Eveline van Dorp

Naloxone treatment in opioid addiction: the risks and benefits

Naloxone is a non-selective, short-acting opioid receptor antagonist that has a long clinical history of successful use and is presently considered a safe drug over a wide dose range (up to 10 mg). In opioid-dependent patients, naloxone is used in the treatment of opioid-overdose-induced respiratory depression, in (ultra)rapid detoxification and in combination with buprenorphine for maintenance therapy (to prevent intravenous abuse). Risks related to naloxone use in opioid-dependent patients are: i) the induction of an acute withdrawal syndrome (the occurrence of vomiting and aspiration is potentially life threatening); ii) the effect of naloxone may wear off prematurely when used for treatment of opioid-induced respiratory depression; and iii) in patients treated for severe pain with an opioid, high-dose naloxone and/or rapidly infused naloxone may cause catecholamine release and consequently pulmonary edema and cardiac arrhythmias. These risks warrant the cautious use of naloxone and adequate monitoring of the cardiorespiratory status of the patient after naloxone administration where indicated.

Naloxone Reversal of Buprenorphine-induced Respiratory Depression

Background:The objective of this investigation was to examine the ability of the opioid antagonist naloxone to reverse respiratory depression produced by the &mgr;-opioid analgesic, buprenorphine, in healthy volunteers. The studies were designed in light of the claims that buprenorphine is relatively resistant to the effects of naloxone. Methods:In a first attempt, the effect of an intravenous bolus dose of 0.8 mg naloxone was assessed on 0.2 mg buprenorphine–induced respiratory depression. Next, the effect of increasing naloxone doses (0.5–7 mg, given over 30 min) on 0.2 mg buprenorphine–induced respiratory depression was tested. Subsequently, continuous naloxone infusions were applied to reverse respiratory depression from 0.2 and 0.4 mg buprenorphine. All doses are per 70 kg. Respiration was measured against a background of constant increased end-tidal carbon dioxide concentration. Results:An intravenous naloxone dose of 0.8 mg had no effect on respiratory depression from buprenorphine. Increasing doses of naloxone given over 30 min produced full reversal of buprenorphine effect in the dose range of 2–4 mg naloxone. Further increasing the naloxone dose (doses of 5 mg or greater) caused a decline in reversal activity. Naloxone bolus doses of 2–3 mg, followed by a continuous infusion of 4 mg/h, caused full reversal within 40–60 min of both 0.2 and 0.4 mg buprenorphine–induced respiratory depression. Conclusions:Reversal of buprenorphine effect is possible but depends on the buprenorphine dose and the correct naloxone dose window. Because respiratory depression from buprenorphine may outlast the effects of naloxone boluses or short infusions, a continuous infusion of naloxone may be required to maintain reversal of respiratory depression.

Morphine-6-glucuronide : Morphine's successor for postoperative pain relief?

In searching for an analgesic with fewer side effects than morphine, examination of morphines active metabolite, morphine-6-glucuronide (M6G), suggests that M6G is possibly such a drug. In contrast to morphine, M6G is not metabolized but excreted via the kidneys and exhibits enterohepatic cycling, as it is a substrate for multidrug resistance transporter proteins in the liver and intestines. M6G exhibits a delay in its analgesic effect (blood-effect site equilibration half-life 4–8 h), which is partly related to slow passage through the blood-brain barrier and distribution within the brain compartment. In humans, M6Gs potency is just half of that of morphine. In clinical studies, M6G is well tolerated and produces adequate and long lasting postoperative analgesia. At analgesic doses, M6G causes similar reduction of the ventilatory response to CO2 as an equianalgesic dose of morphine but significantly less depression of the hypoxic ventilatory response. Preliminary data indicate that M6G is associated less than morphine with nausea and vomiting, causing 50% and 75% less nausea in postoperative and experimental settings, respectively. Although the data from the literature are very promising, we believe that more studies are necessary before we may conclude that M6G is superior to morphine for postoperative analgesia.

Morphine-6-glucuronide (M6G) for postoperative pain relief.

Morphine‐6‐glucuronide (M6G) is morphines active metabolite acting at the μ‐opioid receptor. Recent experimental human studies and 5 of 6 randomized clinical trials indicate that M6G causes adequate and long lasting pain relief comparable to morphine. There are various observations that M6G is associated with a reduction in the severity of side effects normally associated with opioid use, such as reduced postoperative nausea and vomiting (PONV) and reduced respiratory depression. The present drug profile provides a review of the pharmacological properties of M6G, the clinical evidence relating to its efficacy and safety, and discusses its future role in the treatment of postoperative pain.

Naloxone reversal of morphine- and morphine-6-glucuronide-induced respiratory depression in healthy volunteers: a mechanism-based pharmacokinetic-pharmacodynamic modeling study.

