- Any patient requiring cytochrome P450 family 3, subfamily A, polypeptide 4 (CYP3A4) isoform-inducing drugs (e.g. phenytoin, phenobarbital, carbamazepine, rifampin, rifabutin, ketoconazole, St. John's wort) will be excluded; CYP3A4-inducing drugs should be discontinued at least 2 weeks prior to the first cycle of irinotecan