The novel antiepileptic drug imepitoin compares favourably to other GABA-mimetic drugs in a seizure threshold model in mice and dogs
Graphical abstract
Imepitoin potentiates GABAergic inhibition via the BZD site of the GABAA receptor but its affinity and intrinsic efficacy at this site are much lower compared to benzodiazepines.
Introduction
The dog is an important large animal model in various fields of biomedical research and fills a crucial step in the translation of basic research to new treatment regimens [1], [2], [3], [4], [5]. Recently, naturally occurring canine epilepsy has been proposed as a translational platform for human therapeutic trials on new epilepsy treatments [6], [7], [8], [9]. This idea is not new; it has been proposed by us almost 30 years ago [10], [11]. The prevalence and phenomenology of naturally occurring canine epilepsy are very similar to human epilepsy [11], [12], [13], [14], [15], [16]. Epilepsy is the most common chronic neurological disease in dogs, affecting about 0.6–1% of the dog population [11], [13], [16]. However, clinical trials on new antiepileptic drugs (AEDs) in epileptic dogs are as laborious and time-consuming as clinical trials in human patients, necessitating randomized trial designs in which the new drug is compared with either placebo or a standard comparator [17], [18].
One critical decision for all clinical trials is selection of appropriate doses, which has a large impact on the success of drug development programmes [19]. This is also true for clinical trials in dogs with epilepsy, so that a seizure model in dogs would be a valuable tool for pharmaceutical dose selection [20]. Several canine seizure models have been described and used for drug testing, including induction of seizures in healthy dogs by maximal electroshock [20], [21], [22], [23], microinjection of kainate into the amygdala [24], or, most commonly, systemic administration of pentylenetetrazole (PTZ) [21], [25], [26], [27], [28], [29]. We started to use timed intravenous (i.v.) PTZ infusion for determining seizure threshold in dogs in 1982 [30]. Since then, this model has been used by us to study the relationship between GABA and seizure threshold [30], [31], to determine the liability of benzodiazepine (BZD) receptor ligands to induce tolerance and physical dependence during chronic treatment [32], [33], and to determine the anticonvulsant efficacy of BZD receptor ligands such as diazepam (DZP), clonazepam (CLZ), abecarnil and imepitoin [29], [32] or derivatives of the AED valproate (VPA) [34].
For the novel AED imepitoin [35], which acts as a low-affinity partial agonist at the BZD site of the GABAA receptor [36], the timed i.v. PTZ seizure threshold test was used for dose selection for subsequent clinical trials in dogs with epilepsy [29]. Imepitoin (Pexion®) has recently been approved in Europe for treatment of epilepsy in dogs. The only other drug that is approved for monotherapy of canine epilepsy in Europe is phenobarbital (PB), which has been used as an AED for more than 100 years [37]; however, its use in the treatment of human epilepsy has declined because of substantial tolerability issues. In epileptic dogs, imepitoin exerts a similar antiepileptic efficacy as PB, but its tolerability is much higher [29], [35]. The aim of the present study was to compare the anticonvulsant activity of imepitoin and PB in the timed i.v. PTZ seizure threshold test in dogs. Furthermore, the two compounds were compared in the same test in mice to determine whether mice correctly predict efficacy in dogs. In order to allow comparisons between drugs and species, we determined threshold increasing doses (TID) for 20% and 50% increase by dose–effect curves as previously suggested for comparison of drug effects in this model in mice [38], [39]. The high-affinity partial BZD receptor agonist abecarnil [40] was included in the experiments for comparison.
Section snippets
Animals
Male NMRI mice (Hsd:Win NMRI) weighing 20–25 g were obtained from Harlan-Winkelmann (Borchen, Germany) or, after Harlan closed the breeding site in Germany, Harlan in the Netherlands (Horst). Mice were housed under controlled conditions (ambient temperature 23–25 °C, humidity 30–50%, 12 h light cycle with lights on from 6:00 am to 6:00 pm). Food and water were freely available. Animals were purchased from the breeder at least 1 week before being used in the experiments. Age at onset of the
Anticonvulsant effect of PB on PTZ threshold in mice and dogs
In the absence of drug treatment, the PTZ threshold for the first myoclonic twitch was 39.0 ± 2.5 mg/kg in mice (n = 100) and 14.9 ± 0.2 mg/kg in dogs (n = 7), which was significantly different (P < 0.0001). As shown in Table 1 and Fig. 1A, B, PB dose-dependently increased the PTZ seizure threshold in both mice and dogs. In mice, we compared the efficacy of PB after i.p. and p.o. administration (Fig. 1A). Oral administration was only moderately less effective than i.p. injection with TID20 values of 14.8
Discussion
Imepitoin (AWD 131–138 or ELB 138; 1-(4-chlorophenyl)-4-morpholino-imidazolin-2-one) was developed in the 1990s from a series of imidazolinones and shown to exert anticonvulsant and anxiolytic effects in various animal models [46]. Its anticonvulsant activity was confirmed in the U.S. Anticonvulsant Screening Project (ASP) of the Antiepileptic Drug Development (ADD) programme sponsored by the National Institute of Neurological Disorders and Stroke (NINDS) of the National Institutes of Health
Acknowledgements
We thank Ernst Meyer, Doris Möller and Edith Kaczmarek for help with the PTZ seizure-threshold determinations, and Martina Gramer and Maria Hausknecht for HPLC analyses. The study was supported by Schering (Berlin, Germany), Pfizer Animal Health (Kalamazoo, Michigan, USA), Elbion (Radebeul, Germany), and Boehringer Ingelheim Vetmedica (Ingelheim, Germany).
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Present address: Abbott Laboratories, Hannover, Germany.