Opioid Tolerance Research Articles

Opioid dose titration for cancer pain.

Opioid dose titration is a fundamental process of opioid therapy in cancer pain. To assess data opioid dose titration. The principal opioid dose titration methods, outcomes, and modalities of administration regarding the different opioid preparations were examined in different clinical contexts. Most studies suggested that opioid-naive patients should be started at doses of 15-30 mg/day of oral morphine equivalents. Opioid-tolerant patients may receive low or higher doses of oral morphine equivalents, depending on the level of opioid tolerance. Generally, dose increments of 30%-50% seem to be indicated to start dose titration. Some patients with severe excruciating cancer pain may present as an emergency requiring a rapid application of powerful analgesic strategies. The intravenous use of opioids may circumvent this problem providing a faster pain relief, due to the large availability and rapid achievement of effective plasma concentrations. Opioid dose titration is a delicate passage in patients with cancer pain. This approach may be different according to different clinical conditions. Opioid dose titration requires expertise to optimize cancer pain management while minimizing the development of adverse effects. While most approaches are meaningful and partially supported by existing literature, more studies are necessary to establish advantages and disadvantages in different clinical conditions. Optimization of opioid dose titration is of paramount importance. This review provides the most recent insights on the different modalities of opioid dose titration in cancer pain management.

Dealing with "complex" pain patients in eye surgery : Perioperative management of patients with pre-existing chronic pain, opioid consumption and opioid use disorder

The quality of postoperative pain management is still considered insufficient in many cases, also in surgical ophthalmology. Complex constellations and comorbidities, such as pre-existing chronic pain, opioid consumption and opioid use disorders represent aspecial challenge due to psychosocial influencing factors and sometimes psychological and psychiatric comorbidities but also due to pharmacological effects, such as the development of opioid tolerance, the opioid-induced hyperalgesia. This review article aims to impart knowledge on aspects of these comorbidities and the perioperative management to improve the treatment skills of ophthalmologists in the management of pain in these complex patients.

Repurposing EGFR Inhibitors for Oral Cancer Pain and Opioid Tolerance.

Oral cancer pain remains a significant public health concern. Despite the development of improved treatments, pain continues to be a debilitating clinical feature of the disease, leading to reduced oral mobility and diminished quality of life. Opioids are the gold standard treatment for moderate-to-severe oral cancer pain; however, chronic opioid administration leads to hyperalgesia, tolerance, and dependence. The aim of this review is to present accumulating evidence that epidermal growth factor receptor (EGFR) signaling, often dysregulated in cancer, is also an emerging signaling pathway critically involved in pain and opioid tolerance. We presented preclinical and clinical data to demonstrate how repurposing EGFR inhibitors typically used for cancer treatment could be an effective pharmacological strategy to treat oral cancer pain and to prevent or delay the development of opioid tolerance. We also propose that EGFR interaction with the µ-opioid receptor and glutamate N-methyl-D-aspartate receptor could be two novel downstream mechanisms contributing to pain and morphine tolerance. Most data presented here support that repurposing EGFR inhibitors as non-opioid analgesics in oral cancer pain is promising and warrants further research.

Enhancing fentanyl antinociception and preventing tolerance with α-2 adrenoceptor agonists in rats

Fentanyl (FEN) is a potent opioid analgesic used for pain management. Opioid analgesic tolerance poses a significant challenge to the clinical utility of opioid agonists. Preventing the development of tolerance to opioid analgesia is crucial for improving its efficacy and safety. The noradrenergic system is involved in pain regulation. This study examined the effects of α-2 adrenoceptor (AR) agonists, dexmedetomidine (DEX), and xylazine (XYL) on FEN tolerance and antinociception, and their impact on μ-opioid receptor (MOR) expression in the posterior horn of the spinal cord (SC). Male rats were divided into six groups and treated with different drug combinations for three consecutive days. Analgesia tests and motor performance assessments were conducted, followed by SC analysis using immunohistochemistry (IHC). Analgesia tests revealed the development of FEN tolerance on the second day, but the groups receiving combined drugs did not develop tolerance. Instead, FEN antinociception was enhanced, with a prolonged duration of its effects. None of the drugs caused sedation or motor impairment, and SC morphology appeared normal. MOR expression levels did not differ significantly between the groups based on IHC analysis. These findings suggest that changes in the secondary messenger system may play a role in the early development of FEN tolerance. Combining drugs can prevent tolerance, while enhancing FEN's antinociceptive effects. These results have promising implications for chronic pain management; however, further research is needed to explore the molecular effects of α-2 AR agonists on FEN tolerance. Overall, this study sheds light on the mechanism of FEN tolerance and identifies potential avenues for future research.

