Capecitabine-phenytoin interaction is dose dependent with an unexpected time course.

The main objective of this study was to further analyze the drug interaction between capecitabine and phenytoin with special emphasis on magnitude, timing, and the dose effect. A single patient had multiple phenytoin levels at various doses of capecitabine. Phenytoin levels were subjected to analysis of variance and the dose effect was approached using a multiple comparison test. All phenytoin levels meeting the criteria for steady state were subjected to box plot analysis. Analysis of 35 phenytoin levels over a 25-month period demonstrated a statistically significant interaction between the medications. The interaction onset was delayed by 4 weeks and persisted up to 9 weeks after capecitabine dose increases or decreases. The magnitude of interaction was directly proportional to the capecitabine dosage. We concluded that capecitabine caused a marked increase in phenytoin levels that was capecitine dose dependent, with a delayed time course not predicted by phenytoin kinetics, suggesting a novel interaction mechanism. The magnitude and complexity of this interaction suggest that alternate antiepileptic medications should be used in combination with capecitabine.