Effect of prostate-specific membrane antigen (PSMA) radioimmunotherapy on circulating tumor cell (CTC) count.

199 Background: CTC counts with CellSearch methodology are prognostic; changes following therapy may be associated with improved outcomes. Since FDA clearance, baseline and follow up CTC counts were included in studies of radiolabeled J591. The effect of J591 alone (without an effector molecule) is unknown. Methods: Our phase II single-dose 177Lu-J591 (Cohort 3), phase I fractionated dose 177Lu-J591 (expansion cohorts), and phase I fractionated 177Lu-J591 + docetaxel (all pts) trials were analyzed. Any pt with baseline and at least 1 follow-up CTC count (CellSearch) was included in the analysis. A retrospective subset of patients undergoing J591-based imaging without an effector molecule attached was also analyzed. Results: 48 pts received 177Lu-J591 with prospectively measured CTC counts with median age of 73.7 years. 26 of 48 (54.2%) had unfavorable (≥5 CTCs/7.5 mL blood) baseline CTCs (median 29, range 5-449 CTCs/7.5 mL). Of the 23 with ≥5 baseline CTCs and available 4-6 wk post-treatment counts, 22 (9...