Progabide for refractory partial epilepsy: a controlled add-on trial

Proof of principle trials: exploratory open studies

The aims of the observational, early phase IIa studies are usually several in number and often competing, and therefore the priorities should be clearly defined. The preclinical data, particularly the mode(s) of action of the drug and the results on experimental models, can help to build hypotheses on the agent's potential efficacy for individual seizure types and syndromes. For ethical reasons, the population selected for the initial clinical trials of antiepileptic drugs (AEDs) should be patients with continuing seizures (e.g. no less than two to four partial seizures/month) in spite of several therapeutic attempts using other appropriate AEDs, taken in monotherapy and in various combinations. Although the inherent bias in open, observational studies has limited their use in comparing treatments in epilepsy, they have several advantages over controlled trials, including lower cost, greater timeliness and inclusion of a broader range of patients. Observational, open studies with well-chosen patients and an adequate design tailored to answer precise predefined questions (including go/no go criteria) are most valuable for experienced clinicians in determining the clinical benefit of the drug, and for the pharmaceutical company in assessing the commercial merit of proceeding with further clinical testing.

Clobazam, oxcarbazepine, tiagabine, topiramate, and other new antiepileptic drugs

Clinical investigators recently have studied at least 21 new antiepileptic drugs (AEDs) in people with epilepsy. This review briefly examines 15 of these new AEDs: clobazam (CLB), dezinamide, flunarizine (FNR), loreclezole, milacemide (MLM), MK-801, nafimidone, ORG-6370, oxcarbazepine (OCBZ), progabide (PGB), ralitoline, stiripentol, tiagabine (TGB), topiramate (TPM), and zonisamide (ZNS). CLB, PGB, and TGB represent agents that act on the GABA system, and MLM acts on the glycine system. MK-801 and ZNS (in part) are excitatory amino acid antagonists, and FNR is a calcium-channel antagonist. OCBZ is a keto analogue of carbamazepine, which is not metabolized to the epoxide and may have fewer side effects. The remaining agents are novel compounds with a variety of suspected mechanisms. TPM appears especially effective for intractable partial seizures but has a high incidence of cognitive side effects. None of these new AEDs is useful for all patients with inadequate seizure control or ongoing toxicity. The role of each will require further clinical study and experience.

Tiagabine: a novel drug with a GABAergic mechanism of action

The various possibilities for manipulating the gamma-aminobutyric acid (GABA) system to augment GABAergic inhibition have been surveyed with reference to the relevant antiepileptic compounds that have been successfully or unsuccessfully investigated in relation to these different mechanisms of action. The first clinical studies of tiagabine (TGB), a novel GABA-uptake inhibitor are now available. These studies utilized a novel design, the enrichment (Amery) design, which is put into perspective compared to classical clinical trial designs. Possible advantages and disadvantages of TGB, as seen at this stage in development, have been identified.

Innovative designs of controlled clinical trials in epilepsy

Uncontrolled noncomparative clinical observations of investigational antiepileptic drugs (AEDs) often lead to overoptimistic efficacy results and are therefore of very limited value for clinical AED development. The classic add-on trial with placebo as control treatment, in contrast, has provided unequivocal evidence of the efficacy of classic and new AEDs and has also identified less useful AEDs. Drug interactions, carryover effects, difficulty in analyzing individual drug action, and the recognition that monotherapy is by far the more common way of prescribing AEDs have led to the development of classic active control monotherapy trials. A major problem of these trials is a no-difference outcome, which allows no useful interpretation. Recently, two alternative monotherapy designs have been developed to avoid the deadlock of a no-difference outcome. In these designs the active control drug is administered in an attenuated form (low dosage or low concentration) or a placebo control is used when standard treatment is discontinued during presurgical evaluation. Both designs have produced unequivocal evidence of the efficacy of the investigational AED during monotherapy. Ethical concerns are minimized by the introduction of preset escape criteria for patient protection. These designs are valuable new supplements for the clinical development of investigational AEDs for monotherapy in epilepsy. In our opinion, alternative monotherapy designs should be preceded by more than one pivotal add-on, placebo-controlled trial.

