A single intrathecal dose of adenosine 2A receptor (A2AR) agonist was previously reported to produce a multi-week reversal of allodynia in two different models of neuropathic pain in addition to downregulating glial activation markers in the spinal cord. We aimed to determine whether a single intrathecal administration of an A2AR agonist was able to attenuate motor symptoms induced by experimental autoimmune encephalopathy. Two A2AR agonists (CGS21680 and ATL313) significantly attenuated progression of motor symptoms following a single intrathecal administration at the onset of motor symptoms. OX-42, a marker of microglial activation, was significantly attenuated in the lumbar spinal cord following A2AR administration compared to vehicle. Therefore, A2AR agonists attenuate motor symptoms of EAE by acting on A2AR in the spinal cord.

Purpose Interleukin-10 (IL-10) is an anti-inflammatory molecule that has achieved interest as a therapeutic for neuropathic pain. In this work, the potential of plasmid DNA-encoding IL-10 (pDNA-IL-10) slowly released from biodegradable microparticles to provide long-term pain relief in an animal model of neuropathic pain was investigated. Methods PLGA microparticles encapsulating pDNA-IL-10 were developed and assessed both in vitro and in vivo . Results In vitro , pDNA containing microparticles activated macrophages, enhanced the production of nitric oxide, and increased the production of IL-10 protein relative to levels achieved with unencapsulated pDNA-IL-10. In vivo , intrathecally administered microparticles embedded in meningeal tissue, induced phagocytic cell recruitment to the cerebrospinal fluid, and relieved neuropathic pain for greater than 74 days following a single intrathecal administration, a feat not achieved with unencapsulated pDNA. Therapeutic effects of microparticle-delivered pDNA-IL-10 were blocked in the presence of IL-10-neutralizing antibody, and elevated levels of plasmid-derived IL-10 were detected in tissues for a prolonged time period post-injection (>28 days), demonstrating that therapeutic effects are dependent on IL-10 protein production. Conclusions These studies demonstrate that microparticle encapsulation significantly enhances the potency of intrathecally administered pDNA, which may be extended to treat other disorders that require intrathecal gene therapy.

Previous studies of peripheral immune cells have documented that activation of adenosine 2A receptors (A 2A Rs) decrease proinflammatory cytokine release and increase release of the potent anti-inflammatory cytokine, interleukin-10 (IL-10). Given the growing literature supporting that glial proinflammatory cytokines importantly contribute to neuropathic pain and that IL-10 can suppress such pain, we evaluated the effects of intrathecally administered A 2A R agonists on neuropathic pain using the chronic constriction injury (CCI) model. A single intrathecal injection of the A 2A R agonists 4-(3-(6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxytetrahydrofuran-2-yl)-9 H -purin-2-yl)prop-2-ynyl)piperidine-1-carboxylic acid methyl ester (ATL313) or 2- p -(2-carboxyethyl)phenethylamino-5′- N -ethylcarboxamido adenosine HCl (CGS21680), 10–14 d after CCI versus sham surgery, produced a long-duration reversal of mechanical allodynia and thermal hyperalgesia for at least 4 weeks. Neither drug altered the nociceptive responses of sham-operated controls. An A 2A R antagonist [ZM241385 (4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-ylamino]ethyl)phenol)] coadministered intrathecally with ATL313 abolished the action of ATL313 in rats with neuropathy-induced allodynia but had no effect on allodynia in the absence of the A 2A R agonist. ATL313 attenuated CCI-induced upregulation of spinal cord activation markers for microglia and astrocytes in the L4–L6 spinal cord segments both 1 and 4 weeks after a single intrathecal ATL313 administration. Neutralizing IL-10 antibodies administered intrathecally transiently abolished the effect of ATL313 on neuropathic pain. In addition, IL-10 mRNA was significantly elevated in the CSF cells collected from the lumbar region. Activation of A 2A Rs after intrathecal administration may be a novel, therapeutic approach for the treatment of neuropathic pain by increasing IL-10 in the immunocompetent cells of the CNS.

