Antagonists of toll like receptor 4 maybe a new strategy to counteract opioid-induced hyperalgesia and opioid tolerance

Oxycodone-Naloxone Combination Hinders Opioid Consumption in Osteoarthritic Chronic Low Back Pain: A Retrospective Study with Two Years of Follow-Up

Chronic low back pain (CLBP) due to osteoarthritis represents a therapeutic challenge worldwide. Opioids are extensively used to treat such pain, but the development of tolerance, i.e., less susceptibility to the effects of the opioid, which can result in a need for higher doses to achieve the same analgesic effect, may limit their use. Animal models suggest that ultra-low doses of opioid antagonists combined with opioid agonists can decrease or block the development of opioid tolerance. In this retrospective study, we tested this hypothesis in humans. In 2019, 53 patients suffering from CLBP were treated with either Oxycodone and Naloxone Prolonged Release (27 patients, OXN patients) or Oxycodone Controlled Release (26 patients, OXY patients). The follow-up period lasted 2 years, during which 10 patients discontinued the treatment, 5 out of each group. The remaining 43 patients reached and maintained the targeted pain relief, but at 18 and 24 months, the OXY patients showed a significantly higher oxycodone consumption than OXN patients to reach the same level of pain relief. No cases of respiratory depression or opioid abuse were reported. There were no significant differences in the incidence of adverse effects between the two treatments, except for constipation, more common in OXY patients. From our results, we can affirm that a long-term opioid treatment with oxycodone-naloxone combination, when compared with oxycodone only, may significantly hinder the development of opioid tolerance. We were also able to confirm, in our cohort, the well known positive effect of naloxone in terms of opioid-induced bowel dysfunction incidence reduction.

Interactions between the Nociceptin and Toll-like Receptor Systems

Nociceptin and the nociceptin receptor (NOP) have been described as targets for treatment of pain and inflammation, whereas toll-like receptors (TLRs) play key roles in inflammation and impact opioid receptors and endogenous opioids expression. In this study, interactions between the nociceptin and TLR systems were investigated. Human THP-1 cells were cultured with or without phorbol myristate acetate (PMA 5 ng/mL), agonists specific for TLR2 (lipoteichoic acid, LTA 10 µg/mL), TLR4 (lipopolysaccharide, LPS 100 ng/mL), TLR7 (imiquimod, IMQ 10 µg/mL), TLR9 (oligonucleotide (ODN) 2216 1 µM), PMA+TLR agonists, or nociceptin (0.01−100 nM). Prepronociceptin (ppNOC), NOP, and TLR mRNAs were quantified by RT-qPCR. Proteins were measured using flow cytometry. PMA upregulated ppNOC mRNA, intracellular nociceptin, and cell membrane NOP proteins (all p < 0.05). LTA and LPS prevented PMA’s upregulating effects on ppNOC mRNA and nociceptin protein (both p < 0.05). IMQ and ODN 2216 attenuated PMA’s effects on ppNOC mRNA. PMA, LPS, IMQ, and ODN 2216 increased NOP protein levels (all p < 0.05). PMA+TLR agonists had no effects on NOP compared to PMA controls. Nociceptin dose-dependently suppressed TLR2, TLR4, TLR7, and TLR9 proteins (all p < 0.01). Antagonistic effects observed between the nociceptin and TLR systems suggest that the nociceptin system plays an anti-inflammatory role in monocytes under inflammatory conditions.

