The multifunctional peptide DN-9 produced peripherally acting antinociception in inflammatory and neuropathic pain via μ- and κ-opioid receptors

The dual modulating effects of neuropeptide FF on morphine-induced analgesia at the spinal level

Increasing evidence indicates that neuropeptide FF (NPFF) produces analgesic effects and augments opioid-induced analgesia at the spinal level. However, our recent research demonstrated that NPFF exerted complex opioid-modulating effects in an inflammatory pain model after intrathecal (i.t.) injection. Consistent with previous findings, we found that i.t. NPFF dose-dependently attenuated complete Freund's adjuvant-induced pain hypersensitivity. Interestingly, pharmacological results illustrated that NPFF exhibited opposite opioid-modulating effects at the spinal level depending on its administration dosage, wherein i.t. NPFF potentiated morphine-induced anti-allodynia at the dose of 10 nmol, while attenuated morphine analgesia at an ultra-low-dose of 10 pmol. Behavioral results obtained from neuropeptide FF receptor 2 (NPFFR2) knockout animals suggested that both pro- and anti-opioid effects of NPFF were mediated by NPFFR2. Moreover, these modulating effects of spinal NPFFR2 were selectively targeting mu-opioid receptor, had no effect on delta- and kappa-opioid receptor agonist-induced analgesia. Finally, the opioid-modulating effects of NPFF were further verified using in vitro calcium imaging assay, demonstrating that pretreated with NPFF in primary-cultured spinal neurons significantly attenuated the inhibitory effects of morphine on high-K+-induced neuronal excitability. Taken together, our results suggested that NPFF exhibited dual modulating effects on morphine-induced analgesia after i.t. administration, which provides a possible mechanism to explain the complex opioid-modulating effects of endogenous NPFF systems.

Conorphin-66 produces peripherally restricted antinociception via the kappa-opioid receptor with limited side effects

With the current unmet demand for effective pain relief, analgesics without major central adverse effects are highly appealing, such as peripherally restricted kappa-opioid receptor (KOR) agonists. In this study, Conorphin-66, an analog of the selective KOR peptide agonist Conorphin T, was pharmacologically characterized in a series of experiments, with CR845 serving as the reference compound. Firstly, in vitro functional assay indicated that Conorphin-66 selectively activates KOR and exhibits weak β-arrestin2 signaling bias (-1.54 versus -4.35 for CR845). Additionally, subcutaneous Conorphin-66 produced potent antinociception in mouse pain models with ED50 values ranged from 0.02 to 3.28 μmol/kg, including tail-flick test, post-operative pain, formalin pain, and acetic acid-induced visceral pain. Similarly, CR845 exert potent antinociception in mouse pain models ranged from 0.15 to 1.47 μmol/kg. Notably, antagonism studies revealed that the analgesic effects of Conorphin-66 were mainly mediated by the peripheral KOR. Furthermore, Conorphin-66 produced non-tolerance-forming antinociception over 8 days. Unlike CR845, subcutaneous Conorphin-66 did not promote the sedation, anxiogenic effects, depressive-like effects, but did exhibit diuretic activity. Further study showed that Conorphin-66 does not have apparent antipruritic effects in an acute itch model. Overall, Conorphin-66 emerges as a novel peripherally restricted KOR agonist that produced potent antinociception with reduced side effects.

Intrathecal administration of MCRT produced potent antinociception in chronic inflammatory pain models via μ-δ heterodimer with limited side effects

An important goal in the opioid field is to discover effective analgesic drugs with minimal side effects. MCRT demonstrated potent antinociceptive effects with limited side effects, making it a promising candidate. However, its pharmacological properties and how it minimizes side effects remain unknown. Various mouse pain and opioid side effect models were used to evaluate the antinociceptive properties and safety at the spinal level. The targets of MCRT were identified through cAMP measurement, isolated tissue assays, and pharmacological experiments. Immunofluorescence was employed to visualize protein expression. MCRT displayed distinct antinociceptive effects between acute and chronic inflammatory pain models due to its multifunctional properties at the μ opioid receptor (MOR), µ-δ heterodimer (MDOR), and neuropeptide FF receptor 2 (NPFFR2). Activation of NPFFR2 reduced MOR-mediated antinociception, leading to bell-shaped response curves in acute pain models. However, activation of MDOR produced more effective antinociception in chronic inflammatory pain models. MCRT showed limited tolerance and opioid-induced hyperalgesia in both acute and chronic pain models and did not develop cross-tolerance to morphine. Additionally, MCRT did not exhibit addictive properties, gastrointestinal inhibition, and effects on motor coordination. Mechanistically, peripheral chronic inflammation or repeated administration of morphine and MCRT induced an increase in MDOR in the spinal cord. Chronic administration of MCRT had no apparent effect on microglial activation in the spinal cord. These findings suggest that MCRT is a versatile compound that provides potent antinociception with minimal opioid-related side effects. MDOR could be a promising target for managing chronic inflammatory pain and addressing the opioid crisis.

