Investigation of the interaction between the atypical agonist c[YpwFG] and MOR

Synthesis and Antiproliferative Activity against Cancer Cells of Indole-Aryl-Amide Derivatives

Indoles constitute a large family of heterocyclic compounds widely occurring in nature which are present in a number of bioactive natural and synthetic compounds, including anticancer agents or atypical opioid agonists. As a result, exponential increases in the development of novel methods for the synthesis of indole-containing compounds have been reported in the literature. A series of indole-aryl amide derivatives 1-7 containing tryptamine or an indolylacetic acid nucleus were designed, synthesized, and evaluated as opioid ligands. These new indole derivatives showed negligible to very low affinity for μ- and δ-opioid receptor (OR). On the other hand, compounds 2, 5 and 7 showed Ki values in the low μM range for κ-OR. Since indoles are well known for their anticancer potential, their effect against a panel of tumor cell lines was tested. The target compounds were evaluated for their in vitro cytotoxicity in HT29, HeLa, IGROV-1, MCF7, PC-3, and Jurkat J6 cells. Some of the synthesized compounds showed good activity against the selected tumor cell lines, with the exception of IGROV1. In particular, compound 5 showed a noteworthy selectivity towards HT29 cells, a malignant colonic cell line, without affecting healthy human intestinal cells. Further studies revealed that 5 caused the cell cycle arrest in the G1 phase and promoted apoptosis in HT29 cells.

Pharmacological Characterization of µ-Opioid Receptor Agonists with Biased G Protein or β-Arrestin Signaling, and Computational Study of Conformational Changes during Receptor Activation

In recent years, G protein vs. β-arrestin biased agonism at opioid receptors has been proposed as an opportunity to produce antinociception with reduced adverse effects. However, at present this approach is highly debated, a reason why more information about biased ligands is required. While the practical relevance of bias in the case of µ-opioid receptors (MOP) still needs to be validated, it remains important to understand the basis of this bias of MOP (and other GPCRs). Recently, we reported two cyclopeptides with high affinity for MOP, the G protein biased Dmt-c[d-Lys-Phe-pCF3-Phe-Asp]NH2 (F-81), and the β-arrestin 2 biased Dmt-c[d-Lys-Phe-Asp]NH2 (C-33), as determined by calcium mobilization assay and bioluminescence resonance energy transfer-based assay. The biased character of F-81 and C-33 has been further analyzed in the [35S]GTPγS binding assay in human MOP-expressing cells, and the PathHunter enzyme complementation assay, used to measure β-arrestin 2 recruitment. To investigate the structural features of peptide-MOP complexes, we performed conformational analysis by NMR spectroscopy, molecular docking, and molecular dynamics simulation. These studies predicted that the two ligands form alternative complexes with MOP, engaging specific ligand-receptor contacts. This would induce different displays of the cytosolic side of the seven-helices bundle, in particular by stabilizing different angulations of helix 6, that could favor intracellular coupling to either G protein or β-arrestin.

Development of Novel δ Opioid Receptor Inverse Agonists without a Basic Nitrogen Atom and Their Antitussive Effects in Mice

Our previous results showed that naltrindole (NTI) derivatives with certain types of electron-withdrawing groups as an N-substituent showed δ opioid receptor (DOR) inverse agonistic activities. We therefore synthesized N-acylated NTI derivatives 3a-e and observed that N-benzoyl and N-cyclopropanecarbonyl derivatives SYK-736 (3b) and SYK-623 (3c) were DOR full inverse agonists and the N-acryloyl derivative 3d was a DOR partial inverse agonist. SKY-623 was over 110-fold more potent than the reference compound ICI-174,864. Both naltriben (NTB) and 7-benzylidenenaltrexone (BNTX) derivatives with N-benzoyl and N-cyclopropanecarbonyl groups were also DOR full inverse agonists. These N-acylated inverse agonists are interesting compounds because they have no basic nitrogen atom, which has been demonstrated to be an important pharmacophore. NTI and BNTX-type DOR inverse agonists SYK-623 and SYK-723 (12c) showed dose-dependent antitussive effects in a mouse cough model induced by citric acid exposure. The antitussive effects by SYK-623 and SYK-723 were significantly attenuated by pretreatment with DOR agonist SNC80.

