Synthesis and antinociceptive effects of endomorphin-1 analogs with C-terminal linked by oligoarginine

A Novel Multi-Target Mu/Delta Opioid Receptor Agonist, HAGD, Produced Potent Peripheral Antinociception with Limited Side Effects in Mice and Minimal Impact on Human Sperm Motility In Vitro

Pain is a common clinical symptom among patients. Although various opioid analgesics have been developed, their side effects hinder their application. This study aimed to develop a novel opioid analgesic, HAGD (H-Tyr-D-AIa-GIy-Phe-NH₂), with limited side effects. In vivo studies on mouse models as well as in vitro studies on Chinese hamster ovary (CHO) cells expressing human mu, delta, or kappa opioid receptors (CHO_hMOP, CHO_hDOP, and CHO_hKOP, respectively) and human sperm were conducted. Compared with subcutaneous morphine (10 mg/kg), subcutaneous HAGD (10 mg/kg) produced equipotent or even greater antinociception with a prolonged duration by activating mu/delta opioid receptors in preclinical mouse pain models. The analgesic tolerance, rewarding effects (i.e., conditioned place preference and acute hyperlocomotion), and gastrointestinal transit inhibition of HAGD were significantly reduced compared with those of morphine. Both HAGD and morphine exhibited a withdrawal response and had no impacts on motor coordination. In CHO_hMOP and CHO_hDOP, HAGD showed specific and efficient intracellular Ca²⁺ stimulation. HAGD had minimal impact on human sperm motility in vitro, whereas 1 × 10^-7 and 1 × 10^-8 mol/L of morphine significantly declined sperm motility at 3.5 h. Overall, HAGD may serve as a promising antinociceptive compound.

Novel Cyclic Endomorphin Analogues with Multiple Modifications and Oligoarginine Vector Exhibit Potent Antinociception with Reduced Opioid-like Side Effects

Endomorphins (EMs) are potent pharmaceuticals for the treatment of pain. Herein, we investigated several novel EM analogues with multiple modifications and oligoarginine conjugation. Our results showed that analogues 1-6 behaved as potent μ-opioid agonists and enhanced stability and lipophilicity. Analogues 5 and 6 administered centrally and peripherally induced significant and prolonged antinociceptive effects in acute pain. Both analogues also produced long-acting antiallodynic effects against neuropathic and inflammatory pain. Furthermore, they showed a reduced acute antinociceptive tolerance. Analogue 6 decreased the extent of chronic antinociceptive tolerance, and analogue 5 exhibited no tolerance at the supraspinal level. Particularly, they displayed nontolerance-forming antinociception at the peripheral level. In addition, analogues 5 and 6 exhibited reduced or no opioid-like side effects on gastrointestinal transit, conditioned place preference (CPP), and motor impairment. The present investigation established that multiple modifications and oligoarginine-vector conjugation of EMs would be helpful in developing novel analgesics with fewer side effects.

Intracerebroventricular administration of CYX-6, a potent μ-opioid receptor agonist, a δ- and κ-opioid receptor antagonist and a biased ligand at μ, δ & κ-opioid receptors, evokes antinociception with minimal constipation and respiratory depression in rats in contrast to morphine

Mu opioid receptor (MOPr) agonists are thought to produce analgesia via modulation of G-protein-coupled intracellular signalling pathways whereas the β-arrestin2 pathway is proposed to mediate opioid-related adverse effects. Here, we report the antinociception, constipation and respiratory depressant profile of CYX-6, a potent MOPr agonist that is also a delta and a kappa opioid receptor (DOPr/KOPr) antagonist and that lacks β-arrestin2 recruitment at each of the MOPr, DOPr and the KOPr. In anaesthetised male Sprague Dawley rats, an intracerebroventricular (i.c.v.) guide cannula was stereotaxically implanted. After 5-7 days post-surgical recovery, rats received a single i.c.v. bolus dose of CYX-6 (3-30 nmol), morphine (100 nmol) or vehicle. Antinociception was assessed using the warm water tail flick test (52.5 ± 0.5 °C). Constipation was assessed using the charcoal meal gut motility test and the castor oil-induced diarrhoea test. Respiratory depression was measured by whole-body plethysmography in awake, freely moving animals, upon exposure to a hypercapnic gas mixture (8% CO2, 21% O2 and 71% N2). The intrinsic pharmacology of CYX-6 given by the i.c.v. route in rats showed that it produced dose-dependent antinociception. It also produced respiratory stimulation rather than depression and it had a minimal effect on intestinal motility in contrast to the positive control, morphine. CYX-6 is an endomorphin-2 analogue that dissociates antinociception from constipation and respiratory depression in rats. Our findings provide useful insight to inform the discovery and development of novel opioid analgesics with a superior tolerability profile compared with morphine.

