Pentapeptides displaying mu opioid receptor agonist and delta opioid receptor partial agonist/antagonist properties

Structural basis of μ-opioid receptor targeting by a nanobody antagonist

The μ-opioid receptor (μOR), a prototypical G protein-coupled receptor (GPCR), is the target of opioid analgesics such as morphine and fentanyl. Due to the severe side effects of current opioid drugs, there is considerable interest in developing novel modulators of μOR function. Most GPCR ligands today are small molecules, however biologics, including antibodies and nanobodies, represent alternative therapeutics with clear advantages such as affinity and target selectivity. Here, we describe the nanobody NbE, which selectively binds to the μOR and acts as an antagonist. We functionally characterize NbE as an extracellular and genetically encoded μOR ligand and uncover the molecular basis for μOR antagonism by determining the cryo-EM structure of the NbE-μOR complex. NbE displays a unique ligand binding mode and achieves μOR selectivity by interactions with the orthosteric pocket and extracellular receptor loops. Based on a β-hairpin loop formed by NbE that deeply protrudes into the μOR, we design linear and cyclic peptide analogs that recapitulate NbE's antagonism. The work illustrates the potential of nanobodies to uniquely engage with GPCRs and describes lower molecular weight μOR ligands that can serve as a basis for therapeutic developments.

Structural Basis of μ-Opioid Receptor-Targeting by a Nanobody Antagonist

The μ-opioid receptor (μOR), a prototypical member of the G protein-coupled receptor (GPCR) family, is the molecular target of opioid analgesics such as morphine and fentanyl. Due to the limitations and severe side effects of currently available opioid drugs, there is considerable interest in developing novel modulators of μOR function. Most GPCR ligands today are small molecules, however biologics, including antibodies and nanobodies, are emerging as alternative therapeutics with clear advantages such as affinity and target selectivity. Here, we describe the nanobody NbE, which selectively binds to the μOR and acts as an antagonist. We functionally characterize NbE as an extracellular and genetically encoded μOR ligand and uncover the molecular basis for μOR antagonism by solving the cryo-EM structure of the NbE-μOR complex. NbE displays a unique ligand binding mode and achieves μOR selectivity by interactions with the orthosteric pocket and extracellular receptor loops. Based on a β-hairpin loop formed by NbE that deeply inserts into the μOR and centers most binding contacts, we design short peptide analogues that retain μOR antagonism. The work illustrates the potential of nanobodies to uniquely engage with GPCRs and describes novel μOR ligands that can serve as a basis for therapeutic developments.

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

Antinociceptive activity of thiazole-containing cyclized DAMGO and Leu-(Met) enkephalin analogs

Numerous studies demonstrate the promise of opioid peptides as analgesics, but poor oral bioavailability has limited their therapeutic development. This study sought to increase the oral bioavailability of opioid peptides by cyclization, using Hantzsch-based macrocyclization strategies to produce two new series of cyclized DAMGO and Leu/Met-enkephalin analogs. Opioid receptor affinity and selectivity for compounds in each series were assessed in vitro with radioligand competition binding assays. Compounds demonstrated modest affinity but high selectivity for the mu, delta, and kappa opioid receptors (MOR, DOR and KOR), while selectivity for mu opioid receptors varied by structure. Antinociceptive activity of each compound was initially screened in vivo following intracerebroventricular (i.c.v.) administration and testing in the mouse 55 °C warm-water tail-withdrawal test. The four most active compounds were then evaluated for dose- and time-dependent antinociception, and opioid receptor selectivity in vivo. Cyclic compounds 1924-10, 1936-1, 1936-7, and 1936-9 produced robust and long- lasting antinociception with ED₅₀ values ranging from 0.32-0.75 nmol following i.c.v. administration mediated primarily by mu- and delta-opioid receptor agonism. Compounds 1924-10, 1936-1 and 1936-9 further displayed significant time-dependent antinociception after oral (10 mg kg^-1, p.o.) administration. A higher oral dose (30 mg kg^-1. p.o.) of all four cyclic peptides also reduced centrally-mediated respiration, suggesting successful penitration into the CNS. Overall, these data suggest cyclized opioid peptides synthesized by a Hantzsch-based macrocyclization strategy can retain opioid agonist activity to produce potent antinociception in vivo while conveying improved bioavailability following oral administration.

