Structure–Activity Relationship Study of Opiorphin, a Human Dual Ectopeptidase Inhibitor with Antinociceptive Properties

Opiorphin: an endogenous human peptide with intriguing application in diverse range of pathologies

Mammalian zinc ectopeptidases have significant functions in deactivating neurological and hormonal peptide signals on the cell surface. The identification of Opiorphin, a physiological inhibitor of zinc ectopeptidases that inactivate enkephalin, has revealed its strong analgesic effects in both chemical and mechanical pain models. Opiorphin achieves this by increasing the transmission of endogenous opioids, which are dependent on the body's own opioid system. The function of opiorphin is closely linked to the rat sialorphin peptide, which inhibits pain perception by enhancing the activity of naturally occurring enkephalinergic pathways that depend on μ- and δ-opioid receptors. Opiorphin is highly intriguing in terms of its physiological implications within the endogenous opioidergic pathways, particularly in its ability to regulate mood-related states and pain perception. Opiorphin can induce antidepressant-like effects by influencing the levels of naturally occurring enkephalin, which are released in response to specific physical and/or psychological stimuli. This effect is achieved through the modulation of delta-opioid receptor-dependent pathways. Furthermore, research has demonstrated that opiorphin's impact on the cardiovascular system is facilitated by the renin-angiotensin system (RAS), sympathetic ganglia, and adrenal medulla, rather than the opioid system. Hence, opiorphin shows great potential as a solitary candidate for the treatment of several illnesses such as neurodegeneration, pain, and mood disorders.

Time-course of pain and salivary opiorphin release in response to oral capsaicin differ in burning mouth syndrome patients, temporomandibular disorders patients and control subjects

OBJECTIVES

Opiorphin is an analgesic peptide released by salivary glands and capsaicin an agonist of TRPV1 receptors eliciting burning sensations. The primary objective of this study was to assess opiorphin release after stimulation of the tongue by capsaicin (STC). The secondary objectives were to compare opiorphin release after STC in 3 groups of subjects [healthy (CTRL), Burning Mouth Syndrome (BMS), painful Temporomandibular disorders (TMDp)] and pain evoked by STC in these 3 groups.

MATERIALS AND METHODS

Salivary opiorphin was assessed with high-performance liquid chromatography at 3 different time points (baseline, after 5 min and 20 min of STC). Pain was self-reported on a (0-10) numeric rating scale.

RESULTS

Three groups (N = 16) of adults were recruited at the Clinical Hospital Centre and School of Dental Medicine in Zagreb. Opiorphin levels were higher (1) in TMDp compared to CTRL in 1st (2.23 ± 1.72 pg/ul vs. 0.67 ± 0.44 pg/ul, p = 0.002) and 3rd sampling (2.44 ± 2.01 pg/ul vs. 0.74 ± 0.52 pg/ul, p = 0.020) and (2) within BMS group at 3rd sampling vs. baseline (p < 0.025). Pain scores were higher in BMS compared to TMDp (p < 0.025) and CTRL (p < 0.025).

CONCLUSION

This study evidenced (1) a differential basal amount of opiorphin in two pain conditions and control subjects (2) a differential kinetic of release of opiorphin after STC in CTRL, BMS and TMDp (3) a differential pain perception after STC in BMS and TMDp vs. CTRL, which can provide a readout for animal models.

CLINICAL RELEVANCE

The specific regulation of opiorphin release in patients with orofacial painful conditions provides valuable insights for clinicians and researchers in physiology and pathology and encourages further research in this area.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04694274. Registered on 01/05/2021.

Influence of polar side chains modifications on the dual enkephalinase inhibitory activity and conformation of human opiorphin, a pain perception related peptide

The dual inhibitory action of the pain related peptide opiorphin (H-Gln-Arg-Phe-Ser-Arg-OH) against neutral endopeptidase (NEP) and aminopeptidase N (AP-N) was further investigated by a SAR study involving minor modifications on the polar side chains of Arg residues and glycosylation with monosaccharides at Ser. None of them exerted dual or individual inhibitory potency superior than opiorphin. However, the correlations deduced offer further proof for the key role of these residues upon the binding and bioactive conformational stabilization of opiorphin. NMR conformational studies on the glycopeptides suggest that they are still very flexible compounds that may attain their respective bioactive conformations.

Injectable liposomal formulations of opiorphin as a new therapeutic strategy in pain management

BACKGROUND

Conventional and PEGylated liposomes were developed, aimed at improving the pain-killing effect of opiorphin.

METHODS

The antinociceptive action of the formulations was investigated on rats (tail-flick test), and compared with that of opiorphin and morphine aqueous solutions (all at 5 mg/kg).

RESULTS

Opiorphin loading in conventional liposomes enabled a 28% AUC increase with respect to free peptide. PEGylated liposomes provided AUC values 80, 60 and 40% higher than free peptide, morphine and opiorphin-loaded conventional liposomes, respectively. Moreover, opiorphin entrapment in PEGylated liposomes increased analgesic effect duration by more than 50%. These results were attributed to the greater effectiveness of PEGylated liposomes in protecting the drug and prolonging its circulation time.

CONCLUSION

Opiorphin-loaded PEGylated-liposomes can represent a valid alternative to morphine in pain management.

