Interaction of μ-opioid receptor agonists and antagonists with the analgesic effect of buprenorphine in mice

IUPHAR review: Recent progress in the development of Mu opioid receptor modulators to treat opioid use disorders

Opioid Use Disorder (OUD) can be described as intense preoccupation with using or obtaining opioids despite the negative consequences associated with their use. As the number of OUD cases in the U.S. increase, so do the number of opioid-related overdose deaths. In 2022, opioid-related overdose became the No. 1 cause of death for individuals in the U.S. between the ages of 25 and 64 years of age. Because of the introduction of highly potent synthetic opioids (e.g. fentanyl) to the illicit drug market, there is an urgent need for therapeutics that successfully reduce the number of overdoses and can help OUD patients maintain sobriety. Most abused opioids stimulate the mu-opioid receptor (MOR) and activation of this receptor can lead to positive (e.g., euphoria) consequences. However, the negative side effects of MOR stimulation can be fatal (e.g., sedation, respiratory depression). Therefore, the MOR is an attractive target for developing medications to treat OUD. Current FDA drugs include MOR agonists that aid in detoxification and relapse prevention, and MOR antagonists that also serve as maintenance therapies or reverse overdose. These medications are limited by their abuse potential, adverse effects, or pharmacological profiles which leaves ample room for research into designing new chemical entities with optimal physiological effects. These includes, orthosteric ligands that target the primary binding site of the MOR, allosteric ligands that positively, negatively, or "silently" modulate receptor function, and lastly, bitopic ligands target both the orthosteric and allosteric sites simultaneously.

An Examination of the Complex Pharmacological Properties of the Non-Selective Opioid Modulator Buprenorphine

The goal of this review is to provide a recent examination of the pharmacodynamics as well as pharmacokinetics, misuse potential, toxicology, and prenatal consequences of buprenorphine. Buprenorphine is currently a Schedule III opioid in the US used for opioid-use disorder (OUD) and as an analgesic. Buprenorphine has high affinity for the mu-opioid receptor (MOR), delta (DOR), and kappa (KOR) and intermediate affinity for the nociceptin (NOR). Buprenorphine's active metabolite, norbuprenorphine, crosses the blood-brain barrier, is a potent metabolite that attenuates the analgesic effects of buprenorphine due to binding to NOR, and is responsible for the respiratory depressant effects. The area under the concentration curves are very similar for buprenorphine and norbuprenorphine, which indicates that it is important to consider this metabolite. Crowding sourcing has identified a buprenorphine street value (USD 3.95/mg), indicating some non-medical use. There have also been eleven-thousand reports involving buprenorphine and minors (age < 19) at US poison control centers. Prenatal exposure to clinically relevant dosages in rats produces reductions in myelin and increases in depression-like behavior. In conclusion, the pharmacology of this OUD pharmacotherapy including the consequences of prenatal buprenorphine exposure in humans and experimental animals should continue to be carefully evaluated.

Antinociceptive effects of the combined use of butorphanol and buprenorphine in mice

Butorphanol and buprenorphine are mixed opioid receptor agonist-antagonist drugs widely used as analgesics in people and animals. There are few reports concerning the interaction of multiple opioids, and their antinociceptive effects, when combined with other opioids, remain unclear. Therefore, we report the preliminary findings of the antinociceptive effects of the combined use of butorphanol and buprenorphine in C57BL/6JJcl mice. Both drugs were administered either simultaneously or in different orders. Compared with the baseline values, the tail-flick and hot-plate test latencies increased regardless of the order of administration. Furthermore, enhanced latencies were observed on administration of butorphanol followed by buprenorphine. Combined use of these drugs may not attenuate analgesic efficacy. Besides, enhancement of these effects can be obtained by changing the order of the administration of these drugs. It is necessary to further investigate the molecular basis of the underlying mechanism in future definitive studies.

The thermal antinociceptive effects of a high-concentration formulation of buprenorphine alone or followed by hydromorphone in conscious cats

OBJECTIVE

To evaluate the thermal antinociceptive effects of a high-concentration formulation of buprenorphine alone or followed by hydromorphone in conscious cats.

STUDY DESIGN

Randomized, blinded, placebo-controlled crossover study design.

ANIMALS

A total of six purpose-bred, adult female ovariohysterectomized Domestic Short Hair cats.

METHODS

Cats were allocated into three treatments each consisting of two injections, subcutaneous then intravenous (IV) administration, 2 hours apart: treatment SS, two injections of 0.9% saline; treatment BS, buprenorphine (0.24 mg kg^-1, 1.8 mg mL^-1) and saline; and treatment BH, buprenorphine (0.24 mg kg^-1) and hydromorphone (0.1 mg kg^-1). Skin temperature (ST) and thermal threshold (TT) were recorded before (baseline) and for 24 hours following first injection. TT data were analyzed using mixed linear models and a Benjamini-Hochberg sequential adjustment procedure (p < 0.05).

RESULTS

There were no significant differences among treatments for baseline ST and TT values, treatment SS over time and between treatments BS and BH. Compared with baseline, TT was significantly increased at all time points in treatments BH and BS except at 2 hours in treatment BS. TT was significantly higher than SS at 3-18 hours and 4-12 hours for treatments BS and BH, respectively. Maximal increases in TT were 47.5 °C at 2 hours, 53.9 °C at 3 hours and 52.4 °C at 6 hours in treatments SS, BS and BH, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of IV hydromorphone following high-concentration buprenorphine provided no additional antinociception and decreased the duration of effect when compared with high-concentration buprenorphine alone. Alternative analgesics should be considered if additional analgesia is required after administration of high-concentration buprenorphine.

Sustained-release buprenorphine induces acute opioid tolerance in the mouse

Sustained-release buprenorphine is widely used in mice with the intention of providing long-lasting analgesia. Statements about duration of therapeutic efficacy are based on persistence of serum buprenorphine levels over a minimum threshold, but behavioral data demonstrating sustained efficacy is not established. Additionally, chronic opioid exposure can induce tolerance and/or hyperalgesia; mice receiving sustained-release buprenorphine have not been evaluated for these effects. This study assessed clinical efficacy and duration of sustained-release buprenorphine in inflammatory, post-operative, and cancer pain; and screened for centrally-mediated opioid-induced hyperalgesia as well as opioid tolerance. At 1-2 mg/kg sustained-release buprenorphine, statistically significant analgesic efficacy occurred only at time points up to 2 h. These animals showed no changes in von Frey thresholds on the contralateral side, i.e. no centrally-mediated opioid hyperalgesia. To establish whether acute onset opioid tolerance resulted from a single sustained-release buprenorphine administration, we used the tail flick assay, exposing mice to sustained-release buprenorphine or saline on Day 1 and buprenorphine on Day 2. We measured duration and efficacy of 1 mg/kg buprenorphine after 1 mg/kg sustained-release buprenorphine, and also quantified a dose-response curve of buprenorphine (0.1-3 mg/kg) after 2 mg/kg sustained-release buprenorphine. Compared to control animals, mice previously exposed to sustained-release buprenorphine showed diminished analgesic response to buprenorphine; the resultant dose-response curve showed decreased efficacy. Pretreatment with naloxone, an opioid receptor antagonist, blocked sustained-release buprenorphine analgesic action. The short duration of antinociception following administration of sustained-release buprenorphine in mice is caused by the rapid development of tolerance.

