A randomised controlled trial on the efficacy and side-effect profile (nausea/vomiting/sedation) of morphine-6-glucuronide versus morphine for post-operative pain relief after major abdominal surgery*

Peripheralization Strategies Applied to Morphinans and Implications for Improved Treatment of Pain

Opioids are considered the most effective analgesics for the treatment of moderate to severe acute and chronic pain. However, the inadequate benefit/risk ratio of currently available opioids, together with the current 'opioid crisis', warrant consideration on new opioid analgesic discovery strategies. Targeting peripheral opioid receptors as effective means of treating pain and avoiding the centrally mediated side effects represents a research area of substantial and continuous attention. Among clinically used analgesics, opioids from the class of morphinans (i.e., morphine and structurally related analogues) are of utmost clinical importance as analgesic drugs activating the mu-opioid receptor. In this review, we focus on peripheralization strategies applied to N-methylmorphinans to limit their ability to cross the blood-brain barrier, thus minimizing central exposure and the associated undesired side effects. Chemical modifications to the morphinan scaffold to increase hydrophilicity of known and new opioids, and nanocarrier-based approaches to selectively deliver opioids, such as morphine, to the peripheral tissue are discussed. The preclinical and clinical research activities have allowed for the characterization of a variety of compounds that show low central nervous system penetration, and therefore an improved side effect profile, yet maintaining the desired opioid-related antinociceptive activity. Such peripheral opioid analgesics may represent alternatives to presently available drugs for an efficient and safer pain therapy.

Peripheral Opioid Receptor Blockade Enhances Epithelial Damage in Piroxicam-Accelerated Colitis in IL-10-Deficient Mice

Mucosal CD4+ T lymphocytes display a potent opioid-mediated analgesic activity in interleukin (IL)-10 knockout mouse model of inflammatory bowel diseases (IBD). Considering that endogenous opioids may also exhibit anti-inflammatory activities in the periphery, we examined the consequences of a peripheral opioid receptor blockade by naloxone-methiodide, a general opioid receptor antagonist unable to cross the blood-brain barrier, on the development of piroxicam-accelerated colitis in IL-10-deficient (IL-10-/-) mice. Here, we show that IL-10-deficient mice treated with piroxicam exhibited significant alterations of the intestinal barrier function, including permeability, inflammation-related bioactive lipid mediators, and mucosal CD4+ T lymphocyte subsets. Opioid receptor antagonization in the periphery had virtually no effect on colitis severity but significantly worsened epithelial cell apoptosis and intestinal permeability. Thus, although the endogenous opioid tone is not sufficient to reduce the severity of colitis significantly, it substantially contributes to the protection of the physical integrity of the epithelial barrier.

A pharmacokinetic study of morphine-6-O-sulfate in rat plasma and brain

Morphine-6-O-sulfate (M6S), a polar, zwitterionic sulfate ester of morphine, is a powerful and safe analgesic in several rat models of pain. A sensitive liquid chromatography-tandem mass spectrometry bioanalytical method was developed and validated for the simultaneous determination of M6S and morphine (MOR) in rat plasma and brain after M6S administration. Morphine-d₆ was used as internal standard. Multiple reaction monitoring was used for detection and quantitation of M6S, MOR, and morphine-d₆ in the turbo ion spray positive mode. The chromatographic separation was carried out on an Alltech Altima C18 column. The analytical method was validated for linearity, precision, accuracy, specificity, and stability over a concentration range of 3-8000 ng/ml in rat plasma and 10-10,000 ng/ml in brain samples for both M6S and MOR. The validated method was applied to determine the PK profile of M6S in plasma after i.v., i.p., and oral dosing in male Sprague-Dawley rats. Rats were administered M6S by i.p. administration (5.6 and 10.0 mg/kg) or orally (10 and 30 mg/kg) and bioavailability compared to an i.v. injection (1 mg/kg) of M6S. The in vivo results indicate that M6S is not a prodrug of morphine, since M6S is not biotransformed into MOR in plasma after either i.p. or oral administration, and MOR was not detected in brain. The bioavailability of M6S was >93% and about 5% after i.p. and oral dosing, respectively. The low oral bioavailability of M6S may be due to poor permeation of the intestinal epithelial membrane. After i.p.-administration, M6S appears to reach brain tissues in low, but significant, concentrations.

