Epileptic Disorders
MENUAntiepileptogenesis, neuroprotection, and disease modification in the treatment of epilepsy: focus on levetiracetam Volume 5, supplément 1, Supplement, May 2003
Auteur(s) : Henrik Klitgaard1, Asla Pitkänen2
1 Preclinical CNS Research, UCB S.A. Pharma Sector,
Braine L’Alleud, Belgium
2 Epilepsy Research Laboratory, A.I. Virtanen Institute
for Molecular Sciences, University of Kuopio and Department of
Neurology, Kuopio University Hospital, Kuopio, Finland
Introduction
The discovery of new drugs with specific properties requires
appropriate tests of drug function. Traditional screens for
antiepileptic drugs (AEDs) examine anti-seizure properties; i.e,
the ability to suppress expression of experimentally induced
seizures in normal laboratory animals [1, 2]. For this reason,
current drug treatment options for epilepsy predominantly combat
ictogenesis, or the initiation of paroxysmal activity [3]. This has
identified a number of classes of AEDs that primarily suppress
neuronal excitability by blocking Na+ channels or
enhancing inhibitory GABAergic activity [4, 5]. While these
traditional AEDs have had a profound effect by reducing the
expression of epileptic seizures, their function invariably elicits
some impairment of the normal neuronal excitability underlying
cognitive function [6-8]. Since ictogenesis and cognition are both
mediated by neuronal excitability, it may not be possible to
discover optimal non-impairing AEDs using traditional screens. This
may be improved by performing drug screens in animal models of
chronic epilepsy. Thus, by applying genetically modified or kindled
animals [9] it may be possible to discover new AEDs that inhibit
the neuronal hypersynchronization leading to an ictal event,
without interfering with normal neuronal excitability [10]. An
additional approach to the discovery of novel AEDs would be to
examine processes of epileptogenesis in addition to ictogenesis
[11]. Since the development of epilepsy is a multistep progressive
process [12], there may be several mechanisms in addition to the
neuronal excitability and hypersynchronization associated with the
paroxysmal event that are susceptible to pharmacologic
intervention. Thus, it appears possible to devise novel drug
screens that may reveal new classes of AEDs with less compromising
mechanisms of action.
Epileptogenesis refers to the multiphase process in which a normal
brain undergoes alterations to support the generation of
spontaneous seizures. It may be initiated by brain damage produced
by events such as head trauma [13], stroke [14], infection [15,
16], or status epilepticus [17]. Following such an initial insult,
a latency phase without seizures follows and may last for weeks to
years. During these initial stages, progressive brain alterations
result in lowered seizure thresholds which eventually cause
spontaneous seizures [13, 18]. Once seizures occur, the epileptic
disease state probably continues to progress, with each seizure
having the potential to induce additional neuronal alterations that
may further lower seizure thresholds [19].
In order to discover novel AEDs that combat these phases of
epileptogenesis, new drug screening models must be employed. It is
likely that such screens would identify drugs with mechanisms of
action different from traditional AEDs, since the molecular
mechanisms underlying epileptogenesis and ictogenesis are
different. While anticonvulsants reduce the duration or frequency
of seizures by suppressing neuronal excitation or excitability,
real antiepileptogenic agents would act by blocking the initial
epileptogenic process or by altering the epileptic disease state
after the seizure onset [11]. Appropriate screens for
antiepileptogenic drug action would be tests for the ability of
drugs to reduce alterations in molecular, cellular, and network
properties that occur during the epileptogenic process.
The induction of status epilepticus (SE) and kindling represent
two animal paradigms in the preclinical evaluation of AEDs. SE is
defined as long-duration seizures, typically lasting for more than
30 min [20]. Experimentally, SE can be induced by acute
systemic exposure to epileptogenic agents, including drugs that
block GABAergic inhibition or facilitate glutamatergic or
cholinergic excitatory transmission. The glutamate receptor agonist
kainic acid [21] and the cholinergic agonist pilocarpine [22] are
commonly used in SE models. SE can also be induced by electrical
stimulation [1]. Anti-seizure properties of potential AEDs can be
tested by measuring the ability of drugs to suppress SE initiation,
duration, or seizure intensity following administration of
convulsant agents.
