The effect of intrathecal gabapentin and 3-isobutyl gamma-aminobutyric acid on the hyperalgesia observed after thermal injury in the rat

Multifactorial pathways in burn injury-induced chronic pain: novel targets and their pharmacological modulation

Burn injuries are among the highly prevalent medical conditions worldwide that occur mainly in children, military veterans and victims of fire accidents. It is one of the leading causes of temporary as well as permanent disabilities in patients. Burn injuries are accompanied by pain that persists even after recovery from tissue damage which puts immense pressure on the healthcare system. The pathophysiology of burn pain is poorly understood due to its complex nature and lack of considerable preclinical and clinical shreds of evidence, that creates a substantial barrier to the development of new analgesics. Burns damage the skin layers supplied with nociceptors such as NAV1.7, TRPV1, and TRPA1. Burn injury-mediated co-localization and simultaneous activation of TRPA1 and TRPV1 in nociceptive primary afferent C-fibers which contributes to the development and maintenance of chronic pain. Burn injuries are accompanied by central sensitization, a key feature of pain pathophysiology mainly driven by a series of cascades involving aberrations in the glutamatergic system, microglial activation, release of neuropeptides, cytokines, and chemokines. Activation of p38 mitogen-activated protein kinase, altered endogenous opioid signaling, and distorted genomic expression are other pathophysiological factors responsible for the development and maintenance of burn pain. Here we discuss comprehensive literature on molecular mechanisms of burn pain and potential targets that could be translated into near future therapeutics.

Perioperative Multimodal Analgesia Reduces Opioid Use Following Skin Grafting in Nonintubated Burn Patients

Hundreds of patients are treated for burn injuries each year at University of Wisconsin School of Medicine and Public Health. Pain management is particularly challenging during dressing changes and following skin grafting procedures. We performed a retrospective chart review from January 2011 through June 2018 to evaluate the effect of nonopioid analgesic medications on opioid use in nonintubated patients. Our primary outcome was the change in opioid use following the procedure. We found that most patients (69%) report severe pain (Numeric Rating Scale ≥7) immediately after autologous skin grafting. On average, patients required an additional 52 mg of oral morphine equivalents (ME) in the 24 h after the procedure compared with the 24 h before. The use of perioperative nonopioid analgesia varied between patients (acetaminophen 29%, gabapentin 29%, ketamine 35%, and all three 8%). Patients who received either gabapentin or a combination of acetaminophen, gabapentin, and ketamine had a smaller increase in their opioid use than patients who did not receive the medications (-25 ME, 95% confidence interval [-46, -4]; P = .018 and -47 ME, [-81, -11]; P = .010, respectively). These results support using a combination of acetaminophen, gabapentin, and ketamine for perioperative analgesia in burn patients undergoing autologous skin grafting.

Transcriptomic and behavioural characterisation of a mouse model of burn pain identify the cholecystokinin 2 receptor as an analgesic target

Burn injury is a cause of significant mortality and morbidity worldwide and is frequently associated with severe and long-lasting pain that remains difficult to manage throughout recovery. We characterised a mouse model of burn-induced pain using pharmacological and transcriptomic approaches. Mechanical allodynia elicited by burn injury was partially reversed by meloxicam (5 mg/kg), gabapentin (100 mg/kg) and oxycodone (3 and 10 mg/kg), while thermal allodynia and gait abnormalities were only significantly improved by amitriptyline (3 mg/kg) and oxycodone (10 mg/kg). The need for relatively high opioid doses to elicit analgesia suggested a degree of opioid resistance, similar to that shown clinically in burn patients. We thus assessed the gene expression changes in dorsal root ganglion neurons and pathophysiological mechanisms underpinning burn injury-induced pain using a transcriptomic approach. Burn injury was associated with significantly increased expression of genes associated with axon guidance, neuropeptide signalling, behavioural defence response and extracellular signalling, confirming a mixed neuropathic and inflammatory aetiology. Notably, among the pain-related genes that were upregulated post-injury was the cholecystokinin 2 receptor (Cckbr), a G protein-coupled receptor known as a pain target involved in reducing opioid effectiveness. Indeed, the clinically used cholecystokinin receptor antagonist proglumide (30 mg/kg) was effective at reversing mechanical allodynia, with additional analgesia evident in combination with low-dose oxycodone (1 mg/kg), including significant reversal of thermal allodynia. These findings highlight the complex pathophysiological mechanisms underpinning burn injury-induced pain and suggest that cholecystokinin-2 receptor antagonists may be useful clinically as adjuvants to decrease opioid requirements and improve analgesic management.