Chewing gum prophylaxis for postoperative nausea and vomiting in the intensive care unit: a pilot randomised controlled trial

Objective: To test the effectiveness of chewing gum in the prophylaxis of postoperative nausea and vomiting (PONV) in patients admitted to the intensive care unit (ICU) after surgery. Design: Prospective, open label, pilot randomised controlled trial. Setting: Two metropolitan ICUs. Participants: Ninety postoperative adult patients admitted to the ICU. Intervention: Patients administered chewing gum, who chewed for at least 15 minutes every 4 hours, were compared with a control group, who were administered a 20 mL sip of water orally every 4 hours. Main outcome measures: The primary outcome was the number of patient-reported episodes of nausea in the first 24 hours after the operation. Secondary outcomes included vomiting or dry retching episodes, and duration and severity of nausea. Results: Forty-six patients were randomly allocated to chewing gum and 44 patients to water. There was no difference between groups in the number of patients with nausea (10 [22%] chewing gum v 12 [27%] control patients; P = 0.72), nausea episodes (22 episodes; [median, 0; IQR, 0-0] v 21 episodes [median, 0; IQR, 0-1] per patient in each group respectively), vomiting/retching (2 [4%] chewing gum v 6 [14%] control patients; P = 0.24), or duration/severity of nausea. Conclusion: Regular postoperative administration of chewing gum in a surgical ICU patient cohort did not reduce nausea, vomiting or retching. The prevalence of PONV is less than previously reported. Our findings can inform future studies of PONV prophylaxis in post-surgical ICU patients. Trial registration: Australian New Zealand Clinical Trial Registry No. ACTRN12617001185358.

Replacement of current opioid drugs focusing on MOR-related strategies

The scarcity and limited risk/benefit ratio of painkillers available on the market, in addition to the opioid crisis, warrant reflection on new innovation strategies. The pharmacopoeia of analgesics is based on products that are often old and derived from clinical empiricism, with limited efficacy or spectrum of action, or resulting in an unsatisfactory tolerability profile. Although they are reference analgesics for nociceptive pain, opioids are subject to the same criticism. The use of opium as an analgesic is historical. Morphine was synthesized at the beginning of the 19th century. The efficacy of opioids is limited in certain painful contexts and these drugs can induce potentially serious and fatal adverse effects. The current North American opioid crisis, with an ever-rising number of deaths by opioid overdose, is a tragic illustration of this. It is therefore legitimate to develop research into molecules likely to maintain or increase opioid efficacy while improving their tolerability. Several avenues are being explored including targeting of the mu opioid receptor (MOR) splice variants, developing biased agonists or targeting of other receptors such as heteromers with MOR. Ion channels acting as MOR effectors, are also targeted in order to offer compounds without MOR-dependent adverse effects. Another route is to develop opioid analgesics with peripheral action or limited central nervous system (CNS) access. Finally, endogenous opioids used as drugs or compounds that modify the metabolism of endogenous opioids (Dual ENKephalinase Inhibitors) are being developed. The aim of the present review is to present these various targets/strategies with reference to current indications for opioids, concerns about their widespread use, particularly in chronic non-cancer pains, and ways of limiting the risk of opioid abuse and misuse.

Effects of intramuscular morphine in men and women with temporomandibular disorder with myofascial pain

OBJECTIVES

This placebo-controlled randomized double-blinded clinical study assessed the analgesic efficacy of intramuscular morphine in TMD patients with myofascial pain and sex-dependent responses of the morphine treatment.

