Genetic modulation of the pharmacological treatment of pain

Pain management in patients with cancer: focus on opioid analgesics

Cancer pain is generally treated with pharmacological measures, relying on using opioids alone or in combination with adjuvant analgesics. Weak opioids are used for mild-to-moderate pain as monotherapy or in a combination with nonopioids. For patients with moderate-to-severe pain, strong opioids are recommended as initial therapy rather than beginning treatment with weak opioids. Adjunctive therapy plays an important role in the treatment of cancer pain not fully responsive to opioids administered alone (ie, neuropathic, bone, and visceral colicky pain). Supportive drugs should be used wisely to prevent and treat opioids’ adverse effects. Understanding the pharmacokinetics, pharmacodynamics, interactions, and cautions with commonly used opioids can help determine appropriate opioid selection for individual cancer patients.

Pharmacogenetics of new analgesics

Patient phenotypes in pharmacological pain treatment varies between individuals, which could be partly assigned to their genotypes regarding the targets of classical analgesics (OPRM1, PTGS2) or associated signalling pathways (KCNJ6). Translational and genetic research have identified new targets, for which new analgesics are being developed. This addresses voltage-gated sodium, calcium and potassium channels, for which SCN9A, CACNA1B, KCNQ2 and KCNQ3, respectively, are primary gene candidates because they code for the subunits of the respective channels targeted by analgesics currently in clinical development. Mutations in voltage gated transient receptor potential (TRPV) channels are known from genetic pain research and may modulate the effects of analgesics under development targeting TRPV1 or TRPV3. To this add ligand-gated ion channels including nicotinic acetylcholine receptors, ionotropic glutamate-gated receptors and ATP-gated purinergic P2X receptors with most important subunits coded by CHRNA4, GRIN2B and P2RX7. Among G protein coupled receptors, δ-opioid receptors (coded by OPRD1), cannabinoid receptors (CNR1 and CNR2), metabotropic glutamate receptors (mGluR5 coded by GRM5), bradykinin B(1) (BDKRB1) and 5-HT(1A) (HTR1A) receptors are targeted by new analgesic substances. Finally, nerve growth factor (NGFB), its tyrosine kinase receptor (NTRK1) and the fatty acid amide hydrolase (FAAH) have become targets of interest. For most of these genes, functional variants have been associated with neuro-psychiatric disorders and not yet with analgesia. However, research on the genetic modulation of pain has already identified variants in these genes, relative to pain, which may facilitate the pharmacogenetic assessments of new analgesics. The increased number of candidate pharmacogenetic modulators of analgesic actions may open opportunities for the broader clinical implementation of genotyping information.

Micro Opioid Receptor A118G Polymorphism and Post-Operative Pain: Opioids' Effects on Heterozigous Patients

The Single-Nucleotide-Polymorphism (SNP) 118A>G in the μ-1 Opioid Receptor gene (OPRM1) is associated with a decrease in the analgesic effects of opioids. The aim of this study is to assess whether 118A>G polymorphism could influence the analgesic response to opioid-based postoperative pain (POP) therapy. The study consisted of two parts: section α, observational, included 199 subjects undergoing scheduled surgical procedures with pain management standardized on surgery invasiveness and on expected level of postoperative pain; section β, randomized, included 41 women undergoing scheduled caesarean delivery with continuous intra-operative epidural anesthesia and post-operative analgesia (CEA). In both sections, POP was measured over 48 h (T6h-T24h-T48h) by the visual analogue scale (VAS). In section β we also tested the responsiveness of hypothalamic-pituitary-adrenal axis (HPA) expressed by Cortisol levels. In section α, with cluster analysis, subjects were analyzed according to their genotype: a group (#1) of 34 patients reporting VAS score >3 at every time lapse was identified and included only A118G carriers, while wild-type (A118A - absence of 118A>G polymorphism) patients were unevenly distributed between those with cluster #2 (VAS score <3 at every study steps) and those with cluster #3 (VAS score progressively reducing from T6h). In section (3, A118G carriers receiving epidural sufentanil had the lowest VAS scores at T24h; also in these patients, Cortisol levels remained more stable, with a mild decrease at T6h. This study shows that the OPRM1 118A>G polymorphism affects postoperative pain response in heterozygous patients: they have a different postoperative pain response than patients with wild-type genes, which may affect the efficacy of the analgesic therapy.

