Reversible acute renal allograft dysfunction due to gabapentin

BACKGROUND

The use of gabapentin as an effective analgesic agent for neuropathic pain has expanded considerably. Its lack of both anticholinergic side effects and interference with the metabolism of drugs via the cytochrome P450 pathway make it especially useful for transplant recipients.

METHODS AND RESULTS

We describe the case of a renal transplant recipient with a long-term stable functioning allograft who developed reversible acute renal dysfunction after beginning gabapentin therapy for chronic pain due to diabetic neuropathy.

CONCLUSIONS

We suggest that gabapentin may cause acute renal dysfunction by a mechanism involving renal afferent vasoconstriction. Caution should be employed when considering the use of gabapentin in transplant recipients, especially when combined with other agents that may potentiate renal vasoconstriction.

Adjuvant therapy of melanoma with interferon-alpha-2b is associated with mania and bipolar syndromes

BACKGROUND

The use of a high dose regimen of interferon-alpha-2b (IFN) has recently been demonstrated to benefit patients with resected high risk melanoma. The incidence of melanoma is rising rapidly, and the use of this regimen is becoming increasingly common. IFN has been associated with numerous psychiatric side effects.

METHODS

The authors describe four melanoma patients treated with adjuvant IFN who developed a manic-depressive syndrome or mood instability with therapy, and they review the literature on mania and the mixed affective syndromes associated with IFN.

RESULTS

The authors suggest that IFN may induce a mixed affective instability, and that patients risk developing hypomania or mania as IFN doses fluctuate or as IFN-induced depression is treated with antidepressants alone. Mania is particularly associated with dose reductions or pauses in IFN treatment. The risk of mood fluctuation continues after treatment with IFN stops, and patients should be monitored for 6 months following completion of therapy. Gabapentin appeared effective as monotherapy for acute mania, as an antianxiety agent, as a hypnotic, and as a mood stabilizer in these individual cases.

CONCLUSIONS

Mania and mood instability can occur in patients being treated with IFN therapy for melanoma. In this study, gabapentin was an effective mood-stabilizing agent for these patients.

Gabapentin: a new tool in the treatment of neuropathic pain

Human neuropathic pain remains a prevalent and pervasive problem in our society. Pharmacologically there is also no single, uniformly well-tolerated drug that is reliably helpful. Gabapentin has emerged as a useful new tool based on the results of two large multicenter trials in models of human neuropathy. Gabapentin proved to be a significantly better analgesic than placebo, was well tolerated in the elderly population, and had a significant positive impact on several subjective and objective outcome measures. A discussion of the standard treatments and the studies supporting this new tool is the purpose of this review.

Different effects of GABAergic anticonvulsants on 4-aminopyridine-induced spontaneous GABAergic hyperpolarizations of hippocampal pyramidal cells--implication for their potency in migraine therapy

Clinical studies indicate anti-migrane efficacy of the probably GABAergic anticonvulsants valproate and gabapentin. For the GABAergic anticonvulsants vigabatrin and tiagabine, studies about antimigrane efficacy are missing. The aim of this study was to test the GABAergic potency of these drugs in vitro before further clinical studies. Intracellular recordings were obtained from hippocampal pyramidal cells. Spontaneous GABAergic hyperpolarizations (SGH) elicited by 75 microM 4-aminopyridine were used to test the effect of these drugs on GABA-dependent potentials. Tiagabine (0.1 mM) prolonged the duration of SGH. Furthermore, monophasic SGH turned over into triphasic typical GABAergic membrane potential fluctuations within 20 min. In contrast, valproate, gabapentin, and vigabatrin failed to affect SGH up to 60 min of application. The reason for the fast action of tiagabine on SGH may be caused by a faster increase of synaptic GABA levels compared with other drugs. As migraine therapy benefits from an augmentation of GABA activity, we recommend clinical studies of tiagabine as a fast-acting agent in migraine attacks.

