The effect of buprenorphine and benzodiazepines on respiration in the rat

Serotonin therapies for opioid-induced disordered swallow and respiratory depression

Opioids are well-known to cause respiratory depression, but despite clinical evidence of dysphagia, the effects of opioids on swallow excitability and motor pattern are unknown. We tested the effects of the clinically relevant opioid buprenorphine on pharyngeal swallow and respiratory drive in male and female rats. We also evaluated the utility of 5-HT1A agonists (8-OH-DPAT and buspirone) to improve swallowing and breathing following buprenorphine administration. Experiments were performed on 44 freely breathing Sprague-Dawley rats anesthetized with sodium pentobarbital. Bipolar fine wire electrodes were inserted into the mylohyoid, thyroarytenoid, posterior cricoarytenoid, thyropharyngeus, and diaphragm muscles to measure electromyographic (EMG) activity of swallowing and breathing. We evaluated the hypotheses that swallowing varies by stimulus, opioids depress swallowing and breathing, and that 5-HT1A agonists improve these depressions. Our results largely confirmed the following hypotheses: 1) swallow-related EMG activity was larger during swallows elicited by esophageal distension plus oral water infusion than by either stimulus alone. 2) Buprenorphine depressed swallow in both sexes, but females were more susceptible to total swallow suppression. 3) Female animals were also more vulnerable to opioid-induced respiratory depression. 4) 8-OH-DPAT rescued breathing following buprenorphine-induced respiratory arrest, and pretreatment with the partial 5-HT1A agonist buspirone prevented buprenorphine-induced respiratory arrest in female animals. 5) 8-OH-DPAT enhanced mylohyoid and thyropharyngeus EMG amplitude during swallow but did not restore excitability of the swallow pattern generator following total suppression by buprenorphine. Our results highlight sex-specific and behavior-specific effects of buprenorphine and provide preclinical evidence of a 5HT1A agonist for the treatment of respiratory depression and dysphagia.NEW & NOTEWORTHY This is the first study, to our knowledge, to evaluate sex-specific effects of opioid administration on pharyngeal swallow. We expand on a small but growing number of studies that report a lower threshold for opioid-induced respiratory depression in females compared with males, and we are the first to produce this effect with the partial μ-opioid-receptor agonist buprenorphine. This is the first demonstration, to our knowledge, that activation of 5-HT1A receptors can improve swallow and breathing outcomes following systemic buprenorphine administration.

Mechanisms of respiratory depression induced by the combination of buprenorphine and diazepam in rats

BACKGROUND

The safety profile of buprenorphine has encouraged its widespread use. However, fatalities have been attributed to benzodiazepine/buprenorphine combinations, by poorly understood mechanisms of toxicity. Mechanistic hypotheses include (i) benzodiazepine-mediated increase in brain buprenorphine (pharmacokinetic hypothesis); (ii) benzodiazepine-mediated potentiation of buprenorphine interaction with opioid receptors (receptor hypothesis); and (iii) combined effects of buprenorphine and benzodiazepine on respiratory parameters (pharmacodynamic hypothesis).

METHODS

We studied the neuro-respiratory effects of buprenorphine (30 mg kg-1, i.p.), diazepam (20 mg kg-1, s.c.), and diazepam/buprenorphine combination in rats using arterial blood gas analysis, plethysmography, and diaphragm electromyography. Pretreatments with various opioid and gamma-aminobutyric acid receptor antagonists were tested. Diazepam impact on brain 11C-buprenorphine kinetics and binding to opioid receptors was studied using positron emission tomography imaging.

RESULTS

In contrast to diazepam and buprenorphine alone, diazepam/buprenorphine induced early-onset sedation (P<0.05) and respiratory depression (P<0.001). Diazepam did not alter 11C-buprenorphine brain kinetics or binding to opioid receptors. Diazepam/buprenorphine-induced effects on inspiratory time were additive, driven by buprenorphine (P<0.0001) and were blocked by naloxonazine (P<0.01). Diazepam/buprenorphine-induced effects on expiratory time were non-additive (P<0.001), different from buprenorphine-induced effects (P<0.05) and were blocked by flumazenil (P<0.01). Diazepam/buprenorphine-induced effects on tidal volume were non-additive (P<0.01), different from diazepam- (P<0.05) and buprenorphine-induced effects (P<0.0001) and were blocked by naloxonazine (P<0.05) and flumazenil (P<0.05). Compared with buprenorphine, diazepam/buprenorphine decreased diaphragm contraction amplitude (P<0.01).

CONCLUSIONS

Pharmacodynamic parameters and antagonist pretreatments indicate that diazepam/buprenorphine-induced respiratory depression results from a pharmacodynamic interaction between both drugs on ventilatory parameters.

