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© 1999 International Anesthesia Research Society
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Abstract |
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Although local anesthetics can, in some situations, alleviate neuropathic pain, currently available preparations are short-acting and nonselective, producing, for example, motor dysfunction. Clinical studies report that a novel suspension preparation of butamben has the advantage of a prolonged duration of action, and it can be used epidurally, without impairment of motor function. In this behavioral study, we investigated the effect of the epidural administration of a 5% butamben suspension on nerve injury-induced allodynia. Behavioral studies were performed using an established animal model of neuropathic pain, which involves a partial ligation of the sciatic nerve. Nociceptive thresholds to mechanical stimulation were determined by the paw withdrawal method. The allodynia to mechanical stimulation induced by partial nerve ligation was significantly attenuated by daily injections, for 5 days, of 10 µL of butamben suspension. The analgesia lasted at least 7 days after the final injection. Daily injections of 10 µL of vehicle, for 5 days, had no significant effect on allodynia. During the period of daily injections, both the butamben and vehicle treated rats had temporary impairment of motor coordination compared with untreated controls. Motor function recovered after the final injection. Neither daily injections of butamben for 2 or 3 days, nor smaller volumes for 5 days (2.5–5 µL), had a long-lasting effect. We conclude that repeated epidural administration of butamben suspension for several days provides long-lasting analgesia in rats with nerve injury-induced allodynia to mechanical stimulation.
Implications: In this animal behavioral study, using rats with nerve injury-induced pain, we examined the possible long-term analgesic effects of epidural administration of a suspension of the local anesthetic, butamben. We found that multiple doses for several days were required to provide a prolonged analgesia.
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Introduction |
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Neuropathic pain associated with peripheral nerve injury is characterized by spontaneous pain, increased responsiveness to normally noxious stimuli (hyperalgesia), and normally nonnoxious stimuli are perceived as painful (allodynia). Both human and animal studies have shown that these symptoms are relieved, at least temporarily, by regional nerve block (1) or IV administration of local anesthetic (2–5). The actual site of action for the analgesic effects of local anesthetics in this condition is not known. It is suggested that, in normal animals, analgesia after systemic injection of local anesthetic results from an action in the spinal cord, because IV lidocaine, at doses that do not block impulse conduction, inhibits the C-fiber-evoked polysynaptic reflex generated by sural nerve stimulation (6). In animals with nerve injury, however, analgesic doses of IV local anesthetic attenuates the spontaneous activity generated at both the site of injury and the dorsal root ganglia (DRG) (7). It is presumed that such spontaneous activity developing after nerve injury contributes to spontaneous pain and induces and maintains central sensitization (8,9), which possibly mediates, in part, hyperalgesia and allodynia (10). There is also evidence that systemic administration of local anesthetic may act centrally, attenuating the nerve injury-induced abnormal physiology of spinal neurons (11).
Although local anesthetics provide some relief in neuropathic pain syndromes, they, unfortunately, have several disadvantages. In general, local anesthetics tend to be short-acting and nonselective for sensory neurons, and repeated administration results in tachyphylaxis. As a consequence of the nonselectivity, there are significant side effects, with motor dysfunction being the most prominent. Other undesirable aspects of nonselectivity include bladder and bowel dysfunction, self-injury as a result of blockade of Aß-fibers, and hypotension caused by sympathetic blockade. Furthermore, among contemporary local anesthetics, the amino-amides are relatively toxic, with a therapeutic index close to two. Major target organs for toxicity are the central nervous system and cardiovascular system (12). It is evident that an alternative therapeutic drug that is selective and can be used long term is needed for the treatment of nerve injury-induced pain.
