Progabide as an add-on drug for epilepsy refractory to high dose antiepileptic drug therapy

New GABA-Targeting Therapies for the Treatment of Seizures and Epilepsy: II. Treatments in Clinical Development

The inhibitory neurotransmitter γ-aminobutyric acid (GABA) plays an important role in the modulation of neuronal excitability, and a disruption of GABAergic transmission contributes to the pathogenesis of some seizure disorders. Although many currently available antiseizure medications do act at least in part by potentiating GABAergic transmission, there is an opportunity for further research aimed at developing more innovative GABA-targeting therapies. The present article summarises available evidence on a number of such treatments in clinical development. These can be broadly divided into three groups. The first group consists of positive allosteric modulators of GABAA receptors and includes Staccato® alprazolam (an already marketed benzodiazepine being repurposed in epilepsy as a potential rescue inhalation treatment for prolonged and repetitive seizures), the α2/3/5 subtype-selective agents darigabat and ENX-101, and the orally active neurosteroids ETX155 and LPCN 2101. A second group comprises two drugs already marketed for non-neurological indications, which could be repurposed as treatments for seizure disorders. These include bumetanide, a diuretic agent that has undergone clinical trials in phenobarbital-resistant neonatal seizures and for which the rationale for further development in this indication is under debate, and ivermectin, an antiparasitic drug currently investigated in a randomised double-blind trial in focal epilepsy. The last group comprises a series of highly innovative therapies, namely GABAergic interneurons (NRTX-001) delivered via stereotactic cerebral implantation as a treatment for mesial temporal lobe epilepsy, an antisense oligonucleotide (STK-001) aimed at upregulating NaV1.1 currents and restoring the function of GABAergic interneurons, currently tested in a trial in patients with Dravet syndrome, and an adenoviral vector-based gene therapy (ETX-101) scheduled for investigation in Dravet syndrome. Another agent, a subcutaneously administered neuroactive peptide (NRP2945) that reportedly upregulates the expression of GABAA receptor α and β subunits is being investigated, with Lennox-Gastaut syndrome and other epilepsies as proposed indications. The diversity of the current pipeline underscores a strong interest in the GABA system as a target for new treatment development in epilepsy. To date, limited clinical data are available for these investigational treatments and further studies are required to assess their potential value in addressing unmet needs in epilepsy management.

Reappraisal of polytherapy in epilepsy: a critical review of drug load and adverse effects

PURPOSE

We reviewed the literature to determine whether an analysis of published data could clarify the relationship between antiepileptic drug (AED) polytherapy and adverse affects (AE). We highlight the problems encountered.

METHODS

We made a Medline-search for articles published between 1974 and 1994 reporting the number of AE and doses or serum levels of every AED, per patient or treatment group, and used the PDD/DDD ratio to calculate AED load per patient from doses or the OSL/AToxL ratio to do so from serum levels of individual drugs. The PDD/DDD is the sum of ratios of the actual prescribed daily doses divided by the published average therapeutic dose of each drug. The OSL/AToxL is the sum of each observed serum level divided by its average toxic level.

RESULTS

We retrieved 118 trial reports. Most had to be excluded because of incomplete reporting of concomitant medication or AE. The data of the 15 articles selected for further analysis indicate a relationship between drug load and number of AE. We noted no relationship between the number of AEDs administered and AE. In add-on studies, the difference in neurotoxicity between the active and placebo arm may be obscured if the relative increase in drug load is small, as exemplified by the study of McGuire et al..

CONCLUSIONS

Articles reporting add-on trials of new AEDs generally do not provide detailed information about the basic medication to which the new AED is added, which makes calculation of total drug load impossible. Furthermore, often only frequency of AE is reported, not severity or development of tolerance, making it difficult to judge the impact of AE. However, despite the paucity of available information, we present some evidence that toxicity in AED polytherapy may be related to total drug load, rather than to the number of drugs administered. Therefore, the present trend to reject polytherapy for fear of increased toxicity is not warranted, which removes one of the objections to initiating specific research to prove or disprove the value of AED combinations as long as the drug load is appropriate.

