Systemic lidocaine for neuropathic pain relief

A selective Nav1.8 sodium channel blocker, A-803467 [5-(4-chlorophenyl-N-(3,5-dimethoxyphenyl)furan-2-carboxamide], attenuates spinal neuronal activity in neuropathic rats

We have recently reported that systemic delivery of A-803467 [5-(4-chlorophenyl-N-(3,5-dimethoxyphenyl)furan-2-carboxamide], a selective Na(v)1.8 sodium channel blocker, reduces behavioral measures of chronic pain. In the current study, the effects of A-803467 on evoked and spontaneous firing of wide dynamic range (WDR) neurons were measured in uninjured and rats with spinal nerve ligations (SNLs). Administration of A-803467 (10-30 mg/kg i.v.) reduced mechanically evoked (10-g von Frey hair) and spontaneous WDR neuronal activity in SNL rats. In uninjured rats, A-803467 (20 mg/kg i.v.) transiently reduced evoked but not spontaneous firing of WDR neurons. The systemic effects of A-803467 in SNL rats were not altered by spinal transection or by systemic pretreatment with the transient receptor potential vanilloid type 1 (TRPV1) receptor agonist, resiniferatoxin, at doses that impair the function of TRPV1-expressing fibers. To determine sites of action, A-803467 was administered into spinal tissue, into the uninjured L4 dorsal root ganglion (DRG), or into the neuronal receptive field. Injections of A-803467 into the L4 DRG (30-100 nmol/1 mul) or into the hindpaw receptive field (300 nmol/50 mul) reduced evoked but not spontaneous WDR firing. In contrast, intraspinal (50-150 nmol/0.5 mul) injection of A-803467 decreased both evoked and spontaneous discharges of WDR neurons. Thus, Na(v)1.8 sodium channels on the cell bodies/axons within the L4 DRG as well as on peripheral and central terminals of primary afferent neurons regulate the inflow of low-intensity mechanical signals to spinal WDR neurons. However, Na(v)1.8 sodium channels on central terminals seem to be key to the modulation of spontaneous firing in SNL rats.

Attenuating phosphorylation of p38 MAPK in the activated microglia: a new mechanism for intrathecal lidocaine reversing tactile allodynia following chronic constriction injury in rats

Increasing evidences approve the long-term analgesia effects of intrathecal lidocaine in patients with chronic pain and in animal peripheral nerve injury models, but the underlying mechanism remains elusive. Previous evidences suggest that the activation of the p38 MAPK signaling pathway in hyperactive microglia in the dorsal horn of spinal cord involves in nerve injury-induced neuropathic pain. In this study, we demonstrate that attenuating phosphorylation of p38 MAPK in the activated microglia of spinal cord, at least partly, is the mechanism of intrathecal lidocaine reversing established tactile allodynia in chronic constriction injury model of rats. This finding not only provides a new insight into the mechanisms underlying long-term therapeutic effects of lidocaine on neuropathic pain, but also reveals one more specific drug target for analgesia.

Epidural local anesthetic plus corticosteroid for the treatment of cervical brachial radicular pain: single injection versus continuous infusion

BACKGROUND

Efficacy of epidural local anesthetics plus steroids for the treatment of cervicobrachial pain is uncertain.

METHODS

A prospective study randomized 160 patients with cervicobrachial pain resistant to conventional therapy. Patients were divided into 4 groups on the basis of the time between pain onset and treatment initiation: group A, 40 patients with pain onset 15 to 30 days; group B, 40 patients with pain from 31 to 60 days; group C, 40 patients, 61 to 180 days; and group D, 40 patients with pain >180 days. Patients of each group were randomized to receive an epidural block with bupivacaine and methylprednisolone at intervals of 4 to 5 days (Single injection) or continuous epidural bupivacaine every 6, 12, or 24 hours plus methylprednisolone every 4 to 5 days (Continuos epidural). The maximum duration of treatment (9 blocks in Single injection, and 30 days in Continuos epidural) was dependent on achieving Pain Control (PC) > or =80% [PC is defined by this formulae: (100) (VAS(initial)-VAS(final))/VAS(initial)]. Follow-up at 1 month and 6 months compared PC and the number of pain-free hours of sleep.

RESULTS

One hundred forty-one patients completed the study. The 4 groups had similar characteristics. At the 1-month and 6-month follow-up analysis based on the time between pain onset and treatment initiation showed that patients of group D, who received the Continuous epidural treatment, had significantly greater PC and significantly more pain-free hours of sleep compared with similar patients in Single injection.

CONCLUSIONS

Therapy with continuous epidural local anesthetic and methylprednisolone provides better control of chronic cervicobrachial pain compared with Single injection. These results are discussed with respect to the possible mechanism of action of the drugs and may relate to the physiopathologic mechanisms associated with neuronal plasticity that result in chronic pain.

Benzazepinone Nav1.7 blockers: potential treatments for neuropathic pain

A series of benzazepinones were synthesized and evaluated as hNa(v)1.7 sodium channel blockers. Several compounds from this series displayed good oral bioavailability and exposure and were efficacious in a rat model of neuropathic pain.

Central glucocorticoid receptors regulate the upregulation of spinal cannabinoid-1 receptors after peripheral nerve injury in rats

Previous studies have shown that peripheral nerve injury upregulated both glucocorticoid receptors (GR) and cannabinoid-1 receptors (CB1R) within the spinal cord dorsal horn in rats. However, the relationship between the expression of spinal GR and CB1R after nerve injury remains unclear. Here, we examined the hypothesis that the upregulation of spinal CB1R induced by chronic constriction nerve injury (CCI) in rats would be regulated by spinal GR. CCI induced the upregulation of spinal CB1R primarily within the ipsilateral spinal cord dorsal horn as revealed by Western blot and immunohistochemistry. The expression of CB1R in CCI rats was substantially attenuated by intrathecal treatment with either the GR antagonist RU38486 or a GR antisense oligonucleotide given twice daily for postoperative day 1-6, whereas the expression of spinal CB1R was enhanced following intrathecal administration of a GR sense oligonucleotide twice daily for postoperative day 1-6. Furthermore, the upregulation of spinal CB1R after nerve injury was prevented in adrenalectomized rats, which was at least partially restored with the intrathecal administration of an exogenous GR agonist dexamethasone, indicating that corticosteroids (endogenous GR agonists) were critical to spinal GR actions. Since the development of neuropathic pain behaviors in CCI rats was attenuated by either RU38486 or a GR antisense oligonucleotide, these results suggest that CB1R is a downstream target for spinal GR actions contributory to the mechanisms of neuropathic pain.