Brain-derived neurotrophic factor (BDNF) was covalently attached to polyethylene glycol (PEG) in order to enhance delivery to the spinal cord via the cerebrospinal fluid (intrathecal administration). By varying reaction conditions, mixtures of BDNF covalently attached to one (primary), two (secondary), three (tertiary), or more (higher order) PEG molecules were produced. The biological activity of each resulting conjugate mixture was assessed with the goal of identifying a relationship between the number of PEG molecules attached to BDNF and biological activity. A high degree of in vitro biological activity was maintained in mixtures enriched in primary and secondary conjugate products, while a substantial reduction in biological activity was observed in mixtures with tertiary and higher order conjugates. When a biologically active mixture of PEG-BDNF was administered intrathecally, it displayed a significantly improved half-life in the cerebrospinal fluid and an enhanced penetration into spinal cord tissue relative to native BDNF. Results from these studies suggest a PEGylation strategy that preserves the biological activity of the protein while also improving the half-life of the protein in vivo. Furthermore, PEGylation may be a promising approach for enhancing intrathecal delivery of therapeutic proteins with potential for treating disease and injury in the spinal cord.

Multiple Sclerosis (MS) is an autoimmune inflammatory disease that presents clinically with a range of symptoms including motor, sensory, and cognitive dysfunction as well as demyelination and lesion formation in brain and spinal cord. A variety of animal models of MS have been developed that share many of the pathological hallmarks of MS including motor deficits (ascending paralysis), demyelination and axonal damage of central nervous system (CNS) tissue. In recent years, neuropathic pain has been recognized as a prevalent symptom of MS in a majority of patients. To date, there have been very few investigations into sensory disturbances in animal models of MS. The current work contains the first assessment of hind paw mechanical allodynia (von Frey test) over the course of a relapsing-remitting myelin oligodendrocyte glycoprotein induced experimental autoimmune encephalomyelitis (MOG-EAE) rat model of MS and establishes the utility of this model in examining autoimmune induced sensory dysfunction. We demonstrate periods of both decreased responsiveness to touch that precedes the onset of hind limb paralysis, and increased responsiveness (allodynia) that occurs during the period of motor deficit amelioration traditionally referred to as symptom remission. Furthermore, we tested the ability of our recently characterized anti-inflammatory IL-10 gene therapy to treat the autoimmune inflammation induced behavioral symptoms and tissue histopathological changes. This therapy is shown here to reverse inflammation induced paralysis, to reduce disease associated reduction in sensitivity to touch, to prevent the onset of allodynia, to reverse disease associated loss of body weight, and to suppress CNS glial activation associated with disease progression in this model.

This chapter explores how pain can be powerfully amplified by the activation of peripheral immune cells associated with peripheral nerves and by the activation of immune-like glial cells (microglia and astrocytes) within the central nervous system. After review of the basics of pain and pain modulation, the focus will shift to chronic pain, which is poorly if at all controlled by currently available drugs. The thesis will be developed that such pain arises as a consequence of immune and glial activation. The involvement of peripheral immune cells and glia in such pain states is explored. Finally, the very recent finding that glia also oppose the pain-suppressive effects of drugs like morphine is discussed, including the implications of these findings for clinical pain control.

Neuropathic pain is a major clinical problem unresolved by available therapeutics. Spinal cord glia play a pivotal role in neuropathic pain, via the release of proinflammatory cytokines. Anti-inflammatory cytokines, like interleukin-10 (IL-10), suppress proinflammatory cytokines. Thus, IL-10 may provide a means for controlling glial amplification of pain. We recently documented that intrathecal IL-10 protein resolves neuropathic pain, albeit briefly (approximately 2-3 h), given its short half-life. Intrathecal gene therapy using viruses encoding IL-10 can also resolve neuropathic pain, but for only approximately 2 weeks. Here, we report a novel approach that dramatically increases the efficacy of intrathecal IL-10 gene therapy. Repeated intrathecal delivery of plasmid DNA vectors encoding IL-10 (pDNA-IL-10) abolished neuropathic pain for greater than 40 days. Naked pDNA-IL-10 reversed chronic constriction injury (CCI)-induced allodynia both shortly after nerve injury as well as 2 months later. This supports that spinal proinflammatory cytokines are important in both the initiation and maintenance of neuropathic pain. Importantly, pDNA-IL-10 gene therapy reversed mechanical allodynia induced by CCI, returning rats to normal pain responsiveness, without additional analgesia. Together, these data suggest that intrathecal IL-10 gene therapy may provide a novel approach for prolonged clinical pain control.