Background:Opioid-induced respiratory depression is antagonized effectively by the competitive opioid receptor antagonist naloxone. However, to fully understand the complex opioid agonist–antagonist interaction, the effects of various naloxone doses on morphine and morphine-6-glucuronide (M6G)-induced respiratory depression were studied in healthy volunteers. Methods:Twenty-four subjects received 0.15 mg/kg morphine intravenously at t = 0 followed by placebo, 200 or 400 &mgr;g naloxone at t = 30 min. Thirty-two subjects received 0.3 mg/kg M6G intravenously at t = 0 followed by placebo, 25, 100, or 400 &mgr;g naloxone at t = 55 min. There were a total of 8 subjects per treatment group. Respiration was measured on a breath-to-breath basis at constant end-tidal Pco2. A mechanism-based pharmacokinetic–pharmacodynamic model consisting of a part describing biophase equilibration and a part describing receptor association–dissociation kinetics was used to analyze the data. Results:Naloxone reversal of M6G-induced respiratory depression developed more slowly than reversal of the respiratory effect of morphine. A simulation study revealed that this was related to the slower receptor association–dissociation kinetics of M6G (koff M6G = 0.0327 ± 0.00455 min−1 versus morphine 0.138 ± 0.0148 min−1; values are typical ±SE). Duration of naloxone reversal was longer for M6G. This was related to the three- to fourfold greater potency of naloxone as an antagonist against M6G compared with morphine. Increasing the naloxone dose had no effect on the speed of reversal, but it did extend reversal duration. Conclusions:Naloxone reversal of the opioid effect is dependent on the receptor association–dissociation kinetics of the opioid that needs reversal with respect to the rate of reversal. The pharmacodynamics of naloxone determines reversal magnitude and duration.

Morphine-6β-glucuronide Rapidly Increases Pain Sensitivity Independently of Opioid Receptor Activity in Mice and Humans

Background:Previous data indicate that morphine-6&bgr;-glucuronide (M6G), a morphine metabolite with analgesic properties, can paradoxically increase pain sensitivity in mice and humans. The authors tested mice and humans for M6G hyperalgesia and assessed the contribution of N-methyl-d-aspartate receptor activity in mice. Methods:Nociception after acute injection (10 mg/kg) and chronic infusion (1.6 mg/kg per 24 h) of M6G or saline was assayed using the tail-withdrawal test in CD-1 mice implanted with pellets containing the opioid antagonist naltrexone or placebo and in knockout mice lacking &mgr;-, &kgr;-, and &dgr;-opioid receptors and their B6129F1 controls. In volunteers, responses to heat pain were tested after a M6G (0.4 mg/kg) injection in the presence of a continuous high naloxone (0.04-mg/kg bolus followed by 0.04 mg/kg per hour) or saline background infusion. Results:Acute M6G injection evoked analgesia in CD-1 mice implanted with placebo pellets and B6129F1 control mice, whereas it caused hyperalgesia in CD-1 mice treated concurrently with naltrexone and in knockout mice. Continuous M6G infusion produced hyperalgesia within 24 h, lasting for a minimum of 6 days, in both placebo- and naltrexone-pelleted mice. The N-methyl-d-aspartate receptor antagonist MK-801 (0.05 mg/kg) blocked and reversed hyperalgesia after the acute injection and continuous infusion of M6G, respectively. In humans, M6G increased heat pain sensitivity for at least 6 h independently of simultaneous naloxone infusion. Conclusions:These data indicate that M6G causes hyperalgesia independent of previous or concurrent opioid receptor activity or analgesia. In mice, a causal role for the N-methyl-d-aspartate receptor is also indicated.

Arterial [H+] and the ventilatory response to hypoxia in humans: influence of acetazolamide-induced metabolic acidosis

In this study, we investigated possible separate effects of H+ ions and CO2 on hypoxic sensitivity in humans. We also examined whether hypoxic sensitivity, conventionally defined as the ratio of (hypoxic - normoxic) ventilation over (hypoxic - normoxic) Hb oxygen saturation can also be estimated by taking the ratio (hypoxic - normoxic) ventilation over (logPa(O2) hypoxia - logPa(O2) normoxia), enabling one to measure the hypoxic response independently from potential confounding influences of changes in position of the Hb oxygen saturation curve. We used acetazolamide to induce a metabolic acidosis. To determine the acute hypoxic response (AHR), we performed step decreases in end-tidal Po2 to approximately 50 Torr lasting 5 min each at three different constant end-tidal Pco2 levels. Nine subjects ingested 250 mg of acetazolamide or placebo every 8 h for 3 days in a randomized double-blind crossover design. The metabolic acidosis was accompanied by a rise in ventilation, a substantial fall in Pa(CO2), and a parallel leftward shift of the ventilatory CO2 response curve. In placebo, CO2 induced equal relative increases in hypoxic sensitivity (O2-CO2 interaction) regardless of the way it was defined. Acetazolamide shifted the response line representing the relationship between hypoxic sensitivity and arterial [H+] ([H+](a)) to higher values of [H+](a) without altering its slope, indicating that it did not affect the O2-CO2 interaction. So, in contrast to an earlier belief, CO2 and H+ have separate effects on hypoxic sensitivity. This was also supported by the finding that infusion of bicarbonate caused a leftward shift of the hypoxic sensitivity-[H+](a) response lines in placebo and acetazolamide. A specific inhibitory effect of acetazolamide on hypoxic sensitivity was not demonstrated.