SUBJECTS AND METHODS

Men and women with TMD were treated with morphine (1.5 or 5 mg), lidocaine, or saline in the masseter muscle. VAS of pain intensity, PPT, and PPtol were compared between treatment groups and gender. An additional group was treated with morphine in the trapezius muscle to evaluate the systemic effect of morphine that may reduce pain in the masseter muscle.

RESULTS

There was a significant difference in VAS scores between the morphine 5 mg group and the saline group favoring morphine, but not between the morphine 5 mg and lidocaine. Morphine 1.5 and 5 mg treatments led to consistently and significantly elevated PPT and PPtol measures in men, but not in women. Morphine administered in the trapezius muscle did not affect the outcome measures.

CONCLUSION

A single dose intramuscular morphine produced analgesic effects up to 48 hr in patients with myofascial pain. Intramuscular morphine elevated mechanical pain threshold and tolerance in the masseter only in male patients, suggesting sex differences in local morphine effects. No systemic effect of intramuscular morphine was detected.

Evaluation of morphine-like effects of the mixed mu/delta agonist morphine-6-O-sulfate in rats: Drug discrimination and physical dependence

Morphine-6-O-sulfate (M6S) is as a mixed-action mu/delta (μ/δ) opioid receptor agonist with high potency and analgesic efficacy. These studies used assays of drug discrimination and schedule-controlled responding to assess abuse-liability, tolerance, and physical dependence as compared to morphine in rats. Attempts to train 0.3 mg/kg (IP) M6S from saline failed, but all rats rapidly acquired the discrimination when the training dose was changed to 3.0 mg/kg morphine, and substitution tests showed that morphine and fentanyl both fully substituted for the training dose, M6S and M3A6S (3-O-acetyl ester of M6S) only partially substituted, and salvinorin A did not elicit morphine-like effects. Tolerance to response rate-decreasing effects was studied in rats administered either 1.0 or 3.0 mg/kg morphine or M6S before food-reinforced operant sessions. At both unit doses, tolerance to M6S-elicited rate suppression developed more slowly than tolerance to morphine-induced reductions in response rates. To assess dependence, rats were maintained on 1.0 mg/kg morphine or 1.0 mg/kg M6S until food-reinforced response rates were stable for at least 5 days. Rats were then administered saline or increasing doses of the opioid antagonist naltrexone (NTX) (0.3, 1.0, 3.0, or 10.0 mg/kg) in order to determine antagonist-precipitated withdrawal. NTX precipitated withdrawal was similar in both morphine-maintained and M6S-maintained rats. In conclusion, the mixed μ/δ agonist activity of M6S failed to completely protect against the development of physical dependence, but delayed tolerance development to behavioral effects and resulted in decreased morphine-like subjective effects, perhaps implying a decreased abuse liability over μ agonists.

Sublingual sufentanil for postoperative pain relief: first clinical experiences

Background

The sublingual sufentanil tablet system (SSTS) is a novel hand-held patient-controlled analgesia device developed for treatment of moderate-to-severe postoperative pain. Here we present the first results of its clinical use.

Methods

Adult patients undergoing major surgery in five hospitals in the Netherlands received the SSTS for postoperative pain relief as part of multimodal pain management that further included paracetamol and a nonsteroidal anti-inflammatory drug (NSAID). The following variables were collected: postoperative pain scores using the 11-point numerical rating score (NRS) at rest, number of tablets used, occurrence of nausea, and patient satisfaction scores.

Results

We included 280 patients in the study; the majority underwent laparoscopic abdominal (49%) or orthopedic (knee replacement) surgery (34%). The median NRS was 3.5 (interquartile range 2.3–4.0) on the day of surgery, 3.3 (2.3–4.0) on the first postoperative day, and 2.8 (2.0–4.0) on the second postoperative day; pain scores did not differ between surgery types. Mean number of tablets used was 19 (range 0–86). Nausea occurred in 34% of patients, more often in women (45% vs 19%). Overall satisfaction was high in 73% of patients. Satisfaction was correlated with pain relief (p<0.001) and inversely correlated with occurrence of nausea (p=0.01).

