Gabapentin and the Neurokinin 1 Receptor Antagonist CI-1021 Act Synergistically in Two Rat Models of Neuropathic Pain

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1 /02/ $7.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 303, No. 2 Copyright 2002 by The American Society for Pharmacology and Experimental Therapeutics 33134/ JPET 303: , 2002 Printed in U.S.A. Gabapentin and the Neurokinin 1 Receptor Antagonist CI-1021 Act Synergistically in Two Rat Models of Neuropathic Pain MARK J. FIELD, 1 M. ISABEL GONZALEZ, 2 RONALD J. TALLARIDA, and LAKHBIR SINGH Department of Biology, Pfizer Global Research and Development, Cambridge Laboratories, Cambridge University, Cambridge, United Kingdom (M.J.F., M.I.G., L.S.); and Department of Pharmacology, Temple University School of Medicine, Philadelphia, Pennsylvania (R.J.T.) Received January 25, 2002; accepted June 6, 2002 ABSTRACT The present study examines the effect of combinations of gabapentin (Neurontin) and a selective neurokinin (NK) 1 receptor antagonist, 1-(1H-indol-3-ylmethyl)-1-methyl-2-oxo-2-[(1-phenylethyl)amino]ethyl]-2-benzofuranylmethyl ester (CI-1021), in two models of neuropathic pain. Dose responses to both gabapentin and CI-1021 were performed against static allodynia induced in the streptozocin and chronic constriction injury (CCI) models. Theoretical additive lines were calculated from these data. Dose responses to various fixed dose ratios of a gabapentin/ci-1021 combination were then examined in both models. In the streptozocin model, administration of gabapentin/ci-1021 combinations at fixed dose ratios of 1:1 and 60:1 resulted in an additive effect with dose response similar to the theoretical additive line. However, a synergistic interaction was seen after fixed dose ratios of 10:1, 20:1, and 40:1 with static allodynia completely blocked and 1 Current address: Pain Therapeutics, Discovery Biology, Pfizer Global Research and Development, Sandwich Laboratories (ipc 351), Sandwich, Kent CT13 9NJ, United Kingdom. 2 Current address: GlaxoSmithKline, New Frontiers Science Park, Harlow, Essex, CM19 5AW, UK. Article, publication date, and citation information can be found at DOI: /jpet the dose responses shifted approximately 8-, 30-, and 10-fold leftward, respectively, from the theoretical additive values. In the CCI model, after fixed dose ratios of 5:1 and 20:1, combinations of gabapentin and CI-1021 produced an additive response. At the fixed dose ratio of 10:1 static allodynia was completely blocked with an approximate 10-fold leftward shift of the dose response from the theoretical additive value, indicating synergy. The combination of gabapentin with a structurally unrelated NK 1 receptor antagonist, (2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine (CP-99,994), also produced synergy, at a fixed dose ratio of 20:1. This ratio completely blocked streptozocin-induced static allodynia and was approximately shifted leftward 5-fold from the theoretical additive value. These data suggest a synergistic interaction between gabapentin and NK 1 receptor antagonists in animal models of neuropathic pain. Neuropathic pain is a consequence of disease or trauma to peripheral nerves or the central nervous system. Thus, this type of pain affects many people with a wide range of ailments (e.g., trauma, diabetes, stroke, postherpetic neuralgia, and cancer). In the clinic, neuropathic pain has proved to be extremely difficult to treat. Neuropathic pain patients can present with a number of different pain subtypes, with allodynic sensations (pain to previously innocuous stimuli) being among the most common and debilitating. Conventional analgesics, such as opiates and nonsteroidal anti-inflammatory drugs have limited therapeutic value in the management of neuropathic pain. This has led to the use of adjuvant analgesics, such as antidepressants and anticonvulsants, for the management of these types of pains. However, no agent is fully effective in all patients and undesirable side effects are common (James and Page, 1994; Galer, 1995). In 1996, we described studies suggesting that gabapentin may be useful in the treatment of pain and anxiety (Singh et al., 1996). A