Pharmacokinetic study of zonisamide in patients undergoing brain surgery

To test whether the concentration of the anticonvulsant zonisamide in erythrocytes reflects the brain concentration and the clinical response of the drug, its pharmacokinetics were studied in nine patients undergoing surgery for brain tumour. Erythrocyte, total and free serum concentrations in samples drawn on the day of brain surgery were compared with levels on a day after the operation. In three patients zonisamide and its major metabolite, 2-sulphamoylacetylphenol, were also analysed in urine. The area under the curve of the free and the erythrocyte concentration did not differ between the two study phases whereas the area under the curve of the total serum concentration was significantly lower on the day of the operation, and this was associated with significant increases in total clearance (15.4 compared with 12.7 mL kg-1 h-1, P < 0.05, n = 9) and renal clearance (5.4 compared with 3.3 mL kg-1 h-1, P < 0.05, n = 3), and non-significant change in non-renal clearance (7.7 on the day of operation compared with 8.4 mL kg-1 h-1 on the post-operation day, n = 3). Zonisamide distribution was also altered by the operative procedure, as evidenced by a higher volume of distribution (1.48 compared with 0.87 L kg-1, P < 0.05, n = 9). The binding of zonisamide was characterized on both days. Zonisamide binding to erythrocytes seemed to occur by two processes: a saturable process and a non-saturable linear process. The maximum binding capacity to erythrocytes (31.6 vs 29.7 micrograms mL-1) did not differ on the two days; however, increases in the dissociation binding constant (+28%) and the proportionality constant (+24%) were observed on the day of the operation, suggesting that the zonisamide concentration in erythrocytes was greater on the day of the operation. Brain surgery appears to be one of the possible factors altering the rate of elimination of zonisamide and the uptake of the drug by erythrocytes.