Experimentally induced SE can also be used as a model for
epileptogenesis, since SE induces neuronal alterations similar to
those seen in epileptic patients. Further, the long-duration
seizures characteristic of SE produce neuronal damage similar to
Ammon’s horn sclerosis observed in patients with temporal lobe
epilepsy [23, 24]. SE induces cell loss in specific neuronal
populations in multiple brain regions, including the hippocampus,
amygdala, and entorhinal cortex [22, 23]. The damage induced by
kainic acid-induced SE is produced by the evoked seizure activity
and not by direct activation of glutamate receptors by kainic acid
[23]. There are two phases of cell death following SE. Acute
necrotic cell loss occurs during the prolonged seizure event, while
other cells undergo delayed cell death hours or days following
seizure termination. Surviving brain cells undergo morphological
alterations including axonal sprouting and altered density of
dendritic spines. In addition, SE causes widespread changes in gene
expression, the extracellular matrix, and neurogenesis. SE also
causes alteration in non-neuronal brain cells, such as changes in
number and morphology of astrocytes and microglia. Functionally, SE
produces long-lasting deficits in cognition, behavior, and memory.
Critical to the use of SE as a model of epileptogenesis is that
spontaneous seizures develop after a latency following SE. SE can
be used to test for antiepileptogenic properties of potential AEDs
by administering the AED following SE and examining the effect on
neuronal pathology and expression of spontaneous seizures.
A second animal model commonly used for evaluating anti-seizure
properties of AEDs is focal, electrical kindling. In the kindling
model, repeated exposure to an initial sub-convulsive stimulus
eventually evokes seizures [19, 25, 26]. Initially, electrical
kindling stimuli only elicit short-duration afterdischarges
produced by a synchronous neuronal discharge near the site of
stimulation. Each additional kindling stimulation induces longer
afterdischarges which incorporate larger brain regions, with the
limbic system quickly becoming involved. Behavioral seizures
accompany the afterdischarges and become more complex and longer
with repeated stimuli. This progressive increasing sensitivity to a
previously subconvulsant stimuli usually takes a number of days or
weeks and eventually reaches a plateau in which kindling stimuli
evoke seizures and afterdischarges with reproducible behaviors and
durations. The kindling-induced reduction in seizure threshold is
permanent. The seizures evoked by focal, electrical kindling
stimuli in the temporal lobe involve limbic circuits and are
analogous to human complex partial seizures with secondary
generalization [26, 27]. Pharmacologic convulsants or electrical
stimuli can induce kindling [26]. Kindling can be used as a screen
for anti-seizure effects since kindled seizures are inducible and
have durations and electrographic and behavioral manifestations
that are easily characterized. After animals have been fully
kindled, potential AEDs can be administered and the effects on
behavioral and electrographic seizures measured.
The progressive nature of kindling, in which repeated seizures cause a reduction in seizure thresholds over time, may share features with the epileptogenic process in humans. It is possible that the long delay between trauma and seizure expression in posttraumatic epilepsy may reflect a slow kindling process [26]. This idea is supported by the development of generalized seizures in a patient receiving electrical stimulation of the thalamus [28]. Evidence against kindling as a mechanism underlying epilepsy in man relates to the observation that although primates can be kindled, they are much more resistant to kindling stimuli than are rodents [26].
An association between alterations of neuronal circuits and increased seizure susceptibility has also been found in kindling. Even relatively brief kindled seizures, lasting seconds to minutes, have been shown to produce limited neuronal alterations similar to those seen following SE. Kindled seizures induce progressive, but limited, cell loss in limbic regions including the dentate gyrus, hippocampus and entorhinal cortex [29, 30], and sprouting of axonal collaterals in the dentate gyrus [31]. Kindling also induces behavioral alterations and causes long-term deficits in cognitive function [32-34]. In contrast to SE, however, kindling rarely results in the development of spontaneous seizures.
Therefore, kindling stands as a model to investigate the effects
of potential antiepileptogenic compounds on the reorganization of
neuronal circuits which have similarities to those that occur after
SE and lead to the development of spontaneous seizures [11, 35].