number of preclinical reports followed demonstrating that gabapentin, unlike opiates, had no effect on transient physiological pain responses (Field et al., 1997a,b; Hunter et al., 1997; Shimoyama et al., 1997) but possessed antihyperalgesic and antiallodynic properties in animal models of inflammatory, surgical, and neuropathic pain (Xiao and Bennett, 1995; Singh et al., 1996; Field et al., 1997a,b, 1999a,b; Hunter et al., 1997; Shimoyama et al., 1997; Hwang and Yaksh, 1997). In 1998, the first randomized, double blind, placebo-controlled clinical trials were reported that demonstrated that gabapentin showed some efficacy in approximately 50% of patients with postherpetic neuralgia or diabetes-induced neuropathy (Backonja et al., 1998; Row- ABBREVIATIONS: NK, neurokinin; CCI, chronic constriction injury; PWT, paw withdrawal threshold; MED, minimum effective dose; CI-1021, 1-(1H-Indol-3-ylmethyl)-1-methyl-2-oxo-2-[(1-phenylethyl)amino]ethyl]-2-benzofuranylmethyl ester; CP-99994, (2S,3S)-3-(2-methoxybenzylamino)-2- phenylpiperidine. 730

2 Synergy between Gabapentin and NK 1 Receptor Antagonists 731 botham et al., 1998). These data confirmed the preclinical studies and also the growing body of anecdotal clinical evidence. For many years, it has been hypothesized that the peptide substance P plays an important role in pain processing due to its localization in and release from primary afferent fibers after noxious stimuli (Otsuka and Yoshioka, 1993). Substance P selectively interacts with the endogenous neurokinin (NK) 1 receptor. The development of selective, nonpeptide NK 1 receptor antagonists has been critical in elucidating the role of substance P in pain. A large amount of preclinical evidence suggests that these compounds possess antihyperalgesic and antiallodynic properties in models of pain (Ma and Hill, 1999). CI-1021 is a selective NK 1 receptor antagonist (Singh et al., 1997) that has been shown to block the maintenance of static allodynia in models of neuropathic pain (Field et al., 1998; Gonzalez et al., 2000). There have been a limited number of clinical trials of NK 1 receptor antagonists in pain (Dionne, 1999), but the clinical utility of these compounds has yet to be fully determined. Combination therapy is often used to increase the clinical utility of analgesic agents (Eisenach and Gebhart, 1999). The coadministration of two such compounds often achieves analgesia at lower doses than required for either compound alone, leading to enhanced pain relief and a reduction of side effects (Eisenach and Gebhart, 1999). Herein, we describe the effect of coadministration of gabapentin and CI-1021 in two models of neuropathic pain. Part of this work has been published previously in abstract form (Field et al., 2000). Materials and Methods Animals. Male Sprague-Dawley rats ( g), obtained from Charles River (Margate, Kent, UK), were housed in groups of six. All animals were kept under a 12-h light/dark cycle (lights on at 7:00 AM) with food and water ad libitum. All experiments were carried out by an observer who was unaware of drug treatments. Development of Diabetes in Rat. Diabetes was induced in rats by a single i.p. injection of streptozocin (50 mg/kg) as described previously (Field et al., 1999b). Control animals received a similar administration of isotonic saline. CCI Surgery in Rat. CCI surgery was performed as described previously (Field et al., 1999a). Effect of Combinations on the Maintenance of Streptozocin or CCI-Induced Static Allodynia. Dose responses to gabapentin, CI-1021, and CP-99,994 were first performed alone in the CCI and streptozocin models. The dose-response data for both compounds in the combination were used to determine theoretical additive lines using the method described by Tallarida (2000). Combinations were examined after a fixed ratio design. A dose response to each fixed dose ratio of the combination was performed and compared with the theoretical additive line. On each test day, baseline paw withdrawal thresholds (PWTs) to von Frey hairs were determined before drug treatment. All compounds were administered p.o. and PWT reexamined for up to 4 h. CI-1021 or CP-99,994 was administered directly