Nav1.7 and other voltage-gated sodium channels as drug targets for pain relief

INTRODUCTION: Chronic pain is a massive clinical problem. We discuss the potential of subtype selective sodium channel blockers that may provide analgesia with limited side effects. AREAS COVERED: Sodium channel subtypes have been linked to human pain syndromes through genetic studies. Gain of function mutations in Nav1.7, 1.8 and 1.9 can cause pain, whilst loss of function Nav1.7 mutations lead to loss of pain in otherwise normal people. Intriguingly, both human and mouse Nav1.7 null mutants have increased opioid drive, because naloxone, an opioid antagonist, can reverse the analgesia associated with the loss of Nav1.7 expression. EXPERT OPINION: We believe there is a great future for sodium channel antagonists, particularly Nav1.7 antagonists in treating most pain syndromes. This review deals with recent attempts to develop specific sodium channel blockers, the mechanisms that underpin the Nav1.7 null pain-free phenotype and new routes to analgesia using, for example, gene therapy or combination therapy with subtype specific sodium channel blockers and opioids. The use of selective Nav1.7 antagonists together with either enkephalinase inhibitors or low dose opioids has the potential for side effect-free analgesia, as well as an important opioid sparing function that may be clinically very significant

Gaining on Pain

The work of Salvemini and coworkers (Stockstill et al.) is notable in unraveling the mechanisms that contribute to the neuropathic pain caused by bortezomib, an anticancer medication.

In vivo characterization of distinct modality-specific subsets of somatosensory neurons using GCaMP

Mechanistic insights into pain pathways are essential for a rational approach to treating this vast and increasing clinical problem. Sensory neurons that respond to tissue damage (nociceptors) may evoke pain sensations and are typically classified on the basis of action potential velocity. Electrophysiological studies have suggested that most of the C-fiber nociceptors are polymodal, responding to a variety of insults. In contrast, gene deletion studies in the sensory neurons of transgenic mice have frequently resulted in modality-specific deficits. We have used an in vivo imaging approach using the genetically encoded fluorescent calcium indicator GCaMP to study the activity of dorsal root ganglion sensory neurons in live animals challenged with painful stimuli. Using this approach, we can visualize spatially distinct neuronal responses and find that >85% of responsive dorsal root ganglion neurons are modality-specific, responding to either noxious mechanical, cold, or heat stimuli. These observations are mirrored in behavioral studies of transgenic mice. For example, deleting sodium channel Nav1.8 silences mechanical- but not heat-sensing sensory neurons, consistent with behavioral deficits. In contrast, primary cultures of axotomized sensory neurons show high levels of polymodality. After intraplantar treatment with prostaglandin E2, neurons in vivo respond more intensely to noxious thermal and mechanical stimuli, and additional neurons (silent nociceptors) are unmasked. Together, these studies define polymodality as an infrequent feature of nociceptive neurons in normal animals

Silent cold-sensing neurons contribute to cold allodynia in neuropathic pain.

Neuropathic pain patients often experience innocuous cooling as excruciating pain. The cell and molecular basis of this cold allodynia is little understood. We used in vivo calcium imaging of sensory ganglia to investigate how the activity of peripheral cold-sensing neurons was altered in three mouse models of neuropathic pain: Oxaliplatin-induced neuropathy, partial sciatic nerve ligation and ciguatera poisoning. In control mice, cold-sensing neurons were few in number and small in size. In neuropathic animals with cold allodynia, a set of normally silent large-diameter neurons became sensitive to cooling. Many of these silent cold-sensing neurons responded to noxious mechanical stimuli and expressed the nociceptor markers NaV1.8 and CGRPα. Ablating neurons expressing NaV1.8 resulted in diminished cold allodynia. The silent cold-sensing neurons could also be activated by cooling in control mice through blockade of KV1 voltage-gated potassium channels. Thus silent cold-sensing neurons are unmasked in diverse neuropathic pain states and cold allodynia results from peripheral sensitization caused by altered nociceptor excitability

A central mechanism of analgesia in mice and humans lacking the sodium channel NaV1.7

Deletion of SCN9A encoding the voltage-gated sodium channel NaV1.7 in humans leads to profound pain insensitivity and anosmia. Conditional deletion of NaV1.7 in sensory neurons of mice also abolishes pain, suggesting that the locus of analgesia is the nociceptor. Here we demonstrate, using in vivo calcium imaging and extracellular recording, that NaV1.7 knockout mice have essentially normal nociceptor activity. However, synaptic transmission from nociceptor central terminals in the spinal cord is greatly reduced by an opioid-dependent mechanism. Analgesia is also reversed substantially by central but not peripheral application of opioid antagonists. In contrast, the lack of neurotransmitter release from olfactory sensory neurons is opioid independent. Male and female humans with NaV1.7-null mutations show naloxone-reversible analgesia. Thus, inhibition of neurotransmitter release is the principal mechanism of anosmia and analgesia in mouse and human Nav1.7-null mutants

Certolizumab Pegol Remains an Equally Efficacious Treatment of Rheumatoid Arthritis Over a Range of Background Methotrexate Regimens.