We examined the influence of a novel suspension preparation of the amino-ester, butamben, on nerve injury-induced allodynia. Clinical studies of terminally ill patients with cancer show that epidural administration of this suspension preparation has the advantage of inducing a long-acting analgesia without apparent motor dysfunction (13,14). It is proposed that the apparent selectivity and long duration of epidurally administered butamben is a result of the suspension’s being confined to the epidural space and acting as a depot on the dorsal roots, limiting the spread of butamben ventrally and providing a slow release from the small particles in the suspension (13). The aim of this animal behavioral study was to investigate the effect of the epidural administration of a suspension of 5% butamben on nerve injury-induced allodynia to mechanical stimulation. Allodynia was induced by a partial ligation of the sciatic nerve, which is an established model of neuropathic pain (15).
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Methods |
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Implantation of Epidural Catheters
Catheters (18 cm of polyethylene tubing [PE-10]) were implanted epidurally by a method similar to that previously described by Durant and Yaksh (16). Briefly, male Wistar rats (270–300 g; Gore Hill Animal Research Laboratory, Sydney, Australia) were anesthetized (sodium pentobarbitone, 50 mg/kg, intraperitoneal), and the tissue between the two posterior articular processes of L5 was cleared to expose the ligamentum flavum. This ligament was pierced at the base of the L5 spinous process using a right-angled hook constructed from a 25-gauge needle, and the catheter was gently fed cephalad into the lumbar epidural space to a length of 2 cm. The catheter was flushed with saline, containing 10 U/mL heparin, to ensure that there was no leakage into the surrounding tissue. The catheter was secured to the superficial lumbar muscles with 4/0 silk sutures, and the remainder of the catheter was tunneled subcutaneously, with 2 cm exteriorized through a second skin incision midline over the upper cervical region. The cervical end of the catheter was fixed to the fascia of the neck muscles with 4/0 silk sutures, and the cervical and lumbar skin incisions were closed. All animals showed normal behavior upon recovery from the anesthetic. Approval for this study was obtained from Royal North Shore Hospital/University of Technology, Sydney animal ethics committee.
Nerve Injury
For the partial nerve ligation, the dorsal third of the left sciatic nerve was tightly ligated with 4/0 silk. A sham operation was performed on the right sciatic nerve, in which the nerve was exposed but not ligated. The wounds were closed with uninterrupted 3/0 silk, and the rats were allowed to recover.
Behavioral Test
The nociceptive flexion reflex was quantified using an Analgesymeter (Ugo Basile, Comerio-Varese, Italy). A mechanical force, increasing linearly with time, was applied to the dorsum of the rat’s hindpaw by a dome-shaped plunger (1.4-mm diameter, 36o-radius of curvature). The nociceptive threshold was defined as the force, in grams, at which the rat withdrew its paw. The chronic epidural catheter was inserted 1 wk before commencing behavioral studies to allow rats sufficient time for recovery. Nociceptive thresholds were then determined on a daily basis for a 3-wk period. Each day, thresholds were determined at 10 min intervals for a period of 2 h. The mean of the last six measurements represented the nociceptive threshold for that day. After the first 5 days of testing, the sciatic nerve was partially ligated. During the second week, a suspension of butamben or vehicle was administered via the epidural catheter using a 50 µL Hamilton syringe. After each injection, the catheter was flushed with 20 µL of saline (dead space of catheter was 20 µL). The results are expressed as the percent change in mechanical threshold of the injured paw as compared with the sham operated control and calculated as: 
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In separate groups of animals, an accelerating rotarod treadmill (Ugo Basile Rotarod, Comerio-Varese, Italy) was used to test the effect of epidural administration of either butamben (10 µL) or vehicle (10 µL), over 5 days, on motor coordination. The rotarod was set in motion at a constant speed, and the rats were placed onto the rotating drum in individual sections of the apparatus. Once the rats were in position, the timers were set to zero, and the rotarod was switched to the accelerating mode. The rotarod accelerated from 4 to 40 rpm in a period of 5 min. An animal’s performance time in seconds was recorded when the rat, unable to stay on the rotarod, tripped a plate and stopped the timer. Motor performance scores were determined daily at 10 min intervals for 2 h over a 3-wk period.