after gabapentin. The data are expressed at the 1-h time point in the streptozocin model and at the 2-h time point in the CCI model because these times represent the peak antiallodynic effects. Evaluation of Static Allodynia. Static allodynia was measured using Semmes-Weinstein von Frey hairs (Stoelting, Wood Dale, IL). Animals were placed into wire bottom cages allowing access to the underside of their paws. Animals were habituated to this environment before the start of the experiment. Static allodynia was tested by touching the plantar surface of the animals right hind paw with von Frey hairs in ascending order of force (0.7, 1.2, 1.5, 2, 3.6, 5.5, 8.5, 11.8, 15.1, and 29 g) for up to 6 s. Once a withdrawal response was established, the paw was retested, starting with the next descending von Frey hair until no response occurred. The highest force of 29 g lifted the paw as well as eliciting a response, thus representing the cutoff point. The lowest amount of force required to elicit a response was recorded as the PWT in grams. Drugs. CI-1021 and CP-99,994 were synthesized at Pfizer Global Research and Development (Cambridge, UK), and gabapentin was synthesized at Pfizer Global Research and Development (Ann Arbor, MI). CI-1021 was dissolved in gelucire at a temperature of 65 C and once dissolved maintained at 45 C. Gabapentin was dissolved in water. CP-99,994 and streptozocin (Aldrich, Dorset, UK) were dissolved in 0.9% (w/v) NaCl. Drug administrations were made in a volume of 1 ml/kg. Gabapentin was administered orally directly followed by the oral administration of CI Data Analysis. Data for dose responses were subjected to a Mann-Whitney U test to compare drug values with controls. The dose-effect data for the individual compounds (acting alone) were used to calculate the theoretical (regression) line of additivity for each fixed ratio combination (Tallarida et al., 1997; Tallarida, 2000). The calculated potency value from the line of additivity was then compared with the corresponding value obtained experimentally to assess significance using the software package PharmToolsPro (The McCary Group, Elkins Park, PA). Results Effect of Gabapentin and CI-1021 Alone on Streptozocin-Induced Static Allodynia. CI-1021 dose dependently (1 30 mg/kg p.o.) blocked the maintenance of static allodynia with an MED of 3 mg/kg (Fig. 1). The antiallodynic action of CI-1021 lasted for over 3hatthedoses of 10 and 30 mg/kg (Fig. 1). Similar administration of gabapentin also dose dependently ( mg/kg p.o.) blocked the maintenance of static allodynia with an MED of 10 mg/kg (Fig. 1). Effect of Gabapentin and CI-1021 Alone on CCI-Induced Static Allodynia. The oral administration of either CI-1021 ( mg/kg) or gabapentin ( mg/kg) dose dependently blocked the maintenance of static allodynia with MEDs of 30 mg/kg (Fig. 2). The highest dose (100 mg/kg) of gabapentin, but not of CI-1021, completely blocked static allodynia (Fig. 2). Effect of Combinations of Gabapentin and CI-1021 on Streptozocin-Induced Static Allodynia. Gabapentin and CI-1021 had peak antiallodynic actions at 1-h postadministration in the streptozocin model. For clarity, only combination data for this time point are shown. The dose-response data for gabapentin and CI-1021 alone at 1 h were used to calculate theoretical additive lines (Fig. 3a) Gabapentin and CI-1021 were administered at fixed dose ratios of 1:1, 10:1, 20:1, 40:1, and 60:1. After a fixed dose ratio of 1:1 and 60:1, combinations of gabapentin and CI-1021 produced effects close to the theoretical additive line, indicating an additive response (Fig. 3). However, after fixed dose ratios of 10:1 20:1, and 40:1, static allodynia was completely blocked by respective total doses of 11, 3.15, and mg/kg compared with respective theoretical additive total doses of 82.5, 89.9, and 94.6 mg/kg (Fig. 3). This confirms a synergistic interaction between the two compounds with the 10:1, 20:1, and 40:1 ratios shifted approximately 8-, 30-, and 10-fold from the theoretical additive lines (Fig. 3). Both gabapentin and CI-1021 had similar durations of action when administered alone (Fig. 1) and in combination in this model (data not shown).