A Novel Maintenance Therapeutic for Opioid Use Disorder

Opioid use disorder (OUD) is a major socioeconomic burden. An ideal OUD pharmacotherapy will mitigate the suffering associated with opioid-withdrawal, inhibit the effects of high efficacy opioids, and minimize opioid-cravings while being safe and accessible to a diverse patient population. Although current OUD pharmacotherapies inhibit the euphoric effects of opioids of abuse, the extent to which they safely alleviate withdrawal and opioid-cravings corresponds with their intrinsic µ opioid receptor (MOR) efficacy. In addition to inhibiting the euphoric effects of opioids of abuse, the medium efficacy MOR agonist buprenorphine alleviates withdrawal and opioid-cravings, but its intrinsic MOR efficacy is sufficient such that its utility is limited by abuse and safety liabilities. Although the MOR antagonist naltrexone minimizes euphoria and has no abuse liability, it exacerbates suffering associated with withdrawal and opioid cravings. Therefore, a therapeutic with intrinsic MOR activity between the partial agonist (buprenorphine) and the antagonist (naltrexone) would strike a balance between the benefits and liabilities of these two therapeutics. To address this need, we derived RM1490, an MOR agonist based on a nonmorphinan scaffold that exhibits approximately half the intrinsic MOR efficacy of buprenorphine. In a series of preclinical assays, we compared RM1490 with buprenorphine and naltrexone at doses that achieve therapeutic levels of central nervous system MOR occupancy. RM1490 exhibited a behavioral profile consistent with reduced reward, dependence, and precipitated withdrawal liabilities. RM1490 was also more effective than buprenorphine at reversing the respiratory depressant effects of fentanyl and did not suppress respiration when combined with diazepam. SIGNIFICANCE STATEMENT: In preclinical studies, RM1490 has a physiological and behavioral profile suitable for opioid use disorder maintenance therapy.

Buprenorphine-Related Deaths in North Carolina from 2010-2018

Buprenorphine is a commonly prescribed medication for the treatment of opioid-use disorder. As prescriptions increase in North Carolina, buprenorphine is more frequently encountered statewide in routine postmortem casework. Between 2010 and 2018, there were 131 select cases investigated by the Office of the Chief Medical Examiner where buprenorphine was detected in peripheral blood and considered a primary cause of death, with no other opioids present and no other non-opioid substances found in the lethal range. The decedents ranged in age from 14 to 64 years, with 67% male. The mean/median peripheral blood concentrations were 4.1/2.1 ng/mL for buprenorphine and 7.8/3.4 ng/mL for the metabolite, norbuprenorphine. These postmortem blood concentrations overlap antemortem therapeutic concentrations in plasma reported in the literature for opioid-dependent subjects receiving sublingual maintenance therapy. The pathologist considered scene findings, prescription history, autopsy findings, toxicological analysis, and decedent behavior prior to death to conclude a drug-related cause of death. Many of the deaths were complicated by the presence of other central nervous system depressants along with contributory underlying cardiovascular and respiratory disease. The three most prevalent additive substances were alprazolam, ethanol, and gabapentin, found in 67, 36, and 32 cases out of 131, respectively. Interpreting buprenorphine involvement in a death is complex, and instances may be under-estimated in epidemiological data because of the lack of a defined toxic or lethal range in postmortem blood along with its good safety profile. As expansion to access of opioid-use disorder treatment becomes a priority, awareness of the challenges of postmortem interpretation is needed as increased use and diversion of buprenorphine are inevitable.

Associations between prescribed benzodiazepines, overdose death and buprenorphine discontinuation among people receiving buprenorphine

BACKGROUND AND AIMS

Benzodiazepines are commonly prescribed to patients with opioid use disorder receiving buprenorphine treatment, yet may increase overdose risk. However, prescribed benzodiazepines may improve retention in care by reducing buprenorphine discontinuation and thus may prevent relapse to illicit opioid use. We aimed to test the association between benzodiazepine prescription and fatal opioid overdose, non-fatal opioid overdose, all-cause mortality and buprenorphine discontinuation.

DESIGN AND SETTING

This was a retrospective cohort study using five individually linked data sets from Massachusetts, United States government agencies.

PARTICIPANTS

We studied 63 389 Massachusetts residents aged 18 years or older who received buprenorphine treatment between January 2012 and December 2015.

MEASUREMENTS

Filled benzodiazepine prescription during buprenorphine treatment was the main independent variable. The primary outcome was time to fatal opioid overdose. Secondary outcomes were time to non-fatal opioid overdose, all-cause mortality and buprenorphine discontinuation. We defined buprenorphine discontinuation as having a 30-day gap without another prescription following the end date of the previous prescription. We used Cox proportional hazards models to calculate hazards ratios that tested the association between receipt of benzodiazepines and all outcomes, restricted to periods during buprenorphine treatment.

FINDINGS

Of the 63 345 individuals who received buprenorphine, 24% filled at least one benzodiazepine prescription during buprenorphine treatment. Thirty-one per cent of the 183 deaths from opioid overdose occurred when individuals received benzodiazepines during buprenorphine treatment. Benzodiazepine receipt during buprenorphine treatment was associated with an increased risk of fatal opioid overdose adjusted hazard ratio (HR) = 2.92, 95% confidence interval (CI) = 2.10-4.06, non-fatal opioid overdose, adjusted HR = 2.05, 95% CI, 1.68-2.50, all-cause mortality, adjusted HR = 1.90, 95% CI, 1.48-2.44 and a decreased risk of buprenorphine discontinuation, adjusted HR = 0.87, 95% CI, 0.85-0.89.

