What is the minimum duration to be on bicalutamide prior to starting GnRH agonist in de novo metastatic prostate cancer with significant bone disease?
How long before starting GnRH agonist do you initiate bicalutamide in a patient with significant prostate cancer mets?
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The onset of action for bicalutamide is rapid--typically slightly more than a day to reach maximum/peak concentration. In contrast, the pharmacological effect of leuprolide is related to the time to testosterone suppression which occurs approximately 2-4 weeks after starting. So the time to testosterone flare is typically during weeks 1-3 after starting. I typically start both treatments simultaneously and have the patient take bicalutamide for 4 weeks before stopping. This is also when I usually have the patient come back to the clinic to discuss their NGS test results and decide on which intensified therapy combination is optimal for them.
The risk of testosterone flare by starting both at the same time is almost zero because of the very rapid onset of action of bicalutamide. If I am treating a patient in the hospital that has been hospitalized because of tumor-related pain then I will start bicalutamide and then consider adding ADT after discharge or a few days to 1 week after we have better control of their presenting symptom(s).