3 732 Field et al. Fig. 1. Effect of CI-1021 (a) and gabapentin (b) on the maintenance of diabetes-induced static allodynia. Baseline (BL) PWTs to von Frey hairs were determined in streptozocin-treated animals before drug administration. All compounds were administered p.o., and PWTs were reexamined up to 4 h postdrug. Results are expressed as median force (grams) required to induce paw withdrawal in 8 to 10 animals/group (vertical bars represent first and third quartiles)., P 0.05;, P 0.01;, P significantly different (Mann-Whitney U test) from vehicle-treated group at each time point. Effect of Combinations of Gabapentin and CI-1021 on CCI-Induced Static Allodynia. Gabapentin and CI had peak antiallodynic actions at 2 h postadministration in the CCI model. Thus, for clarity all combination data are expressed at this time point. The dose-response data for gabapentin and CI-1021 alone at 2 h were used to calculate theoretical additive lines (Fig. 4). Gabapentin and CI-1021 were administered at fixed dose ratios of 5:1, 10:1, and 20:1. After fixed dose ratios of 5:1 and 20:1, combinations of gabapentin and CI-1021 produced an additive interaction (Fig. 4). However, the fixed dose ratio of 10:1 demonstrated synergy with static allodynia completely blocked by a total dose of 11 mg/kg compared with a theoretical additive total dose of 110 mg/kg (Fig. 4). Both gabapentin and CI-1021 had similar durations of action when administered alone (Fig. 2) and in combination in this model (data not shown). Effect of Combinations of Gabapentin and CP-99,994 on Streptozocin-Induced Static Allodynia. A dose-response analysis with CP-99,994 showed that its peak antiallodynic effect occurs at 1 h post-treatment, similar to gabapentin. Thus, the combination study was expressed at this time point. The dose-response data for gabapentin and CP- 99,994 alone at 1 h were used to calculate theoretical additive line (Fig. 5a). Gabapentin and CP-99,994 were administered at a fixed dose ratio of 20:1. Static allodynia was completely blocked by a total dose of 21 mg/kg compared with a theoretical additive total dose of 105 mg/kg (Fig. 5b). This represents Fig. 2. Effect of CI-1021 (a) and gabapentin (b) on the maintenance of CCI-induced static allodynia. Baseline (BL) PWTs to von Frey hairs were determined in both paws (CL, contralateral paw) of CCI animals before drug administration. All compounds were administered p.o. and (PWT) were reexamined up to 4 h postdrug. Results are expressed as median force (grams) required to induce paw withdrawal in 8 to 10 animals/group (vertical bars represent first and third quartiles)., P 0.05;, P 0.01;, P significantly different (Mann-Whitney U test) from vehicle-treated group at each time point. a synergistic interaction with an approximate 5-fold leftward shift in the dose response compared with the theoretical additive line (Fig. 5b). Discussion We have previously reported that both gabapentin and CI-1021 can block the maintenance of streptozocin-induced static allodynia (Field et al., 1998, 1999b). The present study confirms and expands on these findings by demonstrating that both compounds can also block the maintenance of CCIinduced static allodynia. The major finding of the present study was the synergistic interaction between the two compounds when coadministered in both the streptozocin and CCI models of neuropathic pain. It is known that CI-1021 shows species differences with respect to its affinity for the NK 1 receptor. Thus, it possesses at least 2 orders of magnitude higher affinity for the human and guinea pig than the rodent NK 1 receptor (Singh et al., 1997). This difference is apparent in neuropathic pain models where 100-fold higher doses are required to block pain responses in the rat compared with the guinea pig (Field et al., 1998; Gonzalez et al., 2000). It could be suggested that CI-1021 might loose selectivity at high doses and that the antiallodynic action might not involve the NK 1 receptor. However, results from a number of studies seem to argue against this and support the