CONCLUSIONS

Benzodiazepine receipt appears to be associated with both increased risk of opioid overdose and all-cause mortality and decreased risk of buprenorphine discontinuation among people receiving buprenorphine.

Prediction of Drug-Drug Interactions Between Opioids and Overdosed Benzodiazepines Using Physiologically Based Pharmacokinetic (PBPK) Modeling and Simulation

Background

Researchers have long been interested in the potential drug–drug interactions (DDIs) between opioids and benzodiazepines. However, much remains unknown concerning the interactions between these two drug classes. The objective of this work is to study the mechanism underlying the DDIs between opioids and benzodiazepines from the perspective of their pharmacokinetic (PK) interactions. A PK interaction occurs when two drugs are metabolized by the same cytochrome P450 enzymes and is one of the most common reasons for DDIs.

Methods

We quantitatively predicted the DDIs between three opioids (fentanyl, oxycodone and buprenorphine) and four benzodiazepines (alprazolam, diazepam, midazolam and triazolam) using a physiologically based pharmacokinetic (PBPK) modeling approach. A set of PBPK models was first constructed for these common opioids and benzodiazepines using SimCYP software, and the DDIs between them were then explored at various dosages.

Results

Our simulation results suggested there were no PK interactions between normal doses of opioids and benzodiazepines; but weak interactions can be expected with the combination of opioids and overdosed benzodiazepines. Particular attention should be given to the combination of fentanyl and overdosed alprazolam since a PK interaction can be observed between them.

Conclusion

Our results appear to indicate that pharmacodynamics may play a more important role than PKs in causing DDIs between opioids and benzodiazepines. This study also demonstrated that molecular modeling can be a very useful tool to mitigate the problem of “missing metabolic reaction parameters” in PK modeling and simulation.

Electronic supplementary material

The online version of this article (10.1007/s40268-019-00282-3) contains supplementary material, which is available to authorized users.

Effects of oxycodone and diazepam alone and in combination on operant nociception

Developing effective analgesics with fewer unwanted side effects is a pressing concern. Due to a lack of effective nonopioid options currently available, an alternative approach termed opioid-sparing evaluates the ability of a coadministered drug to reduce the amount of opioid needed to produce an antinociceptive effect. Opioids and benzodiazepines are often coprescribed. Although this approach is theoretically rational given the prevalent comorbidity of chronic pain and anxiety, it also has inherent risks of respiratory depression, which is likely responsible for the substantial percentage of fatal opioid overdoses that have involved benzodiazepines. Moreover, there have been no clinical trials to support the effectiveness of this drug combination nor has there been corroborative preclinical evidence using traditional animal models of nociception. The present studies examined the prescription µ-opioid analgesic oxycodone (0.003-0.1 mg/kg) and the prototypical benzodiazepine anxiolytic diazepam (0.03-1.0 mg/kg), alone and in combination, using an animal model of pain that examines the restoration of conflict-related operant behavior as evidence of analgesia. Results documented significant dose-related increases in thermal threshold following oxycodone treatment. Diazepam treatment alone did not produce significant antinociception. In combination, diazepam pretreatment shifted oxycodone functions upward in a dose-dependent manner, but the additive effects were limited to a narrow dose range. In addition, combinations of diazepam and oxycodone at higher doses abolished responding. Taken together, though intriguing, these findings do not provide sufficient evidence that coadministration of an anxiolytic will result in clinically relevant opioid-sparing for pain management, especially when considering the inherent risks of this drug class combination.

Behavioral Effects of Opioid Full and Partial Agonists During Chronic Buprenorphine Treatment

Buprenorphine, a partial agonist at the μ-opioid receptor, is commonly prescribed for the management of opioid addiction. Notwithstanding buprenorphine's clinical popularity, the relationship between its effectiveness in attenuating relapse-related behavior and its opioid efficacy is poorly understood. Furthermore, changes in the antinociceptive potency or effectiveness of opioid drugs that might occur during buprenorphine treatment have not been characterized. Here, we address these questions by assessing the ability of daily buprenorphine treatment to protect against the reinstatement of drug-seeking behavior by six opioids differing in efficacy (methadone, heroin, oxycodone, buprenorphine, butorphanol, nalbuphine) and, in separate experiments, by determining how such treatment may modify their antinociceptive effects. In one set of experiments, squirrel monkeys were trained to respond under concurrent schedules (choice) of food or intravenous oxycodone presentations. The priming strength of different opioids during sessions in which saline, rather than oxycodone, was available for intravenous self-administration was determined before and during chronic buprenorphine treatment (0.1 or 0.32 mg/kg per day). In other subjects, antinociceptive effects of the different opioids were assessed using cumulative dosing procedures in a modified warm-water tail withdrawal procedure before and during buprenorphine treatment. Results show that, notwithstanding some tolerance, full agonists retain high efficacy in producing priming and antinociceptive effects. In contrast, both the priming strength and antinociceptive effectiveness of partial agonists were decreased. These results suggest that the utility of buprenorphine in the management of opioid addiction, and how it alters the analgesic effects of opioids, can vary depending on the efficacy of the abused or prescribed opioid. SIGNIFICANCE STATEMENT: Our findings indicate that the pharmacological efficacy of abused opioids may predict the ability of buprenorphine to attenuate their relapse-related priming and analgesia-related antinociceptive effects. This information can help inform physicians as to the effectiveness and limitations of buprenorphine as a pharmacotherapy for opioid addiction.

