Anesth Analg 2003;96:191-194
© 2003 International Anesthesia Research Society
PAIN MEDICINE
Intrathecal Ketorolac Enhances Antinociception from Clonidine
Dawn R. Conklin, BA, and
James C. Eisenach, MD
From the Department of Anesthesiology and Center for the Study of Pharmacologic Plasticity in the Presence of Pain, Wake Forest University School of Medicine, Winston-Salem, North Carolina
Address correspondence and reprint requests to James C. Eisenach, MD, Department of Anesthesiology, Wake Forest University School of Medicine, Medical Center Blvd., Winston-Salem, NC 27157–1009. Address e-mail to eisenach{at}wfubmc.edu
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Abstract
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Although both 
2-adrenergic agonists and cyclooxygenase inhibitors produce analgesia, their exact sites of action and interaction remain unclear. A previous report demonstrated a surprising inhibition of antinociception in rats from intrathecal clonidine by co-administered ketorolac. There are no other reports of interaction between these two classes of analgesics. We therefore reexamined this interaction, determining the effect of intrathecal clonidine and ketorolac alone and in combination in normal rats. Clonidine, but not ketorolac, produced antinociception to noxious hind paw thermal stimulation. The addition of ketorolac significantly enhanced the effect of clonidine, indicating a synergistic interaction for analgesia. Although the reasons for the discrepancy between this and the previous report are unclear, these results are consistent with previous studies that indicate an antinociceptive action of intrathecal 2-adrenergic agonists in the normal condition, a lack of such effect for cyclooxygenase inhibitors, and positive reinforcing effects of these two systems when co-stimulated.
IMPLICATIONS: Spinal injection of the 2-adrenergic agonist clonidine and the cyclooxygenase inhibitor ketorolac results in a synergistic interaction for antinociception in normal animals, suggesting that the combination of these drugs will enhance rather than detract from the analgesia of either alone.
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Introduction
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The study of intrathecal application of drugs for the treatment of pain is important for two reasons. First, it is directly relevant to anesthesia practice in that the intrathecal space is often instrumented as part of perioperative, peripartum, or chronic pain care, and currently available intrathecal analgesics have significant shortcomings. Second, it provides important fundamental information regarding mechanisms of analgesic action and of pain transmission, which could guide pharmaceutical development of both intrathecal and systemic drug development. A good example of these rationales is examination of cyclooxygenase (COX) enzyme expression and inhibition in the spinal cord as it relates to pain treatment. COX is expressed in the normal spinal cord in small amounts, both isoforms COX-1 and COX-2, with the latter predominating (1). Indeed, the constitutive presence of COX-2 in the spinal cord has been suggested to underlie the early analgesic effect of COX inhibitors after surgery or other peripheral injury and at times before peripheral COX-2 expression is increased. After peripheral injury, spinal COX-2 expression is greatly enhanced, leading to increased spinal release of prostaglandins with resultant increased substance P release and central sensitization (2). For this reason, spinally administered COX inhibitors produce analgesia after injury. We have recently completed preclinical toxicity screening of a COX inhibitor for intrathecal administration (JC Eisenach, unpublished observations) and, with the Food and Drug Administration, have begun clinical trials with this therapy.
It is becoming increasingly apparent that single drugs are unlikely to produce effective analgesia with minimal side effects, especially in the setting of chronic pain. For this reason, the study of drug interactions is relevant. Intrathecally administered COX inhibitors have been demonstrated to enhance analgesic effects of intrathecal opioids (3). Surprisingly, however, the other class of approved intraspinal analgesics, 2-adrenergic agonists, has been reported to be antagonized by the COX inhibitor ketorolac (4). Although there are complex interactions between noradrenergic and prostaglandin systems in the periphery, it is suggested that spinal prostaglandin production reduces norepinephrine release (5). There is no clear reason, based on known pharmacology, physiology, and anatomy of the 2-adrenergic and COX systems, why these two should interact in an antagonistic manner. The purpose of the current study was to test the broadness of the previous observation of antagonism between intrathecal clonidine and ketorolac in normal animals in a different laboratory and using a slightly different noxious heat stimulus.
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Methods
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After Animal Care and Use Committee approval, adult male Sprague-Dawley rats (240–330 g; Harlan, Indianapolis, IN) were anesthetized with halothane. A catheter was inserted, as previously described, through a small nick in the cisterna magnum membrane into the intrathecal space and advanced 7.5 cm such that its tip lay in the lumbar intrathecal space. Rats recovered uneventfully, and proper catheter tip location was determined by appropriate bilateral lower-extremity motor block from an injection of 10 µL of 2% lidocaine through the catheter on the day after preparation. After catheter implantation, rats were housed individually with a 12 h/12 h light/dark cycle and unlimited supply of water and food. Experiments occurred at least 6 days after surgery.
