Neuropathic pain: strategies in drug discovery and treatment

Investigation of the predictive validity of laser-EPs in normal, UVB-inflamed and capsaicin-irritated skin with four analgesic compounds in healthy volunteers

AIMS

The aim of the present study was to assess the predictivity of laser-(radiant-heat)-evoked potentials (LEPs) from the vertex electroencephalogram, using an algesimetric procedure, testing the anti-nociceptive/anti-hyperalgesic effects of single oral doses of four marketed analgesics (of different compound classes) vs. placebo, in healthy volunteers with three skin types.

METHODS

This was a randomized, placebo-controlled, single-blind, five-way-crossover trial. Twenty-five healthy male/female Caucasians were included (receiving celecoxib 200 mg, pregabalin 150 mg, duloxetine 60 mg, lacosamide 100 mg or placebo) in a Williams design, with CO₂ laser-induced painful stimuli to normal, ultraviolet (UV) B-inflamed and capsaicin-irritated skin. LEPs and visual analogue scale ratings were taken at baseline and hourly for 6 h postdose from all three skin types.

RESULTS

In normal skin, the averaged postdose LEP peak-to-peak-(PtP)-amplitudes were reduced by pregabalin (-2.68 μV; 95% confidence interval (CI) -4.16, 1.19) and duloxetine (-1.73 μV; 95% CI -3.21, -0.26) but not by lacosamide and celecoxib vs. placebo. On UVB-irradiated skin, reflecting inflammatory pain, celecoxib induced a pronounced reduction in LEP PtP amplitudes vs. placebo (-6.2 μV; 95% CI -7.88, -4.51), with a smaller reduction by duloxetine (-4.54 μV; 95% CI -6.21, -2.87) and pregabalin (-3.72 μV; 95% CI -5.40, -2.04), whereas lacosamide was inactive. LEP PtP amplitudes on capsaicin-irritated skin, reflecting peripheral/spinal sensitization, as in neuropathic pain, were reduced by pregabalin (-3.78 μV; 95% CI -5.31, -2.25) and duloxetine (-2.32 μV; 95% CI -3.82, -0.82) but not by celecoxib or lacosamide vs. placebo, which was in agreement with known clinical profiles. Overall, PtP amplitude reductions were in agreement with subjective ratings.

CONCLUSIONS

LEP algesimetry is sensitive to analgesics with different modes of action and may enable the effects of novel analgesics to be assessed during early clinical development.

Restorative effect and mechanism of mecobalamin on sciatic nerve crush injury in mice

Mecobalamin, a form of vitamin B₁₂ containing a central metal element (cobalt), is one of the most important mediators of nervous system function. In the clinic, it is often used to accelerate recovery of peripheral nerves, but its molecular mechanism remains unclear. In the present study, we performed sciatic nerve crush injury in mice, followed by daily intraperitoneal administration of mecobalamin (65 μg/kg or 130 μg/kg) or saline (negative control). Walking track analysis, histomorphological examination, and quantitative real-time PCR showed that mecobalamin significantly improved functional recovery of the sciatic nerve, thickened the myelin sheath in myelinated nerve fibers, and increased the cross-sectional area of target muscle cells. Furthermore, mecobalamin upregulated mRNA expression of growth associated protein 43 in nerve tissue ipsilateral to the injury, and of neurotrophic factors (nerve growth factor, brain-derived nerve growth factor and ciliary neurotrophic factor) in the L_4–6 dorsal root ganglia. Our findings indicate that the molecular mechanism underlying the therapeutic effect of mecobalamin after sciatic nerve injury involves the upregulation of multiple neurotrophic factor genes.

Synthesis and anticonvulsant activity of ethyl 2,2-dimethyl-1-(2-substitutedhydrazinecarboxamido) cyclopropanecarboxylate derivatives

In this study on the development of new anticonvulsants, fourteen ethyl 2,2-dimethyl-1-(2-substitutedhydrazinecarboxamido) cyclopropanecarboxylate derivatives were synthesized and tested for anticonvulsant activity using the maximal electroshock, subcutaneous pentylenetetrazole screens, which are the most widely employed seizure models for early identification of candidate anticonvulsants. Their neurotoxicity was determined applying the rotorod test. Two compounds 6f and 6k showed promising anticonvulsant activities in both models employed for anticonvulsant evaluation. The most active compound 6k showed the maximal electroshock-induced seizures with ED50 value of 9.2 mg/kg and TD50 value of 387.5 mg/kg after intraperitoneally injection to mice, which provided compound 6k with a protective index (TD50/ED50 ) of 42.1 in the maximal electroshock test.