Objective. Anti-tumor necrosis factor agents (anti-TNFs) are frequently used in combination with methotrexate to treat rheumatoid arthritis (RA). We investigated the effect of background methotrexate dose, in combination with anti-TNF certolizumab pegol (CZP), on treatment efficacy and safety in RA patients. Methods. A pre-specified subgroup analysis comparing two methotrexate dose categories (10 and ≤15 mg/week; and >15 mg/week. Results. 638, 635 and 325 patients received CZP 200 mg, CZP 400 mg and placebo, respectively. At Week 24, treatment responses in both CZP groups were uninfluenced by baseline methotrexate dose category, and were superior to placebo group, for all investigated endpoints: ACR20/50/70, DAS28(ESR) and mTSS. TEAE incidence rates were higher in patients receiving methotrexate ≥15 mg/week for most TEAE types, across treatment groups. Conclusion. CZP efficacy was not affected by background methotrexate dose category. It can be hypothesized that to minimize TEAEs, background methotrexate doses coul

Adalimumab reduces hand bone loss in rheumatoid arthritis independent of clinical response: Subanalysis of the PREMIER study

<p>Abstract</p> <p>Background</p> <p>Anti-TNF therapy has been shown to reduce radiographic joint damage in rheumatoid arthritis (RA) independent of clinical response. This has previously not been examined for periarticular bone loss, the other characteristic feature of bone involvement in RA.</p> <p>The objective of this study was to examine if treatment with the TNF-α inhibitor adalimumab also could reduce periarticular bone loss in RA patients independent of disease activity.</p> <p>Methods</p> <p>RA patients were recruited from the PREMIER study and included 214 patients treated with methotrexate (MTX) plus adalimumab and 188 patients treated with MTX monotherapy. Periarticular bone loss was assessed by digital X-ray radiogrammetry metacarpal cortical index (DXR-MCI). Change in DXR-MCI was evaluated in patients with different levels of clinical response, as assessed by changes in DAS28 score at 52 weeks and in mean C-reactive protein (CRP) levels during follow-up.</p> <p>Results</p> <p>In the MTX group, there was a greater median DXR-MCI loss among patients with moderate and high disease activity compared to those in remission or with low disease activity (-3.3% vs. -2.2%, p = 0.01). In contrast, periarticular bone loss was independent of disease activity (-1.9% vs. -2.4%, p = 0.99) in the combination group. In the MTX group patients with a mean CRP of ≥ 10 mg/l lost significantly more DXR-MCI than patients with low CRP (-3.1% vs. -1.9%, p <0.01) whereas in the combination group no significant differences between the two CRP groups was seen (-2.4% vs. -2.0%, p = 0.48).</p> <p>Conclusion</p> <p>Adalimumab in combination with MTX reduces periarticular bone loss independently of clinical response. These results support the hypothesis that TNF-α stimulates the osteoclast not only by the inflammatory pathway but do also have a direct effect on the osteoclast.</p> <p>Trial Registration</p> <p>ClinicalTrials (NCT): <a href="http://www.clinicaltrials.gov/ct2/show/NCT001195663">NCT001195663</a></p

Identifying future models for delivering genetic services: a nominal group study in primary care

BACKGROUND: To enable primary care medical practitioners to generate a range of possible service delivery models for genetic counselling services and critically assess their suitability. METHODS: Modified nominal group technique using in primary care professional development workshops. RESULTS: 37 general practitioners in Wales, United Kingdom too part in the nominal group process. The practitioners who attended did not believe current systems were sufficient to meet anticipated demand for genetic services. A wide range of different service models was proposed, although no single option emerged as a clear preference. No argument was put forward for genetic assessment and counselling being central to family practice, neither was there a voice for the view that the family doctor should become skilled at advising patients about predictive genetic testing and be able to counsel patients about the wider implications of genetic testing for patients and their family members, even for areas such as common cancers. Nevertheless, all the preferred models put a high priority on providing the service in the community, and often co-located in primary care, by clinicians who had developed expertise. CONCLUSION: There is a need for a wider debate about how healthcare systems address individual concerns about genetic concerns and risk, especially given the increasing commercial marketing of genetic tests

Achieving comprehensive disease control in patients with early and established rheumatoid arthritis treated with adalimumab plus methotrexate versus methotrexate alone

Objective: To evaluate the achievement of comprehensive disease control (CDC) following 1 year of treatment with adalimumab+methotrexate versus methotrexate alone and whether early achievement of remission (at week 24 or 26) is associated with CDC at week 52 in patients with either early or established rheumatoid arthritis (RA). Methods: Post hoc analyses were conducted in three clinical studies assessing treatment with adalimumab+methotrexate: DE019 (NCT00195702) enrolled patients with established RA who were methotrexate inadequate responders; OPTIMA (NCT00420927) and PREMIER (NCT00195663) enrolled methotrexate-naive patients with early RA. In OPTIMA, patients not achieving stable low disease activity at weeks 22 and 26 in the placebo+methotrexate group could receive open-label adalimumab+methotrexate for 52 weeks (Rescue ADA arm). CDC was defined as the simultaneous achievement of clinical remission (DAS28(CRP)<2.6), normal function (HAQ-DI<0.5) and absence of radiographic progression (ΔmTSS≤0.5). Results: Regardless of disease duration, significantly more patients receiving adalimumab+methotrexate achieved CDC compared with methotrexate alone. In the adalimumab+methotrexate group, a numerically greater proportion of patients with early RA (~25%) versus established RA (14%) achieved CDC at 1 year; achievement of CDC was notably greater among patients who met criteria for remission at week 24 or 26 (~50% of patients with early RA and 39% with established RA). Conclusion: Treatment with adalimumab+methotrexate increases the likelihood of achieving CDC in patients with either early or established RA. Clinical remission at week 24 or 26 is associated with achievement of CDC at week 52