Polymorphism of the Glutamate Transporter Protein EAAT2 and Migraine Transformation into Chronic Daily Headache

Background and Purpose

The progression of migraine into chronic daily headache involves multiple risk factors, but the main contributor is not known. Glutamate is the major excitatory neurotransmitter in central sensitization, which is an important process in the pathogenesis of migraine transformation. The glutamate transporter protein excitatory amino acid transporter 2 (EAAT2) is the primary modulator of glutamatergic neurotransmission, and genetic polymorphisms of its gene, EEAT2, have been identified. The aim of this study was to determine the effect of EAAT2 polymorphisms on migraine transformation into chronic daily headache.

Methods

We included 74 migraine patients with episodic attack (M-E) and 59 migraine patients with chronic daily headache (M-CDH). After amplifying EAAT2 by polymerase chain reaction, we assessed its genotype frequencies based on restriction fragment length polymorphisms. We reclassified all migraine patients into two groups according to their EAAT2 genotype, either with the A allele (n=62) or without it (n=71), and compared the clinical variables between the two groups.

Results

The genotype frequencies of EAAT2 polymorphisms did not differ between the M-E and M-CDH groups. Comparison between EEAT2 genotypes revealed that the frequency of analgesic usage was significantly higher among migraine patients with the A allele (12.9±1.6 days/month) than in those without the A allele (8.1±1.2 days/month; p=0.019). The other clinical variables of migraine did not differ between the two groups.

Conclusions

The results suggest that EEAT2 polymorphism contributes to the tendency toward frequent analgesic usage in migraine patients. This implies a potential genetic influence on the progression of migraine into chronic daily headache through the development of medication-overuse headache.

Opioid management and dependency among adult patients with sickle cell disease

While pain is one of the most debilitating symptoms of sickle cell disease, narcotics remain an effective although controversial widely practiced intervention. Vaso-occlusive crises are the most common cause for seeking pharmacological treatment. The influence of stigmatization and pseudo addiction in emergency departments and outpatient clinics was reviewed. We analyzed patterns of narcotic utilization in a sample of 63 adult patients with sickle cell disease to determine if their psychological functioning and reports of pain differed as a function of the primary narcotics they were taking for oral pain management. Fifty-one percent of patients reported treatment of Oxycodone, 35% OxyContin, 24% methadone and 11% morphine. Patients who were treated with Oxycodone reported greater sensory reactions to pain (p = 0.001), visual analog scale (VAS) (p = 0.02), and averaged weekly pain intensity ratings than patients who did not use this medication. There were no differences in pain or affective response in patients treated with OxyContin, methadone or morphine. We suggest there are clear differences between the reports of pain in patients with sickle cell disease taking short-acting narcotics for pain management as compared to those who are not, a pattern that does not distinguish patients who are managed with long-acting preparations. We discuss the relevance of addressing narcotic management in the context of the perception of health care providers and patients with sickle cell disease.

Pain, analgesia and genetics

OBJECTIVES

In the clinical setting, there is marked intersubject variability in the intensity of pain reported by patients with apparently similar pain states, as well as widely differing analgesic dosing requirements between individuals to produce satisfactory pain relief with tolerable side-effects. Genetic and environmental factors as well as their interaction are implicated, and these are discussed in this review.

KEY FINDINGS

Pioneering work undertaken in mice more than a decade ago, showed a strong genetic contribution to levels of nociception/hypersensitivity as well as levels of antinociception produced by commonly available analgesic agents. To date more than 300 candidate 'pain' genes have been identified as potentially contributing to heritable differences in pain sensitivity and analgesic responsiveness in animals and humans, with this information available in a publicly accessible database http://www.jbldesign.com/jmogil/enter.html. Since then, many genetic association studies have been conducted in humans to investigate the possibility that single nucleotide polymorphisms (SNPs) in an individual gene may explain drug inefficacy or excessive toxicity experienced by a small subset of the whole population who have the rare allele for a particular SNP.

SUMMARY

Despite the fact that SNPs in more than 20 genes that affect pain sensitivity or contribute to interindividual variability in responses to analgesic medications have been identified in the human genome, much of the data is conflicting. Apart from deficiencies in the design and conduct of human genetic association studies, recent research from other fields has implicated epigenetic mechanisms that facilitate dynamic gene-environment communication, as a possible explanation.