Discussion

In this data set obtained under real-life conditions we show that the SSTS effectively managed postoperative pain in abdominal and orthopedic surgeries. Future studies should determine patient populations that benefit most from the SSTS, assess the added values versus intravenous patient-controlled analgesia, and determine the pharmacoeconomics of the system.

Postoperative local incision analgesia for acute pain treatment in patients with hepatocellular carcinoma

OBJECTIVE

The present study aimed to investigate the analgesic effect and safety of using local incision analgesia to treat acute postoperative pain in patients with hepatocellular carcinoma (HCC).

METHOD

A cohort of 60 patients undergoing liver cancer resection was randomly divided into three groups (n=20 per group): local incision analgesia (LIA) group, which received local infiltration with ropivacaine combined with a postoperative analgesia pump; intravenous patient-controlled analgesia (PCA) group, which received fentanyl intravenous analgesia postoperatively; and the control group, which received tramadol hydrochloride injection postoperatively according to the NRS scoring system. The postoperative analgesic effect in each group was compared and tumor recurrence (survival) was analyzed using the Kaplan-Meier method.

RESULTS

NRS scores, rate of analgesic usage, ambulation time (h) and intestinal function recovery time (h) were significantly reduced in LIA group compared with the control group at each postoperative time point (6, 12, 24 and 48 hours; p<0.05). Additionally, the NRS scores of LIA patients at 12 hours post-surgery was significantly reduced compared with PCA group (p<0.05), and the occurrence of postoperative adverse events in LIA group was significantly lower than that in PCA group (p<0.05). Survival analysis demonstrated that the mean survival time (tumor recurrence) was significantly increased in LIA group compared with the control group (χ2=4.749; p=0.029).

CONCLUSION

Local incision analgesia improves the analgesic effect, causes fewer adverse reactions and increases postoperative survival time. Our study demonstrated that local incision analgesia is a safe and effective method of postoperative pain management following hepatectomy.

Comparison of intravenous and oral acetaminophen for pain control after total knee and hip arthroplasty: A systematic review and meta-analysis

OBJECTIVE

To evaluate the efficacy between intravenous and oral acetaminophen as adjunct to multimodal analgesia regimens for pain management after total knee and hip arthroplasties.

METHODS

We conduct electronic searches of Medline (1966-2017.09), PubMed (1966-2017.09), Embase (1980-2017.09), ScienceDirect (1985-2017.09), and the Cochrane Library. Only randomized controlled trials (RCTs) are included. The quality assessment is performed according to the Cochrane systematic review method. Fixed/random effect model is adopted according to the heterogeneity tested by I statistic. Meta-analysis is performed using Stata 11.0 software.

RESULTS

Two RCTs are included involving 236 patients. The present meta-analysis demonstrated that there were no significant differences between groups regarding pain scores at 12, 24, or 48 hours. No significant differences were observed in terms of opioid consumption at 12, 24, or 48 hours after arthroplasties.

CONCLUSION

Intravenous acetaminophen to multimodal analgesia dose not demonstrate a significant benefit in reducing pain and opioid consumption compared oral formulation after total knee arthroplasty and total hip arthroplasty. Higher-quality RCTs are required for further research.

