Benoit Kenda

Anti‐convulsive and anti‐epileptic properties of brivaracetam (ucb 34714), a high‐affinity ligand for the synaptic vesicle protein, SV2A

Screening of 12 000 compounds for binding affinity to the synaptic vesicle protein 2A (SV2A), identified a high‐affinity pyrrolidone derivative, brivaracetam (ucb 34714). This study examined its pharmacological profile in various in vitro and in vivo models of seizures and epilepsy, to evaluate its potential as a new antiepileptic drug.

SV2A protein is a broad-spectrum anticonvulsant target: functional correlation between protein binding and seizure protection in models of both partial and generalized epilepsy.

SV2A, a synaptic vesicle protein, has been recently identified as a binding target for levetiracetam (Keppra). The specific mechanism by which SV2A binding leads to seizure protection has not yet been fully elucidated. However, a functional correlation between SV2A binding affinity and anticonvulsant potency has been observed in the mouse audiogenic seizure model. The present study was undertaken to test whether similar correlations exist in rodent models of partial and generalized epilepsies. As expected, there was a high degree of correlation between anticonvulsant potency and SV2A binding affinity in the mouse audiogenic seizure model (r(2)=0.77; p<0.001). A similar correlation was also observed in the mouse corneal kindling (r(2)=0.80; p<0.01) and in the rat model of generalized absence epilepsy (GAERS) (r(2)=0.72; p<0.01). Moreover, there were no significant differences between the slopes and intercepts of regression lines in these models. Interestingly, the protective potencies in these three epilepsy models were also well correlated with each other. As such, protective doses of a given SV2A ligand in one model could be easily predicted based on the data obtained in another model. Taken together, these results support the concept that SV2A protein is an important target for both partial and generalized epilepsies and thereby relevant for the generation of new antiepileptic drugs with potential broad-spectrum efficacy.

Brivaracetam: Rationale for discovery and preclinical profile of a selective SV2A ligand for epilepsy treatment

Despite availability of effective antiepileptic drugs (AEDs), many patients with epilepsy continue to experience refractory seizures and adverse events. Achievement of better seizure control and fewer side effects is key to improving quality of life. This review describes the rationale for the discovery and preclinical profile of brivaracetam (BRV), currently under regulatory review as adjunctive therapy for adults with partial‐onset seizures. The discovery of BRV was triggered by the novel mechanism of action and atypical properties of levetiracetam (LEV) in preclinical seizure and epilepsy models. LEV is associated with several mechanisms that may contribute to its antiepileptic properties and adverse effect profile. Early findings observed a moderate affinity for a unique brain‐specific LEV binding site (LBS) that correlated with anticonvulsant effects in animal models of epilepsy. This provided a promising molecular target and rationale for identifying selective, high‐affinity ligands for LBS with potential for improved antiepileptic properties. The later discovery that synaptic vesicle protein 2A (SV2A) was the molecular correlate of LBS confirmed the novelty of the target. A drug discovery program resulted in the identification of anticonvulsants, comprising two distinct families of high‐affinity SV2A ligands possessing different pharmacologic properties. Among these, BRV differed significantly from LEV by its selective, high affinity and differential interaction with SV2A as well as a higher lipophilicity, correlating with more potent and complete seizure suppression, as well as a more rapid brain penetration in preclinical models. Initial studies in animal models also revealed BRV had a greater antiepileptogenic potential than LEV. These properties of BRV highlight its promising potential as an AED that might provide broad‐spectrum efficacy, associated with a promising tolerability profile and a fast onset of action. BRV represents the first selective SV2A ligand for epilepsy treatment and may add a significant contribution to the existing armamentarium of AEDs.

Profile of the new pyrrolidone derivative seletracetam (ucb 44212) in animal models of epilepsy.

Seletracetam is a pyrrolidone derivative with a one-log-unit higher affinity for the synaptic vesicle protein 2A (SV2A) than levetiracetam (Keppra). This study explored its anticonvulsant properties in animal models of epilepsy. Seletracetam reduced both the amplitude and repetitive firing of population spikes induced by a high K(+)/low Ca(2+) concentration fluid (HKLCF) in rat hippocampal slices. The reduction of HKLCF-induced increases in population spike amplitude was particularly pronounced, and occurred at approximately 10 times lower seletracetam concentrations than previously observed for levetiracetam. These invitro data suggest that desynchronisation of epileptiform activity may contribute significantly to the antiepileptic properties of seletracetam. Seletracetam also showed a potent anti-seizure activity in animal models mimicking partial-onset (kindled animals) and generalized epilepsy (audiogenic seizure susceptible mice and genetic absence epilepsy rats from Strasbourg (GAERS)). In amygdala-kindled rats, seletracetam increased the generalized seizure threshold current and decreased the duration of the after-discharge and the seizure severity observed at the after-discharge threshold current, and generally had a much more potent effect than previously observed for levetiracetam. Seletracetam showed no psychomimetic effects and a very high central nervous system (CNS) tolerability in both kindled and GAERS rats, markedly superior to that of levetiracetam and other antiepileptic drugs. These results suggest that seletracetam may represent an effective and very well tolerated broad-spectrum agent for the symptomatic treatment of epilepsy.

