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Homogeneous Immunoassays—Therapeutic Drug Monitoring
QMS®—Latex Enhanced Immunoturbidimetric Assays
Zonisamide Assay System
The ZONISAMIDE Assay System is intended for the quantitative determination of total Zonisamide in serum for therapeutic drug monitoring.
Zonisamide (1, 2-benzisoxazole-3-methanesulfonamide) is a second-generation antiepileptic drug developed by Dainippon Pharmaceuticals. The brand name of zonisamide in the USA is Zonegran. This medicine is available in the dosage form of oral and capsule. It was approved in year 2000 for the use in the United States by FDA as adjunct therapy in the treatment of adults with partial seizures refractory to therapy with traditional anticonvulsants. Oral dosage for zonisamide ranges from 400-600 mg per day with two doses. Bioavailability from animal studies suggests nearly 100% absorption. Steady state levels are achieved within two weeks. Concentrations are the highest in erythrocytes from the affinity of the sulfonamide derivative, then in whole blood and plasma1. Plasma protein binding, primarily to albumin, is from 40 to 60%. The volume of distribution ranges from 0.9 to 1.4 L/Kg. The elimination half-life is from 50 to 62 hours. The therapeutic range is varied, but Dainippon’s range is 10 to 30 µg/mL.
The mechanism of action for zonisamide has not been established, however, it is believed to involve blocking both the voltage sensitive sodium and T-type calcium channels. It may also be an allosteric modulator of the GABA receptor and free radical scavenger. Since there does not appear to be a dose response relationship. Dosage individualization is required to optimize the therapeutic potential. Binding to erytrocytes is saturable, therefore plasma or serum concentrations are recommended for therapeutic monitoring2. Urinary metabolites have been identified as N-acetyl-zonisamide and a glucuronide conjugate with an open isoxazole ring3.
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