Association of human μ-opioid receptor gene polymorphism A118G with fentanyl analgesia consumption in Chinese gynaecological patients

Role of antioxidants in the neurobiology of drug addiction: An update

Relationships between protective enzymatic and non-enzymatic pro-antioxidant mechanisms and addictive substances use disorders (SUDs) are analyzed here, based on the results of previous research, as well as on the basis of our current own studies. This review introduces new aspects of comparative analysis of associations of pro-antixidant and neurobiological effects in patients taking psychoactive substances and complements very limited knowledge about relationships with SUDs from different regions, mainly Europe. In view of the few studies on relations between antioxidants and neurobiological processes acting in patients taking psychoactive substances, this review is important from the point of view of showing the state of knowledge, directions of diagnosis and treatment, and further research needed explanation. We found significant correlations between chemical elements, pro-antioxidative mechanisms, and lipoperoxidation in the development of disorders associated with use of addictive substances, therefore elements that show most relations (Pr, Na, Mn, Y, Sc, La, Cr, Al, Ca, Sb, Cd, Pb, As, Hg, Ni) may be significant factors shaping SUDs. The action of pro-antioxidant defense and lipid peroxidation depends on the pro-antioxidative activity of ions. We explain the strongest correlations between Mg and Sb, and lipoperoxidation in addicts, which proves their stimulating effect on lipoperoxidation and on the induction of oxidative stress. We discussed which mechanisms and neurobiological processes change susceptibility to SUDs. The innovation of this review is to show that addicted people have lower activity of dismutases and peroxidases than healthy ones, which indicates disorders of antioxidant system and depletion of enzymes after long-term tolerance of stressors. We explain higher level of catalases, reductases, ceruloplasmin, bilirubin, retinol, α-tocopherol and uric acid of addicts. In view of poorly understood factors affecting addiction, analysis of interactions allows for more effective understanding of pathogenetic mechanisms leading to formation of addiction and development the initiation of directed, more effective treatment (pharmacological, hormonal) and may be helpful in the diagnosis of psychoactive changes.

Impact of Genetic Variants on Postoperative Pain and Fentanyl Dose Requirement in Patients Undergoing Major Breast Surgery: A Candidate Gene Association Study

BACKGROUND

Postoperative analgesia is crucial for the early and effective recovery of patients undergoing surgery. Although postoperative multimodal analgesia is widely practiced, opioids such as fentanyl are still one of the best analgesics. The analgesic response of fentanyl varies widely among individuals, probably due to genetic and nongenetic factors. Among genetic factors, single nucleotide polymorphisms (SNPs) may influence its analgesic response by altering the structure or function of genes involved in nociceptive, fentanyl pharmacodynamic, and pharmacokinetic pathways. Thus, it is necessary to comprehensively ascertain if the SNPs present in the aforementioned pathways are associated with interindividual differences in fentanyl requirement. In this study, we evaluated the association between 10 candidate SNPs in 9 genes and 24-hour postoperative fentanyl dose (primary outcome) and also with postoperative pain scores and time for first analgesia (secondary outcomes).

METHODS

A total of 257 South Indian women, aged 18-70 years, with American Society of Anesthesiologists (ASA) physical status I-III, undergoing major breast surgery under general anesthesia, were included in the study. Patients were genotyped for candidate SNPs using real-time polymerase chain reaction. All patients received a standardized intravenous fentanyl infusion through a patient-controlled analgesic (PCA) pump, and the 24-hour postoperative fentanyl dose requirement was measured using PCA.

RESULTS

The median 24-hour postoperative fentanyl requirement was higher in rs1799971 carriers (G/G versus A/A + A/G-620 μg [500-700] vs 460 μg [400-580]) with a geometric mean (GM) ratio of 1.91 (95% confidence interval [CI], 1.071-1.327). The median 24-hour pain scores were higher in rs4680 carriers (A/G + A/A versus G/G-34 [30-38] vs 31 [30-38]) with a GM ratio of 1.059 (95% CI, 1.018-1.101) and were lower in rs1045642 carriers (A/A + A/G versus G/G-34 [30-38] vs 30 [30-34]) with a GM ratio of 0.936 (95% CI, 0.889-0.987). The median time for first analgesic was lower in rs734784 carriers [C/C versus T/T + C/T-240 minutes (180-270) vs 240 minutes (210-270)] with a GM ratio of 0.902 (95% CI, 0.837-0.972). Five of 9 clinical factors, namely, history of diabetes, hypertension, hypothyroidism, anesthesia duration, and intraoperative fentanyl requirement were associated with different outcomes individually ( P < .05) and were used to adjust the respective associations.

CONCLUSIONS

The SNP opioid receptor mu-1 ( OPRM1 ) (rs1799971) was associated with higher postoperative fentanyl requirement in South Indian patients undergoing major breast surgery. Twenty-four hour postoperative pain scores were higher in catechol-O-methyl transferase ( COMT ) (rs4680) carriers and lower in ATP binding cassette subfamily B member 1 ( ABCB1 ) (rs1045642) carriers, whereas time for first analgesic was lower in potassium channel subunit 1 ( KCNS1 ) (rs734784) carriers. However, these exploratory findings must be confirmed in a larger study.

Genetic Factors Associated with Morphine Consumption in Women Undergoing Laparoscopic Cholecystectomy: A Prospective Cohort Study

Introduction

Morphine has been a crucial analgesic agent used perioperatively in various surgical procedures. Genetic factors can lead to morphine dose requirement interpatient variability. Our objective was to determine the contribution of genetic polymorphisms in human μ-opioid receptor gene (OPRM1), ATP binding cassette gene (ABCB1) and rs2952768 to the variation of the perioperative morphine consumption in women undergoing laparoscopic cholecystectomy.

Methods

This is a prospective cohort study that included 102 adult Arab females undergoing laparoscopic cholecystectomy. The exposures were carrying the genetic variants of OPRM1, ABCB1 and rs2952768. Our primary outcome was total morphine or morphine equivalent dose required perioperatively. The secondary outcomes were pain score during the first 24 hours and adverse drug reactions. A standardized, general anaesthesia was used for all subjects. In addition to the genetic factors, we also investigated non-genetic factors influencing post-operative pain sensitivity and morphine consumption.

Results

Both (rs1799971, A>G) in OPRM1 and (rs2952768, T>C) showed statistically significant association with intra-operative total morphine dose requirements. Patients carrying the "G" allele in OPRM1 had a significantly higher total morphine mean rank dose compared to the AA genotype [62.9 vs 47.1, p=0.008]. Furthermore, patients homozygous for the rs2952768 (T>C) minor allele "CC" had a higher mean rank compared to the other genotypes [72.7 vs 50.1, p=0.046].