Lidocaine for pain relief in burn injured patients

BACKGROUND

Pain is a major issue for patients suffering from many different types of wounds in particular those with burn injuries. Prompt, aggressive use of opioid analgesics such as morphine has been suggested as critical to avert the cycle of pain and anxiety, but side effects are encountered. It is proposed that newer agents such as lidocaine could be effective in reducing pain and alleviating the escalating opioid dosage requirements in patients with burn injury.

OBJECTIVES

To assess the safety and effectiveness of intravenous lidocaine as a means of pain relief versus no therapy, placebo, other drugs or two or more of the above therapies in combination in patients exposed to burn injury.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 1, 2007), MEDLINE (1966 to March 2007), EMBASE (1980 to 2007), CINAHL (1982 to March 2007).

SELECTION CRITERIA

Those trials that were considered were: randomised controlled trials (RCTs) and controlled clinical trials (CCTs), both published and unpublished studies, which assessed the efficacy of intravenous lidocaine varying doses as a single-agent therapy with no therapy, placebo, other analgesics such as opioids, lidocaine plus another drug, or two or more of the above therapies as a means of pain relief in patients exposed to burn injury.

DATA COLLECTION AND ANALYSIS

The two review authors applied the entry criteria to identified studies.

MAIN RESULTS

No clinically relevant RCTs or CCTs were identified through the above searches.

AUTHORS' CONCLUSIONS

No information is available from the published RCTs or CCTs on clinically relevant primary outcome measures which can influence current burns care practice and management. Therefore, since current clinical evidence is subject to the inherent weaknesses of case series or reports, intravenous lidocaine must be considered a pharmacological agent under investigation in burns care whose effectiveness is yet to be determined in well-designed and conducted clinical trials.

Incidence and Management of Phantom Limb Pain According to World Health Organization Analgesic Ladder in Amputees of Malignant Origin

Antidepressants and anticonvulsants are currently considered to be the drug treatment of choice for neuropathic pain. Opioids are effective in relieving neuropathic pain, including phantom pain in the early postoperative course. The present study of 42 cancer patients with limb amputation was conducted to determine the incidence of phantom limb pain and phantom sensation and to test the utility of the World Health Organization 3-step analgesic ladder in phantom limb pain management. Patients were monitored monthly for the first 2 months postoperatively and every 2 months thereafter for 2 years. The World Health Organization analgesic ladder was followed for pain management. The patients complaining of phantom sensation, phantom pain, and stump pain decreased from 69%, 60%, and 31%, respectively, at 1 month to 32%, 32%, and 5%, at the end of 2 years with the addition of opioids. The World Health Organization analgesic ladder played significant role in phantom limb pain management.

Trigeminal Autonomic Cephalalgias: Current and Future Treatments

The trigeminal autonomic cephalgias include cluster headache, paroxysmal hemicrania, and short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT). The evidence for the current treatment options for each of these syndromes is considered, including oxygen, sumatriptan, and verapamil in cluster headache, indomethacin in paroxysmal hemicrania, and intravenous lidocaine and lamotrigine in SUNCT. Some treatments such as topiramate have an effect in all of these, as well as in migraine and other pain syndromes. The involvement of the hypothalamus in functional imaging studies implies that this may be a substrate for targeting treatment options in the future.

Pharmacological management of chronic neuropathic pain - consensus statement and guidelines from the Canadian Pain Society

Neuropathic pain (NeP), generated by disorders of the peripheral and central nervous system, can be particularly severe and disabling. Prevalence estimates indicate that 2% to 3% of the population in the developed world suffer from NeP, which suggests that up to one million Canadians have this disabling condition. Evidence-based guidelines for the pharmacological management of NeP are therefore urgently needed. Randomized, controlled trials, systematic reviews and existing guidelines focusing on the pharmacological management of NeP were evaluated at a consensus meeting. Medications are recommended in the guidelines if their analgesic efficacy was supported by at least one methodologically sound, randomized, controlled trial showing significant benefit relative to placebo or another relevant control group. Recommendations for treatment are based on degree of evidence of analgesic efficacy, safety, ease of use and cost-effectiveness. Analgesic agents recommended for first-line treatments are certain antidepressants (tricyclics) and anticonvulsants (gabapentin and pregabalin). Second-line treatments recommended are serotonin noradrenaline reuptake inhibitors and topical lidocaine. Tramadol and controlled-release opioid analgesics are recommended as third-line treatments for moderate to severe pain. Recommended fourth-line treatments include cannabinoids, methadone and anticonvulsants with lesser evidence of efficacy, such as lamotrigine, topiramate and valproic acid. Treatment must be individualized for each patient based on efficacy, side-effect profile and drug accessibility, including cost. Further studies are required to examine head-to-head comparisons among analgesics, combinations of analgesics, long-term outcomes, and treatment of pediatric and central NeP.

Discovery of a novel class of benzazepinone Na(v)1.7 blockers: potential treatments for neuropathic pain

A series of benzodiazepines and benzazepinones were synthesized and evaluated as potential sodium channel blockers in a functional, membrane potential-based assay. One member of the benzazepinone series, compound 47, displayed potent, state-dependent block of hNa(v)1.7, and was orally efficacious in a rat model of neuropathic pain.

Intravenous lidocaine for neuropathic pain: Diagnostic utility and therapeutic efficacy

Lidocaine is a use-dependent sodium channel blocker that produces analgesia when administered intravenously to patients with neuropathic pain. This article reviews the role and limitations of intravenous lidocaine infusions for neuropathic pain. Lidocaine infusions rarely provide relief that persists significantly beyond the duration of the infusion. Diagnostically, systemic lidocaine may help establish the presence of neuropathic pain and the responsivity to oral sodium channel blockade. However, the data supporting diagnostic infusions remain sparse. Therapeutically, infusions should generally be restricted to patients with neuropathic pain who are unable to take oral medication.

