Antiepileptic medications in development

Gabapentin for the treatment of pain related to radiation-induced mucositis in patients with head and neck tumors treated with intensity-modulated radiation therapy

BACKGROUND

This retrospective study evaluates the efficacy of gabapentin for the treatment of pain syndrome related to radiation-induced mucositis in patients with head and neck tumors.

METHODS

Thirty cases of head and neck malignancies treated with radiotherapy were analyzed.

RESULTS

By using a median dose of 2700 mg/day of gabapentin, only 10% of patients required additional narcotic pain medications for adequate pain relief during the third and fourth week of treatment, despite grade 2 or higher mucositis present in 56% and 73% of the patients, respectively. Likewise, during the last weeks of intensity-modulated radiation therapy (IMRT), only 35% of patients required additional narcotics for pain control, despite the presence of grade 2 or higher mucositis in 80% of cases.

CONCLUSIONS

Gabapentin appears promising in reducing the need for narcotic pain medication for patients with head and neck malignancies treated with IMRT and should be further evaluated prospectively in controlled clinical trials.

Evaluation of lamotrigine toxicity reported to poison centers

BACKGROUND

Lamotrigine is an antiepileptic drug for the treatment of partial and generalized seizures as well as bipolar disorder. Limited published information exists describing the clinical effects of lamotrigine overdose.

OBJECTIVE

To examine the toxicity of exposures to lamotrigine utilizing national poison center data.

METHODS

Data on single-substance exposures to lamotrigine reported to the American Association of Poison Control Centers Toxic Exposure Surveillance System in 2000 and 2001 were retrospectively analyzed.

RESULTS

There were 493 cases that met the inclusion criteria. The majority of exposures occurred within the age groups 20-59 years old (n = 198, 40.2%), followed by </=4 years old (n = 173, 35.1%). Overall, the majority of patients (52.1%) exposed to lamotrigine in overdose experienced no toxic clinical effects. The most common clinical effects reported in overdose were drowsiness/lethargy (20.9%), vomiting (11%), nausea (5.1%), ataxia (4.9%), dizziness/vertigo (4.5%), and tachycardia (4.3%). Major clinical effects included coma (n = 6), seizures (n = 8), and respiratory depression (n = 3). Medical outcome was reported as minor in 150 (30.4%), moderate in 73 (14.8%), and major in 13 (2.6%) cases. There were no deaths.

CONCLUSIONS

These data demonstrate that the majority of patients exposed to lamotrigine in overdose experienced minor or no clinical effects. Although rare, serious effects can also occur.

Topiramate. A review of its pharmacodynamic and pharmacokinetic properties and clinical efficacy in the management of epilepsy

Topiramate is a sulphamate-substituted monosaccharide derived from D-fructose and is structurally unrelated to other antiepileptic drugs. It acts by multiple mechanisms that suggest it may be effective in several types of epilepsy. In double-blind placebo-controlled trials, add-on therapy with topiramate 400 to 1000 mg/day reduces the seizure rate by > or = 50% in 35 to 52% of adult patients with resistant partial epilepsy (with or without secondarily generalised seizures) compared with 0 to 19% of placebo recipients; a 200 mg/day dosage was less effective. Topiramate has also been shown to be superior in efficacy to placebo in well controlled trials in patients with generalised tonic-clonic seizures, Lennox-Gastaut syndrome and in paediatric patients with partial epilepsy. Efficacy has been maintained for 7 years and some patients may also be satisfactorily treated with topiramate monotherapy. Further study is needed to follow up on the promising results of topiramate use in other paediatric epilepsies. Adverse CNS events are the most common untoward effects during topiramate therapy and are most likely to lead to withdrawal of the drug. However, most adverse events are mild to moderate in severity and lessen with continued drug therapy. In clinical trials, most adverse events which were dose limiting or led to discontinuation of treatment occurred during the titration phase. The overall incidence of adverse events may be reduced by slower upward dosage titration. In summary, topiramate appears to be a suitable agent for add-on therapy in adult patients with partial epilepsy. Preliminary reports support the use of add-on topiramate in adults with generalised epilepsy, in childhood epilepsies and in patients with Lennox-Gastaut syndrome, as well as the use of topiramate monotherapy in patients with partial epilepsy. Thus, topiramate can be considered an important new drug for the management of patients with refractory epilepsy.

Gabapentin. A review of its pharmacological properties and clinical potential in epilepsy

Gabapentin is an antiepileptic drug with an unknown mechanism of action apparently dissimilar to that of other antiepileptic agents, and possessing some desirable pharmacokinetic traits. The drug is not protein bound, is not metabolised and does not induce liver enzymes, diminishing the likelihood of drug interactions with other antiepileptic agents and drugs such as oral contraceptives. Although gabapentin is a structural analogue of the neurotransmitter gamma-aminobutyric acid (GABA), which does not cross the blood-brain barrier, gabapentin penetrates into the CNS and its activity is seemingly distinct from GABA-related effects. Present clinical evaluation is largely restricted to proof of efficacy trials of gabapentin as add-on therapy in patients with partial epilepsy resistant to conventional treatment. Gabapentin (usually 600 to 1800 mg/day) provides notable benefit, reducing seizure frequency by > or = 50% in 18 to 28% of patients with refractory partial seizures, as shown in 3 double-blind, placebo-controlled trials. Overall, seizure frequency decreased by 18 to 32% during 3-month treatment periods. Patients with complex partial seizures, and partial seizures secondarily generalised, are particularly likely to respond to gabapentin. Current experience with the drug in other seizure types, and as monotherapy, is limited. Mild adverse events, commonly somnolence, fatigue, ataxia and dizziness, have been reported in about 75% of gabapentin recipients. While the drug has been well tolerated when administered to a few patients for periods of up to 5 years, its long term tolerability profile has yet to be fully expounded. Thus, with its favourable pharmacokinetic profile, and efficacy in some refractory patients, gabapentin is poised to fill a niche as an adjunct to the treatment of partial epilepsy. Promising results obtained thus far warrant further work to clarify its long term tolerability, its possible efficacy in other seizure types, its position relative to other agents and its use as monotherapy. In the meantime, gabapentin is likely to provide a much-needed option in a therapeutic area requiring complex management.

