Automated enumeration and phenotypic characterization of CTCs and tdEVs in patients with metastatic castration resistant prostate cancer

BACKGROUND: Although most patients with metastatic castration-resistant prostate cancer (mCRPC) initially benefit from treatment with androgen receptor signaling inhibitors (ARSi), resistance inevitably occurs. Hence, we investigated the prognostic value of automated circulating tumor cell (CTC) and tumor-derived extracellular vesicle (tdEV) enumeration and their dynamics, in patients with mCRPC in the context of the initiation of treatment with ARSi. Furthermore, we hypothesize that CTC phenotypic heterogeneity might serve as a measurable biomarker under these circumstances. METHODS: Using an image analysis tool, we reanalyzed all CellSearch images previously acquired in the context of a prospective, multicenter clinical study for patients with mCRPC (n = 170) starting a new line of ARSi, for CTC and tdEV detection and enumeration. CTC (n = 19 129) phenotypic diversity was quantified by the Shannon index (SI). Progression-free survival (PFS) and overall survival (OS) were compared between groups of patients stratified according to CTC, tdEV, and SI levels. RESULTS: Automated CTC enumeration provided similar clinical prognostication compared with operator-based counts. Patients demonstrating high CTC phenotypic heterogeneity before therapy had a shorter median PFS (4.82 vs. 8.49 months, HR 1.79; P = 0.03) and OS (12.6 months vs. not reached, HR 2.32; P = 0.03), compared to patients with low diversity, irrespective of CTC level. Multivariable analysis showed how the prognostic value of the baseline SI was lost by pretreatment chemotherapy status, CTC counts, and PSA levels. CONCLUSIONS: Automated CTC counts are a reliable substitute for reviewer-based enumeration, as they are equally informative for prognosis assessment in patients with mCRPC. Beyond enumeration, we demonstrated the added value of studying CTC phenotypic diversity for patient prognostication, warranting future investigation.