Relationship Between the Firing Frequency of Injured Peripheral Neurons and Inhibition of Firing by Sodium Channel Blockers

Animal models of neuropathic pain in which a peripheral nerve is damaged result in spontaneous activity in primary afferents that can be inhibited by intravenous administration of sodium channel blockers. Many of these compounds exhibit use-dependent block of sodium current, leading to the prediction that they should more readily inhibit neurons that fire at higher frequencies. This prediction was tested in 2 rat models of nerve injury, L5 spinal nerve section and sciatic nerve section. Sciatic nerve section produced average firing frequencies that were higher than spinal nerve section and often manifested as high-frequency bursting. Inhibition of firing by intravenous sodium channel blockers was longer lasting in this model. Within each model, higher frequency of firing did not translate into more effective block. In the spinal nerve section model, there was a robust inverse correlation between frequency and inhibition. Within the sciatic section model, only neurons that fired in rhythmic bursts were inhibited, and again, those firing at lower mean frequencies were more effectively inhibited. These results indicate that the efficacy of sodium channel blockers depends on the nature of the injury and the pattern of the resulting activity rather than simply the frequency of action potentials generated.

PERSPECTIVE

This study examines the ability of frequency-dependent sodium channel blockers to inhibit spontaneous firing of injured peripheral nerves in vivo. It outlines the conditions under which inhibition is more and less effective and will provide insight into conditions under which sodium channel blockers are likely to be therapeutically useful.

Butyl 2-(4-[1.1′-biphenyl]-4-yl-1H-imidazol-2-yl)ethylcarbamate, a potent sodium channel blocker for the treatment of neuropathic pain

A series of 4-arylimidazole carbamates was synthesized and their binding affinities to the site-2 sodium (Na+) channel were determined. SAR studies led to the identification of compound 10, a potent Na+ channel blocker which was efficacious in pain models in vivo.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

Dental impaction pain model as a potential tool to evaluate drugs with efficacy in neuropathic pain

Intravenous lidocaine, a nonspecific Na-channel blocker, was used to assess the dental impaction model for evaluation of neuropathic pain drugs. Sixty patients, experiencing moderate or severe pain after removal of > or = 2 third molars, were randomized (2:2:1:1) to lidocaine (4 mg/kg; maximal dose 300 mg), oxycodone/acetaminophen (10/650 mg), placebo, and active placebo (diphenhydramine, 50 mg). Lidocaine provided a modest degree of pain relief. Predefined endpoints of total pain relief and sum of pain intensity at 2, 4, and 6 hours showed numerically, not statistically significantly, greater pain relief versus placebo. A significantly greater effect over placebo was observed in peak effect and at shorter time points (30 minutes and 1 hour), consistent with the pharmacokinetic profile (plasma concentration of approximately 2 mug/mL). Oxycodone/acetaminophen provided significantly greater analgesia versus placebo, validating study conduct, and significantly greater pain relief was observed versus lidocaine, which is consistent with a smaller portion of dental extraction pain being of neuropathic origin.

Local analgesic efficacy of tramadol following intraplantar injection

Several studies have suggested that systemic tramadol, an opioid, can represent a valuable treatment in severe pain conditions because of their effects on central pain pathways. However, there are not enough studies supporting that tramadol is efficacious when administered locally. Therefore, we studied the potential local analgesic effects of tramadol in peripheral nociception. In addition, we tested the antinociceptive effects of tramadol-CaCl(2) or naloxone combinations after subcutaneous intraplantar injection in a validated rat model of acute thermal nociception. Local analgesic effects of tramadol were compared with those of lidocaine. The effects of tramadol on thermal paw withdrawal latencies were monitored using the plantar test. The antinociceptive potency of tramadol is higher and long-lasting than that of lidocaine. Naloxone was unable to inhibit the increased antinociceptive response produced by tramadol. Ca(2+) modified the effect of tramadol. When Ca(2+) dose was increased in the solution, thermal antinociceptive potency of tramadol, but not lidocaine was prolonged. Thermal nociceptive responses were not affected in the non-injected paws, indicating a lack of systemic effects with doses of tramadol and lidocaine that elicited local analgesia. These results suggest that intraplantar tramadol administration can produce local analgesic effect with a different action mechanism than that of lidocaine. In addition, extracellular Ca(2+) may play an important role in the local analgesic action of tramadol.

Differential blockade of nerve injury-induced shift in weight bearing and thermal and tactile hypersensitivity by milnacipran

Antidepressants such as tricyclic antidepressants have become used to treat a variety of chronic pain conditions. However, the side effects are dose-limiting in the treatment of chronic pain. Milnacipran is a norepinephrine/serotonin reuptake inhibitor that does not have the severe side effects associated with traditional tricyclic antidepressants. The effects of intrathecal and systemic administration of milnacipran on spinal nerve ligation (SNL)-induced thermal and mechanical hypersensitivity and shift in weight bearing were determined. Intrathecal administration of milnacipran was found to reverse both SNL-induced thermal and tactile (to von Frey filaments) hypersensitivity, as well as shift in weight bearing. Acute systemic administration of milnacipran also reversed nerve injury-induced thermal hypersensitivity for up to 5 hours but failed to reverse tactile hypersensitivity or shift in weight bearing. Of note, both intrathecal and subcutaneous administration of milnacipran induced thermal antinociception in both SNL and sham rats. Chronic (daily) systemic administration of milnacipran alleviated both thermal hypersensitivity and shift in weight bearing, with both acute and chronic effects observed on thermal hypersensitivity. However, chronic systemic milnacipran administration failed to alleviate tactile hypersensitivity to von Frey filaments. These results indicate that different mechanisms underlie shift in weight bearing, thermal hypersensitivity, and tactile hypersensitivity.

PERSPECTIVE

These results indicate that the ability of milnacipran to relieve nerve injury-induced allodynia, hyperalgesia, and shift in weight bearing depends on the route of administration and the duration of treatment, with alleviation of SNL-induced tactile hypersensitivity and shift in weight bearing as a result of activity within the central-rather than the peripheral-nervous system.

