Cross-reactivity of fosphenytoin in two human plasma phenytoin immunoassays.
The cross-reactivity of fosphenytoin, a phosphate ester prodrug of phenytoin, was investigated in the Abbott phenytoin TDx/TDxFLx 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 (i.v.) or intramuscular (i.m.) fosphenytoin dosing. The greatest degree of phenytoin concentration overestimation occurred at times when fosphenytoin concentrations were highest: within 1 to 2 h after i.v. infusion or during the first 2 to 4 h after i.m. injection. It is recommended that phenytoin concentrations not be monitored using these or other potentially nonspecific immunoanalytical methods for at least 2 h after i.v. fosphenytoin infusion or 4 h after i.m. fosphenytoin injection.
Kugler
A R
AR
Department of CNS Clinical Research and Development, Parke-Davis Pharmaceutical Research, Division of Warner-Lambert Company, Ann Arbor, MI 48105, USA. Alan.Kugler@wl.com
Annesley
T M
TM
Nordblom
G D
GD
Koup
J R
JR
Olson
S C
SC
D000927:Anticonvulsants / Q000008:administration & dosage / Q000097:blood*; D002851:Chromatography, High Pressure Liquid; D003429:Cross Reactions; D016231:Fluorescence Polarization Immunoassay; D006801:Humans; D007124:Immunoenzyme Techniques; D007274:Injections, Intraperitoneal; D007275:Injections, Intravenous; D008954:Models, Biological; D010672:Phenytoin / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000097:blood; D011237:Predictive Value of Tests; D015203:Reproducibility of Results
D000927:Anticonvulsants; D010672:Phenytoin; C043114:fosphenytoin