Paclitaxel is a commonly used cancer chemotherapy drug that frequently causes painful peripheral neuropathies. The mechanisms underlying this dose-limiting side effect are poorly understood. Growing evidence supports that proinflammatory cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor (TNF), released by activated spinal glial cells and within the dorsal root ganglia (DRG) are critical in enhancing pain in various animal models of neuropathic pain. Whether these cytokines are involved in paclitaxel-induced neuropathy is unknown. Here, using a rat neuropathic pain model induced by repeated systemic paclitaxel injections, we examined whether paclitaxel upregulates proinflammatory cytokine gene expression, and whether these changes and paclitaxel-induced mechanical allodynia can be attenuated by intrathecal IL-1 receptor antagonist (IL-1ra) or intrathecal delivery of plasmid DNA encoding the anti-inflammatory cytokine, interleukin-10 (IL-10). The data show that paclitaxel treatment induces mRNA expression of IL-1, TNF, and immune cell markers in lumbar DRG. Intrathecal IL-1ra reversed paclitaxel-induced allodynia and intrathecal IL-10 gene therapy both prevented, and progressively reversed, this allodynic state. Moreover, IL-10 gene therapy resulted in increased IL-10 mRNA levels in lumbar DRG and meninges, measured 2 weeks after initiation of therapy, whereas paclitaxel-induced expression of IL-1, TNF, and CD11b mRNA in lumbar DRG was markedly decreased. Taken together, these data support that paclitaxel-induced neuropathic pain is mediated by proinflammatory cytokines, possibly released by activated immune cells in the DRG. We propose that targeting the production of proinflammatory cytokines by intrathecal IL-10 gene therapy may be a promising therapeutic strategy for the relief of paclitaxel-induced neuropathic pain.

Research on communication between glia and neurons has increased in the past decade. The onset of neuropathic pain, a major clinical problem that is not resolved by available therapeutics, involves activation of spinal cord glia through the release of proinflammatory cytokines in acute animal models of neuropathic pain. Here, we demonstrate for the first time that the spinal action of the proinflammatory cytokine, interleukin 1 (IL-1) is involved in maintaining persistent (2 months) allodynia induced by chronic-constriction injury (CCI). The anti-inflammatory cytokine IL-10 can suppress proinflammatory cytokines and spinal cord glial amplification of pain. Given that IL-1 is a key mediator of neuropathic pain, developing a clinically viable means of long-term delivery of IL-10 to the spinal cord is desirable. High doses of intrathecal IL-10-gene therapy using naked plasmid DNA (free pDNA-IL-10) is effective, but the dose required limits its potential clinical utility. Here we show that intrathecal gene therapy for neuropathic pain is improved sufficiently using two, distinct synthetic polymers, poly(lactic-co-glycolic) and polyethylenimine, that substantially lower doses of pDNA-IL-10 are effective. In conclusion, synthetic polymers used as i.t. gene-delivery systems are well-tolerated and improve the long-duration efficacy of pDNA-IL-10 gene therapy.

Chronic pathological pain is a major unresolved clinical problem. Spinal cord glial cells are important in diverse forms of enhanced pain states via their release of proinflammatory cytokines. Interleukin-10 (IL-10), a potent anti-inflammatory cytokine, suppresses proinflammatory cytokine production & activity. We have shown that spinal delivery (intrathecal; i.t.) of IL-10 protein resolves animal models of pathological pain. Both i.t. IL-10 protein or i.t. viral vector-delivered IL-10 gene lead to transient therapeutic effects (2hr & 2 wk, respectively). We have recently reported that i.t. naked plasmid DNA encoding IL-10 (pDNA-IL-10) leads to a substantial improvement in the duration of therapeutic pain control (40 days). However, transgene IL-10 may be compromised &/or cleared by cell division. Recently, a chromosomal scaffold/matrix- attached region (S/MAR) linked to the simian virus 40 origin of replication was reported to stably propagate episomally in mammalian cells. Mitotic stability occurs via specific interactions with nuclear matrix proteins. We sought determine if S/MAR inserted into our current plasmid (pDNA-S/MARS-IL10) could improve upon the therapeutic duration of pain control in an animal model of neuropathic pain. The model we use is hypersensitivity to light touch [allodynia] produced by chronic constriction injury [CCI] of the sciatic nerve. Naked pDNA-S/MARS-IL10 was injected peri- spinally (intrathecally; i.t.) after CCI-sensitivity was induced. Behavioral measures were assessed in rats prior to & at 3 & 10 days after induction of CCI. Intrathecal injections were given on Day 10 & Day 12 post-CCI consisting of pDNA-S/MARS-IL10 (100 ug/injection in 18 ul), or pDNA-IL-10 (100 ug/injection in 18 ul). A second i.t. pDNA-S/MARS-IL10 (1 or 25 ug/injection in 5 ul), or pDNA-IL10 (25 ug in 5 ul) injection was given 2 days later. Allodynia was reassessed every 4 days following the second DNA injection. Allodynia was stable through day 10 after CCI induction. An immediate & robust improvement of pain reversal was observed within 24 hr after the first injection of pDNA-S/MARS-IL10 vs pDNA-IL-10. Animals have remained reversed from CCI-induced allodynia after the second i.t. pDNA-S/MARS-IL10 and 25 ug is most effective.Studies are ongoing and will be terminated 90 days after CCI or when CCI-induced allodynia returns. Simultaneous studies are determining whether 1) a single i.t. injection of pDNA-S/ MARS-IL10 is sufficient for long-term pain control, 2) transgene expression by protein analysis can be detected at 1, 2 &/or 3 months in cerebrospinal fluid after injection in behaviorally verified rats, and 3) the distribution of transgene expression in superficial spinal cord or deeper parenchymal layers. We thank Dr. Hans J. Lipps for the gift of plasmid containing pDNA-S/MARS. Support: NIH HL56510, DA018156 & DA015642.