The intravenous ketamine test predicts subsequent response to an oral dextromethorphan treatment regimen in fibromyalgia patients

Fibromyalgia (FM) is a challenging pain syndrome for which no reliable pharmacologic treatment exists. Recent clinical studies suggest that N-methyl-D-aspartate receptors might play a role in the pathogenesis of this disorder. To determine whether an intravenous (IV) ketamine test predicts the response to a therapeutic trial with an oral N-methyl-D-aspartate receptor antagonist, we performed a low-dose (0.1 mg/kg) IV ketamine infusion on 34 consecutive patients with FM, which was subsequently followed by an oral dextromethorphan (DX) treatment regimen. As per previous guidelines, the cutoff value for a positive response to the IV ketamine test was designated to be 67% pain relief, and a positive response to DX treatment was 50% pain reduction at 4- to 6-week follow-up visits. The degree of correlation between pain relief with ketamine and DX was highly significant (Pearson correlation coefficient, 0.66; P < .001). Ten patients responded positively to both ketamine and DX, 19 responded to neither drug, 3 had a positive response to ketamine but not DX, and 2 obtained good pain relief with DX but not ketamine. The sensitivity of the IV ketamine test was 83%, the specificity was 86%, the positive predictive value was 77%, and the negative predictive value was 91%. An association was also found between the development of side effects to the two treatments.

PERSPECTIVE

The response to an IV ketamine infusion was found to predict the subsequent response to an oral dextromethorphan treatment regimen in fibromyalgia patients, with an observed agreement of 83%. Considering the refractory nature of fibromyalgia to conventional pain treatments, the IV ketamine test might enhance patient care by saving time and reducing unnecessary treatment trials.

The clinical challenge of chronic neuropathic pain

BACKGROUND

The clinician often faces the problem that certain types of chronic pains remain refractory to the commonly used analgesic treatment options. Neuropathic pain, which is defined as pain caused by direct nerve lesions, may have different causes and a variety of clinical presentations. A correct management of chronic neuropathic pain requires a thorough understanding of the potential causes, the diagnosis and the pathophysiological mechanisms.

OBJECTIVES

The purpose of this review article is to provide the reader with the latest insights in the diagnostic work-out and the clinical presentation of neuropathic pain. Additionally, the possible pathophysiological changes induced by nerve lesions are explained.

METHODS

An extensive literature review was performed using Pubmed citations.

RESULTS AND CONCLUSIONS

This article, which is based on extensive literature review, aims at providing a concise review of the current knowledge regarding aetiology, diagnosis and pathophysiology of neuropathic pain.

Reversal of neuropathic pain by α-hydroxyphenylamide: A novel sodium channel antagonist

Sodium (Na) channel blockers are known to possess antihyperalgesic properties. We have designed and synthesized a novel Na channel antagonist, alpha-hydroxyphenylamide, and determined its ability to inhibit both TTX-sensitive (TTX-s) and TTX-resistant (TTX-r) Na currents from small dorsal root ganglion (DRG) neurons. alpha-Hydroxyphenylamide tonically inhibited both TTX-s and TTX-r Na currents yielding an IC(50) of 8.2+/-2.2 microM (n=7) and 28.9+/-1.8 microM (n=8), respectively. In comparison, phenytoin was less potent inhibiting TTX-s and TTX-r currents by 26.2+/-4.0% (n=8) and 25.5+/-2.0%, respectively, at 100 microM. alpha-Hydroxyphenylamide (10 microM) also shifted equilibrium gating parameters of TTX-s Na channels to greater hyperpolarized potentials, slowed recovery from inactivation, accelerated the development of inactivation and exhibited use-dependent block. In the chronic constriction injury (CCI) rat model of neuropathic pain, intraperitoneal administration of alpha-hydroxyphenylamide attenuated the hyperalgesia by 53% at 100mg/kg, without affecting motor coordination in the Rotorod test. By contrast, the reduction in pain behavior produced by phenytoin (73%; 100mg/kg) was associated with significant motor impairment. In summary, we report that alpha-hydroxyphenylamide, a sodium channel antagonist, exhibits antihyperalgesic properties in a rat model of neuropathic pain, with favorable sedative and ataxic side effects compared with phenytoin.

Pharmacology of 2-[4-(4-Chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide: A Potent, Broad-Spectrum State-Dependent Sodium Channel Blocker for Treating Pain States

Voltage-gated Na(+) channels may play important roles in establishing pathological neuronal hyperexcitability associated with chronic pain in humans. Na(+) channel blockers, such as carbamazepine (CBZ) and lamotrigine (LTG), are efficacious in treating neuropathic pain; however, their therapeutic utility is compromised by central nervous system side effects. We reasoned that it may be possible to gain superior control over pain states and, in particular, a better therapeutic index, by designing broad-spectrum Na(+) channel blockers with higher potency, faster onset kinetics, and greater levels of state dependence than existing drugs. 2-[4-(4-Chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide (PPPA) is a novel structural analog of the state-dependent Na(+) channel blocker V102862 [4-(4-fluorophenoxy)benzaldehyde semicarbazone]. Tested on recombinant rat Na(v)1.2 channels and native Na(+) currents in cultured rat dorsal root ganglion neurons, PPPA was approximately 1000 times more potent, had 2000-fold faster binding kinetics, and > or =10-fold higher levels of state dependence than CBZ and LTG. Tested in rat pain models against mechanical endpoints, PPPA had minimal effective doses of 1 to 3 mg/kg p.o. in partial sciatic nerve ligation, Freund's complete adjuvant, and postincisional pain. In all cases, efficacy was similar to clinically relevant comparators. Importantly, PPPA did not produce motor deficits in the accelerating Rotarod assay of ataxia at doses up to 30 mg/kg p.o., indicating a therapeutic index >10, which was superior to CBZ and LTG. Our experiments suggest that high-potency, broad-spectrum, state-dependent Na(+) channel blockers will have clinical utility for treating neuropathic, inflammatory, and postsurgical pain. Optimizing the biophysical parameters of broad-spectrum voltage-gated Na(+) channel blockers may lead to improved pain therapeutics.

A high-capacity membrane potential FRET-based assay for NaV1.8 channels

Clinical treatment of neuropathic pain can be achieved with a number of different drugs, some of which interact with all members of the voltage-gated sodium channel (NaV1) family. However, block of central nervous system and cardiac NaV1 channels can cause dose-limiting side effects, preventing many patients from achieving adequate pain relief. Expression of the tetrodotoxin-resistant NaV1.8 subtype is restricted to small-diameter sensory neurons, and several lines of evidence indicate a role for NaV1.8 in pain processing. Given these features, NaV1.8 subtype-selective blockers are predicted to be efficacious in the treatment of neuropathic pain and to be associated with fewer adverse effects than currently available therapies. To facilitate the identification of NaV1.8-specific inhibitors, we stably expressed the human NaV1.8 channel together with the auxiliary human beta1 subunit (NaV beta1) in human embryonic kidney 293 cells. Heterologously expressed human NaV1.8/NaV beta1 channels display biophysical properties that are similar to those of tetrodotoxin-resistant channels present in mouse dorsal root ganglion neurons. A membrane potential, fluorescence resonance energy transfer-based functional assay on a fluorometric imaging plate reader (FLIPR-Tetra, Molecular Devices, Sunnyvale, CA) platform has been established. This highcapacity assay is sensitive to known state-dependent NaV1 modulators and can be used to identify novel and selective NaV1.8 inhibitors.

