TREATMENT OF NEUROPATHIC PAIN WITH N-METHYL-D-ASPARTATE (NMDA) RECEPTOR ANTAGONISTS

Abstract: Central neuropathic pain is treated with an analgesic composition that consists essentially of an N-methyl-D-aspartate (NMDA) receptor antagonist. In one embodiment, the invention includes chronic administration of the (NMDA) receptor antagonist. In another embodiment, the invention is use of an NMDA receptor antagonist or component thereof for the manufacture of a medicament than includes an analgesic component that consists essentially of an NMDA receptor antagonist for the chronic treatment of central neuropathic pain.

Description: TREATMENT OF NEUROPATHIC PAIN WITH N-METHYL-D-ASPARTATE (NMDA) RECEPTOR ANTAGONISTS RELATED APPLICATION This application claims the benefit of U.S. Provisional Application "Treatment of Neuropathic Pain with N-Methyl-D-Aspartate (NMDA) Receptor Antagonists" by Christine N. Sang (Attorney Docket Number 0838.2002-000), filed June 7, 20001, the entire teachings of which are incorporated herein by reference. BACKGROUND OF THE INVENTION Humans with injury to the central nervous system (e.g., brain and spinal cord) can suffer from chronic central neuropathic pain. However, standard analgesics, such as nonsteroidal anti-i Tammatory drugs, opioids, tricyclic antidepressants, anticonvulsants and antispasmodics, are ineffective in relieving the chronic central neuropathic pain, in particular pain associated with spinal cord injury. Further, relief of pain by certain analgesics can result in adverse side effects such as fatigue, confusion, dizziness, somnolence and speech difficulty thereby diminishing the attractiveness of the analgesic to the human. Thus, there is a need to develop new, improved and effective methods of treatments for pain in humans with central nervous system injury which alleviate pain without adverse side effects. SUMMARY OF THE INVENTION The present invention relates to a method of treating chronic central neuropathic pain in humans suffering from spinal cord injury by administering an N-methyl-D- aspartate (NMDA) receptor antagonist. In one embodiment, the method is treatment of central neuropathic pain in a human, the method including administering to the human an analgesic composition, wherein the improvement comprises chronic administration to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagonist and wherein there is essentially no high affinity N-methyl-D-aspartate receptor antagonist in the analgesic composition. In another embodiment, the method is for treating central neuropathic pain in a human, the method including administering to the human an analgesic composition, wherein the improvement comprises administering to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagonist, and wherein the N-methyl-D-aspartate receptor antagonist essentially does not include etamine or a subtype selective N-methyl-D-aspartate receptor antagonist. In an additional embodiment, the invention is the use of an N-methyl-D- aspartate receptor antagonist, or component thereof for the manufacture of a medicament that includes an analgesic component that consists essentially of the N- methyl-D-aspartate receptor antagomst, for the chronic treatment of central neuropathic pain. In still another embodiment, the method is treating central neuropathic pain in a human, comprising the step of acutely administering to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagonist, and wherein the N-methyl-D-aspartate receptor antagonist essentially does not include ketamine. In yet another embodiment, the method is treating central neuropathic pain in a human, comprising chronically administering to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagonist and wherein there is essentially no high affinity N-methyl-D-aspartate receptor antagonist in the analgesic composition. In a further embodiment, the method is treating central neuropathic pain in a human, comprising the step of administering to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagonist, and wherein the N-methyl-D-aspartate receptor antagonist essentially does not include ketamine or a subtype selective N-methyl-D-aspartate receptor antagomst. The invention described herein provides a method of treating chronic neuropathic pain in a human suffering from a spinal cord injury by administering NMDA receptor antagonists. Advantages of the method of the invention include, for example, augmented pain relief with no or significantly reduced side effects (e.g., fatigue, confusion, dizziness, somnolence and speech difficulty) particularly in humans where pain management strategies are difficult to implement. The methods of the invention provide an efficient way to treat and reduce the severity of central neuropathic pain in a human suffering from a spinal cord injury. Thus, treatment of humans with a spinal cord injury who have central neuropathic pain with NMDA receptor antagonists can dimmish their pain without intolerable side effects. BRIEF DESCRIPTION OF FIGURES Figure 1 lists the clinical characteristics of human patients (n=28) with central neuropathic pain. Figure 2 lists the demographic characteristics of human patients (n=18) in the four treatment groups. Figure 3 depicts the weeldy average Gracely pain score for human patients in the four treatment groups. Figure 4 is a summary of Global Pain Intensity and Pain Relief Measures in human patients (n=18) in the four treatment groups rated on the last