The effect of gabapentin and ketorolac on allodynia and conditioned place preference in antibody-induced inflammation

BACKGROUND

Glucose-6-phosphate isomerase and collagen type II antibody-induced arthritis models (K/BxN and CAIA, respectively) have an inflammatory and a post-inflammatory phase. Both phases display robust tactile allodynia. In previous work, inflammatory phase allodynia was reversed by gabapentin and ketorolac, whereas in late phase only gabapentin was effective. Here, we sought to determine if the effects of these two drugs during the early and late phases of the two arthritis models were observed in the conditioned place preference (CPP) paradigm, indicating a differential drug effect on the aversive state.

METHODS

Male C57BL/6 mice received K/BxN serum intraperitoneally, while male BALB/c mice received collagen type II antibody cocktail intravenously. After onset of inflammation and allodynia, we assessed effects of i.p. gabapentin (100 mg/kg) or ketorolac (15 mg/kg) using a CPP paradigm: 2 days adaptation, 2 days conditioning (vehicle in morning and drug in afternoon), preference testing on day 5.

RESULTS

Consistent with the effects upon allodynia, both gabapentin and ketorolac produced a preference for the drug-paired compartment in the early phase of the K/BxN model, while gabapentin, but not ketorolac, resulted in a place preference during late phase. In the CAIA model, consistent with differential effects upon allodynia, gabapentin produced a preference in the early phase and a trend in the late phase, whereas ketorolac was ineffective at either time.

CONCLUSIONS

CPP validated the aversive state in the inflammatory and post-inflammatory phases of the K/BxN and CAIA arthritis models and correspondence between the anti-hyperpathic pharmacology as defined by thresholds and CPP.

Analgesic effect of intrathecal gabapentin in a rat model of persistent muscle pain

OBJECTIVE

To evaluate the analgesic effect of intrathecal gabapentin therapy on secondary hyperalgesia in a rat model of persistent muscle pain.

METHODS

Intrathecal catheters were implanted into rats. Mechanical secondary hyperalgesia was induced by repeated intramuscular injections of acidic solution into the gastrocnemius muscle. Gabapentin was administrated intrathecally. Rats were allocated to control and experimental (gabapentin 30, 100, 300, and 1,000 µg) group. After gabapentin administration, mechanical withdrawal threshold was measured every 15 minutes and the motor function was measured 30 minutes later.

RESULTS

Mechanical hyperalgesia was evoked after the second acidic buffer injection. There was a significant improvement on the mechanical threshold after administration of 100, 300, and 1,000 µg gabapentin compared to pre-injection and the control group. The analgesic effect continued for 105, 135, and 210 minutes, respectively. To discern side effects, motor function was measured. Motor function was preserved in both groups after gabapentin administration, except for rats who received 1,000 µg gabapentin.

CONCLUSION

Intrathecal gabapentin administration produces dose-dependent improvements in mechanical hyperalgesia in a persistent muscle pain rat model. This implicates the central nervous system as having a strong influence on the development of persistent mechanical hyperalgesia. These results are helpful in understanding the pathophysiology of secondary hyperalgesia and in the treatment of patients with chronic muscle pain.

Pregabalin in acute and chronic pain

Pregabalin is a gamma-amino-butyric acid analog shown to be effective in several models of neuropathic pain, incisional injury, and inflammatory injury. In this review, the role of pregabalin in acute postoperative pain and in chronic pain syndromes has been discussed. Multimodal perioperative analgesia with the use of gabapentinoids has become common. Based on available evidence from randomized controlled trials and meta-analysis, the perioperative administration of pregabalin reduces opioid consumption and opioid-related adverse effects in the first 24 h following surgery. Postoperative pain intensity is however not consistently reduced by pregabalin. Adverse effects like visual disturbance, sedation, dizziness, and headache are associated with higher doses. The advantage of the perioperative use of pregabalin is so far limited to laparoscopic, gynecological, and daycare surgeries which are not very painful. The role of the perioperative administration of pregabalin in preventing chronic pain following surgery, its efficacy in more painful surgeries and surgeries done under regional anesthesia, and the optimal dosage and duration of perioperative pregabalin need to be studied. The efficacy of pregabalin in chronic pain conditions like painful diabetic neuropathy, postherpetic neuralgia, central neuropathic pain, and fibromyalgia has been demonstrated.