Conclusion

OPRM1 (rs1799971) and rs2952768 are associated with variation of intra-operative morphine consumption in laparoscopic cholecystectomy.

Clinical Trial Identifier

This study was registered at ClinicalTrials.gov, NCT04621864. https://clinicaltrials.gov/ct2/show/NCT04621864.

Pharmacogenetic Analysis Enables Optimization of Pain Therapy: A Case Report of Ineffective Oxycodone Therapy

Patients suffering from chronic pain may respond differently to analgesic medications. For some, pain relief is insufficient, while others experience side effects. Although pharmacogenetic testing is rarely performed in the context of analgesics, response to opiates, non-opioid analgesics, and antidepressants for the treatment of neuropathic pain can be affected by genetic variants. We describe a female patient who suffered from a complex chronic pain syndrome due to a disc hernia. Due to insufficient response to oxycodone, fentanyl, and morphine in addition to non-steroidal anti-inflammatory drug (NSAID)-induced side effects reported in the past, we performed panel-based pharmacogenotyping and compiled a medication recommendation. The ineffectiveness of opiates could be explained by a combined effect of the decreased activity in cytochrome P450 2D6 (CYP2D6), an increased activity in CYP3A, and an impaired drug response at the µ-opioid receptor. Decreased activity for CYP2C9 led to a slowed metabolism of ibuprofen and thus increased the risk for gastrointestinal side effects. Based on these findings we recommended hydromorphone and paracetamol, of which the metabolism was not affected by genetic variants. Our case report illustrates that an in-depth medication review including pharmacogenetic analysis can be helpful for patients with complex pain syndrome. Our approach highlights how genetic information could be applied to analyze a patient's history of medication ineffectiveness or poor tolerability and help to find better treatment options.

Effect of common OPRM1, COMT, SLC6A4, ABCB1, and CYP2B6 polymorphisms on perioperative analgesic and propofol demands on patients subjected to thyroidectomy surgery

BACKGROUND

Perioperative anesthetic and/or analgesic demand present considerable variation, and part of that variation appears to be genetic in origin. Here we investigate the impact of common polymorphisms in OPRM1, COMT, SLC6A4, ABCB1, and CYP2B6 genes, on the intra-operative consumption of remifentanil and propofol, as well as the postoperative analgesic needs, in patients subjected to thyroidectomy surgery.

METHODS

We conducted a prospective cohort study with 90 patients scheduled to undergo elective thyroidectomy, under total intravenous anesthesia achieved by target control infusion (TCI) of propofol and remifentanil. Postoperative analgesics were administered by protocol and on-demand by the individual patient. Genotyping was established by PCR-RFLP methods. Genotyping data, intra-operative hemodynamics, and total consumption of remifentanil and propofol, as well as postoperative analgesic needs and pain perception, were recorded for each individual.

RESULTS

Patients with the ABCB1 3435TT genotype appeared to experience significantly less pain within one hour post-operatively, compared to C carriers [mean VAS (SD) = 0.86 (1.22) vs. 2.42 (1.75); p = 0.017], a finding limited to those seeking rescue analgesic treatment. Intra-operatively, homozygotes patients for the minor allele of OPRM1 A118G and CYP2B6 G516T appeared to consume less remifentanil [mean (SD) = 9.12 (1.01) vs. 13.53 (5.15), for OPRM1 118GG and A carriers] and propofol [median (range) = 14.95 (11.53, 1359.5) vs. 121.4 (1.43, 2349.4), for CYP2B6 516TT and G carriers, respectively] but the difference was not statistically significant in our sample.

CONCLUSIONS

The ABCB1 C3435T polymorphism appears to affect the postoperative perception of surgical pain among patients with low pain threshold. The small number of minor allele homozygotes for the OPRM1 A118G and CYP2B6 G516T polymorphisms precludes a definitive conclusion regarding the inclusion of the latter in a TCI-programming algorithm, based on the results of this study.

CLINICAL TRIAL REGISTRATION NUMBER

ACTRN12616001598471.

No Association between Genetic Variants of the COMT and OPRM1 Genes and Pain Perception among Patients Undergoing Total Hip or Knee Arthroplasty for Primary Osteoarthritis

Each year approximately 1 million total hip replacements are performed worldwide. The most common indications to choose this procedure are rest pain and pain after activity as well as functional limitations influencing daily activities. Experimental pain is highly variable by individuals, which is partly due to genetics. The aim of the study was to investigate a possible association of the catechol-O-methyltransferase (COMT) and μ-opioid receptor (OPRM1) genotypes with pain perception in patients undergoing total hip replacement and total knee replacement taking into account aspects such as age, sex and diabetes. The study included 207 patients (119 females, 88 males, median age 65 years, range 33−77) that qualified for surgical treatment (total hip replacement and knee arthroplasty) due to osteoarthritis. Pain sensitivity measurement was performed using a standard algometer. The genomic DNA was extracted from the buccal cells.. Single locus analysis was conducted using a general linear model. In the study group, we did not find statistically significant genetic associations between variants of COMT and OPRM1 and pain thresholds/pain tolerance. The analysis of subjective pain perception using the visual analog scale did not show any relationship between the OPRM1 rs1799971A>G variant and COMT rs4680, rs4633, rs4818 and rs6269.

Genetic polymorphisms of OPRM1 on the efficacy and safety of anesthetic and analgesic agents: a systematic review

Aim: This systematic review aimed to outline the outcome of OPRMI (A118G) variants on the effects of anesthetic and analgesic agents used in various procedures. Materials & methods: Literature was obtained from reliable, established databases and reference tracking. Efficacy and side/adverse effects of anesthetic and analgesic drugs intraoperatively or within 48 h postsurgery were the key outcome measures for all populations. Animal studies were excluded. Results: Twenty-nine studies were chosen for inclusion. In association with the efficacy and safety of anesthetic and analgesic agents, gene polymorphism in OPRM1 displayed a strong correlation in reduced analgesic effect and protection against adverse reactions. Conclusion: This systematic review summarized the correlation between genetic polymorphism in the OPRM1 gene and anesthetic/analgesic effects.

