Recovery of phenytoin from an enteral nutrient formula.

The recovery of phenytoin from phenytoin oral suspension dispersed in an enteral nutrient formula was determined. The study was conducted in two phases. In phase 1, diluted phenytoin oral suspension was added to 10 1-mL samples of full-strength Osmolite and 10 1-mL samples of a distilled water control solution to produce a theoretical concentration of 10 micrograms/mL. The samples were filtered through an ultrafiltration membrane and assayed for phenytoin concentration by a homogeneous enzyme-multiplied immunoassay technique. In phase 2, varying amounts of diluted phenytoin oral suspension were added to 30-mL quantities of half-strength Osmolite or control solution to determine the effect of phenytoin concentration on recovery of phenytoin; also, a constant amount of diluted phenytoin oral suspension was added to 30-, 60-, and 90-mL quantities of half-strength Osmolite or control solution to determine the effect of solvent volume on recovery of phenytoin. Duplicate samples of each phase 2 mixture were filtered and assayed in the same manner as phase 1 samples. The mean concentration of phenytoin in phase 1 samples was 3.70 +/- 0.28 microgram/mL for Osmolite and 9.87 +/- 0.27 microgram/mL for control solution; this difference was significant. The percentage of phenytoin recovered from phase 2 samples of Osmolite increased with increasing phenytoin concentration and decreased with increasing volumes of Osmolite. The decreased recovery of phenytoin from the enteral nutrient formula used in this study has potential clinical importance, but further research in humans is needed to substantiate these in vitro observations.