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29 August 2014 No. 27 PHARMACOGENETICS OF PAIN A genetic based approach to personalized pain therapy R Randolph Moderator: A Dunpath CONTENTS INTRODUCTION ................................................................................................... 3 RARE HEREDITARY SYNDROMES OF COMPLETE INSENSITIVITY TO PAIN 4 THE GENETICS OF VARIABLE PAIN SENSITIVITY IN THE GENERAL POPULATION....................................................................................................... 5 GENETIC MODULATION OF DRUGS USED IN PAIN TREATMENT .................. 6 PHARMACOGENETICS OF DRUG INTERACTIONS IN PAIN THERAPY ........ 10 GENETIC TESTING ............................................................................................ 12 CONCLUSION .................................................................................................... 14 REFERENCES.................................................................................................... 15 Page 2 of 18 INTRODUCTION The inadequate treatment of acute and chronic pain remains a major cause of distress and dissatisfaction among our patient population. It has been estimated that one out of ten surgical patients develop chronic postoperative pain, irrespective of the type of surgery.[1] Therefore, effective pain management remains a key issue. Our awareness of the genetic modulation of pain and analgesia has grown over the past decade, so too, has our hope of developing a genetic based approach to pain management. It has been our expectation that genotyping may provide a guide to analgesic choice and dose before the commencement of treatment. Genotyping may also provide a valuable insight into why certain individuals do not respond to conventional therapy. Genetic factors determine ones risk of developing a painful disease as well as its severity from the varied expressive functioning of the nociceptive sensory system (Figure1). Substances administered for pain are under distributional and metabolic processes which involve genetic modulation. Finally, the interaction between the drug and its target site are also under genetic influence. Several foundation studies, mostly animal models, have provided a source of candidate genes for genetic association studies in humans. Mice of 11 inbred strains, showed variable nociceptive responses in 12 different pain models. These differences in nociceptive sensitivity varied as much as 1.2- 54 fold [2]. Various mice strains also showed a variation in their nociceptive response to morphine administration [3]. This has also been seen in humans as opioid dose requirements vary in the clinical setting by as much as 40 fold [4]. It is well recognised in humans that the risk of developing a painful disease varies with certain genotypes. Several twin studies have evaluated the heritability of various pain conditions and have indicated that migraines have a 39-58% genetic contribution[5], lower back pain carries a 21-67% genetic contribution[6] and menstrual pain a 55% genetic contribution [7]. Common phenotypes, like those identified in the average population, are highly complex and multi-genetic. On the other hand, monogenetic heredity have been restricted to rare and extreme phenotypes.[8] A patient’s phenotype may be regarded as the sum or balance of synergistic or antagonistic effects of several genetic variants concomitantly present in an individual. At present, common genetic variants have not been reliably used to predict an individual’s pain and analgesic response. The aim of this review is to highlight the human genetic variants that have been identified in the pharmacotherapy of pain with the focus on the mechanisms involved and the altered clinical effects of analgesics. Page 3 of 18 Figure 1. Pain sensitivity graph [9] RARE HEREDITARY SYNDROMES OF COMPLETE INSENSITIVITY TO PAIN The complete inability to sense pain is an extremely rare phenotype. It involves the interruption of transmission and processing of nociception. Channelopathy associated insensitivity to pain syndrome results from the loss of functional mutations in the alpha subunit of the voltage gated sodium channel Na(v)1.7. These changes are encoded by the gene SCN9A at the locus 2q24.3. [10] It is exclusively expressed in primary nociceptive sensory neurons where it is responsible for the amplification of small depolarisations. A further five rare syndromes are characterized by the absence of pain. Hereditary Sensory and Autonomic Neuropathy, HSAN (types I-V) results from the loss of functional mutations in genes which control the development and homeostasis of the nervous system.