Novel Synthetic Opioids: The Pathologist's Point of View

Background: New Psychoactive Substances (NPS) constitute a broad range of hundreds of natural and synthetic drugs, including synthetic opioids, synthetic cannabinoids, synthetic cathinones, and other NPS classes, which were not controlled from 1961 to 1971 by the United Nations drug control conventions. Among these, synthetic opioids represent a major threat to public health. Methods: A literature search was carried out using public databases (such as PubMed, Google Scholar, and Scopus) to survey fentanyl-, fentanyl analogs-, and other synthetic opioid-related deaths. Keywords including “fentanyl”, “fentanyl analogs”, “death”, “overdose”, “intoxication”, “synthetic opioids”, “Novel Psychoactive Substances”, “MT-45”, “AH-7921”, and “U-47700” were used for the inquiry. Results: From our literature examination, we inferred the frequent implication of fentanyls and synthetic opioids in side effects, which primarily affected the central nervous system and the cardiovascular and pulmonary systems. The data showed a great variety of substances and lethal concentrations. Multidrug-related deaths appeared very common, in most reported cases. Conclusions: The investigation of the contribution of novel synthetic opioid intoxication to death should be based on a multidisciplinary approach aimed at framing each case and directing the investigation towards targeted toxicological analyses.

Unintentional mortality associated with paracetamol and codeine preparations, with and without doxylamine, in Australia

INTRODUCTION

Misuse of paracetamol, codeine and doxylamine combination analgesics may lead to addiction and mortality. This study aimed to (1) identify unintentional deaths in Australia associated with use of combination analgesic products containing paracetamol, codeine and doxylamine; (2) describe cases characteristics, including demographics and additional medication use; and (3) identify common factors associated with misuse and mortality of these medicines in Australia.

DESIGN

This retrospective case series analysed National Coronial Information System data to identify cases of unintentional death attributable to paracetamol, codeine and doxylamine products between 2002 and 2012.

SETTING

Three Eastern Australian states: New South Wales, Queensland, Victoria, comprising a population of approximately 18.6 million people.

RESULTS

441 unintentional deaths attributed to paracetamol/codeine products were identified; doxylamine was detected in 102 cases (23%). Overall unintentional death rates rose from 0.9-per-million in 2002 to 3.6-per-million in 2009, declining to 1.9-per-million in 2012. Median age at time of death was 48, half of all cases occurred between 35-54 years of age, and 57% were female. Concomitant medication use was detected in 79% of cases, including benzodiazepines, other opioids, psychiatric medications, alcohol and illicit drugs. Behaviours consistent with drug misuse including doctor/pharmacy shopping, excessive dosages and extended use, were identified in 24% of cases.

CONCLUSIONS

This study identified 441 deaths associated with codeine-combination analgesic products across three Australian states; with an average of 40 deaths per year. Death commonly involved multiple substance use and abuse behaviours indicative of misuse and dependence.

Overlapping buprenorphine, opioid, and benzodiazepine prescriptions among veterans dually enrolled in Department of Veterans Affairs and Medicare Part D

BACKGROUND

Buprenorphine is a key tool in the management of opioid use disorder, but there are growing concerns about abuse, diversion, and safety. These concerns are amplified for the Department of Veterans Affairs (VA), whose patients may receive care concurrently from multiple prescribers within and outside VA. To illustrate the extent of this challenge, we examined overlapping prescriptions for buprenorphine, opioids, and benzodiazepines among veterans dually enrolled in VA and Medicare Part D.

METHODS

We constructed a cohort of all veterans dually enrolled in VA and Part D who filled an opioid prescription in 2012. We identified patients who received tablet or film buprenorphine products from either source. We calculated the proportion of buprenorphine recipients with any overlapping prescription (based on days supply) for a nonbuprenorphine opioid or benzodiazepine, focusing on veterans who received overlapping prescriptions from a different system than their buprenorphine prescription (Part D buprenorphine recipients receiving overlapping opioids or benzodiazepines from VA and vice versa).

RESULTS

There were 1790 dually enrolled veterans with buprenorphine prescriptions, including 760 (43%) from VA and 1091 (61%) from Part D (61 veterans with buprenorphine from both systems were included in each group). Among VA buprenorphine recipients, 199 (26%) received an overlapping opioid prescription and 11 (1%) received an overlapping benzodiazepine prescription from Part D. Among Part D buprenorphine recipients, 208 (19%) received an overlapping opioid prescription and 178 (16%) received an overlapping benzodiazepine prescription from VA. Among VA and Part D buprenorphine recipients with cross-system opioid overlap, 25% (49/199) and 35% (72/208), respectively, had >90 days of overlap.

CONCLUSIONS

Many buprenorphine recipients receive overlapping prescriptions for opioids and benzodiazepines from a different health care system than the one in which their buprenorphine was filled. These findings highlight a previously undocumented safety risk for veterans dually enrolled in VA and Medicare.