Low-Dose Naltrexone Co-Treatment in the Prevention of Opioid-Induced Hyperalgesia

Opioid-induced hyperalgesia (OIH) is characterized by a heightened sensitivity to pain that occurs in patients following opioid use. Prescription of opioids is currently the standard form of pain management for both neuropathic and nociceptive pain, due to the relief that patients typically report following their use. Opioids, which aim to provide analgesic effects, can paradoxically cause increasing degrees of pain among the users. The increased nociception can be either due to the underlying pain for which the opioid was initially prescribed, or other unrelated pain. As a result, those who are initially prescribed opioids for chronic pain relief may instead be left with no overall relief, and experience additional algesia. While OIH can be treated through the reduction of opioid use, antagonistic treatment can also be utilized. In an attempt to reduce OIH in patients, low doses of the opioid antagonist naltrexone can be given concurrently. This review will analyze the current role and effectiveness of the use of naltrexone in managing OIH in opioid users as described in clinical and non-clinical studies. Additionally, it seeks to characterize the underlying mechanisms that enable opioid antagonist naltrexone to reduce OIH while still allowing opioids to act as an analgesic. The authors find that OIH is a prevalent condition, and in order to effectively combat it, clinicians and patients can benefit from an extended study on how naltrexone can be utilized as a treatment alongside opioids prescribed for pain management.

Toll-like receptor 4: A promising crossroads in the diagnosis and treatment of several pathologies

Toll-like receptor 4 (TLR4) is expressed in a wide variety of cells and is the central component of the mammalian innate immune system. Since its discovery in 1997, TLR4 has been assigned an ever-increasing number of functions that extend from pathogen recognition to tissue damage identification and promotion of the intrinsic "damage repair response" in pain, intestinal, respiratory and vascular disorders. Precisely, the finding of conserved sequence homology among species along with the molecular and functional characterisation of the TLR4 gene enabled researchers to envisage a common operating system in the activation of innate immunity and the initiation of plastic changes at the onset of chronic pain. Malfunctioning in other conditions was conceived in parallel. In this respect, "pivot" proteins and pathway redundancy are not just evolutionary leftovers but essential for normal functioning or cell survival. Indeed, at present, TLR4 single nucleotide polymorphisms (SNP) and their association with certain dysfunctions and diseases are being confirmed in different pools of patients. However, despite its ability to trigger pathogen infection or alternatively tissue injury communications to immune system, TLR4 targeting might not be considered a panacea. This review article represents a compilation of what we know about TLR4 from clinics and basic research on the 20th anniversary of its discovery. Understanding how to fine-tune the interaction between TLR4 and its specific ligands may lead in the next decades to the development of promising new treatments, reducing polypharmacy and probably having an impact on drug use in numerous pathologies.

Minocycline does not affect experimental pain or addiction-related outcomes in opioid maintained patients

RATIONALE

Minocycline, a tetracycline antibiotic, inhibits activation of microglia. In preclinical studies, minocycline prevented development of opioid tolerance and opioid-induced hyperalgesia (OIH). The goal of this study was to determine if minocycline changes pain threshold and tolerance in individuals with opioid use disorder who are maintained on agonist treatment.

METHODS

In this double-blind, randomized human laboratory study, 20 participants were randomized to either minocycline (200 mg/day) or placebo treatment for 15 days. The study had three test sessions (days 1, 8, and 15 of treatment) and one follow-up visit 1 week after the end of treatment. In each test session, participants were assessed on several subjective and cognitive measures, followed by assessment of pain sensitivity using the Cold Pressor Test (CPT). Daily surveys and cognitive measures using Ecological Momentary Assessment (EMA) were also collected four times a day on days 8 through 14 of treatment, and proinflammatory serum cytokines were assessed before and on the last day of treatment.

RESULTS

Minocycline treatment did not change pain threshold or tolerance on the CPT. Similarly, minocycline did not change severity of pain, opioid craving, withdrawal, or serum cytokines. Minocycline treatment increased accuracy on a Go/No-Go task.

CONCLUSIONS

While these findings do not support minocycline's effects on OIH, minocycline may have a potential use as a cognitive enhancer for individuals with opioid use disorder, a finding that warrants further systematic studies.