Submassive hepatic necrosis associated with the use of progabide: a GABA receptor agonist

Progabide, a recently introduced gamma-aminobutyric acid mimetic, is currently undergoing clinical evaluation for a variety of convulsive disorders. We describe a patient in whom severe hepatic failure developed after four weeks of Progabide therapy. The patient's course was marked by encephalopathy, jaundice, hypoglycemia, markedly elevated serum aminotransferase levels, and prolongation of the prothrombin time. Liver biopsy showed extensive hepatocellular necrosis. The patient recovered slowly after discontinuation of the drug. The finding of eosinophilia and increased serum IgE suggests an immunologically mediated mechanism for the Progabide-induced hepatic injury. Alternatively, the lipophilic moiety of Progabide may interact with hepatocyte cell membrane lipids leading to toxic injury. We conclude that Progabide may occasionally cause severe hepatic injury.

Double-blind withdrawal of phenytoin and carbamazepine in patients treated with progabide for partial seizures

Of 30 patients who completed a study of progabide (PGB) as an add-on to both phenytoin (PHT) and carbamazepine (CBZ), 11 volunteered for a double-blind withdrawal protocol in which the PHT and CBZ were to be withdrawn. All patients were receiving 24-32 mg/kg/day PGB in combination with PHT and CBZ. Each patient was randomly assigned to withdrawal of either CBZ or PHT in the first block, and then withdrawal from the other in the second block in an attempt to achieve PGB monotherapy. Seizure occurrence was monitored by sequential analysis, and if a significant increase over baseline seizure frequency occurred, the dose of PGB was increased to a maximum of 45 mg/kg/day. If seizure frequency remained above baseline, the drug being withdrawn was added back and an attempt made to withdraw the other. The study was terminated if these adjustments were not successful in decreasing seizure frequency to baseline. At the conclusion of the study, three patients were being treated with PGB and PHT, two with CBZ and PGB, and six with all three. This study demonstrated the applicability of sequential analysis to antiepileptic drug trials.

Comparison of progabide with other antiepileptic and GABAergic drugs

The effect of the experimental antiepileptic gamma-aminobutyric acid (GABA) agonist drug progabide, [alpha-(chloro-4-phenyl)fluor-5-hydroxy-2-benzilideneamino]-4-buty ramide, on the trigeminal complex of cats was compared with the effect of established antiepileptic drugs and with the effect of various GABA agonists and antagonists. Intravenous administration of 10-40 mg/kg progabide depressed excitatory transmission and descending periventricular inhibition, similar to carbamazepine and phenytoin. However, progabide depressed, rather than facilitated, segmental inhibition. The serum levels of progabide were comparable with those in patients receiving long-term treatment with progabide. The GABA antagonist bicuculline had the opposite effect of progabide on our experimental model, but the other GABA agonists THIP (4,5,6,7-tetrahydroisoxazolo-5,4-C-pyridine-3-ol) and muscimol did not have the same effects as progabide. THIP had no effect on excitatory transmission, periventricular inhibition, or segmental inhibition, whereas muscimol facilitated periventricular inhibition and sometimes segmental inhibition and had no effect on excitatory transmission. Our experiments thus indicate that progabide, but not THIP or muscimol, should have antiepileptic properties, in agreement with the clinical experiences that have been reported. The reason for the differential effect of these three GABA agonists remains to be elucidated.

Evaluation of different GABA receptor agonists in the kindled amygdala seizure model in rats

The effects of three specific GABA receptor agonists, muscimol, progabide, and gaboxadol, on kindled seizures were evaluated in amygdala-kindled rats. The only compound that exerted significant anticonvulsant effects at nonsedative doses was progabide. Thus, after i.p. administration of 100 mg/kg progabide, a significant increase in seizure latency and significant decreases in duration of motor seizures and amygdala afterdischarges were determined. A decrease in severity of motor seizures was found only after 200 mg/kg progabide which, however, gave rise to marked sedation and muscle relaxation. Muscimol and gaboxadol were almost inactive in attenuating kindled seizures even at doses that produced pronounced side effects. Assuming that the amygdala-kindled rat is a useful model of complex partial seizures with secondary generalization in the human, the data suggest that GABA receptor agonists are not effective against this type of epilepsy (muscimol, gaboxadol) or effective only at large doses (progabide).