A friend of mine stopped Zytiga and his PSA went down. I'm familiar with bicalutamide withdrawal syndrome: PSA increases after a while taking Casodex but goes down when Casodex is stopped. This is caused by the Androgen Receptor using bicalutamide to become activated, and is well known. I've even seen reports of this occurring with Xtandi (see below). But Zytiga is not an antiandrogen per se -- it's an enzyme blocker, so I didn't understand how this can happen.
onlinelibrary.wiley.com/doi...
In going over my friend's PSA pattern with his oncologist, his oncologist told me he's seen it too. He said that a metabolite of abiraterone is able to act as a ligand (activator) of the androgen receptor (AR). It is a minor effect compared to the powerful androgen-suppressing effect of abiraterone. But as the AR is amplified and more mutations occur in it, it can sometimes become noticeable.
I was wondering if anyone in this forum has noticed this: PSA went down when Zytiga was stopped.