Chronic pain control is a major unresolved clinical problem. Spinal cord astrocytes & microglia are critically involved in the creation & maintenance of diverse enhanced pain states via the release of proinflammatory cytokines. Interleukin-10 (IL-10), a potent anti-inflammatory cytokine, suppresses proinflammatory cytokine production & activity. We have previously shown that administration of IL-10 protein directly or via a viral vector encoding it into the spinal cord (intrathecal, i.t.) reverses neuropathic pain in the rat; although direct administration of IL-10 protein showed a very short-term reversal. We sought to determine whether neuropathic pain (hypersensitivity to light touch [allodynia] produced by chronic constriction injury [CCI] of the sciatic nerve) could be reversed by: 1) single or repeated i.t. delivery of naked plasmid DNA encoding rat IL-10 (pIL10), 2) single or repeated i.t. delivery of pIL10 treated with the cationic polymer, polyethyleneimine (PEI- pIL10), and 3) single or repeated i.t. pIL10 encapsulated in micro-particles prepared from FDA-approved biodegradable copolymers of polylactide and polyglycolide (PLGA) (PLGA- pIL10). Lastly, we examined whether i.t. rhodamine-labeled PLGA-micro-particles could be visualized in spinal cord using confocal microscopy in naive rats. Behavioral measures were assessed prior to & at 3 & 10 days post CCI. I.t. injections were given on day 10 post-CCI, consisting of pIL10 (100 ug/injection), control plasmid encoding jellyfish green fluorescent protein (pGFP; 100 ug/injection), vehicle (3% sucrose in phosphate buffered saline), PEI- pIL10 (10 ug/injection) or PLGA- pIL10 (350 ug PGLA/injection). For repeated injections, a 2nd injection was given on day 13. Allodynia was reassessed every 1 to 4 days. Allodynia was stable in control treated rats but was reversed (prolonged reversal lasting 40+ days), in rats given 2 i.t. injections of: 1) pIL10 2) PEI- pIL10 & 3) PLGA-IL10. Single i.t. injections of pIL10, PEI- pIL10 or PLGA- pIL10 produced a brief 3-6 day reversal of allodynia. Importantly, rhodamine-labeled PLGA-micro-particles were visible immediately & at 3 days after i.t. injection. Ongoing studies using i.t. rhodamine-labeled PLGA-micro-particles (either without DNA or with pGFP DNA) are aimed at examining the spread, the distribution in superficial spinal cord or deeper parenchymal layers & gene expression. This approach to pain control represents a dramatic departure from all other available therapies. Support: Avigen & NIH HL56510, DA015656, DA018156 & DA015642.

Controlling chronic pain in humans is a major unresolved problem. Spinal cord astrocytes and microglia are critically involved in the creation and maintenance of diverse enhanced pain states via the release of proinflammatory cytokines. Interluekin-10 (IL-10), a potent anti-inflammatory cytokine, suppresses proinflammatory cytokine production and activity. Control of chronic neuropathic pain such as sensitivity to light touch (allodynia) requires chronic spinal delivery of IL-10. Chronic pain can be surgically induced in rats by loose ligature of chromic cat gut around the sciatic nerve in four places. This creates a chronic constriction injury (CCI) as the nerve bundle swells against the irritating ligature. The CCI model produces an enhanced pain phenotype for 3 months. We have demonstrated that this pain is reversed transiently (4hrs.) through intrathecal injection of recombinant IL-10 protein and that two temporally spaced injections of an IL-10 expression vector containing inverted terminal repeats (ITRs) reverses this pain long term (3+ months, Milligan et al., these proceedings). The role of ITRs in gene expression is not well known. However, there is evidence that they possess transcription promoter and enhancer activity (Flotte et al., 1992 Am J Respir Cell Mol Biol. 7:p349-56 and 1993 J Biol Chem. 268: p3781-90). Based on this and other evidence in the literature we sought to determine if the ITRs in our expression vector, which are located 5' of the CMV enhancer and Chicken â-actin promoter (5' ITR) and 3' of the SV 40 poly A tail sequence (3' ITR), influence reversal of pain in the CCI model. To this end, we deleted both ITRs from the expression vector and found that it was incapable of producing pain reversal. We next asked if one or both ITRs are necessary for pain reversal. We found that plasmids containing just the 3' or 5' ITR were both effective in reversing the chronic pain state. Ongoing studies seek to determine the effect of multimerization of the injected plasmid on pain reversal and gene expression and to further characterize the role of ITRs on gene expression and pain reversal in the CCI model.