Antinociception was determined in response to noxious heat stimulus, which was applied using radiant heat from a focused high-intensity lamp. Rats were acclimated to the testing apparatus in which they were placed in Plexiglas containers on a glass surface maintained at 30°C. The lamp and lens were positioned under a hind paw and then activated. The lamp was automatically turned off when the rat rapidly raised its paw from the glass surface or at 30 s, a cutoff used to avoid tissue damage during periods of intense analgesia. Current to the lamp was adjusted between 5.0 and 5.3 A to obtain baseline withdrawal latencies of approximately 10 s and was thereafter not altered during the experiment.
After determination of baseline latency, rats received intrathecal injections in a 5-µL volume, followed by an injection of 15 µL of sterile saline to flush the catheter. Withdrawal latency was determined every 20 min for 80 min thereafter. The effect of clonidine was determined by the intrathecal injection of 1 and 10 µg. We studied the maximum dose of ketorolac (50 µg) obtainable in 10 µL using the commercial preparation of ketorolac, Acular PF, to be used in clinical studies, alone and with 1 and 10 µg of clonidine to test the interaction between these two. This formulation of ketorolac is in preservative-free saline. Drugs were purchased from Sigma Chemical Co (St Louis, MO) or Allergan (AcularPF; Palo Alto, CA) and were prepared in sterile normal saline. Spontaneous behavior (movement in the test apparatus and walking when placed unrestricted on a smooth surface) was also observed after drug treatment as a gross screen for sedative or motor effects. All injections were performed in an open-label manner.
Data were converted to percent maximum possible effect (%MPE, [observed latency - baseline latency]/[30 s - baseline latency]) and are presented as %MPE over time or its integral (area under the %MPE/time curve) as mean ± SE. Data were analyzed by one-way repeated-measures analysis of variance followed by the Dunnett test within individual dose groups to determine time course of effect and by two-way repeated-measures analysis of variance to determine differences between clonidine alone and clonidine plus ketorolac. P < 0.05 was considered significant.
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Results
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All rats recovered uneventfully from surgery, and lidocaine produced bilateral motor block in all cases. There were no prolonged effects observed from any of the tested drugs or doses, nor was gross motor block observed from any of the test drug injections.
Clonidine produced antinociception, as measured by increased latency to paw withdrawal from noxious heat, whereas ketorolac did not (Fig. 1). The addition of ketorolac to clonidine resulted in increased antinociception (Fig. 2).
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Figure 1. Antinociception, measured as percent maximum possible effect (%MPE) after intrathecal injection at time 0 of ketorolac 50 µg (solid squares) or clonidine 1 µg (open circles) or 10 µg (open squares).
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Figure 2. Antinociception, measured as percent maximum possible effect (%MPE) after intrathecal injection of clonidine (open circles) alone or clonidine plus ketorolac 50 µg (closed circles). *P < 0.05 compared with clonidine alone.
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Discussion
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The interaction between clonidine and ketorolac in the current study was technically synergism because ketorolac had no effect alone but enhanced the effect of clonidine. These results therefore disagree with a previous report (4) indicating an antagonism of clonidine-induced antinociception by ketorolac. The reasons for this discrepancy are unclear, yet two factors would lead one to expect enhancement rather than antagonism by the combination of these drugs. First, 2-adrenergic agonists act by a different mechanism than COX inhibitors. 2-adrenergic receptors are both pre- and postsynaptically located relative to primary afferent terminals in the spinal cord (6). Stimulation of 2-adrenergic receptors by agonist exposure in vitro and in vivo reduces release of excitatory neurotransmitters from noxious stimuli in the spinal cord, including substance P (7), calcitonin gene-related peptide (8), and glutamate (9). Additionally, stimulation of 2-adrenergic receptors hyperpolarizes dorsal horn neurons with rostral projections (10), indicating direct postsynaptic effects. 2-adrenergic receptors are coupled primarily to Gi/Go G proteins and reduce adenylyl cyclase activity as well as increasing resting K+ conductance (4,11). In contrast, COX inhibitors prevent synthesis of sensitizing prostaglandins, primarily prostaglandin E2 (PGE2), in the spinal cord. Prostaglandins act both pre- (2) and postsynaptically (12) to enhance excitatory neurotransmission in the spinal cord, reflecting both exaggerated glutamate, calcitonin gene-related peptide (CGRP), and substance P release and exaggerated response to their release.