Impact of Fentanyl Use on Buprenorphine Treatment Retention and Opioid Abstinence

OBJECTIVES

There has been a rapid increase in the presence of illicitly manufactured fentanyl in the heroin drug supply. Buprenorphine is an effective treatment for heroin and prescription opioid use disorder; however, little is known about treatment outcomes among people using fentanyl. We compared 6-month treatment retention and opioid abstinence among people initiating buprenorphine treatment who had toxicology positive for heroin compared to fentanyl at baseline.

METHODS

Retrospective cohort study of 251 adult patients initiating office-based buprenorphine treatment who had available toxicology testing across an academic health system between August 2016 and July 2017. Exposure was assessed at baseline before initiating buprenorphine and was categorized as negative toxicology (n = 184) versus fentanyl positive toxicology (n = 48) versus heroin positive toxicology (n = 19).

RESULTS

Six-month treatment retention rates were not different between the fentanyl positive and heroin positive groups [38% (n = 18) vs 47% (n = 9); P = 0.58], or between the fentanyl positive and the negative toxicology group [38% (n = 18) vs 51% (n = 93); P = 0.14]. Opioid abstinence at 6 months among those who had testing did not differ between the fentanyl positive and the heroin positive group [55% (n = 6) vs 60% (n = 6); P = 0.99]. The fentanyl positive group had a lower abstinence rate at 6 months compared to those with negative toxicology at baseline [55% (n = 6) vs 93% (n = 63); P = 0.004]. Mean initial buprenophine dosage did not differ between groups.

CONCLUSIONS

Buprenorphine treatment retention and abstinence among those retained in treatment is not worse between people using fentanyl compared to heroin at treatment initiation. Both groups have lower abstinence rates at 6 months compared to individuals with negative toxicology at baseline. These findings suggest that people exposed to fentanyl still benefit from buprenorphine treatment.

Perioperative Management of Buprenorphine: Solving the Conundrum

OBJECTIVE

There is no consensus on the optimal perioperative management of patients on buprenorphine (BUP) for opioid use disorder (OUD). This article will review the available literature on BUP and the analgesic efficacy of BUP combined with full mu-opioid agonists and discuss the conflicting management strategies in the context of acute pain and our institution's protocol for the periprocedural management of BUP.

METHODS

We searched published data on BUP periprocedural management from inception through March 2018 without language restrictions. Study selection included publications reporting outcomes on perioperative pain management in OUD patients maintained on BUP.

RESULTS

Our search resulted in four case reports supporting periprocedural discontinuation of BUP and two case series, one secondary observational study, one prospective matched cohort study, and four retrospective cohort studies supporting periprocedural continuation of BUP. No clinical trials were identified.

CONCLUSIONS

Maintaining BUP perioperatively does not lead to worsened clinical outcomes. Patients can receive adequate pain control from mu-opioid agonists while maintained on BUP. Based upon available evidence, we recommend continuing BUP at a reduced dose when indicated to avoid withdrawal symptoms and to facilitate the analgesic efficacy of mu-opioid agonists administered in combination for acute postoperative pain.

Clinical Management of Pain in Rodents

The use of effective regimens for mitigating pain remain underutilized in research rodents despite the general acceptance of both the ethical imperative and regulatory requirements intended to maximize animal welfare. Factors contributing to this gap between the need for and the actual use of analgesia include lack of sufficient evidence-based data on effective regimens, under-dosing due to labor required to dose analgesics at appropriate intervals, concerns that the use of analgesics may impact study outcomes, and beliefs that rodents recover quickly from invasive procedures and as such do not need analgesics. Fundamentally, any discussion of clinical management of pain in rodents must recognize that nociceptive pathways and pain signaling mechanisms are highly conserved across mammalian species, and that central processing of pain is largely equivalent in rodents and other larger research species such as dogs, cats, or primates. Other obstacles to effective pain management in rodents have been the lack of objective, science-driven data on pain assessment, and the availability of appropriate pharmacological tools for pain mitigation. To address this deficit, we have reviewed and summarized the available publications on pain management in rats, mice and guinea pigs. Different drug classes and specific pharmacokinetic profiles, recommended dosages, and routes of administration are discussed, and updated recommendations are provided. Nonpharmacologic tools for increasing the comfort and wellbeing of research animals are also discussed. The potential adverse effects of analgesics are also reviewed. While gaps still exist in our understanding of clinical pain management in rodents, effective pharmacologic and nonpharmacologic strategies are available that can and should be used to provide analgesia while minimizing adverse effects. The key to effective clinical management of pain is thoughtful planning that incorporates study needs and veterinary guidance, knowledge of the pharmacokinetics and mechanisms of action of drugs being considered, careful attention to individual differences, and establishing an institutional culture that commits to pain management for all species as a central component of animal welfare.

Converting from Transdermal to Buccal Formulations of Buprenorphine: A Pharmacokinetic Meta-Model Simulation in Healthy Volunteers

Objective

To develop a model to predict buprenorphine plasma concentrations during transition from transdermal to buccal administration.

Design

Population pharmacokinetic model-based meta-analysis of published data.

Methods

A model-based meta-analysis of available buprenorphine pharmacokinetic data in healthy adults, extracted as aggregate (mean) data from published literature, was performed to explore potential conversion from transdermal to buccal buprenorphine. The time course of mean buprenorphine plasma concentrations following application of transdermal patch or buccal film was digitized from available literature, and a meta-model was developed using specific pharmacokinetic parameters (e.g., absorption rate, apparent clearance, and volumes of distribution) derived from analysis of pharmacokinetic data for intravenously, transdermally, and buccally administered buprenorphine.

Results

Data from six studies were included in this analysis. The final transdermal absorption model employed a zero-order input rate that was scaled to reflect a nominal patch delivery rate and time after patch application (with decline in rate over time). The transdermal absorption rate constant became zero following patch removal. Buccal absorption was a first-order process with a time lag and bioavailability term. Simulations of conversion from transdermal 20 mcg/h and 10 mcg/h to buccal administration suggest that transition can be made rapidly (beginning 12 hours after patch removal) using the recommended buccal formulation titration increments (75-150 mcg) and schedule (every four days) described in the product labeling.

Conclusions

Computer modeling and simulations using a meta-model built from data extracted from publications suggest that rapid and straightforward conversion from transdermal to buccal buprenorphine is feasible.