Inter- and intra-subject variability in gabapentin absorption and absolute bioavailability

Gabapentin (GBP) is a non-metabolized, non-plasma protein bound, renally excreted antiepileptic drug that is actively absorbed via the system L amino acid transporter. Previous studies have demonstrated that gabapentin displays dose-dependent absorption.

OBJECTIVES

These studies were conducted to determine inter- and intra-subject variability of gabapentin absorption. Two prospective clinical studies in healthy adult volunteers were conducted. Coefficient of variation (CV) was used to express variability of gabapentin absorption.

METHODS

Study A: 400-mg single dose, randomized, cross-over study to assess bioavailability of four different gabapentin formulations (n=20, 9 males, 11 females; mean age and weight 41 years, 75.1 kg). Plasma was serially collected up to 48 h and bioavailability (F) calculated post-dose to determine concentration-time curves (AUC). All four formulations were bioequivalent, thus repeated measures analysis was performed to assess inter-and intra-subject variability. Study B: 600-mg single dose study (n=50, 15 males, 35 females; mean age and weight 31.1 years, 72.7 kg) was conducted to determine inter-subject variability in gabapentin F. Urine was collected over 48 h and bioavailability (F) calculated. Urine and plasma gabapentin concentrations were measured by HPLC-UV.

RESULTS

Study A: Overall mean (CV) of GBP AUC values was 34.1+/-24 ug/h per ml. Inter-subject CV for AUC was 22.5% and intra-subject CV was 12.1%. Study B: Overall mean (SD) GBP F was 49.3+/-13.6%. Inter-subject CV of F was 27.6%.

DISCUSSION

The inter-subject variability in gabapentin absorption is substantially less than that of the inter-subject variability. This indicates that one would expect a wide range in gabapentin absorption between subjects; however, a much smaller variability within a subject. The within subject variability of gabapentin is small enough that plasma drug monitoring may be used to assess gabapentin absorption for a given subject and the benefit of dose individualization.

Gabapentin enhances the analgesic effect of morphine in healthy volunteers

The most effective group of drugs for the treatment of severe pain is opioid analgesics. Their use, however, is limited by decreased effects in neuropathic and chronic pain as a result of increased pain and development of tolerance. Gabapentin (GBP) is effective in both experimental models of chronic pain and clinical studies of neuropathic pain. Therefore, we investigated, in a randomized, placebo-controlled, double-blinded study, the pharmacodynamic and pharmacokinetic interaction of GBP and morphine in 12 healthy male volunteers. Morphine (60 mg, controlled release) or placebo was administered at 8:00 AM, and GBP (600 mg) or placebo was administered at 10:00 AM, thus comparing the analgesic effect of placebo + GBP (600 mg) with placebo + placebo and morphine (60 mg) + GBP in comparison to morphine plus placebo by using the cold pressor test. The duration and intensity of the side effects were assessed by using visual analog scales. The analgesic effect was evaluated by the change in the area under the curve (h x %; 0% baseline before Medication 1) of pain tolerance. Placebo + GBP (18.9% x h, 95% confidence interval [CI]: -2.5 to 40.3) did not present any significant analgesic effect compared with placebo + placebo (4.7% x h, 95% CI: -16.7 to 26.1). A significant increase in pain tolerance was observed comparing the combination of morphine and GBP (75.5% x h, 95% CI: 54.0-96.9) with morphine + placebo (40.6% x h, 95% CI: 19. 2-62.0). The observed adverse events after placebo + GBP were not significantly different compared with placebo + placebo. Morphine + placebo led to the expected opioid-mediated side effects. They were significantly more pronounced compared with placebo + placebo but did not differ significantly compared with the combination of morphine + GBP. Concerning the pharmacokinetic variables of morphine and its glucuronides, no significant difference between morphine + placebo and morphine + GBP was observed, whereas the area under the curve of GBP (43.9 +/- 5.3 vs 63.4 +/- 16.2 microg. h(-1). mL(-1), P < 0.05) significantly increased, and apparent oral clearance (230.8 +/- 29.4 mL/min vs 178 +/- 97.9 mL/min, P = 0.06) and apparent renal clearance (86.9 +/- 20.6 vs 73.0 +/- 24.2 mL/min, P = 0.067) of GBP decreased when morphine was administered concomitantly. These results suggest two different sites for the pharmacokinetic interaction-one at the level of absorption and the other at the level of elimination. Our study reveals both a pharmacodynamic and pharmacokinetic interaction between morphine and GBP, leading to an increased analgesic effect of morphine + GBP. These results and the good tolerability of GBP should favor clinical trials investigating the clinical relevance of the combination of morphine and GBP for treating severe pain.

