptor kinases (GRK) and arrestins is likely to be an important component underlying the development of tolerance to opioid drugs. Reconstitution of this process in Xenopus oocytes revealed distinct differences in the kinetics of GRK and arrestin regulation of the closely related opioid receptors mu (MOR), delta (DOR), and kappa (KOR). We demonstrated that under identical conditions, GRK and arrestin-dependent desensitization of MOR proceeds dramatically slower than that of DOR. Furthermore, GRK3 phosphorylation sites required for opioid receptor desensitization also greatly differ. The determinants for DOR and KOR desensitization reside in the carboxyl-terminal tail, whereas MOR depends on Thr-180 in the second intracellular loop. Although this later finding might indicate an inefficient phosphorylation of MOR Thr-180, increasing the amount of arrestin expressed greatly increased the rate of MOR desensitization to a rate comparable with that of DOR. Similarly, coexpression of a constitutively active arrestin 2(R169E) with MOR and DOR desensitized both receptors in an agonist-dependent, GRK-independent manner at rates that were indistinguishable. Together, these data suggest that it is the activation of arrestin, rather than its binding, that is the rate-limiting step in MOR desensitization. In addition, mutation of Thr-161 in DOR, homologous to MOR Thr-180, significantly inhibited the faster desensitization of DOR. These results suggest that DOR desensitization involves phosphorylation of both the carboxyl-terminal tail and the second intracellular loop that together leads to a more efficient activation of arrestin and thus faster desensitization.     

‘Carba’-carfentanil (trans isomer): A μ opioid receptor (MOR) partial agonist with a distinct binding mode

There is strong evidence to indicate that a positively charged nitrogen of endogenous and exogenous opioid ligands forms a salt bridge with the Asp residue in the third transmembrane helix of opioid receptors. To further examine the role of this electrostatic interaction in opioid receptor binding and activation, we synthesized ‘carba’-analogues of the highly potent μ opioid analgesic carfentanil (3), in which the piperidine nitrogen was replaced with a carbon. The resulting trans isomer (8b) showed reduced, but still significant MOR binding affinity (K_i^μ = 95.2 nM) with no MOR versus DOR binding selectivity and was a MOR partial agonist. The cis isomer (8a) was essentially inactive. A MOR docking study indicated that 8b bound to the same binding pocket as parent 3, but its binding mode was somewhat different. A re-evaluation of the uncharged morphine derivative N-formylnormorphine (9) indicated that it was a weak MOR antagonist showing no preference for MOR over KOR. Taken together, the results indicate that deletion of the positively charged nitrogen in μ opioid analgesics reduces MOR binding affinity by 2–3 orders of magnitude and may have pronounced effects on the intrinsic efficacy and on the opioid receptor selectivity profile.     

Role of Opioid Receptors Signaling in Remote Electrostimulation - Induced Protection against Ischemia/Reperfusion Injury in Rat Hearts

Aims

Our previous studies demonstrated that remote electro-stimulation (RES) increased myocardial GSK3 phosphorylation and attenuated ischemia/ reperfusion (I/R) injury in rat hearts. However, the role of various opioid receptors (OR) subtypes in preconditioned RES-induced myocardial protection remains unknown. We investigated the role of OR subtype signaling in RES-induced cardioprotection against I/R injury of the rat heart.

Methods & Results

Male Spraque-Dawley rats were used. RES was performed on median nerves area with/without pretreatment with various receptors antagonists such as opioid receptor (OR) subtype receptors (KOR, DOR, and MOR). The expressions of Akt, GSK3, and PKCε expression were analyzed by Western blotting. When RES was preconditioned before the I/R model, the rat''s hemodynamic index, infarction size, mortality and serum CK-MB were evaluated. Our results showed that Akt, GSK3 and PKCε expression levels were significantly increased in the RES group compared to the sham group, which were blocked by pretreatment with specific antagonists targeting KOR and DOR, but not MOR subtype. Using the I/R model, the duration of arrhythmia and infarct size were both significantly attenuated in RES group. The mortality rates of the sham RES group, the RES group, RES group + KOR antagonist, RES group + DOR/MOR antagonists (KOR left), RES group + DOR antagonist, and RES group + KOR/MOR antagonists (DOR left) were 50%, 20%, 67%, 13%, 50% and 55%, respectively.

