Co-administration of morphine and gabapentin leads to dose dependent synergistic effects in a rat model of postoperative pain

Association Between Gabapentinoids and Postoperative Pulmonary Complications in Patients Undergoing Thoracic Surgery

OBJECTIVE

Perioperative gabapentinoids in general surgery have been associated with an increased risk of postoperative pulmonary complications (PPCs), while resulting in equivocal pain relief. This study's aim was to examine the utilization of gabapentinoids in thoracic surgery to determine the association of gabapentinoids with PPCs and perioperative opioid utilization.

DESIGN

A multicenter retrospective cohort study.

SETTING

Hospitals in the Premier Healthcare Database from 2012 to 2018.

PARTICIPANTS

A total of 70,336 patients undergoing elective open thoracotomy, video-assisted thoracic surgery, and robotic-assisted thoracic surgery.

INTERVENTIONS

Propensity score analyses were used to assess the association between gabapentinoids on day of surgery and the primary composite outcome of PPCs, defined as respiratory failure, pneumonia, reintubation, pulmonary edema, and noninvasive and invasive ventilation. Secondary outcomes included invasive and noninvasive ventilation, hospital mortality, length of stay, opioid consumption on day of surgery, and average daily opioid consumption after day of surgery.

RESULTS

Overall, 8,142 (12%) patients received gabapentinoids. The prevalence of gabapentin on day of surgery increased from 3.8% in 2012 to 15.9% in 2018. Use of gabapentinoids on day of surgery was associated with greater odds of PPCs (odds ratio [OR] 1.19, 95% CI 1.11-1.28), noninvasive mechanical ventilation (OR 1.30, 95% CI 1.16-1.45), and invasive mechanical ventilation (OR 1.14, 95% CI 1.02-1.28). Secondary outcomes indicated no clinically meaningful associations of gabapentinoid use with opioid consumption, hospital mortality, or length of stay.

CONCLUSIONS

Perioperative gabapentinoid administration in elective thoracic surgery may be associated with a higher risk of PPCs and no opioid-sparing effect.

Characterization of hospitalized patients who received naloxone while receiving opioids with or without gabapentinoids

Introduction

Gabapentin and pregabalin (gabapentinoids) can be given with opioids for opioid-sparing and adjuvant analgesic effects. In the context of certain comorbidities and high dosages, coadministration of these agents can lead to respiratory depression or oversedation, necessitating naloxone administration.

Methods

A retrospective chart review from January 2015 to December 2017 was conducted to include patients who received naloxone and opioids with or without gabapentinoids. Exclusion criteria included pregnancy or having received naloxone in the emergency department, intensive care, or pediatrics units. The primary outcome was to characterize differences between groups regarding comorbidities, history of renal or hepatic dysfunction, history of SUD, opioid tolerance, initiation and dose appropriateness of gabapentinoids, and dose intensity of gabapentinoids and opioids. Secondary outcomes were concomitant CNS depressant use and naloxone episodes for documented respiratory depression.

Results

Of 126 patients who met inclusion criteria, 36 received opioids and gabapentinoids (gabapentinoid group) and 90 received opioids alone (nongabapentinoid group). There were 136 naloxone episodes between the 2 groups. More than 50% of the naloxone episodes in the gabapentinoid group involved opioids of at least 90 oral morphine mg equivalents. Respiratory depression accounted for 39% and 15.8% of the naloxone episodes in the gabapentinoid and nongabapentinoid groups, respectively.

Discussion

There may be increased naloxone episodes among patients receiving opioids and gabapentinoids. Future studies are needed to evaluate the incremental risk of respiratory depression and oversedation as it pertains to concomitant medication administration and patient-specific factors.