Novel Cyclic Biphalin Analogues by Ruthenium-Catalyzed Ring Closing Metathesis: in Vivo and in Vitro Biological Profile

In this work we report the application of the ring-closing metathesis (RCM) to the preparation of two cyclic olefin-bridged analogues of biphalin (Tyr-d-Ala-Gly-Phe-NH-NH ← Phe ← Gly ← d-Ala ← Tyr), using the second generation Grubbs' catalyst. The resulting cis- and trans-cyclic isomers were identified, fully characterized, and tested in vitro at μ (ΜΟR), δ (DOR), and κ (KOR) opioid receptors and in vivo for antinociceptive activity. Both were shown to be full agonists at MOR and potential partial antagonists at DOR, with low potency KOR agonism. They also share a strong antinociceptive effect after intracerebroventricular (i.c.v.) and intravenous (i.v.) administration, higher than that of the cyclic biphalin analogues containing a disulfide bridge between the side chains of two d-Cys or d-Pen residues, previously described by our group.

In vivo effects of μ-opioid receptor agonist/δ-opioid receptor antagonist peptidomimetics following acute and repeated administration

BACKGROUND AND PURPOSE

Agonists at μ-opioid receptors (μ-receptors) are used for pain management but produce adverse effects including tolerance, dependence and euphoria. The co-administration of a μ-receptor agonist with a δ-opioid receptor (δ-receptor) antagonist has been shown to produce antinociception with reduced development of some side effects. We characterized the effects of three μ-receptor agonist/δ-receptor antagonist peptidomimetics in vivo after acute and repeated administration to determine if this profile provides a viable alternative to traditional opioid analgesics.

EXPERIMENTAL APPROACH

Three μ-receptor agonist / δ-receptor antagonist peptidomimetics, AAH8, AMB46 and AMB47, and morphine were evaluated for the development of tolerance and dependence after 5 days of twice daily treatment with escalating doses of drug (10-50 mg·kg^-1 ). Antinociceptive effects were measured in the warm water tail withdrawal assay before and after repeated drug treatment. Physical dependence was evaluated by naltrexone-precipitated withdrawal jumping. The rewarding effects of AAH8 were evaluated using a conditioned place preference (CPP) assay with twice daily conditioning sessions performed for 5 days.

KEY RESULTS

Morphine, AAH8, AMB47 and AMB46 all demonstrated acute antinociceptive effects, but repeated administration only produced tolerance in animals treated with morphine and AMB46. Injection of naltrexone precipitated fewer jumps in mice treated repeatedly with AAH8 as compared with morphine, AMB47 or AMB46. Conditioning with morphine, but not AAH8, produced significant CPP.

CONCLUSIONS AND IMPLICATIONS

AAH8 may be a better alternative than traditional opioid analgesics, producing antinociception with less development of tolerance and dependence and may be less rewarding than morphine.

Fentanyls continue to replace heroin in the drug arena: the cases of ocfentanil and carfentanil

Purpose

Ocfentanil and carfentanil are two potent synthetic opioids that are analogues of fentanyl and are actively involved in the recent fentanyl crisis. The aim of this review is to provide all the available information on these two fentanyl analogues.

Methods

All reviewed information was gathered through a detailed search of PubMed and the World Wide Web using relevant keywords.