Benzodiazepine use, misuse, and abuse: A review

Benzodiazepine (BZD) abuse has reached epidemic levels and results in poor outcomes, particularly when combined with concomitant central nervous system depressants. BZDs are abused most commonly in combination with opioids and alcohol. Emergency department visits and related deaths have soared in recent years. In the absence of other medications or illicit substances, BZDs are rarely the sole cause of death. Prescription drug abuse has received more attention in recent years, yet much remains unknown about BZD abuse. BZDs have low abuse potential in most of the general population. A subset is at elevated risk of abuse, especially those with a history of a substance use disorder. Education, prevention, and identification are vital in reducing BZD abuse.

Benzodiazepine use during buprenorphine treatment for opioid dependence: clinical and safety outcomes

BACKGROUND

Prescribing benzodiazepines during buprenorphine treatment is a topic of active discussion. Clinical benefit is unclear. Overdose, accidental injury, and benzodiazepine misuse remain concerns. We examine the relationship between benzodiazepine misuse history, benzodiazepine prescription, and both clinical and safety outcomes during buprenorphine treatment.

METHODS

We retrospectively examined outpatient buprenorphine treatment records, classifying patients by past-year benzodiazepine misuse history and approved benzodiazepine prescription at intake. Primary clinical outcomes included 12-month treatment retention and urine toxicology for illicit opioids. Primary safety outcomes included total emergency department (ED) visits and odds of an ED visit related to overdose or accidental injury during treatment.

RESULTS

The 12-month treatment retention rate for the sample (N=328) was 40%. Neither benzodiazepine misuse history nor benzodiazepine prescription was associated with treatment retention or illicit opioid use. Poisson regressions of ED visits during buprenorphine treatment revealed more ED visits among those with a benzodiazepine prescription versus those without (p<0.001); benzodiazepine misuse history had no effect. The odds of an accidental injury-related ED visit during treatment were greater among those with a benzodiazepine prescription (OR: 3.7, p<0.01), with an enhanced effect among females (OR: 4.7, p<0.01). Overdose was not associated with benzodiazepine misuse history or prescription.

CONCLUSIONS

We found no effect of benzodiazepine prescriptions on opioid treatment outcomes; however, benzodiazepine prescription was associated with more frequent ED visits and accidental injuries, especially among females. When prescribing benzodiazepines during buprenorphine treatment, patients need more education about accidental injury risk. Alternative treatments for anxiety should be considered when possible, especially among females.

Respiratory effects of diazepam/methadone combination in rats: a study based on concentration/effect relationships

BACKGROUND

Methadone may cause respiratory depression and fatalities. Concomitant use of benzodiazepines in methadone-treated patients for chronic pain or as maintenance therapy for opiate abuse is common. However, the exact contribution of benzodiazepines to methadone-induced respiratory toxicity remains debatable.

METHODS

We investigated the respiratory effects of the combination diazepam (20mg/kg)/methadone (5mg/kg) in the rat, focusing on methadone concentration/effect relationships. Respiratory effects were studied using arterial blood gases and whole-body plethysmography. Plasma concentrations of both R- and S-methadone enantiomers were measured using high-performance liquid chiral chromatography coupled to mass spectrometry. To clarify mechanisms of diazepam/methadone interaction, methadone metabolism was investigated in vitro using rat liver microsomes.

RESULTS

Diazepam/methadone co-administration significantly increased methadone-related effects on inspiratory time (p<0.001) but did not significantly alter the other respiratory parameters when compared with methadone alone, despite significant increase in the area under the curve of plasma R-methadone concentrations measured during 240 min (p<0.05). Diazepam/methadone co-incubation with microsomes in vitro resulted in a significant inhibition of methadone metabolism (p<0.01), with 50%-inhibitory diazepam concentrations of 25.02 ± 0.18 μmol/L and 25.18 ± 0.23 μmol/L for R- and S-methadone, respectively.

CONCLUSION

We concluded that co-administration of high-doses of diazepam and methadone in rats is not responsible for additional respiratory depression in comparison to methadone alone, despite significant metabolic interaction between the drugs. In humans, although our experimental data may suggest the relative safety of benzodiazepine/methadone co-prescription, physicians should remain cautious as other underlying conditions may enhance this drug-drug interaction.

Respiratory toxicity of buprenorphine results from the blockage of P-glycoprotein-mediated efflux of norbuprenorphine at the blood-brain barrier in mice

OBJECTIVES

Deaths due to asphyxia as well as following acute poisoning with severe respiratory depression have been attributed to buprenorphine in opioid abusers. However, in human and animal studies, buprenorphine exhibited ceiling respiratory effects, whereas its metabolite, norbuprenorphine, was assessed as being a potent respiratory depressor in rodents. Recently, norbuprenorphine, in contrast to buprenorphine, was shown in vitro to be a substrate of human P-glycoprotein, a drug-transporter involved in all steps of pharmacokinetics including transport at the blood-brain barrier. Our objectives were to assess P-glycoprotein involvement in norbuprenorphine transport in vivo and study its role in the modulation of buprenorphine-related respiratory effects in mice.

SETTING

University-affiliated research laboratory, INSERM U705, Paris, France.

SUBJECTS

Wild-type and P-glycoprotein knockout female Friend virus B-type mice.