Transfer of opiorphin through a blood-brain barrier culture model

Opioid peptides are potent analgesics with therapeutic potential in the treatment of acute and chronic pain. Their efficacy is limited by peptidases (enkephalinases). Opiorphin pentapeptide (QRFSR) is the first characterized human endogenous inhibitor of enkephalinases. The peptide is able to increase the binding and affinity of endogenous opiates to mu opioid receptors; thus, the mechanism of opiorphin may provide a new therapeutic approach in pain management. The analgesic effect of opiorphin was proven in several earlier published in vitro and in vivo studies. Our aim was to test the transfer of opiorphin through a blood-brain barrier model for the first time. The flux of opiorphin was tested on a blood-brain barrier culture model consisting of rat brain endothelial, glial and pericyte cells. Brain endothelial cells in this triple co-culture model form tight monolayers characterized by transendothelial electrical resistance measurement. Relative quantity of the peptide was estimated by mass spectrometry. The transfer of opiorphin through the blood-brain barrier model was estimated to be ∼3%, whereas the permeability coefficient was 0.53 ± 1.36 × 10(-6) cm/s (n = 4). We also observed rapid conversion of N-terminal glutamine into pyroglutamic acid during the transfer experiments. Our results indicate that opiorphin crosses cultured brain endothelial cells in the absence of serum factors in a significant amount. This is in agreement with previous in vivo data showing potentiation of enkephalin-mediated antinociception. We suggest that opiorphin may have a potential as a centrally acting novel drug to treat pain.

Opiorphin analysis in equine plasma and urine using hydrophilic interaction LC–MS

Background: Due to opiorphin's analgesic and antidepressant functions, its illicit use is rumored in some racing jurisdictions. Opiorphin is very difficult to detect due to its hydrophilic nature and rapid degradation in plasma and urine samples. Methodology & Results: We have developed a sensitive, reliable method for opiorphin detection and confirmation in equine samples, using EDTA to inhibit analyte degradation between the time of collection and analysis. Opiorphin was extracted by weak cation exchange followed by analysis using HILIC-MS/MS. The method was validated and the LOD was determined to be 50 pg/ml in equine plasma and urine. Conclusion: The method has good selectivity and precision and is the first reported method for the detection of opiorphin in equine plasma and urine.

Opioids, Neutral Endopeptidase, its Inhibitors and Cancer: Is There a Relationship among them?

The role of endogenous animal opioids in the biology of cancer is widely recognized but poorly understood. This is, among others, because of the short half-life of these peptides, which are quickly inactivated by endopeptidases, e.g., neutral endopeptidase (NEP, CD10). It has been established that NEP is engaged in the modulation of the tumor microenvironment, among others that of colon cancer, by exerting influence on cell growth factors, the extracellular matrix and other biologically active substances. Although there are some discrepancies among the findings on the role of both opioids and NEP in cancer development, authors agree that their role seems to depend on the origin, stage and grade of tumor, and even on the method of examination. Moreover, recently, natural inhibitors of NEP, such as sialorphin, opiorphin and spinorphin have been detected. Their analgesic activity has been established. It is interesting to ask whether there is a relationship among opioid peptides, tumor-associated NEP and its inhibitors.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 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 and thermoregulation (Section 16); and immunological responses (Section 17).

Characterization of the effects of opiorphin and sialorphin and their analogs substituted in position 1 with pyroglutamic acid on motility in the mouse ileum

Opiorphin and sialorphin are two recently discovered endogenous enkephalin-degrading enzyme inhibitors. Our aim was to characterize their effect on the mouse ileum motility and to investigate the role of glutamine in position 1. Opiorphin, sialorphin, and their analogs substituted in position 1 with pyroglutamic acid (pGlu) were synthesized by the solid-phase method using Fmoc chemistry. The effect of peptides on gastrointestinal (GI) motility was characterized using in vitro assays and in mouse model of upper GI transit. Opiorphin and sialorphin, but not their analogs, significantly increased electrical field-stimulated contractions in the mouse ileum in a δ-opioid receptor-dependent manner. Opiorphin, sialorphin, and their analogs did not influence the effect of [Met(5)]enkephalin on smooth muscle contractility in the mouse ileum in vitro. [Met(5)]enkephalin and sialorphin, but not opiorphin injected intravenously (1 mg/kg), significantly inhibited the upper GI transit. The intraperitoneal administration of peptides (3 mg/kg) did not change the mouse upper GI transit. In conclusion, this is the first study investigating the effect of opiorphin and sialorphin on the mouse ileum motility and demonstrating that glutamine in position 1 is crucial for their pharmacological action. Our results may be important for further structure-activity relationship studies on opiorphin and sialorphin and future development of potent clinical therapeutics aiming at the enkephalinergic system.

Opiorphin highly improves the specific binding and affinity of MERF and MEGY to rat brain opioid receptors

Endogenously occurring opioid peptides are rapidly metabolized by different ectopeptidases. Human opiorphin is a recently discovered natural inhibitor of the enkephalin-inactivating neutral endopeptidase (NEP) and aminopeptidase-N (AP-N) (Wisner et al., 2006). To date, in vitro receptor binding experiments must be performed either in the presence of a mixture of peptidase inhibitors and/or at low temperatures, to block peptidase activity. Here we demonstrate that, compared to classic inhibitor cocktails, opiorphin dramatically increases the binding of [(3)H]MERF and [(3)H]MEGY ligands to rat brain membrane preparations. We found that at 0 °C the increase in specific binding is as high as 40-60% and at 24 °C this rise was even higher. In contrast, the binding of the control [(3)H]endomorphin-1, which is relatively slowly degraded in rat brain membrane preparations, was not enhanced by opiorphin compared to other inhibitors. In addition, in homologous binding displacement experiments, the IC(50) affinity values measured at 24 °C were also significantly improved using opiorphin compared to the inhibitor cocktail. In heterologous binding experiments the differences were less obvious, but still pronounced using [(3)H]MERF and MEGY compared to dynorphin(1-11), or naloxone and DAGO competitor ligands.