Results

Like most of the members of the family of fentanyls, they are either sold as heroin to unsuspecting users or used extensively to lace heroin street samples. Despite the fact that ocfentanil was studied clinically in the early 1990s, it did not manage to find its place in clinical practice. On the other hand, carfentanil is mainly used today as an anesthetic agent in large animals. Ocfentanil and carfentanil are used and abused extensively, mainly in Europe and in the United States. As a result, they are the cause of some verified intoxication cases and deaths worldwide. This review provides information concerning chemistry, synthesis, prevalence, pharmacology, and toxicology, as well as the current legal status of these two fentanyl analogues. Analytical methods developed for the determination of ocfentanil and carfentanil in biological specimens and seized materials, as well as related intoxication and lethal cases are also presented.

Conclusions

Ocfentanil and carfentanil are undeniably very dangerous opioid drugs and a very serious matter of concern for public safety. The authorities should take the appropriate actions to avoid the expansion of this threat by taking proper and prompt measures.

The behavioral effects of a mixed efficacy antinociceptive peptide, VRP26, following chronic administration in mice

RATIONALE

VRP26 displays mu opioid receptor agonist and delta opioid receptor antagonist activity in vitro, a pharmacological profile purported to produce reduced tolerance, dependence, and rewarding effects. We hypothesized that VRP26 would display reduced adverse effects after chronic administration as compared with the traditional opioid analgesic fentanyl.

OBJECTIVE

The aim of this study is to explore the development of tolerance, dependence, and conditioned place preference of VRP26 as compared with the traditional opioid analgesic fentanyl.

METHODS

The antinociceptive effects of VRP26 and fentanyl were assessed using the mouse warm water tail withdrawal (WWTW) assay. Measurement of antinociceptive tolerance and physical dependence occurred after 7 days of continuous administration of either fentanyl (0.3 mg/kg/day) or VRP26 (10 mg/kg/day); tolerance was measured by a shift in the antinociceptive dose response curve in the WWTW assay. Physical dependence was determined by observation of withdrawal symptoms after precipitated withdrawal. Rewarding effects were measured by the ability of VRP26 or fentanyl to produce conditioned place preference.

RESULTS

Fentanyl produced significant tolerance and dependence, as well as significant conditioned place preference. VRP26 produced neither tolerance nor physical dependence, nor did it produce significant conditioned place preference.

CONCLUSIONS

These results suggest that chronic treatment with VRP26 may produce less tolerance or physical dependence than chronic treatment with clinically available mu opioid analgesics such as fentanyl. Additionally, VRP26 produces less rewarding effects than fentanyl. This desirable in vivo profile may be due to the mixed efficacy nature of VRP26 and could provide the framework for safer opioid analgesics.

Cyclic Opioid Peptides

For decades the opioid receptors have been an attractive therapeutic target for the treatment of pain. Since the first discovery of enkephalin, approximately a dozen endogenous opioid peptides have been known to produce opioid activity and analgesia, but their therapeutics have been limited mainly due to low blood brain barrier penetration and poor resistance to proteolytic degradation. One versatile approach to overcome these drawbacks is the cyclization of linear peptides to cyclic peptides with constrained topographical structure. Compared to their linear parents, cyclic analogs exhibit better metabolic stability, lower offtarget toxicity, and improved bioavailability. Extensive structure-activity relationship studies have uncovered promising compounds for the treatment of pain as well as further elucidate structural elements required for selective opioid receptor activity. The benefits that come with employing cyclization can be further enhanced through the generation of polycyclic derivatives. Opioid ligands generally have a short peptide chain and thus the realm of polycyclic peptides has yet to be explored. In this review, a brief history of designing ligands for the opioid receptors, including classic linear and cyclic ligands, is discussed along with recent approaches and successes of cyclic peptide ligands for the receptors. Various scaffolds and approaches to improve bioavailability are elaborated and concluded with a discourse towards polycyclic peptides.