INTERVENTIONS

Respiratory effects were studied using plethysmography and the P-glycoprotein role at the blood-brain barrier using in situ brain perfusion.

MEASUREMENTS AND MAIN RESULTS

Norbuprenorphine(≥ 1 mg/kg) and to a lesser extent buprenorphine (≥ 10 mg/kg) were responsible for dose-dependent respiratory depression combining increased inspiratory (TI) and expiratory times (TE). PSC833, a powerful P-glycoprotein inhibitor, significantly enhanced buprenorphine-related effects on TI (p < .01) and TE (p < .05) and norbuprenorphine-related effects on minute volume (VE, p < .05), TI, and TE (p < .001). In P-glycoprotein-knockout mice, buprenorphine-related effects on VE (p < .01), TE (p < .001), and TI (p < .05) and norbuprenorphine-related effects on VE (p < .05) and TI (p < .001) were significantly enhanced. Plasma norbuprenorphine concentrations were significantly increased in PSC833-treated mice (p < .001), supporting a P-glycoprotein role in norbuprenorphine pharmacokinetics. Brain norbuprenorphine efflux was significantly reduced in PSC833-treated and P-glycoprotein-knockout mice (p < .001), supporting P-glycoprotein-mediated norbuprenorphine transport at the blood-brain barrier.

CONCLUSIONS

P-glycoprotein plays a key-protective role in buprenorphine-related respiratory effects, by allowing norbuprenorphine efflux at the blood-brain barrier. Our findings suggest a major role for drug-drug interactions that lead to P-glycoprotein inhibition in buprenorphine-associated fatalities and respiratory depression.

Polydrug abuse: a review of opioid and benzodiazepine combination use

This paper reviews studies examining the pharmacological interactions and epidemiology of the combined use of opioids and benzodiazepines (BZDs). A search of English language publications from 1970 to 2012 was conducted using PubMed and PsycINFO(®). Our search found approximately 200 articles appropriate for inclusion in this paper. While numerous reports indicate that the co-abuse of opioids and BZDs is ubiquitous around the world, the reasons for the co-abuse of these medications are not entirely clear. Though the possibility remains that opioid abusers are using BZDs therapeutically to self-medicate anxiety, mania or insomnia, the data reviewed in this paper suggest that BZD use is primarily recreational. For example, co-users report seeking BZD prescriptions for the purpose of enhancing opioid intoxication or "high," and use doses that exceed the therapeutic range. Since there are few clinical studies investigating the pharmacological interaction and abuse liability of their combined use, this hypothesis has not been extensively evaluated in clinical settings. As such, our analysis encourages further systematic investigation of BZD abuse among opioid abusers. The co-abuse of BZDs and opioids is substantial and has negative consequences for general health, overdose lethality, and treatment outcome. Physicians should address this important and underappreciated problem with more cautious prescribing practices, and increased vigilance for abusive patterns of use.

Mechanisms of respiratory insufficiency induced by methadone overdose in rats

Methadone may cause respiratory depression. We aimed to understand methadone-related effects on ventilation as well as each opioid-receptor (OR) role. We studied the respiratory effects of intraperitoneal methadone at 1.5, 5, and 15 mg/kg (corresponding to 80% of the lethal dose-50%) in rats using arterial blood gases and plethysmography. OR antagonists, including intravenous 10 mg/kg-naloxonazine at 5 minutes (mu-OR antagonist), subcutaneous 30 mg/kg-naloxonazine at 24 hours (micro1-OR antagonist), 3 mg/kg-naltrindole at 45 minutes (delta-OR antagonist) and 5 mg/kg-Nor-binaltorphimine at 6 hours (kappa-OR antagonist) were pre-administered. Plasma concentrations of methadone enantiomers were measured using high-performance liquid chromatography coupled to mass-spectrometry. Methadone dose-dependent inspiratory time (T(I)) increase tended to be linear. Respiratory depression was observed only at 15 mg/kg and characterized by an increase in expiratory time (T(E)) resulting in hypoxemia and respiratory acidosis. Intravenous naloxonazine completely reversed all methadone-related effects on ventilation, while subcutaneous naloxonazine reduced its effects on pH (P < 0.05), PaCO(2) (P < 0.01) and T(E) (P < 0.001) but only partially on T(I) (P < 0.001). Naltrindole reduced methadone-related effects on T(E) (P < 0.001). Nor-binaltorphimine increased methadone-related effects on pH and PaO(2) (P < 0.05) Respiratory effects as a function of plasma R-methadone concentrations showed a decrease in PaO(2) (EC(50): 1.14 microg/ml) at lower concentrations than those necessary for PaCO(2) increase (EC(50): 3.35 microg/ml). Similarly, increased T(I) (EC(50): 0.501 microg/ml) was obtained at lower concentrations than those for T(E) (EC(50): 4.83 microg/ml). Methadone-induced hypoxemia is caused by mu-ORs and modulated by kappa-ORs. Additionally, methadone-induced increase in T(E) is caused by mu1- and delta-opioid receptors while increase in T(I) is caused by mu-ORs.