Conclusion

The mechanism of RES-induced myocardial protection against I/R injury seems to involve multiple target pathways such as Akt, KOR and/or DOR signaling.     

Structure-function studies of linear and cyclized peptide antagonists of the GnRH receptor.

Structurally new analogs of the peptidic GnRH receptor antagonist Cetrorelix as well as conformationally constrained cyclized deca- or pentapeptides were synthesized and selected peptides evaluated comprehensively. To understand how structural variations of the antagonistic peptide effect pharmacodynamic properties, binding affinities and antagonistic potencies toward the human and rat GnRH receptor were determined. Whereas large substituents in position 6 of linear peptides are compatible with high binding affinity (K(D) < 0.5 nM), all cyclized peptides except the cyclo[3-10] analog D-52391 depicted low binding affinity (K(D) > 10 nM). Binding affinity and antagonistic potency in vitro correlated for all peptides and surprisingly no discrimination between human and rat receptor proteins was observed. Since receptor residues W(101) and N(102) are involved in agonist and antagonist binding, equally potent but structurally different antagonists were tested for binding to the respective W(101)A and N(102)A mutants. In contrast to linear decapeptides, residues N(102) and W(101) are not involved in binding of D-23938 and W(101) is the critical residue for D-52391 binding. We conclude that although equally potent, peptidic GnRH receptor antagonists do have distinct interactions within the ligand binding pocket. Finally, selected antagonists were tested for testosterone suppression in male rats. The duration of testosterone suppression below castration levels differed largely from 1 day for Ganirelix to 27 days for D-23487. Systemic availability became evident as the most important parameter for in vivo efficacy.     

Evaluation of N-substituent structural variations in opioid receptor profile of LP1

The benzomorphan scaffold has great potential as lead structure and the nature of the N-substituent is able to influence affinity, potency, and efficacy at all three opioid receptors. Building upon these considerations, we synthesized a new series of LP1 analogues by introducing naphthyl or heteroaromatic rings in propanamide side chain of its N-substituent (9–15). In vitro competition-binding assays in HEK293 cells stably expressing MOR, DOR or KOR showed that in compound 9 the 1-naphthyl ring led to the retention of MOR affinity (K_i^MOR = 38 ± 4 nM) displaying good selectivity versus DOR and KOR. In the electrically stimulated GPI, compound 9 was inactive as agonist but produced an antagonist potency value (pA₂) of 8.6 in presence of MOR agonist DAMGO. Moreover, subcutaneously administered it antagonized the antinociceptive effects of morphine with an AD₅₀ = 2.0 mg/kg in mouse-tail flick test. Modeling studies on MOR revealed that compound 9 fit very well in the binding pocket but in a different way in respect to the agonist LP1. Probably the replacement of its N-substituent on the III, IV and V TM domains reflects an antagonist behavior. Therefore, compound 9 could represent a potential lead to further develop antagonists as valid therapeutic agents and useful pharmacological tools to study opioid receptor function.     

Synthesis and monoamine transporter affinity of front bridged tricyclic 3beta-(4'-halo or 4'-methyl)phenyltropanes bearing methylene or carbomethoxymethylene on the bridge to the 2beta-position

A series of front bridged tricyclic 3beta-(4'-halo or 4'-methyl)phenyltropanes bearing methylene or carbomethoxymethylene on the bridge to the 2beta-position was synthesized, and their binding affinities were determined in cells transfected to express human norepinephrine transporter (NET), serotonin transporter (SERT), and dopamine transporter (DAT) via competition binding assays. All compounds studied in this series exhibit a moderate to high potency at all three transporters with SERT or DAT selectivity. 3beta-(4'-iodo)phenyltropane bearing methylene on the bridge to the 2beta-position (24) presents a particularly attractive pharmacological profile, with very high SERT affinity (K(i) = 0.09 nM) and selectivity versus NET (65-fold) and DAT (94-fold).     