Simultaneous determination of morphine-6-d-glucuronide, morphine-3-d-glucuronide and morphine in human plasma and urine by ultra-performance liquid chromatography-tandem mass spectrometry: Application to M6G injection pharmacokinetic study

A robust ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method for the determination of morphine-6-d-glucuronide (M6G), morphine-3-d-glucuronide (M3G) and morphine (MOR) in human plasma and urine has been developed and validated. The analytes of interest were extracted from plasma by protein precipitation. The urine sample was prepared by dilution. Both plasma and urine samples were chromatographed on an Acquity UPLC HSS T₃ column using gradient elution. Detection was performed on a Xevo TQ-S tandem mass spectrometer in multiple reaction monitoring mode using positive electrospray ionization. Matrix interferences were not observed at the retention time of the analytes and internal standard, naloxone-D5. The lower limits of quantitation of plasma and urine were 2/0.5/0.5 and 20/4/2 ng/mL for M6G/M3G/MOR, respectively. Calibration curves were linear over the concentration ranges of 2-2000/0.5-500/0.5-500 and 20-20,000/4-4000/2-2000 ng/mL for M6G/M3G/MOR in plasma and urine samples, respectively. The precision was <7.14% and the accuracy was within 85-115%. Furthermore, stability of the analytes at various conditions, dilution integrity, extraction recovery and matrix effect were assessed. Finally, this quantitative method was successfully applied to the pharmacokinetic study of M6G injection in Chinese noncancer pain patients.

The efficiency and safety of fascia iliaca block for pain control after total joint arthroplasty: A meta-analysis

BACKGROUND

This meta-analysis aimed to perform a meta-analysis including randomized controlled trials (RCTs) to assess the efficiency and safety of fascia iliaca block (FIB) for pain control in patients undergoing total joint arthroplasty (TJA).

METHODS

A systematic search was performed in Medline (1966-2017.03), PubMed (1966-2017.03), Embase (1980-2017.03), ScienceDirect (1985-2017.03) and the Cochrane Library. Study evaluated the efficiency and safety of FIB in TJA was selected. Meta-analysis was performed using Stata 11.0 software.

RESULTS

Five randomized controlled trials (RCTs) including 270 patients met the inclusion criteria. The present meta-analysis indicated that there were significant differences between groups in terms of visual analog scale (VAS) score at 12 hours (SMD = -0.544, 95% CI: -0.806 to -0.281, P = .000) and 24 hours (SMD = -0.519, 95% CI: -0.764 to -0.273, P = .000), morphine equivalent consumption at 12 hours (SMD = -0.895, 95% CI: -1.164 to -0.626, P = .000) and 24 hours (SMD = -0.548, 95% CI:-0.793 to -0.303, P = .000). In addition, fewer adverse side effect was identified in FIB groups (RD = -0.139, 95% CI: -0.243 to -0.034, P = .009).

CONCLUSION

The application of fascia iliaca block could significantly reduce VAS scores and morphine consumption at 12 and 24 hours following total knee and hip arthroplasty. In addition, there were fewer adverse effects in FIB groups. Due to the limited quality of the evidence currently available, higher quality RCTs are required.

Intestinal inflammation and pain management

Intestinal inflammation results in the production of inflammatory pain-inducing mediators that may directly activate colon sensory neurons. Endogenous opioids produced by mucosal effector CD4(+) T lymphocytes identified as colitogenic may paradoxically counterbalance the local pro-algesic effect of inflammatory mediators by acting on opioid receptors expressed on sensory nerve endings. The review will focus on the endogenous immune-mediated regulation of visceral inflammatory pain, current pain treatments in inflammatory bowel diseases and prospectives on new opioid therapeutic opportunities to alleviate pain but avoiding common centrally-mediated side effects.

Effect of UGT2B7 -900G>A (-842G>A; rs7438135) on morphine glucuronidation in preterm newborns: results from a pilot cohort

AIM

Assess association between UGT2B7 polymorphism -900G>A (rs7438135, also known as -842G>A) with morphine kinetics in preterm newborns undergoing mechanical ventilation.

MATERIALS & METHODS

Thirty-four infants were enrolled in a randomized clinical trial and allocated to rapid sequence intubation with remifentanil (1 µg/kg) or morphine (0.3 mg/kg). The latter group was included in our study.