day of each treatment. Figure 5 lists the mean pain intensities over each five week treatment period in each of the four treatment groups based on functional classification, distribution of pain and presence of evoked pain. Figure 6 depicts hyperalgesia in human patients treated with dextromethorphan alone (DEX), gabapentin alone (GABA) or a dextromethorphan/gabapentin combination (DG) compared to the placebo treatment group. DETAILED DESCRIPTION OF THE INVENTION The features and other details of the invention, either as steps of the invention or as combinations of parts of the invention, will now be more particularly described and pointed out in the claims. It will be understood that the particular embodiments of the invention are shown by way of illustration and not as limitations of the invention. The principle features of this invention can be employed in various embodiments without departing from the scope of the invention. The present invention relates to the discovery that treatment with NMDA receptor antagonists decrease central neuropathic pain of humans, hi particular, the chronic administration of an NMDA receptor antagomst (e.g., dextromethorphan hydrobromide) has been found to decrease chronic central neuropathic pain in humans following spinal cord injury with no or significantly diminished side effects. The NMDA receptor antagonists can be high affinity NMDA receptor antagonists (e.g., ketamine), low affinity NMDA receptor antagonists, such as dextromethorphan hydrobromide (also referred to herein as "dextromethorpan"), amantadine, memantine, remacemide, riluzole; and opioids with NMDA activity (e.g., ketobemindone, methadone, dextropropoxyphene, meperidine). "Central neuropathic" pain refers to pain associated with a disorder, congential defect or injury of the central nervous system (the brain or spinal cord). The central neuropathic pain can be spontaneous or invoked pain. "Central neuropathic" pain can be chronic or acute. Chronic neuropathic pain typically is pain of a duration greater than three months. The central neuropathic pain can be in a human suffering from a stroke, a brain lesion or a spinal cord injury. For example, the spinal cord injury can be the result of trauma to the spinal cord either as a result of direct anatomical disruption of spinal cord or associated spinal cord damage in Syringomyelia. Additionally, or alternatively, the central neuropathic pain can be the result of a lesion, Multiple sclerosis, a tumor within or surrounding the spinal cord or a vascular lesion compressing the spinal cord. "Spinal cord injury"refers to any trauma, damage or wound to any level (e.g., cervical, thoracic, lumbar, sacral) of the spinal cord. The spinal cord injury can also be a congential defect. Using standard medical criteria, one of skill in the art would be capable of diagnosing a human with a central neuropathic pain spinal cord injury. An "analgesic amount" or "analgesic dose" is the quantity of NMDA receptor antagonist which relieves the pain perceived by the human undergoing treatment with the NMDA receptor antagonist. In a preferred embodiment, the analgesic amount is an amount of an NMDA receptor antagonist, such as dextromethorpan, in a range between about greater thanl 20 mg/day and about 1200 mg/day. "Chronic administration" is wherein a single dose is not effective in alleviating central neuropathic pain. Chronic administration can be, for example, the administration of an NMDA receptor antagonist, such as dextromethorpan hydrobromide, for 21 or more days. The NMDA receptor antagonist can be chromcally administered by administration an initial dose then a subsequent dose. "Acute administration" is administration of a dosage that is effective at alleviating central neuropathic pain in a single dose, whether the administration is a single bolus, such as pills or an intramuscular injection, or continuously within a limited period of time, such as by intravenous administration. In particular the invention relates to a method of treating a human with a spinal cord injury suffering from central neuropathic pain by titrating the dose of NMDA receptor antagonist to an analgesic dose which results in minimal side effects (e.g., fatigue, confusion, dizziness, somnolence and speech difficulty). Titration of the dose of NMDA receptor antagonist is accomplished administering to the human an initial dose of NMDA receptor antagonist (e.g., 120 mg/day) followed by an evaluation by the human undergoing treatment of their pain and side effects. A subsequent dose of NMDA receptor antagomst is administered to the human until the pain perceived by the human is relieved or tolerable. In a preferred embodiment, the dose of NMDA receptor antagonist is increased in increments of about 60 mg/day. The method further includes maintaining the human on a dose of NMDA receptor antagonist which results in analgesia with minimal side effects. NMDA receptor antagonists are compounds capable of competing with or counteracting the effect of NMDA receptors. In a preferred embodiment, the NMDR receptor antagonist is dextromethorphan. Dextromethorphan is widely available. Central neuropathic pain can also be treated with subtype specific NMDA receptor antagonists. In a preferred embodiment, administration of the subtype specific NMDA receptor antagonist is used to treat a spinal cord injury, for example, as a result of a trauma to the spinal cord either (e.g., as a result of direct anatomical