IMPLICATIONS

In a randomized, placebo-controlled, double-blinded trial with 12 healthy volunteers, we studied the interaction of morphine and gabapentin using the cold pressor test. The anticonvulsant gabapentin enhanced the acute analgesic effect of morphine. Furthermore, the plasma concentration of gabapentin was increased when morphine was administered concomitantly. Therefore, the well tolerated combination of gabapentin and morphine may improve pain therapy, especially in pain states, like chronic and neuropathic pain, which respond poorly to opioids.

Gabapentin for essential tremor: a multiple-dose, double-blind, placebo-controlled trial

Essential tremor (ET) is a common movement disorder that often becomes refractory to conventional pharmacologic management. Open-label studies suggest that gabapentin is efficacious for ET, but the results of controlled trials have been mixed. To determine the efficacy and tolerability of gabapentin in ET, we conducted a double-blind, placebo-controlled, cross-over trial evaluating two doses (1800 mg per day and 3600 mg per day; N = 25). Patients on other ET medications were maintained on their concurrent medications for 3 months prior to study initiation and throughout the study. Twenty patients (mean age, 69.9 +/- 6.1 yrs) completed the study. Overall, patient global assessments (p <0.05), observed tremor scores (p <0.005), water pouring scores (p <0.05), and activities of daily living scores (p <0.005) significantly improved. Accelerometry scores, spirographs, and investigator global impression scores did not improve. The results were similar for high and low doses. Statistical regression models did not demonstrate any significant predictors for response. Gabapentin may be effective in some cases of ET.

Effect of vigabatrin and gabapentin on phynytoin pharmacokinetics in the dog

The study was aimed at investigating whether or not the kinetics of intravenously administered phenytoin (PT) was altered by oral administration of vigabatrin (VGB) or gabapentin (GBP). A group of five beagle dogs were given a daily dose of PT (12 mg/kg, i.v.) for a period of 1 week. On day 8, plasma samples were serially collected over 24 hr. after administration of the PT dose. PT administration was continued with oral supplementary dose of VGB (60 mg/kg) for another week and then plasma samples were collected for analysis of PT levels. The same protocol was followed for the PT (12 mg/kg, i.v.)-GBP (300 mg caps., p.o.) study on a separate group (n = 5) of dogs. Orally administered GBP did not significantly alter the pharmacokinetic parameters of parental PT. VGB, however markedly changed the drug's kinetics as evidenced by a 31% (P = 0.015) reduction in total body clearance (CL) and increase of over 45% in half-life (t1/2), (P = 0.013) and area under the plasma PT concentration-time curve (AUC), (P = 0.044). GBP does not appear to have any pharmacokinetic interaction with PT, while coadministration of VGB and PT results in marked reduction in systemic clearance of the latter in the dog.

[Gabapentin used in 559 patients with partial seizures. A multicenter observation study. Spanish Gabapentin Work Group]

INTRODUCTION

Gabapentin (GBP) is a new antiepileptic drug whose efficacy and tolerability have been evaluated in clinical trials, although there is little data on its use in everyday clinical practice.

OBJECTIVES

To evaluate the characteristics of GBP in an observational study when used in patients with uncontrolled partial seizures.

PATIENTS AND METHODS

An open multicentric study in which GBP was used in 559 patients of over 12 years of age with uncontrolled partial crises in whom the efficacy, tolerability and quality of life (QOLIE-10) over a period of 6 months were analysed.