Peptide-derived ligands for the discovery of safer opioid analgesics

Drugs targeting the μ-opioid receptor (MOR) remain the most efficacious analgesics for the treatment of pain, but activation of MOR with current opioid analgesics also produces harmful side effects, notably physical dependence, addiction, and respiratory depression. Opioid peptides have been accepted as promising candidates for the development of safer and more efficacious analgesics. To develop peptide-based opioid analgesics, strategies such as modification of endogenous opioid peptides, development of multifunctional opioid peptides, G protein-biased opioid peptides, and peripherally restricted opioid peptides have been reported. This review seeks to provide an overview of the opioid peptides that produce potent antinociception with much reduced side effects in animal models and highlight the potential advantages of peptides as safer opioid analgesics.

Structure-Activity Relationships of a Novel Cyclic Hexapeptide That Exhibits Multifunctional Opioid Agonism and Produces Potent Antinociceptive Activity

The cyclic peptide c[d-Lys2, Asp5]-DN-9 has recently been identified as a multifunctional opioid/neuropeptide FF receptor agonist, displaying potent analgesic activity with reduced side effects. This study utilized Tyr-c[d-Lys-Gly-Phe-Asp]-d-Pro-NH2 (0), a cyclic hexapeptide derived from the opioid pharmacophore of c[d-Lys2, Asp5]-DN-9, as a chemical template. We designed, synthesized, and characterized 22 analogs of 0 with a single amino acid substitution to investigate its structure-activity relationship. Most of these cyclic hexapeptide analogs exhibited multifunctional activity at μ and δ opioid receptors (MOR and DOR, respectively) and produced antinociceptive effects following subcutaneous administration. The lead compound analog 15 showed potent agonistic activities at the MOR, κ opioid receptor (KOR), and DOR in vitro and produced a strong and long-lasting analgesic effect through peripheral MOR and KOR in the tail-flick test. Further biological evaluation identified that analog 15 did not cause significant side effects such as tolerance, withdrawal, or reward liability.

All-Hydrocarbon Stapled Peptide Multifunctional Agonists at Opioid and Neuropeptide FF Receptors: Highly Potent, Long-Lasting Brain Permeant Analgesics with Diminished Side Effects

Our previous study reported the multifunctional agonist for opioid and neuropeptide FF receptors DN-9, along with its cyclic peptide analogues c[D-Cys2, Cys5]-DN-9 and c[D-Lys2, Asp5]-DN-9. These analogues demonstrated potent antinociceptive effects with reduced opioid-related side effects. To develop more stable and effective analgesics, we designed, synthesized, and evaluated seven hydrocarbon-stapled cyclic peptides based on DN-9. In vitro calcium mobilization assays revealed that most of the stapled peptides, except 3, displayed multifunctional agonistic activities at opioid and neuropeptide FF receptors. Subcutaneous administration of all stapled peptides resulted in effective and long-lasting antinociceptive activities lasting up to 360 min. Among these stapled peptides, 1a and 1b emerged as the optimized compounds, producing potent central antinociception following subcutaneous, intracerebroventricular, and oral administrations. Additionally, subcutaneous administration of 1a and 1b caused nontolerance antinociception, with limited occurrence of constipation and addiction. Furthermore, 1a was selected as the final optimized compound due to its wider safety window compared to 1b.

The Downregulation of Opioid Receptors and Neuropathic Pain

Neuropathic pain (NP) refers to pain caused by primary or secondary damage or dysfunction of the peripheral or central nervous system, which seriously affects the physical and mental health of 7-10% of the general population. The etiology and pathogenesis of NP are complex; as such, NP has been a hot topic in clinical medicine and basic research for a long time, with researchers aiming to find a cure by studying it. Opioids are the most commonly used painkillers in clinical practice but are regarded as third-line drugs for NP in various guidelines due to the low efficacy caused by the imbalance of opioid receptor internalization and their possible side effects. Therefore, this literature review aims to evaluate the role of the downregulation of opioid receptors in the development of NP from the perspective of dorsal root ganglion, spinal cord, and supraspinal regions. We also discuss the reasons for the poor efficacy of opioids, given the commonness of opioid tolerance caused by NP and/or repeated opioid treatments, an angle that has received little attention to date; in-depth understanding might provide a new method for the treatment of NP.