PREVALENCE, TREATMENT, AND SURVIVAL OF BURNED WILDLIFE PRESENTING TO REHABILITATION FACILITIES FROM 2015 TO 2018

In recent decades, wildfires have increased in frequency and geographic scale across the globe. The human health implications and ecological succession after wildfires are well documented and studied, but there is a lack of empirical research about the direct effects of wildfires on wildlife. Recent wildfires have demonstrated the need to better understand animal burn injuries and innovations in veterinary burn treatment. An online survey was distributed to wildlife rehabilitation facilities internationally to collect baseline information about the number and type of burned wildlife cases admitted, treatments used, and survivorship of wildlife affected by wildfires. Approximately 80% (n = 49) of all respondents (n = 61) reported admitting cases of burned wildlife from 2015 to 2018. Respondents included facilities from six different countries and roughly 43% of facilities reported having a veterinarian on staff. Electrical burns were most commonly reported with 89% of respondents stating that they had seen electrical burns while 38% of respondents reported seeing wildfire-source thermal burns in wildlife patients. Respondents were asked about their frequency of use of different treatment methods. Bandages, colloid fluids, and opioids were used at significantly higher rates at facilities with veterinarians compared with facilities that did not report having a veterinarian; however, survival of burned wildlife patients did not significantly differ based on the factor of having a veterinarian on staff. Long-term and short-term complications were commonly reported for wildlife burn patients; 88% of facilities reported scarring, 81% reported alopecia, and 61% reported sepsis. Burned animals admitted to facilities were reported to have equal odds of dying and surviving. Burn care recommendations have changed considerably in recent decades. This study provided a unique opportunity to compare contemporary recommendations in human medicine with current methods used in wildlife rehabilitation facilities to identify potential areas of further investigation and improvement for wildlife medicine.

Spinal administration of the multi-functional opioid/neuropeptide FF agonist BN-9 produced potent antinociception without development of tolerance and opioid-induced hyperalgesia

Chronic opioids treatment is impeded by the development of analgesic tolerance and opioid-induced hyperalgesia. Recent studies have shown that multi-functional opioid compounds produce analgesic activities with limited side effects. We developed a novel multi-functional peptide targeting opioid and neuropeptide FF receptors named BN-9, which produced potent and non-tolerance forming antinociceptive effect after supraspinal and systemic administrations. In the present study, the analgesic properties and potential side effects of intrathecal BN-9 were investigated in a range of preclinical rodent models. In complete Freund's adjuvant-induced inflammatory pain model, intrathecal BN-9 dose-dependently produced analgesic effect via opioid receptors, and the spinal antinociceptive effect was augmented by the neuropeptide FF receptor antagonist RF9. In contrast, in plantar incision-induced postoperative pain model, BN-9 exhibited potent anti-allodynic effect via opioid receptors and, at least partially, neuropeptide FF receptors. In mouse models of acetic acid-induced visceral pain and formalin pain, BN-9-induced spinal antinociception was mainly mediated by opioid receptors, independent of neuropeptide FF receptors. Furthermore, at the spinal level, chronic treatments with BN-9 did not lead to analgesic tolerance and cross-tolerance to morphine. Moreover, opioid-induced hyperalgesia was observed after repeated administration of morphine, but not BN-9. Taken together, our present study suggests that intrathecal BN-9 produces potent and non-tolerance forming antinociception, and does not cause opioid-induced hyperalgesia. Thus, BN-9 might serve as a promising lead compound in the development of multi-functional opioid analgesics with minimized side effects.

Complex encephalopathy arising from the combination of opioids and gabapentin

The interactions between opioids and gabapentin are more clinically relevant than ever. Prescriptions dispensed for gabapentin increased from 39 million in 2012 to 64 million in 2018 in the USA and are ever increasing. Authors present a challenging case of these interactions. A 58-year-old man presented to the emergency department with acute respiratory failure and altered mental status. He was on high dose opioids and gabapentin as prescription medications. Despite full intensive care support and resolution of his respiratory failure with non-invasive positive pressure ventilation, the patient did not regain consciousness. After ruling out other causes, the diagnosis of gabapentin withdrawal was considered. Gabapentin was administered by a nasogastric tube that quickly resulted in a reversal of his symptoms. We concluded that severe gabapentin withdrawal should be considered in patients on higher doses of gabapentin when it is stopped abruptly. In such patients, gabapentin should be replaced. As most patients are unable to swallow in this situation and intravenous formulation is not available, nasogastric tube can be used for replacement.

