LAUSR

273Background: A previous analysis of 161 patients (pts) tested for nuclear-localized AR-V7(+) CTCs showed a therapy interaction between AR-V7 positivity and improved overall survival (OS) on taxane chemotherapy vs. androgen receptor signaling inhibitors (ARSI). To validate the use of the biomarker result for physician decision making, we prospectively analyzed an independent, multicenter, blinded, cross-sectional cohort (n = 225) to confirm a therapy interaction with AR-V7. We corrected for possible pt selection bias by the treating physician by analyzing the association of therapy to OS in low and high risk groups defined by the test cohort. Methods: Two analyses were performed: (1) the validation of a therapy interaction between AR-V7 positivity and superior OS benefit for pts treated with taxanes in the context of use for 2nd+ line pts; and (2) as the choice between ARSI or taxanes was at the discretion of the attending physician, pt risk was incorporated into the predictive biomarker assessment. A pt...