Impact of sex on pain and opioid analgesia: a review

Chronic pain is a debilitating condition that impacts tens of millions each year, resulting in lost wages for workers and exacting considerable costs in health care and rehabilitation. A thorough understanding of the neural mechanisms underlying pain and analgesia is critical to facilitate the development of therapeutic strategies and personalized medicine. Clinical and epidemiological studies report that women experience greater levels of pain than men and have higher rates of pain-related disorders. Studies in both rodents and humans report sex differences in the anatomical and physiologic properties of the descending antinociceptive circuit, mu opioid receptor (MOR) expression and binding, morphine metabolism, and immune system activation, all of which likely contribute to the observed sex differences in pain and opioid analgesia. Although more research is needed to elucidate the underlying mechanisms, these sex differences present potential therapeutic targets to optimize pain management strategies for both sexes.

Sex differences in innate immunity and its impact on opioid pharmacology

Morphine has been and continues to be one of the most potent and widely used drugs for the treatment of pain. Clinical and animal models investigating sex differences in pain and analgesia demonstrate that morphine is a more potent analgesic in males than in females. In addition to binding to the neuronal μ-opioid receptor, morphine binds to the innate immune receptor toll-like receptor 4 (TLR4), located on glial cells. Activation of glial TLR4 initiates a neuroinflammatory response that directly opposes morphine analgesia. Females of many species have a more active immune system than males; however, few studies have investigated glial cells as a potential mechanism driving sexually dimorphic responses to morphine. This Mini-Review illustrates the involvement of glial cells in key processes underlying observed sex differences in morphine analgesia and suggests that targeting glia may improve current treatment strategies for pain. © 2016 Wiley Periodicals, Inc.

Withdrawal-associated injury site pain (WISP): a descriptive case series of an opioid cessation phenomenon

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This descriptive case series among adults documents that pain can return temporarily at healed, previously pain-free injury sites during acute opioid withdrawal.

Withdrawal pain can be a barrier to opioid cessation. Yet, little is known about old injury site pain in this context. We conducted an exploratory mixed-methods descriptive case series using a web-based survey and in-person interviews with adults recruited from pain and addiction treatment and research settings. We included individuals who self-reported a past significant injury that was healed and pain-free before the initiation of opioids, which then became temporarily painful upon opioid cessation—a phenomenon we have named withdrawal-associated injury site pain (WISP). Screening identified WISP in 47 people, of whom 34 (72%) completed the descriptive survey, including 21 who completed qualitative interviews. Recalled pain severity scores for WISP were typically high (median: 8/10; interquartile range [IQR]: 2), emotionally and physically aversive, and took approximately 2 weeks to resolve (median: 14; IQR: 24 days). Withdrawal-associated injury site pain intensity was typically slightly less than participants' original injury pain (median: 10/10; IQR: 3), and more painful than other generalized withdrawal symptoms which also lasted approximately 2 weeks (median: 13; IQR: 25 days). Fifteen surveyed participants (44%) reported returning to opioid use because of WISP in the past. Participants developed theories about the etiology of WISP, including that the pain is the brain's way of communicating a desire for opioids. This research represents the first known documentation that previously healed, and pain-free injury sites can temporarily become painful again during opioid withdrawal, an experience which may be a barrier to opioid cessation, and a contributor to opioid reinitiation.

Sex Differences in Microglia Activity within the Periaqueductal Gray of the Rat: A Potential Mechanism Driving the Dimorphic Effects of Morphine