Treating Chronic Pain: An Overview of Clinical Studies Centered on the Buprenorphine Option

The buprenorphine receptor binding profile is unique in that it binds to all three major opioid receptors (mu, kappa, delta), and also binds to the orphan-like receptor, the receptor for orphanin FQ/nociceptin, with lower affinity. Within the mu receptor group, buprenorphine analgesia in rodents is dependent on the recently discovered arylepoxamide receptor target in brain, which involves a truncated 6-transmembrane mu receptor gene protein, distinguishing itself from morphine and most other mu opioids. Although originally designed as an analgesic, buprenorphine has mainly been used for opioid maintenance therapy and only now is increasingly recognized as an effective analgesic with an improved therapeutic index relative to certain potent opioids. Albeit a second-, third-, or fourth-line analgesic, buprenorphine is a reasonable choice in certain clinical situations. Transdermal patches and buccal film formulations are now commercially available as analgesics. This review discusses buprenorphine pharmacodynamics and pharmacokinetics, use in certain populations, and provides a synopsis of systematic reviews and randomized analgesic trials. We briefly discuss postoperative management in patients receiving buprenorphine maintenance therapy, opioid equivalence to buprenorphine, rotations to buprenorphine from other opioids, and clinical relevance of buprenorphine-related QTc interval changes.

What Parenteral Opioids to Use in Face of Shortages of Morphine, Hydromorphone, and Fentanyl

Parenteral potent opioid availability is becoming an issue in acute pain management. Two opioids, nalbuphine and buprenorphine, are available which can be substituted for hydromorphone, fentanyl, and morphine. There are advantages and disadvantages in using these 2 opioids which are discussed, and potential dosing strategies are outlined.

Use of immediate-release opioids as supplemental analgesia during management of moderate-to-severe chronic pain with buprenorphine transdermal system

Background

The buprenorphine transdermal system (BTDS) is approved in the US for the management of chronic pain. Due to its high affinity for μ-opioid receptors with a slow dissociation profile, buprenorphine may potentially displace or prevent the binding of competing μ-opioid-receptor agonists, including immediate-release (IR) opioids, in a dose-dependent manner. Health care professionals may assume that the use of IR opioids for supplemental analgesia during BTDS therapy is not acceptable.

Materials and methods

This post hoc analysis evaluated the use of IR opioids as supplemental analgesia during the management of moderate–severe chronic pain with BTDS at 52 US sites (BUP3015S, NCT01125917). Patients were categorized into IR-opioid and no-IR-opioid groups. At each visit of the extension phase, adverse events, concomitant medications, and information from the Brief Pain Inventory (BPI) were recorded.

Results

The most common supplemental IR opioids prescribed during BTDS treatment (n=354) were hydrocodone–acetaminophen and oxycodone–acetaminophen. The mean daily dose of IR opioids (morphine equivalents) for supplemental analgesia was 22 mg. At baseline, BPI – pain intensity and BPI – interference scores were higher for patients in the IR-opioid group. In both treatment groups, scores improved by week 4, and then were maintained throughout 6 months of the open-label extension trial. The incidence of treatment-emergent adverse events was similar in both groups.

Conclusion

Patients who were prescribed IR opioids reported lower scores for BPI pain intensity and pain interference to levels similar to patients receiving BTDS without IR opioids, without increasing the rate or severity of treatment-emergent adverse events. Patients prescribed concomitant use of IR opioids with BTDS had greater treatment persistence. The results of this post hoc analysis provide support for the concomitant use of IR opioids for supplemental analgesia during the management of moderate–severe chronic pain with BTDS.

Effects of buprenorphine on QT intervals in healthy subjects: results of 2 randomized positive- and placebo-controlled trials

OBJECTIVES

To study the effect of transdermal buprenorphine on QTc prolongation at dose levels of 10, 40, and 80 mcg/h, (BTDS 10, BTDS 40, BTDS 80).

METHODS

Two randomized, placebo- and positive-controlled, parallel-group, dose-escalating clinical studies evaluated healthy adult subjects randomized to BTDS, placebo, or moxifloxacin in the first study; and to BTDS only, BTDS plus naltrexone, naltrexone alone at the same dose, placebo, or moxifloxacin in the second study. QT intervals were corrected for heart rate using data from each individual subject (QTcI).

RESULTS

In the first study (n = 44), the maximum upper bounds of the 90% confidence interval (CI) for mean placebo-corrected change from baseline in QTcI across 13 time points over 24 h were: 10.0 msec for BTDS 10 (Day 6) and 13.3 msec for BTDS 40 (Day 13); and 17.0 msec (Day 6) and 15.5 msec (Day 13) for moxifloxacin, respectively.  Similarly, in the second study (n = 66), the upper bound of the 90% CI for mean placebo-corrected change from baseline for QTcI was under 10 msec at all time points for BTDS 10 (maximum upper bound, 5.63 msec), over 10 msec at 5 time points for BTDS 40 (maximum 11.81 msec) and over 10 msec at all 13 time points for BTDS 80 (maximum, 14.14 msec). Naltrexone administered with BTDS eliminated the QTcI prolongation seen with supratherapeutic BTDS doses (BTDS 40, BTDS 80) administered without naltrexone.

CONCLUSIONS

At the therapeutic dose of 10 mcg/h, BTDS has no clinically significant effect on QTc. At supratherapeutic doses of 40 and 80 mcg/h, BTDS treatment produces prolongation of QTcI similar in magnitude to that produced by a 400 mg dose of moxifloxacin. Despite the modest, dose-dependent increase in QTcI noted in these studies, transdermal buprenorphine has not been associated with proarrhythmic effects.

Antinociceptive effects of intravenous administration of hydromorphone hydrochloride alone or followed by buprenorphine hydrochloride or butorphanol tartrate to healthy conscious cats

OBJECTIVE

To evaluate antinociceptive effects of IV administration of hydromorphone alone or followed by buprenorphine or butorphanol to cats.

ANIMALS

6 healthy adult cats.

PROCEDURES

In a randomized, blinded crossover design, cats received each of 4 treatments in which 2 IV injections were given 30 minutes apart: 2 of saline (0.9% NaCl) solution (Sal-Sal) or 1 each of hydromorphone HCl and saline solution (H-Sal), hydromorphone and buprenorphine HCl (H-Bupre), or hydromorphone and butorphanol tartrate (H-Butor). Skin temperature and thermal threshold were recorded before (baseline) and for 12 hours after the first injection. Percentage of maximum possible effect (%MPE) and thermal excursion (TE) were compared among treatments and measurement points.

RESULTS

Compared with baseline values, skin temperature was higher from 0.75 to 2 hours after the first injection for H-Sal; at 0.5, 1, 3, and 4 hours for H-Bupre; from 0.5 to 3 hours for H-Butor; and from 0.5 to 1 hours for Sal-Sal. Thermal excursion was higher than at baseline from 0.25 to 2 hours for H-Sal and H-Bupre and 0.25 to 0.75 hours for H-Butor; %MPE increased from 0.25 to 2 hours for H-Sal, 0.25 to 3 hours for H-Bupre, and 0.25 to 0.75 hours for H-Butor. Results were similar for comparisons with Sal-Sal, except TE was greater for H-Sal versus Sal-Sal and TE and %MPE were greater for H-Bupre versus Sal-Sal from 0.25 to 1 hours after the first injection.