Characteristics and comparative severity of respiratory response to toxic doses of fentanyl, methadone, morphine, and buprenorphine in rats

Opioids are known to induce respiratory depression. We aimed to characterize in rats the effects of four opioids on arterial blood gases and plethysmography after intraperitoneal administration at 80% of their LD(50) in order to identify opioid molecule-specific patterns and classify response severity. Opioid-receptor (OR) antagonists, including intravenous 10 mg kg(-1)-naloxonazine at 5 min [mu-OR antagonist], subcutaneous 30 mg kg(-1)-naloxonazine at 24 h [mu1-OR antagonist], subcutaneous 3 mg kg(-1)-naltrindole at 45 min [delta-OR antagonist], and subcutaneous 5 mg kg(-1)-Nor-binaltorphimine at 6 h [kappa-OR antagonist] were pre-administered to test the role of each OR. Methadone, morphine, and fentanyl significantly decreased PaO(2) (P<0.001) and increased PaCO(2) (P<0.05), while buprenorphine only decreased PaO(2) (P<0.05). While all opioids significantly increased inspiratory time (T(I), P<0.001), methadone and fentanyl also increased expiratory time (T(E), P<0.05). Intravenous 10 mg kg(-1)-naloxonazine at 5 min completely reversed opioid-related effects on PaO(2) (P<0.05), PaCO(2) (P<0.001), T(I) (P<0.05), and T(E) (P<0.01) except in buprenorphine. Subcutaneous 30 mg kg(-1)-naloxonazine at 24 h completely reversed effects on PaCO(2) (P<0.01) and T(E) (P<0.001), partially reversed effects on T(I) (P<0.001), and did not reverse effects on PaO(2). Naltrindole reversed methadone-induced T(E) increases (P<0.01) but worsened fentanyl's effect on PaCO(2) (P<0.05) and T(I) (P<0.05). Nor-binaltorphimine reversed morphine- and buprenorphine-induced T(I) increases (P<0.001) but worsened methadone's effect on PaO(2) (P<0.05) and morphine (P<0.001) and buprenorphine's (P<0.01) effects on pH. In conclusion, opioid-related respiratory patterns are not uniform. Opioid-induced hypoxemia as well as increases in T(I) and T(E) are caused by mu-OR, while delta and kappa-OR roles appear limited, depending on the specific opioid. Regarding severity of opioid-induced respiratory effects at 80% of their LD(50), all drugs increased T(I). Methadone and fentanyl induced hypoxemia, hypercapnia, and T(E) increases, morphine caused both hypoxemia and hypercapnia while buprenorphine caused only hypoxemia.

A community-based evaluation of sudden death associated with therapeutic levels of methadone

BACKGROUND

Published case reports have associated the therapeutic use of methadone with the occasional occurrence of sudden cardiac death. Because of the established utility of this drug and with the eventual goal of enhancing safety of use, we performed a community-based study to evaluate this association.

METHODS

During a 4-year period, we prospectively evaluated all patients who consecutively had sudden cardiac death and underwent investigation by the medical examiner in the metropolitan area of Portland, Ore. Case subjects of interest were those with a therapeutic blood level of methadone (<1 mg/L), and case comparison subjects were those with no methadone identified. Patients with recreational drug use or any drug overdose were excluded from either group. Detailed autopsies were conducted, including the detection and quantification of all substances in the blood.

RESULTS

A total of 22 sudden cardiac death cases with therapeutic levels of methadone (mean 0.48+/-0.22 mg/L; range 0.1-0.9 mg/L) were identified (mean age 37.0+/-10 years, 68% were male) and compared with 106 consecutive sudden cardiac death cases without evidence of methadone (mean age 42+/-13 years, 69% were male). The most common indication for methadone use was pain control (n=12, 55%). Among cases receiving methadone therapy, sudden death-associated cardiac abnormalities were identified in only 23% (n=5), with no clear cause of sudden cardiac death in the remaining 77% (n=17). Among cases with no methadone, sudden death-associated cardiac abnormalities were identified in 60% (n=64, P=.002).

CONCLUSION

The significantly lower prevalence of cardiac disease in the case group implicates methadone, even at therapeutic levels, as a likely cause of sudden death. These findings point toward an association between methadone and occurrence of sudden death in the community. Clinical safeguards and further prospective studies specifically designed to enhance safety of methadone use are warranted.

Concurrent buprenorphine and benzodiazepines use and self-reported opioid toxicity in opioid substitution treatment

AIMS

To examine concurrent buprenorphine and benzodiazepine consumption and to compare opioid toxicity symptoms induced by methadone and buprenorphine, examining factors associated with the reporting of these symptoms.

DESIGN

Self-report cross-sectional survey.

SETTING

Five needle syringe programmes and five opioid substitution treatment services in Melbourne, Australia.

PARTICIPANTS

A total of 250 people who had experience with methadone or buprenorphine. Eligibility criteria were current or previous methadone or buprenorphine use.

MEASUREMENTS

Structured questionnaire covering: demographic characteristics; current treatment and drug use; concurrent use of buprenorphine and benzodiazepines, including route of administration and source of medications; and opioid toxicity symptoms reported in association with methadone and buprenorphine consumption.

