Due to the unique trimeric structure shared between the TNF ligand and the TNF receptor (both belonging to the TNF/TNFR SPF), the transmembrane portion of TNF molecule (mTNF), besides being a ligand, is capable of acting as a receptor for a soluble form of TNF (sTNF) in a "reverse-signaling" manner [
169], which then inhibits phosphorylation of p38 and hence expression of TNF protein. This unique phenomenon makes it possible to use gene therapy with a herpes simplex virus vector carrying a p55 sTNFR gene to transfect DRG neurons of rats [
170]. As a result, over-expressed p55 sTNFR (sTNFR2) binds to the mTNF of DRG and down-regulates overall production of TNF by reverse signaling, significantly reducing the allodynia and hyperalgesia responses to CCI [
170,
171]. Following a similar logic, a fusion protein (ELP-sTNFR2) has been developed wherein a soluble form of TNFR2 (sTNFR2) is conjugated to a temperature-sensitive elastin-like polypeptide (ELP), which can be thermally triggered to form a deposit around the peri-neural site of injection [
172]. This fusion protein has been reported to be able to mitigate levels of TNF-α in DRG of injured nerve in rat models [
173]. Indeed, many studies have demonstrated that local or spinal administration of agents that antagonize TNF-α will attenuate pain behaviors in neuropathic animal models [
174‐
177]. Mechanical allodynia in the rat model of central neuropathic pain due to T13 spinal cord hemisection is attenuated by immediate, but not delayed, intrathecal administration of etanercept (a fusion protein blocker of TNF-α) at 1-4 weeks post spinal cord injury [
178]. Propentofylline is a methylxanthine that inhibits lipopolysaccharide (LPS)-induced release of both TNF-α and IL-1β in a dose-dependent manner in glial cultures [
179] and abates allodynia in rat spinal nerve transection models by modulating glial activation [
180,
181]. Propentofylline was initially evaluated for treating dementia [
182], but was eventually withdrawn from further clinical studies due to patent issues [
183], and its efficacy in animal neuropathic pain models has yet to be tested in humans. Thalidomide, once banned in 1963 due to its teratogenicity, is now regaining favor in neuropathic pain research due to its ability to cross the BBB and its inhibitory effects on TNF-α (in vitro and in-vivo) and on IL-1/IL-6 (in-vitro only) [
184,
185]. In the rat model of CCI, systemic thalidomide reduces the hyperalgesia response coincident with reductions in TNF-α levels, unchanged levels of IL-1/IL-6 and increased levels of IL-10 [
186,
187]. Clinically, there have been sporadic reports of success in using thalidomide to treat complex regional pain syndromes [
188]. However, the balance of thalidomide's efficacy versus safety in treating in chronic and neuropathic pain needs further clinical study [
189], especially in view of its paradoxical neurotoxicity [
189,
190]. Methotrexate is a well-known drug for treating cancer that is derived from glutamic acid. It is capable of crossing the BBB [
191] and has anti-rheumatoid and anti-inflammatory actions through its inhibition of production of TNF-α via adenosine nucleotides [
192,
193] and its ability to antagonize the actions of IL-1 [
194]. Intrathecal administration of methotrexate reduces classic CCI-induced allodynia in rats [
195] but its value in treating neuropathic pain is severely offset by its propensity
per se to induce astrocytic proliferation [
196] and hence neurotoxicity [
197,
198].
The role of TNF-α in chronic pain seems irrefutable in view of abundant data from various neuropathic animal models, and with the actual isolation of TNF-α from neuropathic nerves [
85] and perineural fat from radiculopathic nerve roots [
199] in humans. An initial pilot study using subcutaneous etanercept to treat patients admitted to the hospital with acute severe sciatica showed improved pain scores [
200]. Similarly, an open-label study with infliximab (an antibody to TNF-α) revealed promising results [
201]. Subsequent randomized controlled trials failed to support the benefits of systemic anti-TNF-α treatment [
202‐
205], but a recent report did show positive benefits of epidurally administered etanercept in the treatment of sciatica [
206]. To date we are unaware of any randomized controlled clinical trials of infliximab or etanercept in treating other types of neuropathic pain. AV411(ibudilast), a trial drug that was originally developed as a non-selective phosphodiesterase inhibitor for treating bronchial asthma, has been studied in phase I and phase 2a clinical trials in the US and in Australia for treatment of diabetic neuropathic pain [
207], based on findings that AV411 also suppresses glial cell activation and reduces the production of pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) in rat neuropathic pain model [
208].