Zonisamide is a sulfonamide derivative that was introduced into the Japanese market in 1989 and received licensure in the US and Europe in 2000 and 2005, respectively.
With blockage of voltage-sensitive sodium channels and T-type calcium channels, zonisamide possesses a unique mode of action among the currently available anticonvulsant drugs. There is evidence that ZNS also exhibits direct effects on synthesis, release and degradation of the neurotransmitters glutamate, gamma aminobutyric acid (GABA), dopamine, serotonin and acetylcholine, thereby promoting synaptic inhibition. Furthermore, ZNS has been ascribed neuroprotective effects.
Zonisamide has less efficacy than other drugs such as phenobarbital, diazepam or potassium bromide, but zonisamide monotherapy is effective in some dogs with idiopathic epilepsy with few side-effects at the recommended dose, primariyl transient neurological signs (e.g., transient sedation, ataxia, vomiting) and elevated liver enzymes.
However, severe side effects have been reported in some dogs, such as acute renal tubular acidosis and acute idiosyncratic hepatic necrosis have been reported. Side-effects commonly reported at dose of 30 mg/kg or higher.
Caution may be necessary when zonisamide is given with phenobarbital and when antiepileptic therapy is changed from phenobarbital to zonisamide.
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