CONCLUSIONS AND CLINICAL RELEVANCE

Butorphanol administration decreased the duration of antinociception achieved with hydromorphone administration in cats. This opioid interaction and its impact on pain management require additional investigation.

Effect of pretreatment with hydromorphone or buprenorphine on thermal antinociception induced by fentanyl in awake cats

OBJECTIVES

The aim of this study was to determine the effect of pretreatment with hydromorphone or buprenorphine on thermal antinociception induced by fentanyl in cats.

METHODS

Seven healthy cats received five different treatments consisting of two drugs. Drug 1 was administered intravenously 1 h before drug 2. Drug 2 was given as an intravenous loading dose followed by an infusion for 4 h. The drug combinations used were: buprenorphine 20 µg/kg followed by fentanyl (BF; 7 μg/kg, 7 μg/kg/h), buprenorphine 20 µg/kg followed by 0.9% saline solution (B), hydromorphone 0.07 mg/kg followed by 0.9% saline solution (H), hydromorphone 0.07 mg/kg followed by fentanyl (HF; 7 μg/kg, 7 μg/kg/h) and 0.9% saline solution followed by fentanyl (F; 7 μg/kg, 7 μg/kg/h). Thermal thresholds were obtained prior to treatment and at predetermined times up to 7 h after drug 1.

RESULTS

Thermal thresholds were significantly higher than baseline in all treatment groups as follows: H from 0.25 to 2.50 h; B from 0.25 to 2.50 h; F from 1.25 to 5.50 h; HF from 0.25 to 5.50 h and BF from 0.25 to 5.25 h. Thermal thresholds were significantly higher in HF and BF than in F treatment before the fentanyl infusion was commenced (from 0.25 to 1.00 h). During the fentanyl infusion thermal thresholds in BF compared with F were lower at 1.75 h and from 2.50 to 3.50 h. After the constant rate infusion of fentanyl was started, thermal thresholds were significantly lower in HF compared with F at one time point (3 h).

CONCLUSIONS AND RELEVANCE

Pretreatment with buprenorphine did partially inhibit the antinociceptive action of fentanyl. Hydromorphone did inhibit the antinociceptive action of fentanyl at one single time point in cats.

Use of CO2 Laser as an Adjunctive Treatment for Caudal Stomatitis in a Cat

Lasers have become a popular tool in veterinary practice, particularly the carbon dioxide (CO2) laser. In humans, the CO2 laser is used most commonly in oral and maxillofacial soft tissue surgery due to its favorable interactions with oral soft tissues. Other types of lasers are better suited for use on hard tissues such as enamel and dentin. This article reviews the history of laser use, physics of laser-tissue interaction, delivery systems, and laser types used in dentistry and oral surgery. This is followed by a case report describing the use of CO2 laser as an adjunctive treatment for therapy of refractory caudal stomatitis in a cat.

Long-term administration of high doses of transdermal buprenorphine in cancer patients with severe neuropathic pain

Background

Buprenorphine is often administered by the transdermal route (transdermal buprenorphine [TB]) in cancer patients with severe neuropathic pain. However, high doses of TB of 140 µg/h are rarely used.

Patients and methods

Three cancer patients with severe neuropathic Numeric Rating Scale (NRS) pain scores of 8–10 who were successfully treated with high doses of TB up to 140 µg/h along with other opioids and adjuvant analgesics.

Results

TB was administered for a long period of follow-up (9 months to 4 years, including 34–261 days of treatment with the dose of 140 µg/h), which allowed achievement of satisfactory analgesia (NRS 3–5) and good treatment tolerance. In all three patients, TB dose was gradually titrated from 35 to 140 µg/h, and all patients used morphine at least for some time for breakthrough and background pain management along with adjuvant analgesics. Two patients continued the treatment with TB until the end of life, and one patient is still receiving the treatment.

Conclusion

TB at doses of up to 140 µg/h in cancer patients with severe neuropathic pain seems to be effective and safe in combination with other opioids and with adjuvant analgesics, and may significantly improve patients’ quality of life. Clinical studies may explore higher than maximal 140 µg/h TB doses recommended by a manufacturer, and also in combination with other opioids and adjuvant analgesics.

A successful Switch From Transdermal Fentanyl to Transdermal Buprenorphine in a Patient with Neuropathic Pain

Buprenorphine is a frequently used opioid in the treatment of neuropathic pain component that is often present in patients with cancer. A case of a 41-year-old patient was depicted whose pain syndrome was associated with the chondrosarcoma growth originating from the sacral bone and numerous surgical interventions and radiotherapy. Improvement in analgesia and good toleration of therapy were observed after switching from transdermal fentanyl to transdermal buprenorphine while maintaining treatment with antidepressants and anticonvulsants. This case report indicates a possibility of a safe switch of transdermal opioids at home, which may provide benefits in terms of analgesia and adverse effects and in consequence have positive impact on the patients’ quality of life. This is also accompanied by constant psychological, social, and spiritual support provided to the patient and family.

Transdermal buprenorphine in chronic pain: indications and clinical experience

Transdermal buprenorphine has been shown to be effective in managing moderate-to-severe cancer pain and severe pain that is unresponsive to nonopioid analgesics. In clinical trials, it provided better pain relief than placebo, despite a higher consumption of rescue analgesia by placebo patients. Analgesia was rated as satisfactory or better by 90% of patients in a long-term follow-up study and 94.6% considered the buprenorphine matrix patch to be user friendly. Transdermal buprenorphine is well tolerated; most adverse events are transient local reactions to the patch or systemic effects typical of treatment with opioids. Even in opioid-experienced volunteers, buprenorphine does not cause respiratory depression at doses up to 70-times higher than those used for analgesia. No problems have been encountered when switching from another opioid to transdermal buprenorphine, or in combining the buprenorphine patch with intravenous morphine or tramadol for breakthrough pain. There is a growing body of evidence that transdermal buprenorphine may be particularly useful for managing neuropathic pain. Most notably, it appears to be effective in treating hyperalgesic states and syndromes characterized by pronounced central sensitization.

Buprenorphine—Clinically useful but often misunderstood

Background

There are a number of false myths about buprenorphine based on unconfirmed animal data, even from isolated animal organs, and early clinical research. These myths came into textbooks on pharmacology and pain about 30 years ago and have been difficult to eradicate. Animal models of pain and pain relief are notoriously unreliable as predictors of human clinical effects. The fact is that in clinical practice there is NO bell-shaped dose-response curve, there is NO plateau on the dose-response curve, and there is NO antagonist effect from buprenorphine on other mu-opioid agonists.

Methods

This narrative, topical review of relevant research publications evaluates new knowledge on the pharmacodynamics and pharmacokinetics of buprenorphine of importance in clinical practice.

Results

Buprenorphine is a potent opioid analgesic acting on all four opioid receptors: it is an agonist on the mu-, the delta, and the ORL-1 receptors. It is an antagonist at the kappa-receptor. Buprenorphine has a number of active metabolites with different effects on the four opioid receptors; all except the norbup-3-glu are analgesic. Buprenorphine itself is not a respiratory depressant or sedative, but some of its active metabolites are. Buprenorphine and its active metabolites are not excreted by the kidney. Therefore buprenorphine may be used in patients with advanced renal failure.

