Antiepileptic Drug Discovery and Development: Past Achievements and Future Challenges
Henrik Klitgaard, Ph.D., Vice President, Neurosciences Therapeutic Area, UCB Pharma, Braine-l’Alleud,Belgium
Preclinical screening for new antiepileptic drugs (AEDs) was first introduced by Merritt and Putnam in 1937 when they tested a number of compounds from Parke Davis against maximal electroshock seizures (MES) in cats, leading to the discovery of phenytoin. Thes.c.pentylenetetrazole (PTZ) test was later used in mice and revealed first the anticonvulsant properties of trimethadione and later valproate, whereas phenytoin was found to be inactive in this model. This proved that these two seizure tests can identify AEDs with a different clinical profile. For that reason, the MES and PTZ tests have for several decades constituted the two key preclinical tests applied in rodents for random screening aimed at detection of new AEDs. Together with structural variation of known AEDs and rational drug discovery this has led to the identification of a significant number of AEDs, in particular of third generation AEDs introduced during the last two decades. This has expanded the armamentarium of AEDs providing more treatment options and AEDs associated with improved ease of use and tolerability, lower risk for hypersensitivity reactions and detrimental drug-drug interactions. However, none of these AEDs have been able to significantly eliminate the issue of drug refractory epilepsy or to provide a preventive and/or curative pharmacological treatment of the disease. This highlights the need for future antiepileptic drug discovery to focus on identifying AEDs that target the key remaining unmet medical need by new treatment paradigms that prevent or reverse drug refractory epilepsy and/or epileptogenesis. New treatment paradigms may derive from novel targets identified from existing hypothesis for drug refractory epilepsy and knowledge of the known disease biology of epileptogenesis, research on genetics, transcriptomics and epigenetics or mechanisms relevant for other therapy areas. These targets should be explored in antiepileptic drug discovery strategies that exploit the promise of novel technologies to develop medium- to high throughput screening models and that also contain relevant in vivo testing in kindling-, genetic-, status epilepticus- or insult specific models mimicking different aspects of drug refractory epilepsy and/or epileptogenesis. In order to minimize the high attrition rate associated with drug development, which mainly relate to failure of demonstrating sufficient clinical efficacy of new treatments, it is important to define new integrated strategies for antiepileptic drug discovery and development. These should focus on identifying drug candidates for viable patient populations and end points permitting conduct of pivotal trials targeting indications in drug refractory epilepsy, disease modification and epileptogenesis. An important tool for successful execution of integrated antiepileptic drug discovery and development strategies will be the availability of relevant biomarkers that enable to assess to what extent a drug candidate engage with its target and modulate associated biological processes, disease activity and regulatory end points. This will permit a continuum of proof of concept approaches during early clinical testing to rapidly confirm or reject preclinical findings, important to de-risk the development efforts. In conclusion, new integrated antiepileptic drug discovery and development strategies are needed to target the remaining unmet medical need for treatments that prevent or reverse drug refractory epilepsy and/or epileptogenesis. This will require significant interactions and efforts among preclinical and clinical scientists that hold the promise to be rewarded by the identification of improved treatments for epilepsy!
Nadia Fredsø Andersen
Epilepsy in dogs – new treatment (?)
Nadia Fredsø Andersen, DVM, PhD student & Mette Berendt, Professor MSO, Department of Veterinary and Clinical Animal Science, University of Copenhagen
Epilepsy is a common neurological disorder in dogs 1,2. Epilepsy as it occurs spontaneously in dogs share many similarities with human epilepsy with respect to aetiology, seizure types and seizure phenomenology, as have been reported by The Danish Canine Epilepsy Research Group, University of Copenhagen3,4.The prevalence of epilepsy in dogs has been estimated to around 1-2 % in a referral hospital population5, but in specific breeds with genetic epilepsy the prevalence is much higher6-13. The exact prevalence of epilepsy in the general dog population is unknown. As in humans, seizure freedom can be difficult to achieve medically, and dogs with insufficient seizure control are at an increased risk of being euthanized14.
Up till now antiepileptic treatment in veterinary medicine has mainly been restricted to the use of the two older antiepileptic drugs, phenobarbital and potassium bromide used as mono-therapy or in combination. Numerous antiepileptic drugs are available in human medicine, but unfortunately most are not efficient in dogs due to inappropriate pharmacokinetics such as e.g. a very a short half-life or toxicity.
Although effective in general, phenobarbital and potassium bromide possess undesirable side-effects, and a number of dogs are resistant to therapy with these drugs. Therefore, there is a great need for alternative treatment options.
At Department of Veterinary and Clinical Animal Science, University of Copenhagen, we have initiated a study that investigates the potential of an alternative mono-therapy treatment for epilepsy in dogs. This prospective treatment study uses a single blinded parallel group design. Dogs with newly diagnosed epilepsy are included in the study and randomised to one of two treatments, A and B. Dogs are followed in a longitudinal study with regular control visits.
Displacement of [11C]yohimbine binding in brain reveals noradrenaline release by vagal nerve stimulation
Anne M Landau, Suzan Dyve, Steen Jacobsen, Aage Alstrup, Arne Møller, Doris Doudet, Albert Gjedde
We tested the hypothesis that VNS-evoked release of noradrenaline explains the salutary effects of VNS. We used inhibition plots to estimate the release of noradrenaline in brains of Landrace pigs and Göttingen minipigs by means of the novel PET tracer [11C]yohimbine, which is a selective antagonist of the alpha-2 adrenoceptors. First, we determined the displacement of the tracer from the receptors after amphetamine administration and, second, we used VNS to test the alleged elevation of noradrenaline in cerebral cortex associated with the stimulation.