Distribution of morphine and methadone to the brain in a developmental chicken embryo model

The use and/or misuse of opioids by pregnant women would expose the fetuses to these drugs during critical stages of development with serious effects for the newborn, like the neonatal abstinence syndrome (NAS). We have revisited an established chicken model for NAS to describe the distribution of morphine and methadone to the brain and explore its validity as a valuable alternative to rodent models. For this purpose, chicken eggs were injected with a single dose of 10 mg/kg or 20 mg/kg morphine or 20 mg/kg methadone onto the chorioallantoic membrane (CAM) on embryonal day 13. Whole brains and lungs were harvested and the concentrations of morphine, methadone and their subsequent metabolites (morphine-3-glucuronide and EDDP, respectively) determined in the brain and lungs at different time points using LC-MS/MS. Morphine and methadone, as well as their metabolites, were detected both in the brain and lungs, with significantly higher concentrations in the lungs. Pharmacokinetic modelling showed that the distribution of morphine to the brain followed a first-order absorption with transit compartments and linear elimination, with concentrations linearly dependent on dose. Moreover, methadone, but not morphine, reduced μ receptor (the main morphine receptor) binding, which can be of relevance for opioid tolerance. The present study is the first to report the brain distribution of morphine, which can be described by standard pharmacokinetic processes, and methadone in the developing chicken embryo. The present findings supplement the already established model and support the use of this chicken model to study NAS.

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. Invivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

Novel multi-objective affinity approach allows to identify pH-specific μ-opioid receptor agonists

Opioids are essential pharmaceuticals due to their analgesic properties, however, lethal side effects, addiction, and opioid tolerance are extremely challenging. The development of novel molecules targeting the mu-opioid receptor (MOR) in inflamed, but not in healthy tissue, could significantly reduce these unwanted effects. Finding such novel molecules can be achieved by maximizing the binding affinity to the MOR at acidic pH while minimizing it at neutral pH, thus combining two conflicting objectives. Here, this multi-objective optimal affinity approach is presented, together with a virtual drug discovery pipeline for its practical implementation. When applied to finding pH-specific drug candidates, it combines protonation state-dependent structure and ligand preparation with high-throughput virtual screening. We employ this pipeline to characterize a set of MOR agonists identifying a morphine-like opioid derivative with higher predicted binding affinities to the MOR at low pH compared to neutral pH. Our results also confirm existing experimental evidence that NFEPP, a previously described fentanyl derivative with reduced side effects, and recently reported beta-fluorofentanyls and -morphines show an increased specificity for the MOR at acidic pH when compared to fentanyl and morphine. We further applied our approach to screen a >50K ligand library identifying novel molecules with pH-specific predicted binding affinities to the MOR. The presented differential docking pipeline can be applied to perform multi-objective affinity optimization to identify safer and more specific drug candidates at large scale.

Tryptophan Substitution in CJ-15,208 (cyclo[Phe-D-Pro-Phe-Trp]) Introduces δ-Opioid Receptor Antagonism, Preventing Antinociceptive Tolerance and Stress-Induced Reinstatement of Extinguished Cocaine-Conditioned Place Preference.

The macrocyclic tetrapeptide CJ-15,208 (cyclo[Phe-D-Pro-Phe-Trp]) and its D-Trp isomer exhibit kappa opioid receptor (KOR) antagonism which prevents stress-induced reinstatement of extinguished cocaine-conditioned place preference. Here, we evaluated the effects of substitution of Trp and D-Trp on the peptides' opioid activity, antinociceptive tolerance, and the ability to prevent relapse to extinguished drug-CPP. Six analogs were synthesized using a combination of solid-phase peptide synthesis and cyclization in solution. The analogs were evaluated in vitro for opioid receptor affinity in radioligand competition binding assays, efficacy in the [35S]GTPγS assay, metabolic stability in mouse liver microsomes, and for opioid activity and selectivity in vivo in the mouse 55 °C warm-water tail-withdrawal assay. Potential liabilities of locomotor impairment, respiratory depression, acute tolerance, and conditioned place preference (CPP) were also assessed in vivo, and the ameliorating effect of analogs on the reinstatement of extinguished cocaine-place preference was assessed. Substitutions of other D-amino acids for D-Trp did not affect (or in one case increased) KOR affinity, while two of the three substitutions of an L-amino acid for Trp decreased KOR affinity. In contrast, all but one substitution increased mu opioid receptor (MOR) affinity in vitro. The metabolic stabilities of the analogs were similar to those of their respective parent peptides, with analogs containing a D-amino acid being much more rapidly metabolized than those containing an L-amino acid in this position. In vivo, CJ-15,208 analogs demonstrated antinociception, although potencies varied over an 80-fold range and the mediating opioid receptors differed by substitution. KOR antagonism was lost for all but the D-benzothienylalanine analog, and the 2'-naphthylalanine analog instead demonstrated significant delta opioid receptor (DOR) antagonism. Introduction of DOR antagonism coincided with reduced acute opioid antinociceptive tolerance and prevented stress-induced reinstatement of extinguished cocaine-CPP.