Effects of systemic administration of lidocaine and QX-314 on hyperexcitability of spinal dorsal horn neurons after incision in the rat

Although systemic lidocaine has been shown to suppress postoperative pain in a clinical setting, the mechanisms of action of lidocaine have not been elucidated. The present study was therefore designed to determine the relative contribution of central and peripheral sites to the action of lidocaine on incision-induced hyperexcitation of spinal dorsal horn (SDH) neurons in the rat. Receptive field (RF) areas, spontaneous activities, and responses of single wide-dynamic-range (WDR) neurons of the SDH to nonnoxious and noxious stimuli were recorded before and after longitudinal incisions of 1cm through the skin, fascia, and muscle had been made in the center of their RFs of the hindquarters. Significant increases in spontaneous activities, RF sizes, and responses of WDR neurons to both nonnoxious and noxious stimuli were observed at 30 min after the incision (P<0.001). Systemic administration of lidocaine (1 mg/kg bolus plus 0.5 mg/kg/h and 2 mg/kg bolus plus 1 mg/kg/h) and QX-314 (1mg/kg bolus plus 0.5 mg/kg/h and 2 mg/kg bolus plus 1 mg/kg/h) significantly but temporarily suppressed and reversed the increases in spontaneous activity, responses to nonnoxious, and noxious stimuli and RF sizes (P<0.01). Systemic administration of the same doses of lidocaine and QX-314 did not affect responses of WDR neurons to nonnoxious or noxious stimuli or their RF sizes in sham-operated animals in which an incision had not been made. The results suggest that systemic administration of lidocaine has suppressive effects on postoperative pain mainly through peripheral sites of action.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

A randomized, double-masked, placebo-controlled pilot trial of extended IV lidocaine infusion for relief of ongoing neuropathic pain

OBJECTIVES

To determine the dose-response effect and safety of IV lidocaine at different dose infusion rates on spontaneous ongoing neuropathic pain.

METHODS

In this double-masked, placebo-controlled, parallel study conducted in an outpatient clinical research center, patients with peripheral neuropathic pain received a 6-hour infusion of three doses (1, 3, and 5 mg/kg) of lidocaine or placebo. The main outcome measure was relief of pain intensity (percentage pain intensity difference [PID %]). Other measures were responder rate, adverse events, and correlation between lidocaine levels and PID %.

RESULTS

There was a significant difference in the median PID % between the group treated with lidocaine 5 mg/kg/h (-34.60) and the placebo group (-11.96, P=0.012). Such effect began 4 hours after the onset of treatment and lasted until the end of the study. Lidocaine at lower infusion rates was no better than placebo in relieving pain. A modest but significant correlation was found between methylethylglycinexylidide (MEGX) levels and pain relief (R=0.60). There were no serious adverse events, but in two patients lidocaine was stopped prematurely.

CONCLUSIONS

Lidocaine at 5 mg/kg/h was more effective than placebo at relieving neuropathic pain. The effect started 4 hours after the onset of treatment and continued for at least 4 hours after the end of the infusion. Additional research is needed using higher infusion rates with larger sample sizes to confirm these results and to explore the role of MEGX in the relief of neuropathic pain.

Translating basic research on sodium channels in human neuropathic pain

A number of experimental studies in animals have suggested that voltage-gated sodium channels may play a crucial role in neuropathic pain. However, it is still difficult to translate the pathophysiological mechanisms identified in animal studies to the clinic and several questions regarding the role of sodium channels in neuropathic pain must therefore be addressed primarily in the clinical setting. Despite providing indirect information, pharmacologic challenge using sodium channel blockers, such as some antiepileptics, local anesthetics and derivatives, is the best way to investigate the role of sodium channels in the various clinical symptoms of neuropathies (eg, spontaneous pain, mechanical or thermal allodynia, and hyperalgesia). Randomized controlled trials have demonstrated the efficacy of these compounds for various neuropathic pain conditions. Recent psychophysical studies in which symptoms and signs are more accurately assessed indicate that these compounds act as antihyperalgesic agents rather than as simple analgesics. They also show that the sensitivity to these drugs is not affected by the aetiology of pain and the peripheral or central location of the nerve lesion. These data emphasize the role of peripheral and central sodium channels in neuropathic pain. Studies using more selective sodium channel blockers are required to gain further insight into the role of the various subtypes of sodium channel in these pain conditions.

PERSPECTIVE

Pharmacological challenge using sodium channel blockers is the best way to translate basic research on sodium channels in human neuropathic pain. To date, the role of sodium channels in neuropathic pain symptoms/signs is mostly documented for mechanical static and dynamic allodynia, and either peripheral or central sodium channels may be involved.

A comparative study of the action of tolperisone on seven different voltage dependent sodium channel isoforms

The specific, acute interaction of tolperisone, an agent used as a muscle relaxant and for the treatment of chronic pain conditions, with the Na(v1.2), Na(v1.3), Na(v1.4), Na(v1.5), Na(v1.6), Na(v1.7), and Na(v1.8) isoforms of voltage dependent sodium channels was investigated and compared to that of lidocaine. Voltage dependent sodium channels were expressed in the Xenopus laevis oocyte expression system and sodium currents were recorded with the two electrode voltage clamp technique. Cumulative dose response relations revealed marked differences in IC(50) values between the two drugs on identical isoforms, as well as between isoforms. A detailed kinetic analysis uncovered that tolperisone as well as lidocaine exhibited their blocking action not only via state dependent association/dissociation with voltage dependent sodium channels, but a considerable fraction of inhibition is tonic, i.e. permanent and basic in nature. Voltage dependent activation was affected to a minor extent only. A shift in steady-state inactivation to more negative potentials could be observed for most drug/isoform combinations. The contribution of this shift to overall block was, however, small at drug concentrations resulting in considerable overall block. Recovery from inactivation was affected notably by both drugs. Lidocaine application led to a pronounced prolongation of the time constant of the fast recovery process for the Na(v1.3), Na(v1.5), and Na(v1.7) isoforms, indicating common structural properties in the local anesthetic receptor site of these three proteins. Interestingly, this characteristic drug action was not observed for tolperisone.

