238pcharacterization of Circulating Tumor Cells (ctcs) of Metastatic Castration Resistant Prostate Cancer (mcrpc) Patients in First, Second & Third Line Systemic Therapies.

Aim: New androgen signaling directed therapies (AR Tx), including abiraterone acetate plus prednisone (AA + P) and enzalutamide (E), prolong survival in patients (pts) with mCRPC. Used sequentially, response to E given after AA + P is of shorter duration and less frequent. Assessing the evolution of the disease to a more resistant phenotype following each treatment is necessary to develop more effective treatment(s). We examined changes in CTC subtypes along with AR expression (exp) and AR localization (LOC) in pts receiving the 1, 2 and 3+ line of systemic therapy for mCRPC. Methods: 110 blood samples from 88 unique pts (26 with no prior tx, 32 with 1 prior tx, and 52 with 2+ prior tx) were analyzed for CTCs utilizing the Epic Sciences platform and CellSearch®. Epic analysis included identification of four specific CTC subtypes: traditional CTCs (CK + , CD45cells, intact nuclei, morph distinct), CKCTCs (CK-, CD45-, intact nuclei, morph distinct), small CTCs (CK + , CD45-, intact nuclei, small cell size), and CTC clusters. CTC AR exp and AR subcellular LOC were evaluated. Results: CTCs and CTC subtype frequency increased with lines of systemic therapies. Heterogeneity of CTC subtypes, AR exp and LOC also increased.