Phenytoin interaction with enteral feedings administered through nasogastric tubes

Inadequate drug plasma levels have been associated with the administration of phenytoin with enteral feedings through nasogastric (NG) tubes. It is demonstrated in this study that loss of phenytoin to tubing is a function of pH. Nonionized phenytoin is irreversibly bound to NG tubing from solution at the pH of enteral nutrient solutions while this is not the case for anionic phenytoin in unbuffered water or saline. In experiments pulsing phenytoin through glass vs NG tubing perfused with buffer at varying pH, reversible loss to tubing was observed at high pH while irreversible loss was observed at low pH. In addition, the irreversible loss of phenytoin was greater in NG tubing than glass particularly at low pH. It is suggested that in those cases where tubing is placed into the duodenum, inadequate gastrointestinal residence time for dissolution of phenytoin solid and suspension dosage forms coupled with irreversible drug loss from solution to NG tubing will result in decreased phenytoin absorption and subsequently lower drug plasma levels.