disruption of spinal cord or associated spinal cord damage in Syringomyelia) a tumor within or surrounding the spinal cord or a vascular lesion compressing the spinal cord. The methods of the invention can be accomplished by the administration of the NMDA receptor antagonist by enteral or parental means. A preferred method of administration is by oral ingestion of a capsule, tablet or drink. Alternatively, or additionally, the NMDA receptor antagonist can be administered intramuscularly or intraperitoneally. The NMDA receptor antagonists can be administered alone or as admixtures with conventional excipients (e.g., water, salt solutions) which do not deleteriously react with the NMDA receptor antagonist. The present invention is further illustrated by the following examples, which are not intended to be limiting in any way. EXEMPLIFICATION Approximately 183,000 to 230,000 persons in the United States have sustained spinal cord injuries (SCI) (DeVivo MJ, et al. ArchNeurol. 37:707-708 (1980); Harvey C, et al. Paraplegia 28:537-544 (1990); Lasfarques IB, et al. Paraplegia 33:62-68 (1995)). Direct injury to the spinal cord can result in a chronic neuropathic pain condition, typically described as constant "burning," "tight," "constricting," and/or "shooting and stabbing," often in combination with pain due to a stimulus which does not normally evoke pain (allodynia), or augmented pain to a normally painful stimulus (hyperalgesia). Central neuropathic pain following SCI generally occurs in one of two anatomic distributions: 1) Circumferential ("band-like"), at the border of normal sensation and anesthetic skin; and 2) below the level of the spinal cord lesion, perceived diffusely in anesthetic regions (Siddall PJ, et al. Spinal Cord 35(2):69-75 (1997)). The prevalence of chronic pain following SCI has been reported to vary between 34% 5 and 90% (Botterell EH, et al. Proc R Soc Med. 47:281-288(1953)). Importantly, disability associated with SCI more often is due to its associated pain rather than loss of function (Rose M, et al. Pain 34:101-102(1988)). and may be a major factor in causing unemployment and depression. (Ravenscroft A, et al. Spinal Cord 38(10):611-4 (2000)). There is currently no chronically-administered analgesic regimen that has systematically been shown to be effective for the treatment of chronic central neuropathic pain following SCI. Results of the few randomized controlled trials evaluating chronic oral agents in SCI pain have been negative, including studies of valproate (Drewes AM, et al. Paraplegia 32:565-569(1994)), mexiletine (Chiou-Tan FY, et al. Am J Phys Med Rehabil. 75:84-87 (1996)), and trazodone (Davidoff G, et al, Pain 29: 151-161 (1997)). Data in animal models of SCI have shown that restricting the extent of excitotoxicity after SCI with NMDA receptor antagonists could alleviate behaviors associated with spontaneous pain and hyperalgesia, the behavioral correlate of central nervous system excitation (Liu S, et al. Brain Res. 756: 160-167 (1997); Bennett AD, et al. Brain Res. 859:72-82(2000); Hao JX, et al. Pain 45: 175-185 (1991);and Hao JX, and Xu XJ. Pain 66:279-285 (1996)). Specifically, a study by Hao and Xu (Hao JX, and Xu XJ. Pain 66:279-285 (1996)) have shown that, of a series of NMDA receptor antagonists, only dextromethorphan preserves motor function in spinally injured rats at doses that relieved touch-evoked pain. However, the clinical data in patients with peripheral neuropathic pain show a marginal effect (Nelson KA, et al. Neurology 48:1212-1218 (1997) and Sang CN, et al. American Pain Society Abstracts, 1997)). In patients (also referred to herein as "human" or "human patient"), a recent study evaluating the acute parenteral administration of the NMDA receptor antagomst ketamine (Eide PK, et al. Neurosurg. 37 : 1080- 1087( 1995)) reduced both spontaneous and evoked SCI pain, but ketamine has limited clinical utility because of its psychotomimetic side effects. Repeated doses of the antiepileptic agent gabapentin, whose mechanism is still being elucidated, also alleviated chronic hypersensitivity in spinally injured rats (Hao, JX, et al. Neurosci Lett 280(3):211-4 (2000)). To date, there are no data which systematically demonstrate an effect of either dextromethorphan or gabapentin on central neuropathic pain. Moreover, the simultaneous use of multiple standard analgesics is the most frequent form of treatment in central and peripheral neuropathic pain states, and there has until now been no systematic approach to assess combination therapy. The present study is based on the hypothesis that the use of combinations of drugs acting through different pharmacologic mechanisms may result in an augmented analgesia without augmented toxicity. A single-center, randomized, double-blind, double-dummy, 2x2 factorial crossover study evaluating the combination of dextromethorphan and gabapentin, dextromethorphan alone, gabapentin alone, and placebo in the treatment of central neuropathic pain following traumatic spinal cord injury was performed. Central neuropathic pain following traumatic spinal cord injury (SCI) may be severe and refractory to standard analgesics. Pharmacologic agents that target distinct pathophysiological pain mechanisms theoretically could provide pain relief for these patients. The analgesic efficacy of dextromethorphan and gabapentin in combination, dextromethoφhan alone, gabapentin alone, and placebo, in patients with pain following