A randomized controlled trial to compare pregabalin with gabapentin for postoperative pain in abdominal hysterectomy

Background:

Pregabalin is a potent ligand for alpha-2-delta subunit of voltage-gated calcium channels in the central nervous system, which exhibits potent anticonvulsant, analgesic and anxiolytic activity. The pharmacological activity of pregabalin is similar to that of gabapentin and shows possible advantages. Although it shows analgesic efficacy against neuropathic pain, very limited evidence supports its postoperative analgesic efficacy. We investigated its analgesic efficacy in patients experiencing acute pain after abdominal hysterectomy and compared it with gabapentin and placebo.

Methods:

A randomized, double-blind, placebo-controlled study was conducted in 90 women undergoing abdominal hysterectomy who were anaesthetized in a standardized fashion. Patients received 300 mg pregabalin, 900 mg gabapentin or placebo, 1–2 hours prior to surgery. Postoperative analgesia was administered at visual analogue scale (VAS) ≥3. The primary outcome was analgesic consumption over 24 hours and patients were followed for pain scores, time to rescue analgesia and side effects as secondary outcomes.

Results:

The diclofenac consumption was statistically significant between pregabalin and control groups, and gabapentin and control groups; however, pregabalin and gabapentin groups were comparable. Moreover, the consumption of tramadol was statistically significant among all the groups. Patients in pregabalin and gabapentin groups had lower pain scores in the initial hour of recovery. However, pain scores were subsequently similar in all the groups. Time to first request for analgesia was longer in pregabalin group followed by gabapentin and control groups.

Conclusion:

A single dose of 300 mg pregabalin given 1–2 hours prior to surgery is superior to 900 mg gabapentin and placebo after abdominal hysterectomy. Both the drugs are better than placebo.

Effects of gabapentin on brain hyperactivity related to pain and sleep disturbance under a neuropathic pain-like state using fMRI and brain wave analysis

Neuropathic pain is the most difficult pain to manage in the pain clinic, and sleep problems are common among patients with chronic pain including neuropathic pain. In the present study, we tried to visualize the intensity of pain by assessing neuronal activity and investigated sleep disturbance under a neuropathic pain-like state in mice using functional magnetic resonance imaging (fMRI) and electroencephalogram (EEG)/electromyogram (EMG), respectively. Furthermore, we investigated the effect of gabapentin (GBP) on these phenomena. In a model of neuropathic pain, sciatic nerve ligation caused a marked decrease in the latency of paw withdrawal in response to a thermal stimulus only on the ipsilateral side. Under this condition, fMRI showed that sciatic nerve ligation produced a significant increase in the blood oxygenation level-dependent (BOLD) signal intensity in the pain matrix, which was significantly decreased 2 h after the i.p. injection of GBP. Based on the results of an EEG/EMG analysis, sciatic nerve-ligated animals showed a statistically significant increase in wakefulness and a decrease in non-rapid eye movement (NREM) sleep during the light phase, and the sleep disturbance was almost completely alleviated by a higher dose of GBP in nerve-ligated mice. These findings suggest that neuropathic pain associated with sleep disturbance can be objectively assessed by fMRI and EEG/EMG analysis in animal models. Furthermore, GBP may improve the quality of sleep as well as control pain in patients with neuropathic pain.

Gabapentin improves cold-pressor pain responses in methadone-maintained patients

Individuals on methadone maintenance for the treatment of addiction (MM) are demonstrated to be hyperalgesic to cold-pressor pain in comparison to matched controls and ex-opioid addicts, a finding described as clinical evidence of opioid-induced hyperalgesia (OIH). Interestingly, opioids induce hyperalgesia via many of the same neuro-inflammatory and central sensitization processes that occur with the development of neuropathic pain. Evaluated in this study was the efficacy of a key pharmacotherapy for neuropathic pain, gabapentin (GPN), to reverse OIH in MM patients. Utilizing a clinical trial design and double blind conditions, changes in cold-pressor pain threshold and tolerance following a 5-week trial of GPN (titrated to 2400mg/day) were evaluated at peak and trough methadone plasma levels in a well-characterized MM sample. Drug abstinence was encouraged via an escalating payment schedule, and compliance monitored via pill counts and GPN plasma levels; entered into the analyses were only those subjects compliant and abstinent throughout the study (approximately 45%). Utilizing change scores from baseline, significant improvements in cold-pressor pain threshold and pain tolerance were observed at both peak and trough methadone levels (p<0.05). Notably, drop-out rates due to medication side effects were low (2%) and the medication was well-tolerated. These results support that GPN, as prescribed for the treatment of neuropathic pain, is effective in decreasing OIH in patients who are abstinent and stable in methadone treatment.