Butamben Suspension
A sterile 5% butamben injectable suspension was supplied by Abbott Laboratories (North Chicago, IL). Each mL of suspension contained 50 mg butamben; 0.25 mg polysorbate 80, and 25 mg polyethylene glycol 3350 as suspending agents; and 9 mg NaCl for tonicity adjustment. This preparation, without butamben, was used as the vehicle in control experiments. The suspension had a pH of approximately 6.0 and a mean particle size of approximately 40 microns.
The results are expressed as the mean ± SEM. Statistical analysis was performed using one-way analysis of variance (ANOVA) followed by Scheffé test post hoc comparisons.
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Results |
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Partial nerve injury induced a significant decrease in mechanical thresholds, compared with thresholds determined in the week before nerve injury (n = 12, P < 0.05, Figure 1). Daily epidural injections of 10 µL of butamben, for 5 days, significantly attenuated nerve injury-induced allodynia, compared with untreated controls (n = 18, P < 0.05, Figure 1A). A significant analgesic effect persisted for at least 7 days after the last epidural injection. In a control group of animals, daily injections of 10 µL of the vehicle (n = 16), for 5 days, was found to have no long-term effect on nerve injury-induced allodynia compared with untreated controls (Figure 1B).
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The actual mechanical thresholds, in grams, of the sham operated control and ligated paws in both butamben treated and untreated animals are depicted in Figure 1, C and D. In the untreated rats, there was a significant difference in mechanical threshold between the ligated and sham operated paw (Figure 1C, P < 0.05, n = 12). Butamben attenuated the nerve injury-induced decrease in threshold of the ligated paw. There was no significant difference in threshold between ligated and sham operated paws of the butamben treated rats (Figure 1D, n = 18). Moreover, butamben did not significantly alter the mechanical threshold of the sham operated paw during Weeks 2 and 3 compared with thresholds determined in Week 1.
Animals injected with either 10 µL butamben or vehicle, for 5 days, were also tested for motor coordination (Figure 2). We found that there was a significant impairment of motor function during Week 2 in animals injected with either vehicle (P < 0.05, n = 4) or butamben (P < 0.05, n = 4), compared with animals receiving no injections (n = 4). There was no significant difference between the butamben and vehicle treated groups. Motor function recovered during Week 3, and there was no significant difference between treated and untreated animals during this time. We also observed that the animals treated with either butamben or vehicle did not appear to have impaired bladder or bowel function.
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We found that volumes less than 10 µL of butamben had no significant long-lasting effect on nerve injury-induced allodynia. Daily injections of either 2.5 (n = 7) or 5 µL (n = 10) of butamben suspension, for 5 days, had no significant effect on the allodynia to mechanical stimulation determined in Week 3 (Figure 3, A–C). It was also evident that daily epidural injections of butamben for several days were required to maintain analgesia during Week 3 of the protocol. Injections of 10 µL butamben given daily, for 2 (n = 4) or 3 (n = 4) days, had no significant long-term effect on nerve injury-induced allodynia compared with untreated controls (Figure 3, D and E).
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Discussion |
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This study has demonstrated the analgesic effect in rats of epidural butamben on nerve injury-induced allodynia to mechanical stimulation. Multiple doses for several days were required to provide a prolonged analgesia. Although there are no comparable clinical studies examining the influence of epidural butamben on neuropathic pain, repeated epidural doses of butamben have been shown to provide prolonged analgesia in terminally ill patients with cancer (13,14). Korsten et al. (13), reported that the analgesic effects of epidural butamben in patients with cancer required multiple injections for 2–14 days and, in some cases, the analgesia lasted several months after the last injection. The prolonged duration of analgesia induced by butamben is proposed to be caused by either the slow release from a depot localized on the dorsal aspect of the spinal cord (13) and/or an irreversible effect on sodium channels (17).
