Fosphenytoin is the water-soluble disodium phosphate ester of phenytoin. Although it is water-soluble, less than 5% of fosphenytoin is excreted via direct renal clearance in healthy subjects,39 probably because it is rapidly converted to phenytoin. The rate of conversion of fosphenytoin to phenytoin may be greater in patients with renal insufficiency than in healthy subjects because protein binding of fosphenytoin is decreased with renal insufficiency.50
The extent of removal of fosphenytoin by hemodialysis is unknown.49 Its water solubility would suggest that it is subject to removal, but its rapid conversion to phenytoin, which is not removed, may suggest that little fosphenytoin would be removed during hemodialysis. It may be prudent to use injectable sodium phenytoin rather than fosphenytoin to administer phenytoin during dialysis.
Reviewed and revised February 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.
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