For those of you who are/were on Zytiga and Xtandi at the same time, I have a question. Were the doses and/or frequency adjusted to comply with the different metabolism rates of the two drugs when given together or were you treated with the standard doses of each drug? My Oncologist told me that he could not prescribe both drugs at the same time because they interact adversely because of their metabolism rates. I believe he said that the blood serum level of one goes up and one goes down. I do not remember which one went up and which one went down.
From the Full Prescribing Information under Drug Interactions for Xtandi and Zytiga is the following information.
From the Full Prescribing Information for Xtandi - Drug Interactions:
Avoid CYP3A4, CYP2C9 and CYP2C19 substrates with a narrow therapeutic index, as XTANDI may decrease the plasma exposures of these drugs.
From the Full Prescribing Information for Zytiga - Drug Interactions:
CYP3A4 Inducers: Avoid concomitant strong CYP3A4 inducers during ZYTIGA treatment. If a strong CYP3A4 inducer must be co-administered, increase the ZYTIGA dosing frequency.
CYP3A4 is also known as Cytochrome P450 3A4.