SCI was evaluated in a randomized, double-blind, double-dvrmmy, 2x2 factorial, Latin Square crossover trial, we evaluated the four treatments. Each treatment was titrated over 4 weeks and maintained at the individual's minimum intolerated dose until the end of Week 5. Each treatment period was followed by a 1-week washout period or until pam returned to baseline. Primary efficacy measure was mean pain intensity during the 5-week treatment period, measured by the 20-point Gracely pain intensity scale. Eighteen of 23 randomized subjects completed all 4 treatments. Mean daily doses were 416 mg for dextromethoφhan, 2657 mg for gabapentin, and 401 mg of dextromethoφhan with 2007 mg of gabapentin for the combination. The combination resulted in significantly reduced pain intensities over dextromethoφhan (p=0.004), gabapentin (p=0.02), and placebo (p=0.001) during Week 1, which persisted to Week 4. During Week 5, the combination significantly reduced mean pain intensities of gabapentin only (p=0.02). Eleven of 18 (61 %) patients receiving the combination had at least moderate or better pain relief, in contrast to that of dextromethoφhan 9/18 (50%), gabapentin 7/18 (39%), and placebo 2/18 (11%). Pain relief scores for the combination were significantly better than gabapentin (p=0.04) but not for dextromethoφhan (p=0.27). Overall patient satisfaction at the end of each treatment period, which takes into account both side effects and pain relief, was significantly better for dextromethoφhan alone (pθ.001), gabapentin alone (p<0.01) and the combination (p<0.001) compared to placebo. Chronic oral administration of dextromethoφhan, gabapentin, and the dextromethoφhan-gabapentin combination provided a sigmficant reduction of SCI pain compared to placebo and greater reduction than either component alone. This is the first systematic demonstration of any successful chronic treatment of refractory central pain following SCI. METHODS Patients Patients with 1) central neuropathic pain secondary to traumatic SCI in persons > 18 years of age, 2) moderate pain for at least 50% of the day for at least 3 months, 3) concurrent use of no more than two analgesics at a stable dose (e.g. tricyclic antidepressants, anticonvulsants, nonsteroidal antiinflammatory drugs, or limited use of low-potency short-acting opioids), and 4) 100% compliance in rating their overall pain intensity 5-times daily in a diary for 1 week, were recruited nationwide between July 1997 and April 1999 using written announcements and by physician referrals. Exclusion criteria included: 1) Presence of another type of pain of equal severity as that caused by SCI, such as musculoskeletal pain; 2) pregnancy or breast feeding; 3) hepatic or renal dysfunction; 4) significant cardiac disease; 5) signs or symptoms of another central neurological disorder; 6) severe psychological disorder requiring treatment; 7) concurrent use of monoamine oxidase inhibitors or phenothiazines; history of hypersensitivity or intolerance to dextromethoφhan or gabapentin; and chronic substance abuse, including alcohol. Women of childbearing potential agreed to use adequate contraception during the study. The study was in compliance with the Declaration of Helsinki and was approved by the Partners Human Research Committee. All patients gave written informed consent. Study Design All patients were followed as outpatients. Patients were randomly assigned to a sequence of the 4 treatments. Each of the 4 treatment periods consisted of 5 weeks. Each treatment period was separated by a 1-week washout period, with the requirement that pain return to baseline level. Dextromethoφhan (Endo Inc., Neptune, NJ; and its externally identical placebo) and gabapentin (Neurontin, Pfizer, Inc., Ann Arbor, MI; and its externally identical placebo) were dispensed using a double-dummy design; thus, the dextromethoφhamgabapentin dose-ratio was maintained at 1:5. Subjects started at 120 mg/day of dextromethoφhaα vs. placebo and 600 mg/day of gabapentin vs. placebo, and titrated according to a fixed schedule until they reached the ceiling doses equivalent to 600 mg/day of dextromethoφhan or 3000 mg/day of gabapentin or their maximally tolerated dose (the dose just causing side effects, MTD). The titration regimen and the number of capsules per dose for each treatment were the same in each treatment group, in order to maintain double blinded conditions. A nurse-clinician blinded to the study drug called each patient at least twice- weekly to instruct each patient on dose increases, encourage compliance, encourage consistency in dosage of concurrent analgesics, assess side effects, and answer any questions relating to the study protocol. Up to 2,000 mg of acetaminophen were allowed as rescue medication, on a case-by-case basis, for pain intensity scores of 18/20 or higher. Endpoints The primary endpoint was mean spontaneous pain intensity averaged weekly, from the start of study drug until the end of Week 5 for each treatment period and overall for the 5 weeks, assessed in a diary in which pain ratings were recorded 5-times daily, and evaluated with the 20-point Gracely pain scale based on 13 words ("faint" to "extremely intense") (Gracely RH, et al. Pain 5:5-18(1978)). Secondary endpoints were assessed at a clinic visit on the final day of each treatment period, and included: 1) Global Pain Intensity (6-item categorical scale); 2) Global Pain Relief (6-item categorical scale); 3) pain intensity (Gracely Scale) of individual pain descriptors (burning pain; aching pain; tingling pain; cold pain; brief lancinating pain; deep stabbing pain; constricting pain; and touch-evoked pain (allodynia)); 4) areas of evoked pain : a) allodynia and b) pinprick-evoked hyperalgesia; 5) quality of life, assessed using the Duke Health Profile (Parkerson, GR, et al. Medical Care 28:1056-1069(1990)). 