Pregabalin: in the treatment of postherpetic neuralgia

Pregabalin is the pharmacologically active S-enantiomer of 3-aminomethyl-5-methyl-hexanoic acid. It has a similar pharmacological profile to that of its developmental predecessor gabapentin, but had greater analgesic activity in rodent models of neuropathic pain. Pregabalin is thought to act by reducing the excessive release of several excitatory neurotransmitters by binding to the alpha(2)-delta protein subunit of voltage-gated calcium channels. Oral pregabalin 150-600 mg/day, administered in two or three divided doses, was significantly more effective than placebo in relieving pain and improving pain-related sleep interference in four randomized, double-blind, multicentre studies of 4-13 weeks' duration in patients with postherpetic neuralgia (PHN). Pregabalin achieved a faster onset of pain relief than placebo. The median times to the onset of pain relief with fixed and flexible doses of pregabalin were 1.5 and 3.5 days compared with >4 weeks with placebo. Pregabalin was generally well tolerated when titrated over 1 week to fixed dosages (maximum 600 mg/day) in clinical trials in mostly elderly PHN patients. Adverse events were usually mild to moderate in severity.

Spinal p38 mitogen-activated protein kinase mediates allodynia induced by first-degree burn in the rat

Activation of p38 mitogen-activated protein kinase (MAPK) in the spinal cord has been implicated in the development and maintenance of pain states. In this study, we tested whether p38 MAPK is involved in the response to first-degree burn of the hind paw. This injury induces central sensitization leading to tactile allodynia and is mediated by activation of Ca(2+) permeable AMPA/kainate receptors through PKC and PKA. We demonstrate that p38 MAPK is rapidly and robustly activated in the superficial spinal dorsal horn after mild thermal injury to the hind paw. Activated p38 MAPK was localized primarily to microglia and to a lesser extent in oligodendrocytes and lamina II neurons. Astrocytes were not involved in the p38 MAPK response. Intrathecal pretreatment of pharmacological inhibitors of p38 MAPK (SB203580, SD-282) dose-dependently blocked development of tactile allodynia, a characteristic of the first-degree burn model. The effects of the inhibitors on tactile allodynia were lost when they were administered after injury. These studies identify p38 MAPK as a major mediator of tactile allodynia, most likely activated downstream of AMPA/kainate receptors.

Successful use of gabapentin in acute pain management following burn injury: a case series

Pain after burn injury has multiple qualities, including neuropathic and hyperalgesic elements. This element of the burn patients' pain experience is frequently difficult to manage and contributes significantly to their suffering. The onset may be either immediate or delayed. Gabapentin has established efficacy in the reduction of burn-induced hyperalgesia and allodynia in animal and human experimental burn models. This article reports a case series of six patients who, following admission to hospital with burn injury, described burning dysesthesia at either the injury or graft donor site. These patients were prescribed gabapentin in addition to standard analgesia. The use of gabapentin resulted in a rapid reduction in the severity of the neuropathic element of the pain. The medication was well tolerated, with no severe adverse reactions. Conclusions. This case series introduces the use of gabapentin as a potentially important therapy in the management of neuropathic pain following burn injury. Further research is required to define the use of gabapentin in this specific setting.

Gabapentin evoked changes in functional activity in nociceptive regions in the brain of the anaesthetized rat: an fMRI study

BACKGROUND AND PURPOSE

Gabapentin (GBP; 1-(aminomethyl)cyclohexane acetic acid) is used clinically in the treatment of pain. Nevertheless, the sites and mechanisms of action of GBP are poorly defined. Herein, the effects of GBP on brain activation have been studied.

EXPERIMENTAL APPROACH

Changes in blood oxygen level dependent (BOLD) haemodynamic signal following intravenous infusion of GBP (equivalent to 30 mg kg(-1) p.o., followed by 100 mg kg(-1) p.o.), compared to saline control, were studied in isofluorane anaesthetized rats (n=8 per group). Effects of GBP on mean arterial blood pressure (MAP) were also recorded.