Kinetic analysis of the two compartments of accumulated radioligand in brain in vivo distinguishes between the quantity of radioligand dissolved in the “non-displaceable” volume of distribution (VND) and the quantity of radioligand bound to receptors, which in principle is displaceable by inhibition of the radioligand binding. Together the two compartments define the total volume of distribution (VT). The magnitudes of the total volume in the absence and presence of displacement, VT(baseline) and VT(inhibition), are related by the degree of receptor saturation elicited by the displacing competitor, alleged to be noradrenaline in the studies summarized here. The relationship linearizes to the form, VT(inhibition) = (1-s) VT(baseline) + s VND where s is the saturation of the receptor (Gjedde & Wong 2000). Linear regression to matching measures of VT(baseline) and VT(inhibition) yielded independent estimates of s and VND, which we used to calculate binding potential indices (BPND) of the release of noradrenaline.
Four groups of pigs were studied; including inhibition by exogenous antagonist competition in Landrace pigs reported by Jakobsen et al. (2006), inhibition after amphetamine in two groups, one reported by Landau et al. (2012) for Landrace pigs, and one unpublished for minipigs, and inhibition during VNS in minipigs with stimulators implanted in the neck. Dynamic PET scanning began at injection of high specific activity (>50 GBq/μmol) [11C]yohimbine with multiple frames of progressively increasing duration for 90 min. In the tests of amphetamine, the animals received standard doses of 2 or 10 mg/kg d-amphetamine as an intravenous bolus 20 minutes before the second [11C]yohimbine PET scan. We obtained the (VT) for each region with the Logan graphical analysis during the 25–90-min post-[11C]yohimbine injection period, using arterial samples as the input function.
In the first study, the exogenous antagonist competitors occupied from 55% to 80% of the alpha-2 receptors. In the first amphetamine challenge with 10 mg/kg amphetamine, the putative noradrenaline occupied from 55% to 70% of the receptors, and in the second challenge with 2 and 10 mg/kg, from 35% to 40%. With the average value VND of 1.8 ml/g, the acute VNS led to an average saturation of 19% of the receptors, presumably due to release of noradrenaline as predicted.
There is increasing evidence that vagal nerve stimulation (VNS) is useful in some forms of intractable major depression and epilepsy. Among the monoamines, noradrenaline is thought to play a particularly important role in the pathogenesis as well as the treatment of both disorders. Recent findings indicate that noradrenaline acts by shifting the balance between background and stimulus-evoked signals by inhibiting the spontaneous spiking of glycinergic interneurons, such that elimination of background spiking relieved inhibitory synapses from depression and thereby enhanced stimulus-evoked inhibition (Kuo & Trussell). The pre-clinical determination of binding potentials and saturability of alpha-2-adrenoceptors by selective labeling with selective alpha-2-adrenoceptor ligand yohimbine and subsequent competition from exogenous antagonists or the endogenous neurotransmitter noradrenaline, as released after amphetamine administration, is the basis for the conclusion that the displacement of the radioligand during VNS is due to release of noradrenaline. There, the monoamine occupies close to one-fifth of the receptors, where it subsequently elevates the signal-to-noise ratio of neuronal excitability.
Vagus nerve stimulation (and other forms of neurostimulation) in adults with epilepsy
Due to the lack of adequate treatments forl refractory epilepsy patients, the general search for less invasive treatments in medicine and the progress in biotechnology have led to an renewed and increasing interest in neurostimulation as a therapeutic option.
For all types of neurostimulation currently being investigated, major issues remain unresolved. The ideal targets and stimulation parameters for a specific type of patient, seizure or epilepsy syndrome are unknown. The characterisation of the full and long-term side effects profile especially for DBS need to be further investigated. The elucidation of the mechanism of action of different neurostimulation techniques requires more basic research in order to demonstrate its potential to achieve long-term changes and true neuromodulation.
It can be concluded that VNS is an efficacious and safe treatment for patients with refractory epilepsy. VNS appears to be a broad-spectrum treatment; identification of responders on the basis of type of epilepsy or specific patient characteristics is still slucive. VNS is a safe treatment and lacks the typical cognitive side effects associated with many other antiepileptic treatments. Moreover, there is generally a positive effect of VNS on mood, alertness and memory. In contrast to many pharmacological compounds, treatment tolerance does not develop inVNS. Instead, efficacy tends to increase with longer treatment.
However, on the basis of currently available data the responder rate in patients treated with VNS is not substantially higher compared to recently marketed anti-epileptic drugs. To increase efficacy, rational stimulation paradigms should be further investigated. With a rapidly evolving biomedical world, various neurostimulation modalities will be applied in patients with refractory epilepsy. Future studies will have ZAto show the position of VNS in comparison to treatment such as deep brain stimulation and transcranial magnetic stimulation.
Deep brain stimulation is now a treatment option for patients with refractory epilepsy. The finalisation of several pilot trials in different epilepsy centers have lead conclusion of pivotal trials and approval in Europe.
MEG & epilepsy
Magnetoencephalography (MEG) measures the changes in the magnetic field caused by the activity of the neurons in the brain. The most important clinical application of MEG is the diagnostic workup of patients with epilepsy, especially the ones with therapy resistant focal epilepsy who are candidates for surgical treatment (resection of the epileptic focus). MEG yields information that is complementary to the EEG: MEG records tangential dipoles (sources parallel to the surface), while EEG is mainly sensitive for radial dipoles (sources perpendicular to the surface). Thus MEG can record epileptiform discharges that are not visible in the EEG recordings. Imaging of the MEG sources has several advantages compared to the imaging of EEG sources: the high number of MEG sensors allows better spatial resolution, and the problem of different conductivity of the CSF and scalp only affects the electric source analysis.