Complex encephalopathy arising from the combination of opioids and gabapentin

The interactions between opioids and gabapentin are more clinically relevant than ever. Prescriptions dispensed for gabapentin increased from 39 million in 2012 to 64 million in 2018 in the USA and are ever increasing. Authors present a challenging case of these interactions. A 58-year-old man presented to the emergency department with acute respiratory failure and altered mental status. He was on high dose opioids and gabapentin as prescription medications. Despite full intensive care support and resolution of his respiratory failure with non-invasive positive pressure ventilation, the patient did not regained consciousness. After ruling out other causes, the diagnosis of gabapentin withdrawal was considered. Gabapentin was administered by a nasogastric tube that quickly resulted in a reversal of his symptoms. We concluded that severe gabapentin withdrawal should be considered in patients on higher doses of gabapentin when it is stopped abruptly. In such patients, gabapentin should be replaced. As most patients are unable to swallow in this situation and intravenous formulation is not available, nasogastric tube can be used for replacement.

Effects of subthalamic deep brain stimulation with gabapentin and morphine on mechanical and thermal thresholds in 6-hydroxydopamine lesioned rats

Chronic pain is the most common non-motor symptom among Parkinson's disease (PD) patients, with 1.85 million estimated to be in debilitating pain by 2030. Subthalamic deep brain stimulation (STN DBS) programmed for treating PD motor symptoms has also been shown to significantly improve pain scores. However, even though most patients' pain symptoms improve or disappear, 74% of patients treated develop new pain symptoms within 8 years. Previously we have shown that duloxetine and STN high frequency stimulation (HFS) significantly increase mechanical thresholds more than either alone. The current project specifically investigates the effects of gabapentin and morphine alone and with high (150 Hz; HFS) and low (50 Hz; LFS) frequency stimulation in the 6-hydroxydopamine rat model for PD. We found that HFS, LFS, gabapentin 15 mg/kg and morphine 1 mg/kg all independently improve von Frey (VF) thresholds. Neither drug augments the HFS response significantly. Morphine at 1 mg/kg showed a trend to increasing thresholds compared to LFS alone (p = 0.062). Interestingly, gabapentin significantly reduced (p = 0.019) the improved VF thresholds and Randall Selitto thresholds seen with LFS. Thus, though neither drug augments DBS, we found effects of both compounds independently increase VF thresholds, informing use of our model of chronic pain in PD. Gabapentin's reversal of LFS effects warrants further exploration.

Bivalent ligand that activates mu opioid receptor and antagonizes mGluR5 receptor reduces neuropathic pain in mice

The mu opioid receptor (MOR) and metabotropic glutamate receptor 5 (mGluR5) are well-established pharmacological targets in the management of chronic pain. Both receptors are expressed in the spinal cord. MMG22, a bivalent ligand containing 2 pharmacophores separated by 22 atoms, which simultaneously activates MOR and antagonizes mGluR5, has been shown to produce potent reversal of tactile hypersensitivity in rodent models of lipopolysaccharide (LPS)-and bone cancer-induced chronic pain. This study assessed whether intrathecal MMG22 also is effective in reducing pain of neuropathic origin. Furthermore, we theorized that MMG22 should reduce hyperalgesia in nerve-injured mice in a manner consistent with a synergistic interaction between MOR and mGluR5. Several weeks after spared nerve injury, tactile hypersensitivity was reversed in mice by the intrathecal injection of MMG22 (0.01-10 nmol) but also by its shorter spacer analog, MMG10, with similar potency. The potencies of the bivalent ligands were 10- to 14-fold higher than those of the compounds upon which the bivalent structure was based, the MOR agonist oxymorphone and the mGluR5 antagonist MPEP. Coadministration of oxymorphone and MPEP demonstrated analgesic synergism, an interaction confirmed by isobolographic analysis. This study indicates that in the spared nerve injury-induced model of neuropathic pain, the 2 pharmacophores of the bivalent ligands MMG22 and MMG10 target MOR and mGluR5 as separate receptor monomers. The observed increase in the potency of MMG22 and MMG10, compared with oxymorphone and MPEP, may reflect the synergistic interaction of the 2 pharmacophores of the bivalent ligand acting at their respective separate receptor monomers.