Progress, Challenges, and Prospects of Research on the Effect of Gene Polymorphisms on Adverse Reactions to Opioids

The abuse of opioids has become one of the most serious concerns in the world. Opioid use can cause serious adverse reactions, including respiratory depression, postoperative nausea and vomiting, itching, and even death. These adverse reactions are also important complications of clinical application of opioid drugs that may affect patient safety and recovery. Due to the fear of adverse reactions of opioids, clinicians often do not dare to use opioids in an adequate or appropriate amount, thus affecting the clinical medication strategy and the quality of treatment for patients. The prediction of adverse reactions to opioids is one of the most concerned problems in clinical practice. At present, the correlation between gene polymorphism and the efficacy of opiates has been widely studied and preliminarily confirmed, but the research on the effect of gene polymorphism on the adverse reactions of opiates is relatively limited. Existing studies have made encouraging progress in predicting the incidence and severity of adverse opioid reactions and clinical management by using genetic testing, but most of these studies are single-center, small-sample clinical studies or animal experiments, which have strong limitations. When the same receptor or enzyme is studied by different experimental methods, different or even opposite conclusions can be drawn. These phenomena indicate that the correlation between gene polymorphism and adverse opioid reaction still needs further research and demonstration. At present, it is still too early to use genetic testing to predict opioid adverse reactions in clinic. In this paper, the correlation between gene polymorphism and adverse opioid reactions and a small number of clinical applications were reviewed in terms of pharmacokinetics and pharmacodynamics, in order to provide some suggestions for future research and clinical drug decision making.

Reviewing pharmacogenetics to advance precision medicine for opioids

BACKGROUND

Adequate opioid prescribing is critical for therapeutic success of pain management. Despite the widespread use of opioids, optimized opioid therapy remains unresolved with risk of accidental lethal overdosing. With the emergence of accumulating evidence linking genetic variation to opioid response, pharmacogenetic based treatment recommendations have been proposed.

OBJECTIVE

The aim of this review is to evaluate pharmacogenetic evidence and provide an overview on genes involved in the pharmacokinetics and pharmacodynamics of opioids.

METHODS

For this review, a systematic literature search of published articles was used in PubMed®, with no language restriction and between the time period of January 2000 to December 2020. We reviewed randomized clinical studies, study cohorts and case reports that investigated the influence of genetic variants on selected opioid pharmacokinetics and pharmacodynamics. In addition, we reviewed current CPIC clinical recommendations for pharmacogenetic testing.

RESULTS

Results of this review indicate consistent evidence supporting the association between selected genetic variants of CYP2D6 for opioid metabolism. CPIC guidelines include recommendations that indicate the avoidance of tramadol use, in addition to codeine, in CYP2D6 poor metabolizers and ultrarapid metabolizers, and to monitor intermediate metabolizers for less-than-optimal response. While there is consistent evidence for OPRM1 suggesting increased postoperative morphine dosing requirements in A118G G-allele carriers, the clinical relevance remains limited.

CONCLUSION

There is emerging evidence of clinical relevance of CYP2D6 and, to a lesser extent, OPRM1 polymorphism in personalized opioid drug dosing. As a result, first clinics have started to implement pharmacogenetic guidelines for CYP2D6 and codeine.

Influence of Genetic Polymorphisms on the Response to Tramadol, Ibuprofen, and the Combination in Patients With Moderate to Severe Pain After Dental Surgery

PURPOSE

We aimed to elucidate the influence on analgesic effect of genetic polymorphisms in enzymes responsible for biotransformation of tramadol and ibuprofen or other possible genes involved in their mechanism of action.

METHODS

The study population comprised 118 patients from a multicenter, randomized, double-blind, placebo-controlled, Phase III clinical trial that assessed the analgesic efficacy and tolerability of a single dose of ibuprofen (arginine)/tramadol 400/37.5 mg compared with ibuprofen arginine 400 mg alone, tramadol 50 mg alone, and placebo in patients with moderate to severe pain after dental surgery. We analyzed 32 polymorphisms in the cytochrome P450 (CYP) enzymes COMT, ABCB1, SLC22A1, OPRM1, and SLC22A1.

FINDINGS

We did not find any statistically significant difference among CYP2C9 phenotypes related to ibuprofen response, although CYP2C9 poor metabolizers had a longer effect (higher pain relief at 6 hours). Likewise, we did not find any statistically significant difference among PTGS2 genotypes, contradicting previously publications.

IMPLICATIONS

There was not a clear effect of CYP2D6 phenotype on tramadol response, although CYP2D6 poor metabolizers had a slower analgesic effect. Concerning the transport of CYP2D6, we observed a better response in individuals carrying ABCB1 mutated alleles, which might correlate with higher tramadol plasma levels. Finally, we found a statistically significant better response in patients carrying the OPRM1 A118G G allele, which contradicts the previous reports. Measuring the active metabolite O-desmethyl-tramadol formation would be of great importance to better evaluate this association because O-desmethyl-tramadol has a higher μ-opioid receptor affinity compared with the parent drug. EudraCT.ema.europa.eu identifier: 2013-004637-33.

Single nucleotide polymorphisms associated with postoperative inadequate analgesia after single-port VATS in Chinese population

Background

Postoperative inadequate analgesia following video-assisted thoracoscopic surgery (VATS) is a common and significant clinical problem. While genetic polymorphisms may play role in the variability of postoperative analgesia effect, few studies have evaluated the associations between genetic mutations and inadequate analgesia after single-port VATS.

Methods

Twenty-eight single nucleotide polymorphisms (SNPs) among 18 selected genes involved in pain perception and modulation were genotyped in 198 Chinese patients undergoing single-port VATS. The primary outcome was the occurrence of inadequate analgesia in the first night and morning after surgery which was defined by a comprehensive postoperative evaluation. Multivariable logistic regression analyses were used to identify the association between genetic variations and postoperative inadequate analgesia.

Results

The prevalence of postoperative inadequate analgesia was 45.5% in the present study. After controlling for age and education level, association with inadequate analgesia was observed in four SNPs among three genes encoding voltage-gated sodium channels. Patients with the minor allele of rs33985936 (SCN11A), rs6795970 (SCN10A), and 3312G > T (SCN9A) have an increased risk of suffering from inadequate analgesia. While the patients carrying the minor allele of rs11709492 (SCN11A) have lower risk experiencing inadequate analgesia.

Conclusions

We identified that SNPs in SCN9A, SCN10A, and SCN11A play a role in the postoperative inadequate analgesia after single-port VATS. Although future larger and long-term follow up studies are warranted to confirm our findings, the results of the current study may be utilized as predictors for forecasting postoperative analgesic effect for patients receiving this type of surgery.

Trial registration

This study was retrospectively registered in the ClinicalTrials.gov Registry (NCT03916120) on April 16, 2019.

Effects of Single Nucleotide Polymorphisms on Surgical and Postsurgical Opioid Requirements: A Systematic Review and Meta-Analysis

OBJECTIVES

There is great heterogeneity in the way individuals respond to medications. Inherited differences, such as single nucleotide polymorphisms (SNP), can influence the efficacy and toxicity of drugs. This meta-analysis aims to collate data from studies investigating the effect of SNPs on postoperative and/or intraoperative opioid requirements.