6) Patient Satisfaction (5-item categorical scale); 7) patients' assessment of best treatment. . Allodynia was mapped by stroking the skin with the distal 5 mm of the comer of a 2x2 gauze sponge at standard pressure, at a rate of 1 cm/sec. Pinprick-evoked hyperalgesia was mapped using a standard safety pin that was pressed against the skin until dimpling was visible. Ambient temperature was maintained at 24 degrees centigrade. All mapping took place within a 30-minute period. The delineated areas of evoked pain were recorded using photographs of 4 views (anterior/posterior/right lateral/left lateral), which were scanned, and the areas of each of the 4 views were summed to determine the composite area. Presence and intensity (5-item categorical scale) of adverse effects were assessed continuously using an open question. Evaluation of Adequacy of Blinding Patient questionnaires to evaluate the adequacy of blinding of treatments, and whether their answers were influenced by side effects or analgesia, were admimstered at the completion of each treatment period and at the end of the study. Statistical Analysis A difference between mean treatment of "mild" and "moderate" pain, with greater than 80% power was detected. Our sample size of 18 was determined by selecting a Type I error = 0.05, Type II error = 0.2, and a within-subject standard deviation (SD) of 0.15. For the weekly average Gracely pain scores, a mixed model repeated measures analysis of variance (ANOVA) was used. In order to conduct pairwise comparisons between treatments at each week, a separate 4x4 Latin Square was built into the overall repeated measures model at each week. To obtain a mean difference between treatments across weeks, the weekly pairwise differences were averaged across weeks using contrasts in the overall repeated measures model. All end of period assessments were analyzed using Latin Square ANOVA. For binomial parameters, differences between treatments were examined using the McNemar Q test. Potential carry over effects were examined using the method of Cochran and Cox (Cochran, WG and GM Cox. Experimental Designs. New York: John Wiley & Sons, 1968, pg 135-139)). RESULTS Subjects Twenty-eight subjects were screened; five were not randomized because of an unwillingness to risk potential side effects (1), a predominance of musculoskeletal or overuse pain syndromes (1), and abnormal laboratory tests (3). Twenty-three subjects were each randomized to one of the 4 treatment sequence groups. Their clinical characteristics are listed in Figure 1. Eighteen of the randomized subjects completed evaluations of all of the four treatments Figure 2. Median age was 51 (range, 34-68) years; median time since injury was 6.8 (1.7-30.3) years. There were no significant differences between any of the treatment sequence groups or between the intent-to-treat and completer data sets with respect to baseline demographic and clinical characteristics. Of the 5 dropouts (all for unacceptable cognitive side effects), three received 1 treatment (2 received dextromethoφhan; 1 received placebo), and two received 2 treatments (combination, dextromethoφhan; placebo, combination). Treatment After the 4 weeks of titration to MTD, mean doses during Week 5 were 416 + 34 mg/day (maximum daily dose administered, 470 + 29 mg/day) for dextromethoφhan, 2657 ± 155 mg/day for gabapentin (maximum, 2717 ± 148 mg/day), and 401 + 32 mg/day of dextromethoφhan (maximum, 440 + 30 mg/day) with 2007 + 158 mg/day of gabapentin (maximum, 2200 + 150 mg/day) for the combination. The doses of gabapentin were significantly different when used either alone or in combination (p=0.001), while those of dextromethoφhan were not (p=0.72). Efficacy Primary endpoint The dextromethoφhan-gabapentin combination resulted in significantly reduced spontaneous pain intensities over dextromethoφhan alone (p=0.004), gabapentin alone (p=O.02), and placebo (p=0.001) during the first week of therapy (Week 1). The mean pain intensities during both Week 5 and over the entire treatment period were significantly lower for the combination compared to gabapentin (Week 5, p=0.02; Weeks 1-5, p=0.001). Although the combination was also significantly better during the entire treatment period compared to dextromethoφhan (Weeks 1-5, p=0.001) the reduction in pain intensity was no longer significant during Week 5. (Figures 3, 4 and 5). Figure 3 shows the mean pain intensities over each 5-week treatment period for subgroups based on functional classification, distribution of pain, and presence of evoked pain. Eight subjects had both complete SCI (i.e. lacking sacral innervation) and segmental pain; all 8 had allodynia. All subjects with segmental pain had allodynia. Five of 9 subjects with complete SCI had allodynia. Three subjects had a sacral distribution of pain (data not shown). Eleven of 18 (61%) patients receiving the combination had at least moderate or better pain relief, in contrast to that of dextromethoφhan 9/18 (50%), gabapentin 7/18 (39%), and placebo 2/18 (11%). Pain relief scores for the combination were