4 Synergy between Gabapentin and NK 1 Receptor Antagonists 733 Fig. 3. Effect of fixed dose ratios of gabapentin and CI-1021 on the maintenance of diabetes-induced static allodynia. All data are expressed at the 1-h time point postdrug administration. a, dose-response data for gabapentin and CI-1021 alone. Fixed dose ratios of 1:1 (b), 10:1 (c), 20:1 (d), 40:1 (e), and 60:1 (f) gabapentin and CI-1021 combinations. Theoretical additive lines were calculated from dose-response data in a. All compounds were administered p.o., and PWTs to von Frey hairs were examined 1 h postdrug administration. Results are expressed as median force (grams) required to induce paw withdrawal in six to eight animals per group (vertical bars represent first and third quartiles). involvement of the NK 1 receptor. Radioligand binding studies have shown that CI-1021 is a highly selective NK 1 receptor antagonist (Singh et al., 1997), and we have demonstrated that CI-1021 dose dependently blocks the tactile and thermal hypersensitivity induced by activation of central NK 1 receptors in the rat (Field et al., 1998; Gonzalez et al., 1998). Moreover, this antagonism was complete and required the same doses as those that blocked allodynia/ hyperalgesia in models of neuropathic pain. Furthermore, at these doses CI-1021 had no effect on the NK 2 receptormediated tactile hypersensitivity (Field et al., 1998) and only a partial and transient effect on the NK 2 receptormediated thermal hypersensitivity (Gonzalez et al., 1998). The most compelling evidence that the synergy between CI-1021 and gabapentin reported herein is related to an interaction with at the NK 1 receptor is provided by our findings that the structurally unrelated NK 1 receptor antagonist CP-99,994 also synergizes with gabapentin. The mechanisms responsible for synergistic interactions are poorly understood, but a number of hypotheses have been

5 734 Field et al. Fig. 4. Effect of fixed dose ratios of gabapentin and CI-1021 on the maintenance of CCI-induced static allodynia. All data are expressed at the 2-h time point postdrug administration. a, dose-response data for gabapentin and CI-1021 alone. Fixed dose ratios of 5:1 (b), 10:1 (c), and 20:1 (d) gabapentin and CI-1021 combinations. Theoretical additive lines were calculated from dose-response data. All compounds were administered p.o., and PWTs to von Frey hairs were examined 2 h postdrug administration. Results are expressed as median force (grams) required to induce paw withdrawal in six to eight animals per group (vertical bars represent first and third quartiles). developed to explain these effects (Yaksh and Malmberg, 1994). These include the interaction being pharmacokinetic, where one drug increases the active levels of the other by reducing its rate of clearance or altering its metabolism. The interaction may also be pharmacodynamic where concurrent activation of distinct systems may modulate a common pathway or one compound may enhance the affinity or coupling of the other. It may also be due to physiological interactions where a third endogenous substance is released by the combination. Although a number of analgesic drug combinations have been demonstrated to produce a synergistic interaction, the mechanism(s) for their synergism are not generally known (Tallarida, 2001). In fact, synergism is actually difficult to demonstrate precisely. This demonstration requires an experimental design and a statistical analysis that distinguishes between simple additivity and superadditivity. Indeed, many claims of synergism lack the precision that is required in this demonstration (Gebhart, 1992a,b; Tallarida, 1992). Our results and experiments demonstrate that the NK1 receptor (and substance P) is involved, because we showed that two structurally different NK 1 antagonists produced the synergism. Equally important is our demonstration that the enhanced response is not merely a property of gabapentin and the NK 1 antagonists. The enhancement also depends on the ratio of the constituents in