[10] Unfortunately, these syndromes have no practical significance in terms of pharmacotherapy as they are extremely rare and those that are affected do not require pain treatment. However, in defining their molecular basis for insensitivity, new target sites may be considered in the development of novel analgesic drugs. Page 4 of 18 THE GENETICS OF VARIABLE PAIN SENSITIVITY IN THE GENERAL POPULATION The genetics of variable pain sensitivity are influenced by various factors which have been identified in the general population (Table 1). These include the large number of genes which encode for the structural components of the nociceptive system, enzymes producing or degrading nociceptive transmitters and components that regulate pain pathogenesis. Mu - opioid receptors Opioid receptors provide a natural site of action for endogenous ligands such as endorphin, dynorphin and encephalin. Variable pain sensitivity in the general population has been associated with common variants of the mu opioid receptor gene. A variant, exon 1 of the mu opioid receptor gene (OPRM1, 118A>Grs1799971) has been associated with a 0.8 fold lower pressure pain intensity, especially in females[11]. A 0.5 fold smaller amplitude of pain related cortical potentials following stimulation of specific nasal nociception to carbon dioxide, has also been noted.[12] Another study found lower intrathecal fentanyl dose requirements for labour analgesia in females carrying this variant.[13] Melanocortin 1 receptors (MCR1) Kappa agonists , nalbuphine and pentazocine produce pronounced analgesia in females carrying melanocortin 1 receptor gene variant on chromosome 16q24.3.[14] This variant is known to abolish MC1R functionality and is seen in red hair, fair skin phenotypes. It has been suggested that the activation of the MC1R by endogenous neuromodulators exerts anti -opioid activity. This has been demonstrated by the binding of kappa opioid receptor ligand, dynorphin, to nonmutated MCR1s.The greater effect of Kappa agonists in those carrying this mutation may be attributed to the omission of this anti –opioid activity due to mutated nonfunctional MCR1s. Guanosine triphosphate cyclohydrolase1 (GCH1) A haplotype of the GCH1 gene on chromosome 14q22.1-q22.2 was found to have an allelic frequency of 15.4%. GCH1 is a rate limiting enzyme in the production of tetrahydrobiopterin (BH4) an essential cofactor of nitric oxide synthase. The accumulation of excessive amounts of BH4 in peripheral nerves following axonal injury (increase in calcium influx and nitric oxide production), is associated with the development of neuropathic pain.[15] A decrease in pain has been demonstrated with reduced GCH1 function in defined cohorts after lumbar disc surgery.[16] Transient receptor potential cation (TRPV1) At least 6 polymorphisms in human TRPV1 gene have been discovered. TRPV1 integrates a number of noxious stimuli such as capsaicin, heat, protons and leukotrienes. People with complete insensitivity to capsaicin showed several point mutations in the second intron of their TRPV1 gene. [17] Page 5 of 18 Table 1: Genetic modulations affecting common human pain phenotype [8] GENETIC MODULATION OF DRUGS USED IN PAIN TREATMENT Pharmacokinetics Genetic variations may regulate the uptake, transport, metabolism and excretion of a drug. However, due to the wide therapeutic range of analgesics not all measurable fluctuations in drug concentration have clinical consequences. Therefore, focus must be given to variants that have demonstrated an altered clinical effect. Drug metabolising enzymes There are several enzymes that are responsible for the metabolic clearance of analgesics. It is important to note that if the compound concerned is an active principle drug then decreased metabolism should result in a greater clinical effect due to slower systemic elimination. On the other hand, if the compound in question is a pro-drug then the opposite is true due to the decreased production of its active metabolite. Page 6 of 18 Pro-drug activation A pro-drug is a substance that is administered in an almost inactive from and requires metabolic activation before its clinical effects are recognised. Codeine, tilidine, parecoxib, metamizole, and tramadol(less typical) are all examples of pro-drugs. Codeine is extensively metabolised by the cytochrome P450 isoenzyme 2D6. After the ingestion of 30mg of codeine, 81% was transformed into codeine 6 glucuronide, 2.2% into norcodeine, 0.6% transformed into free morphine, 2.1% into morphine 3 glucuronide, 0.8% into morphine 6 glucuronide and 2.4% transformed into normorphine.