The management of pain during pediatric hematopoietic stem cell transplantation: A qualitative study of contextual factors that influenced pain management practices.

Children hospitalized following hematopoietic stem cell transplantation (HSCT) experience complex and prolonged pain in response to the intensity of this treatment. To describe how pain was managed for children during HSCT therapy and how contextual factors related to the clinical environment influenced healthcare providers' and parents' pain management practices. A qualitative case study was conducted and involved semi-structured interviews at two time points following transplantation (30 and 90days) with parents (n=10) and naturalistic observations of pain-related care provided to children (n=29) during HSCT therapy by their healthcare providers (n=10). Semi-structured interviews were also conducted with healthcare providers (n=14). The effectiveness of pain management interventions was hindered by the multifactorial nature of pain children experienced, a gap in the provision of psychosocial interventions for pain and a lack of evidence-based guidelines for the sustained, and often long-term, administration of opioids and adjuvant medications. Misconceptions were demonstrated by healthcare providers about escalating pain management according to pain severity and differentiating between opioid tolerance and addiction. Parents were active in the management of pain for children, especially the provision of nonpharmacological interventions. Collaboration with external pain services and the impact of caring for children in protective isolation delayed timely management of pain. There is a pressing need to create evidence-based supportive care guidelines for managing pain post transplantation to optimize children's relief from pain. If parents and children are to be involved in managing pain, greater efforts must be directed toward building their capacity to make informed decisions.

Safety and Efficacy of Rapid Methadone Titration for Opioid Use Disorder in an Inpatient Setting: A Retrospective Cohort Study.

Inpatient guidelines for methadone titration do not exist, whereas outpatient guidelines lack flexibility and do not consider individual opioid tolerance. The evaluation of rapid, adaptable titration protocols may allow more patient-centered and effective treatment for opioid use disorder in the fentanyl era. This study performed a retrospective chart review of patients 18 years or older with opioid use disorder who were initiated on methadone at a single academic urban hospital using a rapid divided dose protocol between November 2019 and November 2020. The primary outcome was adverse events associated with methadone, specifically opioid toxicity or sedation requiring increased medical observation or intervention. The secondary outcome was total daily dose of methadone received on day 7 of titration. Ninety-eight patients were included for a total of 168 visits. Sixty-five (66%) were male, with a median age of 38 years (interquartile range, 31-42 years). Sedation occurred in 2 patients (1%), who required either naloxone administration or transfer to an intensive care unit for monitoring. Of the 135 visits where patients received at least 7 days of methadone, the mean dose on day 1 was 41 mg (SD, 9.6 mg) and on day 7 was 65 mg (SD, 20.9 mg). In this inpatient cohort, rapid methadone titration was well tolerated and resulted in patients reaching higher doses of methadone than would be possible with a standard schedule, with few adverse events. Given the known effective dose range, this approach may result in shorter time to clinical stabilization and suggests that alternative methadone titration schedules may be safe and effective in appropriately selected patients.

Development of tolerance to chemokine receptor antagonists: current paradigms and the need for further investigation.