A randomized, double-blind, placebo-controlled pilot study of IV morphine-6-glucuronide for postoperative pain relief after knee replacement surgery.

ObjectivesTo determine the dose-response effect of intravenous morphine-6-glucuronide (M6G) on acute postoperative pain. MethodsPatients undergoing knee replacement surgery under spinal anesthesia were randomly assigned to 1 of 4 single intravenous M6G doses, 0 (placebo), 10, 20, or 30 mg/70 kg, administered 150 minutes after the spinal anesthetic was given. Analgesic effects were evaluated by determining the cumulative patient controlled analgesia (PCA) morphine dose, consumed over a 12 and 24 hours period, after the initial dose of M6G. For pain assessments, a 10 cm visual analog scale was used. ResultsData from 41 patients were evaluated (n=10, 10, 10, and 11 in the 0, 10, 20, and 30 mg M6G groups). Only at the highest M6G dose (30 mg/70 kg), morphine PCA consumption was significantly less compared with placebo: over the first 12 postoperative hours mean PCA morphine consumption was 3.0±2.0 mg/h after placebo and 1.4±0.5 mg/h after 30 mg M6G (P=0.03); over the first 24 h mean PCA morphine consumption was 2.5±2.1 mg after placebo and 1.0±0.4 mg after 30 mg M6G (P=0.04) (mean±SD). Visual analog scale values were similar across all groups during these time periods. DiscussionThe analgesic effect of M6G in postoperative pain was demonstrated with 30 mg/70 kg M6G superior to placebo. At this dose, M6G has a long duration of action as determined by a reduction in the use of morphine PCA over 12 and 24 hours.

A retrospective study on persistent pain after childbirth in the Netherlands

Reported prevalence rates of persistent postpartum pain (PPP) range from less than 1% to almost 20%. The aim of this study was to examine the prevalence of PPP in a Dutch cohort and to evaluate a possible causal role for specific risk factors on the development of chronic pain after childbirth. A questionnaire was sent to 960 postpartum women approximately 2 years after delivery. Primary outcome was pain that arose from childbirth at follow-up, and secondary outcomes included quality of life (QoL) and Hospital Anxiety and Depression Scale scores. Tested risk factors included mode of labor analgesia, history of negative effect, history of chronic pain, delivery route, parity, and ethnicity. A total of 495 (51.6%) women participated. At a mean time of 2.3 postpartum years, 7.3% of women reported any pain and 6.1% reported significant pain related to the delivery. Compared to spontaneous delivery, cesarean delivery provided protection against persistent pain (odds ratio, 0.12; 95% CI, 0.01–0.63, P<0.05). None of the other risk factors, including remifentanil use for labor pain, were of influence on the prevalence of persistent pain. Women with PPP experienced greater negative effects and had lower QoL scores compared to women without pain. In this cohort of Dutch patients, PPP is a serious problem with a great impact on the physical and mental health of women.

Arterial and venous pharmacokinetics of morphine-6-glucuronide and impact of sample site on pharmacodynamic parameter estimates.

BACKGROUND: In pharmacokinetic–pharmacodynamic modeling studies, venous plasma samples are sometimes used to derive pharmacodynamic model parameters. In the current study the extent of arteriovenous concentration differences of morphine-6-glucuronide (M6G) was quantified. We used simulation studies to estimate possible biases in pharmacodynamic model parameters when linking venous versus arterial concentrations to effect. METHODS: Seventeen healthy volunteers received an IV 90-second infusion of 0.3 mg/kg morphine-6-glucuronide (M6G). Arterial and venous blood samples, from the radial artery and cubital vein, respectively, were obtained. An extended pharmacokinetic model was constructed linking arterial and venous compartments. The extent of bias in pharmacodynamic model parameter estimates was explored in simulation studies with NONMEM, simulating M6G effect using first-order effect-compartment–inhibitory sigmoid EMAX models. M6G effect was simulated at various values for the arterial blood-effect-site equilibration half-lifes (t½kE0), ranging from 5 to 240 minutes. RESULTS: Arteriovenous concentration differences were apparent, with higher arterial plasma concentrations just after infusion, whereas at later times (>60 minutes) venous M6G concentrations exceeded arterial concentrations. The extended pharmacokinetic model adequately described the data and consisted of 3 arterial compartments, 1 central venous compartment, and 1 peripheral venous compartment. The simulation studies revealed large biases in model parameters derived from venous concentration data. The biases were dependent on the value of t½kE0. Assuming that the true values of M6G t½kE0 range from 120 to 240 minutes (depending on the end point measured), we would have underestimated t½kE0 by 30%, whereas the potency parameter would have been overestimated by about 40%, when using venous plasma samples. CONCLUSIONS: Because of large arteriovenous differences in M6G plasma, concentration biases in pharmacodynamic model parameters will occur when linking venous concentration to effect, using a traditional effect-compartment model.