[18] Of interest only 6% of codeine is transformed into morphine and its glucuronides. Codeines clinical effect is determined by its conversion to morphine as morphine has a 200 fold higher affinity and 50 fold higher intrinsic activity at the mu receptor compared to codeine.[19] As the CYP2D6 system which metabolizes codeine is genetically polymorphic, the effects of codeine are under pharmacogenetic infleunce.[20] Genetically determined codeine effects may result from absent, decreased or highly increased CYP2D6 activity when compared with the population average. The decreased effects of codeine can be seen in 7-11% of the Caucasian population as they display a poor metabolizer phenotype (PM) as very low levels or no morphine is formed after the administration of codeine. [21] On the other hand 7% of the Caucasian population has an extremely active CYP2D6 ultrafast metabolizer phenotype (UM) with very high levels of morphine being formed.[22] Therefore, one in seven Caucasians are at risk of treatment failure or toxicity from codeine.[23] Tramadol a mu receptor agonist has a lower receptor affinity than its metabolite odesmethyltramadol. However, tramadol is not without effect should the CYP2D6 display a poor metabolizer phenotype.[24] This is because non-opioid dependent mechanisms involve serotonin and noradrenaline pain inhibition in the brain stem. Tilidine is metabolized to nortilidine and paraecoxib to valdecoxib via CYP3A. This enzyme system is genetically polymorphic. Individuals with at least one CYP3A5*1 allelele copy produce high levels of full CYP3A5 MRnA and display active CYP3A5 functionality.[25] 95% of the Caucasian population have no active CYP3A5 due to a premature stop cordon.[26] Positive associations between prodrug activation and analgesic activity have not been reported. Drug inactivation Morphine, codeine and buprenorphine undergo glucuronidation which is mediated by UDP glucuronosyl transferase (UGT)2B7.[27] A couple of genetic polymorphisms have been discovered. In the case of morphine, the main metabolites are morphine 3 glucuronide and morphine 6 glucuronide. Both metabolites are active, but seem to display opposite effects. Page 7 of 18 Morphine 3 glucuronide has an anti-analgesic and excitatory effect when compared to the opioid agonistic effects of morphine 6 glucuronide. Functional polymorphisms at the UGT2B7 gene have been associated with a change in plasma concentration ratios of opioids and their glucuronide metabolites[28] The Tyr268 UGT2B7 glucuronidates buprenorphine at a 10 fold higher rate in vitro than the His268 isoform.[28] In vivo variants of the 5 untranslated region of UGT2B7 are associated with reduced morphine 6 glucuronide to morphine ratio.[29] UGT variants are restricted to alterations in plasma concentrations and none of the variants alone have been associated with altered analgesic effect. Increased enzyme activity associated with the CYP3A581 allele may cause the accelerated elimination of CYP3A substrates.[30] These include alfentanil, fentanyl and sufentanil. Positive associations between CYP3A polymorphisms and analgesic activity have not been reported. Transmembrane transporters P – glycoprotein ( P-gp) a transmembrane transporter is located in organs that have an excretory function like the liver, kidney and gastrointestinal tract. At the blood brain barrier it forms an outward tract. P-gp is coded by the ABCB1 gene.[31] Functional impairment of P-gp may lead to the increased bioavailability of orally administered drugs, decreased renal clearance and increased brain concentrations. One would therefore expect an increased clinical effect and decreased dose requirements for P-gp substrates if its functionality were impaired. Methadone dosing in heroin substitution may be decreased in carriers ABCB1 variant.