Chemokine G-protein coupled receptors are validated drug targets for many diseases, including cancer, neurological, and inflammatory disorders. Despite much time and effort spent on therapeutic development, very few chemokine receptor antagonists are approved for clinical use. Among potential reasons for the slow progress in developing chemokine receptor inhibitors, antagonist tolerance, a progressive reduction in drug efficacy after repeated administration, is likely to play a key role. The mechanisms leading to antagonist tolerance remain poorly understood. In many cases, antagonist tolerance is accompanied by increased receptor concentration on the cell surface after prolonged exposure to chemokine receptor antagonists. This points to a possible role of altered receptor internalization and presentation on the cell surface, as has been shown for agonist (primarily opioid) tolerance. In addition, examples of antagonist tolerance in the context of other G-protein coupled receptors suggest the involvement of noncanonical signal transduction in opposing the effects of the antagonists. In this review, we summarize the available progress and challenges in therapeutic development of chemokine receptor antagonists, describe the available knowledge about antagonist tolerance, and propose new avenues for future investigation of this important phenomenon. Furthermore, we highlight the modern methodologies that have the potential to reveal novel mechanisms leading to antagonist tolerance and to propel the field forward by advancing the development of potent "tolerance-free" antagonists of chemokine receptors.

City‐wide medication use process standardization and implementation of intravenous patient‐controlled analgesia consensus recommendations at an academic medical center

AbstractIntroductionIndianapolis Coalition for Patient Safety, Inc. (ICPS) identified practice variation across participating health‐systems related to intravenous Patient‐Controlled Analgesia (PCA) in 2018. No evidence‐based published recommendations were available to provide optimization for the medication use process during this evaluation, leading to development of an ad hoc interdisciplinary group to address gaps.ObjectivesPrimary objectives were to operationalize safe use of PCA and evaluate implementation of ICPS PCA consensus recommendations at an academic medical center by assessing PCA prescribing patterns following policy and electronic health record (EHR) changes. Secondary objectives focused on markers of adoption changes and employee engagement.MethodsICPS formed an interdisciplinary, diverse group of content experts representing six health‐systems to review current PCA processes. Significant variations were identified, and a gap‐analysis was completed to develop consensus recommendations. To evaluate implementation of these recommendations, a retrospective pre‐/post‐analysis was completed of patients at a safety‐net, academic medical center from March 2021 to July 2022 who received an order for PCA. Prescribing patterns and electronic documentation were analyzed.ResultsThe ICPS PCA Safety Summit developed 45 consensus recommendations addressing the following PCA medication use processes: ordering, patient assessment, administration, EHR design, documentation tools, smart pump drug library settings, policies, adverse event monitoring, and education. Evaluation included a total of 155 patients. The ordered total number of morphine milligram equivalent (MME) potentially available per day to a patient from PCA, electronic documentation of opioid tolerance, and PCA smart infusion pump review all significantly improved post‐implementation.ConclusionCity‐wide standardization and development of PCA consensus guidelines filled a gap in practice to provide direction and optimization for the medication use process surrounding these high‐alert medications. Organizational results following implementation provide evidence that using new EHR order sets and incorporating opioid tolerance resulted in more intentional PCA prescribing and more thorough electronic nursing documentation.

Patient-controlled subcutaneous analgesia with hydromorphone versus oral oxycodone for opioid titration of cancer pain: A prospective multicenter randomized trial.

12124 Background: Studies have shown that oral oxycodone sustained-release (SR) tablets can be used for opioid titration. The European Society for Medical Oncology (ESMO) guidelines for adult cancer pain recommend opioid titration through the parenteral route, usually the intravenous or subcutaneous route. Patient-controlled subcutaneous analgesia (PCSA) with hydromorphone needs further evaluation for opioid titration. This prospective multicenter study was designed to compare the efficacy and safety of hydromorphone PCSA with oral oxycodone SR tablets for opioid titration of cancer pain. Methods: Eligible patients with cancer pain were randomly assigned in a 1:1 ratio to the PCSA group or the oxycontin group for dose titration. Different titration methods were given in both groups depending on whether the patient had an opioid tolerance. The primary endpoint of this study was time to successful titration (TST). Results: A total of 256 patients completed this study. The PCSA group had a significantly lower TST compared with the oxycontin group (median [95% confidence interval (CI)], 5.5 [95%CI: 2.5-11.5] hours vs. 16.0 [95%CI: 11.5-22.5] hours; p< 0.001). PCSA reduced the frequency of breakthrough pain (Btp), especially in patients with opioid tolerance. The number of Btp was 121±28 times and 186±31 times in the PCSA and oxycontin groups, respectively ( p< 0.001). The 12-hour numeric rating scale (NRS) score was significantly lower in the PCSA group than that in the oxycontin group (median [95%CI], 2.5 [95%CI: 1.4-2.9] vs. 4.4[95%CI: 3.2-5.8]) ( p< 0.001). Similar equivalent morphine consumption and significantly improved quality of life were observed in the two groups. No between-group difference in the incidence of opioid-related adverse effects was observed. Conclusions: Compared with oral oxycodone SR tablet, the use of PCSA with hydromorphone achieved faster pain relief and a shorter titration duration for patients with cancer pain without increasing adverse events. Clinical trial information: ChiCTR2000037845 .