Characterization of voltage-gated sodium-channel blockers by electrical stimulation and fluorescence detection of membrane potential

Voltage-gated ion channels regulate many physiological functions and are targets for a number of drugs. Patch-clamp electrophysiology is the standard method for measuring channel activity because it fulfils the requirements for voltage control, repetitive stimulation and high temporal resolution, but it is laborious and costly. Here we report an electro-optical technology and automated instrument, called the electrical stimulation voltage ion probe reader (E-VIPR), that measures the activity of voltage-gated ion channels using extracellular electrical field stimulation and voltage-sensitive fluorescent probes. We demonstrate that E-VIPR can sensitively detect drug potency and mechanism of block on the neuronal human type III voltage-gated sodium channel expressed in human embryonic kidney cells. Results are compared with voltage-clamp and show that E-VIPR provides sensitive and information-rich compound blocking activity. Furthermore, we screened approximately 400 drugs and observed sodium channel-blocking activity for approximately 25% of them, including the antidepressants sertraline (Zoloft) and paroxetine (Paxil).

Synthesis and SAR of 1,2-trans-(1-hydroxy-3-phenylprop-1-yl)cyclopentane carboxamide derivatives, a new class of sodium channel blockers

Novel cyclopentane-based 3-phenyl-1-hydroxypropyl compounds were evaluated for inhibitory activity against the peripheral nerve sodium channel Na(V)1.7 and off-target activity against the cardiac potassium channel hERG. The stereochemistry of the hydroxyl group and substitution on the phenyl rings with either fluorinated O-alkyl or alkyl groups were found to be critical for conferring potency against Na(V)1.7. A benchmark compound from this series displayed efficacy in rat models of inflammatory and neuropathic pain.

Local anesthetics suppress nerve growth factor-mediated neurite outgrowth by inhibition of tyrosine kinase activity of TrkA

Local anesthetics (LAs) suppress sympathetic sprouting, which correlates with neuropathic pain. However, the precise mechanism of the suppression is unknown. Nerve growth factor (NGF) contributes to the sympathetic sprouting, and NGF signaling starts with NGF-stimulated autophosphorylation of TrkA, which is a high affinity receptor of NGF. We examined the effects of lidocaine, bupivacaine, and procaine on NGF signaling under suppression of NGF-stimulated neurite outgrowth in PC12 cells, which is a cellular model of sympathetic sprouting. To investigate the effect of these LAs on NGF-mediated neurite outgrowth of PC12 cells, cells were incubated with 40, 400, and 4000 microM of each LA. The effect of LAs on NGF-stimulated TrkA activity was examined to analyze autophosphorylation of TrkA using immunoprecipitation and immunoblotting. Cytotoxic effects of LAs on PC12 cells were also assessed by lactate dehydrogenase release and by propidium iodide staining. Lidocaine (400 microM), bupivacaine (40 and 400 microM), or procaine (4000 microM) suppressed either neurite outgrowth or autophosphorylation significantly without cytotoxicity. The inhibition of NGF-stimulated tyrosine kinase activity of TrkA might be involved in the mechanisms of suppression of neurite outgrowth induced by LAs.

Ion Channel Targets and Treatment Efficacy in Neuropathic Pain

Chronic neuropathic pain due to injury or dysfunction of the nervous system remains a formidable treatment challenge in spite of a growing range of medication choices. We review current clinical research supporting the use of ion channel modulators for neuropathic pain states. New modes of local drug delivery, novel Ca2+ channel targets, and increased choices for drugs with activity at Na+channels are transforming this longstanding therapeutic strategy. Clinical decision making is increasingly informed by a more nuanced understanding of the role of voltage-gated Na+channels (VGSCs) and Ca2+ channels (VGCCs) in the pathophysiology of nerve injury. Although holding great promise for the future, mechanism-based approaches to treatment will require greater understanding of the analgesic mechanisms of drug action and of the relationships between pathophysiologic mechanisms and clinical presentation.

PERSPECTIVE

Treatment options for neuropathic pain targeting ion channels have grown rapidly in the past decade. An evolving body of clinical research supports the widespread use of this longstanding therapeutic strategy. Improved efficacy of ion channel modulators hinges upon further elucidation of the relationship between signs and symptoms of pain and underlying pathophysiology.

Systemic administration of local anesthetics to relieve neuropathic pain: a systematic review and meta-analysis

We reviewed randomized controlled trials to determine the efficacy and safety of systemically administered local anesthetics compared with placebo or active drugs. Of 41 retrieved studies, 27 trials of diverse quality were included in the systematic review. Ten lidocaine and nine mexiletine trials had data suitable for meta-analysis (n = 706 patients total). Lidocaine (most commonly 5 mg/kg IV over 30-60 min) and mexiletine (median dose, 600 mg daily) were superior to placebo (weighted mean difference on a 0-100 mm pain intensity visual analog scale = -10.60; 95% confidence interval: -14.52 to -6.68; P < 0.00001) and equal to morphine, gabapentin, amitriptyline, and amantadine (weighted mean difference = -0.60; 95% confidence interval: -6.96 to 5.75) for neuropathic pain. The therapeutic benefit was more consistent for peripheral pain (trauma, diabetes) and central pain. The most common adverse effects of lidocaine and mexiletine were drowsiness, fatigue, nausea, and dizziness. The adverse event rate for systemically administered local anesthetics was more than for placebo but equivalent to morphine, amitriptyline, or gabapentin (odds ratio: 1.23; 95% confidence interval: 0.22 to 6.90). Lidocaine and mexiletine produced no major adverse events in controlled clinical trials, were superior to placebo to relieve neuropathic pain, and were as effective as other analgesics used for this condition.