Comparative study of the anticonvulsant effect of gamma-aminobutyric acid agonists in the feline kindling model of epilepsy

We made a comparative study of the anticonvulsant effect of GABA agonists on feline amygdala or hippocampal kindled seizures. Progabide (PGB) [gamma-aminobutyric acid (GABA) receptor agonist 25-100 mg/kg intraperitoneally, i.p.] significantly reduced both the kindled seizure stage and after discharge (AD) duration in a dose-dependent manner. SKF89976A (GABA uptake inhibitor 0.5-2.0 mg/kg i.p.) also significantly reduced the kindled seizure stage. Toxic doses of SKF89976A caused generalized paroxysmal EEG discharges and myoclonus, but AD generation in the kindled focus was suppressed completely. Furthermore, gamma-vinyl GABA (GABA catabolic enzyme inhibitor, GVG 50-200 mg/kg i.p.) significantly reduced the seizure stage, while causing prolongation of the AD duration. In contrast, baclofen (selective GABAB receptor agonist, 1 or 5 mg/kg) did not show anticonvulsant effects on any parameters of kindled seizures. Therefore, these GABA agonists, which potentiate the inhibitory function of the GABAA systems, have potent anticonvulsant effects on partial onset and secondarily generalized limbic seizures.

Interaction of the anticonvulsant ameltolide with standard anticonvulsants

The newly characterized anticonvulsant ameltolide was studied in mice in combination with the standard antiepileptic drugs (AEDs), phenytoin (PHT), carbamazepine (CBZ), and valproate (VPA). In combination with either PHT or CBZ, ameltolide produced dose-additive effects in the maximal electroshock (MES) test and in the horizontal screen (HS) test for neurologic impairment. The large separation between the doses for the anticonvulsant effects and the neurologically impairing effects (protective index, PI) were maintained as well in the combinations as in the individual compounds. VPA was impotent in the MES test and did not have a clear separation between the doses that produce the anticonvulsant effects and those that are neurologically impairing (low PI). When VPA and ameltolide were combined, the effects were less than additive by isobolographic analysis on both the MES and HS tests. At high oral doses (20 and 40 mg/kg, p.o.), ameltolide produced impairment on the HS test and decreased body temperature. The effects on the HS test were enhanced twofold, whereas the effects on body temperature were not markedly enhanced, by coadministration of the MES ED95 of PHT and CBZ. VPA (MES ED95) appeared to antagonize the temperature-lowering effects of ameltolide. These interaction studies suggest that ameltolide would be safe, with no unexpected effects, when used in epileptic patients concurrently receiving these standard AEDs. These studies also suggest that the effects of ameltolide would be lessened by simultaneous administration of VPA.

Double-blind, placebo-controlled, cross-over trial of progabide as add-on therapy in epileptic patients

In a double-blind, cross-over trial, progabide (PGB) and placebo were compared as add-on therapy in 59 patients with moderate to severe epilepsy. Eight patients did not complete the study, 4 because of adverse drug reactions (elevation of liver transaminases, 2; gastritis, 1; and acute psychosis, 1) and 4 because of administrative reasons. Among the remaining 51 patients, seizure frequency was reduced greater than 50% in 18 patients with PGB treatment and in 8 patients with placebo (p less than 0.05). The number of days with seizures was significantly (p = 0.034) reduced during PGB treatment. Both patients' and physicians' preferences at the end of the trial were in favor (p less than 0.01) of PGB. Mild clinical side effects were present in 54.7% of the patients treated with PGB and in 37.7% with placebo. Increase in liver transaminases was observed in 2 patients during the double-blind study and in 1 during the follow-up period. Our data show that PGB, as previously reported, is useful in 30-40% of patients who are not responding completely to other antiepileptic drugs (AEDs). The compound is well tolerated, but liver function must be monitored.

An analysis of anticonvulsant actions of GABA agonists (progabide and baclofen) in the kindling model of epilepsy

The anticonvulsant action of progabide, an agonist of gamma-aminobutyric acid (GABA)A and GABAB receptors, was investigated in the kindling model of epilepsy in rats. Progabide shortened afterdischarge durations and attenuated the severity of the accompanying convulsive responses in previously kindled rats from the amygdala (AM), frontal cortex (FC), ventral and dorsal hippocampus (HIPP), in a dose-dependent manner. Although progabide was less effective in the dorsal HIPP kindled seizures, the efficacy was potent in AM, FC and ventral HIPP kindled seizures. On the other hand, the anticonvulsant action of baclofen, a selective agonist of GABAB receptors, was relatively weak in terms of the measurement of the afterdischarge duration of AM and HIPP kindled seizures even at toxic doses, compared with progabide. In addition, the anticonvulsant effects of progabide were partially reversed by treatment with the antagonist of benzodiazepine receptors, Ro 15-1788, whereas Ro 15-1788 administration alone did not alter AM kindled seizures. We concluded that the action of progabide may be mediated via the GABA/benzodiazepine receptor complex. These results support the hypothesis that a failure of GABAA-mediated inhibition is one of the bases of induction and generalization of seizures.