[32] Loperamide an opioid used for its antidiarrheal properties does not produce high CNS concentrations because of its poor absorption and rapid elimination. However, in the ABCB1 343577 variant it produced significant CNS opioid effects due to the inhibition of P-pg.[33] Those individuals that carry the ABCB1 345C>T variant are at increased risk of fentanyl associated respiratory depression. [34] Pharmacodynamics Genetic factors may decrease the effects of analgesics. They can result in inadequate receptor binding, activation and signalling. Opioid receptors OPRM1 gene is highly polymorphic and so far 1799 Human SNPs have been identified.[35] Of consequence, genetic mutations that code for the 3 rd intracellular loop of the mu receptor have resulted in decreased G protein coupling, signalling and desensitization. OPRM1 variant 118A>G SNP is present in >5% of the population. Carriers of this variant display decreased clinical effects to various opioids in experimental and clinical models.[36] Page 8 of 18 G protein coupled receptor kinase (GRK) and G protein coupled inwardly rectifying potassium channels (GIRK) are involved in intracellular opioid receptor, signalling cascade. GIRKs are the primary post synaptic effectors of opioids in the CNS while GRK are involved with opioid receptor desensitization. GRK2,3,6 have shown to modulate opioid tolerance[37].GRK2 and 6 may play a role in inflammation and allodynia.[38] GIRK1 and GIRK2 are involved in opioid induced analgesia.[39] GIRK3 are modulators of pain sensitivity.[40] GIRK and GRK are currently being examined as candidates in experimental models for the genetic modulation of pain and opioid effect. Catechol-o-methyl transferase As mentioned earlier, COMT degrades catecholamine neurotransmitters. Reduced COMT functionality increases dopamine concentrations that suppress the production of endogenous opioid peptides.[41] As a result, opioid receptors are up regulated. This has been demonstrated with the V158M variant of COMT. Patients with cancer pain carrying the variant V158M needed less morphine than those that did not.[42] Cyclooxygenases Prostaglandin endoperoxide synthase 2 gene (PTGS2) codes for cycloxygenase 2 (COX2). COX2 modulates inflammation and provides a target site for antiinflammatory drugs especially COX2 inhibitors. PTGS2-765G>C SNP resulted in a twofold decrease in COX2 expression[43]. Not surprisingly carriers of the -765G variant lacked the analgesic effects of Rofecoxib.[44]. Co analgesics Co analgesics are often used in the treatment of chronic pain. Several antidepressants used in the treatment of chronic neuropathic pain are substrates of polymorphic CYP2D6 or CYP2C19.[45] It terms of their pharmacodynamics, antidepressant target structures, are norephinephrine (SLC6A2) and serotonin(SLC6A4) transporters.[46] These have been found to display genetic variability. This has led to a specific genotype dependant dosage recommendation. Unfortunately, these are based on psychiatric response rather than analgesic. Page 9 of 18 PHARMACOGENETICS OF DRUG INTERACTIONS IN PAIN THERAPY There are three main types of enzyme interactions. A substrate (Table 2) is any drug that is metabolized by an enzyme. An inducer (Table 3) is a drug that accelerates the metabolism of another. This results in the increased breakdown and clearance as well as a decreased duration of action. However, if the drug is a pro-drug, accelerated metabolism would lead to an increase in activity. An inhibitor (Table 4) is a medication that reduces the metabolism of another drug. This may result in excessively high concentrations, increased effect and toxicity. If the drug in question is a pro-drug there may be a decreased effect as a result of its inhibition. The incidence of potential drug interactions increase with the number of medications being used by an individual. There are 7 clinically relevant enzymes responsible for the metabolism of commonly used drugs. These include CYP1A2, CYP2C8, CYP2C9,CYP2C19, CYP2D6,CYP2E1 and CYP3A4. St. John’s wort, used in the treatment of depression induces CYP1A2, 2C9 and 3A4. The induction of CYP2C9 and CYP1A2 leads to the accelerated metabolism of warfarin with decreased blood concentration and a greater risk of clot development.[47] Smoking is a potent inducer of CYP1A2 resulting in decreased caffeine levels. This may explain the increased agitation experienced when smoking stops and caffeine levels begin to rise.