Discovery of Heterocyclic Nonacetamide Synaptic Vesicle Protein 2A (SV2A) Ligands with Single-Digit Nanomolar Potency: Opening Avenues towards the First SV2A Positron Emission Tomography (PET) Ligands

The role of the synaptic vesicle protein 2A (SV2A) protein, target of the antiepileptic drug levetiracetam, is still mostly unknown. Considering its potential to provide in vivo functional insights into the role of SV2A in epileptic patients, the development of an SV2A positron emission tomography (PET) tracer has been undertaken. Using a 3D pharmacophore model based on close analogues of levetiracetam, we report the rationale design of three heterocyclic non‐acetamide lead compounds, UCB‐A, UCB‐H and UCB‐J, the first single‐digit nanomolar SV2A ligands with suitable properties for development as PET tracers.

[11C]UCB-A, a novel PET tracer for synaptic vesicle protein 2 A

INTRODUCTION Development of a selective and specific high affinity PET tracer, [(11)C]UCB-A, for the in vivo study of SV2A expression in humans. Radiochemistry and preclinical studies in rats and pigs including development of a tracer kinetic model to determine VT. A method for the measurement of percent intact tracer in plasma was developed and the radiation dosimetry was determined in rats. RESULTS 3-5GBq of [(11)C]UCB-A could be produced with radiochemical purity exceeding 98% with a specific radioactivity of around 65GBq/μmol. In vitro binding showed high selective binding towards SV2A. [(11)C]UCB-A displayed a dose-dependent and reversible binding to SV2A as measured with PET in rats and pigs and the VT could be determined by Logan analysis. The dosimetry was favorable and low enough to allow multiple administrations of [(11)C]UCB-A to healthy volunteers, and the metabolite analysis showed no sign of labeled metabolites in brain. CONCLUSIONS We have developed the novel PET tracer, [(11)C]UCB-A, that can be used to measure SV2A expression in vivo. The dosimetry allows up to 5 administrations of 400MBq of [(11)C]UCB-A in humans. Apart from measuring drug occupancy, as we have shown, the tracer can potentially be used to compare SV2A expression between individuals because of the rather narrow range of baseline VT values. This will have to be further validated in human studies.

Discovery of Indolone Acetamides as Novel SV2A Ligands with Improved Potency Toward Seizure Suppression

A major goal in epilepsy research is to develop new antiepileptic drugs (AEDs) that combine improved seizure control with enhanced tolerability while avoiding drug–drug interactions. Although the prognosis for seizure control is acceptable in up to 70 % of patients, approximately 30 % suffer from intractable pharmaco-resistant epilepsy. 3] Furthermore, the clinical use of most older AEDs is hampered by their limited tolerability; this most commonly consists of CNS-related adverse effects, idiosyncratic reactions such as skin rashes, and the potential for unfavorable drug–drug interactions. It was previously reported that the AED levetiracetam (2 ; Keppra ) has a unique brainspecific binding site. This site was recently identified as the synaptic vesicle protein 2A (SV2A). A strong correlation between the affinity of levetiracetam analogues for SV2A and their anti-seizure potency in the audiogenic mouse model of epilepsy has been reported. This suggests that levetiracetam’s interaction with SV2A has a major functional role in its anticonvulsant mechanism of action. SV2, a protein specific to synaptic vesicles, is a 12-transmembrane region glycoprotein present in all neural cells, and it is present in three isoforms: SV2A, SV2B, and SV2C. SV2A is the most widely distributed isoform and is ubiquitous in the CNS, but is also present in endocrine cells. Although the exact molecular role of SV2A is still unknown, it is believed to play a role in the exocytosis of neurotransmitters and to act as a modulator of vesicle fusion. Given the proven clinical efficacy of levetiracetam as an AED, its unique mechanism of action, and its excellent tolerability, we decided to initiate a drug discovery program focused on SV2A as a novel molecular target. The aim was to identify a new generation of SV2A ligands with an equal or better tolerability profile than 2 and an improved potency toward seizure suppression in animal models. At the outset of our work in the SV2A field, we systematically investigated the various positions of the pyrrolidone acetamide scaffold of 2. Among others, we discovered the importance of the carboxamide moiety on 2 and the preferred substitution position a to the carboxamide. This research lead to the identification of two compounds currently in clinical development: brivaracetam (3) and seletracetam (4), both of which are more potent than 2 in vitro toward SV2A and in vivo as anticonvulsant agents in audiogenic seizure-prone mice. (Figure 1)