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

Stapling of leu-enkephalin analogs with bifunctional reagents for prolonged analgesic activity

The design and synthesis of leu-enkephalin analogs by replacing the glycine residues with N-(2-thioethyl)glycines and opening the cyclisation potential is presented. The cyclization (stapling) was achieved using bifunctional reagents (hexafluorobenzene and trithiocyanuric acid derivatives). The CD conformational studies of the stapled analogs suggest that the peptides adopt the type I β-turn conformation, which is in agreement with the theoretical analysis. The analog containing a trithiocyanuric acid derivative with a benzyl substituent shows potent analgesic activity.

Modification and Delivery of Enkephalins for Pain Modulation

Chronic pain negatively affects patient's quality of life and poses a significant economic burden. First line pharmaceutical treatment of chronic pain, including NSAIDs or antidepressants, is often inefficient to reduce pain, or produces intolerable adverse effects. In such cases, opioids are frequently prescribed for their potent analgesia, but chronic opioid use is also frequently associated with debilitating side effects that may offset analgesic benefits. Nonetheless, opioids continue to be widely utilized due to the lack of effective alternative analgesics. Since their discovery in 1975, a class of endogenous opioids called enkephalins (ENKs) have been investigated for their ability to relieve pain with significantly reduced adverse effects compared to conventional opioids. Their low metabolic stability and inability to cross biological membranes, however, make ENKs ineffective analgesics. Over past decades, much effort has been invested to overcome these limitations and develop ENK-based pain therapies. This review summarizes and describes chemical modifications and ENK delivery technologies utilizing ENK conjugates, nanoparticles and ENK gene delivery approaches and discusses valid lessons, challenges, and future directions of this evolving field.

Bis-Cyclic Guanidine Heterocyclic Peptidomimetics as Opioid Ligands with Mixed μ-, κ- and δ-Opioid Receptor Interactions: A Potential Approach to Novel Analgesics

The design and development of analgesics with mixed-opioid receptor interactions has been reported to decrease side effects, minimizing respiratory depression and reinforcing properties to generate safer analgesic therapeutics. We synthesized bis-cyclic guanidine heterocyclic peptidomimetics from reduced tripeptides. In vitro screening with radioligand competition binding assays demonstrated variable affinity for the mu-opioid receptor (MOR), delta-opioid receptor (DOR), and kappa-opioid receptor (KOR) across the series, with compound 1968-22 displaying good affinity for all three receptors. Central intracerebroventricular (i.c.v.) administration of 1968-22 produced dose-dependent, opioid receptor-mediated antinociception in the mouse 55 °C warm-water tail-withdrawal assay, and 1968-22 also produced significant antinociception up to 80 min after oral administration (10 mg/kg, p.o.). Compound 1968-22 was detected in the brain 5 min after intravenous administration and was shown to be stable in the blood for at least 30 min. Central administration of 1968-22 did not produce significant respiratory depression, locomotor effects or conditioned place preference or aversion. The data suggest these bis-cyclic guanidine heterocyclic peptidomimetics with multifunctional opioid receptor activity may hold potential as new analgesics with fewer liabilities of use.

Endogenous opiates and behavior: 2019

This paper is the forty-second consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2019 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

An Effective and Safe Enkephalin Analog for Antinociception

Opioids account for 69,000 overdose deaths per annum worldwide and cause serious side effects. Safer analgesics are urgently needed. The endogenous opioid peptide Leu-Enkephalin (Leu-ENK) is ineffective when introduced peripherally due to poor stability and limited membrane permeability. We developed a focused library of Leu-ENK analogs containing small hydrophobic modifications. N-pivaloyl analog KK-103 showed the highest binding affinity to the delta opioid receptor (68% relative to Leu-ENK) and an extended plasma half-life of 37 h. In the murine hot-plate model, subcutaneous KK-103 showed 10-fold improved anticonception (142%MPE·h) compared to Leu-ENK (14%MPE·h). In the formalin model, KK-103 reduced the licking and biting time to ~50% relative to the vehicle group. KK-103 was shown to act through the opioid receptors in the central nervous system. In contrast to morphine, KK-103 was longer-lasting and did not induce breathing depression, physical dependence, and tolerance, showing potential as a safe and effective analgesic.

Cyclizing Painkillers: Development of Backbone-Cyclic TAPS Analogs

Painkillers are commonly used medications. Native peptide painkillers suffer from various pharmacological disadvantages, while small molecule painkillers like morphine are highly addictive. We present a general approach aimed to use backbone-cyclization to develop a peptidomimetic painkiller. Backbone-cyclization was applied to transform the linear peptide Tyr-Arg-Phe-Sar (TAPS) into an active backbone-cyclic peptide with improved drug properties. We designed and synthesized a focused backbone-cyclic TAPS library with conformational diversity, in which the members of the library have the generic name TAPS c(n-m) where n and m represent the lengths of the alkyl chains on the nitrogens of Gly and Arg, respectively. We used a combined screening approach to evaluate the pharmacological properties and the potency of the TAPS c(n-m) library. We focused on an in vivo active compound, TAPS c(2-6), which is metabolically stable and has the potential to become a peripheral painkiller being a full μ opioid receptor functional agonist. To prepare a large quantity of TAPS c(2-6), we optimized the conditions of the on-resin reductive alkylation step to increase the efficiency of its SPPS. NMR was used to determine the solution conformation of the peptide lead TAPS c(2-6).