Although morphine remains the primary drug prescribed for alleviation of severe or persistent pain, both preclinical and clinical studies have shown that females require two to three times more morphine than males to produce comparable levels of analgesia. In addition to binding to the neuronal μ-opioid receptor, morphine binds to the innate immune receptor toll-like receptor 4 (TLR4) localized primarily on microglia. Morphine action at TLR4 initiates a neuroinflammatory response that directly opposes the analgesic effects of morphine. Here, we test the hypothesis that the attenuated response to morphine observed in females is the result of increased microglia activation in the periaqueductal gray (PAG), a central locus mediating the antinociceptive effects of morphine. We report that, whereas no overall sex differences in the density of microglia were noted within the PAG of male or female rats, microglia exhibited a more "activated" phenotype in females at baseline, with the degree of activation a significant predictor of morphine half-maximal antinociceptive dose (ED₅₀) values. Priming microglia with LPS induced greater microglia activation in the PAG of females compared with males and was accompanied by increased transcription levels of IL-1β and a significant rightward shift in the morphine dose-response curve. Blockade of morphine binding to PAG TLR4 with (+)-naloxone potentiated morphine antinociception significantly in females such that no sex differences in ED₅₀ were observed. These results demonstrate that PAG microglia are sexually dimorphic in both basal and LPS-induced activation and contribute to the sexually dimorphic effects of morphine in the rat.SIGNIFICANCE STATEMENT We demonstrate that periaqueductal gray (PAG) microglia contribute to the sexually dimorphic effects of morphine. Specifically, we report that increased activation of microglia in the PAG contributes to the attenuated response to morphine observed in females. Our data further implicate the innate immune receptor toll-like receptor 4 (TLR4) as an underlying mechanism mediating these effects and establish that TLR4 inhibition in the PAG of females reverses the sex differences in morphine responsiveness. These data suggest novel methods to improve current opioid-based pain management via inhibition of glial TLR4 and illustrate the necessity for sex-specific research and individualized treatment strategies for the management of pain in men and women.

Opioid adjuvant strategy: improving opioid effectiveness

Opioid analgesics continue to be the mainstay of pharmacologic treatment of moderate to severe pain. Many patients, particularly those suffering from chronic pain, require chronic high-dose analgesic therapy. Achieving clinical efficacy and tolerability of such treatment regimens is hampered by the appearance of opioid-induced side effects such as tolerance, hyperalgesia and withdrawal syndrome. Among the therapeutic options to improve the opioid effectiveness, this current review focuses on strategies combining opioids to other drugs that can modulate opioid-mediated effects. We will discuss about experimental evidences reported for several potential opioid adjuvants, including N-methyl-d-aspartate receptor antagonists, 5-HT7 agonists, sigma-1 antagonists, I2-R ligands, cholecystokinin antagonists, neuropeptide FF-R antagonists and toll-like receptor 4 antagonists.

A randomized clinical trial of the effects of ultra-low-dose naloxone infusion on postoperative opioid requirements and recovery

BACKGROUND

Tolerance to remifentanil during sevoflurane anesthesia may increase postoperative analgesic requirements. Low-dose naloxone not only has been shown to block the development of acute opioid tolerance but also to ameliorate undesired opioid-induced side effects. We hypothesized that naloxone prevents the acute opioid tolerance produced by a large dose of remifentanil, and reduces the incidence of opioid-induced side effects.

METHODS

Seventy-two patients undergoing open colorectal surgery were randomly assigned to receive intraoperative remifentanil (1) small dose at 0.1 μg/kg/min; (2) large dose at 0.30 μg/kg/min; or (3) large dose at 0.30 μg/kg/min combined with low-dose naloxone at 0.25 μg/kg/h just after the induction. Cumulative morphine consumption, postoperative pain scores, incidence of opioid-related side effects, time to recovery of bowel function, and length of hospital stay were recorded.

RESULTS

Cumulative morphine consumption at 24 h after surgery was higher in the large-dose remifentanil group (28 ± 12 mg) compared with the small-dose remifentanil group (17 ± 12 mg), and large-dose remifentanil-naloxone group (18 ± 9 mg), (P < 0.001). The median time to return of bowel function was shorter in the large-dose remifentanil-naloxone group than the other two groups (P < 0.05). The median length of hospital stay was lower in the large-dose remifentanil-naloxone group (8 [interquartile range: 8-12] days) compared with the small-dose remifentanil group (12 [interquartile range: 9-15] days) and large-dose remifentanil group (12 [interquartile range: 10-13] days), (P < 0.001).

CONCLUSION

Naloxone infusion prevented the acute opioid tolerance, provided a quicker recovery of bowel function, and reduced the length of hospital stay after open colorectal surgery.