Isocyanide-Based Multicomponent Reactions for the Synthesis of Heterocycles

Multicomponent reactions (MCRs) are extremely popular owing to their facile execution, high atom-efficiency and the high diversity of products. MCRs can be used to access various heterocycles and highly functionalized scaffolds, and thus have been invaluable tools in total synthesis, drug discovery and bioconjugation. Traditional isocyanide-based MCRs utilize an external nucleophile attacking the reactive nitrilium ion, the key intermediate formed in the reaction of the imine and the isocyanide. However, when reactants with multiple nucleophilic groups (bisfunctional reactants) are used in the MCR, the nitrilium intermediate can be trapped by an intramolecular nucleophilic attack to form various heterocycles. The implications of nitrilium trapping along with widely applied conventional isocyanide-based MCRs in drug design are discussed in this review.

Rapid Synthesis of Boc-2',6'-dimethyl-l-tyrosine and Derivatives and Incorporation into Opioid Peptidomimetics

The unnatural amino acid 2',6'-dimethyl-l-tyrosine has found widespread use in the development of synthetic opioid ligands. Opioids featuring this residue at the N-terminus often display superior potency at one or more of the opioid receptor types, but the availability of the compound is hampered by its cost and difficult synthesis. We report here a short, three-step synthesis of Boc-2',6'-dimethyl-l-tyrosine (3a) utilizing a microwave-assisted Negishi coupling for the key carbon-carbon bond forming step, and employ this chemistry for the expedient synthesis of other unnatural tyrosine derivatives. Three of these derivatives (3c, 3d, 3f) have not previously been examined as Tyr(1) replacements in opioid ligands. We describe the incorporation of these tyrosine derivatives in a series of opioid peptidomimetics employing our previously reported tetrahydroquinoline (THQ) scaffold, and the binding and relative efficacy of each of the analogues at the three opioid receptor subtypes: mu (MOR), delta (DOR), and kappa (KOR).

Multitarget opioid ligands in pain relief: New players in an old game

Still nowadays pain is one of the most common disabling conditions and yet it remains too often unsolved. Analgesic opioid drugs, and mainly MOR agonists such as morphine, are broadly employed for pain management. MOR activation, however, has been seen to cause not only analgesia but also undesired side effects. A potential pain treatment option is represented by the simultaneous targeting of different opioid receptors. In fact, ligands possessing multitarget capabilities led to an improved pharmacological fingerprint. This review focuses on the examination of multitarget opioid ligands which have been distinguished in peptide and non-peptide and further listed as bivalent and bifunctional ligands. Moreover, the potential of these compounds, both as analgesic drugs and pharmacological tools to explore heteromer receptors, has been stressed.

Further Optimization and Evaluation of Bioavailable, Mixed-Efficacy μ-Opioid Receptor (MOR) Agonists/δ-Opioid Receptor (DOR) Antagonists: Balancing MOR and DOR Affinities

In a previously described peptidomimetic series, we reported the development of bifunctional μ-opioid receptor (MOR) agonist and δ-opioid receptor (DOR) antagonist ligands with a lead compound that produced antinociception for 1 h after intraperitoneal administration in mice. In this paper, we expand on our original series by presenting two modifications, both of which were designed with the following objectives: (1) probing bioavailability and improving metabolic stability, (2) balancing affinities between MOR and DOR while reducing affinity and efficacy at the κ-opioid receptor (KOR), and (3) improving in vivo efficacy. Here, we establish that, through N-acetylation of our original peptidomimetic series, we are able to improve DOR affinity and increase selectivity relative to KOR while maintaining the desired MOR agonist/DOR antagonist profile. From initial in vivo studies, one compound (14a) was found to produce dose-dependent antinociception after peripheral administration with an improved duration of action of longer than 3 h.

