- No cytochrome P450 3A4 (CYP3A4) inducers within 7 days prior to starting protocol therapy and while on protocol treatment. CYP3A4 inducers: phenytoin, carbamazepine, barbiturates, rifampicin, dexamethasone, and St John's Wort; single doses of dexamethasone used as an antiemetic are permitted