Analgesic tolerance and cross-tolerance to the bifunctional opioid/neuropeptide FF receptors agonist EN-9 and μ-opioid receptor ligands at the supraspinal level in mice

The chimeric peptide EN-9 was reported as a κ-opioid/neuropeptide FF receptors bifunctional agonist that modulated chronic pain with no tolerance. Many lines of evidence have shown that the effect of the κ-opioid receptor is mediated by not only its specific activation but also downstream events participation, especially interaction with the μ-opioid receptor pathway in antinociception and tolerance on most occasions. The present study investigated the acute and chronic cross-tolerance of EN-9 with μ-opioid receptor agonist EM-2, DAMGO, and morphine after intracerebroventricularly (i.c.v) injection in the mouse tail-flick test. In the acute tolerance test, EN-9 showed symmetrical acute cross-tolerance to DAMGO but no cross-tolerance to EM2. In the chronic tolerance test, EN-9 had no tolerance after eight days of repeated administration. However, EN-9 illustrated complete cross-tolerance to morphine and symmetrical cross-tolerance to EM2. In addition, inhibition of NPFF receptor could induce the tolerance development of EN-9. These findings indicated that supraspinal EN-9-induced antinociception contains additional components, which are mediated by the downstream μ-opioid receptor pathway both in acute and chronic treatment, whereas the subtypes of μ-opioid receptor or NPFF system pathway involved in antinociceptive effects induced by EN-9 are complex. Identifying the receptor mechanism could help design preferable bifunctional opioid compounds.

OCP002, a Mixed Agonist of Opioid and Cannabinoid Receptors, Produces Potent Antinociception With Minimized Side Effects

BACKGROUND

Increasing attention has been attracted to the development of bifunctional compounds to minimize the side effects of opioid analgesics. Pharmacological studies have verified the functional interaction between opioid and cannabinoid systems in pain management, suggesting that coactivation of the opioid and cannabinoid receptors may provide synergistic analgesia with fewer adverse reactions. Herein, we developed and characterized a novel bifunctional compound containing the pharmacophores of the mu-opioid receptor agonist DALDA and the cannabinoid peptide VD-Hpα-NH2, named OCP002.

METHODS

The opioid and cannabinoid agonistic activities of OCP002 were investigated in calcium mobilization and western blotting assays, respectively. Moreover, the central and peripheral antinociceptive effects of OCP002 were evaluated in mouse preclinical models of tail-flick test, carrageenan-induced inflammatory pain, and acetic acid-induced visceral pain, respectively. Furthermore, the potential opioid and cannabinoid side effects of OCP002 were systematically investigated in mice after intracerebroventricular (ICV) and subcutaneous (SC) administrations.

RESULTS

OCP002 functioned as a mixed agonist toward mu-opioid, kappa-opioid, and cannabinoid CB1 receptors in vitro. ICV and SC injections of OCP002 produced dose-dependent antinociception in mouse models of nociceptive (the median effective dose [ED50] values with 95% confidence interval [CI] are 0.14 [0.12-0.15] nmol and 0.32 [0.29-0.35] μmol/kg for ICV and SC injections, respectively), inflammatory (mechanical stimulation: ED50 values [95% CI] are 0.76 [0.64-0.90] nmol and 1.23 [1.10-1.38] μmol/kg for ICV and SC injections, respectively; thermal stimulation: ED50 values [95% CI] are 0.13 [0.10-0.17] nmol and 0.23 [0.08-0.40] μmol/kg for ICV and SC injections, respectively), and visceral pain (ED50 values [95% CI] are 0.0069 [0.0050-0.0092] nmol and 1.47 [1.13-1.86] μmol/kg for ICV and SC injections, respectively) via opioid and cannabinoid receptors. Encouragingly, OCP002 cannot cross the blood-brain barrier and exerted nontolerance-forming analgesia over 6-day treatment at both supraspinal and peripheral levels. Consistent with these behavioral results, repeated OCP002 administration did not elicit microglial hypertrophy and proliferation, the typical features of opioid-induced tolerance, in the spinal cord. Furthermore, at the effective analgesic doses, SC OCP002 exhibited minimized opioid and cannabinoid side effects on motor performance, body temperature, gastric motility, physical and psychological dependence, as well as sedation in mice.

CONCLUSIONS

This study demonstrates that OCP002 produces potent and nontolerance-forming antinociception in mice with reduced opioid- and cannabinoid-related side effects, which strengthen the candidacy of bifunctional drugs targeting opioid/cannabinoid receptors for translational-medical development to replace or assist the traditional opioid analgesics.

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.