Synthesis of Carfentanil Amide Opioids Using the Ugi Multicomponent Reaction

We report a novel approach to synthesize carfentanil amide analogues utilizing the isocyanide-based four-component Ugi multicomponent reaction. A small library of bis-amide analogues of carfentanil was created using N-alkylpiperidones, aniline, propionic acid, and various aliphatic isocyanides. Our lead compound showed high affinity for mu (MOR) and delta opioid receptors (DOR) with no appreciable affinity for kappa (KOR) receptors in radioligand binding assays. The compound was found to be a mixed MOR agonist/partial DOR agonist in [(35)S]GTPγS functional assays, and it showed moderate analgesic potency in vivo. The compound showed no visible signs of physical dependence or constipation in mice. In addition, it produced less respiratory depression than morphine. Most mixed MOR/DOR opioids reported in the literature are peptides and thereby systemically inactive. Our approach utilizing a multicomponent reaction has the promise to deliver potent and efficacious small-molecule analgesics with potential clinical utility.

Structural Requirements for CNS Active Opioid Glycopeptides

Glycopeptides related to β-endorphin penetrate the blood-brain barrier (BBB) of mice to produce antinociception. Two series of glycopeptides were assessed for opioid receptor binding affinity. Attempts to alter the mu-selectivity of [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO)-related glycopeptides by altering the charged residues of the amphipathic helical address were unsuccessful. A series of pan-agonists was evaluated for antinociceptive activity (55 °C tail flick) in mice. A flexible linker was required to maintain antinociceptive activity. Circular dichroism (CD) in H2O, trifluoroethanol (TFE), and SDS micelles confirmed the importance of the amphipathic helices (11s → 11sG → 11) for antinociception. The glycosylated analogues showed only nascent helices and random coil conformations in H2O. Chemical shift indices (CSI) and nuclear Overhauser effects (NOE) with 600 MHz NMR and CD confirmed helical structures in micelles, which were rationalized by molecular dynamics calculations. Antinociceptive studies with mice confirm that these glycosylated endorphin analogues are potential drug candidates that penetrate the BBB to produce potent central effects.

Synthesis of mixed opioid affinity cyclic endomorphin-2 analogues with fluorinated phenylalanines

As part of our continuing studies on the structure-activity relationships of cyclic pentapeptides based on the structure of endomorphin-2 (EM-2), we report here the synthesis and biological activities of a new series of analogues of a general sequence Tyr/Dmt-c[d-Lys-Phe-Phe-Asp]NH2 (where Dmt = 2',6'-dimethyltyrosine), incorporating fluorinated amino acids: 4-fluorophenylalanine (4-F-Phe), 2,4-difluorophenylalanine (2,4-F-Phe), or 4-trifluoromethylphenylalanine (4-CF3-Phe) instead of the Phe residue in position 3 or 4. Depending on the fluorinated amino acid residue and its position in the sequence, analogues were mixed, high affinity MOP/KOP receptor agonists, MOP/DOP/KOP agonists, or selective KOP agonists. The in vitro potencies and efficacies of all novel analogues were assessed in calcium mobilization assay. The most potent analogues, Dmt-c[d-Lys-Phe-4-F-Phe-Asp]NH2 and Dmt-c[d-Lys-Phe-2,4-F-Phe-Asp]NH2, were tested in vivo in the mouse hot-plate test. They produced strong antinociceptive effect not only after intracerebroventricular but also after intraperitoneal injection, indicating that they were able to cross the blood-brain barrier.

Pharmacological characterization of endomorphin-2-based cyclic pentapeptides with methylated phenylalanine residues

As part of our continuing studies on the structure-activity relationships of cyclic pentapeptides based on the structure of endomorphin-2, we report here the synthesis and biological activities of a new series of analogs incorporating 2', 3' or 4'-methylphenylalanine (MePhe) residues into positions 3 or 4 of the parent cyclopeptide, Dmt-c[d-Lys-Phe-Phe-Asp]NH2 (Dmt=2',6'-dimethyltyrosine). Analogs with MePhe in position 4 showed a row of magnitude increased μ-opioid receptor (MOP receptor) affinity as compared with a parent compound. The in vitro potencies of the new analogs were determined in calcium mobilization assay performed in Chinese Hamster Ovary (CHO) cells expressing human recombinant opioid receptors and chimeric G proteins. All analogs were strong μ/κ (MOP/KOP) receptor agonists and weak δ (DOP) receptor agonists. In the in vivo hot-plate test in mice, the MePhe(4)-modified peptides showed remarkable antinociceptive activity after intracerebroventricular (i.c.v.) administration which was most likely due to the concomitant activation of more than one opioid receptor type.