Despite many decades of drug development, effective therapies for neuropathic pain remain elusive. The recent recognition of spinal cord glia and glial pro-inflammatory cytokines as important contributors to neuropathic pain suggests an alternative therapeutic strategy; that is, targeting glial activation or its downstream consequences. While several glial-selective drugs have been successful in controlling neuropathic pain in animal models, none are optimal for human use. Thus the aim of the present studies was to explore a novel approach for controlling neuropathic pain. Here, an adeno-associated viral (serotype II; AAV2) vector was created that encodes the anti-inflammatory cytokine, interleukin-10 (IL-10). This anti-inflammatory cytokine is known to suppress the production of pro-inflammatory cytokines. Upon intrathecal administration, this novel AAV2-IL-10 vector was successful in transiently preventing and reversing neuropathic pain. Intrathecal administration of an AAV2 vector encoding beta-galactosidase revealed that AAV2 preferentially infects meningeal cells surrounding the CSF space. Taken together, these data provide initial support that intrathecal gene therapy to drive the production of IL-10 may prove to be an efficacious treatment for neuropathic pain.

The present series of experiments examined whether the complement cascade might play a key role in the expression of mechanical allodynia. Soluble complement receptor 1 (sCR1) was used to block the activation of the membrane attack pathway of the complement cascade. In doing so, sCR1 prevents the formation of the biologically active end products C3a, C5a, and membrane attack complexes (MACs). Intrathecal sCR1 had no effect on the behavioral responses of control groups. In contrast, blockade of this pathway abolished the expression of mechanical allodynia induced by peripheral nerve inflammation (sciatic inflammatory neuropathy model), partial sciatic nerve injury (chronic constriction injury model), and intrathecal injection of human immunodeficiency virus type 1 gp120, a viral envelope protein that activates glia. The fact that enhanced nociception was prevented or reversed in all 3 paradigms suggests that complement might be broadly involved in spinally mediated pain enhancement. The mechanisms whereby complement activation might potentially affect the functioning of microglia, astrocytes, and neurons are discussed. The complement cascade has not been previously implicated in spinal sensitization. These data suggest that complement activation within the spinal cord might contribute to enhanced pain states and provide additional evidence for immune regulation of pain transmission.

Spinal cord glia, via proinflammatory cytokines (interleukin-1, tumor necrosis factor, & interleukin-6), critically mediate the creation/maintenance of pathological pain such as sciatic inflammatory neuropathy (SIN) & chronic constriction injury (CCI). The anti-inflammatory cytokine, IL-10, suppresses proinflammatory cytokine production & function. We have used a variety of gene therapy approaches to examine the efficacy of IL-10 in rat models of neuropathic pain. Behavior was assessed before & after induction of various pain models. Adenoviral vectors expressing the IL-10 gene block &/or reverse exaggerated pain (mechanical allodynia [von Frey test] & thermal hyperalgesia [Hargreaves test]) produced by intrathecal (IT) lumbosacral spinal (LS) delivery of gp120, SIN, & CCI. IT LS delivery of AAV2 encoding IL-10 (AAV2-IL-10) was able to block SIN allodynia [amp] reverse CCI allodynia [amp] thermal hyperalgesia. However, the effects of viral IL-10 delivery systems resulted in reversals in the pain state of short duration. An IL-10-encoding non-viral plasmid vector (NVV) was also introduced IT in the CCI model. In contrast to the viral vectors, this approach resulted in long-lasting reversals of the pain state. Ongoing experiments will assess additional effects of NVV-driven IL-10 protein expression, including the identification of transfected tissue(s). These results suggest that NVV-based IL-10 gene therapy may be a potent addition to the treatments available to neuropathic pain patients.