Systemic administration of local anesthetic agents to relieve neuropathic pain

BACKGROUND

Lidocaine, mexiletine, tocainide, and flecainide are local anesthetics which give an analgesic effect when administered orally or parenterally. Early reports described the use of intravenous lidocaine or procaine to relieve cancer and postoperative pain (Keats 1951; Gilbert 1951; De Clive-Lowe 1958; Bartlett 1961). Interest reappeared decades later when patient series and clinical trials reported that parenteral lidocaine and its oral analogs tocainide, mexiletine, and flecainide relieved neuropathic pain in some patients (Boas 1982; Lindblom 1984; Petersen 1986; Dunlop 1988; Bach 1990; Awerbuch 1990). With the recent publication of clinical trials with high quality standards, we have reviewed the use of systemic lidocaine and its oral analogs in neuropathic pain to update our knowledge, to measure their benefit and harm, and to better define their role in therapy.

OBJECTIVES

To evaluate pain relief and adverse effect rates between systemic local anesthetic-type drugs and other control interventions.

SEARCH STRATEGY

We searched MEDLINE (1966 through 15 May 2004), EMBASE (January 1980 to December 2002), Cancer Lit (through 15 December 2002), Cochrane Central Register of Controlled Trials (2nd Quarter, 2004), System for Information on Grey Literature in Europe (SIGLE), and LILACS, from January 1966 through March 2001. We also hand searched conference proceedings, textbooks, original articles and reviews.

SELECTION CRITERIA

We included trials with random allocation, that were double blinded, with a parallel or crossover design. The control intervention was a placebo or an analgesic drug for neuropathic pain from any cause.

DATA COLLECTION AND ANALYSIS

We collected efficacy and safety data from all published and unpublished trials. We calculated combined effect sizes using continuous and binary data for pain relief and adverse effects as primary and secondary outcome measurements, respectively.

MAIN RESULTS

Thirty-two controlled clinical trials met the selection criteria; two were duplicate articles. The treatment drugs were intravenous lidocaine (16 trials), mexiletine (12 trials), lidocaine plus mexiletine sequentially (one trial), and tocainide (one trial). Twenty-one trials were crossover studies, and nine were parallel. Lidocaine and mexiletine were superior to placebo [weighted mean difference (WMD) = -11; 95% CI: -15 to -7; P <0.00001], and limited data showed no difference in efficacy (WMD = -0.6; 95% CI: -7 to 6), or adverse effects versus carbamazepine, amantadine, gabapentin or morphine. In these trials, systemic local anesthetics were safe, with no deaths or life-threatening toxicities. Sensitivity analysis identified data distribution in three trials as a probable source of heterogeneity. There was no publication bias.

AUTHORS' CONCLUSIONS

Lidocaine and oral analogs were safe drugs in controlled clinical trials for neuropathic pain, were better than placebo, and were as effective as other analgesics. Future trials should enroll specific diseases and test novel lidocaine analogs with better toxicity profiles. More emphasis is necessary on outcomes measuring patient satisfaction to assess if statistically significant pain relief is clinically meaningful.

NOTES

Notes 2017 A restricted search in June 2017 did not identify any potentially relevant studies likely to change the conclusions. Therefore, this review has now been stabilised following discussion with the authors and editors. If appropriate, we will update the review if new evidence likely to change the conclusions is published, or if standards change substantially which necessitate major revisions. 2019 We performed another restricted search in September 2019 but did not identify any potentially relevant studies likely to change the conclusions. Therefore, this review has now been stabilised following discussion with the authors and editors, and we will reassess the review for updating in 2020. If appropriate, we will update the review sooner if new evidence likely to change the conclusions is published, or if standards change substantially which necessitate major revisions. 2020 We performed another restricted search in September 2020 but again did not identify any potentially relevant studies likely to change the conclusions. This area of research is not active and therefore this review has now been stabilised following discussion with the authors and editors; we will reassess the review for updating in 2025. If appropriate, we will update the review sooner if new evidence likely to change the conclusions is published, or if standards change substantially which necessitate major revisions.

Neuropathic pain: early spontaneous afferent activity is the trigger

Intractable neuropathic pain often results from nerve injury. One immediate event in damaged nerve is a sustained increase in spontaneous afferent activity, which has a well-established role in ongoing pain. Using two rat models of neuropathic pain, the CCI and SNI models, we show that local, temporary nerve blockade of this afferent activity permanently inhibits the subsequent development of both thermal hyperalgesia and mechanical allodynia. Timing is critical-the nerve blockade must last at least 3-5 days and is effective if started immediately after nerve injury, but not if started at 10 days after injury when neuropathic pain is already established. Effective nerve blockade also prevents subsequent development of spontaneous afferent activity measured electrophysiologically. Similar results were obtained in both pain models, and with two blockade methods (200mg of a depot form bupivacaine at the injury site, or perfusion of the injured nerve just proximal to the injury site with TTX). These results indicate that early spontaneous afferent fiber activity is the key trigger for the development of pain behaviors, and suggest that spontaneous activity may be required for many of the later changes in the sensory neurons, spinal cord, and brain observed in neuropathic pain models. Many pre-clinical and clinical studies of pre-emptive analgesia have used much shorter duration of blockade, or have not started immediately after the injury. Our results suggest that effective pre-emptive analgesia can be achieved only when nerve block is administered early after injury and lasts several days.

Novel cyclopentane dicarboxamide sodium channel blockers as a potential treatment for chronic pain

A series of new voltage-gated sodium channel blockers were prepared based on the screening lead succinic diamide BPBTS. Replacement of the succinimide linker with the more rigid cyclic 1,2-trans-diamide linker was well tolerated. N-Methylation on the biphenylsulfonamide side of the amide moiety significantly reduced the clearance rate in rat pharmacokinetic studies.

Discovery of potent and use-dependent sodium channel blockers for treatment of chronic pain

A new series of voltage-gated sodium channel blockers with potential for treatment of chronic pain is reported. Systematic structure-activity relationship studies, starting with compound 1, led to identification of potent analogs that displayed use-dependent block of sodium channels, were efficacious in pain models in vivo, and most importantly, were devoid of activity against the cardiac potassium channel hERG.