Development of a bioavailable μ opioid receptor (MOPr) agonist, δ opioid receptor (DOPr) antagonist peptide that evokes antinociception without development of acute tolerance

We have previously described a cyclic tetrapeptide, 1, that displays μ opioid receptor (MOPr) agonist and δ opioid receptor (DOPr) antagonist activity, a profile associated with a reduced incidence of opioid tolerance and dependence. Like many peptides, 1 has poor bioavailability. We describe here an analogue of 1 with an added C-terminal β-glucosylserine residue, Ser(β-Glc)NH2, a modification that has previously been shown to improve bioavailability of opioid peptides. The resulting peptide, 4, exhibits full antinociceptive efficacy in the mouse warm water tail withdrawal assay after intraperitoneal administration with potency similar to that of morphine. Further, 4 does not give rise to acute tolerance and thus represents a promising lead for the development of opioid analgesics with reduced side effects.

Translation of structure-activity relationships from cyclic mixed efficacy opioid peptides to linear analogues

Most opioid analgesics used in the treatment of pain are mu opioid receptor (MOR) agonists. While effective, there are significant drawbacks to opioid use, including the development of tolerance and dependence. However, the coadministration of a MOR agonist with a delta opioid receptor (DOR) antagonist slows the development of MOR-related side effects, while maintaining analgesia. We have previously reported a series of cyclic mixed efficacy MOR agonist/DOR antagonist ligands. Here we describe the transfer of key features from these cyclic analogs to linear sequences. Using the linear MOR/DOR agonist, Tyr-DThr-Gly-Phe-Leu-Ser-NH2 (DTLES), as a lead scaffold, we replaced Phe(4) with bulkier and/or constrained aromatic residues shown to confer DOR antagonism in our cyclic ligands. These replacements failed to confer DOR antagonism in the DTLES analogs, presumably because the more flexible linear ligands can adopt binding poses that will fit in the narrow binding pocket of the active conformations of both MOR and DOR. Nonetheless, the pharmacological profile observed in this series, high affinity and efficacy for MOR and DOR with selectivity relative to KOR, has also been shown to reduce the development of unwanted side effects. We further modified our lead MOR/DOR agonist with a C-terminal glucoserine to improve bioavailability. The resulting ligand displayed high efficacy and potency at both MOR and DOR and no efficacy at KOR.

The role of imidazole in peptide cyclization by transesterification: parallels to the catalytic triads of serine proteases

The improved bioavailability, stability and selectivity of cyclic peptides over their linear counterparts make them attractive structures in the design and discovery of novel therapeutics. In our previous work, we developed an imidazole-promoted preparation of cyclic depsipeptides in which we observed that increasing the concentration of imidazole resulted in the concomitant increase in the yield of cyclic product and reduction in dimerization, but also resulted in the generation of an acyl-substituted side product. In this work, we used transition state analysis to explore the mechanism of the imidazole-catalyzed esterification of one such peptide, Ac-SAFYG-SCH2φ, and determined the acyl substitution product to be an intermediate in a competing reaction pathway involving acyl substitution of the thioester by imidazole. Our findings indicate that imidazole plays an essential role in this side-chain to C-terminal coupling, and by extension, in transesterifications in general, through a concerted mechanism wherein imidazole deprotonates the nucleophile as the nucleophile attacks the carbonyl. The system under study is identical to the histidine-serine portion of the catalytic triads in serine proteases and it is likely that these enzymes employ the same concerted mechanism in the first step of peptide cleavage. Additionally, relatively high concentrations of imidazole must be used to effectively catalyze reactions in aprotic solvents since the overall reaction involves imidazole acting both as an acid and as a base, existing in solution as an equilibrium distribution between the neutral form and its conjugate acid.