NaV1.7 Channel Blocker [Ala5, Phe6, Leu26, Arg28]GpTx-1 Attenuates CFA-induced Inflammatory Hypersensitivity in Rats via Endogenous Enkephalin Mechanism

Venom-derived Na_V1.7 channel blockers have promising prospects in pain management. The 34-residue tarantula peptide GpTx-1 is a potent Na_V1.7 channel blocker. Its powerful analog [Ala⁵, Phe⁶, Leu²⁶, Arg²⁸]GpTx-1 (GpTx-1-71) displayed excellent Na_V1.7 selectivity and analgesic properties in mice. The current study aimed to elucidate the anti-hyperalgesic activities of GpTx-1-71 in inflammatory pain and reveal the underlying mechanisms. Our results demonstrated that intrathecal and intraplantar injections of GpTx-1-71 dose-dependently attenuated CFA-induced inflammatory hypersensitivity in rats. Moreover, GpTx-1-71-induced anti-hyperalgesia was significantly reduced by opioid receptor antagonists and the enkephalin antibody and diminished in proenkephalin (Penk) gene knockout animals. Consistently, GpTx-1-71 treatment increased the enkephalin level in the spinal dorsal horn and promoted the Penk transcription and enkephalin release in primary dorsal root ganglion (DRG) neurons, wherein sodium played a crucial role in these processes. Mass spectrometry analysis revealed that GpTx-1-71 mainly promoted the secretion of Met-enkephalin but not Leu-enkephalin from DRG neurons. In addition, the combination of subtherapeutic Met-enkephalin and GpTx-1-71 produced synergistic anti-hyperalgesia in CFA-induced inflammatory hypersensitivity. These findings suggest that the endogenous enkephalin pathway is essential for GpTx-1-71-induced spinal and peripheral analgesia in inflammatory pain. PERSPECTIVE: This article presents a possible pharmacological mechanism underlying Na_V1.7 blocker-induced analgesia in inflammatory pain, which helps us to better understand and develop venom-based painkillers for incurable pain.

OFP011 Cyclic Peptide as a Multifunctional Agonist for Opioid/Neuropeptide FF Receptors with Improved Blood-Brain Barrier Penetration

Mounting evidence indicates that the neuropeptide FF (NPFF) system is involved in the side effects of opioid usage, including antinociceptive tolerance, hyperalgesia, abuse, constipation, and respiratory depression. Our group recently discovered that the multitarget opioid/NPFF receptor agonist DN-9 exhibits peripheral antinociceptive activity. To improve its metabolic stability, antinociceptive potency, and duration, in this study, we designed and synthesized a novel cyclic disulfide analogue of DN-9, OFP011, and examined its bioactivity through in vitro cyclic adenosine monophosphate (cAMP) functional assays and in vivo behavioral experiments. OFP011 exhibited multifunctional agonistic effects at the μ-opioid and the NPFF₁ and NPFF₂ receptors and partial agonistic effects at the δ- and κ-opioid in vitro, as determined via the cAMP functional assays. Pharmacokinetic and pharmacological experiments revealed improvement in its blood-brain barrier permeability after systemic administration. In addition, subcutaneous OFP011 exhibited potent and long-lasting antinociceptive activity via the central μ- and κ-opioid receptors, as observed in different physiological and pathological pain models. At the highest antinociceptive doses, subcutaneous OFP011 exhibited limited tolerance, gastrointestinal transit, motor coordination, addiction, reward, and respiration depression. Notably, OFP011 exhibited potent oral antinociceptive activities in mouse models of acute, inflammatory, and neuropathic pain. These results suggest that the multifunctional opioid/NPFF receptor agonists with improved blood-brain barrier penetration are a promising strategy for long-term treatment of moderate to severe nociceptive and pathological pain with fewer side effects.

An analgesic peptide H-20 attenuates chronic pain via the PD-1 pathway with few adverse effects

The lack of effective and safe analgesics for chronic pain management has been a health problem associated with people's livelihoods for many years. Analgesic peptides have recently shown significant therapeutic potential, as they are devoid of opioid-related adverse effects. Programmed cell death protein 1 (PD-1) is widely expressed in neurons. Activation of PD-1 by PD-L1 modulates neuronal excitability and evokes significant analgesic effects, making it a promising target for pain treatment. However, the research and development of small molecule analgesic peptides targeting PD-1 have not been reported. Here, we screened the peptide H-20 using high-throughput screening. The in vitro data demonstrated that H-20 binds to PD-1 with micromolar affinity, evokes Src homology 2 domain-containing tyrosine phosphatase 1 (SHP-1) phosphorylation, and diminishes nociceptive signals in dorsal root ganglion (DRG) neurons. Preemptive treatment with H-20 effectively attenuates perceived pain in naïve WT mice. Spinal H-20 administration displayed effective and longer-lasting analgesia in multiple preclinical pain models with a reduction in or absence of tolerance, abuse liability, constipation, itch, and motor coordination impairment. In summary, our findings reveal that H-20 is a promising candidate drug that ameliorates chronic pain in the clinic.