Effect of sodium channel blocker (mexiletine) on pathological ectopic firing pattern in a rat chronic constriction nerve injury model

We studied the efficacy of mexiletine as a sodium channel blocker for neuropathic pain by investigating the effect of mexiletine on the pathological ectopic firing pattern in a chronic constriction nerve injury (CCI) model. The experiment was conducted with 60 male Wistar rats. The CCI model was created by loosely ligating the sciatic nerve. After breeding 7 days, the frequency and pattern of ectopic firing antidromically recorded from the sural nerve and the amplitude of antidromic sensory nerve-evoked potential were analyzed. The CCI rats were given an intravenous injection of normal saline and mexiletine (5 or 15 mg/kg). Mexiletine significantly suppressed spontaneous firing frequency, an on-off firing pattern that consisted of cyclic bursting spikes and ectopic firing generation under the hypoxic condition. Mexiletine did not influence the amplitude of A-delta component in the antidromic sensory nerve-evoked potential. Mexiletine suppressed ectopic firing by blocking activity of the abnormal sodium channel at the nerve-injured site and dorsal root ganglion without blocking nerve conduction. This study suggests that mexiletine is useful for treating neuropathic pain in peripheral neuropathy.

The Inhibitory Effect of Intravenous Lidocaine Infusion on Tinnitus after Translabyrinthine Removal of Vestibular Schwannoma: A Double-Blind, Placebo-Controlled, Crossover Study

OBJECTIVE

Intravenous infusion of lidocaine has previously been demonstrated to have a transient inhibitory effect on tinnitus in 60% of individuals. The site of action has variously been proposed as the cochlea, the cochlea nerve, and the central auditory pathways. To determine whether a central site of action exists, this study investigated the effect of intravenous infusion of lidocaine in individuals with tinnitus who had previously undergone translabyrinthine excision of a vestibular schwannoma, which involves division of the cochlear nerve.

STUDY DESIGN

Double-blind, placebo-controlled, crossover study.

SETTING

University hospital.

PATIENTS

Patients who had undergone translabyrinthine removal of a unilateral, sporadic, and histologically proven vestibular schwannoma in the last decade and who had reported postoperative tinnitus at follow-up were identified from a departmental database. Sixteen patients participated (12 men and 4 women). The mean age (+/- standard deviation) of the patients was 58 +/- 8.6 years, and the meantime since operation was 24.3 +/- 7.3 months.

INTERVENTION

Solutions of 2% lidocaine hydrochloride and sodium chloride 0.9% were prepared in identical randomized vials. The volume required for 1.5 ml/kg body weight lidocaine was calculated, and this volume was given over 5 minutes for either vial. Blood pressure, pulse oximetry, and cardiac monitoring were set up and performed throughout the infusions. All investigators were blinded.

OUTCOME MEASURES

Patient-completed visual analogue scale measures of tinnitus intensity, pitch, and distress, performed before infusion, 5 minutes after infusion onset, and 20 minutes after infusion onset.

RESULTS

A significant difference (Wilcoxon signed-rank test, p < 0.05) between placebo and lidocaine infusion conditions was demonstrated for change in visual analogue scale estimates (preinfusion versus 5 min postinfusion) of tinnitus loudness (p = 0.036), pitch (p = 0.026), and distress (p = 0.04). No significant difference between placebo and lidocaine infusion conditions was demonstrated for change in visual analogue scale estimates (preinfusion versus 20 min postinfusion) of tinnitus loudness (p = 0.066), pitch (p = 0.173), and distress (p = 0.058). The indication is of a short-lasting inhibitory effect on tinnitus of lidocaine infusion compared with saline placebo in patients who have undergone translabyrinthine excision of a vestibular schwannoma.

CONCLUSION

Intravenous infusion of lidocaine has a statistically significant inhibitory effect on tinnitus in patients who have previously undergone translabyrinthine removal of a vestibular schwannoma. The site of action of lidocaine in this instance must be in the central auditory pathway, as the cochlear and vestibular nerves are sectioned during surgery, and this finding has important implications for the task of identifying other agents that will have a similar tinnitus-inhibiting effect.

The Evolving and Important Role of Anesthesiology in Palliative Care

A small but clinically significant proportion of dying patients experience severe physically or psychologically distressing symptoms that are refractory to the usual first-line therapies. Anesthesiologists, currently poorly represented in the rapidly evolving specialties of hospice and palliative medicine, are uniquely qualified to contribute to the comprehensive care of patients who are in this category. Anesthesiologists' interpersonal capabilities in the management of patients and families under duress, their knowledge and comfort level with the application of potent analgesic and consciousness-altering pharmacology, and their titrating and monitoring skills would add a valuable dimension to palliative care teams. This article summarizes the state of the art and means by which anesthesiologists might contribute to improvements in the important end-of-life outcome of safe and comfortable dying.

The intravenous ketamine test: a predictive response tool for oral dextromethorphan treatment in neuropathic pain

IV infusion tests performed to predict subsequent response to oral analgesics are an increasingly popular method used to enhance medical care and conserve resources. Because no infusion test is completely accurate, the potential benefits of these tests must be weighed against the frustration and waste in resources encountered with false-positive results, and the failure to use a potentially beneficial treatment with false-negative results. In recent years, drugs that act antagonistically at N-methyl-d-aspartate receptors have been shown to be valuable adjuncts in the treatment of pain. To determine the predictive value of small-dose (0.1 mg/kg) IV ketamine on an oral dextromethorphan (DX) treatment regimen, we analyzed the analgesic response to these drugs in 25 patients at 2 tertiary care military treatment facilities, institutions at which DX is not readily accessible. When >/=50% response for both drugs was used as the outcome measure for success, the positive predictive value of the ketamine test was 64%, the negative predictive value 73%, and the observed agreement 68%. However, when >/=67% relief with ketamine was used as an outcome measure (as determined by a receiver operating characteristic curve), the positive predictive value was 90%, the negative predictive value 80%, and the observed agreement increased to 84%. Based on these results, we conclude that an IV ketamine test may be useful in predicting response to oral DX. More research is needed to determine the ideal candidates for such a test, and the optimal dose and cutoff value for the response to ketamine.

Management of neuropathic pain following treatment for breast cancer in the absence of recurrence: A challenge for the radiation oncologist

This report reviews various management options for treatment-induced neuropathic pain in breast cancer. First-line options include tricyclic antidepressants and anticonvulsant drugs. Opioids should be prescribed according to published guidelines. Second-line treatments include lignocaine, mexiletine and ketamine. Sympatholytic therapies are available to patients with features of chronic regional pain syndrome. Anti-inflammatory agents are used for neurogenic inflammation. Surgical interventions are considered for refractory neuropathic pain. Interdisciplinary management is appropriate when persisting pain causes physical and psychosocial disabilities.