FINDINGS

Of those reporting buprenorphine use, two-thirds reported concurrent benzodiazepine use, with a median dose reported of 30 mg diazepam equivalents. A greater number of opioid toxicity symptoms were reported in relation to methadone consumption compared with buprenorphine. Those reporting opioid toxicity with buprenorphine were more likely to report intravenous use compared with those reporting opioid toxicity with methadone.

CONCLUSIONS

The risk of opioid toxicity appeared greater with methadone compared with buprenorphine, despite high levels of benzodiazepine consumption and injection being reported in relation to buprenorphine use. The prevalence of buprenorphine injection and the normalization of methadone-induced sedation are two findings that merit further investigation. Establishing recommendations as to the safest and most effective way to manage benzodiazepine-using people in opioid substitution treatment is necessary for the optimization of treatment for opioid dependence in polydrug-using individuals.

Does high-dose buprenorphine cause respiratory depression?: possible mechanisms and therapeutic consequences

Buprenorphine is an opioid agonist-antagonist with a 'ceiling effect' for respiratory depression. Compared with methadone, its unique pharmacology offers practical advantages and enhanced safety when prescribed as recommended and supervised by a physician. Buprenorphine has been approved in several countries as an efficient and safe maintenance therapy for heroin addiction. Its use resulted in a salutary effect with a reduction in heroin overdose-related deaths in countries that implemented office-based buprenorphine maintenance. In France, however, where high-dose buprenorphine has been marketed since 1996, several cases of asphyxic deaths were reported among addicts treated with buprenorphine. Death resulted from buprenorphine intravenous misuse or concomitant sedative drug ingestion, such as benzodiazepines. In these situations of abuse, misuse, or in association with elevated doses of psychotropic drugs, buprenorphine may cause severe respiratory depression. Unlike other opiates, the respiratory effects from buprenorphine are not responsive to naloxone. However, the exact mechanism of buprenorphine-induced effects on ventilation is still unknown. The role of norbuprenorphine, the main N-dealkylated buprenorphine metabolite with potent respiratory depressor activity, also remains unclear. Experimental studies investigating the respiratory effects of combinations of high doses of buprenorphine and benzodiazepines suggested that this drug-drug interaction may result from a pharmacodynamic interaction. A pharmacokinetic interaction between buprenorphine and flunitrazepam is also considered. As there are many questions regarding the possible dangers of death or respiratory depression associated with buprenorphine use, we aimed to present a comprehensive critical review of the published clinical and experimental studies on buprenorphine respiratory effects.

Interactions of buprenorphine and dipotassium clorazepate on anxiety and memory functions in the mouse

Buprenorphine, a partial mu-receptor agonist widely substituted for heroin in the treatment of addiction, is often misused in combination with benzodiazepines. Improved hedonic properties may result, but only at the cost of increased buprenorphine toxicity. In order to elucidate the appeal of the benzodiazepine-buprenorphine combination, the present study looked at its neuropsycho-pharmacological effects on various emotional and cognitive parameters in the mouse. On the basis of previous dose-response studies, the regimen used was buprenorphine 0.3mg/kg, s.c. plus dipotassium clorazepate 1, 4 and 16 mg/kg, i.p. Anxiety-like behaviour was assessed using the black and white test box, and memory processes were examined via the spontaneous alternation paradigm in the Y-maze, and passive avoidance tests. Spontaneous locomotor activity was also evaluated. High doses of clorazepate impaired buprenorphine-induced hyperactivity and anxiogenic-like effects. They also increased buprenorphine-induced spontaneous alternation impairment, but did not modify its impact on long-term memory processes. These results suggest that the positive reinforcement experienced with the buprenorphine-benzodiazepine combination may be attributable, at least in part, to an increase in buprenorphine's sedative effect associated with a decrease in anxiogenicity.

A survey of buprenorphine related deaths in Singapore

Buprenorphine is available in Singapore as substitution treatment for opioid dependence since 2002. This study surveys buprenorphine related deaths in Singapore between September 2003 and December 2004. The aims are to establish the autopsy prevalence of buprenorphine related deaths and the demographical and toxicological profile of the cases. Toxicological screening was performed for all unnatural deaths, deaths involving known drug addicts, as well as when autopsy revealed no obvious cause of death. Twenty-one cases had buprenorphine detected in post-mortem blood and/or urine samples. Eighteen were sudden deaths. There were two fatal falls from height and one death by hanging. All subjects were male. The age range was 24-48 years. Fourteen subjects were between 30 and 39 years of age. The mean age was 35 years. The majority (62%) were Chinese. Eleven (52%) were known drug abusers. For sudden deaths, two groups were identified. Six cases died from natural causes. Blood buprenorphine levels ranged from undetected (detected in urine) to 3.2 ng/mL (mean 1.4 ng/mL). Twelve cases were attributed directly and indirectly to mixed drug poisoning. Blood buprenorphine levels ranged from undetected (detected in urine) to 17 ng/mL (mean 3.2 ng/mL). Nineteen cases showed concurrent abuse of buprenorphine and benzodiazepine, diazepam being the most frequently detected, followed by nitrazepam and midazolam. The availability of buprenorphine as substitution therapy is associated with an increase in buprenorphine related deaths. The danger of co-abuse of buprenorphine and benzodiazepines is highlighted.