Spinally produced PGE2 acts at several receptor subtypes. EP1 antagonists block allodynia from acute blockade of 
-aminobutyric acid receptors with intrathecal bicuculline (13) and from partial sciatic nerve section (14) or sciatic nerve constriction injury (15). Mice lacking the EP1 receptor gene exhibit decreased response to intraperitoneal acetic acid (16) and decreased allodynia from intrathecal PGE2 (17). There is also evidence for other subtypes in pain transmission: EP2 antagonists selectively block postsynaptic excitation in dorsal horn neurons induced by PGE2 (12), and mice lacking the EP3 receptor show decreased thermal hyperalgesia from intrathecal PGE2 (17). In most cases, these receptors are coupled to different second messenger systems than 2-adrenergic receptors, so one would expect enhancement by combining these drugs that act via different mechanisms.
Second, spinally released norepinephrine stimulates PGE2 synthesis by an action on 1-adrenoceptors (5), and this PGE2 synthesis is thought to reduce the net antinociceptive effect of norepinephrine. Clonidine, although selective for 2-adrenergic receptors, has some 1-adrenergic agonist activity at large concentrations (18), such as might occur after intrathecal injections. One would expect that this spillover into 1-adrenergic receptor stimulation by clonidine would diminish its net antinociception and that this reduction would be abolished by the prevention of PGE2 synthesis by ketorolac.
It is conceivable that the difference between the current report (synergy between ketorolac and clonidine) and the previous report (antagonism of clonidine by ketorolac) (4) could also reflect the different proportions at which they were mixed (50:1 or 5:1 in the current report compared with 1:1 in the previous report). Drug interactions vary with ratio of their combination, not only quantitatively, but also qualitatively, and there are other examples of drugs that interact with antagonism at one ratio but with additivity or synergy at another ratio (19). This explanation would suggest that several ratios of these drugs be investigated in humans rather than focus on a single ratio.
The lack of efficacy of intrathecal ketorolac alone in the current study was not surprising given the lack of efficacy of intrathecal COX inhibitors to tail-flick, hot-plate, or paw withdrawal to noxious heat (20). In contrast, intrathecal COX inhibitors are effective after inflammation (21) or peripheral nerve injury models of mechanical hypersensitivity and neuropathic pain (22). Because intrathecally administered 2-adrenergic agonists increase in potency and efficacy in such models (23) and epidural clonidine effectively treats patients with neuropathic pain (24), interactions between 2-adrenergic agonists and COX inhibitors might be even more positive in hypersensitivity states.
In summary, intrathecal ketorolac lacks efficacy in normal rats subjected to acute, noxious heat stimuli but enhances the effect of intrathecal clonidine. These data refute a previous report and provide a rationale for clinical study of the combination of these drugs.
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Acknowledgments
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Supported, in part, by NIH grants GM35523 and NS41386.
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References
|
|---|
- Maihöfner C, Tegeder I, Euchenhofer C, et al. Localization and regulation of cyclo-oxygenase-1 and -2 and neuronal nitric oxide synthase in mouse spinal cord. Neuroscience 2000; 101: 1093–1108.[ISI][Medline]
- Malmberg AB, Yaksh TL. Capsaicin-evoked prostaglandin E2 release in spinal cord slices: relative effect of cyclooxygenase inhibitors. Eur J Pharmacol 1994; 271: 293–9.[ISI][Medline]
- Malmberg AB, Yaksh TL. Pharmacology of the spinal action of ketorolac, morphine, ST-91, U50488H, and L-PIA on the formalin test and an isobolographic analysis of the NSAID interaction. Anesthesiology 1993; 79: 270–81.[ISI][Medline]
- Tejwani GA, Rattan AK. Antagonism of antinociception produced by intrathecal clonidine by ketorolac in the rat: the role of the opioid system. Anesth Analg 2000; 90: 1152–6.[Abstract/Free Full Text]