RESULTS

The response rate (> 50% reduction in seizures) was 71% with 35.3% seizure-free patients. There were no differences related to age, aetiology, previous frequency of seizures or duration of the epilepsy. In 18.8% there were adverse effects (somnolence, dizziness, headache, blurred vision, diplopia and nausea), which were generally well-tolerated, but in 4.5% of the patients led to the drug being suspended. In the 65 patients in whom quality of life was evaluated there was improvement, both overall and in each aspect studied.

CONCLUSION

Under normal conditions of everyday clinical practice, GBP is an effective drug which is well tolerated by adults and adolescents with refractory partial seizures.

Gabapentin's effects on hot flashes and hypothermia

The author describes six cases in which gabapentin treatment reduced the frequency of hot flashes. In addition, gabapentin treatment enhanced the frequency of hypothermic episodes in a separate patient with known hypothalamic dysfunction. Gabapentin may act directly upon temperature regulatory centers.

A microdialysis study on the mechanism of action of gabapentin

To gain insight into the mechanism of action of the anti-epileptic, gabapentin, the effects of gabapentin on the in vivo extracellular gamma-aminobutyric acid (GABA) levels in the rat substantia nigra reticulata were studied using microdialysis. In order to investigate possible interference with different GABA-ergic compartments in the substantia nigra reticulata, we studied the effects of gabapentin under basal, K(+)-, nipecotic acid- and glutamate-stimulated conditions. Intraperitoneally (i.p.) administered gabapentin, at a dose of 100 mg/kg, did not significantly affect extracellular GABA levels under any condition. Thus, our data do not support the involvement of nigral GABA release in the mechanism of action of the anti-epileptic gabapentin.

Anorgasmia in a patient with bipolar disorder type 1 treated with gabapentin

Gabapentin is a relatively new anticonvulsant indicated for adjunctive therapy in the treatment of partial seizures, with and without secondary generalization, in adults with epilepsy. Overall, it has a minimal side effect profile compared with other anticonvulsant agents. Postmarketing surveillance is needed to further delineate the spectrum of adverse events that may be experienced by patients treated with this medication. This is a case report of a 25-year-old man with a 10-year history of mood swings that progressively worsened and resulted in a suicide attempt 8 months before his first appointment. A diagnosis of bipolar disorder was established, and a clinical interview ruled out other mental disorders. The patient was administered gabapentin 300 mg/day, and the dose was titrated upward to 900 mg/day. A follow-up appointment revealed improved control of his bipolar symptoms. However, the patient reported that he could not have an orgasm during sexual intercourse. The medication was changed to valproic acid 250 mg three times daily. His bipolar symptoms remained under control and the anorgasmia resolved. This was maintained at the next follow-up appointment. The side effect profile and therapeutic monitoring requirements of gabapentin are favorable when compared with those of other anticonvulsant agents. However, because this agent is relatively new, especially for use in the treatment of bipolar disorder, a more thorough development of its side effect profile is needed. Observing, recording, and reporting atypical adverse events and side effects are critical to postmarketing surveillance and enhance the clinician's ability to make rational therapeutic decisions.

Efficacy of gabapentin as adjunctive therapy in a large, multicenter study. The Steps Study Group

The objective of this study was to determine the efficacy of gabapentin as adjunctive therapy in doses required to achieve the most effective seizure control. There were 2016 patients with partial seizures requiring adjunctive therapy who received gabapentin at doses up to 3600 mg/day in this open-label, multicenter, 16-week study. Of the 1055 patients evaluable for efficacy, 573 received gabapentin < or =1800 mg/day and 482 received > 1800 mg/day as the highest dose received. For the overall efficacy evaluable population, the percentage of patients achieving at least a 50% reduction in seizure frequency was 76.0%; 46.4% of the patients were seizure free. Patients whose highest gabapentin dose did not require > 1800 mg/day had, at baseline, fewer seizures and were receiving fewer concomitant antiepileptic drugs (AEDs) at baseline than those patients requiring > 1800 mg/day. This suggests that patients requiring higher doses of gabapentin were more refractory to drug treatment at the start of the study. Gabapentin was well tolerated at all doses in this study. The results of the study demonstrate that gabapentin is effective as adjunctive therapy in patients with partial seizures whose seizures are inadequately controlled by traditional AEDs.