Drugs with antiepileptogenic properties may inhibit the development
of kindling. Some antiepileptogenic drugs might function to block
spread of the synchronous neuronal discharge underlying seizure
activity or prevent the formation of secondary foci. This model is
confounded by the fact that during kindling development, it is the
kindled seizures that induce the neuronal alterations underlying
lowered afterdischarge thresholds. Therefore, drugs with
anti-seizure effects might inhibit kindling development simply by
preventing, or shortening, the expression of seizures, not by
inhibiting the effects of seizures. In this sense, anti-seizure
compounds might have disease-modifying effects by shortening
seizure duration. However, this problem may be solved by continued
evaluation of kindling inhibition after cessation of treatment with
an AED. It has consistently been reported that AEDs which enhance
GABAergic transmission delay development of kindling [36]. In
contrast, most AEDs that block Na+ channels do not delay
the development of kindling [37-40].
The main problem with kindling as a model of epileptogenesis is
that kindled seizures must be induced. Since the emergence of
spontaneous seizures following kindling is rare, it may be
questioned if kindling produces a true epileptic state [41]. It is
possible that the neuronal alterations produced by kindling,
including cell loss and aberrant axonal sprouting, are relatively
mild and may not be sufficient to mediate epileptogenesis [29-31].
Furthermore, it may be argued that the neuronal damage in the
kindling model is the result of, and not the cause of,
seizures.
Animal models for testing neuroprotective effects of AEDs
A wide range of brain insults, including SE, head trauma, and stroke, produce a pattern of brain damage. Different initial events may induce a similar sequence of events, including acute neuronal necrosis, followed by delayed glutamate release and excitotoxicity, which commonly results in the death of specific neuronal populations. Long-term alterations, evoked by activity-induced gene expression [42] or compensatory responses to cell damage and death, appears to produce changes in neuronal circuits [17]. It is likely that at least a subset of these alterations underlie the reduced seizure thresholds and expression of spontaneous seizures that define the epileptogenic disease state. For example, altered neuronal circuitry from axonal sprouting and aberrant excitatory synapse formation may produce hyperexcitable recurrent circuits [43]. Altered glial cell function observed following SE may disrupt extracellular K+ buffering contributing to neuronal hyperexcitability. The multistep process of epileptogenesis provides a number of sites for potential pharmacologic intervention. Drug screens may be designed to specifically target the discovery of agents that inhibit the initial damage produced by brain insults. Alternatively, antiepileptogenic drug screens may seek compounds that block excitotoxic cell death or other secondary damage. Other agents may prevent or reverse the compensatory alterations in neuronal circuits that contribute to lowered seizure thresholds.
Ischemia models
Screens for antiepileptogenic drugs may identify compounds that
protect against altered neuronal circuits and neuronal damage. SE
models can be used to test drugs for effects against SE-induced
neuronal death, morphological alterations, altered excitability,
and seizure expression. Temporary global ischemia in rodents
produced by arterial occlusion or cardiac arrest is used as a model
of stroke. Neuronal pathology following global ischemia has many
similarities to damage following SE [44-47]. Both can lead to
expression of spontaneous seizures [14]. The ability of drugs to
block the ischemia-induced neuronal damage or the emergence of
neurological deficits and seizures in SE models can be considered
as a screen for antiepileptogenic drug properties. In that respect,
tests of traditional AEDs in ischemia models has found that
Na+ channel blockers (carbamazepine, phenytoin,
lamotrigine) [48] and GABAergic transmission enhancers (clonazepam,
tiagabine, topiramate, vigabatrin) [46, 49-51] reduce ischemic
damage.
In addition to attenuating the initial alterations in neuronal
circuits and brain damage preceding the first spontaneous seizures,
antiepileptogenic drugs also might function after the epileptic
state has been established to change the underlying disease state.
Antiepileptogenic agents may alter neuronal circuits, making them
less seizure-prone, and neuroprotective agents may reduce further
seizure-induced damage. It remains to be determined to what extent
these two approaches may alleviate the consequences of the
epileptogenic process in man.