- Taiwo YO, Levine JD. Prostaglandins inhibit endogenous pain control mechanisms by blocking transmission at spinal noradrenergic synapses. J Neurosci 1988; 8: 1346–9.[Abstract]
- Howe JR, Yaksh TL, Go VL. The effect of unilateral dorsal root ganglionectomies or ventral rhizotomies on 2-adrenoceptor binding to, and the substance p, enkephalin, and neurotensin content of, the cat lumbar spinal cord. Neuroscience 1987; 21: 385–94.[ISI][Medline]
- Kuraishi Y, Hirota N, Sato Y, et al. Noradrenergic inhibition of the release of substance P from the primary afferents in the rabbit spinal dorsal horn. Brain Res 1985; 359: 177–82.[ISI][Medline]
- Bourgoin S, Pohl M, Mauborgne A, et al. Monoaminergic control of the release of calcitonin gene- related peptide- and substance P-like materials from rat spinal cord slices. Neuropharmacology 1993; 32: 633–40.[ISI][Medline]
- Kamisaki Y, Hamada T, Maeda K, et al. Presynaptic 2 adrenoceptors inhibit glutamate release from rat spinal cord synaptosomes. J Neurochem 1993; 60: 522–6.[ISI][Medline]
- North RA, Yoshimura M. The actions of noradrenaline on neurones of the rat substantia gelatinosa in vitro. J Physiol 1984; 349: 43–55.[Abstract/Free Full Text]
- Uhlén S, Wikberg JES. Inhibition of cyclic AMP production by 2-adrenoceptor stimulation in the guinea-pig spinal cord slices. Pharmacol Toxicol 1988; 63: 178–82.[ISI][Medline]
- Baba H, Kohno T, Moore KA, Woolf CJ. Direct activation of rat spinal dorsal horn neurons by prostaglandin E2. J Neurosci 2001; 21: 1750–6.[Abstract/Free Full Text]
- Zhang ZZ, Hefferan MP, Loomis CW. Topical bicuculline to the rat spinal cord induces highly localized allodynia that is mediated by spinal prostaglandins. Pain 2001; 92: 351–61.[ISI][Medline]
- Syriatowicz J-P, Hu D, Walker JS, Tracey DJ. Hyperalgesia due to nerve injury: role of prostaglandins. Neuroscience 1999; 94: 587–94.[ISI][Medline]
- Kawahara H, Sakamoto A, Takeda S, et al. A prostaglandin e(2) receptor subtype ep(1) receptor antagonist (ono- 8711) reduces hyperalgesia, allodynia, and c-fos gene expression in rats with chronic nerve constriction. Anesth Analg 2001; 93: 1012–7.[Abstract/Free Full Text]
- Stock JL, Shinjo K, Burkhardt J, et al. The prostaglandin E2 EP1 receptor mediates pain perception and regulates blood pressure. J Clin Invest 2001; 107: 325–31.[ISI][Medline]
- Nakano H, Kobayashi T, Sugimoto Y, et al. Characterization of EP receptor subtypes responsible for prostaglandin E2-induced pain responses by use of EP1 and EP3 receptor knockout mice. Br J Pharmacol 2001; 133: 438–44.[ISI][Medline]
- Kobinger W, Pichler L. 1/2 Selectivity ratio in a series of agonists and their relation to pre/postsynaptic activity ratios. Eur J Pharmacol 1983; 91: 129–33.[ISI][Medline]
- Tallarida RJ, Porreca F, Cowan A. Statistical analysis of drug-drug and site-site interactions with isobolograms. Life Sci 1989; 45: 947–61.[ISI][Medline]
- McCormack K. Non-steroidal anti-inflammatory drugs and spinal nociceptive processing. Pain 1994; 59: 9–43.[ISI][Medline]
- Yaksh TL, Dirig DM, Conway CM, et al. The acute antihyperalgesic action of nonsteroidal, anti-inflammatory drugs and release of spinal prostaglandin E2 is mediated by the inhibition of constitutive spinal cyclooxygenase-2 (COX-2) but not COX-1. J Neurosci 2001; 21: 5847–53.[Abstract/Free Full Text]
- Zhao Z, Chen SR, Eisenach JC, et al. Spinal cyclooxygenase-2 is involved in development of allodynia after nerve injury in rats. Neuroscience 2000; 97: 743–8.[ISI][Medline]
- Puke MJC, Wiesenfeld-Hallin Z. The differential effects of morphine and the 2-adrenoceptor agonists clonidine and dexmedetomidine on the prevention and treatment of experimental neuropathic pain. Anesth Analg 1993; 77: 104–9.[Abstract/Free Full Text]
- Eisenach JC, DuPen S, Dubois M, et al. Epidural clonidine analgesia for intractable cancer pain. Pain 1995; 61: 391–9.[ISI][Medline]
Accepted for publication September 10, 2002.
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Abstract
Introduction
