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 androgen-deprivation therapy–free survival or progression-free survival (PFS) in patients with metachronous oligometastatic castration-sensitive prostate cancer (omCSPC). Although most patients with omCSPC 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 prostate-specific membrane antigen-positive (PSMA⁺) extracellular vesicles (EV) and PSA in a biomarker correlative study using blood samples from three independent patient cohorts. Experimental Design: Plasma samples from 46 patients with omCSPC on the ORIOLE trial and 127 patients with omCSPC on the STOMP trial protocol treated with SABR were included in the study. Pre-SABR PSMA⁺EV levels (EV/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, the median biochemical PFS were 26.1 and 15.0 months (P ¼ 0.005), and the median radiographic PFS were 36.0 and 25.0 months (P ¼ 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 a lower risk of radiographic progression (HR, 0.34, 95% confidence interval, 0.18–0.58; P ¼ 0.0002). In the ORIOLE cohort, which included both an SABR arm and an observation arm, low PSMA⁺EV was predictive of benefit from SABR (P ¼ 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 about identifying patients who will achieve a durable benefit from consolidative SABR alone.