MATERIALS AND METHODS

A meta-analysis was conducted following PRISMA guidelines. Eligibility criteria for studies included were reporting amount of postoperative and/or intraoperative opioid used as the primary outcome and genotyping patients for SNPs in one of the following genes; OPRM1, CYP2D6, CYP3A4, CYP3A5, COMT, UGT2B7, or ABCB1. A comprehensive systematic search for articles using keywords "opioid-sensitivity," "polymorphisms," "post-operative opioid," "post-surgical opioid," "post-operative pain," and "post-surgical pain" was performed.

RESULTS

Fifty-one studies were included. Individuals homozygous for AA at the OPRMI (rs1799971) polymorphisms required less postsurgical opioid compared with those homozygous for GG (Hedges g, -0.270; 95% confidence interval, -0.433 to -0.108; P=0.001). Polymorphisms in CYP2D6, CYP3A4, CYP3A5, COMT, UGT2B7, and ABCB1 did not affect opioid requirements.

DISCUSSION

Investigation of single changes in 1 gene can only yield limited information regarding genetic effects on opioid requirements. Rapid development of whole genome sequencing enables information on all genetic modifications that may affect analgesic response to be collected. The information collected must include data on the individual's metabolic enzymes, as well as information on drug receptors and enzymes responsible for drug degradation, so that a personal profile can be built up which will predict individual response to drugs, and guide clinicians on the type and dosage of drug to use.

Pharmacogenetics in obstetric anesthesia

The 21st century has been billed as the era of "precision/personalized medicine." Genetic investigation of clinical syndromes may guide therapy as well as reveal previously unknown biological or pharmacological pathways that may result in novel drug therapies. Several clinical issues in obstetrics and obstetric anesthesiology have been targets for genetic investigations. These include evaluation of the genetic effects on preterm labor and the progression of labor, spinal anesthesia-induced hypotension and the response to medications used to treat hypotension, and the effect of gene variants on pain and analgesic responses. Most studies have examined specific single nucleotide polymorphisms. Findings have revealed modest effects of genetic variation without tangible impact on current clinical practice. Over the next decade, increased availability of whole exome and genome sequencing, epigenetics, large genetic databases, computational biology and other information technology, and more rapid methods of real-time genotyping may increase the impact of genetics in the clinical arena of obstetrics and obstetric anesthesia.

Association of Single-Nucleotide Polymorhism C3435T in the ABCB1 Gene with Opioid Sensitivity in Treatment of Postoperative Pain

BACKGROUND

The minimal effective analgesic concentration of opioids required for satisfactory analgesia may differ significantly among the patients. Genetic factors may contribute to the variable response to opioids by affecting their pharmacokinetics or pharmacodynamics.

METHODS

Ninety nine patients undergoing abdominal surgery with colorectal anastomosis because of colorectal carcinoma were enrolled in the present study. C34535T was genotyped in all subjects and the patients were divided into three groups according to their genotype: CC-wild type homozygous, CT-mutant heterozygous and TT-mutant homozygous. Intravenous fentanyl, patient controlled analgesia was provided postoperatively for pain control in the first 24 hour after surgery. Opioid consumption, pain scores and the adverse side effects were evaluated.

RESULTS

Our main result is that the patients in the CC genotype group consumed significantly more fentanyl (375.0 μg ± 43.1) than the patients in the TT group (295.0 μg ± 49.1) and the CT (356.4 μg ± 41.8) group in the treatment of postoperative pain. The patients in the TT group had lower VAS scores at 6h, 12h, 18 h and 24h postoperatively. There were no significant differences in the side effects among the three groups regarding the vomiting and the sedation score. The patients in the TT group had more frequently nausea score 1, than the patients in the other two groups.

CONCLUSION

Our study indicates that the C3435T SNPs of the ABCB1 gene is associated with differences in the opioid sensitivity. The ABCB1 polymorphism may serve as an important genetic predictor to guide the acute pain therapy in postoperative patients.

The Opposing Roles of IVS2+691 CC Genotype and AC/AG Diplotype of 118A>G and IVS2+691G>C of OPRM1 Polymorphisms in Cold Pain Tolerance Among Opioid-Dependent Malay Males on Methadone Therapy

Introduction

We recently reported that a majority of opioid-dependent Malay males on methadone therapy are cold pain sensitive. It is postulated that common OPRM1 polymorphisms may be responsible. This study investigated the association between 118A>G (dbSNP rs1799971) and IVS2+691G>C (dbSNP rs2075572) variants on cold pain responses among opioid-dependent Malay males on methadone maintenance therapy.

Methods

Cold pain responses including pain threshold, pain tolerance, and pain intensity were measured using the cold pressor test. DNA was extracted from the venous blood before polymerase chain reaction genotyping. Repeated measures analysis of variance was used to compare the cold pain responses and OPRM1 polymorphisms (118A>G and IVS2+691G>C) using models including genotype dominant and recessive models, allelic additive models, and analysis of haplotypes and diplotypes.

Results

A total of 148 participants were recruited. With the recessive model, those with IVS2+691 homozygous CC genotype had a shorter cold pain tolerance time than those without CC genotype (i.e., GG/GC genotype; 29.81 vs. 43.08 s, respectively, P = 0.048). On the other hand, with diplotype analysis, participants with combined homozygous 118 AA genotype and heterozygous IVS2+691 GC genotype (i.e., AC/AG diplotype) had a longer cold pain tolerance time than those without this diplotype (49.34 vs. 31.48 s, respectively, P = 0.043). Cold pain threshold was not associated with any of the 118A>G and IVS2+691G>C variations despite being analyzed using various models (all P > 0.05).

Conclusion

The IVS2+691 CC genotype and AC/AG diplotype of 118A>G and IVS2+691G>C seem to have opposing roles in pain tolerance among opioid-dependent Malay males on methadone therapy. Haplotypes of OPRM1 may be associated with altered binding affinity.

Electronic supplementary material

The online version of this article (doi:10.1007/s40122-015-0041-y) contains supplementary material, which is available to authorized users.

Pharmacodynamics and clinical efficacy of fentanyl iontophoretic transdermal system for post-operative pain in hospitalized patients

INTRODUCTION

The fentanyl iontophoretic transdermal system (ITS) is a patient-controlled transdermal system allowing needle-free administration of on-demand doses of Fentanyl of 40 µg over a 10-min period up to 80 doses or over a 24-h period. It is indicated in opioid naïve patients for the treatment of acute postoperative pain in the hospitalized patients for up to 72 h.

AREAS COVERED

It has been demonstrated to be effective and safe in randomized trials and to provide comparable analgesia versus morphine intravenous (i.v.) patient-controlled analgesia (PCA) with adverse events similar between groups.