Synthesis and anticonvulsant activity evaluation of 8-alkoxy-5-(4H-1,2,4-triazol-4-yl)quinoline derivatives

Two series of 8-alkoxy-5-(4H-1,2,4-triazol-4-yl)quinolines and 8-alkoxy-5-(2H-1,2,4-triazol-3-one-4-yl)quinolines were synthesized. The anticonvulsant activity of these compounds was evaluated with maximal electroshock seizure test and rotarod test. Among the synthesized compounds, 8-octoxy-5-(4H-1,2,4-triazol-4-yl)quinoline (4g) was the most active compound with ED(50) of 8.80 mg/kg, TD(50) of 176.03 mg/kg and protective index of 20.0. Its neurotoxicity was lower than all other synthesized compounds and also markedly lower than that of the reference drug carbamazepine. In addition, the potency of compound 4g against seizures induced by pentylenetetrazole, 3-mercaptopropionic acid, and bicuculline suggested its broad spectrum activity, and the mechanisms of action including inhibition of voltage-gated ion channels and modulation of GABAergic activity might involve in its anticonvulsant activity.

Discovery of Fused Triazolo-thiadiazoles as Inhibitors of TNF-alpha: Pharmacophore Hybridization for Treatment of Neuropathic Pain

Introduction

Neuropathic pain is a complex, chronic pain state that is usually accompanied by tissue injury. With neuropathic pain, the nerve fibers themselves may be damaged, dysfunctional, or injured.

Methods

A series of pharmacophoric hybrids of substituted aryl semicarbazides incorporated into a fused triazolo-thiadiazole nucleus were synthesized and evaluated for neuropathic pain activity. After the assessment of neurotoxicity and peripheral analgesic activity, the compounds were evaluated in two peripheral neuropathic pain models, the chronic constriction injury and partial sciatic nerve ligation, to assess their antiallodynic and antihyperalgesic potential.

Results

Selected compounds exhibiting promising efficacies (4b, 6a, and 7e) revealed median effective dose (ED₅₀) values ranging from 7.62–28.71 mg/kg in four behavioral assays of allodynia and hyperalgesia (spontaneous pain, tactile allodynia, cold allodynia, and mechanical hyperalgesia). Studies carried out to assess the underlying mechanism revealed that compounds suppressed the inflammatory component of the neuropathic pain by inhibiting tumor necrosis factor (TNF)-alpha and preventing oxidative and nitrosative stress.

Conclusion

Using a hybrid design approach, the present study identified novel chemical compounds that could be a potential lead for the treatment of neuropathic pain.

Current approaches with the glutamatergic system as targets in the treatment of neuropathic pain

Glutamate is the most widely distributed and a major excitatory neurotransmitter in the CNS. It has been found to play a critical role in various physiological functions in which increased glutamate or its subsequent stimulation is thought to have a role in pathophysiological mechanism of various CNS diseases like epilepsy, stroke, depression and pain. Early attempts to develop glutamatergic antagonists failed in clinical studies due to nonselective or competitive antagonism and have a lot of safety issues like loss of cognitive functions, psychomimetic effect and sedation. Neuropathic pain can be described as pain associated with damage or permanent alteration of the peripheral or central nervous system. At present, there are very few effective therapies for neuropathic pain. The current approach includes targeting specific or alternate binding sites of glutamate receptors, resulting in reduced CNS liabilities. Targeting the glutamatergic system shows a better efficacy and fewer side effects, compared with classical drugs for the treatment of neuropathic pain. This review discusses the various targets on glutamatergic system, which includes the receptors, transporters and enzymes, for the treatment of neuropathic pain and their advantages over classical glutamatergic antagonists. The review also highlights the newer drugs in clinical trials for neuropathic pain.

Drugability of extracellular targets: discovery of small molecule drugs targeting allosteric, functional, and subunit-selective sites on GPCRs and ion channels

Beginning with the discovery of the structure of deoxyribose nucleic acid in 1953, by James Watson and Francis Crick, the sequencing of the entire human genome some 50 years later, has begun to quantify the classes and types of proteins that may have relevance to human disease with the promise of rapidly identifying compounds that can modulate these proteins so as to have a beneficial and therapeutic outcome. This so called 'drugable space' involves a variety of membrane-bound proteins including the superfamily of G-protein-coupled receptors (GPCRs), ion channels, and transporters among others. The recent number of novel therapeutics targeting membrane-bound extracellular proteins that have reached the market in the past 20 years however pales in magnitude when compared, during the same timeframe, to the advancements made in the technologies available to aid in the discovery of these novel therapeutics. This review will consider select examples of extracellular drugable targets and focus on the GPCRs and ion channels highlighting the corticotropin releasing factor (CRF) type 1 and gamma-aminobutyric acid receptors, and the Ca(V)2.2 voltage-gated ion channel. These examples will elaborate current technological advancements in drug discovery and provide a prospective framework for future drug development.