significantly better than gabapentin (p=0.04) but not for dextromethoφhan (p=0.27). Overall patient satisfaction at the end of each treatment period, which takes into account both side effects and pain relief, was significantly better for dextromethoφhan alone (pO.OOl), gabapentin alone (p<0.01) and the combination (pO.OOl) compared to placebo. Analyses of potential biases of the crossover design showed no carryover or period effects for intensity of spontaneous pain or intensity of evoked allodynia. Secondary endpoints Spontaneous Pain Global Pain Intensity (of spontaneous pain), Global Pain Relief, and Patient Satisfaction measures were all significantly better in the combination and in the individual components alone compared to placebo (Figure 4). The combination was statistically superior to both the placebo and gabapentin alone for the Global Pain Relief rating. Among the descriptors of pain, the intensity of burning pain (p=0.005) and brief paroxysms of lancinating pain (p=0.03) were both significantly improved with the combination compared to placebo, but we were unable to detect an effect of the individual components. Dextromethoφhan alone also provided sigmficant relief for these two types of pain compared to placebo (burning pain, p=0.001; brief paroxysms of lancinating pain, p=0.003) Evokedpain Intensity of touch-evoked allodynia Among the 14 subjects with allodynia at their screening visit, the combination and each individual component significantly reduced the intensity of allodynia compared to placebo (dextromethoφhan, p=0.03, gabapentin, p=0.01; combination, p=0.001) Figure 4). of allodynia and hyperalgesia Mean areas of allodynia after treatment were reduced from baseline (362 + 496 cm 2 ) by 50% for the combination (182 ± 373 cm 2 ), 49% for dextromethoφhan (183 + 321 cm 2 ), 71%) for gabapentin (106 + 231 cm 2 ); they were increased by 6% for placebo (383 + 557 cm 2 ). Mean areas of pinprick hyperalgesia after treatment were reduced from baseline (371 ± 523 cm 2 ) by 43% for the combination (210 + 421 cm 2 ), 67% for dextromethoφhan (122 + 287 cm 2 ), 59% for gabapentin (152 + 332 cm 2 ); they were increased by 7% for placebo (395 + 586 cm 2 ). There was a significant difference in areas of pinprick hyperalgesia for all of the treatment groups (p=0.03 for the combination, p=0.02 for dextromethoφhan, p=0.03 for gabapentin). There was a strong trend in favor of a reduction of allodynia for the combination over placebo (p=0.05), and a significant difference from placebo for gabapentin (p=0.02) Quality of life Both the combination and dextromethoφhan alone were significantly better than placebo for the General Health (combination, p=0.01 ; dextromethoφhan, p=0.04) and Physical Health (combination, p=0.001; dextromethoφhan, p=0.02) measures, in contrast to gabapentin. The combination, dextromethoφhan, and gabapentin were all significantly better than placebo for the Pain domain (combination, p=0.002; dextromethoφhan, p=0.001; gabapentin, p=0.03). Assessment of blinding Subjects guessed the identity of the treatment drugs correctly in 27/72 (38%) of the individual treatment periods, a finding of less than chance for the 4 treatments. There was no significant difference in the proportions of patients with moderate or better pain relief who could not correctly guess their treatments. Choice of best treatment When considering which of the four treatments provided the best pain relief at the completion of the study, 7/18 (38.9%) chose the combination, 6/18 (33.3%) chose dextromethoφhan alone, 5/18 (27.8%) chose gabapentin alone, and 2/18 (11.8%) chose placebo as their best treatment (p = 0.04). When considering the balance between analgesia and side effects at the completion of the study, 8/18 (47.1%) chose the combination, 3/18 (17.5%) chose dextromethoφhan alone, 4/18 (23.5%) chose gabapentin alone, and 2/18 (11.8%) chose placebo as their best treatment (p = 0.04). Adverse effects Side effects associated with the dextromethoφhan-gabapentin combination occurred at doses that approached the MTD of the individual components. A significantly larger proportion of patients taking the combination (p<0.002) and dextromethoφhan alone (p<0.002) experienced cognitive side effects compared to either placebo or gabapentin, including fatigue (combination 35%, dextromethoφhan 52%o, gabapentin 33%, placebo 15%>); confusion (combination 30%>, dextromethoφhan 33%), gabapentin 22%, placebo 5%); dizziness (combination 45%, dextromethoφhan 61%, gabapentm 22%, placebo 20% o ); somnolence (combination 70%, dextromethoφhan 62%, gabapentin 33%, placebo 10%); euphoria (combination 50%, dextromethoφhan 24%, gabapentin 6%, placebo 5%); and speech difficulty (combination 25%, dextromethoφhan 10%, gabapentin 0%, placebo 0%). A significantly larger proportion of patients taking the combination (p=0.01) and dextromethoφhan alone (p=0.01) experienced other side effects compared to placebo, including urinary incontinence (combination 20%, dextromethoφhan 5%, gabapentin 11%), placebo 5%) and urinary tract infection (combination 35%, dextromethorphan 38%, gabapentm 22%, placebo 15%). There were no clinically significant differences in any of the laboratory studies obtained, including a hematology panel (leukocyte count, hemoglobin, platelet count), electrolytes (sodium, potassium, bicarbonate, chloride, blood urea nitrogen, creatinine), and chemistry panel (alkaline phosphatase, transaminases, total bifirubin). DISCUSSION