Rising to the Pain Challenge: the 5Th Congress of the European Federation of International

Rising to the Pain Challenge: the 5th Congress of the European Federation of International Association for the Study of Pain (IASP) Chapters

September 13-16, 2006
Istanbul, Turkey

· Summary

· Introduction

· New Targets and Mechanisms in Pain

· Chemokines

· Gene Microarrays and the BH4 Synthetic Pathway

· References

Summary

The 5th Congress of the European Federation of the Association for the Study of Pain (IASP) Chapters 2006 took place in Istanbul, Turkey, from September 13 to 16, 2006. This meeting brought together international members from different disciplines, all interested in improving the understanding of pain mechanisms, and the management of acute and chronic pain. This report will focus on some of the highlights from this meeting.

Introduction

The European Federation of the Association for the Study of Pain (IASP) Chapters held its fifth meeting in September 2006 in Istanbul, Turkey. Bringing together international members from different disciplines, the meeting sought to improve the understanding of pain mechanisms, and held presentations on the management of acute and chronic pain.

New Targets and Mechanisms in Pain

Homer-1a

In neurons the activity of group I metabotropic glutamate receptors (mGluRs) is controlled by a family of small proteins, the Homer proteins, Homer-1, Homer-2 and Homer-3. Homer-1 and Homer-2 proteins are subfamilies of splice variants. Homer-1 consists of long-form proteins, Homer 1b and 1c, which are expressed constitutively, and a short-form protein, Homer-1a, which in comparison is dynamically regulated in response to synaptic activity. Homer-1a appears to act as a dominant-negative element by inducing constitutive mGluR activity and disrupting the interaction of mGluRs with other Homer proteins. Immunohistochemical studies have demonstrated that Homer-1a is selectively upregulated in the spinal cord of rats following intraplantar administration of Freund’s complete adjuvant (FCA) in rats, a model of chronic inflammatory pain. Recent studies have shown that short-interfering RNA-mediated inhibition of Homer1a in vivo in mice increases inflammatory pain, while targeted Homer1a gene transfer to the spinal cord is capable of reducing inflammatory hyperalgesia. Further investigation revealed that Homer1a attenuates Ca2+ mobilization downstream of mGluR activation in the spinal cord by diminishing the phosphorylation and nuclear translocation of extracellular signal regulated kinases 1 and 2 (Erk1/2). Therefore, Homer1a could represent a novel feedback mechanism in pain signaling plasticity, warranting further study as a therapeutic target for the treatment of chronic inflammatory pain states.1,2

PAR4

Four subtypes of protease-activated receptors (PARs) have been identified to date, PAR1-4. Researchers from Canada have identified a novel neuronal role for PAR4, which is activated by thrombin, trypsin and cathepsin G, with expression and a functional role described in platelets (activation/aggregation), endothelial cells and leukocytes (inflammation, edema) and epithelial cells (cytokine production). Intraplantar application of a selective PAR4 agonist peptide, at a subinflammatory dose (50 µg), in a model of carrageenan paw edema was reported to inhibit associated reductions in paw withdrawal latencies to mechanical stimuli and delay the onset of allodynia. Further studies in isolated dorsal root ganglia (DRG) sensory neurons revealed that PAR4 agonism attenuates neuronal hyperexcitability in response to capsaicin, via inhibition of Ca2+ mobilization. These data suggest that selective PAR4 agonists may be useful therapeutic strategies for further investigation due to their potential analgesic properties.3

Chemokines

Chemokines are a large family of proteins characterized by 4-cysteine motif and are important in the chemotaxis of leukocytes and activation of immune cells. CCL2 (chemokine [C-C motif] ligand 2), formally known as monocyte chemoattractant protein (MCP)-1, is a preferential ligand for the chemokine receptor CCR2, which is expressed on monocytes, dendritic cells and memory T cells. Recent studies have also shown that CCL2 is induced de novo in sensory neurons following sciatic nerve injury. CCL2 was shown to accumulate proximally to the nerve ligation due to axonal anterograde transport, while mRNA expression increased in damaged DRGs, predominantly in small nociceptive neurons of a nonpeptidergic phenotype. Interestingly, CCL2 is not induced by inflammatory states. Further investigation of its role following nerve injury (in this case spinal nerve ligation) indicated that CCL2 is released from central terminals of damaged hyperexcitable nerve fibers. Moreover, intraspinal CCL2 can activate microglia, resulting in the emergence of mechanical hyperalgesia. CCR2 knockout blocks the development of mechanical hyperalgesia following partial nerve injury, and neutralization of CCL2 can partially reverse mechanical hyperalgesia following CCI.4 Therefore, CCL2/CCR2 appears to be a key nociceptive pathway in the spinal cord, recruiting microglia and delivering large amounts of proinflammatory and pronociceptive cytokines.5 Clinical programs involving CCR2 antagonists are ongoing for syndromes such as rheumatoid arthritis, multiple sclerosis and neuropathic pain at Merck & Co., Incyte, Pfizer, Teijin Pharma, Compugen and sanofi-aventis (Table I lists CCR2 antagonists currently under development).

