The C-terminal amidated analogue of the substance P (SP) fragment SP(1-7) attenuates the expression of naloxone-precipitated withdrawal in morphine dependent rats

The amino-terminal heptapeptide of the algesic substance P provides analgesic effect in relieving chronic neuropathic pain

Of painful conditions, somatic pain of acute nociceptive origin can be effectively managed clinically, while neuropathic pain of chronic neuropathy origin is difficult to control. For molecules involved in pain sensation, substance P (SP) is algesic, exacerbating painful sensation, while its amino-terminal fragment, heptapeptide SP(1-7), confers biological activities different from its full-length parent neuropeptide precursor. We previously demonstrated SP(1-7) interaction with pain processing to alleviate chronic pain. Here we evaluated SP(1-7) and its C-terminal amidated analogue SP(1-7)amide, together with SP and opioid agonist DAMGO. We tested mouse behaviors of both acute somatic pain in tail-flick latency assay, and neuropathic pain in sciatic nerve injury model of chronic constriction injury (CCI). DAMGO produced dose-dependent analgesia for somatic pain as expected, so did both SP(1-7) and its analogue SP(1-7)amide, while SP yielded the opposite effect of algesia, in a phenomenon we termed 'contrintus', meaning 'opposite from within' to denote that two peptides of the same origin (SP and its metabolic fragment SP(1-7)) produced opposite effects. In CCI model, DAMGO showed a general reduction in allodynia sensitivity for both nerve-injured and normal paws, without selective effect for neuropathic pain, consistent with clinical observation that opioids are less effective for chronic neuropathic pain. On the other hand, both SP(1-7) and SP(1-7)amide displayed dose-dependent anti-allodynia effect that is selective for neuropathic pain. These findings suggest that SP(1-7) and its analogue may be useful for developing pharmaceuticals to treat neuropathic pain.

Synthesis and Preclinical Evaluation of the First Carbon-11 Labeled PET Tracers Targeting Substance P1-7

Two potent SP_1–7 peptidomimetics have been successfully radiolabeled via [¹¹C]CO₂-fixation with excellent yields, purity, and molar activity. l-[¹¹C]SP_1–7-peptidomimetic exhibited promising ex vivo biodistribution profile. Metabolite analysis showed that l-[¹¹C]SP_1–7-peptidomimetic is stable in brain and spinal cord, whereas rapid metabolic degradation occurs in rat plasma. Metabolic stability can be significantly improved by substituting l-Phe for d-Phe, preserving 70% more of intact tracer and resulting in better brain and spinal cord tracer retention. Positron emission tomography (PET) scanning confirmed moderate brain (1.5 SUV; peak at 3 min) and spinal cord (1.0 SUV; peak at 10 min) uptake for l- and d-[¹¹C]SP_1–7-peptidomimetic. A slight decrease in SUV value was observed after pretreatment with natural peptide SP_1–7 in spinal cord for l-[¹¹C]SP_1–7-peptidomimetic. On the contrary, blocking using cold analogues of l- and d-[¹¹C]tracers did not reduce the tracers’ brain and spinal cord exposure. In summary, PET scanning of l- and d-[¹¹C]SP_1–7-peptidomimetics confirms rapid blood–brain barrier and blood–spinal-cord barrier penetration. Therefore, further validation of these two tracers targeting SP_1–7 is needed in order to define a new PET imaging target and select its most appropriate radiopharmaceutical.

Neuropeptides: metabolism to bioactive fragments and the pharmacology of their receptors

The proteolytic processing of neuropeptides has an important regulatory function and the peptide fragments resulting from the enzymatic degradation often exert essential physiological roles. The proteolytic processing generates, not only biologically inactive fragments, but also bioactive fragments that modulate or even counteract the response of their parent peptides. Frequently, these peptide fragments interact with receptors that are not recognized by the parent peptides. This review discusses tachykinins, opioid peptides, angiotensins, bradykinins, and neuropeptide Y that are present in the central nervous system and their processing to bioactive degradation products. These well-known neuropeptide systems have been selected since they provide illustrative examples that proteolytic degradation of parent peptides can lead to bioactive metabolites with different biological activities as compared to their parent peptides. For example, substance P, dynorphin A, angiotensin I and II, bradykinin, and neuropeptide Y are all degraded to bioactive fragments with pharmacological profiles that differ considerably from those of the parent peptides. The review discusses a selection of the large number of drug-like molecules that act as agonists or antagonists at receptors of neuropeptides. It focuses in particular on the efforts to identify selective drug-like agonists and antagonists mimicking the effects of the endogenous peptide fragments formed. As exemplified in this review, many common neuropeptides are degraded to a variety of smaller fragments but many of the fragments generated have not yet been examined in detail with regard to their potential biological activities. Since these bioactive fragments contain a small number of amino acid residues, they provide an ideal starting point for the development of drug-like substances with ability to mimic the effects of the degradation products. Thus, these substances could provide a rich source of new pharmaceuticals. However, as discussed herein relatively few examples have so far been disclosed of successful attempts to create bioavailable, drug-like agonists or antagonists, starting from the structure of endogenous peptide fragments and applying procedures relying on stepwise manipulations and simplifications of the peptide structures.