Tryptophan-Containing Non-Cationizable Opioid Peptides - a new chemotype with unusual structure and in vivo activity

Recently, a new family of opioid peptides containing tryptophan came to the spotlight for the absence of the fundamental protonable tyramine 'message' pharmacophore. Structure-activity relationship investigations led to diverse compounds, characterized by different selectivity profiles and agonist or antagonist effects. Substitution at the indole of Trp clearly impacted peripheral/central antinociceptivity. These peculiarities prompted to gather all the compounds in a new class, and to coin the definition 'Tryptophan-Containing Non-Cationizable Opioid Peptides', in short 'TryCoNCOPs'. Molecular docking analysis suggested that the TryCoNCOPs can still interact with the receptors in an agonist-like fashion. However, most TryCoNCOPs showed significant differences between the in vitro and in vivo activities, suggesting that opioid activity may be elicited also via alternative mechanisms.

Ring size in cyclic endomorphin-2 analogs modulates receptor binding affinity and selectivity

The study reports the solid-phase synthesis and biological evaluation of a series of new side chain-to-side chain cyclized opioid peptide analogs of the general structure Tyr-[D-Xaa-Phe-Phe-Asp]NH2, where Xaa = Lys (1), Orn (2), Dab (3), or Dap (4) (Dab = 2,4-diaminobutyric acid, Dap = 2,3-diaminopropionic acid), containing 17- to 14-membered rings. The influence of the ring size on binding to the MOP, DOP and KOP opioid receptors was studied. In general, the reduction of the size of the macrocyclic ring increased the selectivity for the MOP receptor. The cyclopeptide incorporating Xaa = Lys displayed subnanomolar MOP affinity but modest selectivity over the KOP receptor, while the analog with the Orn residue showed increased affinity and selectivity for MOP. The analog with Dab was a weak MOP agonist and did not bind to the other two opioid receptors. Finally, the peptide with Xaa = Dap was completely MOP receptor-selective with subnanomolar affinity. Interestingly, the deletion of one Phe residue from 1 led to the 14-membered Tyr-c[D-Lys-Phe-Asp]NH2 (5), a potent and selective MOP receptor ligand. The in vitro potencies of the new analogs were determined in a calcium mobilization assay performed in Chinese Hamster Ovary (CHO) cells expressing human recombinant opioid receptors and chimeric G proteins. A good correlation between binding and the functional test results was observed. The influence of the ring size, solid support and the N-terminal protecting group on the formation of cyclodimers was studied.

Cyclic endomorphin analogs in targeting opioid receptors to achieve pain relief

Endomorphins, the endogenous ligands of the µ-opioid receptor, are attractive candidates for opioid-based pain-relieving agents. These tetrapeptides, with their remarkable affinity for the µ-opioid receptor, display favorable antinociceptive activity when injected directly into the brain of experimental animals. However, the application of endomorphins as clinical analgesics has been impeded by their instability in body fluids and inability to reach the brain after systemic administration. Among numerous modifications of the endomorphin structure aimed at improving their pharmacological properties, cyclization can be viewed as an interesting option. Here, we have summarized recent advances in obtaining endomorphin-based cyclic peptide analogs.