Opioid-containing immune cells migrate preferentially to inflamed sites, where they release â-endorphin and met-enkephalin, which activate peripheral opioid receptors to inhibit pain.6 These cells produce these peptides via pro-opioid melanocortin (POMC), which is processed in the endoplasmic reticulum and the Golgi apparatus to release immature secretory granules.7 Recent studies have described how, in early inflammation, peripheral opioid-mediated antinociception is critically dependent on polymorphonuclear (PMN) cells and their recruitment by CXC chemokine receptor 2 (CXCR2) chemokines.8 Further studies also showed that CXCL8, a member of the CXC chemokine family, induces opioid release from human PMN cells via its ligation with CXCR1/2 chemokine receptors,9 which are predominantly located on granulocytes in the periphery. These observations could be applied clinically for pain therapeutics, by influencing or improving immune cell migration or increasing levels of chemokines that mediate opioid antinociception. Furthermore, enzyme inhibitors that reduce the degradation of opioid peptides could also be considered.10,11

Gene Microarrays and the BH4 Synthetic Pathway

The search is on for novel analgesic targets via the use of gene microarray systems or “gene chips” that sample, as an example, DRG tissue to identify mRNA modulation following nerve injury to identify key molecular components of the pain pathway. Pioneering work at Massachusetts General Hospital and Harvard Medical School is ongoing to compare temporal gene expression profiles across three rat models of neuropathic pain (spared nerve injury, spinal nerve ligation and chronic constriction injury paradigms). Data from these studies have identified a core genetic response to nerve injury, with modulation of approximately 125 genes in the DRG and 55 genes in the dorsal horn. The research group is currently carrying out functional assays to assess the values of these identified genes as pain therapy targets. Dr. Costigan, who is involved in this research line, presented a new candidate identified from this work: the BH4 synthetic pathway. Tetrahydrobiopterin (BH4) is an essential cofactor required for the production of nitric oxide by each of the nitric oxide synthase (NOS) isoforms (for review see12). Synthesis of BH4 involves a multistep process in which guanosine-5'-triphosphate cyclohydrolase I (GTPCH I) is the rate-limiting enzyme required for the initial step in conversion GTP to BH4. Gene chips have identified that the gene for GTPCH I is significantly upregulated following nerve injury, for a period of up to 40 days. This led the research group to hypothesize that GTPCH I may play a downstream role in the activation of NOS enzymes, via augmented BH4 levels, and the subsequent generation of NO that has been shown to sensitize pain pathways.13 Therefore, inhibiting the GTPCH I cascade may prevent painful symptoms associated with nerve injury. Their hypothesis was proven right upon the application of diamino-6-hydroxypyrimidine (DAHP), a selective GTPCH I inhibitor, with a marked reduction in tactile allodynia and heat hyperalgesia following spared nerve injury, an effect, they report, to be the best ever tested in this model. Blocking GTPCH I in naive animals had no effect on normal pain behavior. Thus, the BH4 synthetic pathway represents a promising new target for achieving analgesia. Further work was carried out to assess the human nociceptive profile of the GTPCH I gene in patients with lumbar pain, before and after discectomy. Interestingly, patients possessing a heterozygous version (15.4% of the population) exhibited 50% reduced pain. Additional studies in healthy volunteers revealed that GTPCH I homozygotes display reduced mechanical ischemic pain or heat pain, upon repeated application of a heat stimulus to the hand. Immortalized leukocytes taken from patients and stimulated with forskolin confirmed that those extracted from GTPCH I homozygotes release reduced levels of GTPCH I. These human studies further support the hypothesis that excess levels of GTPCH I and BH4 are important in the generation of pain.14

Cytokine modulation as a therapeutic strategy for pain-an update

Pain and neural hyperexcitability are associated with the activation of glia cells. Previous studies have shown that inhibition of glia cells can block pain symptoms states via a reduction in their release of pro-inflammatory cytokines, interleukin (IL)-1â, IL-6 and tumor necrosis factor (TNF)-á.15 Researchers from the University of Colorado and Avigen are working together to develop a cytokine-based nonviral gene therapy for pain that works on the basis of “cooling down” activated glia following injury or inflammation. AV-333, which delivers the antiinflammatory cytokine IL-10 to glia upon intrathecal administration, has been shown to reverse painful symptoms in every neuropathic pain model tested to date. IL-10 itself has a short half-life in vivo, while AV-333 allows for prolonged release and isolated exposure. In the neuropathic pain model of chronic sciatic nerve constriction in the rat, three injections of AV-333 provide 1 month reversal of associated hypersensitivity, while two doses, given very close together in time, reduce neuropathic pain for up to 3 months.16