RESULTS

Random effect analysis revealed that the lower dose of GBP produced significant (P<0.001) increases in BOLD signal intensity in several brain regions, including the thalamus and periaqueductal grey (PAG), compared to basal. This dose of GBP also produced significant (P<0.001) decreases in BOLD signal intensity in the amygdala and the entorhinal cortex. Increasing the dose of GBP (100 mg kg(-1)) produced significantly greater changes in BOLD signal intensity in several brain regions including the thalamus and PAG. MAP was not significantly altered by GBP, compared to saline.

CONCLUSIONS AND IMPLICATIONS

GBP had marked positive and negative effects on BOLD signal intensity in a number of brain regions in naïve rats. The activation of key areas involved in nociceptive processing indicate a supraspinal site of action of GBP and this may contribute to its well-described analgesic effects in animal models of pain and clinical studies.

Management of pain after burn injury

PURPOSE OF REVIEW

Burn pain is often under treated. Burn patients suffer from daily background pain as well as procedural pain. Direct mechanical and chemical stimulation to peripheral nociceptors, peripheral- and central sensitization contribute to the pathophysiology of pain. The purpose of this review is to discuss the current management of burn pain and also to stimulate future studies.

RECENT FINDINGS

Background pain is best treated with mild to moderate potent analgesics administered regularly to maintain a steady plasma drug concentration. Procedural pain should be treated vigorously with intravenous opioids, local or even general anesthesia if needed. Opioids are the mainstay of treatment for severe acute pain. PCA should be used wherever applicable. Further opioids should not be substituted by high dose NSAIDs in the management of procedural pain. Hypnosis, therapeutic touch, massage therapy, distracting techniques and other behavioral cognitive techniques have demonstrated some intriguing impact on acute as well as chronic burn pain treatment.

SUMMARY

There is no clear evidence to show that the use of opioids in acute pain may increase the likelihood of developing opioid dependency. Thus, pain after burn injury should be aggressively treated using pharmacologic and non-pharmacologic approaches. Further controlled studies are yet to be conducted to define appropriate treatments for different burn patients and to establish standard treatment protocols for burn pain.

Inhibitory effect of gabapentin on N-methyl-D-aspartate receptors expressed in Xenopus oocytes

BACKGROUND

Gabapentin (GBP) is a prescription drug used for the treatment of neuropathic and post-operative pain. However, the mechanism by which it exerts its analgesic action is not well understood. Because intrathecal administration of GBP has been shown to exert antinociceptive effects in animal studies, we hypothesized that the spinal cord may be a plausible action site.

METHODS

We examined the effects of GBP on neurotransmitter-gated ion channels and G protein-coupled inwardly rectifying potassium (GIRK) channels distributed in the spinal cord and involved in pain modulation. Recombinant human NR1/NR2A N-methyl-D-aspartate (NMDA), alpha(1)beta(2)gamma(2S)gamma-aminobutyric acid type A (GABA(A)) or alpha(1) glycine receptors, or GIRK1/GIRK2 channels were expressed in Xenopus laevis oocytes and the effects of GBP (0.1-1000 microM) on them were assessed using a two-electrode, voltage-clamp system.

RESULTS

GABA(A) and glycine receptors and GIRK channels were not affected by GBP, even at the highest concentrations. Conversely, NMDA receptors were inhibited by GBP in a concentration-dependent manner, with significant inhibition observed at 10 microM. At 30 microM, GBP inhibited the glutamate-concentration response curve without changing the half-maximal effective concentration or the Hill coefficient, indicating a non-competitive inhibition. Glycine decreased the inhibitory effect in a concentration-dependent manner.

CONCLUSIONS

These findings suggest that the inhibitory effect of GBP on NMDA receptors may play an important role in the antinociceptive property of GBP; however, it does not appear that GABA(A) and glycine receptors or GIRK channels contribute to the pharmacological properties of GBP.