EXPERT OPINION

Fentanyl ITS has shown high patient satisfaction rates, and was described by patients and investigators as easy and convenient to use. These properties make this technology interesting when considering perioperative pain management. In the present health care environment additional data are required to establish the cost-benefit ratio of this technology in optimizing patient's recovery from surgery.

Association of single nucleotide polymorphisms of ABCB1, OPRM1 and COMT with pain perception in cancer patients

Pain perception is influenced by multiple factors. The single nucleotide polymorphisms (SNPs) of some genes were found associated with pain perception. This study aimed to examine the association of the genotypes of ABCB1 C3435T, OPRM1 A118G and COMT V108/158M (valine 108/158 methionine) with pain perception in cancer patients. We genotyped 146 cancer pain patients and 139 cancer patients without pain for ABCB1 C3435T (rs1045642), OPRM1 A118G (rs1799971) and COMT V108/158M (rs4680) by the fluorescent dye-terminator cycle sequencing method, and compared the genotype distribution between groups with different pain intensities by chi-square test and pain scores between groups with different genotypes by non-parametric test. The results showed that in these cancer patients, the frequency of variant T allele of ABCB1 C3435T was 40.5%; that of G allele of OPRM1 A118G was 38.5% and that of A allele of COMT V108/158M was 23.3%. No significant difference in the genotype distribution of ABCB1 C3435T (rs1045642) and OPRM1 A118G (rs1799971) was observed between cancer pain group and control group (P=0.364 and 0.578); however, significant difference occurred in the genotype distribution of COMT V108/158M (rs4680) between the two groups (P=0.001). And the difference could not be explained by any other confounding factors. Moreover, we found that the genotypes of COMT V108/158M and ABCB1 C3435T were associated with the intensities of pain in cancer patients. In conclusion, our results indicate that the SNPs of COMT V108/158M and ABCB1 C3435T significantly influence the pain perception in Chinese cancer patients.

OPRM1 A118G Gene Variant and Postoperative Opioid Requirement

Background:

Although a number of studies have investigated the association of the OPRM1 A118G polymorphism with pain response, a consensus has not yet been reached.

Methods:

The authors searched PubMed, EMBASE, and the Cochrane Library to identify gene-association studies that explored the impact of the OPRM1 A118G polymorphism on postoperative opioid requirements through July 2013. Two evaluators independently reviewed and selected articles on the basis of prespecified selection criteria. The authors primarily investigated the standardized mean difference (SMD) of required amounts of opioids between AA homozygotes and G-allele carriers. The authors also performed subgroup analyses for race, opioid use, and type of surgery. Potential bias was assessed using the Egger’s test with a trim and fill procedure.

Results:

Three hundred forty-six articles were retrieved from databases, and 18 studies involving 4,607 participants were included in the final analyses. In a random-effect meta-analysis, G-allele carriers required a higher mean opioid dose than AA homozygotes (SMD, −0.18; P = 0.003). Although there was no evidence of publication bias, heterogeneity was present among studies (I2 = 66.8%). In the subgroup meta-analyses, significance remained robust in Asian patients (SMD, −0.21; P = 0.001), morphine users (SMD, −0.29; P &lt;0.001), and patients who received surgery for a viscus (SMD, −0.20; P = 0.008).

Conclusions:

The OPRM1 A118G polymorphism was associated with interindividual variability in postoperative response to opioids. In a subpopulation, identifying OPRM1 A118G polymorphism may provide valuable information regarding the individual analgesic doses that are required to achieve satisfactory pain control.

µ-opioid receptor gene variant OPRM1 118 A>G: a summary of its molecular and clinical consequences for pain

The human µ-opioid receptor variant 118 A>G (rs1799971) has become one of the most analyzed genetic variants in the pain field. At the molecular level, the variant reduces opioid receptor signaling efficiency and expression, the latter probably via a genetic-epigenetic interaction. In experimental settings, the variant was reproducibly associated with decreased effects of exogenous opioids. However, this translates into very small clinical effects (meta-analysis of 14 studies: Cohen's d = 0.096; p = 0.008), consisting of slightly higher opioid dosing requirements in peri- and post-operative settings. An effect can neither be maintained for chronic analgesic therapy nor for opioid side effects. It seems unlikely that further studies will reveal larger effect sizes and, therefore, further analyses appear unwarranted. Thus, due to its small effect size, the SNP is without major clinical relevance as a solitary variant, but should be regarded as a part of complex genotypes underlying pain and analgesia.

Postoperative Changes in Sleep-disordered Breathing and Sleep Architecture in Patients with Obstructive Sleep Apnea

Background:

Anesthetics, analgesics, and surgery may profoundly affect sleep architecture and aggravate sleep-related breathing disturbances. The authors hypothesized that patients with preoperative polysomnographic evidence of obstructive sleep apnea (OSA) would experience greater changes in these parameters than patients without OSA.

Methods:

After obtaining approvals from the Institutional Review Boards, consented patients underwent portable polysomnography preoperatively and on postoperative nights (N) 1, 3, 5, and 7 at home or in hospital. The primary and secondary outcome measurements were polysomnographic parameters of sleep-disordered breathing and sleep architecture.

Results:

Of the 58 patients completed the study, 38 patients had OSA (apnea hypopnea index [AHI] &gt;5) with median preoperative AHI of 18 events per hour and 20 non-OSA patients had median preoperative AHI of 2. AHI was increased after surgery in both OSA and non-OSA patients (P &lt; 0.05), with peak increase on postoperative N3 (OSA vs. non-OSA, 29 [14, 57] vs. 8 [2, 18], median [25th, 75th percentile], P &lt; 0.05). Hypopnea index accounted for 72% of the postoperative increase in AHI. The central apnea index was low (median = 0) but was significantly increased on postoperative N1 in only non-OSA patients. Sleep efficiency, rapid eye movement sleep, and slow-wave sleep were decreased on N1 in both groups, with gradual recovery.

Conclusions:

Postoperatively, sleep architecture was disturbed and AHI was increased in both OSA and non-OSA patients. Although the disturbances in sleep architecture were greatest on postoperative N1, breathing disturbances during sleep were greatest on postoperative N3.

Factors Associated with Postoperative Exacerbation of Sleep-disordered Breathing

Introduction:

The knowledge on the mechanism of the postoperative exacerbation of sleep-disordered breathing may direct the perioperative management of patients with obstructive sleep apnea. The objective of this study is to investigate the factors associated with postoperative severity of sleep-disordered breathing.

Methods:

After obtaining approvals from Institutional Review Boards, consenting patients underwent portable polysomnography preoperatively, and on postoperative nights 1 and 3 in hospital or at home. The primary outcomes were polysomnography parameters measuring the sleep-disordered breathing. They were treated as repeated measurement variables and analyzed for associated factors by mixed models.

