A change is suggested by Dr. However, Since Xtandi has a 1/2 life about 6 days, from the day you stop the amount of Xtandi starts decaying. So, Apalutamide has a half life of about 3 days, but has a fast absorption rate. This means , if you stop xtandi on day 1 and start Apalutamide on day 2, there is a period where both meds overlap in your system.
you could wait, 3-4 half life’s of xtandi for example. So, I think then you have a decay 1/2x1/2=1/4x1/2=1/8x1/2=1/16th in your body left on average, and so on. So, in this case, you have waited 3x6=18 days on avg, before starting Apalutamide. This would be a drug interaction safety approach.
on the other side of this balance beam , is your Pca safety. Assume the drug switch is due to sense that xtandi is failing to some extent. If you take a wait 3 or 4 xtandi 1/2 lives to take Apalutamide, there may be a risk that your shield against your Pca is being lowered to some extent , presumably giving your Pca a new defense hole to come through, thereby increasing danger in the cancer risk side.
So how do you find the optimal transition timing?