ORL1 and opioid receptor preferences of nociceptin and dynorphin A analogues with Dmp substituted for N-terminal aromatic residues

Nociceptin (NOC) and dynorphin A (DYN) analogues containing 2',6'-dimethylphenylalanine (Dmp) in place of Phe or Tyr in position 1 and/or 4 were synthesized and their metabolic stability and receptor-binding properties were investigated. [Dmp1]NOC(1-13)-NH2 (1) possessed high ORL1 receptor affinity comparable to that of the parent peptide with substantially improved affinities for kappa-, mu-, and delta-opioid receptors. However, Dmp4 substitution of NOC peptide (2) reduced ORL1 receptor affinity. [Dmp1]DYN(1-13)-NH2 (4) and its Dmp4 analogue (5) possessed a 3-fold greater kappa-opioid receptor affinity and improved kappa-receptor selectivity compared to the parent peptide. Analogue 4 however exhibited an unexpectedly low in vitro bioactivity (GPI assay), suggesting, the phenolic hydroxyl group at the N-terminal residue in DYN peptide is extremely important for activation of the kappa-opioid receptor. Analogue 5 possessed an improved kappa-opioid receptor selectivity with an IC50 ratio of 1(kappa)/509(mu)/211598(delta); thus, this peptide may serve as a highly selective kappa-receptor agonist for pharmacological study. Dmp1 substitution in both the NOC and DYN peptides improved metabolic stability toward these peptides, while Dmp4 substitution provided no additional metabolic stability.     

Structure-activity relationship of linear peptide Bu-His6-DPhe7-Arg8-Trp9-Gly10-NH2 at the human melanocortin-1 and -4 receptors: DPhe7 and Trp9 substitution

A series of pentapeptides, based on hMC4R pentapeptide agonist (Bu-His(6)-DPhe(7)-Arg(8)-Trp(9)-Gly(10)-NH(2)), was prepared in which either DPhe(7) or Trp(9) residue was systematically substituted. A number of interesting DPhe surrogates (D-Thi, D-3-CF(3)Phe, D-2-Nal and D-3,4-diClPhe) as well as Trp surrogates (2-Nal and Bta) were identified in this study.     

Delta-Opioid Receptor Analgesia Is Independent of Microglial Activation in a Rat Model of Neuropathic Pain

The analgesic effect of delta-opioid receptor (DOR) ligands in neuropathic pain is not diminished in contrast to other opioid receptor ligands, which lose their effectiveness as analgesics. In this study, we examine whether this effect is related to nerve injury-induced microglial activation. We therefore investigated the influence of minocycline-induced inhibition of microglial activation on the analgesic effects of opioid receptor agonists: morphine, DAMGO, U50,488H, DPDPE, Deltorphin II and SNC80 after chronic constriction injury (CCI) to the sciatic nerve in rats. Pre-emptive and repeated administration of minocycline (30 mg/kg, i.p.) over 7 days significantly reduced allodynia and hyperalgesia as measured on day 7 after CCI. The antiallodynic and antihyperalgesic effects of intrathecally (i.t.) administered morphine (10–20 µg), DAMGO (1–2 µg) and U50,488H (25–50 µg) were significantly potentiated in rats after minocycline, but no such changes were observed after DPDPE (10–20 µg), deltorphin II (1.5–15 µg) and SNC80 (10–20 µg) administration. Additionally, nerve injury-induced down-regulation of all types of opioid receptors in the spinal cord and dorsal root ganglia was not influenced by minocycline, which indicates that the effects of opioid ligands are dependent on other changes, presumably neuroimmune interactions. Our study of rat primary microglial cell culture using qRT-PCR, Western blotting and immunocytochemistry confirmed the presence of mu-opioid receptors (MOR) and kappa-opioid receptors (KOR), further we provide the first evidence for the lack of DOR on microglial cells. In summary, DOR analgesia is different from analgesia induced by MOR and KOR receptors because it does not dependent on injury-induced microglial activation. DOR agonists appear to be the best candidates for new drugs to treat neuropathic pain.