Long March Toward Safe and Effective Analgesia by Enhancing Gene Expression of Kcc2: First Steps Taken

Low intraneuronal chloride in spinal cord dorsal horn pain relay neurons is critical for physiologic transmission of primary pain afferents because low intraneuronal chloride dictates whether GABA-ergic and glycin-ergic neurotransmission is inhibitory. If the neuronal chloride elevates to pathologic levels, then spinal cord primary pain relay becomes leaky and exhibits the behavioral hallmarks of pathologic pain, namely hypersensitivity and allodynia. Low chloride in spinal cord dorsal horn neurons is maintained by proper gene expression of Kcc2 and sustained physiologic function of the KCC2 chloride extruding electroneutral transporter. Peripheral nerve injury and other forms of neural injury evoke greatly diminished Kcc2 gene expression and subsequent corruption of inhibitory neurotransmission in the spinal cord dorsal horn, thus causing derailment of the gate function for pain. Here I review key discoveries that have helped us understand these fundamentals, and focus on recent insights relating to the discovery of Kcc2 gene expression enhancing compounds via compound screens in neurons. One such study characterized the kinase inhibitor, kenpaullone, more in-depth, revealing its function as a robust and long-lasting analgesic in preclinical models of nerve injury and cancer bone pain, also elucidating its mechanism of action via GSK3β inhibition, diminishing delta-catenin phosphorylation, and facilitating its nuclear transfer and subsequent enhancement of Kcc2 gene expression by de-repressing Kaiso epigenetic transcriptional regulator. Future directions re Kcc2 gene expression enhancement are discussed, namely combination with other analgesics and analgesic methods, such as spinal cord stimulation and electroacupuncture, gene therapy, and leveraging Kcc2 gene expression-enhancing nanomaterials.

Spinal microglia-derived TNF promotes the astrocytic JNK/CXCL1 pathway activation in a mouse model of burn pain

Burn injury-induced pain (BIP) is an extremely complicated condition usually resistant to analgesic drugs, while its pathogenesis remains unknown. Considerable attention has been attracted to elucidate the glial mechanisms in chronic pain. In this study, we initiatively used a mouse model of second-degree BIP to investigate the underlying non-neuronal mechanisms at the spinal cord level. Our behavioral results showed that hind-paw burn injury caused persistent allodynia and hyperalgesia for 2 weeks in mice. Further studies revealed that both microglia and astrocytes activated in a spatially- and temporally-dependent manner in spinal cord after burn injury. In addition, the phosphorylated p38 mitogen-activated protein kinase (MAPK)-mediated tumor necrosis factor (TNF) release in spinal microglia is essentially attributed to the early stage of BIP, while the c-Jun N-terminal kinase (JNK) MAPK-dependent chemokine CXCL1 expression is mainly involved in the maintenance of pain hypersensitivity. Most strikingly, burn injury-induced pain symptoms and the activation of astrocytes were significantly suppressed by TNF inhibitor Thalidomide. On the contrary, intrathecal injection of TNF caused apparent pain hypersensitivity, accompanied by the activation of astrocytes and the upregulation of CXCL1 via the JNK MAPK signaling pathway, indicating that TNF is the key cytokine in the interaction between microglia and astrocytes at the spinal level. Moreover, treatment with the CXCR2 receptor antagonist SB225002 to block the biological activities of CXCL1 significantly attenuated the mechanical allodynia and thermal hyperalgesia in this BIP model. Taken together, this study indicates that intervention of glial pathways provides a new perspective in the management of BIP.

NOP01, a NOP receptor agonist, produced potent and peripherally restricted antinociception in a formalin-induced mouse orofacial pain model

Orofacial pain is one of the most common medical challenges. A preliminary report indicates that the NOP receptor may act as a therapeutic target in orofacial pain. Previous studies have shown that [(pF)Phe⁴, Aib⁷, Aib¹¹, Arg¹⁴, Lys¹⁵]N/OFQ-NH₂ (NOP01) functions as a potent NOP receptor peptide agonist. This work aims to investigate the antinociception of NOP01 and its possible action mechanisms in a formalin-induced mouse orofacial pain model at different levels. Our results demonstrated that local, intraperitoneal (i.p.) or intrathecal (i.t.) injection of NOP01 produced dose-related antinociception in both phases of the formalin pain, which could be inhibited by the NOP receptor antagonist but not the classical opioid receptor antagonist. Furthermore, the antinociception induced by systemic NOP01 was blocked by local but not spinal pretreatment with the NOP receptor antagonist, suggesting the involvement of the peripheral NOP receptor in NOP01-induced systemic antinociception. Moreover, local injection of NOP01 markedly suppressed the expression of c-Fos protein induced by formalin in ipsilateral trigeminal ganglion (TG) neurons. In conclusion, this work suggests that NOP01 exerts significant antinociception on orofacial pain at both peripheral and spinal levels via the NOP receptor. Notably, NOP01 cannot readily penetrate the blood-brain barrier. Thus, NOP01 may behave as a potential compound for developing peripherally restricted analgesics.

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.