Cancer pain in the emergency department: An EMR-based opioid dosing intervention and the effects on patient outcomes.

e24167 Background: Pain is a common reason for patients with cancer to seek care in the Emergency Department (ED). Many of these patients are on chronic opioids and may have developed opioid tolerance, which makes appropriate dosing challenging for emergency physicians. Therefore, we designed an electronic medical record (EMR) based best practice advisory (BPA) intervention to recommend appropriate opioid dosing in the ED, based on a patient’s prescription opioid use. Methods: We conducted a retrospective cohort study to evaluate our cancer pain intervention at two academic EDs from May 2020 to May 2022. Our novel BPA algorithm identified ED cancer patients who were taking prescription opioids with an EMR-calculated morphine equivalent daily dose (MEDD) of at least 100. If an ED provider ordered an opioid for one of these patients, the BPA would fire and recommend an opioid dose based on the patient’s individual MEDD. The ED provider would then have the option of accepting or cancelling the BPA. We compared outcomes based on whether patients received BPA-guided increased opioid dosing. We utilized the chi-squared test with an alpha of < 0.05 to assess the relationship between BPA-guided increased opioid dosing and pain scores, hospital admission rates and ED bounce-back rates (return visit within 7 days). Results: We identified 399 patients who met our inclusion criteria, representing 705 BPA alerts. Those patients that received increased opioid dosing were similar to those that did not, with respect to age, sex, race/ethnicity and ECOG score. Patients who received BPA-guided increased opioid dosing experienced a greater improvement in pain (72.5% vs 65.1%, p = 0.03) and were admitted less frequently (58.5% vs 63.9%, p = 0.04). However, among discharged patients, those that received increased opioid dosing bounced back to the ED more frequently (16.2% vs 7.7%, p < .01). Conclusions: This EMR-based BPA intervention was associated with improved pain scores and decreased admission rates among cancer patients visiting the ED. The same intervention, however, was associated with increased ED return visits within 7 days, suggesting that pain management strategies may need to be implemented or modified upon ED discharge to assure that patients’ symptoms are adequately managed at home.

768 Intraoperative Use of Intravenous Methadone and Post-Operative Opioid Requirements in Adult Burn Patients

Abstract Introduction Post-operative pain management can be a significant challenge in patients undergoing burn excision. Standard pharmacologic pain management strategies include both opioid and non-opioid medications. Given the national overuse of opioids and the associated negative repercussions, it is prudent that we find ways manage pain with fewer or no opioids. We hypothesize that intraoperative administration of intravenous methadone will reduce the total morphine milligram equivalents (MME) used in the 36 hours following surgery. Methods This is a retrospective single-center cohort study of adult burn patients who underwent a first excision of full thickness burn between January 2019 through January 2021. The exposure group received intraoperative methadone while the control group did not. The primary outcome was total MME utilized in the 36 hours following surgery. Secondary outcomes included average pain score and post-anesthesia care unit (PACU) total MME utilized. Chi squared tests were used for statistical analysis of categorical variables and unpaired t-tests were used for continuous variables. Results The control group contained 35 subjects and the methadone group contained 19 subjects who did not differ in baseline characteristics. The average burned total body surface area (TBSA) was 9% in the control group and 18% in the methadone group (t(53)=-3.9, p < 0.01). Intraoperative narcotic requirements did differ between groups due to intravenous methadone utilization in the intervention group (t(53)=-15, p < 0.01). In the early post-operative period, the methadone group received 155 MME while the control group received 137 MME (t(53)=-0.36, p = 0.72). While in the PACU, patients in the methadone group received 5 MME while the control group received 12 MME (t(53)=1.60, p=0.12). Patient pain scores did not differ significantly between the groups (t(53)=-0.15, p=0.88). Conclusions Intraoperative utilization of methadone for burn surgery did not affect post-operative opioid usage in the first 36 hours or post-operative pain scores in a statistically significant manner. We acknowledge the data set is not large enough to power the study to detect a significant difference; additional data collection is ongoing to include several hundred subjects. Confounding variables such as different multimodal pain regimen techniques, % TBSA, extent of surgical excision, and history of opioid tolerance may exist within this data set. Further analysis is warranted to adequately power this study and account for potential confounding variables. Applicability of Research to Practice Intraoperative use of intravenous methadone may reduce use of opioid medications in the early post-operative period.