Antiepileptic drugs as analgesics/adjuvants in inflammatory pain: current preclinical evidence

Inflammatory pain is the most common type of pain that is treated clinically. The use of currently available treatments (classic analgesics - NSAIDs, paracetamol and opioids) is limited by insufficient efficacy and/or side effects/tolerance development. Antiepileptic drugs (AEDs) are widely used in neuropathic pain treatment, but there is substantial preclinical evidence on their efficacy against inflammatory pain, too. In this review we focus on gabapentinoids (gabapentin and pregabalin) and dibenzazepine AEDs (carbamazepine, oxcarbazepine, and recently introduced eslicarbazepine acetate) and their potential for relieving inflammatory pain. In models of somatic, visceral and trigeminal inflammatory pain, that have a translational value for inflammatory conditions in locomotor system, viscera and head/face, AEDs have demonstrated analgesic activity. This activity was mostly consistent, dependent on the dose and largely independent on the site of inflammation and method of its induction, nociceptive stimuli, species, specific drug used, its route of administration and dosing schedule. AEDs exerted comparable efficacy with classic analgesics. Effective doses of AEDs are lower than toxic doses in animals and, when expressed as equivalent human doses, they are largely overlapping with AEDs doses already used in humans for treating epilepsy/neuropathic pain. The main mechanism of antinociceptive/antihyperalgesic action of gabapentinoids in inflammatory pain models seems to be α₂δ-dependent suppression of voltage-gated calcium channels in primary sensory neurons that leads to reduced release of neurotransmitters in the spinal/medullar dorsal horn. The suppression of NMDA receptors via co-agonist binding site primarily at spinal sites, activation of various types of K⁺ channels at spinal and peripheral sites, and activation of noradrenergic and serotonergic descending pain modulatory pathways may also contribute. Inhibition of voltage-gated sodium channels along the pain pathway is probably the main mechanism of antinociceptive/antihyperalgesic effects of dibenzazepines. The recruitment of peripheral adrenergic and purinergic mechanisms and central GABAergic mechanisms may also contribute. When co-administered with classic/other alternative analgesics, AEDs exerted synergistic/additive interactions. Reviewed data could serve as a basis for clinical studies on the efficacy/safety of AEDs as analgesics/adjuvants in patients with inflammatory pain, and contribute to the improvement of the treatment of various inflammatory pain states.

Feasibility of Exposure-Response Analyses for Clinical Dose-Ranging Studies of Drug Combinations

The exposure-response relationship of combinatory drug effects can be quantitatively described using pharmacodynamic interaction models, which can be used for the selection of optimal dose combinations. The aim of this simulation study was to evaluate the reliability of parameter estimates and the probability for accurate dose identification for various underlying exposure-response profiles, under a number of different phase II designs. An efficacy variable driven by the combined exposure of two theoretical compounds was simulated and model parameters were estimated using two different models, one estimating all parameters and one assuming that adequate previous knowledge for one drug is readily available. Estimation of all pharmacodynamic parameters under a realistic, in terms of sample size and study design, phase II trial, proved to be challenging. Inaccurate estimates were found in all exposure-response scenarios, except for situations where no pharmacodynamic interaction was present, with the drug potency and interaction parameters being the hardest to estimate. When previous knowledge of the exposure-response relationship of one of the monocomponents is available, such information should be utilized, as it enabled relevant improvements in parameter estimation and in correct dose identification. No general trends for classification of the performance of the tested study designs across different scenarios could be identified. This study shows that pharmacodynamic interactions models can be used for the exposure-response analysis of clinical endpoints especially when accompanied by appropriate dose selection in regard to the expected drug potencies and appropriate trial size and if information regarding the exposure-response profile of one monocomponent is available.