Development of Multifunctional and Orally Active Cyclic Peptide Agonists of Opioid/Neuropeptide FF Receptors that Produce Potent, Long-Lasting, and Peripherally Restricted Antinociception with Diminished Side Effects

We previously reported that a multifunctional opioid/neuropeptide FF receptor agonist, DN-9, achieved peripherally restricted analgesia with reduced side effects. To develop stable and orally bioavailable analogues of DN-9, eight lactam-bridged cyclic analogues of DN-9 between positions 2 and 5 were designed, synthesized, and biologically evaluated. In vitro cAMP assays revealed that these analogues, except 7, were multifunctional ligands that activated opioid and neuropeptide FF receptors. Analogue 1 exhibited improved potency for κ-opioid and NPFF₂ receptors. All analogues exhibited potent, long-lasting, and peripherally restricted antinociception in the tail-flick test without tolerance development after subcutaneous administration and produced oral analgesia. Oral administration of the optimized compound analogue 1 exhibited powerful, peripherally restricted antinociceptive effects in mouse models of acute, inflammatory, and neuropathic pain. Remarkably, orally administered analogue 1 had no significant side effects, such as tolerance, dependence, constipation, or respiratory depression, at effective analgesic doses.

Effects of intrathecal and intracerebroventricular microinjection of kaempferol on pain: possible mechanisms of action

Background and purpose:

Kaempferol (KM), a flavonoid, has an anti-inflammatory and anticancer effect and prevents many metabolic diseases. Nonetheless, very few studies have been done on the antinociceptive effects of KM. This research aimed at assessing the involvement of opioids, gamma-aminobutyric acid (GABA) receptors, and inflammatory mediators in the antinociceptive effects of KM in male Wistar rats.

Experimental approach:

The intracerebroventricular and/or intrathecal administration of the compounds was done for examining their central impacts on the thermal and chemical pain by the tail-flick and formalin paw tests. For assessing the role of opioid and GABA receptors in the possible antinociceptive effects of KM, several antagonists were used. Also, a rotarod test was carried out for assessing motor performance.

Findings/Results:

The intracerebroventricular and/or intrathecal microinjections of KM (40 μg/rat) had partially antinociceptive effects in the tail-flick test in rats (P < 0.05). In the formalin paw model, the intrathecal microinjection of KM had antinociceptive effects in phase 1 (20 and 40 μg/rat; P < 0.05 and P < 0.01, respectively) and phase 2 (20 and 40 μg/rat; P < 0.01 and P < 0.001, respectively). Using naloxonazine and/or bicuculline approved the involvement of opioid and GABA receptors in the central antinociceptive effects of KM, respectively. Moreover, KM reduced the expression levels of caspase 6, interleukin-1β, tumor necrosis factor-α, and interleukin-6. The antinociceptive effects of KM were not linked to variations in the locomotor activity.

Conclusion and implications:

It can be concluded that KM has remarkable antinociceptive effects at a spinal level, which is associated with the presence of the inflammatory state. These impacts were undetectable following injections in the lateral ventricle. The possible mechanisms of KM antinociception are possibly linked to various modulatory pathways, including opioid and GABA receptors.

Central opioid receptors mediate morphine-induced itch and chronic itch via disinhibition

Opioids such as morphine are mainstay treatments for clinical pain conditions. Itch is a common side effect of opioids, particularly as a result of epidural or intrathecal administration. Recent progress has advanced our understanding of itch circuits in the spinal cord. However, the mechanisms underlying opioid-induced itch are not fully understood, although an interaction between µ-opioid receptor (MOR) and gastrin-releasing peptide receptor (GRPR) in spinal GRPR-expressing neurons has been implicated. In this study we investigated the cellular mechanisms of intrathecal opioid-induced itch by conditional deletion of MOR-encoding Oprm1 in distinct populations of interneurons and sensory neurons. We found that intrathecal injection of the MOR agonists morphine or DAMGO elicited dose-dependent scratching as well as licking and biting, but this pruritus was totally abolished in mice with a specific Oprm1 deletion in Vgat+ neurons [Oprm1-Vgat (Slc32a1)]. Loss of MOR in somatostatin+ interneurons and TRPV1+ sensory neurons did not affect morphine-induced itch but impaired morphine-induced antinociception. In situ hybridization revealed Oprm1 expression in 30% of inhibitory and 20% of excitatory interneurons in the spinal dorsal horn. Whole-cell recordings from spinal cord slices showed that DAMGO induced outward currents in 9 of 19 Vgat+ interneurons examined. Morphine also inhibited action potentials in Vgat+ interneurons. Furthermore, morphine suppressed evoked inhibitory postsynaptic currents in postsynaptic Vgat- excitatory neurons, suggesting a mechanism of disinhibition by MOR agonists. Notably, morphine-elicited itch was suppressed by intrathecal administration of NPY and abolished by spinal ablation of GRPR+ neurons with intrathecal injection of bombesin-saporin, whereas intrathecal GRP-induced itch response remained intact in mice lacking Oprm1-Vgat. Intrathecal bombesin-saporin treatment reduced the number of GRPR+ neurons by 97% in the lumber spinal cord and 91% in the cervical spinal cord, without changing the number of Oprm1+ neurons. Additionally, chronic itch from DNFB-induced allergic contact dermatitis was decreased by Oprm1-Vgat deletion. Finally, naloxone, but not peripherally restricted naloxone methiodide, inhibited chronic itch in the DNFB model and the CTCL model, indicating a contribution of central MOR signalling to chronic itch. Our findings demonstrate that intrathecal morphine elicits itch via acting on MOR on spinal inhibitory interneurons, leading to disinhibition of the spinal itch circuit. Our data also provide mechanistic insights into the current treatment of chronic itch with opioid receptor antagonist such as naloxone.

