Most people taking Xtandi (enzalutamide) see their PSA levels begin to drop within the first few weeks of treatment, with the most meaningful changes typically measured at the 90-day mark. In clinical trials, PSA was checked as early as one week after starting treatment, then again at weeks 5 and 13, because declines can happen that quickly. But the speed and depth of the drop vary significantly from person to person, and those differences carry real meaning for long-term outcomes.
What Happens in the First 4 to 12 Weeks
Xtandi works by blocking the androgen receptor, the protein that testosterone uses to fuel prostate cancer growth. It does this at multiple points: it prevents testosterone from attaching to the receptor, stops the receptor from entering the cell nucleus, and blocks it from activating the DNA that drives tumor growth and PSA production. Because PSA is itself produced through this androgen signaling pathway, shutting down the pathway causes PSA levels to fall.
In a study of patients with metastatic castration-resistant prostate cancer, researchers grouped men by how much their PSA dropped within the first 90 days. About 60% of men on enzalutamide achieved at least a 30% PSA decline in the first four weeks alone. Among those who didn’t hit that threshold at four weeks, roughly a third still reached it by 12 weeks, meaning the drug was working but needed more time to show measurable results.
This matters because prostate cancer guidelines specifically discourage making treatment decisions based on PSA changes in the first 12 weeks. Early PSA fluctuations, including temporary rises, can occur and don’t necessarily mean the drug isn’t working. The standard clinical approach is to check PSA at weeks 1, 5, and 13, then every 12 weeks after that.
Why the Speed of Your PSA Drop Matters
The depth of PSA decline in those first 90 days is one of the strongest early signals of how well Xtandi is working. In a large analysis of men with metastatic castration-resistant disease who had previously received chemotherapy, any PSA decline within 90 days was linked to significantly longer overall survival, longer time before the cancer progressed on imaging, and better pain control compared to men whose PSA didn’t drop at all.
The bigger the drop, the better the outlook. Men who achieved a 90% or greater PSA decline within 90 days had the best pain response rates (about 63%) compared to 32% among men whose PSA rose or stayed flat. Median overall survival for men with no PSA decline was 12.7 months. For men with confirmed PSA declines of any size, median survival hadn’t even been reached at the time of analysis, meaning most were still alive.
On the flip side, no PSA decline by 90 days was a clear warning sign. It didn’t guarantee the drug was failing, but it was a poor prognostic indicator that typically prompted doctors to reassess the treatment plan.
Can PSA Rise Before It Falls?
A temporary PSA increase in the early weeks of treatment is possible, though it’s more commonly discussed with chemotherapy drugs like docetaxel, where PSA flares occur in roughly one-third of patients. With Xtandi, true PSA flares are less common, but they do happen. This is one reason clinical guidelines recommend waiting at least 12 weeks before drawing conclusions about whether the drug is effective.
If your PSA rises slightly in the first month or two, it doesn’t automatically mean Xtandi isn’t working. Some men who fail to show a 30% decline at four weeks go on to achieve that decline by week 12, and their long-term survival is significantly better than men who never see a drop at either time point.
How Long PSA Stays Suppressed
Even when Xtandi successfully lowers PSA, the suppression doesn’t last forever. In the PREVAIL trial, which studied men with metastatic castration-resistant prostate cancer who hadn’t yet received chemotherapy, the median time until PSA started rising again was 11.2 months. The median duration patients stayed on the drug was 16.6 months, meaning many continued treatment even after PSA began to creep up, as long as clinical benefit continued.
For men with castration-sensitive disease (cancer that still responds to standard hormone therapy), Xtandi is now also approved and tends to produce deeper, longer-lasting PSA responses. In one specific setting, non-metastatic castration-sensitive prostate cancer with high-risk biochemical recurrence, treatment can actually be paused if PSA becomes undetectable (below 0.2 ng/mL) after 36 weeks. Treatment restarts if PSA climbs back above 2.0 ng/mL after surgery or above 5.0 ng/mL after radiation.
What to Expect at Your PSA Checks
Your oncologist will likely order PSA blood tests frequently in the early months: often at one month, then around five weeks, then at three months. After that initial period, testing typically shifts to every three months. These intervals mirror the schedule used in major clinical trials and give enough time to see whether the drug is producing a meaningful response.
A few practical points to keep in mind. First, a single PSA reading is less important than the trend over multiple tests. Second, PSA alone doesn’t tell the whole story. Some men have cancer that progresses on imaging even while PSA stays stable, and some have rising PSA without measurable disease progression. Your doctor will weigh PSA alongside imaging and how you’re feeling overall. Third, the 90-day window is the key early milestone. If your PSA has dropped meaningfully by then, it’s a genuinely encouraging sign for your longer-term response to the drug.