Clinical pharmacology of analgesic medicines in older people: impact of frailty and cognitive impairment

Pain is highly prevalent in frail older people who often have multiple co-morbidities and multiple medicines. Rational prescribing of analgesics in frail older people is complex due to heterogeneity in drug disposition, comorbid medical conditions, polypharmacy and variability in analgesic response in this population. A critical issue in managing older people with pain is the need for judicious choice of analgesics based on a comprehensive medical and medication history. Care is needed in the selection of analgesic medicine to avoid drug-drug or drug-disease interactions. People living with dementia and cognitive impairment have suboptimal pain relief which in part may be related to altered pharmacodynamics of analgesics and challenges in the systematic assessment of pain intensity in this patient group. In the absence of rigorously controlled trials in frail older people and those with cognitive impairment a pharmacologically-guided approach can be used to optimize pain management which requires a systematic understanding of the pharmacokinetics and pharmacodynamics of analgesics in frail older people with or without changes in cognition.

Genomics and pain research in sickle cell disease: an explanation of heterogeneity?

Sickle cell disease (SCD) is a chronic illness, and the major complication, pain, results in complex multidimensional problems that affect an individual's ability to maintain adequate quality of life in multiple areas. Chronic SCD pain is inadequately treated, because it is not well understood, and the degree of chronic pain, clinical presentation, and sequela complications can vary from patient to patient, even among individuals with the same SCD genotype. The reason for this variation is unknown, but the underlying cause might be genetic. Researchers have not explored the contribution of a genomic variable to the occurrence of heterogeneous chronic SCD pain. Previous research on the guanosine triphosphate cyclohydrolase (GCH1) gene suggests that in some cases, phenotypic heterogeneity in human sensitivity to pain correlates with underlying genotypic variations in the GCH1 gene. These findings imply that genotypic variations might also explain why some SCD patients experience more chronic pain than others.

Inhibition of human drug metabolizing cytochrome P450 enzymes by plant isoquinoline alkaloids

The human cytochrome P450 (CYP) enzymes play a major role in the metabolism of endobiotics and numerous xenobiotics including drugs. Therefore it is the standard procedure to test new drug candidates for interactions with CYP enzymes during the preclinical development phase. The purpose of this study was to determine in vitro CYP inhibition potencies of a set of isoquinoline alkaloids to gain insight into interactions of novel chemical structures with CYP enzymes. These alkaloids (n=36) consist of compounds isolated from the Papaveraceae family (n=20), synthetic analogs (n=15), and one commercial compound. Their inhibitory activity was determined towards all principal human drug metabolizing CYP enzymes: 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6 and 3A4. All alkaloids were assayed in vitro in a 96-well plate format using pro-fluorescent probe substrates and recombinant human CYP enzymes. Many of these alkaloids inhibited the CYP3A4 form, with 30/36 alkaloids inhibiting CYP3A4 with at least moderate potency (IC₅₀ < 10 μM) and 15/36 inhibiting CYP3A4 potently (IC₅₀ < 1 μM). Among them corydine, parfumine and 8-methyl-2,3,10,11-tetraethoxyberbine were potent and selective inhibitors for CYP3A4. CYP2D6 was inhibited with at least moderate potency by 26/34 alkaloids. CYP2C19 was inhibited by 15/36 alkaloids at least moderate potently, whereas CYP1A2, CYP2B6, CYP2C8, and CYP2C9 were inhibited to a lesser degree. CYP2A6 was not significantly inhibited by any of the alkaloids. The results provide initial structure-activity information about the interaction of isoquinoline alkaloids with major human xenobiotic-metabolizing CYP enzymes, and illustrate potential novel structures as CYP form-selective inhibitors.

Drug response profiles to experimental pain are opioid and pain modality specific

Given our limited ability to predict analgesic efficacy, further research is needed to understand factors influencing analgesic response patterns. The aim of this study was to better understand the relationship between morphine and butorphanol analgesic efficacy tested against multiple pain modalities within the same individuals. Participants included healthy men (n = 72) and women (n = 67) who underwent thermal, pressure, and ischemic experimental pain testing before and after the double-blind administration of morphine and butorphanol during separate testing sessions. Factor analysis revealed 6 factors with analgesic effects grouped primarily by pain modality and specific to either morphine or butorphanol. Hierarchical cluster analysis of individual factor scores led to 4 distinct drug response profiles. Three groups displayed exceptional analgesic efficacy produced by 1 type of opioid on 1 pain stimulus modality, whereas the fourth drug response profile was characterized by average analgesic efficacy across all pain modalities for both opioids. These findings suggest that opioids with varying efficacy at the μ and κ receptors produce independent effects on unique pain mechanisms and that individual responsiveness for some is dependent on pain mechanism and opioid type, although a subset of the population is moderately responsive to opioids regardless of efficacy of receptor binding or predominant pain mechanism being activated.