Multifunctional Opioid-Derived Hybrids in Neuropathic Pain: Preclinical Evidence, Ideas and Challenges

When the first- and second-line therapeutics used to treat neuropathic pain (NP) fail to induce efficient analgesia—which is estimated to relate to more than half of the patients—opioid drugs are prescribed. Still, the pathological changes following the nerve tissue injury, i.a. pronociceptive neuropeptide systems activation, oppose the analgesic effects of opiates, enforcing the use of relatively high therapeutic doses in order to obtain satisfying pain relief. In parallel, the repeated use of opioid agonists is associated with burdensome adverse effects due to compensatory mechanisms that arise thereafter. Rational design of hybrid drugs, in which opioid ligands are combined with other pharmacophores that block the antiopioid action of pronociceptive systems, delivers the opportunity to ameliorate the NP-oriented opioid treatment via addressing neuropathological mechanisms shared both by NP and repeated exposition to opioids. Therewith, the new dually acting drugs, tailored for the specificity of NP, can gain in efficacy under nerve injury conditions and have an improved safety profile as compared to selective opioid agonists. The current review presents the latest ideas on opioid-comprising hybrid drugs designed to treat painful neuropathy, with focus on their biological action, as well as limitations and challenges related to this therapeutic approach.

Peripherally acting opioid analgesics and peripherally-induced analgesia

The management of pain, particularly chronic pain, is still an area of medical need. In this context, opioids remain a gold standard for the treatment of pain. However, significant side effects, mainly of central origin, limit their clinical use. Here, we review recent progress to improve the therapeutic and safety profiles of opioids for pain management. Characterization of peripheral opioid-mediated pain mechanisms have been a key component of this process. Several studies identified peripheral µ, δ, and κ opioid receptors (MOR, DOR, and KOR, respectively) and nociceptin/orphanin FQ (NOP) receptors as significant players of opioid-mediated antinociception, able to achieve clinically significant effects independently of any central action. Following this, particularly from a medicinal chemistry point of view, main efforts have been directed towards the peripheralization of opioid receptor agonists with the objective of optimizing receptor activity and minimizing central exposure and the associated undesired effects. These activities have allowed the characterization of a great variety of compounds and investigational drugs that show low central nervous system (CNS) penetration (and therefore a reduced side effect profile) yet maintaining the desired opioid-related peripheral antinociceptive activity. These include highly hydrophilic/amphiphilic and massive molecules unable to easily cross lipid membranes, substrates of glycoprotein P (a extrusion pump that avoids CNS penetration), nanocarriers that release the analgesic agent at the site of inflammation and pain, and pH-sensitive opioid agonists that selectively activate at those sites (and represent a new pharmacodynamic paradigm). Hopefully, patients with pain will benefit soon from the incorporation of these new entities.

Central and peripheral modulation of gastrointestinal transit in mice by DN-9, a multifunctional opioid/NPFF receptor agonist

BACKGROUND

The nonapeptide DN-9 functions as a multifunctional agonist to opioid and neuropeptide FF (NPFF) receptors and exhibits antinociceptive effects at the central and peripheral levels.

METHODS

The effects of DN-9 on small and colonic intestinal transit were evaluated using the upper gastrointestinal (GI) transit test and colonic bead expulsion assay, respectively. Opioid and NPFF receptor antagonists were used to investigate the mechanisms of DN-9-induced GI inhibition. Furthermore, the agonism of the DN-9 analog [Phg⁹ ]-DN-9 to opioid and NPFF receptors was tested by the cAMP assay.

KEY RESULTS

Intracerebroventricular administration of DN-9 dose-dependently slowed upper GI transit and colonic expulsion via mu- and kappa-opioid receptors in the brain, independent of the delta-opioid receptor. Similarly, intraperitoneal injection of DN-9 dose-dependently inhibited GI propulsion via the peripheral opioid receptors. DN-9-induced GI transit inhibitions were significantly aggravated by the NPFF receptor antagonist RF9. Moreover, the DN-9 analog [Phg⁹ ]-DN-9, an agonist at mu-, delta-, and kappa-opioid receptors but not NPFF receptors, inhibited GI more potently than DN-9. In addition, intracerebroventricular NPFF significantly attenuated the central inhibitory effects induced by [Phg⁹ ]-DN-9 and morphine. However, central and peripheral injections of NPFF or RF9 almost had no significant effects on GI transit by itself.

