Dynamics of Circulating Tumor Cell Subsets Defined by PSMA and EpCAM Predict Survival in Metastatic Castration-Resistant Prostate Cancer Patients Treated with 177Lu-PSMA-617

Background: 177Lu-PSMA-617 radioligand treatment represents a major advancement in the management of metastatic castration-resistant prostate cancer (mCRPC), but key questions remain about tumor cell dynamics and evolution in patients who have received this drug. Here, we used a circulating tumor cell (CTC) capture assay to assess the prognostic value of CTC subsets defined by PSMA and EpCAM, and their dynamics, during 177Lu-PSMA-617 treatment in a cohort of mCRPC patients. Methods: Using a deep learning-based holographic imaging platform combined with in-flow protein marker analysis, we prospectively analyzed serial CTC samples from patients receiving 177Lu-PSMA-617 therapy at pre-treatment and on-treatment time points (n = 39 of 100 enrolled patients). PSMA and EpCAM expression on CTCs was determined by immunofluorescence staining. Proportions and absolute counts of CTC subsets delineated by PSMA and EpCAM expression were assessed in PSA50 responders (n = 19) and non-responders (n = 20). Within-patient changes in CTC subsets were compared using Wilcoxon signed-rank tests with Benjamini–Hochberg correction. Associations of CTC subset abundance and dynamics with overall survival were assessed using landmark Kaplan–Meier estimates (time zero at on-treatment sample collection) with log-rank tests and univariable Cox proportional hazards models. Results: PSMA and EpCAM expression defined three CTC subsets: PSMA+/EpCAM+, PSMA+/EpCAM−, and PSMA−/EpCAM+. The mean proportion of PSMA−/EpCAM+ CTCs increased following 177Lu-PSMA-617 in both PSA50 responders (6.2% to 18.7%; p = 0.041) and non-responders (7.9% to 15.4%; p = 0.023). This corresponded to a decrease in the proportion of PSMA+/EpCAM− CTCs, whereas the proportion of PSMA+/EpCAM+ CTCs did not decline. PSMA+ CTCs remained detectable in on-treatment samples from 37/39 patients, including 17/19 PSA50 responders. Pre-treatment counts of >5 CTCs, relative to ≤5 CTCs, were associated with shorter overall survival in the PSMA+/EpCAM+ subset (HR 3.66, 95%CI 1.35–9.94, p = 0.011), with a similar but nonsignificant trend in the PSMA+/EpCAM− subset (HR 7.00, 95%CI 0.94–51.9, p = 0.057), whereas on-treatment counts were not significantly associated with survival. Finally, on-treatment declines in PSMA+/EpCAM+ (HR 0.852, p = 0.731) and PSMA+/EpCAM− (HR 0.66, p = 0.305) CTC subsets were not significantly associated with overall survival relative to sustained high levels of these cells. Conclusions: In mCRPC patients receiving 177Lu-PSMA-617, PSMA+ CTCs frequently persist during treatment, and the pre-treatment abundance of PSMA+ CTC subsets may have prognostic value. These exploratory findings warrant validation in larger cohorts.