Levetiracetam Mechanisms of Action: From Molecules to Systems

Abstract

Epilepsy is a chronic disease that affects millions of people worldwide. Antiepileptic drugs (AEDs) are used to control seizures. Even though parts of their mechanisms of action are known, there are still components that need to be studied. Therefore, the search for novel drugs, new molecular targets, and a better understanding of the mechanisms of action of existing drugs is still crucial. Levetiracetam (LEV) is an AED that has been shown to be effective in seizure control and is well-tolerable, with a novel mechanism of action through an interaction with the synaptic vesicle protein 2A (SV2A). Moreover, LEV has other molecular targets that involve calcium homeostasis, the GABAergic system, and AMPA receptors among others, that might be integrated into a single mechanism of action that could explain the antiepileptogenic, anti-inflammatory, neuroprotective, and antioxidant properties of LEV. This puts it as a possible multitarget drug with clinical applications other than for epilepsy. According to the above, the objective of this work was to carry out a comprehensive and integrative review of LEV in relation to its clinical uses, structural properties, therapeutical targets, and different molecular, genetic, and systemic action mechanisms in order to consider LEV as a candidate for drug repurposing.

Keywords: GABAergic system; SV2A; antiepileptic drugs; calcium homeostasis; levetiracetam; neuroinflammation; neuroprotection.

Figure 1

Chemical structures of racetams and molecular modifications. (A) Molecular structures of different SV2A ligands used in the text. (B) SAR map reported by Mittrapalli, 2014. (C) Updated SAR map according to our bibliographic search.

Figure 2

Schematic representation of the racetam binding site in SV2A found by molecular modeling. Despite having the same components, LEV binds differently than the rest of the racetams in a pocket in front of the racetam binding site that had some polar amino acids that can interact with polar groups in position 4, as in UCB-30889.

Figure 3

SV2A function. SV2A regulates the readily releasable pool (RRP) size (1) and during priming (2) facilitates the progression to the release-competent state, both allowing normal neurotransmission. In the exocytosis (3), SV2A functions as a target for residual calcium and finally, in the endocytosis (4), regulates the vesicle content of the calcium-sensor synaptotagmin protein.

Figure 4

Hypothetical integrated molecular mechanisms of action of LEV. (1) LEV diffuses throughout the blood–brain barrier and neuron membrane or enters during (2) exocytosis and endocytosis processes, subsequently exerts its action by various mechanisms. (3) LEV could decrease the function of SV2A during vesicular priming and thus diminishes the readily releasable pool and therefore the release of neurotransmitters (purple terminal). Another possibility is that LEV stabilizes SV2A and improves its function during (4) exocytosis, and (5) endocytosis modulating the expression and traffic of synaptotagmin protein. (6) Moreover, it has been reported that LEV blocks the voltage-dependent calcium channels, decreasing the synaptic transmission. (7) LEV reduces potassium currents inducing a decrease in the repetitive action potential generation. With respect to calcium intracellular systems, LEV reduces the calcium transients of (8) ryanodine and (9) IP₃ receptors. In the GABAergic system, LEV modulates the region-dependent (10) glutamic acid decarboxylase (GAD) and increases (11) GABA transaminase (GABA-T). In the post-synapse, LEV blocks the effect of the (12) GABA_A receptor antagonists. In the glutamatergic synapse, LEV modulates (13) AMPA receptors and decreases the excitatory current. (14) Finally, LEV interacts with noradrenaline, adenosine and serotonin receptors in post-synapse involved in pain system.

Figure 5

Hypothetical acti-ictogenic and neuroprotector effect of levetiracetam (LEV). (A) The anti-ictogenic effect of LEV seems to be an integrative mechanism, that involved the interaction to this AED with different pharmacological targets, resulting in a decrease in the excitability in the epileptic circuit. Although the exact mechanism throughout LEV can decrease the excitability remain unclear, the evidence suggests that SV2A plays a critical role. In epileptic nerve terminal, the binding LEV-SV2A could result in augment or decrease in neurotransmitter release. Moreover, LEV blocks the voltage calcium and decreases the release of the neurotransmitters. (B) GABA and glutamate are the principal neurotransmitters responsible for maintaining a balance between inhibition and excitation, respectively. In epilepsy, as well as in other pathologies that cause neuronal damage, there is an imbalance between these systems, which results in increased excitability and neuronal death. LEV decreases both events, perhaps by modulating neurotransmitter release by binding to SV2A, which may result in increased neurotransmitter release, particularly GABA, or in a decreased of neurotransmitter release, particularly glutamate. Regardless, whether LEV particularly increases GABA release or decreases glutamate release, the final effect is decreased excitability, seizures, and neuronal death by restored the balance between excitatory and inhibitory systems.

Figure 6

Putative antiepiletogenic levetiracetam (LEV) mechanism. (A) LEV is one of the few antiepileptic drugs (AED) able to retard or inhibit the generation of epileptic neural circuits. The mechanism through which it does is not completely elucidated, but apparently inhibiting the excessive synaptic transmission. (B) The binding LEV-SV2A improve the SV2A effects, diminishing the hyperexcitability and thus delays epileptogenesis. (C) LEV inhibit epileptic foci formation by suppressing BDNF synthesis and consequently the mossy fiber sprouting.

Figure 7

Anti-inflammatory effect of LEV. Seizures induces the activation of microglia, which in turn releases proinflammatory mediators, such as TNF-α, IL-1β, IL-6, etc. These cytokines promote the brain blood– brain barrier (BBB) damage and sustained neuronal inflammation leading to seizures and neurodegeneration, which activates further inflammation, establishing a vicious circle. The anti-inflammatory mechanism of LEV may be related to the voltage-activated Ca²⁺ channel inhibition present in the microglia (purple cell). This results in glia non-activation and thus the attenuation of inflammation. LEV also inhibits the JAK2-STAT3 signaling pathway. LEV can act as an HDAC inhibitor promoting the transcription of antioxidant and cytoprotective genes, restoring the oxidation–reduction balance.