[47] Many of the drugs commonly used are substrates, inhibitors or inducers of medication used in pain therapy. Due to genetic variability many enzymes lose their compensatory capacity when additional drugs are added which target the same pharmacological system. For example the effects of loperamide on the CNS are increased by the administration of quinidine a P-gp blocker in carriers of the P-gp gene variant ABCB1 3435C>T. [55] Non functional variants of CYP2C9 may increase the plasma concentration of COX inhibitors like ibuprofen, diclofenac or celecoxib. As these drugs have a broad therapeutic range, changes in clinical effect are unlikely unless warfarin also a CYP2C9 substrate is added, then the risk of bleeding may increase exponentially.[48] Page 10 of 18 Table 2: Common substrates of CYP enzymes [50] Table 3: Common Inducers of the CYP enzymes [50] Page 11 of 18 Table 4: Common inhibitors of CYP enzymes [50] GENETIC TESTING Genetic polymorphism may cause absent or reduced enzyme activity, resulting in drugs being metabolised slowly. These individuals are at increased risk of side effects or treatment failure. Knowledge of an individual’s polymorphism could aid in selecting the appropriate agent, at a safe dosage. Patients can be classified according to how effectively they metabolize a drug. This depends on the number of normal alleles they inherit. An extensive metabolizer (EM) has two normal alleles where as an intermediate metabolizer (IM) has 1 normal and 1 reduced allele or 2 partially deficient alleles. A poor metabolizer (PM) has 2 mutant alleles leading to a limited or loss of activity. Finally, Ultra rapid metabolizer (UM) may have multiple copies of functional alleles leading to excessive activity. There appears to be an ethnic distribution of polymorphism among metabolizes, as 7 -10% of whites are PMs, with a CYP2D6 deficiency compared to 1-2 % of Asians and 2-4% of blacks. Approximately 30% of Asians and blacks are IMs of CYP2D6. Approximately 10% of Southern Europeans and 1-2% of Northern Europeans are UMs.[51] Page 12 of 18 Genotype based dose adjustment would offer a standard dose, for example of 2 tablets for an EM where as a PM could be offered a single tablet. An IM could need 1.5 tablets and a UM three or more tablets to achieve the same clinical effect.[52] Quantitative urine drug screening may offer clues as to the genetic make-up of an individual. If there is poor conversion of hydrocodone to hydromorphone or poor conversion of oxycodone to oxymorphone, this would suggest a CYP2D6 inhibition or deficiency. If this patient were to complain of poor analgesia then changing to hydromorphone or oxymorphone would bypass the CYP2D6 system and improve analgesic response. DNA testing has become economically feasible with several SNPs available, these include CYP enzymes, 2D6, 2C9, 2C19 and 3A4. Testing can be used to determine or explain ineffective or high opioids use. Patients with a CYP2D6 deficiency would be expected to display a poor response to tramadol, codeine, hydrocodone and oxycodone. On the other hand, patients with an UM, CYP2D6 enzyme may be at risk of developing high levels of morphine from codeine metabolism. By changing to an opioid not metabolised by that particular enzyme a more effective and better side effect profile may be achieved. As mentioned previously, patients with an inactive OPRM1 allele displayed a poor response to the mu agonist morphine and my benefit from switching to a K agonist like buprenorphine. Page 13 of 18 CONCLUSION Genetic research has improved our understanding of the molecular pathways in nociception and analgesia. Unfortunately, the multi genetic heredity of pain in our population coupled with the small effect of each variant, remains a particularly challenging target for genetic based personalized therapy. The impact of pharmacogenetics for pain therapy may be further reduced by additional factors, such as disease, environment and the concurrent use of medication. However, there are some practical uses for genetic testing. Our current pharmacogenetic knowledge of the CYP2D6, MC1R and PTGS2 may guide our choice for the correct analgesic drug. Currently, our patients are given a trial and error based analgesic challenge. 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