the combination and on the model used. Further work is required to elucidate the exact mechanism of synergy between gabapentin and NK 1 receptor antagonists. However, the lack of change in duration of action of each active fixed dose ratio suggests a pharmacodynamic rather than a pharmacokinetic interaction. Further studies examining plasma and central nervous system levels of each compound after administration of the combinations are required to fully evaluate this. Previous studies have suggested that both compounds work via central mechanisms of action but involve distinct mechanisms (Field et al., 1998, 1999b), thus it is likely that this synergy is mediated centrally. The present data demonstrate a synergistic interaction between gabapentin and NK 1 receptor antagonists in animal models of neuropathic pain. Preclinical data clearly demonstrate an important role for substance P in the mediation of pain (Ma and Hill, 1999). However, the role of substance P in the human remains to be clearly determined. The limited clinical studies to date with NK 1 receptor antagonists have been disappointing (Hill, 2000), but it has been suggested that the compounds used as well as the design of the trials may not have been optimal (Urban and Fox, 2000). It is possible that substance P plays more of a modulatory role in the human compared with the rat. If this is the case then monotherapy with NK 1 receptor antagonists may not demonstrate any clinical efficacy. Gabapentin is effective in the treatment of neuropathic pain (Backonja et al., 1998; Rowbotham et al., 1998). These studies have demonstrated that gabapentin has a good side effect profile compared with other

6 Synergy between Gabapentin and NK 1 Receptor Antagonists 735 Fig. 5. Effect of a fixed dose ratio of gabapentin and CP-99,994 on the maintenance of diabetes-induced static allodynia. All data are expressed at the 1-h time point postdrug administration. a, dose response for gabapentin and CP-99,994. b, fixed dose ratio of 20:1 gabapentin and CP- 99,994 combination. Theoretical additive lines were calculated from doseresponse data. All compounds were administered p.o., and PWTs to von Frey hairs were examined 1-h postdrug administration. Results are expressed as median force (grams) required to induce paw withdrawal in six to eight animals per group (vertical bars represent first and third quartiles). existing agents used to treat neuropathic pain with somnolence and sedation being the main side effects. It is interesting to note that in these controlled clinical trials gabapentin was effective in approximately 50% of patients. This represents a good response rate for neuropathic pain treatment but suggests there are still patients refractory to treatment. In conclusion, the present data demonstrate a synergistic interaction between gabapentin and NK 1 receptor antagonists in animal models of neuropathic pain. It is suggested that the clinical utility of gabapentin may be enhanced if it is coadministered with an NK 1 receptor antagonist such as CI References Backonja M, Beydoun A, Edwards KR, Schwartz SL, Fonseca V, Hes M, LaMoreaux L, and Garofalo E (1998) Gabapentin for the treatment of painful neuropathy in patients with diabetes mellitus. J Am Med Assoc 280: Dionne RA (1999) Clinical analgesic trials of NK 1 antagonists. Curr Opin Investig Drugs 1: Eisenach JC and Gebhart GF (1999) Spinal drug interactions, in Novel Aspects of Pain Management: Opioids and Beyond (Sawynok J and Cowan A eds) pp , Wiley-Liss, New York. Field MJ, Bramwell S, Hughes J, and Singh L (1999a) Detection of static and dynamic components of mechanical allodynia in rat models of neuropathic pain: are they signaled by distinct primary sensory neurones? Pain 83: Field MJ, Holloman EF, McCleary S, Hughes J, and Singh L (1997a) Evaluation of gabapentin and S-( )-3-isobutylgaba in a rat model of postoperative pain. J Pharmacol Exp Ther 282: Field MJ, McCleary S, Boden P, Suman-Chauhan N, and Singh L (1998) Involvement of the central Tachykinin NK 1 receptor during the