High androgen levels can dramatically reduce ARSI AR-blocking potency.
A 2019 study found that increasing the synthetic androgen R1881 from 0.1 to 10 nM (a 100-fold increase) raised Nubeqa's IC50 from 80 to 6,560 nM. Study has data for Nubeqa, Xtandi, Erleada, and Casodex.
That's an 82-fold increase in the concentration of Nubeqa needed to achieve 50% inhibition of androgen receptor activity.
Implication for BAT: Supraphysiologic androgen levels can substantially overcome Nubeqa's competitive AR blockade, supporting the rationale for separating Nubeqa and high-testosterone treatment windows.
These were laboratory experiments, not clinical measurements of AR blockade.
Source: Sugawara et al. (2019), International Journal of Cancer, Table 1.