Validation of a pharmacological imaging challenge using 11C-buprenorphine and 18F-2-fluoro-2-deoxy-D-glucose positron emission tomography to study the effects of buprenorphine to the rat brain.

Buprenorphine mainly acts as an agonist of mu-opioid receptors (mu-OR). High dose buprenorphine does not cause respiratory depression and can be safely administered to elicit typical opioid effects and explore pharmacodynamics. Acute buprenorphine, associated with functional and quantitative neuroimaging, may therefore provide a fully translational pharmacological challenge to explore the variability of response to opioids in vivo. We hypothesized that the CNS effects of acute buprenorphine could be monitored through changes in regional brain glucose metabolism, assessed using 18F-FDG microPET in rats. First, level of receptor occupancy associated with a single dose of buprenorphine (0.1 mg/kg, s.c) was investigated through blocking experiments using 11C-buprenorphine PET imaging. Behavioral study using the elevated plus-maze test (EPM) was performed to assess the impact of the selected dose on anxiety and also locomotor activity. Then, brain PET imaging using 18F-FDG was performed 30 min after injection of unlabeled buprenorphine (0.1 mg/kg, s.c) vs. saline. Two different 18F-FDG PET acquisition paradigms were compared: (i) 18F-FDG injected i.v. under anesthesia and (ii) 18F-FDG injected i.p. in awake animals to limit the impact of anesthesia. The selected dose of buprenorphine fully blocked the binding of 11C-buprenorphine in brain regions, suggesting complete receptor occupancy. This dose had no significant impact on behavioral tests used, regardless of the anesthetized/awake handling paradigm. In anesthetized rats, injection of unlabeled buprenorphine decreased the brain uptake of 18F-FDG in most brain regions except in the cerebellum which could be used as a normalization region. Buprenorphine treatment significantly decreased the normalized brain uptake of 18F-FDG in the thalamus, striatum and midbrain (p < 0.05), where binding of 11C-buprenorphine was the highest. The awake paradigm did not improve sensitivity and impact of buprenorphine on brain glucose metabolism could not be reliably estimated. Buprenorphine (0.1 mg/kg, s.c) combined with 18F-FDG brain PET in isoflurane anesthetized rats provides a simple pharmacological imaging challenge to investigate the CNS effects of full receptor occupancy by this partial mu-OR agonist. Sensitivity of the method was not improved in awake animals. This strategy may be useful to investigate de desensitization of mu-OR associated with opioid tolerance in vivo.

Noncoding RNAs: Novel Targets for Opioid Tolerance.

As a global health problem, chronic pain is one of the leading causes of disability, and it imposes a huge economic and public health burden on families and society. Opioids represent the cornerstone of analgesic drugs. However, opioid tolerance caused by long-term application of opioids is a major factor leading to drug withdrawal, serious side effects caused by dose increases, and even the death of patients, placing an increasing burden on individuals, medicine, and society. Despite efforts to develop methods to prevent and treat opioid tolerance, no effective treatment has yet been found. Therefore, understanding the mechanism underlying opioid tolerance is crucial for finding new prevention and treatment strategies. Noncoding RNAs (ncRNAs) are important parts of mammalian gene transcriptomes, and there are thousands of unique noncoding RNA sequences in cells. With the rapid development of high-throughput genome technology, research on ncRNAs has become a hot topic in biomedical research. In recent years, studies have shown that ncRNAs mediate physiological and pathological processes, including chromatin remodeling, transcription, posttranscriptional modification and signal transduction, which are key regulators of physiological processes in developmental and disease environments and have become biomarkers and potential therapeutic targets for various diseases. An increasing number of studies have found that ncRNAs are closely related to the development of opioid tolerance. In this review, we have summarized the evidence that ncRNAs play an important role in opioid tolerance and that ncRNAs may be novel targets for opioid tolerance.