The compound AV-411 constitutes another glia targeting strategy for pain management. AV-411 was approved in Japan over 15 years ago (ibudilast), and has been used for bronchial asthma and stroke. It is a njava-scriptive phosphodiesterase (PDE) inhibitor and therefore a glial cell attenuator.17 Pharmacokinetic profiling in rats following intraperitoneal administration revealed an overlapping profile in the plasma, brain and spinal cord, with a half-life of approximately 2.4 hours. With regards to efficacy, in the CCI model it dose-dependently reduces mechanical allodynia, with sustained effects. Further examination upon oral daily dosing indicated efficacy in the Chung model and a paclitaxel chemotherapy-induced neuropathy model. Additional investigations confirmed that AV-411 does not modulate other pain targets, and in vitro studies in microglia primary cultures revealed that this agent attenuates the production of TNFá and MCP-1 (EC50s approx. 4 µM) following stimulation with lipopolysaccharide (LPS). The strong potential for AV-411 as a pain therapy has facilitated the initiation of clinical studies for neuropathic pain (phase I/II) in Adelaide, Australia; and Avigen is currently in talks with research clinics in the Netherlands to investigate this compound in complex regional pain syndrome (CRPS). Avigen plans to file a U.S. Investigational New Drug application (IND) at the beginning of 2007 and is now working on AV-411 analogues with enhanced PDE selectivity.18

A subsequent presentation by Celgene highlighted the clinical data on immune modulation for pain, focusing on the thalidomide story. Despite its bad publicity, thalidomide is emerging more and more in pain pharmacoinvestigation, due to its abolition of TNFá, a pain-producing proinflammatory cytokine released from microglia and immune cells. The company is now exploring the potential of thalidomide analogues, in a bid to sidestep the associated adverse events. These include CC-4047, CC-11060 (which to date have reached phase II and phase I clinical studies, respectively, for other indications; Fig. 1) and lenalidomide (CC-5013, Revlimid®; which is FDA approved for the treatment of patients with transfusion-dependent anemia and in combination with dexamethasone for the treatment of multiple myeloma; it has also reached phase II/III pain trials; Fig. 1).19,20 These small-molecule, orally available compounds inhibit LPS-mediated TNFá production in human peripheral blood mononuclear cells with IC50s of 13 nM, 110 nM and 930 pM, respectively (compared with thalidomide’s IC50 of 194 µM). Lenalidomide also inhibits a broad range of other proinflammatory cytokines and elevates IL-10.

Fig. 1. Celgene's immunomodulatory agents-the thalidomide analogs

There is abundant literature on thalidomide; for example, it has been shown to be effective in complex regional pain syndrome (CRPS) in open label clinical experiments;21 therefore, testing these novel thalidomide analogues may be an interesting new line of study in CRPS. So far, a six-center open label study has assessed the clinical benefits of lenalidomide in CRPS. A total of 40 patients were included at the beginning of the study with an average CRPS duration of 70 months. At a dose of 10 mg/day, nine of these patients discontinued due to adverse events, while 21 elected for study extension; and now that the second year of this study has been reached, 14 patients continue treatment, with 19% pain relief, improvements in sleep, functionality and mood and a significant reduction in touch-evoked allodynia (-24%). Controlled double-blind studies are currently ongoing and nearing closure in chronic radiculopathy and CRPS (n = 180 each cohort). Therefore, future studies would focus on the therapeutic benefits, potency and safety of these new-generation immunomodulatory drugs.22

Overcoming the adverse effects of opioid therapies

Chronic noncancer pain is increasingly being treated with opioids due to their ability to reduce pain intensity.23 However, associated gastrointestinal side effects and patient burden is severely underestimated. In addition to the analgesic benefits of opioid binding in the central nervous system, agonism of µ-opioid receptors within the enteric nervous system decreases gastrointestinal tract motility and secretions, resulting in gastrointestinal side effects. Opioid-induced gastrointestinal side effects are characterized by symptoms ranging from constipation (the incidence of which ranges from 29-90% in the literature with oral and transdermal opioid therapies), straining, abdominal pain and bloating to gastroesophageal reflux, dyspepsia, nausea, decreased appetite and weight loss. Laxative treatment, a common choice to combat these side effects, is not optimal and decreases quality of life. Thus, there appears to be a painful “trade-off” or “vicious circle” with regards to opioid use (Fig. 2).

Fig. 2. The painful "trade-off" or "vicious circle" of opioid use. GI, gastrointestinal.

The PROBE I Survey conducted by GlaxoSmithKline, which involved a total of 502 subjects from the European Union and 184 from the United States revealed that the vast majority of side effects related to opioid use were moderate or severe, having a severe impact on people’s lives. Interestingly this survey demonstrated that approximately one-third of patients modify their opioid intake in response to gastrointestinal side effects, as a result 92% suffer an increase in their pain. Therefore, opioids are effective for nonmalignant and malignant pain but at the moment are limited by their effects on the gut. Previous reports have suggested efficacy for the peripherally acting µ-opioid receptor (PAM-OR) naloxone; however, naloxone’s peripheral selectivity is questionable, as reduced analgesia and respiratory depression can take the place of gastrointestinal side effects.