Conformational similarities and dissimilarities between the stereoisomeric forms of endomorphin-2

In this study, taking into account both the l-d and cis-trans isomerisms, a comprehensive structural characterization and a comparative conformational analysis were performed on the 32 stereoisomeric forms of opioid tetrapeptide, endomorphin-2. For all stereoisomers, the Φ-Ψ and χ conformational spaces were explored, in the course of which the conformational distributions, as well as the rotamer states of aromatic side chains were characterized in detail. Furthermore, the typical β- and γ-turn structures, as well as the characteristic intramolecular interactions (i.e., H-bonds, aromatic-aromatic and proline-aromatic interplays) were determined. The afore-mentioned structural and conformational features identified for each stereoisomeric form were compared with one another, considering all 32 stereoisomers. The results obtained from this comparative study indicated that both similarities and dissimilarities could be observed between the stereoisomeric forms, with regard to their structural and conformational properties. This theoretical work supplied several valuable observations concerning the effects of both l-d and cis-trans isomerisms on the three-dimensional structure of parent peptide and its stereoisomers. Nevertheless, in the course of this structural investigation, it was clarified how the structural and conformational features of stereoisomeric forms differed from one another.

Physics-based scoring of protein–ligand interactions: explicit polarizability, quantum mechanics and free energies

The ability to accurately predict the interaction of a ligand with its receptor is a key limitation in computer-aided drug design approaches such as virtual screening and de novo design. In this article, we examine current strategies for a physics-based approach to scoring of protein–ligand affinity, as well as outlining recent developments in force fields and quantum chemical techniques. We also consider advances in the development and application of simulation-based free energy methods to study protein–ligand interactions. Fuelled by recent advances in computational algorithms and hardware, there is the opportunity for increased integration of physics-based scoring approaches at earlier stages in computationally guided drug discovery. Specifically, we envisage increased use of implicit solvent models and simulation-based scoring methods as tools for computing the affinities of large virtual ligand libraries. Approaches based on end point simulations and reference potentials allow the application of more advanced potential energy functions to prediction of protein–ligand binding affinities. Comprehensive evaluation of polarizable force fields and quantum mechanical (QM)/molecular mechanical and QM methods in scoring of protein–ligand interactions is required, particularly in their ability to address challenging targets such as metalloproteins and other proteins that make highly polar interactions. Finally, we anticipate increasingly quantitative free energy perturbation and thermodynamic integration methods that are practical for optimization of hits obtained from screened ligand libraries

Peripheral antinociceptive effects of the cyclic endomorphin-1 analog c[YpwFG] in a mouse visceral pain model

We previously described a novel cyclic endomorphin-1 analog c[Tyr-D-Pro-D-Trp-Phe-Gly] (c[YpwFG]), acting as a mu-opioid receptor (MOR) agonist. This study reports that c[YpwFG] is more lipophilic and resistant to enzymatic hydrolysis than endomorphin-1 and produces preemptive antinociception in a mouse visceral pain model when injected intraperitoneally (i.p.) or subcutaneously (s.c.) before 0.6% acetic acid, employed to evoke abdominal writhing (i.p. ED(50)=1.24 mg/kg; s.c. ED(50)=2.13 mg/kg). This effect is reversed by the selective MOR antagonist β-funaltrexamine and by a high dose of the mu(1)-opioid receptor-selective antagonist naloxonazine. Conversely, the kappa-opioid receptor antagonist nor-binaltorphimine and the delta-opioid receptor antagonist naltrindole are ineffective. c[YpwFG] produces antinociception when injected i.p. after acetic acid (ED(50)=4.80 mg/kg), and only at a dose of 20mg/kg did it elicit a moderate antinociceptive response in the mouse, evaluated by the tail flick assay. Administration of a lower dose of c[YpwFG] (10mg/kg i.p.) apparently produces a considerable part of antinociception on acetic acid-induced writhes through peripheral opioid receptors as this action is fully prevented by i.p. naloxone methiodide, which does not readily cross the blood-brain barrier; whereas this opioid antagonist injected intracerebroventricularly (i.c.v.) is not effective. Antinociception produced by a higher dose of c[YpwFG] (20mg/kg i.p.) is partially reversed by naloxone methiodide i.c.v. administered. Thus, only at the dose of 20mg/kg c[YpwFG] can produce antinociception through both peripheral and central opioid receptors. In conclusion, c[YpwFG] displays sufficient metabolic stability to be effective after peripheral administration and demonstrates the therapeutic potential of endomorphin derivatives as novel analgesic agents to control visceral pain.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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).