Gabapentin is effective in treating nocturnal painful spasms in multiple sclerosis

In-patients with MS nocturnal spasms (NPS) occur frequently, primarily during the night and may influence the ability to and/or quality of sleep. We enrolled in an open label trial with GBP (up to 600 mg/day) 24 MS patients with NPS. We obtained patient reports of subjective discomfort at pre-treatment and following 2- (T1) and 8 weeks (T2), utilizing a 3-point analogue scale. Twenty of the 22 patients who completed the study reported resolution or amelioration of discomfort. Clinical improvement occurred 1 - 5 days following initial treatment. Three patients experienced adverse effects but completed the minimal follow-up period (2 weeks). Two patient dropped out of the study due to no compliance or adverse effects. A very low dose of GBP may be effective treatment for MS patients with NPS who may benefit from rapid improvement of discomfort with minimal risk of adverse effects.

Gabapentin use in neuropathic pain syndromes

The development of neuropathic pain involves a series of changes including primary and secondary hyperalgesia, peripheral and central sensitization, and wind-up phenomena. Neurotransmitters play a critical role in this process. For example, glutaminergic subtypes of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and neurokinin prime the N-methyl-D-aspartate (NMDA) receptor by triggering the release of intracellular calcium ions, thus unblocking the magnesium ion plug on the NMDA receptor and allowing Ca2+ influx into the cell. Ca2+ ions acting as secondary messengers initiate protein kinase C activation, phospholipase C and nitric oxide synthetase production, and proto-oncogene expression. The activation of the NMDA receptor thereby increases the responsiveness of the nociceptive system. Anticonvulsant drugs--including carbamazepine, phenytoin, and felbamate--have been used to treat neuropathic pain. Gabapentin is a novel anticonvulsant that may have a unique effect on voltage-dependent Ca2+ channel currents at postsynaptic dorsal horn neurons. Thus, gabapentin may interrupt an entire series of events, not just a single process, that lead to the development of neuropathic pain. Preclinical models of anti-inflammatory and neuropathic pain indicate that gabapentin effectively antagonizes the maintenance of this pain. Additionally, in preemptive surgical models, gabapentin has been shown to prevent the induction of pain. Gabapentin has been shown to be efficacious in numerous smaller clinical studies, case reports, and chart reviews in a variety of neuropathic pain syndromes. Two large multicenter studies, one in postherpetic neuralgia (PHN) and one in diabetic peripheral neuropathy (DPN), support preclinical findings. In the PHN study, patients treated with gabapentin demonstrated a significant difference (P<0.001) in their average daily pain score at endpoint compared to placebo patients. In the DPN trial, mean weekly pain was significantly (P<0.001) different for gabapentin-treated patients compared to placebo-treated patients at endpoint. Consistent with the known side-effect profile of gabapentin, the most common adverse events noted in both studies were dizziness and somnolence. Gabapentin should be considered an important addition to the management of neuropathic pain syndromes.

A comparison of four new antiepileptic medications

In order to select a new medication for a patient with epilepsy, it would be helpful to have an idea of which drug might have the greatest overall chance for success. Since epilepsy is a chronic disorder, the long-term effectiveness and tolerability of the medications are very important. Here, we compared gabapentin, lamotrigine, topiramate and vigabatrin using Kaplan-Meier survival analysis to see how long patients chose to stay on each drug and if they stopped, why they stopped. The results seem to suggest the type of responses to be expected in a hospital seizure clinic.

