Opioid receptor binding and antinociceptive activity of the analogues of endomorphin-2 and morphiceptin with phenylalanine mimics in the position 3 or 4

Current overview of opioids in progression of inflammatory bowel disease; pharmacological and clinical considerations

Inflammatory bowel diseases (IBD) belong to a subgroup of persistent, long-term, progressive, and relapsing inflammatory conditions. IBD may spontaneously develop in the colon, resulting in tumor lesions in inflamed regions of the intestine, such as invasive carcinoma. The benefit of opioids for IBD treatment is still questionable, thereby we investigated databases to provide an overview in this context. This review demonstrates the controversial role of opioids in IBD therapy, their physiological and pharmacological functions in attenuating the IBD symptoms, and in improving inflammatory, oxidative stress, and the quality of life factors in IBD subjects. Data were extracted from clinical, in vitro, and in vivo studies in English, between 1995 and 2019, from PubMed, Google Scholar, Scopus, and Cochrane library. Based on recent reports, there are promising opportunities to target the opioid system and control the IBD symptoms. This study suggests a novel approach for future treatment of functional and inflammatory disorders such as IBD.

Design, synthesis, and biological activity of new endomorphin analogs with multi-site modifications

Endomorphin (EM)-1 and EM-2 are the most effective endogenous analgesics with efficient separation of analgesia from the risk of adverse effects. Poor metabolic stability and ineffective analgesia after peripheral administration were detrimental for the use of EMs as novel clinical analgesics. Therefore, here, we aimed to establish new EM analogs via introducing different bifunctional d-amino acids at position 2 of [(2-furyl)Map⁴]EMs. The combination of [(2-furyl)Map⁴]EMs with D-Arg² or D-Cit² yielded analogs with enhanced binding affinity to the μ-opioid receptor (MOR) and increased stability against enzymatic degradation (t_1/2 > 300 min). However, the agonistic activities of these analogs toward MOR were slightly reduced. Similar to morphine, peripheral administration of the analog [D-Cit², (2-furyl)Map⁴]EM-1 (10) significantly inhibited the pain behavior of mice in multiple pain models. In addition, this EM-1 analog was associated with reduced tolerance, less effect on gastrointestinal mobility, and no significant motor impairment. Compared to natural EMs, the EM analogs synthesized herein had enhanced metabolic stability, bioavailability, and analgesic properties.

Enantioselective Addition of Bromonitromethane to Aliphatic N-Boc Aldimines Using a Homogeneous Bifunctional Chiral Organocatalyst

This report details the enantioselective synthesis of β-amino-α-bromo nitroalkanes with β-alkyl substituents, using homogeneous catalysis to prepare either antipode. Use of a bifunctional Brønsted base/acid catalyst allows equal access to either enantiomer of the products, enabling the use of Umpolung Amide Synthesis (UmAS) to prepare the corresponding L- or D-α-amino amide bearing alkyl side chains - overall, in only 4 steps from aldehyde. The approach also addresses an underlying incompatibility between bromonitromethane and solid hydroxide bases.

Anti-inflammatory action of a novel orally available peptide 317 in mouse models of inflammatory bowel diseases

BACKGROUND

The endogenous opioid system constitutes an attractive target in the treatment of GI disorders, including inflammatory bowel diseases (IBD). The aim of our study was to characterize the anti-inflammatory and antinociceptive effect of P-317, a novel cyclic analog of opioid peptide morphiceptin, in animal models of IBD.

METHODS

The anti-inflammatory effect of P-317 after intraperitoneal (ip) and oral (po) administration was assessed in two mouse models of IBD - Crohn's disease, induced by intracolonic instillation of trinitrobenzenesulfonic acid (TNBS) and ulcerative colitis, induced by addition of dextran sodium sulfate (DSS) into drinking water. The antinociceptive action of P-317 was characterized in mice with acute colitis using mustard oil-induced pain test. Real time RT PCR was used to assess semiquantitatively the expression of IL-1β and TNF-α mRNA in mouse colonic samples. To translate our results to clinical conditions, MOP and KOP mRNA were quantified in human colonic biopsies from IBD patients.

RESULTS

P-317 (0.1mg/kg, ip and 1mg/kg, po) alleviated colonic inflammation in TNBS- and DSS-treated mice in the opioid receptor-dependent manner. The anti-inflammatory effect of P-317 was associated with the decrease in mRNA expression of proinflammatory cytokines. The antinociceptive effect of P-317 was observed after ip and po administration in mice with acute colitis.

