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Because clinical data to confirm the safety and effectiveness of fosphenytoin, a prodrug of phenytoin, are insufficient, the length of administration of fosphenytoin is restricted. Nevertheless, some cases require fosphenytoin administration for more than a few days. The aim of this study was to retrospectively investigate the serum concentration of phenytoin in adult Japanese patients who received intravenous fosphenytoin therapy for more than 3 days.Patients injected with intravenous fosphenytoin for more than 3 days at Gifu University Hospital between January 2012 and September 2014 were enrolled. Individual pharmacokinetic parameters were predicted by Bayesian estimation using NONMEM software, and the maintenance dose of fosphenytoin required to maintain the therapeutic trough concentration (10–20 mcg/mL) was calculated from the parameters.Among a total of 8 patients, the serum trough concentration of phenytoin decreased with each day after repeated injection of fosphenytoin. The incidence rate of significant convulsive seizures was increased time dependently (0% on day 1, 12.5% on day 2, 25% on day 3, and 66.7% on day 4 and after). Phenytoin clearance showed a time-dependent increase. The maintenance dose of fosphenytoin required to maintain the therapeutic trough concentration was simulated to be 779.8 ± 316.8 mg/d, a dose that was markedly higher than the actual maintenance dose (414.1 ± 55.7 mg/d).Prolonged use of fosphenytoin, for such patients as those with autoimmune-mediated encephalopathy accompanied with reflux disease and/or ileus, time dependently decreased the serum concentration of phenytoin and increased the risk of convulsion. Therefore, the maintenance dose should be increased to maintain the therapeutic serum concentration.