Zonisamide. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in epilepsy

Zonisamide is a 1,2 benzisoxazole derivative and the first agent of this chemical class to be developed as an antiepileptic drug. It has shown activity in various animal models of epilepsy, and although a detailed mode of action awaits clarification it appears to block the propagation/spread of seizure discharges and to suppress the epileptogenic focus. Clinical experience with zonisamide in Japan has documented its efficacy in the treatment of partial seizures (partial-onset generalised tonic-clonic, simple partial and/or complex partial seizures), and to a more variable extent, generalised tonic-clonic, generalised tonic (mainly seen in symptomatic generalised epilepsies including Lennox-Gastaut Syndrome) and compound/combination seizures (including those refractory to treatment with other antiepileptic drugs). Other generalised seizure types have also responded to therapy with zonisamide, although only small patient numbers were studied. Zonisamide has demonstrated efficacy equivalent to that of carbamazepine in patients with (mainly) partial seizures, and to that of valproic acid in a small study of children (n = 32) with generalised seizures. Animal studies suggest that zonisamide possesses a more favourable therapeutic index than most other antiepileptic drugs. However, clinical trials conducted to date, have not confirmed any overt tolerability advantage. Indeed, whereas the recommended therapeutic plasma zonisamide concentration is 20 mg/L, clinical investigations have associated adverse events with plasma zonisamide concentrations of > 30 mg/L, suggesting the usefulness of therapeutic drug monitoring. Moreover, although plasma concentrations of zonisamide are empirically regarded to be proportional to therapeutic doses in patients in Japan, nonlinear pharmacokinetics have been reported for this drug in patients in the US and may further complicate its use in this patient population. Additional pharmacokinetic studies will help to establish the change in pharmacokinetic profile that occurs with dosage titration in patients outside Japan. Among 700 patients treated with zonisamide in Europe/US, a high incidence of renal calculi (1.9%) has been noted, however, the causal relationship to zonisamide is disputed. Indeed, although urinary lithiasis has also been recorded for patients in Japan, the aetiology, incidence and spontaneous regression of this condition suggest that it is not a serious problem for this patient population. Until this difference is clarified, it is likely that zonisamide will find its greatest use in the treatment of patients in Japan. Like many other established antiepileptic drugs, available data suggest the propensity for zonisamide to alter the pharmacokinetic profile of other anticonvulsant agents, although severe interactions appear to be unlikely.(ABSTRACT TRUNCATED AT 400 WORDS)

Vigabatrin

OBJECTIVE

To introduce the reader to the use of a new agent, vigabatrin, in the treatment of refractory complex partial seizures. Clinical trials and pharmacokinetic data are reviewed, as well as neuropathology, adverse effects, drug interactions, and dosage guidelines.

DATA SOURCES

A MEDLINE search through March 1992 was used to identify pertinent English-language literature, including clinical trials, reviews, abstracts, and conference proceedings. Indexing terms included vigabatrin and anticonvulsants.

STUDY SELECTIONS

All clinical trials (total of 21) were reviewed, as were all pharmacokinetic studies (total of 8). Selected studies highlighting chemistry, pharmacology, neuropathology, and adverse effects were also reviewed.

DATA EXTRACTION

Performed subjectively by the author. Trials were assessed by design, sample size, types of seizures of the subjects, and clinical response.

DATA SYNTHESIS

Vigabatrin represents the first of a new class of antiepileptic drugs (AEDs)--the gamma-aminobutyric acid transaminase (GABA-T) inhibitors. Vigabatrin works by selective, irreversible inhibition of GABA-T, thus preventing the breakdown of GABA. It has been shown to produce dose-dependent increases in cerebrospinal fluid GABA concentrations, and decreases in GABA-T activity. Vigabatrin may also cause a decrease in excitation-related amino acids. It is well absorbed, is not protein bound, and is eliminated by glomerular filtration. However, even with a short half-life (5-7 h), vigabatrin may be given once or twice daily because of its mechanism of action. Few drug interactions have been reported with this agent, although decreases in phenytoin concentration may reach clinical significance. Concern over neuropathologic findings (microvacuolization of white matter) in animals caused trials of vigabatrin to be halted in 1983, but trials have now resumed as there is no evidence of toxicity in humans. Clinical efficacy of vigabatrin has been evaluated in controlled trials and appears to be most effective in complex partial seizures, producing a 50 percent or greater reduction in seizure frequency in approximately 50 percent of the adult patients studied. Efficacy in children with partial seizures also appears promising, and one uncontrolled study suggests that further study of vigabatrin in infantile spasms may be warranted.

CONCLUSIONS

Vigabatrin appears to be effective in treating refractory complex partial seizures in adults and refractory partial seizures in children. Its relatively benign adverse-effect profile and few known drug interactions may given this agent an advantage over existing anticonvulsants. However, definitive conclusions about the role of vigabatrin in epilepsy treatment should await the completion of ongoing Phase II and Phase III trials.