Targeting Opioid-Induced Hyperalgesia in Clinical Treatment: Neurobiological Considerations

Opioid analgesics have become a cornerstone in the treatment of moderate to severe pain, resulting in a steady rise of opioid prescriptions. Subsequently, there has been a striking increase in the number of opioid-dependent individuals, opioid-related overdoses, and fatalities. Clinical use of opioids is further complicated by an increasingly deleterious profile of side effects beyond addiction, including tolerance and opioid-induced hyperalgesia (OIH), where OIH is defined as an increased sensitivity to already painful stimuli. This paradoxical state of increased nociception results from acute and long-term exposure to opioids, and appears to develop in a substantial subset of patients using opioids. Recently, there has been considerable interest in developing an efficacious treatment regimen for acute and chronic pain. However, there are currently no well-established treatments for OIH. Several substrates have emerged as potential modulators of OIH, including the N-methyl-D-aspartate and γ-aminobutyric acid receptors, and most notably, the innate neuroimmune system. This review summarizes the neurobiology of OIH in the context of clinical treatment; specifically, we review evidence for several pathways that show promise for the treatment of pain going forward, as prospective adjuvants to opioid analgesics. Overall, we suggest that this paradoxical state be considered an additional target of clinical treatment for chronic pain.

Codeine-induced hyperalgesia and allodynia: investigating the role of glial activation

Chronic morphine therapy has been associated with paradoxically increased pain. Codeine is a widely used opioid, which is metabolized to morphine to elicit analgesia. Prolonged morphine exposure exacerbates pain by activating the innate immune toll-like receptor-4 (TLR4) in the central nervous system. In silico docking simulations indicate codeine also docks to MD2, an accessory protein for TLR4, suggesting potential to induce TLR4-dependent pain facilitation. We hypothesized codeine would cause TLR4-dependent hyperalgesia/allodynia that is disparate from its opioid receptor-dependent analgesic rank potency. Hyperalgesia and allodynia were assessed using hotplate and von Frey tests at days 0, 3 and 5 in mice receiving intraperitoneal equimolar codeine (21 mg kg(-1)), morphine (20 mg kg(-1)) or saline, twice daily. This experiment was repeated in animals with prior partial nerve injury and in TLR4 null mutant mice. Interventions with interleukin-1 receptor antagonist (IL-1RA) and glial-attenuating drug ibudilast were assessed. Analyses of glial activation markers (glial fibrillary acid protein and CD11b) in neuronal tissue were conducted at the completion of behavioural testing. Despite providing less acute analgesia (P=0.006), codeine induced similar hotplate hyperalgesia to equimolar morphine vs saline (-9.5 s, P<0.01 and -7.3 s, P<0.01, respectively), suggesting codeine does not rely upon conversion to morphine to increase pain sensitivity. This highlights the potential non-opioid receptor-dependent nature of codeine-enhanced pain sensitivity-although the involvement of other codeine metabolites cannot be ruled out. IL-1RA reversed codeine-induced hyperalgesia (P<0.001) and allodynia (P<0.001), and TLR4 knock-out protected against codeine-induced changes in pain sensitivity. Glial attenuation with ibudilast reversed codeine-induced allodynia (P<0.001), and thus could be investigated further as potential treatment for codeine-induced pain enhancement.

Capsaicin-based therapies for pain control

The TRPV1 receptor is known to play a role in nociceptive transmission in multiple organ systems, usually in response to the pain of inflammation. TRPV1 antagonism has so far shown limited benefit in antinociception. Capsaicin, a TRPV1 agonist, has been shown to induce a refractory period in the nerve terminal expressing TRPV1 and even, in sufficient dosing, to create long-term nerve terminal defunctionalization. This has led to research into topical capsaicin as a treatment for multiple painful conditions. The majority of work has focused on musculoskeletal pain and neuropathic pain and has revealed that although low-dose topical capsaicin has limited effectiveness as an analgesic, high-dose capsaicin, when tolerated, has the potential for long-term analgesia in certain types of neuropathic pain.