Is there a future for mGlu5-positive allosteric modulators in absence epilepsy? A comparison with ethosuximide

Ethosuximide is the drug of choice in the treatment of various types of absence seizures. However, there is plenty of room for other anti-absence drugs, considering that not all subjects (57–74%) become seizure-free and about 47% of ethosuximide therapy fails. New anti-absence drugs may target or modulate glutamatergic and or GABAergic neurotransmission, the key players in the circuitry involved in the cortico-thalamo-cortical oscillations responsible for the highly stereotyped spike–wave discharges (SWDs). Cortical highly excitable cells in the focal region form the trigger for the occurrence of SWDs. In contrast, enhanced tonic inhibition is dominant in the thalamus. Biochemical studies have shown that symptomatic WAG/Rij rats differ from age-matched controls in metabotropic glutamate (mGlu) receptor expression and function: mGlu5 receptor expression and function are increased in the somatosensory cortex, and mGlu1 receptor expression is decreased in the thalamus. The two group I mGlu receptor-positive allosteric modulators (PAMs) VU0360172 and RO0711401 have an interesting profile in acute and (sub)chronic pharmacological studies and produce a dose-dependent decrease of SWDs. Moreover, both compounds are effective in reducing SWDs in the cortex and thalamus. Interestingly, the GABA reuptake blocker tiagabine reduces SWDs in the cortex and not in the thalamus, while the efficacy of ethosuximide is higher in the cortex than in the thalamus. It is thought that VU0360172 stimulates cortex GABA interneurons, which inhibit highly excitable cortical neurons in the focal area. In the thalamus, VU0360172 most likely reduces tonic inhibition. Thus, group I mGlu receptor PAMs might be further developed as anti-absence drugs, with putative disease-modifying effects on epileptogenesis. The preclinical profile of group I mGlu receptor PAMS deserves to be further explored in models of generalized epilepsy and focal types of epilepsy.