Opioid peptidomimetics: leads for the design of bioavailable mixed efficacy μ opioid receptor (MOR) agonist/δ opioid receptor (DOR) antagonist ligands

We have previously described opioid peptidomimetic, 1, employing a tetrahydroquinoline scaffold and modeled on a series of cyclic tetrapeptide opioid agonists. We have recently described modifications to these peptides that confer a μ opioid receptor (MOR) agonist, δ opioid receptor (DOR) antagonist profile, which has been shown to reduce the development of tolerance to the analgesic actions of MOR agonists. Several such bifunctional ligands have been reported, but none has been demonstrated to cross the blood-brain barrier. Here we describe the transfer of structural features that evoked MOR agonist/DOR antagonist behavior in the cyclic peptides to the tetrahydroquinoline scaffold and show that the resulting peptidomimetics maintain the desired pharmacological profile. Further, the 4R diastereomer of 1 was fully efficacious and approximately equipotent to morphine in the mouse warm water tail withdrawal assay following intraperitoneal administration and thus a promising lead for the development of opioid analgesics with reduced tolerance.

Hantzsch based macrocyclization approach for the synthesis of thiazole containing cyclopeptides

An innovative macrocyclization approach via high-yielding solid-phase intramolecular thioalkylation reaction is described. The reaction of S-nucleophiles with newly generated N-terminal 4-chloromethyl thiazoles leads to the desired cyclic products in high purities and good yields.

Variation of the net charge, lipophilicity, and side chain flexibility in Dmt(1)-DALDA: Effect on Opioid Activity and Biodistribution

The influence of the side chain charges of the second and fourth amino acid residues in the peptidic μ opioid lead agonist Dmt-d-Arg-Phe-Lys-NH(2) ([Dmt(1)]-DALDA) was examined. Additionally, to increase the overall lipophilicity of [Dmt(1)]-DALDA and to investigate the Phe(3) side chain flexibility, the final amide bond was N-methylated and Phe(3) was replaced by a constrained aminobenzazepine analogue. The in vitro receptor binding and activity of the peptides, as well as their in vivo transport (brain in- and efflux and tissue biodistribution) and antinociceptive properties after peripheral administration (ip and sc) in mice were determined. The structural modifications result in significant shifts of receptor binding, activity, and transport properties. Strikingly, while [Dmt(1)]-DALDA and its N-methyl analogue, Dmt-d-Arg-Phe-NMeLys-NH(2), showed a long-lasting antinociceptive effect (>7 h), the peptides with d-Cit(2) generate potent antinociception more rapidly (maximal effect at 1h postinjection) but also lose their analgesic activity faster when compared to [Dmt(1)]-DALDA and [Dmt(1),NMeLys(4)]-DALDA.

Modulation of opioid receptor ligand affinity and efficacy using active and inactive state receptor models

Mu opioid receptor (MOR) agonists are widely used for the treatment of pain; however, chronic use results in the development of tolerance and dependence. It has been demonstrated that coadministration of a MOR agonist with a delta opioid receptor (DOR) antagonist maintains the analgesia associated with MOR agonists, but with reduced negative side-effects. Using our newly refined opioid receptor models for structure-based ligand design, we have synthesized several pentapeptides with tailored affinity and efficacy profiles. In particular, we have obtained pentapeptides 8, Tyr-c(S-S)[DCys-1Nal-Nle-Cys]NH(2), and 12, Tyr-c(S-S)[DCys-1Nal-Nle-Cys]OH, which demonstrates high affinity and full agonist behavior at MOR, high affinity but very low efficacy for DOR, and minimal affinity for the kappa opioid receptor (KOR). Functional properties of these peptides as MOR agonists/DOR antagonists lacking undesired KOR activity make them promising candidates for future in vivo studies of MOR/DOR interactions. Subtle structural variation of 12, by substituting D-Cys(5) for L-Cys(5), generated analog 13, which maintains low nanomolar MOR and DOR affinity, but which displays no efficacy at either receptor. These results demonstrate the power and utility of accurate receptor models for structure-based ligand design, as well as the profound sensitivity of ligand function on its structure.