Suppression of 4-aminopyridine-induced epileptogenesis by the GABAA agonist muscimol

Bath application of muscimol to hippocampal slices taken from immature rats suppressed both spontaneous and electrically evoked epileptiform activity produced by 4-aminopyridine (4-AP). Epileptiform events consisted of ictal-like discharges that were up to 30 sec in duration and interictal burst-like discharges. The latter were often followed by brief synchronized afterdischarges that were less than 1 sec in duration. During the transition period to suppression, individual pyramidal cells did not hyperpolarize in response to muscimol, but instead underwent a gradual depolarization that averaged 8.5 mV. At the same time, the input conductance of these cells increased 2-3-fold. Concurrently the ictal-like discharges transiently increased in duration and then abruptly ceased. In most instances the ictal-like events were replaced by large slow depolarizing events. Orthodromic stimulation recruited these slow depolarizations in a graded manner. However, at high stimulus intensities and at unusually long latencies ictal-like discharges were evoked. This suggests that muscimol raises the threshold for the generation of ictal-like discharges but leaves the underlying physiological processes intact. The combined use of 4-AP and muscimol in immature hippocampal slices may prove to be useful for the study of the various physiologic processes that contribute to the genesis of seizures in immature hippocampus. In addition, results of these studies are relevant in light of the proposed use of GABA agonists in anticonvulsant therapy.

Disinhibitory actions of the GABAA agonist muscimol in immature hippocampus

Bath application of muscimol (10-20 microM) to hippocampal slices obtained from rats on postnatal days 10-15 produced epileptiform activity in the form of multiple population spikes in 20% of slices tested, concurrent with marked disinhibition. The disinhibition occurred in nearly 100% of cases tested at muscimol concentrations that produced epileptiform activity. Paired pulse analysis of GABAergic recurrent inhibition revealed a muscimol-induced disinhibitory effect involving a decrease in maximum possible inhibition. Spontaneous and antidromically elicited inhibitory postsynaptic potentials (IPSPs) recorded intracellularly were suppressed by muscimol. Current-voltage analysis of the recurrent IPSPs suggests that muscimol acted at a number of sites to produce disinhibition. The input conductance of the postsynaptic pyramidal cell increased due to muscimol, creating a current shunt which likely decreased the efficacy of synaptic currents. Muscimol also caused a decrease in the conductance due to the IPSP as well as a shift in the depolarizing direction of the equilibrium potential of the IPSP. The data indicate that muscimol, a GABAA agonist, can produce disinhibition resulting from the multiple consequences of its action. We conclude that the physiologic consequences of GABAA agonist treatment are complex. On the other hand, neurons are likely to be inhibited by a tonic increase in membrane conductance. However, since recurrent inhibition is simultaneously compromised, excitatory vollies of sufficient intensity may overcome the tonic inhibition and produce a hyperexcitable state. In some cases, this disinhibition may induce epileptiform activity. These observations are relevant in light of the proposed use of GABA agonists clinically to control seizures.

Progabide-induced changes in carbamazepine metabolism

Carbamazepine (CBZ) and carbamazepine 10,11 epoxide (CBZ-E) concentrations were measured during a safety and efficacy trial of progabide. The average CBZ and CBZ-E serum concentrations were calculated from serial measurements during placebo and active treatment periods. Significant decreases in CBZ and significant increases in CBZ-E were observed after the first dose of progabide, and these changes persisted during 3 months of active treatment. Ninety-five percent of the patients had increases in the epoxide/CBZ ratio at the end of 3 months of treatment. These changes are consistent with displacement of CBZ from protein binding sites and inhibition of CBZ-E metabolism induced by progabide and are analagous to the interaction between valproate and CBZ.

Anticonvulsant action of a non-competitive antagonist of NMDA receptors (MK-801) in the kindling model of epilepsy

Anticonvulsant action of MK-801, a novel non-competitive antagonist of N-methyl-D-aspartate (NMDA) receptors, was investigated in the kindling model of epilepsy in rats. The results obtained were as follows. (1) Both the seizure stage and afterdischarge duration of previously kindled seizures from the amygdala were significantly suppressed following systemic injection of MK-801 (0.25-4 mg/kg) in a dose-dependent manner. The maximum effects were observed between 2 and 4 after the injection. (2) The MK-801 also showed significant anticonvulsant effects on kindled seizures from the frontal cortex and the ventral and dorsal hippocampus. The efficacy, however, significantly differed between these kindled sites. (3) Daily treatment of MK-801 (0.25 and 1 mg/kg) prior to each electrical stimulation of the amygdala significantly retarded kindling seizure development and increased the total amount of afterdischarge (accumulated AD) required to reach the first stage 5 seizure. During drug sessions of 1 mg/kg MK-801 for 19 days, all rats showed only partial seizures and the growth of afterdischarge was strongly prevented. (4) Pretreatment with reserpine did not antagonize the anticonvulsant effects of MK-801 on previously kindled seizures from the amygdala, suggesting that the effects may not be mediated by catecholaminergic systems. These results indicate that MK-801 has potent anticonvulsant actions on kindled seizures from both limbic and cortical foci, the NMDA system may play a critical role in the seizure-triggering mechanism of kindling. The possible application of NMDA antagonists in clinical epilepsy is suggested.