Buprenorphine has a slow onset and a long offset. These properties are advantageous, except sometimes when treating severe acute pain. Its agonist effect on the ORL-1 receptor reduces reward-effects and slows the development of tolerance to the analgesic effects.

Buprenorphine inhibits voltage-gated sodium-channels and enhances and prolongs peripheral nerve blocks. Its ORL-1 -effect at the spinal cord may do the same.

Buprenorphine is well suited for treatment of chronic pain, especially chronic neuropathic pain and cancer pain. The beneficial effects as a co-medication during treatment of the opioid-abuse disease are due to its slow onset (less “kick-effect”). Its prolonged offset-time reduces the likelihood of acute withdrawal problems and reduces the “craving” of opioids.

Adverse effects

Buprenorphine, being a mu-agonist, may induce or maintain opioid addiction. Illegally obtained high-dose transmucosal buprenorphine, intended for treatment of addiction, is dissolved and injected by opioid abusers. This is an increasing problem in some countries.

Conclusions

Buprenorphine’s unusual pharmacodynamics and pharmacokinetics make it an ideal opioid for treatment of most chronic pain conditions where opioid therapy is indicated.

Implications

Buprenorphine is a well studied and often misunderstood analgesic opioid drug. The evidence base predicts that it will be an increasingly important alternative for treatment of chronic pain conditions caused by cancer and non-cancer diseases. It will continue to be an attractive alternative to methadone for opioid abuse rehabilitation.

Buprenorphine/naloxone as a promising therapeutic option for opioid abusing patients with chronic pain: reduction of pain, opioid withdrawal symptoms, and abuse liability of oral oxycodone

Few studies have examined abuse of prescription opioids among individuals with chronic pain under buprenorphine/naloxone (Bup/Nx) maintenance. The current 7-week inpatient study assessed oral oxycodone self-administration by patients with chronic pain who had a history of opioid abuse. Participants (n=25) were transitioned from their preadmission prescribed opioid to Bup/Nx. All of the participants were tested under each of the sublingual Bup/Nx maintenance doses (2/0.5, 8/2 or 16/4 mg) in random order. During each maintenance period, participants could self-administer oxycodone orally (0, 10, 20, 40 or 60 mg prescription opioids) or receive money during laboratory sessions. Drug choice (percentage) was the primary dependent variable. Subjective ratings of clinical pain and withdrawal symptoms also were measured. Mann-Whitney tests compared percentage of drug choice for each active oxycodone dose to placebo. Logistic regression analyses identified correlates of oxycodone preference, defined as 60% or greater choice of oxycodone compared to money. Pain was significantly reduced while participants were maintained on Bup/Nx compared to preadmission ratings. No differences in percentage drug choice were observed between the active oxycodone doses and placebo under each Bup/Nx maintenance dose. However, factors associated with oxycodone preference were lower Bup/Nx maintenance dose, more withdrawal symptoms and more pain. These data suggest that Bup/Nx was effective in reducing pain and supplemental oxycodone use. Importantly, adequate management of pain and withdrawal symptoms by Bup/Nx may reduce oxycodone preference in this population.

A combination of buprenorphine and naltrexone blocks compulsive cocaine intake in rodents without producing dependence

Buprenorphine, a synthetic opioid that acts at both μ and κ opioid receptors, can decrease cocaine use in individuals with opioid addiction. However, the potent agonist action of buprenorphine at μ opioid receptors raises its potential for creating opioid dependence in non-opioid-dependent cocaine abusers. Here, we tested the hypothesis that a combination of buprenorphine and naltrexone (a potent μ opioid antagonist with weaker δ and κ antagonist properties) could block compulsive cocaine self-administration without producing opioid dependence. The effects of buprenorphine and various doses of naltrexone on cocaine self-administration were assessed in rats that self-administered cocaine under conditions of either short access (noncompulsive cocaine seeking) or extended access (compulsive cocaine seeking). Buprenorphine alone reproducibly decreased cocaine self-administration. Although this buprenorphine-alone effect was blocked in a dose-dependent manner by naltrexone in both the short-access and the extended-access groups, the combination of the lowest dose of naltrexone with buprenorphine blocked cocaine self-administration in the extended-access group but not in the short-access group. Rats given this low dose of naltrexone with buprenorphine did not exhibit the physical opioid withdrawal syndrome seen in rats treated with buprenorphine alone, and naltrexone at this dose did not block κ agonist-induced analgesia. The results suggest that the combination of buprenorphine and naltrexone at an appropriate dosage decreases compulsive cocaine self-administration with minimal liability to produce opioid dependence and may be useful as a treatment for cocaine addiction.

Interleukin-1 beta-induced up-regulation of opioid receptors in the untreated and morphine-desensitized U87 MG human astrocytoma cells

Background

Interleukin-1beta (IL-1β) is a pro-inflammatory cytokine that can be produced in the central nervous system during inflammatory conditions. We have previously shown that IL-1β expression is altered in the rat brain during a morphine tolerant state, indicating that this cytokine may serve as a convergent point between the immune challenge and opiate mediated biological pathways. We hypothesized that IL-1β up-regulates opioid receptors in human astrocytes in both untreated and morphine-desensitized states.

Methods

To test this hypothesis, we compared the basal expression of the mu (MOR), delta (DOR), and kappa (KOR) opioid receptors in the human U87 MG astrocytic cell line to SH-SY5Y neuronal and HL-60 immune cells using absolute quantitative real time RT-PCR (AQ-rt-RT-PCR). To demonstrate that IL-1β induced up-regulation of the MOR, DOR and KOR, U87 MG cells (2 x 10⁵ cells/well) were treated with IL-1β (20 ng/mL or 40 ng/mL), followed by co-treatment with interleukin-1 receptor antagonist protein (IL-1RAP) (400 ng/mL or 400 ng/mL). The above experiment was repeated in the cells desensitized with morphine, where U87 MG cells were pre-treated with 100 nM morphine. The functionality of the MOR in U87 MG cells was then demonstrated using morphine inhibition of forksolin-induced intracellular cAMP, as determined by radioimmunoassay.

Results

U87 MG cells treated with IL-1β for 12 h showed a significant up-regulation of MOR and KOR. DOR expression was also elevated, although not significantly. Treatment with IL-1β also showed a significant up-regulation of the MOR in U87 MG cells desensitized with morphine. Co-treatment with IL-1β and interleukin-1 receptor antagonist protein (IL-1RAP) resulted in a significant decrease in IL-1β-mediated MOR up-regulation.

Conclusion

Our results indicate that the pro-inflammatory cytokine, IL-1β, affects opiate-dependent pathways by up-regulating the expression of the MOR in both untreated and morphine-desensitized U87 MG.