Development and in vitro characterization of a novel bifunctional μ-agonist/δ-antagonist opioid tetrapeptide

The development of tolerance to and dependence on opioid analgesics greatly reduces their long-term usefulness. Previous studies have demonstrated that co-administration of a μ-opioid receptor (MOR) agonist and δ-opioid receptor (DOR) antagonist can decrease MOR agonist-induced tolerance and dependence development after chronic exposure. Clinically, a single ligand displaying multiple efficacies (e.g., MOR agonism concurrently with DOR antagonism) would be of increased value over two drugs administered simultaneously. Guided by modeling of receptor-ligand complexes we have developed a series of potent non-selective opioid tetrapeptides that have differing efficacy at MOR and DOR. In particular, our lead peptide (KSK-103) binds with equal affinity to MOR and DOR but acts as a MOR agonist with similar efficacy but greater potency than morphine and a DOR antagonist in cellular assays measuring both G protein stimulation and adenylyl cyclase inhibition.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Two-Steps Hantzsch Based Macrocyclization Approach for the Synthesis of Thiazole Containing Cyclopeptides

Macrocyclization via an efficient high-yielding solid-phase intramolecular thioalkylation reaction is described. The reaction of S-nucleophiles with newly generated N-terminal 4-chloromethyl thiazoles led to the desired macrocyclization products 5 in high purities and good overall yields.

Two-step Hantzsch based macrocyclization approach for the synthesis of thiazole-containing cyclopeptides

Macrocyclization via an efficient high-yielding solid-phase intramolecular thioalkylation reaction is described. The reaction of S-nucleophiles with newly generated N-terminal 4-chloromethyl thiazoles led to the desired macrocyclization products 5 in high purities and good overall yields.

Evaluation of N-substitution in 6,7-benzomorphan compounds

6,7-benzomorphan derivatives, exhibiting different mu, delta, and kappa receptor selectivity profiles depending on the N-substituent, represent a useful skeleton for the synthesis of new and better analgesic agents. In this work, an aromatic ring and/or alkyl residues have been used with an N-propanamide or N-acetamide spacer for the synthesis of a new series of 5,9-dimethyl-2'-hydroxy-6,7-benzomorphan derivatives (12-22). Data obtained by competition binding assays showed that the mu opioid receptor seems to prefer an interaction with the 6,7-benzomorphan ligands having an N-substituent with a propanamide spacer and less hindered amide. Highly stringent features are required for delta receptor interaction, while an N-acetamide spacer and/or bulkier amide could preferentially lead to kappa receptor selectivity. In the propanamide series, compound 12 (named LP1) displayed high mu affinity (Ki=0.83 nM), good delta affinity (Ki=29 nM) and low affinity for the kappa receptor (Ki=110 nM), with a selectivity ratio delta/mu and kappa/mu of 35.1 and 132.5, respectively. Further, in the adenylyl cyclase assay, LP1 displayed a mu/delta agonist profile, with IC50 values of 4.8 and 12 nM at the mu and delta receptors, respectively. The antinociceptive potency of LP1 in the tail-flick test after sc administration in rat was comparable with the potency of morphine (ED50=2.03 and 2.7 mg/kg, respectively), and was totally reversed by naloxone. LP1, possessing a mu/delta agonist profile, could represent a lead in further developing benzomorphan-based ligands with potent in vivo analgesic activity and a reduced tendency to induce side effects.