Results:

Three hundred seventy-six patients, 168 men and 208 women, completed polysomnography on preoperative and postoperative night 1. Age was 59 ± 12 yr (mean ± SD). Preoperative apnea–hypopnea index (AHI) was 12 (4, 26) (median [25th, 75th percentile]) events per hour. Thirty-five patients had minor surgeries, 292 intermediate surgeries, and 49 major surgeries, with 210 general anesthesia and 166 regional anesthesia. The 72-h opioid dose was 55 (14, 85) mg intravenous morphine-equivalent dose. Preoperative AHI, age, and 72-h opioid dose were associated with postoperative AHI. Preoperative central apnea index, male sex, and general anesthesia were associated with postoperative central apnea index. Slow wave sleep percentage was inversely associated with postoperative AHI and central apnea index.

Conclusions:

Patients with a higher preoperative AHI were predicted to have a higher postoperative AHI. Preoperative AHI, age, and 72-h opioid dose were positively associated with postoperative AHI. Preoperative central apnea, male sex, and general anesthesia were associated with postoperative central apnea index.

Pharmacogenetics of OPRM1

Pharmacogenetic research has the potential to explain the variation in treatment efficacy within patient populations. Understanding the interaction between genetic variation and medications may provide a method for matching patients to the most effective therapeutic options and improving overall patient outcomes. The OPRM1 gene has been a target of interest in a large number of pharmacogenetic studies due to its genetic and structural variation, as well as the role of opioid receptors in a variety of disorders. The mu-opioid receptor (MOR), encoded by OPRM1, naturally regulates the analgesic response to pain and also controls the rewarding effects of many drugs of abuse, including opioids, nicotine, and alcohol. Genetic variants in OPRM1, particularly the non-synonymous polymorphism A118G, have been repeatedly associated with the efficacy of treatments for pain and various types of dependence. This review focuses on the current understanding of the pharmacogenetic impact of OPRM1, primarily with regard to the treatment of pain and addiction.

Genetic contributions to labor pain and progress

Studies on genetic contributions to labor analgesia have essentially evaluated the μ-opioid receptor gene (OPRM1), with some evidence that p.118A/G of OPRM1 influences the response to neuraxial opioids. As for labor progress, the β2-adrenergic receptor gene (ADRB2) is associated with preterm labor and delivery, and impacts the course of labor. Taken together though, there is no evidence that pharmacogenetic testing is needed or beneficial in the context of obstetric anesthesia; however, realizing the influence of genetic variants on specific phenotypes provides the rationale for a more cautious interpretation of clinical studies that attempt to find a dose-regimen that fits all.

Effects of OPRM1 A118G polymorphism on epidural analgesia with fentanyl during labor: a meta-analysis

BACKGROUND

Emerging evidence has shown that the most common polymorphism (A118G; rs1799971 A>G) in the μ-opioid receptor (OPRM1) gene may influence the response to labor analgesia, but individually published studies showed inconclusive results.

OBJECTIVE

This meta-analysis aimed to derive a more precise estimation of the effects of the OPRM1 A118G polymorphism on epidural analgesia with fentanyl during labor.

METHODS

A literature search was conducted on PubMed, Embase, Web of Science, and China BioMedicine databases before April 1st, 2013. The crude standardized mean difference (SMD) or odds ratio (OR) with 95% confidence interval (CI) was calculated.

RESULTS

Six clinical studies were included with a total 838 women who received epidural analgesia with fentanyl during labor. The meta-analysis results indicated that women carrying the G allele (AG+GG) of the OPRM1 A118G polymorphism required less fentanyl doses to achieve adequate pain relief compared with those with the AA homozygote (SMD=-0.24, 95% CI [-0.44, -0.03], p=0.022). The 118G variant was associated with a decreased ED50 of fentanyl for labor analgesia (SMD=-1.56, 95% CI [-1.97, -1.15], p<0.001). The analgesia satisfaction in women carrying the G allele (AG+GG) was higher than those with the AA homozygote (SMD=0.22, 95% CI [0.05, 0.39], p=0.012). However, there were no statistically significant differences between an AA homozygote and a G carrier (AG+GG) in the incidence of nausea and vomiting (OR=1.99, 95% CI [0.88, 4.52], p=0.101).

CONCLUSION

In conclusion, the current meta-analysis indicates that women carrying the G allele (AG+GG) of OPRM1 A118G polymorphism may have a good response to epidural analgesia with fentanyl during labor. The OPRM1 A118G polymorphism may help predict individuals' response to epidural labor analgesia and so optimize postoperative pain control.

Pharmacokinetics of non-intravenous formulations of fentanyl

Fentanyl was structurally designed by Paul Janssen in the early 1960s as a potent opioid analgesic (100-fold more potent than morphine). It is a full agonist at μ-opioid receptors and possesses physicochemical properties, in particular a high lipophilicity (octanol:water partition coefficient >700), which allow it to cross quickly between plasma and central nervous target sites (transfer half-life of 4.7-6.6 min). It undergoes first-pass metabolism via cytochrome P450 3A (bioavailability ~30 % after rapid swallowing), which can be circumvented by non-intravenous formulations (bioavailability 50-90 % for oral transmucosal or intranasal formulations). Non-intravenous preparations deliver fentanyl orally-transmucosally, intranasally or transdermally. Passive transdermal patches release fentanyl at a constant zero-order rate for 2-3 days, making them suitable for chronic pain management, as are iontophoretic transdermal systems. Oral transmucosal and intranasal routes provide fast delivery (time to reach maximum fentanyl plasma concentrations 20 min [range 20-180 min] and 12 min [range 12-21 min], respectively) suitable for rapid onset of analgesia in acute pain conditions with time to onset of analgesia of 5 or 2 min, respectively. Intranasal formulations partly bypass the blood-brain barrier and deliver a fraction of the dose directly to relevant brain target sites, providing ultra-fast analgesia for breakthrough pain. Thanks to the development of non-intravenous pharmaceutical formulations, fentanyl has become one of the most successful opioid analgesics, and can be regarded as an example of a successful reformulation strategy of an existing drug based on pharmacokinetic research and pharmaceutical technology. This development broadened the indications for fentanyl beyond the initial restriction to intra- or perioperative clinical uses. The clinical utility of fentanyl could be expanded further by more comprehensive mathematical characterizations of its parametric pharmacokinetic input functions as a basis for the rational selection of fentanyl formulations for individualized pain therapy.

Consequences of the 118A>G polymorphism in the OPRM1 gene: translation from bench to bedside?