RESULTS

Morphine plasma concentrations at 20 min post intubation were associated with postnatal age (p=0.017) and UGT2B7 -900G>A (p=0.036). UGT2B7 -900A allele carriers (n=13) had lower morphine levels compared with UGT2B7 -900G/G patients (n=2). Morphine-3-glucuronide and morphine-6-glucuronide plasma concentrations were only found to be associated with gestational and postnatal age. However, -900A allele carriers had a higher morphine-3-glucuronide:morphine metabolic ratio compared with patients genotyped as -900G/G (p=0.005), as determined by linear regression.

CONCLUSION

Our small pilot study illustrates that in addition to gestational and postnatal age, the UGT2B7 -900G>A polymorphism significantly alters morphine pharmacokinetics in preterm infants.

Reporting of sample size calculations in analgesic clinical trials: ACTTION systematic review

Sample size calculations determine the number of participants required to have sufficiently high power to detect a given treatment effect. In this review, we examined the reporting quality of sample size calculations in 172 publications of double-blind randomized controlled trials of noninvasive pharmacologic or interventional (ie, invasive) pain treatments published in European Journal of Pain, Journal of Pain, and Pain from January 2006 through June 2013. Sixty-five percent of publications reported a sample size calculation but only 38% provided all elements required to replicate the calculated sample size. In publications reporting at least 1 element, 54% provided a justification for the treatment effect used to calculate sample size, and 24% of studies with continuous outcome variables justified the variability estimate. Publications of clinical pain condition trials reported a sample size calculation more frequently than experimental pain model trials (77% vs 33%, P < .001) but did not differ in the frequency of reporting all required elements. No significant differences in reporting of any or all elements were detected between publications of trials with industry and nonindustry sponsorship. Twenty-eight percent included a discrepancy between the reported number of planned and randomized participants. This study suggests that sample size calculation reporting in analgesic trial publications is usually incomplete. Investigators should provide detailed accounts of sample size calculations in publications of clinical trials of pain treatments, which is necessary for reporting transparency and communication of pre-trial design decisions.

PERSPECTIVE

In this systematic review of analgesic clinical trials, sample size calculations and the required elements (eg, treatment effect to be detected; power level) were incompletely reported. A lack of transparency regarding sample size calculations may raise questions about the appropriateness of the calculated sample size.

Opioids for the treatment of arthritis pain

INTRODUCTION

Centrally acting opioids are well established in the treatment of acute, surgical and cancer pain. However, their use in chronic noncancer pain (CNCP) is controversial because of side effects such as tolerance, somnolence, respiratory depression, confusion, constipation and addiction. Chronic arthritis and other musculoskeletal diseases are among the leading causes of CNCP.

AREAS COVERED

This manuscript will discuss the role of conventional opioids in chronic arthritis. In addition, future developments and strategies exploiting peripheral effects of opioids on pain and inflammation will be outlined.

EXPERT OPINION

Aims in drug development include the design of peripherally restricted opioid agonists, selective targeting of endogenous opioids to sites of painful injury and the augmentation of peripheral ligand and receptor synthesis, for example, by gene therapy. Although a large number of peripherally acting opioid compounds have been developed, clinical Phase III studies have not been published so far. Another strategy is to augment the effects of endogenously released opioid peptides by the inhibition of their degrading enzymes. Technology-oriented research is needed to find novel ways of peripheral restriction of opioids. Such analgesics would be desirable for their lack of central side effects and of adverse effects typical of nonsteroidal anti-inflammatory drugs (gastrointestinal ulcers, bleeding, myocardial infarction and stroke).