Small constrained SP1-7 analogs bind to a unique site and promote anti-allodynic effects following systemic injection in mice

Previous results have shown that the substance P (SP) N-terminal fragment SP1-7 may attenuate hyperalgesia and produce anti-allodynia in animals using various experimental models for neuropathic pain. The heptapeptide was found to induce its effects through binding to and activating specific sites apart from any known neurokinin or opioid receptor. Furthermore, we have applied a medicinal chemistry program to develop lead compounds mimicking the effect of SP1-7. The present study was designed to evaluate the pharmacological effect of these compounds using the mouse spared nerve injury (SNI) model of chronic neuropathic pain. Also, as no comprehensive screen with the aim to identify the SP1-7 target has yet been performed we screened our lead compound H-Phe-Phe-NH2 toward a panel of drug targets. The extensive target screen, including 111 targets, did not reveal any hit for the binding site among a number of known receptors or enzymes involved in pain modulation. Our animal studies confirmed that SP1-7, but also synthetic analogs thereof, possesses anti-allodynic effects in the mouse SNI model of neuropathic pain. One of the lead compounds, a constrained H-Phe-Phe-NH2 analog, was shown to exhibit a significant anti-allodynic effect.

Exploration and pharmacokinetic profiling of phenylalanine based carbamates as novel substance p 1-7 analogues

The bioactive metabolite of Substance P, the heptapeptide SP1-7 (H-Arg-Pro-Lys-Pro-Gln-Gln-Phe-OH), has been shown to attenuate signs of hyperalgesia in diabetic mice, which indicate a possible use of compounds targeting the SP1-7 binding site as analgesics for neuropathic pain. Aiming at the development of drug-like SP1-7 peptidomimetics we have previously reported on the discovery of H-Phe-Phe-NH2 as a high affinity lead compound. Unfortunately, the pharmacophore of this compound was accompanied by a poor pharmacokinetic (PK) profile. Herein, further lead optimization of H-Phe-Phe-NH2 by substituting the N-terminal phenylalanine for a benzylcarbamate group giving a new type of SP1-7 analogues with good binding affinities is reported. Extensive in vitro as well as in vivo PK characterization is presented for this compound. Evaluation of different C-terminal functional groups, i.e., hydroxamic acid, acyl sulfonamide, acyl cyanamide, acyl hydrazine, and oxadiazole, suggested hydroxamic acid as a bioisosteric replacement for the original primary amide.

The dipeptide Phe-Phe amide attenuates signs of hyperalgesia, allodynia and nociception in diabetic mice using a mechanism involving the sigma receptor system

Background

Previous studies have demonstrated that intrathecal administration of the substance P amino-terminal metabolite substance P_1-7 (SP_1-7) and its C-terminal amidated congener induced antihyperalgesic effects in diabetic mice. In this study, we studied a small synthetic dipeptide related to SP_1-7 and endomorphin-2, i.e. Phe-Phe amide, using the tail-flick test and von Frey filament test in diabetic and non-diabetic mice.

Results

Intrathecal treatment with the dipeptide increased the tail-flick latency in both diabetic and non-diabetic mice. This effect of Phe-Phe amide was significantly greater in diabetic mice than non-diabetic mice. The Phe-Phe amide-induced antinociceptive effect in both diabetic and non-diabetic mice was reversed by the σ₁ receptor agonist (+)-pentazocine. Moreover, Phe-Phe amide attenuated mechanical allodynia in diabetic mice, which was reversible by (+)-pentazocine. The expression of spinal σ1 receptor mRNA and protein did not differ between diabetic mice and non-diabetic mice. On the other hand, the expression of phosphorylated extracellular signal-regulated protein kinase 1 (ERK1) and ERK2 proteins was enhanced in diabetic mice. (+)-Pentazocine caused phosphorylation of ERK1 and ERK2 proteins in non-diabetic mice, but not in diabetic mice.

Conclusions

These results suggest that the spinal σ₁ receptor system might contribute to diabetic mechanical allodynia and thermal hyperalgesia, which could be potently attenuated by Phe-Phe amide.

Substance P N-terminal fragment SP(1-7) attenuates chronic morphine tolerance and affects dynorphin B and nociceptin in rats

The N-terminal substance P fragment SP(1-7) is known to modulate hyperalgesia and opioid withdrawal in animal models. This study examined the effects of intraperitoneal (i.p.) injections of SP(1-7) on chronic morphine tolerance and on the levels of dynorphin B (DYN B) and nociceptin/orphanin FQ (N/OFQ) in various brain areas of male Sprague-Dawley rats. Morphine tolerance was induced by subcutaneous injections of the opioid (10mg/kg) twice daily for 7 days. SP(1-7) injected i.p. (185 nmol/kg) 30 min prior to morphine reduced the development of morphine tolerance. Immunoreactive (ir) DYN B and N/OFQ peptide levels were measured in several areas of the central nervous system. Levels of ir DYN B in rats treated with SP(1-7) and morphine were decreased in the nucleus accumbens, substantia nigra and ventral tegmental area and increased in the frontal cortex. The ir N/OFQ levels were increased in the periaqueductal gray and decreased in the nucleus accumbens. Since the concentration profiles of the two peptides were altered by SP(1-7) in the areas that are implicated in the modulation of opioid tolerance and analgesia, it is suggested that DYN B and N/OFQ systems may be involved in the effects of SP(1-7) on opioid tolerance.