The 118A>G single nucleotide polymorphism (SNP) in the μ-opioid receptor (OPRM1) gene has been the most described variant in pharmacogenetic studies regarding opioid drugs. Despite evidence for an altered biological function encoded by this variant, this knowledge is not yet utilized clinically. The aim of the present review was to collect and discuss the available information on the 118A>G SNP in the OPRM1 gene, at the molecular level and in its clinical manifestations. In vitro biochemical and molecular assays have shown that the variant receptor has higher binding affinity for β-endorphins, that it has altered signal transduction cascade, and that it has a lower expression compared with wild-type OPRM1. Studies using animal models for 118A>G have revealed a double effect of the variant receptor, with an apparent gain of function with respect to the response to endogenous opioids but a loss of function with exogenous administered opioid drugs. Although patients with this variant have shown a lower pain threshold and a higher drug consumption in order to achieve the analgesic effect, clinical experiences have demonstrated that patients carrying the variant allele are not affected by the increased opioid consumption in terms of side effects.

Associations between single-nucleotide polymorphisms and epidural ropivacaine consumption in patients undergoing breast cancer surgery

Up to date, few published studies indicated the associations between genetic polymorphisms and epidural local anesthetics consumption. In this study, we investigated the associations between seven single-nucleotide polymorphisms (SNPs) and epidural ropivacaine consumption during breast cancer surgery in women from northeastern China. These seven SNPs (rs3803662 and rs12443621 in TNCR9, rs889312 in MAP3K1, rs3817198 in LSP1, rs13387042 at 2q35, rs13281615 at 8q24, and rs2046210 at 6q25.1) were identified by recent genome-wide association studies associated with tumor susceptibility. A total of 418 breast cancer women received thoracic epidural anesthesia with ropivacaine for elective mastectomy with axillary clearance. Their blood samples were genotyped for the seven SNPs using the SNaPshot method. For SNP rs13281615, the subjects with genotype AG and GG consumed a greater amount of the total epidural ropivacaine and the mean ropivacaine dose than the subjects with genotype AA (p=0.047 and p=0.003, respectively). Furthermore, no statistical differences were found in the total dose of ropivacaine, the mean consumption of ropivacaine, the onset of ropivacaine, or the initial dose of lidocaine among the three genotypic groups for the other six SNPs studied. Our study indicated that SNP rs13281615 at 8q24 was associated with the consumption of epidural ropivacaine during breast cancer surgery in northeastern Chinese women. It might provide new insights into the mechanisms of ropivacaine action and metabolism and facilitate the development of personalized medicine.

The correlation between post-operative fentanyl requirements and μ-opioid receptor gene A118G polymorphism in patients undergoing radical gastrectomy

The aim of this study was to investigate the effect of the μ-opioid receptor gene (OPRM1) A118G polymorphism on the requirement for post-operative fentanyl analgesia in patients undergoing radical gastrectomy. One hundred and twenty-eight gastric cancer patients scheduled to undergo radical gastrectomy under general anesthesia were enrolled in the study. Post-operative, patient-controlled intravenous analgesia of fentanyl was provided for satisfactory analgesia until 48 h after surgery. OPRM1 A118G was screened by DNA sequence analysis of polymerase chain reaction (PCR)-amplified DNA. Differences in fentanyl consumption and adverse effects were compared among the different genotypes at 24 and 48 h after surgery. The ranges of fentanyl dose in the 128 patients at 24 and 48 h after surgery were 5.4-17.3 μg/kg and 12.4-29.9 μg/kg, respectively. Among these patients, there were 54 wild-type homozygotes (AA), 53 heterozygotes (AG) and 21 mutant homozygotes (GG). The frequency of the G allele was 0.371 in the OPRM1 polymorphism. There were no significant differences in fentanyl dose or adverse effects, including nausea, vomiting and dizziness, for the OPRM1 A118G polymorphism (P>0.05). The OPRM1 A118G polymorphism does not play a significant role in post-operative fentanyl analgesic dose or post-operative nausea, vomiting and dizziness in patients undergoing radical gastrectomy.

Effect of CYP3A4*18B polymorphisms and interactions with OPRM1 A118G on postoperative fentanyl requirements in patients undergoing radical gastrectomy

The present study aimed to investigate the effect of cytochrome P450 3A4 (CYP3A4)*18B polymorphisms and the interaction of the µ opioid receptor gene (OPRM1) A118G and CYP3A4*18B polymorphisms on postoperative fentanyl analgesia in Chinese Han patients undergoing radical gastrectomy. In total, 97 patients scheduled to undergo radical gastrectomy under general anesthesia were enrolled in this study. Post‑operative patient‑controlled intravenous analgesia of fentanyl was administered as analgesia up to 48 h following surgery. Venous blood (2 ml) was obtained from each patient to measure the OPRM1 A118G and CYP3A4*18B genotypes. The differences in fentanyl consumption and adverse effects were compared among the genotypes at 24 and 48 h following surgery. In the first 48 h following surgery, patients in the CYP3A4*18B/*18B group consumed significantly less fentanyl compared with patients in the *1/*1 group (P=0.032). With regards to the joint genetic effect, during the 48‑h period, patients with AA and *1*18B polymorphisms received fewer fentanyl doses compared with those with AG and *1*1 (P=0.049), while patients with AG and *1*18B polymorphisms received significantly fewer fentanyl doses compared with those with AG and *1*1 (P=0.010), and patients with *18B*18B polymorphisms received significantly fewer fentanyl doses compared with those with AA and *1*1 (P=0.024) or those with AG and *1*1 polymorphisms (P=0.006). No correlation between OPRM1 A118G and CYP3A4*18B and postoperative nausea, vomiting and dizziness was found. Results demonstrated that 48 h following surgery, patients with the CYP3A4*18B/*18B genotype required less fentanyl than patients with the CYP3A4*1/*1 genotype to control pain. Additionally, the combined genotype of CYP3A4*18B and OPRM1 A118G may affect fentanyl doses administered for pain control, but not postoperative nausea, vomiting and dizziness.

Pharmacogenomic considerations in opioid analgesia

Translating pharmacogenetics to clinical practice has been particularly challenging in the context of pain, due to the complexity of this multifaceted phenotype and the overall subjective nature of pain perception and response to analgesia. Overall, numerous genes involved with the pharmacokinetics and dynamics of opioids response are candidate genes in the context of opioid analgesia. The clinical relevance of CYP2D6 genotyping to predict analgesic outcomes is still relatively unknown; the two extremes in CYP2D6 genotype (ultrarapid and poor metabolism) seem to predict pain response and/or adverse effects. Overall, the level of evidence linking genetic variability (CYP2D6 and CYP3A4) to oxycodone response and phenotype (altered biotransformation of oxycodone into oxymorphone and overall clearance of oxycodone and oxymorphone) is strong; however, there has been no randomized clinical trial on the benefits of genetic testing prior to oxycodone therapy. On the other hand, predicting the analgesic response to morphine based on pharmacogenetic testing is more complex; though there was hope that simple genetic testing would allow tailoring morphine doses to provide optimal analgesia, this is unlikely to occur. A variety of polymorphisms clearly influence pain perception and behavior in response to pain. However, the response to analgesics also differs depending on the pain modality and the potential for repeated noxious stimuli, the opioid prescribed, and even its route of administration.

