Background: LuPARP (NCT03874884) is evaluating the safety and efficacy of olaparib in combination with ¹⁷⁷Lu-PSMA-617 in patients (pts) with mCRPC who have progressed on an androgen receptor pathway inhibitors and docetaxel. We aim to identify potential prognostic and predictive biomarkers from serial CTC profiling. Methods: Twenty-six pts with high PSMA expression on PSMA-PET received 7.4 GBq of ¹⁷⁷Lu-PSMA-617 every 6 weeks (wk) together with an escalating dose-schedule of olaparib (50mg BD – 300mg BD, days 2 to 14 or days -4 to 14) for up to 6 cycles. For CTC analysis, 10 mls of blood was collected at baseline, 12-weekly for 48 wk, and thereafter every 24 wk and at disease progression. CK+, CD45-, and DAPI+ CTCs were enumerated from 3 mls of blood and immunoassayed for PSMA expression using the Epic Sciences platform. Correlations between total and PSMA+ CTC counts and PSA50 response (PSA reduction of ≥ 50%) were evaluated. Low pass whole genome sequencing (lpWGS) was performed on baseline and longitudinal CTC samples to identify copy number alterations. Results: At baseline, 23 of 26 pts (88%) had detectable CTCs (median 2.9 CTC/ml, range 0-30), and of these 23 pts, 17 (74%) had detectable PSMA+ CTCs (median 0.9 CTC/ml, range 0.3-27), indicating heterogeneity of PSMA expression in CTCs. The CTC positivity rates (% of cases ≥1 CTC) at week (w)12, w24, w36, w48 and at disease progression were 57%, 58%, 75%, 36% and 100%, and the PSMA positivity rates (% of cases ≥1 PSMA+ CTC) were 38%, 36%, 22%, 50% and 56%, respectively. Fifteen of 20 (75%) evaluable pts with paired baseline and w12 CTCs had a ≥50% decline in total CTC count, of which 9/15 (60%) pts also achieved a PSA50 response. Five of these 9 (56%) pts had 100% CTC clearance at w12. In terms of PSMA+ CTCs, 14/15 (93%) evaluable pts had ≥50% PSMA+ CTC decline with 13 pts (93%) achieving complete CTC clearance by w12. Six of the 13 pts with complete PSMA+ CTC clearance at w12 also achieved a PSA50 response. Beyond PSMA expression, genomic heterogeneity was evident in baseline and on-treatment CTCs, including recurrent loss of PTEN, TP53, BRCA2, ATM, and RB1, and gain of AR and MYC. lpWGS in 10 baseline samples identified 5 cases with BRCA2 loss and 5 cases with ATM loss. Two pts with BRCA2 loss and 3 pts with ATM loss had a PSA50 response by w12. Conclusions: We observed high rates of total and PSMA+ CTCs in PSMA-expressing mCRPC. Total and PSMA+ CTCs decline with combined olaparib and ¹⁷⁷Lu-PSMA-617 treatment. Early declines in total and PSMA+ CTCs including 12w CTC clearance paralleled PSA responses. Notably, total and PSMA+ CTCs rise again in the setting of disease progression. Serial CTC profiling may assist in tracking response to ¹⁷⁷Lu-PSMA-617 therapy. Clinical trial information: NCT03874884.