Background: EC1169 is a small molecule drug conjugate designed to bind to prostate cancer (PCa) cells via prostate specific membrane antigen (PSMA) and, following endocytosis, to then release tubulysin B hydrazide (potent antimitotic). EC1169 administration to mice bearing PSMA-positive LNCaP and MDA PCa 2b xenografts has resulted in complete remissions and cures. EC0652, a novel tracer composed of a technetium-99m chelator and a PSMA-targeting moiety, is being developed to characterize whole body PSMA expression in real time. Rapid PSMA-specific uptake of ^99mTc-EC0652 into tumor tissue and rapid clearance from normal tissue results in enhanced tumor-to-background ratios to localize PSMA expressing tissue on SPECT imaging. Methods: Key inclusion criteria are ECOG PS 0-1, adequate organ function, and a diagnosis of mCRPC. Patients (pts) undergo ^99mTc-EC0652 SPECT scan prior to therapy. EC1169 is administered intravenously on 1 of 2 schedules every 21 days: 3 times weekly with 1 week off (TIW regimen), or once a week for 2 weeks with 1 week off (QW regimen). Dose escalation is based upon the “3+3” method. Study objectives include determination of EC1169’s maximal tolerated dose, safety, pharmacokinetics, and anti-tumor activity. ^99mTc-EC0652 safety will be characterized. Results: Twenty-one pts are evaluable for cycle 1 toxicity (TIW: 5 pts; QW: 16 pts). All pts are Caucasian. Median age is 69.0 (range: 53-82). Median time on study is 6.7 weeks (range: 0.1-33.1). Median number (range) of administered EC1169 cycles is 4 (3-4) for TIW pts, and 2 (1-11) for QW pts. 9 of 21 pts have had drug-related (DR) adverse events (AE’s). No DRAE has occurred in > 1 TIW pt. Vomiting and fatigue were the only DRAE’s reported in ≥ 2 QW pts. There have been no on-study deaths, DR Grade 3-4 toxicity or serious adverse events, or dose limiting toxicity. 3 pts have demonstrated stable disease lasting more than 4 cycles. To date, no patient has demonstrated a 50% decrease in PSA. Dose proportionate increases in both Cmax and AUC for EC1169 have been observed. Conclusions: Both EC1169 and ^99mTc-EC0652 appear to be well tolerated in mCRPC pts. Dose escalation and PK analyses are ongoing. Clinical trial information: NCT02202447