The human μ-opioid receptor gene polymorphism (A118G) is associated with head pain severity in a clinical cohort of female migraine with aura patients

Migraine is a painful and debilitating, neurovascular disease. Current migraine head pain treatments work with differing efficacies in migraineurs. The opioid system plays an important role in diverse biological functions including analgesia, drug response and pain reduction. The A118G single nucleotide polymorphism (SNP) in exon 1 of the μ-opioid receptor gene (OPRM1) has been associated with elevated pain responses and decreased pain threshold in a variety of populations. The aim of the current preliminary study was to test whether genotypes of the OPRM1 A118G SNP are associated with head pain severity in a clinical cohort of female migraineurs. This was a preliminary study to determine whether genotypes of the OPRM1 A118G SNP are associated with head pain severity in a clinical cohort of female migraineurs. A total of 153 chronic migraine with aura sufferers were assessed for migraine head pain using the Migraine Disability Assessment Score instrument and classified into high and low pain severity groups. DNA was extracted and genotypes obtained for the A118G SNP. Logistic regression analysis adjusting for age effects showed the A118G SNP of the OPRM1 gene to be significantly associated with migraine pain severity in the test population (P = 0.0037). In particular, G118 allele carriers were more likely to be high pain sufferers compared to homozygous carriers of the A118 allele (OR = 3.125, 95 % CI = 1.41, 6.93, P = 0.0037). These findings suggest that A118G genotypes of the OPRM1 gene may influence migraine-associated head pain in females. Further investigations are required to fully understand the effect of this gene variant on migraine head pain including studies in males and in different migraine subtypes, as well as in response to head pain medication.

Ethnicity interacts with the OPRM1 gene in experimental pain sensitivity

Robust interindividual variation in pain sensitivity has been observed, and recent evidence suggests that some of the variability may be genetically mediated. Our previous data revealed significantly higher pressure pain thresholds among individuals possessing the minor G allele of the A118G SNP of the mu-opioid receptor gene (OPRM1) compared with those with 2 consensus alleles. Moreover, ethnic differences in pain sensitivity have been widely reported. Yet, little is known about the potential interactive associations of ethnicity and genotype with pain perception. This study aimed to identify ethnic differences in OPRM1 allelic associations with experimental pain responses. A total of 247 healthy young adults from three ethnic groups (81 African Americans; 79 non-white Hispanics; and 87 non-Hispanic whites) underwent multiple experimental pain modalities (thermal, pressure, ischemic, cold pressor). Few African Americans (7.4%) expressed the rare allele of OPRM1 compared to non-Hispanic whites and Hispanics (28.7% vs. 27.8%, respectively). Across the entire sample, OPRM1 genotype did not significantly affect pain sensitivity. However, analysis in each ethnic group separately revealed significant genotype effects for most pain modalities among non-Hispanic-whites (P<.05) but not Hispanics or African Americans. The G allele was associated with decreased pain sensitivity among whites only; a trend in the opposite direction emerged in Hispanics. The reasons for this dichotomy are unclear; they may involve ethnic differences in haplotypic structure, or A118G may be a tag-SNP linked to other functional polymorphisms. These findings demonstrate an ethnicity-dependent association of OPRM1 genotype with pain sensitivity. Additional research is warranted to uncover the mechanisms influencing these relationships.

Pharmacological consequence of the A118G μ opioid receptor polymorphism on morphine- and fentanyl-mediated modulation of Ca²⁺ channels in humanized mouse sensory neurons

BACKGROUND

The most common functional single nucleotide polymorphism of the human OPRM1 gene, A118G, has been shown to be associated with interindividual differences in opioid analgesic requirements, particularly with morphine, in patients with acute postoperative pain. The purpose of this study was to examine whether this polymorphism would modulate the morphine and fentanyl pharmacological profile of sensory neurons isolated from a humanized mouse model homozygous for either the 118A or 118G allele.

METHODS

The coupling of wild-type and mutant μ opioid receptors to voltage-gated Ca channels after exposure to either ligand was examined by employing the whole cell variant of the patch-clamp technique in acutely dissociated trigeminal ganglion neurons. Morphine-mediated antinociception was measured in mice carrying either the 118AA or 118GG allele.

RESULTS

The biophysical parameters (cell size, current density, and peak current amplitude potential) measured from both groups of sensory neurons were not significantly different. In 118GG neurons, morphine was approximately fivefold less potent and 26% less efficacious than that observed in 118AA neurons. On the other hand, the potency and efficacy of fentanyl were similar for both groups of neurons. Morphine-mediated analgesia in 118GG mice was significantly reduced compared with the 118AA mice.

CONCLUSIONS

This study provides evidence to suggest that the diminished clinical effect observed with morphine in 118G carriers results from an alteration of the receptor's pharmacology in sensory neurons. In addition, the impaired analgesic response with morphine may explain why carriers of this receptor variant have an increased susceptibility to become addicted to opioids.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 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).

Elucidation of mu-Opioid Gene Structure: How Genetics Can Help Predict Responses to Opioids

Opioid drugs are among the most commonly used and effective human analgesics. To date, the clinical benefits of opioid analgesics have not been fully realized due to substantial individual variations in the responses to opioids, insufficient drug dosing, and a high rate (up to 66%) of adverse events. As such, there is a substantial need to identify the genetic and molecular biological mechanisms that mediate individual responses to opioid therapy. Recent discoveries show that genetic variations in the μ-opioid receptor (OPRM1) gene locus play an essential role in inter-individual responses. The majority of genetic association studies have focused on the A118G polymorphism, which codes for a non-synonymous change in OPRM1 exon 1. In addition to the A118G polymorphism, another functional SNP (rs563649), which is located within an alternatively-spliced OPRM1 isoform (MOR-1K), has been identified. The MOR-1k isoform codes for 6TM OPRM1 isoforms that display excitatory rather than the inhibitory cellular effects, which are characteristic of the canonical 7TM isoforms. Thus, stimulation of the 6TM isoforms may engage the molecular mechanisms mediating opioid-dependent hyperalgesia, tolerance and dependence. Future clinical and basic studies that seek to identify the functional genetic variants within OPRM1 locus, and associated molecular mechanisms, will result in a better understanding of individual responses to opioid therapy and ultimately to the development new pharmacotherapeutics and diagnostic tools.