Botulinum toxin type A and gabapentin attenuate postoperative pain and NK1 receptor internalization in rats

Treatment of postoperative pain remains a challenge in clinic. Botulinum toxin type A (BoNT/A) and gabapentin regulate the release of neurotransmitters from primary afferent neurons, but their effects of on postoperative pain are not clear. In the current study, using pain behavioral tests, Western blot analysis, and immunocytochemistry, we examined whether BoNT/A, alone or in combination with intrathecal gabapentin, inhibited pain hypersensitivity and attenuated the increase in neurokinin 1 (NK1) receptor internalization in dorsal horn neurons after plantar incision. Our data showed that pretreatment of rats with an intraplantar (2 U) 24 h before plantar incision or intrathecal (0.5 U) injection of BoNT/A 48 h before plantar incision induced a prolonged (3-5 days) decrease in pain scores and mechanical hypersensitivity, as compared to those observed with saline pretreatment. Both intraplantar and intrathecal BoNT/A pretreatment reduced synaptosomal-associated protein 25 levels in the ipsilateral lumbar dorsal root ganglia and spinal cord dorsal horn, and attenuated the increase in NK1 receptor internalization in dorsal horn neurons. Intrathecal administration of a sub-effective dose of gabapentin (50 μg) with BoNT/A (0.5 U) induced greater inhibition of pain hypersensitivity and NK1 receptor internalization than BoNT/A alone. These findings suggest that pretreatment with BoNT/A, alone or in combination with intrathecal gabapentin, may present a promising multimodal analgesia regimen for postoperative pain treatment.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Postoperative pain-from mechanisms to treatment

INTRODUCTION

Pain management after surgery continues to be suboptimal; there are several reasons including lack of translation of results from basic science studies and scientific clinical evidence into clinical praxis.

OBJECTIVES

This review presents and discusses basic science findings and scientific evidence generated within the last 2 decades in the field of acute postoperative pain.

METHODS

In the first part of the review, we give an overview about studies that have investigated the pathophysiology of postoperative pain by using rodent models of incisional pain up to July 2016. The second focus of the review lies on treatment recommendations based on guidelines and clinical evidence, eg, by using the fourth edition of the "Acute Pain Management: Scientific Evidence" of the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine.

RESULTS

Preclinical studies in rodent models characterized responses of primary afferent nociceptors and dorsal horn neurons as one neural basis for pain behavior including resting pain, hyperalgesia, movement-evoked pain or anxiety- and depression-like behaviors after surgery. Furthermore, the role of certain receptors, mediators, and neurotransmitters involved in peripheral and central sensitization after incision were identified; many of these are very specific, relate to some modalities only, and are unique for incisional pain. Future treatment should focus on these targets to develop therapeutic agents that are effective for the treatment of postoperative pain as well as have few side effects. Furthermore, basic science findings translate well into results from clinical studies. Scientific evidence is able to point towards useful (and less useful) elements of multimodal analgesia able to reduce opioid consumption, improve pain management, and enhance recovery.

CONCLUSION

Understanding basic mechanisms of postoperative pain to identify effective treatment strategies may improve patients' outcome after surgery.

Review about gabapentin misuse, interactions, contraindications and side effects

The current work is targeted to review the risks of gabapentin misuse, its potential interactions with other drugs, side effects and use contraindications. This review consists of a total of 99 biographical references (from the year 1983 to 2016). A publication search of PubMed was performed from January 1983 to December 2016. It included animal studies, clinical studies, case studies and reviews related to gabapentin misuse, potential interactions, side effects and use contraindications. The search terms were gabapentin, anticonvulsant and antiepileptic. In general, it seems that gabapentin has risks of being misused based on the increased level of prescriptions, related fatalities, recreational misuse and higher doses of self-administration. The main reasons for gabapentin misuse are as follows: getting high, alleviating opioid withdrawal symptoms and potentiating methadone effects. Some of the main substances that interact with gabapentin are morphine, caffeine, losartan, ethacrynic acid, phenytoin, mefloquine and magnesium oxide. Some of the side effects caused by gabapentin are teratogenicity, hypoventilation, respiratory failure and myopathy. Finally, reports in general contraindicate the use of gabapentin in conditions such as myasthenia gravis and myoclonus.