Pathophysiological and neurochemical mechanisms of postoperative nausea and vomiting

Clinical research shows that postoperative nausea and vomiting (PONV) is caused primarily by the use of inhalational anesthesia and opioid analgesics. PONV is also increased by several risk predictors, including a young age, female sex, lack of smoking, and a history of motion sickness. Genetic studies are beginning to shed light on the variability in patient experiences of PONV by assessing polymorphisms of gene targets known to play roles in emesis (serotonin type 3, 5-HT3; opioid; muscarinic; and dopamine type 2, D2, receptors) and the metabolism of antiemetic drugs (e.g., ondansetron). Significant numbers of clinical trials have produced valuable information on pharmacological targets important for controlling PONV (e.g., 5-HT3 and D2), leading to the current multi-modal approach to inhibit multiple sites in this complex neural system. Despite these significant advances, there is still a lack of fundamental knowledge of the mechanisms that drive the hindbrain central pattern generator (emesis) and forebrain pathways (nausea) that produce PONV, particularly the responses to inhalational anesthesia. This gap in knowledge has limited the development of novel effective therapies of PONV. The current review presents the state of knowledge on the biological mechanisms responsible for PONV, summarizing both preclinical and clinical evidence. Finally, potential ways to advance the research of PONV and more recent developments on the study of postdischarge nausea and vomiting (PDNV) are discussed.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 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 and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Pharmacology in the very young: anaesthetic implications

Anaesthesia dosing in infants (0-2 years) should be based on pharmacokinetic-pharmacodynamic considerations and adverse effects profiles. Disease processes and treatments in this group are distinct from those in adults. Absorption, distribution and clearance change dramatically during this period because of maturation of anatomical and physiological processes as well as behavioural changes. Pharmacogenomic expression also matures in this period. Population-based and physiological-based pharmacokinetic modelling has improved the understanding of maturation and subsequent dose approximation. Postmenstrual, rather than postnatal, age is a reasonable measure for maturation. There remains a need for clinically applicable tools to assess pharmacodynamics which can provide response feedback; this has been achieved for neuromuscular monitoring, but not yet fully for depth of anaesthesia, sedation or pain. Morbidity and mortality associated with paediatric anaesthesia have historically been highest in this age group and continue to be so. Some of this morbidity was attributable to a poor understanding of developmental pharmacology; this facet continues to plague the specialty.

Repeated exposure to codeine alters morphine glucuronidation by affecting UGT gene expression in the rat

We have previously found that phenantrenic opioids, such as heroin or naltrexone, modulate morphine glucuronidation in the rat. Here we further investigated the effects of phenantrenic opioids on morphine glucuronidation comparing the effects of codeine and heroin. In particular, we measured the synthesis of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) from morphine: in the liver microsomal preparations obtained from rats repeatedly treated with two different doses of codeine (ex vivo study); in primary cultures of rat hepatocytes previously incubated for 72h with codeine, or heroin (in vitro study); in the latter conditions, the levels of expression of genes coding for uridine-5'-diphosphate-glucuronosyltransferases (UGTs) A1, A6, A7 and 2B1 were also determined; finally, the levels of glucuronic acid in rat hepatocytes previously incubated for 72h with codeine or heroin were assessed. The ex vivo study shows that codeine exposure in vivo stimulated liver microsomal M3G formation and de novo synthesis of M6G. Differently, in primary hepatocyte cultures both codeine and heroin inhibited M3G formation, whereas heroin only stimulated de novo synthesis of M6G; moreover, codeine significantly reduced UGT2B1 expression at 6h and caused a trend toward inhibition of UGT1A1 expression at 72h; heroin enhanced UGT2B1 expression and inhibited that of UGT1A1 at 72h; finally, both codeine and heroin depleted UDPGA content of hepatocytes. In conclusion, codeine affects liver glucuronidation of morphine enlightening the possible contribution of changes in the spectrum of UGT gene expression and co-factor synthesis in this phenomenon.

Opioid genetics: the key to personalized pain control?

There are now several strong opioids available to choose from for the relief of moderate to severe pain. On a population level, there is no difference in terms of analgesic efficacy or adverse reactions between these drugs; however, on an individual level there is marked variation in response to a given opioid. The genetic influences to this variation are complex, and although current research has shown some promising results, these have not been replicated across larger studies and as such the ultimate aim of personalized prescribing remains elusive. If personalized prescribing could be achieved this would have a major impact at an individual level to facilitate safe, effective and rapid symptom control. This review presents some of the recent positive advances in opioid pharmacogenetic studies, focusing on associations between candidate genes and the three main elements of opioid response: analgesic, upper gastrointestinal and central adverse reactions.