Chronic oral administration of dextromethoφhan, gabapentin, and the dextromethoφhan and gabapentin combination in a fixed 1:5 dose-ratio were all superior to placebo in the treatment of spontaneous pain following SCI. Moreover, when compared to placebo and each component alone during the entire 5-week treatment period, the combination was better than either component alone, with an onset of action as early as the first week of therapy. Moreover, that the combination and the individual components relieved the intensity and spread of pinprick-evoked hyperalgesia, as well as the trend of the combination and dextromethoφhan toward significance for touch-evoked allodynia. These data support the chronic oral use of gabapentin and dextromethoφhan, and supports the hypothesis that targeting complementary but independent pain mechanisms may be an even more effective strategy than using single-drug regimens in SCI pain. Because of the heterogeneity of pain mechanisms, combination therapy used in a fixed dose ratio may provide a more robust analgesic effect by acting at different sites without reaching a critical threshold for toxicity at any site. The components would have a dose-ratio based in part on how the various components of pain mechanisms were weighted in the patients. We chose a dose-ratio of 1 :5 based on clinical experience and current data (Nelson KA, et al. Neurology 48:1212-1218 " (1997); Sang CN, et al. American Pain Society Abstracts, 1997; (Rowbotham M, et al. JAMA. 280(21):1837-42(1998); BackonjaM, et al. JAMA 280(21): 1831-6 (1998)) of ceiling doses in peripheral neuropathic pain. Findings in animal models of SCI and peripheral neuropathic pain demonstrate that blockade of excitatory amino acid transmission with NMDA receptor antagonists relieve allodynia and hyperalgesia (Liu S, et al. Brain Res. 756:160-167 (1997); Bennett AD, et al. Brain Res. 859:72-82(2000); Hao JX, et al. Pain 45:175-185 (1991);and Hao JX, and Xu XJ. Pain 66:279-285 (1996)) The most comprehensive pharmacological work in SCI has been in the ischemic rat model (Xu XJ, et al. Anesth Analg. 74:649-652 (1992) and (Xu XJ, et al. J Pharmacol Exp Ther. 267:140-144 (1993)). Of the three candidate NMDA receptor antagonists evaluated in the ischemic SCI model, dextromethoφhan was the only NMDA receptor antagonist which preserved motor function (Hao JX, et al. Pain 45:175-185 (1991) and Hao JX and Xu XJ. Pain 66:279-285 (1996)). All of the currently available NMDA receptor antagonists with affinity at the phencyclidine site (such as dextromethoφhan, ketamine, and amantadine) are limited by dose-related side effects. In the context of epilepsy, Rogawski (Rogawski MA. Trends Pharmacol Sci. 14:325-331 (1993)) proposed that the low-affinity channel-blocking antagonists such as dextromethoφhan may be less toxic than the higher affinity NMDA receptor antagonists such as ketamine. Despite this, dextromethoφhan' s efficacy in patients with peripheral neuropathic pain is limited by a ceiling analgesic effect, thought to be related to its dose-limiting toxicity (Nelson KA, et al. Neurology 48:1212-1218 (1997); Sang CN, et al. American Pain Society Abstracts, (1997)). However, high doses may be necessary to achieve analgesia, as high concentrations of dextromethoφhan in the central nervous system are required for neuroprotection (Steinberg GK, et al. J Neurosurg. 84:860-866 (1996)). Thus, inadequate dosing may have accounted for, in part, the inability of McQuay et al. (McQuay HJ, et al. Pain 59:127-133 (1994)) to demonstrate a sigmficant analgesic effect of dextromethoφhan alone at 81 mg/day in patients with neuropathic pain. Thus, the addition of a second agent that may reduce excitation via different mechanisms of action may have additive or synergistic therapeutic benefit. Because the doses of dextromethoφhan were not significantly different when given alone or in combination with gabapentin, we cannot clearly infer synergism of dextromethoφhan and gabapentin from this study. On the other hand, we can infer that there may less than additivity for the side effects of these two drugs. In addition to the possibility of independent mechanistic effects of the individual study drugs, either dextromethoφhan or gabapentin may also be enhancing the analgesic effects of concomitant analgesic medications that subjects were taking, although we were unable to detect such an effect. Prior studies in animal neuropathic pain models and patients with postoperative and chronic pain show that dextromethoφhan enhances opioid analgesia (Price DD, et al. J Pain Symptom Manage. 19(1 Suppl):S7-ll (2000) and Caruso FS. JPain Symptom Manage 19(1 Suppl):S31-6 (2000)). This study is the first randomized controlled clinical trial clearly demonstrating the analgesic and anti-hyperalgesic efficacy of any treatment for central neuropathic pain following spinal cord injury. In addition, this study has shown that a combination of analgesic compounds selective for at least two distinct mechanisms of action widens the therapeutic ratio, and infer that the combination produces at least additivity for analgesia but not for toxicity. Future studies may assess a potential synergistic relationship between component drugs. The results of this study offer hope of analgesic therapy for the treatment of SCI pain, which to date remains refractory to currently available therapies. EQUIVALENTS While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims: CLAIMS What is claimed is: 1. hi a method for treating central neuropathic pain in a human, the method including administering to the human an analgesic composition: Wherein the improvement comprises chronic administration to the human an analgesic composition that consists essentially of an N-methyl- D-aspartate receptor antagonist, and wherein there is essentially no high- affinity N-methyl-D-asparate receptor antagonist in the analgesic composition. 