Efficacy of Intrathecal Morphine in a Model of Surgical Pain in Rats

Concerns over interactions between analgesics and experimental outcomes are a major reason for withholding opioids from rats undergoing surgical procedures. Only a fraction of morphine injected intravenously reaches receptors responsible for analgesia in the central nervous system. Intrathecal administration of morphine may represent a way to provide rats with analgesia while minimizing the amount of morphine injected. This study aimed to assess whether morphine injected intrathecally via direct lumbar puncture provides sufficient analgesia to rats exposed to acute surgical pain (caudal laparotomy).In an initial blinded, randomised study, pain-free rats received morphine subcutaneously (MSC, 3mg.kg-1, N = 6), intrathecally (MIT, 0.2mg.kg-1, N = 6); NaCl subcutaneously (NSC, N = 6) or intrathecally (NIT, N = 6). Previously validated pain behaviours, activity and Rat Grimace Scale (RGS) scores were recorded at baseline, 1, 2, 4 and 8h post-injection. Morphine-treated rats had similar behaviours to NaCl rats, but their RGS scores were significantly different over time and between treatments. In a second blinded study, rats (N = 28) were randomly allocated to one of the following four treatments (N = 7): MSC, 3mg.kg-1, surgery; MIT, 0.2mg.kg-1, surgery; NIT, surgery; NSC, sham surgery. Composite Pain Behaviours (CPB) and RGS were recorded as previously. CPB in MIT and MSC groups were not significantly different to NSC group. MSC and MIT rats displayed significantly lower RGS scores than NIT rats at 1 and 8h postoperatively. RGS scores for MIT and MSC rats were not significantly different at 1, 2, and 8h postoperatively. Intraclass correlation value amongst operators involved in RGS scoring (N = 9) was 0.913 for total RGS score. Intrathecal morphine was mostly indistinguishable from its subcutaneous counterpart, providing pain relief lasting up to 8 hours in a rat model of surgical pain. Further studies are warranted to clarify the relevance of the rat grimace scale for assessing pain in rats that have received opioid analgesics.

Population Pharmacokinetic Modelling of Morphine, Gabapentin and their Combination in the Rat

PURPOSE

The combination of morphine and gabapentin seems promising for the treatment of postoperative and neuropathic pain. Despite the well characterised pharmacodynamic interaction, little is known about possible pharmacokinetic interactions. The aim of this study was to evaluate whether co-administration of the two drugs leads to modifications of their pharmacokinetic profiles.

METHODS

The pharmacokinetics of morphine, morphine-3-glucuronide and gabapentin were characterised in rats following subcutaneous injections of morphine, gabapentin or their combination. Non-linear mixed effects modelling was applied to describe the pharmacokinetics of the compounds and possible interactions.

RESULTS

The plasma-concentration-time profiles of morphine and gabapentin were best described using a three- and a one-compartment disposition model respectively. Dose dependencies were found for morphine absorption rate and gabapentin bioavailability. Enterohepatic circulation of morphine-3-glucuronide was modelled using an oscillatory model. The combination did not lead to pharmacokinetic interactions for morphine or gabapentin but resulted in an estimated ~33% diminished morphine-3-glucuronide formation.

CONCLUSIONS

The finding of a lack of pharmacokinetic interaction strengthens the notion that the combination of the two drugs leads to better efficacy in pain treatment due to interaction at the pharmacodynamic level. The interaction found between gabapentin and morphine-3-glucuronide, the latter being inactive, might not have any clinical relevance.