CONCLUSION AND INFERENCES

Intracerebroventricular and intraperitoneal administrations of DN-9 inhibit GI transit via opioid receptors in mice by central and peripheral mechanisms, respectively. In addition, the NPFF agonism of DN-9 possesses antiopioid effects on GI transit, which might explain the reduced constipation at the antinociceptive doses.

VF-13, a chimeric peptide of VD-hemopressin(α) and neuropeptide VF, produces potent antinociception with reduced cannabinoid-related side effects

Pharmacological evidence indicated a functional interaction between neuropeptide FF (NPFF) and cannabinoid systems, and the cannabinoids combined with the NPFF receptor agonist neuropeptide VF (NPVF) produced antinociception without tolerance. In the present study, VF-13, a chimeric peptide containing the pharmacophores of the endogenous cannabinoid peptide VD-hemopressin(α) (VD-Hpα) and NPVF, was synthesized and pharmacologically evaluated. In vitro, VF-13 significantly upregulated the phosphorylated level of extracellular signal-regulated kinase 1/2 (ERK1/2) in CHO cells stably expressing CB1 receptors and inhibited forskolin-induced cAMP accumulation in HEK293 cells stably expressing NPFF₁ or NPFF₂ receptors. Moreover, VF-13 induced neurite outgrowth in Neuro 2A cells via CB1 and NPFF receptors. These results suggest that VF-13 exhibits multifunctional agonism at CB1, NPFF₁ and NPFF₂ receptors in vitro. Interestingly, intracerebroventricular VF-13 produced dose-dependent antinociception in mouse models of tail-flick and carrageenan-induced inflammatory pain via the TRPV1 receptor. In contrast, the reference compound (m)VD-Hpα-NH₂ induced CB1 receptor-mediated supraspinal antinociception. Additionally, subcutaneous injection of (m)VD-Hpα-NH₂ and VF-13 produced significant antinociception in carrageenan-induced inflammatory pain model. In the tetrad assay, our data demonstrated that VF-13 elicited hypothermia, but not catalepsy and hypoactivity after intracerebroventricular injection. Notably, VF-13 produced non-tolerance forming antinociception over 6 days treatment in both acute and inflammatory pain models. Furthermore, VF-13 had no apparent effects on gastrointestinal transit, pentobarbitone-induced sedation, food intake, and motor coordination at the supraspinal level. In summary, VF-13, a novel chimeric peptide of VD-Hpα and NPVF, produced non-tolerance forming antinociception in preclinical pain models with reduced cannabinoid-related side effects.

The multifunctional peptide DN-9 produced peripherally acting antinociception in inflammatory and neuropathic pain via μ- and κ-opioid receptors

BACKGROUND AND PURPOSE

Considerable effort has recently been directed at developing multifunctional opioid drugs to minimize the unwanted side effects of opioid analgesics. We have developed a novel multifunctional opioid agonist, DN-9. Here, we studied the analgesic profiles and related side effects of peripheral DN-9 in various pain models.

EXPERIMENTAL APPROACH

Antinociceptive effects of DN-9 were assessed in nociceptive, inflammatory, and neuropathic pain. Whole-cell patch-clamp and calcium imaging assays were used to evaluate the inhibitory effects of DN-9 to calcium current and high-K⁺ -induced intracellular calcium ([Ca²⁺ ]_i ) on dorsal root ganglion (DRG) neurons respectively. Side effects of DN-9 were evaluated in antinociceptive tolerance, abuse, gastrointestinal transit, and rotarod tests.

KEY RESULTS

DN-9, given subcutaneously, dose-dependently produced antinociception via peripheral opioid receptors in different pain models without sex difference. In addition, DN-9 exhibited more potent ability than morphine to inhibit calcium current and high-K⁺ -induced [Ca²⁺ ]_i in DRG neurons. Repeated treatment with DN-9 produced equivalent antinociception for 8 days in multiple pain models, and DN-9 also maintained potent analgesia in morphine-tolerant mice. Furthermore, chronic DN-9 administration had no apparent effect on the microglial activation of spinal cord. After subcutaneous injection, DN-9 exhibited less abuse potential than morphine, as was gastroparesis and effects on motor coordination.

CONCLUSIONS AND IMPLICATIONS

DN-9 produces potent analgesia with minimal side effects, which strengthen the candidacy of peripherally acting opioids with multifunctional agonistic properties to enter human studies to alleviate the current highly problematic misuse of classic opioids on a large scale.