PSMA+ Extracellular Vesicles are a Biomarker for SABR in Oligorecurrent Prostate Cancer Analysis from the STOMP-like and ORIOLE trial cohorts

Purpose: Two randomized clinical trials (STOMP and ORIOLE) demonstrated that stereotactic ablative radiotherapy (SABR) can prolong ADT-free survival or progression-free survival (PFS) in patients with metachronous oligometastatic prostate cancer (omCSPC) patients. While most omCSPC patients have a more modest delay in progression, a small subset achieves a durable response following SABR. We investigated the prognostic and predictive value of circulating PSMA-positive extracellular vesicles (PSMA+EV) and prostate specific antigen (PSA) in a biomarker correlative study using blood samples from three independent patient cohorts. Methods: Plasma samples from 46 patients on the ORIOLE trial and 127 patietns on the STOMP trial protocol with omCSPC patients treated with SABR. Pre-SABR PSMA+EV levels (EVs/ml) were measured by nanoscale flow cytometry. Kaplan-Meier curves and logistic regression models were used to determine the association of PSMA+EV and PSA levels with clinical outcomes. Results: In the pooled cohorts, median bPFS were 26.1 and 15.0 months (p=0.005) and median rPFS were 36.0 and 25.0 months (p=0.003) for PSMA+EV low and high groups, respectively. The combination of pre-SABR low levels of both PSMA+EV and PSA was associated with lower risk of radiographic progression (HR=0.34, 95% CI: 0.18-0.58, p=0.0002). In the ORIOLE cohort, which included both a SABR arm and an observation arm, low PSMA+EV was predictive of benefit from SABR (p=0.012). Conclusions: PSMA+EV is a novel prognostic and predictive biomarker of radiographically occult tumor burden in omCSPC. PSMA+EV may inform clinical decisions regarding which patients achieve a durable benefit from consolidative SABR alone.