Preclinical findings with levetiracetam (LEV)
The novel AED LEV has interesting properties that may suggest both anti-seizure and antiepileptogenic properties. LEV differs from most AEDs in that it has no anti-seizure effect in the acute maximal chemoconvulsive or electroshock seizure tests [52, 53], but it markedly suppresses seizures in kindled and genetically epileptic animals [52-54]. The ability of LEV to delay the development of kindling [35] suggests that it has the potential to interfere with circuitry modifications underlying the progressive development of lowered seizure threshold. Of particular interest is the finding that, unlike any other currently available AED, LEV treatment results in a persistent suppression of afterdischarge duration in kindled brain, even after the termination of treatment. Further support for an antiepileptogenic potential of LEV derives from recent observations showing that LEV attenuates both hippocampal cell death and enhancement in hippocampal excitability following a pilocarpine-induced SE [55].
Safety of LEV in animal models
One of the promising features of LEV is a highly favorable safety profile in animal models. LEV elicits only mild sedation at doses more than 50 to 100 times higher than the anti-seizure dose [53]. LEV demonstrates low toxicity in rats and mice in an Irwin-type observation test, the rotarod test, and open-field exploration [52, 53, 56]. Thus, LEV induces only mild behavioral alterations in normal and amygdala kindled rats at anti-seizure doses [52, 53]. In corneally kindled mice, LEV had a high safety margin between rotarod impairment and seizure suppression [53]. Furthermore, at doses which produced seizure suppression, LEV did not alter cognitive performance of normal and amygdala kindled rats in the Morris water maze test [57]. Furthermore, at clinically relevant doses, LEV also did not affect induction of long-term potentiation in rat hippocampal slices, a model of memory [57].
LEV mechanisms of action
Although LEV’s mechanism of action is still not fully elucidated, it appears to differ from that of other known AEDs. LEV has a specific membrane binding site within the brain [58], but it does not directly affect glutamate – or GABA – receptor mediated synaptic transmission at therapeutically relevant concentrations [59, 60]. Furthermore, LEV does not alter Na+ channel current properties [61]. LEV produces a limited reduction in high-voltage-activated Ca2+ currents [62] but not low-voltage-activated calcium currents [61]. Although LEV has little direct effect on GABA-receptor mediated currents, it opposes the action of negative modulators of GABA and glycine receptors [60]. Conflicting reports exist as to LEV’s ability to induce a modest inhibition of the delayed rectifier K+ current [63] LEV’s antiepileptic action appears mediated through selective inhibition of neuronal burst firing and blocking synchronized firing of populations of neurons [10, 64]. Indeed, the ability of LEV to selectively suppress synchronized and burst firing interferes with spike propagation from the hippocampus to cortex [64] and may underlie both its unique anti-seizure and antiepileptogenic effects.
Comparison of LEV to other AEDs
LEV’s mechanism of action appears to be distinct from the other new AEDs (table 1), including topiramate, gabapentin, lamotrigine, and oxcarbazepine, which appear to directly affect neuronal excitability.
Table 1. Mechanism of action and
properties of levetiracetam (LEV) and other antiepileptic drugs
(AEDs)
AED | Mechanism of action | Effects on kindling |
---|---|---|
LEV | Specifically reduces the N-type
high-voltage-activated Ca2+ current Opposes the action of negative modulators of GABA and glycine receptors |
Increases afterdischarge threshold Decreases seizure severity Reduces seizure spread Increases threshold for secondary generalized seizures Suppresses kindling development |
Topiramate | Na+ channel blocker, Enhances GABAA receptor currents Inhibits kainate and AMPA receptors Reduces the high voltage-activated Ca2+ current Inhibits type II and IV carbonic anhydrase |
Suppresses kindling development Increases afterdischarge threshold Decreases seizure severity and duration Reduces seizure spread |
Gabapentin | Increases GABA levels Reduces L-type Ca2+ currents |
Increases afterdischarge threshold Decreases seizure severity Reduces seizure spread Increases threshold for secondary generalized seizures |
Lamotrigine | Na+ channel blocker Reduces Ca2+ conductances involved in transmitter release |
Suppresses completed kindled seizures Blocks, has no effect, or facilitates kindling development Increases afterdischarge threshold Has no effect or decreases seizure severity and duration |
Oxcarbazepine | Na+ channel blocker Increases K+ conductance Modulation of high voltage-activated Ca2+ channels Reduces Ca2+ conductances involved in transmitter release |
Does not block (may facilitate) kindling
development |
Topiramate is principally a Na+ channel blocker that
may also enhance GABAA-receptor currents [65]. The mechanism of
action of gabapentin is unclear but relates to reduction in L-type
Ca2+ currents and increases in GABA levels [66].