Curiously, prolonged alleviation of neuropathic pain has also been reported with systemic administration of other local anesthetics, such as lidocaine (4,18). This is puzzling, because the duration of the analgesia is beyond the half-life of the drug. For example, in an animal study, a one-hour IV infusion of lidocaine was shown to suppress allodynia for 21 days (4). Clinically, however, protracted analgesia from IV administration of local anesthetics is rare and unpredictable. One study examining IV infusion of lidocaine in patients with peripheral nerve injury demonstrated that only a quarter of the patients had long-term analgesia, which lasted one to four weeks (18). It is difficult to assess the significance of these results, because the study did not include adequate placebo controls or psychological testing of the patients before inclusion in the study. Moreover, such prolonged analgesia is not routinely reported in clinical studies (5,10,19), and clearly a more reliable therapeutic agent is required.
Although we found impairment in motor performance during the period of injections, the butamben-induced analgesia persisted after recovery of motor function. This is in contrast to previous animal studies in which spinal administration of either a lidocaine solution (4) or lidocaine suspension preparation (20) induced paralysis, with no analgesia upon recovery of motor function. We also found that epidural injections of both butamben and vehicle induced temporary impairment of motor coordination, suggesting that the impairment is not caused by an anesthetic effect on motor neurons, but was more likely a result of, for example, a pressure block. It should be noted that the motor impairment induced by the vehicle injections was not associated with analgesia. The spinal canal in rats is very small, and, obviously, epidural administration of even microliter amounts of a viscous material may compress the spinal cord. The motor impairment seems to be peculiar to the rat, because epidural administration of butamben in humans has not been reported to be associated with such motor deficits (13,14).
Our data also show that butamben attenuated the allodynia in the ligated paw but had no effect on the threshold to mechanical stimulation of the sham operated control paws. These findings suggest that the analgesia induced by butamben results from an interference with a mechanism that develops after nerve injury, rather than an anesthetic effect on sensory neurons. The site of action is uncertain, and studies investigating systemic effects of local anesthetics suggest that these drugs act at both a peripheral and central site to alleviate neuropathic pain (2,7,11). For example, lidocaine, at doses that do not block conduction, attenuates spontaneous activity generated at both the site of injury and at the DRG (7). Assuming this is a general feature of local anesthetics, epidural butamben may also act by reducing the activity generated at the DRG. Several studies suggest that the spontaneous activity is caused, in part, by an altered expression in sodium channel subtypes (17,21–23), which are more sensitive to local anesthetics (21). Abnormal physiology, such as increased spontaneous activity and afterdischarges, also occur in dorsal horn neurons ipsilateral to a nerve injury (11,24,25). Systemic administration of local anesthetics attenuates the nerve injury-induced increase in spontaneous activity of ipsilateral wide dynamic range neurons, whereas the baseline activity of wide dynamic range neurons on the uninjured contralateral side was unaffected. Furthermore, the inhibitory effect of the local anesthetic occurred even after rhizotomy, suggesting a central action, rather than just an attenuation of the peripheral input (11). Although the site of action of the epidural administration of butamben needs verification, it is conceivable that abnormal physiology of both sensory and spinal neurons ipsilateral to the nerve injury can be attenuated by local anesthetics, seemingly at doses that do not alter normal nociceptive processes.
In contrast to butamben, a suspension preparation of lidocaine tested in dogs produced paralysis after the first epidural injection (20). Presumably this is caused by the high solubility of lidocaine leading to a rapid absorption into the cerebrospinal fluid (CSF). It is evident that the physicochemical properties of butamben, a low pKa, low water solubility, and a high partition coefficient, are likely to be important for a slow release and small amounts entering the CSF. Butamben may also be an improved alternative to spinally administered opioid drugs, because systemic absorption and cephalad migration of opioid in CSF also results in central nervous system effects. Clinical trials indicate that a suspension preparation of butamben may be a suitable alternative therapeutic agent for cancer pain (13,14). It is anticipated that a clinically useable suspension of butamben may also prove a promising therapeutic drug for severe chronic neuropathic pain in the future.
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Acknowledgments |
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This work was supported by Abbott Laboratories (North Chicago, IL).
The authors wish to thank Professor Arthur Duggan for helpful discussions.
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References |
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