2. The method of Claim 1 , wherein the central neuropathic pain occurs acutely. 3. The method of Claim 1 , wherein the central neuropathic pain is chronic . 4. The method of Claim 1 , wherein the central neuropathic pain is caused by injury. 5. The method of Claim 1 , wherein the central neuropathic pain is caused by injury resulting in at least one member selected from the group consisting of trauma, surgery and lesion. 6. The method of Claim 4, wherein the injury is a spinal cord injury. 7. The method of Claim 4, wherein the spinal cord injury is selected from the group consisting of trauma, surgery and lesion. 8. The method of Claim 7, wherein the spinal cord injury is a trauma. 9. The method of Claim 8, wherein the trauma is a direct anatomical disruption of the spinal cord. 10. The method of Claim 8, wherein the trauma is associated with spinal cord damage. 11. The method of Claim 7, wherein the spinal cord injury is a lesion. 12. The method of Claim 11, wherein the lesion is syringomyelia. 13. The method of Claim 11 , wherein the lesion is selected from the group consisting of multiple sclerosis, a tumor within or surrounding the spinal cord, and a vascular lesion compressing the spinal cord. 14. The method of Claim 1 , wherein the central neuropathic pain is consequent to brain injury. 15. The method of Claim 14, wherein the brain injury is at least one member selected from the group consisting of trauma, surgery and lesion. 16. The method of Claim 1 , wherein the N-methyl-D-aspartate receptor antagonist is a subtype-selective N-methyl-D-aspartate receptor antagonist and wherein the . central neuropathic pain is consequent to spinal cord injury. 17. The method of Claim 1 , wherein the N-methyl-D-aspartate receptor antagomst is a low-affinity N-methyl-D-aspartate receptor antagonist. 18. The method of Claim 17, wherein the N-methyl-D-aspartate receptor antagonist is at least one member selected for the group consisting of dextromethoφhan hydrobromide, amantadine, memantine, remacemide, and riluzole. 19. The method of Claim 18, wherein the N-methyl-D-aspartate receptor antagonist consists essentially of dextromethoφhan hydrobromide. 20. The method of Claim 19, wherein the dextromethoφhan hydrobromide is administered at a dosage of greater than about 120 mg/day. 21. The method of Claim 20, wherein the dextromethoφan hydrobromide is administered for at least about twenty-eight days. 22. The method of Claim 21 , wherein the dosage is increased incrementally at least until an analgesic effect is obtained. 23. The method of Claim 22 wherein the dosage is increased about 60 mg per day. 24. The method of Claim 1, wherein the dosage is increased incrementally at least until an analgesic effect is obtained. 25. The method of Claim 24, wherein the increase in dosage is terminated when at least one side-effect is identified. 26. The method of Claim 25, wherein the side effect is at least one member of the group consisting of fatigue, confusion, dizziness, somnolence, and speech difficulty, psychosis, and other cognitive effects. 27. hi a method for treating central neuropathic pain in a human, the method including administering to the human an analgesic composition: Wherem the improvement comprises administering to the human an analgesic composition that consists essentially of an N-methyl-D- aspartate receptor antagonist and wherein the N-methyl-D-aspartate receptor antagonist does not include ketamine or a sub-type selective N- methyl-D-asparate receptor antagonist, 28. The method of Claim 27, wherein the administration occurs acutely. 29. Use of an N-methyl-D-aspartate receptor antagomst, or component thereof for the manufacture of a medicament that includes an analgesic component that consists essentially of the N-methyl-D-aspartate receptor antagonist, for the chronic treatment of central neuropathic pain. 30. A method of treating central neuropathic pain in a human, comprising the step of acutely administering to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagonist, and wherein the N- methyl-D-aspartate receptor antagonist essentially does not include ketamine. 31. A method of treating central neuropathic pain in a human, comprising the step of chronically administering to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagonist, and wherein there is essentially no high-affinity N-methyl-D-aspartate receptor antagonist in the analgesic composition. 32. A method of treating central neuropathic pain in a human, comprising the step of administering to the human an analgesic composition that consists essentially of an N-methyl-D-aspartate receptor antagomst, and wherein the N-methyl-D- aspartate receptor antagonist essentially does not include ketamine or a sub-type selective N-methyl-D-aspartate receptor antagonist.

Inventors: SANG CHRISTINE N, SANG, CHRISTINE, N.
Applicants: GEN HOSPITAL CORP, SANG CHRISTINE N, SANG, CHRISTINE, N., THE GENERAL HOSPITAL CORPORATION
Application year: 2001
Main classification: A61K45
IPC codes: A61K31, A61K45, A61P25