CONCLUSION

Our results show a potent anti-inflammatory and antinociceptive effect of P-317 in mouse models of colitis upon activation of opioid receptors. The unique bioavailability of P-317 after oral administration suggests that it is a promising drug candidate for future treatment of IBD.

Engineering endomorphin drugs: state of the art

INTRODUCTION

Although endomorphins-1 (EM-1; H-Tyr-Pro-Phe-Trp-NH(2)) and -2 (EM-2; H-Tyr-Pro-Phe-Phe-NH(2)) are primarily considered agonists for the μ-opioid receptor (MOR), systematic alterations to specific residues provided antagonists and ligands with mixed μ/δ-opioid properties, suitable for application to health-related topics. While the application of endomorphins as antinociceptive agents and numerous biological endpoints were experimentally delineated in laboratory animals and in vitro, clinical use is currently absent. However, structural alterations provide enhanced stability; formation of MOR antagonists or mixed and dual μ/δ-acting ligands could find considerable therapeutic potential.

AREAS COVERED

This review attempts to succinctly provide insight on the development and bioactivity of endomorphin analogues during the past decade. Rational design approaches will focus on the engineering of endomorphin agonists, antagonists and mixed ligands for their application as a multi-target ligand.

EXPERT OPINION

Aside from alleviating pain, EM analogues open new horizons in the treatment of medical syndromes involving neural reward mechanisms and extraneural regulation effects on homeostasis. Highly selective MOR antagonists may be promising to reduce inflammation, attenuate addiction to drugs and excess consumption of high-caloric food, ameliorate alcoholism, affect the immune system and combat opioid bowel dysfunction.

Synthesis and activity of endomorphin-2 and morphiceptin analogues with proline surrogates in position 2

The opioid agonists endomorphins (Tyr-Pro-Trp-Phe-NH(2); EM1 and Tyr-Pro-Phe-Phe-NH(2); EM2) and morphiceptin (Tyr-Pro-Phe-Pro-NH(2)) exhibit an extremely high selectivity for mu-opioid receptor. Here a series of novel EM2 and morphiceptin analogues containing in place of the proline at position 2 the S and R residues of beta-homologues of proline (HPro), of 2-pyrrolidinemethanesulphonic acid (HPrs) and of 3-pyrrolidinesulphonic acid (betaPrs) have been synthesized and their binding affinity and functional activity have been investigated. The highest micro-receptor affinity is shown by [(S)betaPrs(2)]EM2 analogue (6e) which represents the first example of a beta-sulphonamido analogue in the field of opioid peptides.

Synthesis and evaluation of new endomorphin-2 analogues containing (Z)-alpha,beta-didehydrophenylalanine (Delta(Z)Phe) residues

New endomorphin-2 (EM-2) analogues incorporating (Z)-alpha,beta-didehydrophenylalanine (Delta(Z)Phe) in place of the native phenylalanine in EM-2 are reported. Tyr-Pro-Delta(Z)Phe-Phe-NH(2) {[Delta(Z)Phe(3)]EM-2} (1), Tyr-Pro-Phe-Delta(Z)Phe-NH(2) {[Delta(Z)Phe(4)]EM-2} (2), and Tyr-Pro-Delta(Z)Phe-Delta(Z)Phe-NH(2) {[Delta(Z)Phe(3,4)]EM-2}(3) have been synthesized, their opioid receptor binding affinities and tissue bioassay activities were determined, and their conformational properties were examined. Compound 2 shows high mu opioid receptor selectivity and mu agonist activity comparable to those of the native peptide. The conformation adopted in solution and in the crystal by N-Boc-Tyr-Pro-Delta(Z)Phe-Phe-NH(2) (8) is reported.

