- GDC-0449 inhibits cytochrome P450, family 2, subfamily C, polypeptide 8 (CYP2C8), cytochrome P450, family 2, subfamily C, polypeptide 9 (CYP2C9), and cytochrome P450, family 2, subfamily C, polypeptide 19 (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