Intranasal Leptin Prevents Opioid-Induced Respiratory Depression in Obese, Opioid Tolerant Mice

Opioid-induced respiratory depression (OIRD) is the primary cause of death associated with opioids and individuals with obesity are particularly susceptible due to comorbid obstructive sleep apnea (OSA). Repeated exposure to opioids, as in the case of pain management, results in diminished therapeutic effect and/or the need for higher doses to maintain the same effect. With limited means to address the negative impact of repeated exposure it is critical to develop drugs that prevent deaths induced by opioids without reducing beneficial analgesia or causing withdrawal symptoms. We have previously shown that intranasal (IN) leptin can reverse apneas, hypoventilation, and upper airway obstruction while enhancing analgesia following acute morphine administration. Here we hypothesize that OIRD as a result of chronic opioid use can be attenuated by administration of IN leptin without acute withdrawal while also maintaining analgesia in both lean mice and mice with diet-induced obesity (DIO). To test this hypothesis, an opioid tolerance protocol was developed and a model of OIRD in mice chronically receiving morphine and tolerant to morphine analgesia was established. Subsequently, sleep and breathing were recorded by barometric plethysmography following acute or chronic administration of IN leptin. Operant behavioral assays and Pavlovian conditioning procedures were used to determine the impact of IN leptin on the analgesic efficacy of morphine and opioid withdrawal symptoms. Acute administration of IN leptin significantly attenuated OIRD in DIO male mice decreasing the apnea index by 58.9% and apnea time by 60.1%. In lean mice leptin was ineffective. Chronic IN leptin induced &gt; 50% reduction in the OIRD in morphine tolerant DIO male mice suggesting the protective effect of leptin remained after the two-week treatment. Morphine caused a complete loss of temperature aversion which was not reduced by intranasal leptin indicating IN leptin does not decrease morphine analgesia. IN leptin did not modify naloxone conditional place aversion or naloxone withdrawal symptoms, suggesting that IN treatment does not induce opioid withdrawal. We conclude that IN leptin is ineffective in lean mice but prevents OIRD in obesity by increasing hypercapnic sensitivity when leptin resistance at the BBB is present without reducing analgesia or inducing withdrawal. NIH R61HL156240; NIH R01HL128970; NIH R41DA056239 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Telmisartan Is a Promising Agent for Managing Neuropathic Pain and Delaying Opioid Analgesic Tolerance in Rats.

Despite the large arsenal of analgesic medications, neuropathic pain (NP) management is not solved yet. Angiotensin II receptor type 1 (AT1) has been identified as a potential target in NP therapy. Here, we investigate the antiallodynic effect of AT1 blockers telmisartan and losartan, and particularly their combination with morphine on rat mononeuropathic pain following acute or chronic oral administration. The impact of telmisartan on morphine analgesic tolerance was also assessed using the rat tail-flick assay. Morphine potency and efficacy in spinal cord samples of treated neuropathic animals were assessed by [35S]GTPγS-binding assay. Finally, the glutamate content of the cerebrospinal fluid (CSF) was measured by capillary electrophoresis. Oral telmisartan or losartan in higher doses showed an acute antiallodynic effect. In the chronic treatment study, the combination of subanalgesic doses of telmisartan and morphine ameliorated allodynia and resulted in a leftward shift in the dose-response curve of morphine in the [35S]GTPγS binding assay and increased CSF glutamate content. Telmisartan delayed morphine analgesic-tolerance development. Our study has identified a promising combination therapy composed of telmisartan and morphine for NP and opioid tolerance. Since telmisartan is an inhibitor of AT1 and activator of PPAR-γ, future studies are needed to analyze the effect of each component.

Microglial inflammation modulates opioid analgesic tolerance.

As we all know, opioids are the drugs of choice for treating severe pain. However, very often, opioid use leads to tolerance, dependence, and hyperalgesia. Therefore, understanding the mechanisms underlying opioid tolerance and designing strategies for increasing the efficacy of opioids in chronic pain are important areas of research. Microglia are brain macrophages that remove debris and dead cells from the brain and participate in immune defense of the central nervous system during an insult or injury. However, recent studies indicate that microglial activation and generation of proinflammatory molecules (e.g., cytokines, nitric oxide, eicosanoids, etc.) in the brain may contribute to opioid tolerance and other side effects of opioid use. In this review, we will summarize the evidence and possible mechanisms by which proinflammatory molecules produced by activated microglia may antagonize the analgesic effect induced by opioids, and thus, lead to opioid tolerance. We will also delineate specific examples of studies that suggest therapeutic targets to counteract the development of tolerance clinically using suppressors of microglial inflammation.