Experimental studies and controlled clinical testing of valproate and vigabatrin

Gamma-aminobutyric acid (GABA) is the most important inhibitory transmitter, quantitatively, in the CNS. Evidence exists that decreased GABAergic neurotransmission may play a role in some forms of epilepsy. Consequently, manipulating the GABA system may be a therapeutic possibility in the treatment of this disease. Inhibition of the major GABA degrading enzyme, GABA-transaminase (GABA-T), seems to be the most promising approach. Currently, 2 antiepileptic drugs, valproate (VPA) and vigabatrin, gamma-vinyl GABA (GVG), are available, which are supposed to inhibit the degradation of GABA. Both drugs cause an increase in the total concentration of GABA in the brain, but to a different extent. VPA produces a moderate elevation, which seems to be the result of a marked increase in the transmitter-related GABA pool, while the pronounced elevation in GABA concentration observed during treatment with GVG seems to be caused mainly by an increase in the non-transmitter-related (glial) GABA pool. In order to investigate this apparently differential influence of VPA and GVG on the GABA system, a number of studies were undertaken in selectively cultured astrocytes and neurons from mice. For both drugs neuronal GABA-T proved far more sensitive with regard to inhibition than glial GABA-T. In order to obtain a more direct measure of a potential GABAergic mechanism of action of VPA and GVG, synaptic release of endogenous GABA was determined after culturing neurons in the presence of clinically relevant concentrations of the drugs. GVG caused a significant increase in GABA release, even at concentrations as low as 25 microM. For VPA only the highest of the investigated concentrations (300 microM) augmented GABA release. It is concluded that the antiepileptic effect of GVG seems to be caused by a direct GABAergic mechanism of action. For VPA an influence on the GABA system may play a role in the antiepileptic effect of the drug. However, the lack of definite data on human brain levels of VPA after chronic treatment, combined with evidence that VPA exhibits a number of other effects that may be relevant for its antiepileptic properties, makes the interpretation of a GABAergic mechanism of action difficult. Controlled clinical trials have been increasingly applied within all areas of medicine. In 1982 a survey of the literature identified 29 studies of antiepileptic drugs, where the design involved randomization, the double-blind principle and a statistical analysis of the results.(ABSTRACT TRUNCATED AT 400 WORDS)

Intractable epilepsy: etiology, risk factors and treatment

The data emerging from our study are the following: the presence of an identifiable cause is important: complications like tuberous sclerosis or signs of marked cerebral damage represent an adverse risk factor for IE. The presence of epilepsy among relatives, evidence of pre- or perinatal cerebral damage, mental retardation, and early onset, long periods of uncontrolled seizures before starting an adequate therapy and frequency of seizures appear to be indicative of an adverse prognosis, since differences between the two groups of responsive or unresponsive patients are statistically significant. On the contrary, the occurrence of febrile convulsions in the past history does not seem to have an adverse prognosis. Temporal lobe epilepsy and IS bear the worst prognosis. ME, CPS, GTCS, SPS, LGS and PM have a progressively better outcome in responsiveness to AEDs. Concerning therapy in patients with IE, studies indicate the results of high dose monotherapy appear to be equal or better than with polypharmacy. Because of the gravity of the situation, trials with unconventional drugs have been performed, but it is too early to draw definite conclusions about the long-term usefulness of most of them. In conclusion, our data indicate that the appearance of an IE can be predicted utilizing the above mentioned criteria, considered either alone or in combination. The issue of IE remains undoubtedly an important one among the group of convulsive disorders. Further studies considering a greater number of patients and new therpeutic strategies are to be recommended.

Pharmacokinetic interactions of progabide with other antiepileptic drugs

The influence of progabide, a new antiepileptic drug, on the pharmacokinetic profiles of phenobarbital, phenytoin, carbamazepine, and valproic acid was evaluated in four separate studies, each including six young healthy volunteers. The pharmacokinetic parameters of the associated antiepileptic drugs were measured before and after repeated administration of progabide (600 mg t.i.d.) for 15 days. A significant reduction of the total body clearance of both phenytoin and phenobarbital and a higher Cmax value of carbamazepine epoxide and phenobarbital were observed. No modifications were noticed for the kinetic profiles of carbamazepine and valproic acid. These modifications may be of clinical relevance and suggest that, as a general rule, when progabide is added to an established treatment with phenobarbital or phenytoin or carbamazepine, an adjustment of previous posology may be necessary.