# Efficacy of Switching to Levetiracetam After S-1-Induced Phenytoin Concentration Increase: A Case Report

## TL;DR

A patient with gastric cancer successfully switched from phenytoin to levetiracetam to avoid drug interactions with S-1, maintaining seizure control.

## Contribution

This case report presents a successful strategy for managing phenytoin level increases caused by S-1 through switching to levetiracetam.

## Key findings

- Switching from phenytoin to levetiracetam resolved symptoms caused by elevated phenytoin levels due to S-1 interaction.
- The patient remained seizure-free for nine treatment cycles after the switch to levetiracetam.
- Gradual phenytoin dose reduction and levetiracetam introduction improved patient outcomes.

## Abstract

S-1, an oral anticancer drug, interacts with phenytoin (PHT) to increase PHT serum concentration. Although the PHT dosage is usually adjusted, few studies have examined the effects of switching from PHT to another antiepileptic drug. Here, we report the details of a case in which a patient with gastric cancer continued adjuvant chemotherapy after switching from PHT to levetiracetam (LEV) to prevent interactions with S-1. A man in his 60s with advanced gastric cancer received adjuvant chemotherapy with S-1 120 mg/day (cycles of two weeks of administration followed by one week of rest). He had experienced generalized tonic-clonic seizures 48 years prior and had been taking PHT (170 mg/day) and carbamazepine (250 mg/day). On day 22 of treatment, the PHT concentration increased from 3.72 to 11.76 µg/mL. On day 25, he developed dizziness and fell. Gradual PHT dose reduction and a switch to LEV improved his symptoms, and he remained seizure-free over nine treatment cycles. The findings of this case report suggest an approach for switching from PHT to another antiepileptic drug when PHT levels increase due to interactions with S-1. Switching to LEV may result in fewer interactions.