Roles of μ-opioid receptors and nociceptin/orphanin FQ peptide receptors in buprenorphine-induced physiological responses in primates

Buprenorphine is known as a μ-opioid peptide (MOP) receptor agonist, but its antinociception is compromised by the activation of nociceptin/orphanin FQ peptide (NOP) receptors in rodents. The aim of this study was to investigate the roles of MOP and NOP receptors in regulating buprenorphine-induced physiological responses in primates (rhesus monkeys). The effects of MOP antagonist (naltrexone), NOP antagonist [(±)-1-[(3R*,4R*)-1-(cyclooctylmethyl)-3-(hydroxymethyl)-4-piperidinyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (J-113397)], and NOP agonists [(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5] decan-4-one (Ro 64-6198) and 3-endo-8-[bis(2-methylphenyl)methyl]-3-phenyl-8-azabicyclo[3.2.1]octan-3-ol (SCH 221510)] on buprenorphine were studied in three functional assays for measuring analgesia, respiratory depression, and itch in primates. Over the dose range of 0.01 to 0.1 mg/kg, buprenorphine dose-dependently produced antinociception, respiratory depression, and itch/scratching responses, and there was a ceiling effect at higher doses (0.1-1 mg/kg). Naltrexone (0.03 mg/kg) produced similar degrees of rightward shifts of buprenorphine's dose-response curves for all three endpoints. Mean pK(B) values of naltrexone (8.1-8.3) confirmed that MOP receptors mediated mainly buprenorphine-induced antinociception, respiratory depression, and itch/scratching. In contrast, J-113397 (0.1 mg/kg) did not change buprenorphine-induced physiological responses, indicating that there were no functional NOP receptors in buprenorphine-induced effects. More importantly, both NOP agonists, Ro 64-6198 and SCH 221510, enhanced buprenorphine-induced antinociception without respiratory depression and itch/ scratching. The dose-addition analysis revealed that buprenorphine in combination with the NOP agonist synergistically produced antinociceptive effects. These findings provided functional evidence that the activation of NOP receptors did not attenuate buprenorphine-induced antinociception in primates; instead, the coactivation of MOP and NOP receptors produced synergistic antinociception without other side effects. This study strongly supports the therapeutic potential of mixed MOP/NOP agonists as innovative analgesics.

Imaging drugs with and without clinical analgesic efficacy

The behavioral response to pain is driven by sensory and affective components, each of which is mediated by the CNS. Subjective pain ratings are used as readouts when appraising potential analgesics; however, pain ratings alone cannot enable a characterization of CNS pain circuitry during pain processing or how this circuitry is modulated pharmacologically. Having a more objective readout of potential analgesic effects may allow improved understanding and detection of pharmacological efficacy for pain. The pharmacological/functional magnetic resonance imaging (phMRI/fMRI) methodology can be used to objectively evaluate drug action on the CNS. In this context, we aimed to evaluate two drugs that had been developed as analgesics: one that is efficacious for pain (buprenorphine (BUP)) and one that failed as an analgesic in clinical trials aprepitant (APREP). Using phMRI, we observed that activation induced solely by BUP was present in regions with μ-opioid receptors, whereas APREP-induced activation was seen in regions expressing NK(1) receptors. However, significant pharmacological modulation of functional connectivity in pain-processing pathways was only observed following BUP administration. By implementing an evoked pain fMRI paradigm, these drugs could also be differentiated by comparing the respective fMRI signals in CNS circuits mediating sensory and affective components of pain. We report a correlation of functional connectivity and evoked pain fMRI measures with pain ratings as well as peak drug concentration. This investigation demonstrates how CNS-acting drugs can be compared, and how the phMRI/fMRI methodology may be used with conventional measures to better evaluate candidate analgesics in small subject cohorts.

Current knowledge of buprenorphine and its unique pharmacological profile

Despite the increasing clinical use of transdermal buprenorphine, questions have persisted about the possibility of a ceiling effect for analgesia, its combination with other μ-opioid agonists, and the reversibility of side effects. In October 2008, a consensus group of experts met to review recent research into the pharmacology and clinical use of buprenorphine. The objective was to achieve consensus on the conclusions to be drawn from this work. It was agreed that buprenorphine clearly behaves as a full μ-opioid agonist for analgesia in clinical practice, with no ceiling effect, but that there is a ceiling effect for respiratory depression, reducing the likelihood of this potentially fatal adverse event. This is entirely consistent with receptor theory. In addition, the effects of buprenorphine can be completely reversed by naloxone. No problems are encountered when switching to and from buprenorphine and other opioids, or in combining them. Buprenorphine exhibits a pronounced antihyperalgesic effect that might indicate potential advantages in the treatment of neuropathic pain. Other beneficial properties are the compound's favorable safety profile, particularly in elderly patients and those with renal impairment, and its lack of effect on sex hormones and the immune system. The expert group agreed that these properties, as well as proven efficacy in severe pain and favorable tolerability, mean that buprenorphine can be considered a safe and effective option for treating chronic cancer and noncancer pain.

Effects of intraperitoneal and intracerebroventricular injection of crocin on acute corneal pain in rats

In this study, the effects of intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) injection of crocin in separate and combined treatments with i.p. injections of morphine (an opioid receptor agonist) and naloxone (an opioid receptor antagonist) were investigated on acute corneal pain in rats. Acute corneal pain was induced by local application of a drop of 5 M NaCl solution on the corneal surface. The number of eye wipes was taken as a pain response, and counted during the first 30 s. Crocin injected i.p. and i.c.v. and morphine injected i.p. significantly (p < 0.05) decreased the number of eye wipes. Morphine (i.p.)-induced antinociception was significantly (p < 0.05) increased by the systemically and centrally injected crocin. The antinociceptive effects induced by i.p. and i.c.v. injections of crocin were not reversed by i.p. injection of naloxone. These findings indicated that both crocin and morphine attenuated hypertonic saline-induced corneal pain. The opioid receptors may not be involved in the analgesic mechanism of crocin.

Morbidity and mortality rates associated with serial bleeding from the superficial temporal vein in mice

The authors reviewed 3 years' worth of records of superficial temporal vein phlebotomies that had been carried out in Balb/c mice. Mice that had undergone the phlebotomies had a mortality rate of approximately 1% (4 deaths per 490 procedures). Although the mice were being used in a drug study, necropsy and blood assay results suggest that the four deaths were caused by failure to stop the bleeding after the phlebotomies. The authors offer suggestions for improving morbidity and mortality associated with temporal vein phlebotomy.

Buprenorphine transdermal system for opioid therapy in patients with chronic low back pain

OBJECTIVE

The present randomized, double-blinded, crossover study compared the efficacy and safety of a seven-day buprenorphine transdermal system (BTDS) and placebo in patients with low back pain of moderate or greater severity for at least six weeks.

METHODS

Prestudy analgesics were discontinued the evening before random assignment to 5 microg/h BTDS or placebo, with acetaminophen 300 mg/codeine 30 mg, one to two tablets every 4 h to 6 h as needed, for rescue analgesia. The dose was titrated to effect weekly, if tolerated, to 10 microg/h and 20 microg/h BTDS. Each treatment phase was four weeks.

