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Switching between phenytoin generics in patients with epilepsy may lead to increased risk of breakthrough seizure: chart analysis and practice recommendations.

OBJECTIVE
The Food and Drug Administration (FDA) only requires bioequivalence testing of generic substitutions in order for them to be deemed equivalen to the original product. There may be a large difference of bioavailability among the generic drugs that especially have a narrow therapeutic index, and this may affect clinical outcomes. We aimed to determine whether switching from generic-to-generic equivalent anti-epileptic drugs (AEDs) in patients with epilepsy is associated with clinical outcomes.


METHODS
We performed a retrospective study using the electronic medical records of a tertiary hospital. Adults with a history of epilepsy who used a generic phenytoin and whose therapy was switched to another generic phenytoin between January 2012 and June 2013 were included (n = 80). We compared the drug concentration of phenytoin and seizure events before and after the switch.


RESULTS
After switching their generic phenytoin, 33 out of 80 patients (41%) suffered from increasing seizure events (pre-interchange period, 0.44 ± 0.97; post-interchange period, 1.24 ± 2.05; p < 0.0001). The number of medical visits for acute seizure significantly increased in the post-interchange period. The phenytoin serum concentration of all the patients was lesser in the post-interchange period than in the pre-interchange period. (pre-interchange period, 12.79 μg/mL; post-interchange period, 6.36 μg/mL; p < 0.0001). Among the patients with drug resistant epilepsy (DRE), 17 patients (84.2%) had increasing seizure events in the post-interchange period.


CONCLUSIONS
We confirmed that there was a significant difference in bioavailability between generic phenytoin. Therefore, when using or switching generic anti-epileptic drugs, therapeutic drug monitoring must be done, and the patients' condition must be considered.