The pharmacologic activity of fosphenytoin, a new phosphate ester pro-drug of phenytoin, is due to in vivo conversion to phenytoin. Fosphenytoin concentrations cannot be accurately estimated by phenytoin immunoassays (fluorescence polarization and chemiluminescence) owing to the nonlinear relation between fosphenytoin concentration and the observed cross-reactivity. The problem of slow conversion of fosphenytoin to phenytoin in serum in vitro can be circumvented by rapidly converting fosphenytoin to phenytoin in vitro by alkaline phosphatase. Drug-free serum, heparin, EDTA, or citrated plasma were supplemented with 2 concentrations of fosphenytoin. Then to 1-mL aliquots of specimen, no enzyme (control), 10 microL, or 25 microL of enzyme solution was added. The specimens were incubated, and phenytoin concentrations were measured by fluorescence polarization and chemiluminescent assays. In the absence of enzyme, we observed little conversion of fosphenytoin to phenytoin, but in the presence of only 10 microL of enzyme, the conversion of fosphenytoin to phenytoin was complete in 5 minutes. We also observed complete conversion of fosphenytoin to phenytoin by alkaline phosphatase in heparin, EDTA, and citrated plasma. If clinically indicated, the phenytoin concentration can be measured before and after addition of enzyme to roughly estimate the rate of conversion.
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