Expression of 5-HT and SP in the rectum of rats during heroin abstinence, detoxification or relapse

AIM: To investigate the expression of 5-hydroxytryptamine (5-HT) and substance P (SP) in the rectum of rats during heroin abstinence, detoxication, and relapse.

METHODS: Male Sprague-Dawley rats were divided into normal control group (NCG), saline control group (SCG), and experiment group (EG). The EG group was further divided into heroin abstinence group (HAG), methadone detoxication group (MDG), and heroin relapse group (HRG). Rectum tissue samples were taken from each group to perform immunohistochemistry to detect the expression of 5-HT and SP.

RESULTS: Compared with the NCG and SCG groups, the immunostaining density of 5-HT- and SP-positive cells was greater, the numbers of 5-HT- and SP-positive cells increased (5-HT: 10 d: 5.09 ± 3.39 vs 3.16 ± 2.05, 2.80 ± 2.13; 24 d: 5.05 ± 3.04 vs 3.16 ± 2.05, 2.81 ± 1.85; SP: 10 d: 2.55 ± 1.35 vs 1.29 ± 0.86, 1.37 ± 0.93; 24 d: 2.57 ± 1.27 vs 1.29 ± 0.86, 1.39 ± 0.79, all P < 0.05), and the mean grey degree of 5-HT- and SP-positive cells decreased (F = 36.642, 4.583, P < 0.05) in the HAG and HRG groups. Compared with the NCG and SCG groups, there were no statistical significances in the immunostaining density, numbers, and mean grey degree of 5-HT- and SP- positive cells in the MDG group (all P > 0.05).

CONCLUSION: Altered expression of 5-HT and SP in the tunica mucosa recti of rats during heroin abstinence or relapse suggests that 5-HT and SP can regulate the recovery of digestive function during heroin abstinence, detoxification, and relapse.

The effect of substance P1-7 amide on nociceptive threshold in diabetic mice

We previously demonstrated that intrathecal treatment with substance P metabolite substance P(1-7) induced anti-hyperalgesia in diabetic mice. In the present study, we have used a synthetic analog of this peptide, the substance P(1-7) amide, showing higher binding affinity than the native heptapeptide, for studies of the tail-flick response in diabetic and non-diabetic mice. Intrathecal injection of substance P(1-7) amide produced prolongation of the tail-flick latency in both diabetic and non-diabetic mice, an effect that was more pronounced in diabetic mice than non-diabetic mice. Moreover, the observed antinociceptive potency of the substance P(1-7) amide was higher in both diabetic and non-diabetic mice in comparison with the native substance P(1-7). The antinociceptive effect of substance P(1-7) amide was reversed by naloxone but not by the selective opioid receptor antagonist β-funaltrexamine, naltrindole or nor-binaltorphimine, selective for the μ-, δ- or κ-opioid receptor, respectively. In addition, the antinociceptive effect induced by substance P(1-7) amide was partly reversed by the σ(1) receptor agonist (+)-pentazocine, suggesting a possible involvement of the σ(1) receptor for the action of this peptide. These results suggest that the actions of substance P(1-7) amide mimic the effects of the native substance P fragment but with higher potency and that the mechanisms for its action may involve the σ(1) receptor system.

Discovery of dipeptides with high affinity to the specific binding site for substance P1-7

Substance P 1-7 (SP(1-7), H-Arg-Pro-Lys-Pro-Gln-Gln-Phe-OH) is the major bioactive metabolite of substance P. The interest in this heptapeptide originates from the observation that it modulates, and in certain cases opposes the effects of the parent peptide, e.g., the nociceptive effect. The mu-opioid receptor agonist endomorphin-2 (EM-2, H-Tyr-Pro-Phe-Phe-NH(2)) has been found to also interact with the specific binding site of SP(1-7) with only a 10-fold lower affinity compared to the native peptide. Considering the smaller size of EM-2 compared to the target heptapeptide, it was selected as a lead compound in the development of low-molecular-weight ligands to the SP(1-7) binding site. An alanine scan and truncation study led to the unexpected discovery of the dipeptide H-Phe-Phe-NH(2) (K(i) = 1.5 nM), having equal affinity as the endogenous heptapeptide SP(1-7.) Moreover, the studies show that the C-terminal phenylalanine amide is crucial for the affinity of the dipeptide.