RESULTS

Fifty-three patients (28 men, 25 women, mean [+/- SD] age 54.5+/-12.7 years) were evaluable for efficacy (completed two weeks or more in each phase). Baseline pain was 62.1+/-15.5 mm (100 mm visual analogue scale) and 2.5+/-0.6 (five-point ordinal scale). BTDS resulted in lower mean daily pain scores than in the placebo group (37.6+/-20.7 mm versus 43.6+/-21.2 mm on a visual analogue scale, P=0.0487; and 1.7+/-0.6 versus 2.0+/-0.7 on the ordinal scale, P=0.0358). Most patients titrated to the highest dose of BTDS (59% 20 microg/h, 31% 10 microg/h and 10% 5 microg/h). There were improvements from baseline in pain and disability (Pain Disability Index), Pain and Sleep (visual analogue scale), Quebec Back Pain Disability Scale and Short-Form 36 Health Survey scores for both BTDS and placebo groups, without significant differences between treatments. While there were more opioid-related side effects with BTDS treatment than with placebo, there were no serious adverse events. A total of 82% of patients chose to continue BTDS in a long-term open-label evaluation, in whom improvements in pain intensity, functionality and quality of life were sustained for up to six months without analgesic tolerance.

CONCLUSION

BTDS (5 microg/h to 20 microg/h) represents a new treatment option for initial opioid therapy in patients with chronic low back pain.

Buprenorphine is a weak partial agonist that inhibits opioid receptor desensitization

Buprenorphine is a weak partial agonist at mu-opioid receptors that is used for treatment of pain and addiction. Intracellular and whole-cell recordings were made from locus ceruleus neurons in rat brain slices to characterize the actions of buprenorphine. Acute application of buprenorphine caused a hyperpolarization that was prevented by previous treatment of slices with the irreversible opioid antagonist beta-chlornaltrexamine (beta-CNA) but was not reversed by a saturating concentration of naloxone. As expected for a partial agonist, subsaturating concentrations of buprenorphine decreased the [Met](5)enkephalin (ME)-induced hyperpolarization or outward current. When the ME-induced current was decreased below a critical value, desensitization and internalization of mu-opioid receptors was eliminated. The inhibition of desensitization by buprenorphine was not the result of previous desensitization, slow dissociation from the receptor, or elimination of receptor reserve. Treatment of slices with subsaturating concentrations of etorphine, methadone, oxymorphone, or beta-CNA also reduced the current induced by ME but did not block ME-induced desensitization. Treatment of animals with buprenorphine for 1 week resulted in the inhibition of the current induced by ME and a block of desensitization that was not different from the acute application of buprenorphine to brain slices. These observations show the unique characteristics of buprenorphine and further demonstrate the range of agonist-selective actions that are possible through G-protein-coupled receptors.

Analgesic Properties of New Pyrrolidinomorphinane Derivatives: Revealing Potential Pathways

The pyrrolidinomorphinane derivatives have been found to be active in the writhing tests in mice with the analgesic effect being antagonized by naloxone. This suggests that analgesia is mediated by the opioid system. The compounds also displayed anti-inflammatory activity in formalin and histamine edema experimental models and the data may be evidence for anti-inflammatory activity involvement in the inhibition of pain reaction.

Association analysis of genes encoding the nociceptin receptor (OPRL1) and its endogenous ligand (PNOC) with alcohol or illicit drug dependence

Recent studies in animal models have shown that the nociceptin system, comprising nociceptin (or OFQ/N, encoded by PNOC) and the nociceptin receptor (an opioid receptor-like protein encoded by OPRL1), may be involved in alcohol and other drug reward pathways. To determine whether the nociceptin system is associated with alcohol or illicit drug dependence in humans, we analyzed 10 single nucleotide polymorphisms (SNPs) in OPRL1 and 15 SNPs in PNOC in a sample of 1923 European Americans from 219 multiplex alcohol dependent families ascertained by the Collaborative Study on the Genetics of Alcoholism. The SNPs spanned both genes and several kb of their flanking sequences, and were in high linkage disequilibrium. Neither gene was associated with alcohol or illicit drug dependence, although two SNPs in PNOC showed marginal association with alcoholism and one with illicit drug dependence (P = 0.04-0.05). Secondary analyses suggested that two adjacent SNPs in intron 1 of OPRL1 were marginally associated with opioid dependence (P = 0.05); none of the SNPs in PNOC were associated with opioid dependence.

Mechanism-based pharmacokinetic-pharmacodynamic modelling of the reversal of buprenorphine-induced respiratory depression by naloxone : a study in healthy volunteers

BACKGROUND AND OBJECTIVE

Respiratory depression is a potentially life-threatening adverse effect of opioid therapy. It has been postulated that the difficulty of reversing buprenorphine-induced respiratory depression is caused by slow receptor association-dissociation kinetics at the opioid mu receptor. The aim of this study was to characterise the pharmacodynamic interaction between buprenorphine and naloxone in healthy volunteers.

METHODS

A competitive pharmacodynamic interaction model was proposed to describe and predict the time course of naloxone-induced reversal of respiratory depression. The model was identified using data from an adaptive naloxone dose-selection trial following intravenous administration of buprenorphine 0.2mg/70kg or 0.4mg/70kg.

RESULTS

The pharmacokinetics of naloxone and buprenorphine were best described by a two-compartment model and a three-compartment model, respectively. A combined biophase equilibration-receptor association-dissociation pharmacodynamic model described the competitive interaction between buprenorphine and naloxone at the opioid mu receptor. For buprenorphine, the values of the rate constants of receptor association (k(on)) and dissociation (k(off)) were 0.203 mL/ng/min and 0.0172 min(-)(1), respectively. The value of the equilibrium dissociation constant (K(D)) was 0.18 nmol/L. The half-life (t((1/2))) of biophase equilibration was 173 minutes. These estimates of the pharmacodynamic parameters are similar to values obtained in the absence of naloxone co-administration. For naloxone, the half-life of biophase distribution was 6.5 minutes.

CONCLUSIONS

Because of the slow receptor association-dissociation kinetics of buprenorphine in combination with the fast elimination kinetics of naloxone, naloxone is best administered as a continuous infusion for reversal of buprenorphine-induced respiratory depression.

Switching from transdermal drugs: an observational "N of 1" study of fentanyl and buprenorphine

The aim of this study was to confirm that the concomitant presence of transdermal fentanyl (TTS FE) and buprenorphine (TTS BU) may be feasible without important consequences, using doses presumed to be equianalgesic. A prospective "N of 1" study was carried out in a sample of volunteers with cancer pain receiving stable doses of TTS FE or TTS BU, with adequate pain and symptom control. In the study design, each patient provided data before and after a switch from one opioid to the other and then back to the previous one. Sixteen patients receiving daily stable doses of 0.6 or 1.2mg of TTS FE were switched to TTS BU using an FE-BU ratio of 0.6-0.8. After three days, the TTS BU patch was removed and TTS FE patch was placed for another three days. Six patients receiving TTS BU were switched to TTS FE and then rotated back to TTS BU with the same dosing considerations. No statistical differences in changes in pain and symptom intensity during switching and between the two different sequences were observed. No significant changes in rescue doses of oral morphine were reported at the same intervals. Cancer patients receiving stable doses of TTS FE or TTS BU can be safely switched to the alternative transdermal opioid. Further studies should be performed to gather data about the use of TTS BU with other opioids, at different doses, and in different clinical conditions.