PERSPECTIVE

This investigation provides a foundation for understanding patterns of opioid efficacy in varying types of pain. Our findings suggest that opioid response patterns are more complex than originally thought with about half of individuals exhibiting opioid and pain modality specific analgesic response profiles.

Human opioid μ-receptor A118G polymorphism may protect against central pruritus by epidural morphine for post-cesarean analgesia

BACKGROUND

Intrathecal or epidural morphine used for post-operative analgesia frequently induces central type pruritus. The purpose of this study was to investigate the association between the severity of central type pruritus induced by epidural morphine for post-cesarean analgesia and the A118G polymorphism of the human μ-opioid receptor gene (OPRM1).

METHODS

Pregnant women (212) received pure epidural morphine (2 mg) twice per day for post-cesarean analgesia. Blood samples were collected and sequenced with high-resolution melting analysis to detect three different genotypes of OPRM1 (AA, AG and GG). We interviewed all candidates 24 h post-operatively to record the clinical phenotype with subjective complaints and objective observations.

RESULTS

The genotyping revealed that 99 women (46.7%) were AA, 88 (41.5%) were AG and 25 (11.8%) were GG. Sixty-two of 212 women suffered from significant pruritus (29.2%), and 150 of 212 women had non-significant pruritus (70.8%). In genotype AA, 33 patients (53.2%) experienced significant pruritus, 26 (41.9%) in genotype AG and 3 (4.8%) in genotype GG. The G allele was a statistically independent protective factor for individuals developing pruritus, and the multivariate-adjusted odds ratio was 0.27. There was a trend for progressively decreasing severity scores among the three groups, with the lowest severity score (0.72) for pruritus in the GG group.

CONCLUSIONS

The incidence of significant pruritus in the recessive type (GG) was significantly lower compared with the dominant types (AA+AG). The recessive G allele in the A118G polymorphism may have protective effects against significant pruritus after epidural morphine for post-cesarean analgesia.

Genetics of pain, opioids, and opioid responsiveness

Pain is an integral part of the defense mechanisms required for survival. Several hereditary syndromes of complete or almost complete insensitivity to pain have been identified and include channelopathy-associated pain insensitivity, of which the most likely candidate gene is the α-subunit of the voltage-gated sodium channel known as Na(v)1.7. Five hereditary sensory and autonomic neuropathy syndromes have been described. Variable pain sensitivity in the general population has been linked to common variants of the μ-opioid receptor and of the catecholamine-O-methyltransferase genes potentially leading to increased opioid tonus. Variants of the guanosine triphosphate cyclohydrolase 1/dopa-responsive dystonia gene appear to regulate nociception. Other candidate genes are the transient receptor potential cation channel, subfamily 5 member 1, gene and the melanocortin-1 receptor gene. Candidate genes for predicting opioid efficacy are drug-metabolizing enzymes and transporters-including cytochrome P450, uridine 5'-diphosphate-glucuronosyltransferases, and adenosine triphosphate-binding cassette transporters-that are involved in opioid metabolism. Most current knowledge on the genetic regulation of pain has been derived from animal models developed mainly in mice. Genomics has the potential to contribute to therapeutic advances with the promising approach of using small interfering RNA in the control of neuropathic pain. Knowledge of the genetic factors that affect opioid efficacy, metabolism, and adverse effects has the potential for personalizing both acute and chronic pain management, and for designing more useful opiate pain medications with lower adverse event profiles.

Personalized therapy in pain management: where do we stand?

Genomic variations influencing response to pharmacotherapy of pain are currently under investigation. Drug-metabolizing enzymes represent a major target of ongoing research in order to identify associations between an individual’s drug response and genetic profile. Polymorphisms of the cytochrome P450 enzymes (CYP2D6) influence metabolism of codeine, tramadol, hydrocodone, oxycodone and tricyclic antidepressants. Blood concentrations of some NSAIDs depend on CYP2C9 and/or CYP2C8 activity. Genomic variants of these genes associate well with NSAIDs’ side effect profile. Other candidate genes, such as those encoding (opioid) receptors, transporters and other molecules important for pharmacotherapy in pain management, are discussed; however, study results are often equivocal. Besides genetic variants, further variables, for example, age, disease, comorbidity, concomitant medication, organ function as well as patients’ compliance, may have an impact on pharmacotherapy and need to be addressed when pain therapists prescribe medication. Although pharmacogenetics as a diagnostic tool has the potential to improve patient therapy, well-designed studies are needed to demonstrate superiority to conventional dosing regimes.