Induction of morphine-6-glucuronide synthesis by heroin self-administration in the rat

RATIONALE

Heroin is rapidly metabolized to morphine that in turn is transformed into morphine-3-glucuronide (M3G), an inactive metabolite at mu-opioid receptor (MOR), and morphine-6-glucuronide (M6G), a potent MOR agonist. We have found that rats that had received repeated intraperitoneal injections of heroin exhibit measurable levels of M6G (which is usually undetectable in this species).

OBJECTIVE

The goal of the present study was to investigate whether M6G synthesis can be induced by intravenous (i.v.) heroin self-administration (SA).

MATERIALS AND METHODS

Rats were trained to self-administer either heroin (50 μg/kg per infusion) or saline for 20 consecutive 6-h sessions and then challenged with an intraperitoneal challenge of 10 mg/kg of heroin. Plasma levels of heroin, morphine, 6-mono-acetyl morphine, M3G, and M6G were quantified 2 h after the challenge. In vitro morphine glucuronidation was studied in microsomal preparations obtained from the liver of the same rats.

RESULTS

Heroin SA induced the synthesis of M6G, as indicated by detectable plasma levels of M6G (89.7 ± 37.0 ng/ml vs. 7.35 ± 7.35 ng/ml after saline SA). Most important, the in vitro V (max) for M6G synthesis was correlated with plasma levels of M6G (r (2) = 0.78). Microsomal preparations from saline SA rats produced negligible amounts of M6G.

CONCLUSION

Both in vivo and in vitro data indicate that i.v. heroin SA induces the synthesis of M6G. These data are discussed in the light of previous studies conducted in heroin addicts indicating that in humans heroin enhances the synthesis of the active metabolite of heroin and morphine.

Analgesic properties of loperamide differ following systemic and local administration to rats after spinal nerve injury

BACKGROUND

The analgesic properties and mechanisms of loperamide hydrochloride, a peripherally acting opioid receptor agonist, in neuropathic pain warrant further investigation.

METHODS

We examined the effects of systemic or local administration of loperamide on heat and mechanical hyperalgesia in rats after an L5 spinal nerve ligation (SNL).

RESULTS

 (1) Systemic loperamide (0.3-10 mg/kg, subcutaneous in the back) dose dependently reversed heat hyperalgesia in SNL rats, but did not produce thermal analgesia. Systemic loperamide (3 mg/kg) did not induce thermal antinociception in naïve rats; (2) systemic loperamide-induced anti-heat hyperalgesia was blocked by pretreatment with intraperitoneal naloxone methiodide (5 mg/kg), but not by intraperitoneal naltrindole (5 mg/kg) or intrathecal naltrexone (20 μg/10 μL); (3) local administration of loperamide (150 μg), but not vehicle, into plantar or dorsal hind paw tissue induced thermal analgesia in SNL rats and thermal antinociception in naïve rats; (4) the analgesic effect of intraplantar loperamide (150 μg/15 μL) in SNL rats at 45 min, but not 10 min, post-injection was blocked by pretreatment with an intraplantar injection of naltrexone (75 μg/10 μL); (5) systemic (3.0 mg/kg) and local (150 μg) loperamide reduced the exaggerated duration of hind paw elevation to noxious pinprick stimuli in SNL rats. Intraplantar injection of loperamide also decreased the frequency of pinprick-evoked response in naïve rats.

CONCLUSIONS

These findings suggest that both systemic and local administration of loperamide induce an opioid receptor-dependent inhibition of heat and mechanical hyperalgesia in nerve-injured rats, but that local paw administration of loperamide also induces thermal and mechanical antinociception.