- GDC-0449 inhibits CYP2C8, CYP2C9, and CYP2C19 drug metabolism enzymes in vitro at concentrations that may be clinically relevant; therefore, caution should be exercised when dosing GDC-0449 concurrently with medications that are substrates of CYP2C8, CYP2C9, and CYP2C19 and have narrow therapeutic windows; in addition, GDC-0449 is a substrate of CYP3A4; however, the in vitro metabolic conversion of GDC-0449 is low; effects of CYP inducers (e.g., carbamazepine, phenobarbital, phenytoin, rifabutin, rifampin, St. John's wort, and troglitazone) on clinical concentrations of GDC-0449 are unknown; likewise, the effects of strong inhibitors of CYP3A4 (e.g., clarithromycin, erythromycin, itraconazole, ketoconazole, nefazodone, and telithromycin) on GDC-0449 clinical concentrations are unknown, and caution should be exercised when dosing GDC-0449 concurrently with inhibitors of CYP3A4