Lamotrigine is also principally a Na+ channel blocker
[67]. Oxcarbazepine is a Na+ channel blocker that also
increases K+ conductance and modulates high-voltage
activated Ca2+ channels [38].
Some of these AEDs may possess antiepileptogenic properties. For
example, topiramate suppresses kindling development [68]. It acts
primarily by blocking the spread of seizures. When administered
after SE, topiramate attenuates seizure-induced hippocampal cell
death [69]. Oxcarbazepine, however, prolongs afterdischarge
duration during kindling induction and increases the rate of
kindling development [37]. Lamotrigine has been reported to
increase, decrease [70], or have no effect on kindling development
[39, 40]. It is interesting that the AEDs that are Na+
channel blockers have primarily anti-seizure properties, while AEDs
that modulate GABAergic transmission also appear to possess
antiepileptogenic properties. Indeed, most AEDs that enhance
GABAergic transmission have neuroprotective effects against
SE-induced neuronal damage.
LEV pretreatment significantly reduced the infarct volume induced
by transient cerebral artery occlusion [71]. Topiramate
post-treatment has also been reported to protect against global
ischemia-induced hippocampal cell death and motor impairment [45,
46] and to reduce the severity of seizures induced by ischemic
insults [46]. Topiramate post-treatment also reduced the
hippocampal damage when administered 140 min after the onset
of SE induced by unilateral hippocampal stimulation [69].
Gabapentin has been shown to reduce glutamate release in
hippocampal models of ischemia but not in vivo [72].
Lamotrigine post-treatment has been shown to be neuroprotective
both in focal and global ischemia models in rats and gerbils [44,
48, 73-75]. Furthermore, lamotrigine administration before or after
electrical stimulation-induced SE protected against cell death in
the hippocampus and piriform cortex, but did not alter subsequent
memory impairments [76].
Conclusions
Traditional epilepsy treatment has focused on seizure
suppression using anti-seizure drugs. With the understanding that
epilepsy arises as a progressive change in neural circuits and
frequently manifests as neuronal damage, it may be more appropriate
to complement this treatment with antiepileptogenic and
neuroprotective drugs. The molecular basis of epileptogenesis and
ictogenesis have a very different neurobiologic basis and may
therefore be addressed by different classes of drugs or drug
actions. Thus, novel antiepileptogenic compounds may be found by
using screens specifically designed to test for neuroprotection or
the ability to alter the reorganization of neuronal circuits
underlying the development of lowered seizure threshold. Such new
drugs would be important as prophylactic antiepileptogenic drugs
following head trauma, stroke, cerebral infection, and SE to
prevent the potential development of spontaneous seizures.
Importantly, continual antiepileptogenic and neuroprotective drug
administration may be required, since molecular, cellular, and
network reorganization continues after the diagnosis of epilepsy,
particularly in patients who are not seizure-free. Reducing the
ongoing circuitry reorganization in this difficult-to-treat
subpopulation of patients may result in less severe epilepsy
expression. Neuroprotection may constitute a critical component of
epileptogenesis alleviation, of neuronal loss after brain-damaging
insults, and alleviation of the continuous remodeling of neuronal
circuits in established epilepsies.
The novel AED LEV may be the first of a new class of drugs which
meet these needs. Whether the permanent shortening of
afterdischarge duration by LEV treatment during kindling
development is associated with antiepileptogenesis in models in
which the spontaneous seizure development is triggered by brain
damage remains an intriguing hypothesis [35]. Furthermore, whether
this reflects a significant disease-modifying effect (i.e.,
seizures will be shorter) remains to be confirmed in spontaneous
seizure models. LEV also supports the notion that drugs which do
not act directly to suppress neuronal excitability may have more
favorable safety profiles. It is likely that the application of
drug screens specifically testing for antiepileptogenesis may yield
additional promising AEDs.