The manufacturers predict that
phenytoin (and therefore possibly fosphenytoin) may lower the
serum levels of
sirolimus, probably because rifampicin (
rifampin), another CYP3A4 inducer, has been shown to do so. This prediction seems to be confirmed by 2 patients who needed an increase in their
sirolimus doses when they were given
phenytoin.
The extent of any change in
sirolimus levels is uncertain, but it would seem prudent to increase the frequency of monitoring
sirolimus levels, both during concurrent use and also if
phenytoin is withdrawn.