Pharmacological modulation of pain-related brain activity during normal and central sensitization states in humans

Abnormal processing of somatosensory inputs in the central nervous system (central sensitization) is the mechanism accounting for the enhanced pain sensitivity in the skin surrounding tissue injury (secondary hyperalgesia). Secondary hyperalgesia shares clinical characteristics with neurogenic hyperalgesia in patients with neuropathic pain. Abnormal brain responses to somatosensory stimuli have been found in patients with hyperalgesia as well as in normal subjects during experimental central sensitization. The aim of this study was to assess the effects of gabapentin, a drug effective in neuropathic pain patients, on brain processing of nociceptive information in normal and central sensitization states. Using functional magnetic resonance imaging (fMRI) in normal volunteers, we studied the gabapentin-induced modulation of brain activity in response to nociceptive mechanical stimulation of normal skin and capsaicin-induced secondary hyperalgesia. The dose of gabapentin was 1,800 mg per os, in a single administration. We found that (i) gabapentin reduced the activations in the bilateral operculoinsular cortex, independently of the presence of central sensitization; (ii) gabapentin reduced the activation in the brainstem, only during central sensitization; (iii) gabapentin suppressed stimulus-induced deactivations, only during central sensitization; this effect was more robust than the effect on brain activation. The observed drug-induced effects were not due to changes in the baseline fMRI signal. These findings indicate that gabapentin has a measurable antinociceptive effect and a stronger antihyperalgesic effect most evident in the brain areas undergoing deactivation, thus supporting the concept that gabapentin is more effective in modulating nociceptive transmission when central sensitization is present.

Synergic Antinociceptive Interaction between Tramadol and Gabapentin after Local, Spinal and Systemic Administration

The possible interaction between tramadol and gabapentin on formalin-induced nociception in the rat was assessed. Tramadol, gabapentin or a fixed-dose ratio combination of gabapentin and tramadol were administered peripherally, spinally and orally to rats, and the antinociceptive effect was determined in the 1% formalin test. Isobolographic analyses were used to define the nature of the interactions between drugs. Tramadol, gabapentin and tramadol-gabapentin combinations produced a dose-dependent antinociceptive effect when administered locally, spinally or orally. ED30 values were estimated for the individual drugs and isobolograms were constructed. Theoretical ED30 values for the combination estimated from the isobolograms were 126.8 +/- 11.1 microg/paw, 23.1 +/- 2.6 microg/rat, and 2.23 +/- 0.32 mg/kg for the local, intrathecal and oral routes, respectively. These values were significantly higher than the actually observed ED30 values which were 13.3 +/- 2.1 microg/paw, 8.1 +/- 0.6 microg/rat and 0.71 +/- 0.10 mg/kg, indicating a synergistic interaction. Although efficacy was not improved, local peripheral administration resulted in the highest increase in potency, being about tenfold. Spinal and systemic administration increased potency threefold. Data indicate that low doses of the tramadol-gabapentin combination can interact synergistically to reverse formalin-induced nociception and may represent a therapeutic advantage for clinical treatment of inflammatory pain.

Pregabalin for Pain Management

Pregabalin is a new analog of the neurotransmitter gamma-aminobutyric acid (GABA). It is an alpha2-delta (alpha2-delta) ligand that has analgesic, anticonvulsant, and anxiolytic activity. Alpha2-delta is an auxiliary protein associated with voltage-gated calcium channels. Pregabalin binds potently to the alpha2-delta subunit resulting in modulation of calcium channels and reduction in the release of several neurotransmitters, including glutamate, norepinephrine, serotonin, dopamine, and substance P. This review discusses the pharmacology of this medication as well as available studies in patients.

Pregabalin: in the treatment of postherpetic neuralgia

Pregabalin, the pharmacologically active S-enantiomer of 3-aminomethyl-5-methyl-hexanoic acid, has a similar pharmacological profile to that of its developmental predecessor gabapentin, but showed greater analgesic activity in rodent models of neuropathic pain. The exact mechanism of action of pregabalin is unclear, although it may reduce excitatory neurotransmitter release by binding to the alpha2-delta protein subunit of voltage-gated calcium channels. Oral pregabalin 150-600 mg/day, administered twice or three times daily, was superior to placebo in relieving pain and improving pain-related sleep interference in three randomised, double-blind, placebo-controlled, multicentre studies of 8-13 weeks' duration in a total of 776 evaluable patients with postherpetic neuralgia (PHN). Weekly mean pain scores (primary endpoint; assessed in all three studies) and weekly mean sleep interference scores (assessed in two studies) were significantly improved at 1 week. In two studies, significant improvements in daily mean pain scores were apparent on the first or second day of treatment with pregabalin administered three times daily. Pregabalin was generally well tolerated when force-titrated over 1 week to fixed dosages (maximum 600 mg/day) in clinical trials that enrolled most elderly PHN patients. Dizziness, somnolence and peripheral oedema of mild-to-moderate intensity were the most common adverse events.

