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Clinical Chemistry 44: 1474-1480, 1998;
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Right arrow Drug Monitoring and Toxicology
(Clinical Chemistry. 1998;44:1474-1480.)
© 1998 American Association for Clinical Chemistry, Inc.

Drug Monitoring and Toxicology

Cross-reactivity of fosphenytoin in two human plasma phenytoin immunoassays

Alan R. Kugler1,a, Thomas M. Annesley2, Gerald D. Nordblom3, Jeffrey R. Koup3, and Stephen C. Olson3

1 Department of CNS Clinical Research and Development, Parke-Davis Pharmaceutical Research, Division of Warner-Lambert Company, Ann Arbor, MI 48105.

2 Department of Pathology, University of Michigan, Ann Arbor, MI 48109.

3 Department of Pharmacokinetics, Dynamics, and Metabolism, Parke-Davis Pharmaceutical Research, Division of Warner-Lambert Company, Ann Arbor, MI 48105.
a Address correspondence to this author at: 2800 Plymouth Road, Ann Arbor, MI 48105. Fax 734-622-7428; e-mail Alan.Kugler{at}wl.com.

The cross-reactivity of fosphenytoin, a phosphate ester prodrug of phenytoin, was investigated in the Abbott phenytoin TDx®/TDxFLxTM fluorescence polarization immunoassay (TDx) and the Behring Diagnostics phenytoin Emit® 2000 enzyme-multiplied immunoassay (Emit). The first part of our study investigating cross-reactivity utilized in vitro correlation of the two immunoassays with a validated and specific phenytoin HPLC method used to assay plasma samples prepared in several phenytoin and fosphenytoin concentration combinations. Fosphenytoin cross-reacted with both immunoassays, but to a greater extent with TDx. In the second part of the study, empirically-derived models that best explained the in vitro data were used to predict "immunoassay-derived" phenytoin concentrations in plasma samples collected from actual patients after intravenous (IV) or intramuscular (IM) fosphenytoin dosing. The greatest degree of phenytoin concentration overestimation occurred at times when fosphenytoin concentrations were highest: within 1 to 2 h after IV infusion or during the first 2 to 4 h after IM injection. It is recommended that phenytoin concentrations not be monitored using these or other potentially nonspecific immunoanalytical methods for at least 2 h after IV fosphenytoin infusion or 4 h after IM fosphenytoin injection.




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T. M. Annesley, S. Kurzyniec, G. D. Nordblom, N. Buchanan, W. Pool, M. Reily, R. Talaat, and W. L. Roberts
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W. L. Roberts, B. K. De, J. P. Coleman, and T. M. Annesley
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