maintenance of tactile allodynia induced by diabetic neuropathy in the rat. J Pharmacol Exp Ther 285: Field MJ, McCleary S, Hughes J, and Singh L (1999b) Gabapentin and pregabalin, but not morphine and amitriptyline, block both static and dynamic components of mechanical allodynia induced by streptozocin in the rat. Pain 80: Field MJ, McCleary S, and Singh L (2000) Gabapentin and the NK 1 receptor antagonist CI-1021 act synergistically to block allodynia induced in a rat model of neuropathic pain. Br J Pharmacol 129 (Suppl):79P. Field MJ, Oles RJ, Lewis AS, McCleary S, Hughes J, and Singh L (1997b) Gabapentin (neurontin) and S-( )-3-isobutylgaba represent a novel class of selective antihyperalgesic agents. Br J Pharmacol 121: Galer BS (1995) Neuropathic pain of peripheral origin: advances in pharmacologic treatment. Neurology 45 (Suppl 9):S17 S25. Gebhart GF (1992a) Topical reviews: a new type of report in pain. Pain 49:1. Gebhart GF (1992b) Comments on the isobole method. Pain 51:381. Gonzalez MI, Field MJ, Holloman EF, Hughes J, Oles RJ, and Singh L (1998) Evaluation of PD , a NK 1 receptor antagonist, in a rat model of postoperative pain. Eur J Pharmacol 344: Gonzalez MI, Field MJ, Hughes J, and Singh L (2000) Evaluation of the selective NK1 receptor antagonist CI-1021 in animal models of inflammatory and neuropathic pain. J Pharmacol Exp Ther 294: Hill RG (2000) NK1 (substance P) receptor antagonists why are they not analgesic in humans? Trends Pharmacol 21: Hunter JC, Gorgas KR, Hedley LR, Jacobson LO, Kassotakis L, Thompson J, and Fontana DJ (1997) The effect of novel anti-epileptic drugs in rat experimental models of acute and chronic pain. Eur J Pharmacol 324: Hwang JH and Yaksh TL (1997) Effect of subarachnoid gabapentin on tactile-evoked allodynia in a surgically induced neuropathic model in the rat. Reg Anesth 22: James JS and Page JC (1994) Painful diabetic peripheral neuropathy. A stepwise approach to treatment. J Am Podiatr Med Assoc 84: Ma Q-P and Hill RG (1999) Neurokinin antagonists as potential agents for use in pain management. Curr Opin Investig Drugs 1: Otsuka M and Yoshioka K (1993) Neurotransmitter functions of mammalian tachykinins. Physiol Rev 73: Rowbotham M, Harden N, Stacey B, Bernstein P, and Magnus-Miller L (1998) Gabapentin for the treatment of postherpetic neuralgia. J Am Med Assoc 280: Shimoyama N, Shimoyama M, Davis AM, Inturrisi CE, and Elliott KJ (1997) Spinal gabapentin is antinociceptive in the rat formalin test. Neurosci Lett 222: Singh L, Field MJ, Ferris P, Hunter JC, Oles RJ, Williams RG, and Woodruff GN (1996) The antiepileptic agent gabapentin (Neurontin) possesses anxiolytic-like and antinociceptive actions that are reversed by D-Serine. Psychopharmacology 127:1 9. Singh L, Field MJ, Hughes J, Kuo B, Suman-Chauhan N, Tuladhar BR, Wright DS, and Naylor RJ (1997) The tachykinin NK 1 receptor antagonist PD blocks cisplatin-induced delayed emesis in the ferret. Eur J Pharmacol 321: Tallarida RJ (1992) Statistical analysis of drug combinations for synergism. Pain 49: Tallarida RJ (2000) Drug Synergism and Dose-Effect Data Analysis, CRC Press, Boca Raton, FL. Tallarida RJ (2001) Drug synergism: its detection and application. Perspectives in pharmacology. J Pharmacol Exp Ther 298: Tallarida RJ, Stone DJ, and Raffa RB (1997) Efficient designs for studying synergistic drug combinations. Life Sci 61:PL417 PL425. Urban LA and Fox AJ (2000) NK1 receptor antagonists are they really without effect in the pain clinic. Trends Pharmacol Sci 21: Xiao W and Bennett GI (1995) Gabapentin relieves abnormal pains in a rat model of painful peripheral neuropathy. Soc Neurosci Abstr 21:356. Yaksh TL and Malmberg AB (1994) Interaction of spinal modulatory receptor systems, in Progress in Pain Research and Management (Fields HL and Liebeskind JC eds) pp , IASP Press, Seattle. Address correspondence to: Mark Field, Discovery Biology, Pfizer Global Research and Development, Sandwich Laboratories (ipc 351), Sandwich, Kent CT13 9NJ, United Kingdom. mark_j_field@sandwich.pfizer.com