Strategies for the treatment of cancer pain in the new millennium

As was the case in the era before us, in the new millennium we will continue to see an abundance of patients experiencing cancer-related pain for different reasons. Although much needless pain and suffering still affects many of those with cancer, we are presented with a medical dichotomy. With the analgesic drugs available today, and the relatively simple and effective guidelines to treat cancer pain published and disseminated by the World Health Organization, why do people with cancer continue to experience pain? As we search for the answer, the horizon may hold promising new drugs, 'old drugs' with new interest and applications, and new strategies for the field of pain therapy. Possibilities include the isolation and development of analgesics or analgesic combinations that may minimise the adverse effects which are often associated with the current therapeutic class of opioid analgesics. In addition, current research points to promising results identifying the N-methyl D-aspartate non-opioid receptor as a likely component of neuropathic pain. Drugs such as gabapentin, the mechanism of action of which is not well known, have found favour within the clinical community for their analgesic properties and good tolerability. Methadone, in a phase of resurgence, has garnered the attention of the clinical community because of its unique receptor activity and pharmacoeconomic benefits. A number of clinical studies have demonstrated that methadone has a valuable role in treating cancer pain. Perhaps, an unbalanced focus on the risks of inappropriate use, rather than the benefits, should not compromise or distract from the use of methadone as an alternative to morphine. Studies are on going to assess the potential role of methadone in treating neuropathic pain. Drugs such as cannabinoids, although currently applicable for patients with anorexia, nausea and/or vomiting, may offer benefits to patients experiencing pain. Other opportunities exist with such compounds as alpha2-adrenergic agonists, nicotine, lidocaine and ketamine. New strategies such as the switching opioids and/or their route of administration may offer improved analgesia with fewer adverse effects, thus providing therapeutic alternatives for the clinical community. In addition, there is interest in the co-administration of opioids that act on different receptors. For instance, oxycodone appears to be a kappa opioid receptor agonist and may offer enhanced analgesia when combined with morphine.

Gabapentin for neuropathic pain: systematic review of controlled and uncontrolled literature

OBJECTIVE

To assess the efficacy/effectiveness and side effects of gabapentin for the treatment of neuropathic pain.

DESIGN

Systematic review of the literature.

METHODS

Extensive search of several electronic databases located both controlled and uncontrolled studies. Efficacy was assessed through meta-analysis of randomized controlled trials (RCTs), whereas the effectiveness of gabapentin in uncontrolled studies was assessed via a novel system of dichotomous classification of "bad" versus "good" results.

FINDINGS

Thirty-five papers involving 727 patients with multiple neuropathic pain conditions met the inclusion criteria. The meta-analysis of the 2 high-quality, placebo-controlled RCTs showed positive effect of gabapentin in diabetic neuropathy and post-herpetic neuralgia. The addition of 2 low-quality, placebo-controlled RCTs did not alter the magnitude or direction of observed effect. The uncontrolled studies demonstrated positive effect on pain in different neuropathic syndromes, as well as benefit on different types of neuropathic pain; highest dose administered and rate-of-dose escalation showed wide variability between prescribers. Fewer and less severe side effects were reported in the uncontrolled studies.

CONCLUSIONS

Gabapentin seems to be effective in multiple painful neuropathic conditions. The variable prescribing patterns of the uncontrolled studies raise the suspicion that effectiveness may be reduced if one limits administration of the drug to very low doses, whereas rapid dose escalation may be associated with increased central nervous system side effects. Well-designed controlled trials may provide insight into differential symptom sensitivity to the drug.

The pharmacological basis of contemporary pain management

Pain management has become an increasingly well researched area in medicine over recent years, and there have been advances in a number of areas. While opioids remain an integral part of pain-management strategies, there is now an emphasis on the use of adjuvant drugs, such as paracetamol and anti-inflammatory agents, which through physiological or pharmacological synergism, both enhance pain control and reduce opioid use. The management of neuropathic pain continues to be a challenge. Anti-epileptics and antidepressants, together with clonidine and ketamine, provide the foundations for treatment. Another area of interest has been the widespread use of patient-controlled analgesia and the administration of some drugs, especially opioids, by means other than traditional oral and parenteral routes. The number of new drugs that have reached the stage of clinical trials has been small, yet they offer exciting possibilities. The epibatidine analogue ABT-594 and zinconitide both offer novel approaches to the management of neuropathic pain states, while selective cyclo-oxygenase-2 inhibitors and nitroaspirins may see advances in the management of nociceptive pain states.