Inhibition of neuronal Ca(2+) influx by gabapentin and subsequent reduction of neurotransmitter release from rat neocortical slices

Cytosolic calcium ion concentrations ([Ca(2+)](i)) were measured in rat neocortical synaptosomes using fura-2, and depolarization of synaptosomal membranes was induced by K(+) (30 mM). The release of the endogenous excitatory amino acids glutamate and aspartate was evoked by K(+) (50 mM) and determined by HPLC. The release of [(3)H]-noradrenaline from rat neocortical synaptosomes or slices was evoked by K(+) (15 and 25 mM) and measured by liquid scintillation counting. Gabapentin produced a concentration-dependent inhibition of the K(+)-induced [Ca(2+)](i) increase in synaptosomes (IC(50)=14 microM; maximal inhibition by 36%). The inhibitory effect of gabapentin was abolished in the presence of the P/Q-type Ca(2+) channel blocker omega-agatoxin IVA, but not by the N-type Ca(2+) channel antagonist omega-conotoxin GVIA. Gabapentin (100 microM) decreased the K(+)-evoked release of endogenous aspartate and glutamate in neocortical slices by 16 and 18%, respectively. Gabapentin reduced the K(+)-evoked [(3)H]-noradrenaline release in neocortical slices (IC(50)=48 microM; maximal inhibition of 46%) but not from synaptosomes. In the presence of the AMPA receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2, 3-dioxo-6-nitro-1,2,3,4-tetrahydro[f]quinoxaline-7-sulphonamide (NBQX), gabapentin did not reduce [(3)H]-noradrenaline release. Gabapentin did, however, cause inhibition in the presence of the NMDA receptor antagonist DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849). Gabapentin is concluded to reduce the depolarization-induced [Ca(2+)](i) increase in excitatory amino acid nerve terminals by inhibiting P/Q-type Ca(2+) channels; this decreased Ca(2+) influx subsequently attenuates K(+)-evoked excitatory amino acid release. The latter effect leads to a reduced activation of AMPA receptors which contribute to K(+)-evoked noradrenaline release from noradrenergic varicosities, resulting in an indirect inhibition of noradrenaline release.

Anticonvulsant efficacy of gabapentin and levetiracetam in phenytoin-resistant kindled rats

We evaluated the anticonvulsant efficacy of the new antiepileptic drugs (AEDs) gabapentin and levetiracetam in amygdala kindled rats that had been preselected with respect to their response to phenytoin. Anticonvulsant response was tested by determining the afterdischarge threshold (ADT), i.e. a sensitive measure for drug effects on focal seizure activity. By repeated testing with the phenytoin prodrug fosphenytoin, three groups of kindled rats were separated: rats in which consistent anticonvulsant effects were obtained (phenytoin responders), rats which showed no anticonvulsant response (phenytoin nonresponders), and rats with variable responses (variable phenytoin responders). The latter, largest group was used to evaluate at which doses gabapentin and levetiracetam exerted significant anticonvulsant effects on ADT 1 h after i.p. drug administration. Effective doses were then used for drug testing in phenytoin responders and nonresponders. Both gabapentin and levetiracetam proved to be effective anticonvulsant drugs in the kindling model by significantly increasing the ADT. In addition, both drugs markedly decreased seizure severity recorded at ADT currents, indicating that these drugs affect seizure threshold in the epileptic focus and seizure spread from the focus in the kindling model. When the threshold for secondary generalized seizures (GST) was determined in addition to ADT, gabapentin and levetiracetam strikingly increased this threshold compared to predrug control. In phenytoin nonresponders, gabapentin and levetiracetam significantly increased ADT and GST, which is in line with their proven efficacy in patients with refractory partial epilepsy in whom older AEDs have failed. In phenytoin responders, gabapentin tended to be more efficacious in increasing ADT and GST than in nonresponders, substantiating that the difference between these groups of kindled rats extends to other AEDs. In contrast to gabapentin, levetiracetam was more efficacious in increasing ADT in nonresponders than in responders. The data of this study substantiate that phenytoin nonresponders are a unique model for the search of new AEDs with improved efficacy in refractory partial epilepsy.