Padma Nair

A Structure–Activity Relationship Study and Combinatorial Synthetic Approach of C-Terminal Modified Bifunctional Peptides That Are δ/μ Opioid Receptor Agonists and Neurokinin 1 Receptor Antagonists

A series of bifunctional peptides with opioid agonist and substance P antagonist bioactivities were designed with the concept of overlapping pharmacophores. In this concept, the bifunctional peptides were expected to interact with each receptor separately in the spinal dorsal horn where both the opioid receptors and the NK1 receptors were found to be expressed, to show an enhanced analgesic effect, no opioid-induced tolerance, and to provide better compliance than coadministration of two drugs. Compounds were synthesized using a two-step combinatorial method for C-terminal modification. In the method, the protected C-terminal-free carboxyl peptide, Boc-Tyr( tBu)- d-Ala-Gly Phe-Pro-Leu-Trp(Boc)-OH, was synthesized as a shared intermediate using Fmoc solid phase chemistry on a 2-chlorotrityl resin. This intermediate was esterified or amidated in solution phase. The structure-activity relationships (SAR) showed that the C-terminus acted as not only a critical pharmacophore for the substance P antagonist activities, but as an address region for the opioid agonist pharmacophore that is structurally distant from the C-terminal. Among the peptides, H-Tyr- d -Ala-Gly-Phe-Pro-Leu-Trp-NH-Bzl ( 3) demonstrated high binding affinities at both delta and mu receptors ( K i = 10 and 0.65 nM, respectively) with efficient agonist functional activity in the mouse isolated vas deferens (MVD) and guinea pig isolated ileum (GPI) assays (IC 50 = 50 and 13 nM, respectively). Compound 3 also showed a good antagonist activity in the GPI assay with substance P stimulation ( K e = 26 nM) and good affinity for the hNK1 receptor ( K i = 14 nM). Consequently, compound 3 is expected to be a promising and novel type of analgesic with bifunctional activities.

Spinal or systemic TY005, a peptidic opioid agonist/neurokinin 1 antagonist, attenuates pain with reduced tolerance.

BACKGROUND AND PURPOSE The use of opioids in treating pain is limited due to significant side effects including somnolence, constipation, analgesic tolerance, addiction and respiratory depression. Pre‐clinical studies have shown that neurokinin 1 (NK1) receptor antagonists block opioid‐induced antinociceptive tolerance and may inhibit opioid‐induced rewarding behaviours. Here, we have characterized a bifunctional peptide with both opioid agonist and NK1 antagonist pharmacophores in a rodent model of neuropathic pain.

The Importance of Micelle-Bound States for the Bioactivities of Bifunctional Peptide Derivatives for δ/μ Opioid Receptor Agonists and Neurokinin 1 Receptor Antagonists

To provide new insight into the determining factors of membrane-bound peptide conformation that might play an important role in peptide-receptor docking and further biological behaviors, the dodecylphosphocholine (DPC) micelle-bound conformations of bifunctional peptide derivatives of delta-preferring opioid agonists and NK1 antagonists (1: Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bzl(CF 3) 2; 2: Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-3,5-Bzl(CF 3) 2; 3: Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-Bzl) were determined based on 2D NMR studies. Although the differences in the primary sequence were limited to the C-terminus, the obtained NMR conformations were unexpectedly different for each compound. Moreover, their biological activities showed different trends in direct relation to the compound-specific conformations in DPC micelles. The important result is that not only were the NK1 antagonist activities different (the pharmacophore located at the C-terminus)but the opioid agonist activities (this pharmacophore was at the structurally preserved N-terminus) also were shifted, suggesting that a general conformational change in the bioactive state was induced due to relatively small and limited structural modifications.

Building a Better Analgesic: Multifunctional Compounds that Address Injury-Induced Pathology to Enhance Analgesic Efficacy while Eliminating Unwanted Side Effects

The most highly abused prescription drugs are opioids used for the treatment of pain. Physician-reported drug-seeking behavior has resulted in a significant health concern among doctors trying to adequately treat pain while limiting the misuse or diversion of pain medications. In addition to abuse liability, opioid use is associated with unwanted side effects that complicate pain management, including opioid-induced emesis and constipation. This has resulted in restricting long-term doses of opioids and inadequate treatment of both acute and chronic debilitating pain, demonstrating a compelling need for novel analgesics. Recent reports indicate that adaptations in endogenous substance P/neurokinin-1 receptor (NK1) are induced by chronic pain and sustained opioid exposure, and these changes may contribute to processes responsible for opioid abuse liability, emesis, and analgesic tolerance. Here, we describe a multifunctional mu-/delta-opioid agonist/NK1 antagonist compound [Tyr-d-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-Bn(CF3)2 (TY027)] that has a preclinical profile of excellent antinociceptive efficacy, low abuse liability, and no opioid-related emesis or constipation. In rodent models of acute and neuropathic pain, TY027 demonstrates analgesic efficacy following central or systemic administration with a plasma half-life of more than 4 hours and central nervous system penetration. These data demonstrate that an innovative opioid designed to contest the pathology created by chronic pain and sustained opioids results in antinociceptive efficacy in rodent models, with significantly fewer side effects than morphine. Such rationally designed, multitargeted compounds are a promising therapeutic approach in treating patients who suffer from acute and chronic pain.

Novel Bifunctional Peptides as Opioid Agonists and NK-1 Antagonists

Introduction Inadequate treatment of pain is an important and urgent problem, which needs to be addressed. Opiates are still the drugs of choice for the treatment of moderate to severe acute pain; however their use are generally accompanied by undesired side effects like analgesic tolerance. Several pharmacological studies have suggested the role of substance P, in pain transmission [1]. It has been observed that coadministration of neurokinin-1 receptor antagonists and opioid agonists augmented the acute effects of opioids and also prevented opioid induced tolerance [2]. In view of these and other observations we have designed novel bifunctional peptides with mixed opioid μ/δ agonist activity and neurokinin-1 antagonist activity [3]. Earlier reports of molecules with opioid μ/δ agonist activity and neurokinin-1 antagonist activity did not have balanced activity at both the receptor’s systems [4]. In our studies the opioid pharmacophore chosen has the sequence H-Tyr-DAla-Gly-Phe which is a substructure of biphalin, a highly potent agonist at both δ and μ opioid receptors [5]. A 3,5-(bistrifluoromethyl) benzyl ester of N-acylated tryptophans was chosen as the NK1 pharmacophore [6]. These two pharmacophores were combined with amino acids acting as possible address moieties they could be part of the two pharmacophores. Sequences of some of the bifunctional peptides synthesized are represented below (Fig. 1)