Endomorphin-1 analogs with oligoarginine-conjugation at C-terminus produce potent antinociception with reduced opioid tolerance in paw withdrawal test

For clinical use, it is essential to develop potent endomorphin (EM) analogs with reduced antinociceptive tolerance. In the present study, the antinociceptive activities and tolerance development of four potent EM-1 analogs with C-terminal oligoarginine-conjugation was evaluated and compared in the radiant heat paw withdrawal test. Following intracerebroventricular (i.c.v.) administration, all analogs 1-4 produced potent and prolonged antinociceptive effects. Notably, analogs 2 and 4 with the introduction of D-Ala in position 2 exhibited relatively higher analgesic potencies than those of analogs 1 and 3 with β-Pro substitution, consistent with their μ-opioid binding characteristic. In addition, at a dose of 50 μmol/kg, endomorphin-1 (EM-1) failed to produce any significant antinociceptive activity after peripheral administration, whereas analogs 1-4 induced potent antinociceptive effects with an increased duration of action. Herein, our results indicated the development of antinociceptive tolerance to EM-1 and morphine at the supraspinal level on day 7. By contrast, analogs 1-4 decreased the antinociceptive tolerance. Furthermore, subcutaneous (s.c.) administration of morphine at 50 μmol/kg also developed the antinociceptive tolerance, whereas the extent of tolerance developed to analogs 1-4 was largely reduced. Especially, analog 4 exhibited non-tolerance-forming antinociception after peripheral administration. The present investigation gave the evidence that C-terminal conjugation of EM-1 with oligoarginine vector will facilitate the development of novel opioid analgesics with reduced opioid tolerance.

A new tridecapeptide with an octaarginine vector has analgesic therapeutic potential and prevents morphine-induced tolerance

A growing body of evidence suggests that peptides may possess analgesic effects without tolerance development. The synthetic tetrapeptide Tyr-d-Arg-Phe-Gly-NH₂ was modified with the inclusion of a (d-Arg)₈ vector to prevent the action of endopeptidase and to increase the duration of the analgesic action of the tetrapeptide when administered orally. The aim of this study was to estimate the analgesic efficacy of the tetrapeptide with (d-Arg)₈ (tridecapeptide, TDP) in experimental models of acute and chronic pain. The analgesic effects of TDP were estimated using a model of acute visceral pain in mice (writhing test) and a model of chronic neuropathic pain (chronic constriction injury (CCI) of the sciatic nerve) in rats. The intravenous administration of morphine (0.32-1mg/kg) and TDP (0.32-1.8mg/kg) produced significant dose-related antinociceptive effects in the writhing test. The potency of TDP after i.g. administration was lower than that after i.v. administration but comparable with that of i.g. morphine. In the CCI model, TDP (0.1, 1 and 10mg/kg, i.g.) induced marked analgesia with repeated administration without any signs of tolerance. The single administration of TDP after morphine treatment (7days) produced a significant analgesic effect in morphine-tolerant rats, indicating the absence of cross-tolerance between these two drugs. The combined administration of TDP and morphine resulted in the reduction of analgesic tolerance to morphine. The absence of cross-tolerance to morphine and the ability to prevent morphine tolerance allows this compound to be a prospective candidate for chronic pain therapy. In order to find the target receptors for TDP, a docking study was performed. It was found that the molecule can bind to the NMDA receptor using electrostatic, hydrogen bonding and hydrophobic interactions.

Novel endomorphin-1 analogs with C-terminal oligoarginine-conjugation display systemic antinociceptive activity with less gastrointestinal side effects

In recent study, in order to improve the bioavailability of endomorphin-1 (EM-1), we designed and synthesized a series of novel EM-1 analogs by replacement of L-Pro(2) by β-Pro, D-Ala or Sar, together with C-terminal oligoarginine-conjugation. Our results indicated that the introduction of D-Ala and β-Pro in position 2, along with oligoarginine-conjugation, didn't significantly decrease the μ-affinity and in vitro bioactivity, and the enhancement of arginine residues did not markedly influence the μ-affinity of these analogs. All analogs displayed a significant enhancement of stability, which may be due to increased resistance to proline-specific enzymatic degradation. Moreover, following intracerebroventricular (i.c.v.) administration, analogs 1, 2, 4 and 5 produced significant antinociception and increased duration of action, with the ED50 values being about 1.8- to 4.2-fold less potent than that of EM-1. In addition, our results indicated that no significant antinociceptive activity of EM-1 was seen following subcutaneous (s.c.) injection, whereas analogs 1, 2, 4 and 5 with equimolar dose induced significant and prolonged antinociception by an opioid and central mechanism. Herein, we further examined the gastrointestinal transit and colonic propulsive latencies of EM-1 and its four analogs administered centrally and peripherally. I.c.v. administration of EM-1 and analogs 1, 2, 4 and 5 significantly delayed gastrointestinal transit and colonic bead propulsion in mice, but the inhibitory effects induced by these analogs were largely attenuated. It is noteworthy that no significant gastrointestinal side effects induced by these four analogs were observed after s.c. administration. Our results demonstrated that combined modifications of EM-1 with unnatural amino acid substitutions and oligoarginine-conjugation gave an efficient strategy to improve the analgesic profile of EM-1 analogs but with less gastrointestinal side effects when administered peripherally.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 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 (Section 16); and immunological responses (Section 17).

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.