Background: In preclinical models of prostate cancer (PC), DSF reduced tumor growth when co-administered with copper (Cu). Further, intracellular Cu uptake is partially regulated by androgen-receptor signaling. Given these data, we conducted a phase Ib clinical trial of mCRPC patients (pts) receiving Cu with DSF. Methods: Pts with mCRPC were assigned to 1 of 3 cohorts: neuroendocrine PC (NEPC) (A), adenocarcinoma (adenoCA) mCRPC with non-liver/peritoneal metastases (B), and adenoCA mCRPC with liver and/or peritoneal metastases (C). IV CuCl₂ was given weekly for 3 doses with oral daily DSF. After CuCl₂ dosing, daily oral Cu gluconate was started and DSF continued until disease progression as defined by Prostate Cancer Working Group Three (PCWG3). DSF and metabolite MeDDC levels in plasma were sampled at 0, 2, 4, 6 and 8 hours after the first dose and on Cycle 2 Day 1, and measured by HPLC. DSF and MeDDC were evaluated for cytotoxicity in 3 PC cell lines (22Rv1, LnCAP, and PC3) and a prostate epithelial cell line (PWR-1E). mCRPC Cu avidity was measured by ⁶⁴Cu PET scan at baseline for all pts. Results: We treated 9 pts with mCRPC, 6 on cohort B and 3 on cohort C. No confirmed PSA declines or radiographic responses were observed in either cohort. Common adverse events included fatigue and psychomotor depression; no grade 4/5 AEs were observed. PK analysis: No DSF was detected in plasma (LOQ = 0.032 ng/mL, LOD = 0.01 ng/mL), whereas MeDDC was measurable in all study samples (LOQ = 0.512 ng/mL). MeDDC exhibited no cytotoxicity activity in PC cell lines. On ⁶⁴Cu PET scans, bone metastases showed differential and heterogeneous Cu uptake. Lymph node and pulmonary metastases were evaluable, but not liver metastases due to significant Cu uptake in the liver. Conclusions: Oral DSF is not an effective treatment for mCRPC because it is quickly metabolized into the non-cytotoxic inactive metabolite, MeDDC. As such, this trial has stopped enrollment. Further work is needed to identify a stable DSF formulation so that the conditional lethality of Cu and DSF may be evaluated for treatment of mCRPC. Clinical trial information: NCT02963051