Clinical efficacy of buprenorphine to minimize distress in MRL/lpr mice

MRL/MpJ-Fas(lpr) (MRL/lpr) mice are an accepted animal model to study human systemic lupus erythematosus. We tested if a commonly used analgesic (buprenorphine hydrochloride) would reduce pain and distress in these mice without impacting the progression of autoimmune disease. Female MRL/lpr mice were randomly separated into four groups. Experimental groups received cyclophosphamide (25 mg/kg i.p. weekly), buprenorphine (0.09 mg/kg/mouse/day via drinking water), or cyclophosphamide+buprenorphine from 11 to 21 weeks of age. Controls received no treatments. Mice were monitored daily by a licensed veterinarian (blinded observer) and assigned a score weekly on parameters associated with pain and distress as well as progression of disease. Proteinuria was measured weekly, and serum anti-dsDNA antibody levels were determined at 11, 15, and 18 weeks of age. At 21 weeks of age, the animals were euthanized and the kidneys and spleens were removed for evaluation. Regardless of the parameter observed, buprenorphine did not significantly decrease distress when compared to the controls. Buprenorphine did not alter the progression of autoimmune disease, based on characteristics of splenic architecture and splenocyte cell profiles, development of lymphadenopathy, or kidney histology as compared to controls. This study indicates that buprenorphine at this dose and route of administration was ineffective in reducing distress associated with disease progression in the MRL/lpr strain. More studies are needed to determine if, at a different dose or route, buprenorphine would be useful as adjunctive therapy in reducing distress in MRL/lpr mice.

Pharmacological evaluation of opioid and non-opioid analgesics in a murine bone cancer model of pain

The intramedulary injection of osteosarcoma cells in the mouse femur has served as a laboratory model to study bone cancer pain. However, the efficacy of different classes of analgesics has not fully been analyzed in this model. Therefore, the acute antinociceptive properties of different classes of drugs were evaluated on post-inoculation day 15 when the degrees of spontaneous pain and mechanical hypersensitivity in the ipsilateral inoculated hind paw reached almost their maximal effects. At high doses, the opioids fentanyl, morphine, and tramadol had full efficacies for all pain parameters tested. Antagonism experiments with naloxone (10 mg/kg s.c.) or its peripheral analogue methylnaltrexone (10 mg/kg s.c.), suggest that the analgesic effects of fentanyl were predominantly mediated by centrally located mu-opiate receptors. Acetaminophen, the non-steroidal anti-inflammatory drug indomethacin, and the COX-2-inhibitor celecoxib did not significantly improve pain behavior. The tricyclic antidepressants amitriptyline and desipramine significantly reduced spontaneous pain behavior but this only at sedative doses; the serotonin reuptake inhibitor fluoxetine had limited efficacy. Also with the anticonvulsants lamotrigine, topiramate, and gabapentin limited or no efficacies were found. In conclusion, the present study provided integrated information about the tumor-induced bone pain in mice, and clarified acute efficacies of different categories of analgesics for the spontaneous lifting, limb-use impairment, and mechanical hypersensitivity. Moreover, the finding that bone cancer-pain behaviors are attenuated by various established compounds further supports the validity of the murine bone cancer model for the study of bone cancer pain and its use for the identification of novel treatments.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Is there a ceiling effect of transdermal buprenorphine? Preliminary data in cancer patients

OBJECTIVE

The aim of this preliminary study was to explore the possibility of using higher doses of transdermal buprenorphine (TD-BUP) than those commonly used and available as manufactured patches, which are based on the assumption that BUP may have a ceiling effect that has never been determined yet.

MATERIALS AND METHODS

Ten patients who were already receiving TD-BUP (70 microg/h, which is about 1.6 mg/day) and were no longer responsive to this dosage were administered higher doses up to a maximum of 140 microg/h within 6 days, when the study was completed.

RESULTS

In six patients, dose increments of TD-BUP were effective, and patients achieved adequate analgesia within 6 days. Four patients discontinued the treatment due to inefficacy of TD-BUP 140 microg/h and were switched to other opioids until achieving stabilization (oxycodone 320 and 400 mg/day, methadone 120 mg/day, transdermal fentanyl 200 microg/h). This group of patients required higher doses than those chosen for TD-BUP, underlying the need to escalate the dose rapidly, a modality not accomplished with transdermal drugs. Adverse effects did not change and were similar to those observed before increasing the dose of TD-BUP. On the basis of these preliminary data, patients requiring doses higher than 70 microg/h of TD-BUP, in the range of 105-140 microg/h, may still have an analgesic benefit without important consequences in terms of adverse effects. It cannot be excluded that even higher doses may be effective, as some patients required rapid titration with higher morphine equivalent doses, and according to the protocol, other opioids were provided to facilitate this process. Further studies should clarify the role and the benefit of TD-BUP in specific clinical circumstances.

Safety and effectiveness of intravenous morphine for episodic breakthrough pain in patients receiving transdermal buprenorphine

Supplemental dosing of an opioid is the main treatment suggested to manage breakthrough pain in cancer patients. The intravenous route has been proven to be safe and effective, providing rapid analgesia in patients receiving oral morphine. Transdermal buprenorphine (TTS-BUP) is increasingly used in cancer pain management, but this drug has been labeled as a difficult drug to use in combination with other opioids. The aim of this open-label study was to verify the safety and effectiveness of intravenous morphine (IV-MO) for the treatment of episodic pain in cancer patients receiving TTS-BUP. A consecutive sample of 29 cancer patients, who were treated with TTS-BUP, reported an acceptable basal analgesia, and presented with episodic pains were selected for the study. The IV-MO dose was one-fifth of the morphine equivalent oral daily dose calculated using a ratio of TTS-BUP/oral morphine of 1:75, and a morphine IV/oral ratio of 1:3. For each episode, pain intensity and opioid-related adverse effects were recorded when severe pain occurred (T0), and 15 minutes after. One hundred six breakthrough events in the 29 patients (3.7 episodes per patient, on average) were recorded during admission. The mean pain intensity decreased from an initial value of 7.3 (confidence interval [CI] 95% 7.0-7.5) to 2.9 (CI 95% 2.5-3.3) 15 minutes after IV-MO. Ninety-eight episodes (92.4%) were considered treated successfully, defined as a reduction of more than 33% within 15 minutes; 88 of these episodes (83.0%) had more than 50% pain intensity decrease. No differences in age, gender, pain mechanism, and time of events were found. Eight episodes (7.5%) did not respond effectively within 15 minutes, and required further doses. The occurrence of adverse effects for each episode treated was not frequent and intensity was not relevant. IV-MO was effective and safe in most cancer patients receiving TTS-BUP who experienced pain exacerbation.