Synthesis and immunosuppressive activity of cyclolinopeptide A analogues containing homophenylalanine

Immune response suppressors are used in the medical praxis to prevent graft rejection after organ transplantation and in the therapy of some autoimmune diseases. Cyclolinopeptide A, naturally existing immunomodulatory peptide, was modified with homophenylalanine in positions 3 (4), 4 (5) or both 3 and 4 (6). The conformational influence of the replacement of Phe by Hphe was analyzed by NMR spectroscopy. Peptides 4-6 exist as single isomers with all trans peptide bonds except cis Pro-Pro peptide bond. The peptides were tested for their ability to suppress the proliferative response of mouse splenocytes to T- and B-cell mitogens and the secondary humoral immune response to sheep erythrocytes in vitro in parallel with a reference drug--cyclosporine A. The substitution of Phe with Hphe in positions 3 and 4 of CLA led to three different activities in the studied immunological assays. Very potent inhibition of AFC number of peptide 4 was not associated with cell toxicity. This compound caused a complete block of T- and B-cell proliferation. Peptides 5 and 6, containing Hphe in position 3 or 3 and 4, respectively, gave similar effects on the proliferative response of splenocytes to mitogens. Peptide 6 was a moderate suppressor of the humoral immune response, peptide 5 was exceptionally inhibitory. The presence of Hphe in position 4 of CLA backbone markedly reduced the viability of the tested cell line, however addition of the second Hphe in position 3 improved cell survival in comparison with the solvent.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 neurological 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).

Bifunctional [2',6'-dimethyl-L-tyrosine1]endomorphin-2 analogues substituted at position 3 with alkylated phenylalanine derivatives yield potent mixed mu-agonist/delta-antagonist and dual mu-agonist/delta-agonist opioid ligands

Endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2) and [Dmt1]EM-2 (Dmt = 2',6'-dimethyl-l-tyrosine) analogues, containing alkylated Phe3 derivatives, 2'-monomethyl (2, 2'), 3',5'- and 2',6'-dimethyl (3, 3', and 4', respectively), 2',4',6'-trimethyl (6, 6'), 2'-ethyl-6'-methyl (7, 7'), and 2'-isopropyl-6'-methyl (8, 8') groups or Dmt (5, 5'), had the following characteristics: (i) [Xaa3]EM-2 analogues exhibited improved mu- and delta-opioid receptor affinities. The latter, however, were inconsequential (Kidelta = 491-3451 nM). (ii) [Dmt1,Xaa3]EM-2 analogues enhanced mu- and delta-opioid receptor affinities (Kimu = 0.069-0.32 nM; Kidelta = 1.83-99.8 nM) without kappa-opioid receptor interaction. (iii) There were elevated mu-bioactivity (IC50 = 0.12-14.4 nM) and abolished delta-agonism (IC50 > 10 muM in 2', 3', 4', 5', 6'), although 4' and 6' demonstrated a potent mixed mu-agonism/delta-antagonism (for 4', IC50mu = 0.12 and pA2 = 8.15; for 6', IC50mu = 0.21 nM and pA2 = 9.05) and 7' was a dual mu-agonist/delta-agonist (IC50mu = 0.17 nM; IC50delta = 0.51 nM).

Conformational analysis of endomorphin-2 analogs with phenylalanine mimics by NMR and molecular modeling

We investigated a series of conformations of endomorphin-2 (EM-2) analogs substituted by phenylglycine (Phg) and homophenylalanine (Hfe) in the position 3 or 4 by two-dimensional (1)H NMR spectroscopy and molecular modeling. Evaluating the aromatic interactions and the dihedral angles in these phenylalanine mimics, we have observed that the conformations in trans isomer have varied from extended to folded as bioactivity decreases. It is suggested that the flexibility of aromatic side chain affects the backbone of EM-2 to adopt folded structures, which may block the ligands in binding to micro-opioid receptor.

Synthesis and Analgesic Activities of Endomorphin-2 and Its Analogues

Endomorphin-2 (1; H-Tyr-Pro-Phe-Phe-NH2; EM2) and its novel cyclic asparagine (cycloAsn) analogues, H-Tyr-cAsn(CHPh)-Phe-Phe-NH2 (2) and H-Tyr-cAsn(CHMe2)-Phe-Phe-NH2 (3), were synthesized via liquid-phase synthesis. The structures of the products and intermediates were characterized by IR, 1H-NMR, MS, and HR-MS analyses. The antinociceptive activity of EM2 and its cyclic asparagine analogues were assessed in AcOH-induced abdominal constriction tests in mice with i.p. injection. The results show that the antinociceptive activities of EM2 and its cyclic asparagine analogue 2 were higher than those of aspirine and meperidine. Analogue 2 was observed to be a stronger analgesic with dose-dependence than EM2. The test mice did not show any tendency to be addicted while administrated of analogue 2 repeatedly and regularly.