First-in-human PET imaging of ⁶⁸ Ga-FC516, a novel ACP3-targeted radiotracer for prostate cancer.

5128 Background: Prostatic acid phosphatase (ACP3) is overexpressed in more than 85% of prostate cancers, and largely absent in normal tissues, resulting in over 100-fold tumor-to-background specificity. ACP3 represents a differentiated imaging biomarker that may complement prostate-specific membrane antigen (PSMA), particularly in PSMA-low or PSMA-treated patients. We developed 68 Ga-FC516, a novel ACP3-targeted radiotracer, and evaluated its preclinical characteristics and first-in-human imaging performance. Methods: 68 Ga-FC516 was studied preclinically using in vitro assays (radiochemical stability, enzyme inhibition, cellular uptake, internalization, and efflux) and in vivo PET/CT studies, including blocking and biodistribution experiments. A first-in-human exploratory clinical trial was conducted in healthy volunteers and prostate cancer patients. The primary objective was to assess safety and radiation dosimetry; secondary objectives included biodistribution assessment and lesion detection. PET/CT imaging results were compared with PSMA radiotracers or 68 Ga-OncoACP3-DOTA when possible. Results: 68 Ga-FC516 was labeled efficiently with radiochemical purity exceeding 95%. In in vitro studies, 68 Ga-FC516 demonstrated potent target binding, long cellular retention and high stability. ACP3-specific tumor uptake of 68 Ga-FC516 was confirmed via an in vivo blocking experiment. The tracer showed low background uptake in liver, kidneys, and salivary glands, with tumor uptake exceeding 16.4% ID/g in a biodistribution study. Gallbladder uptake was markedly lower than that of 68 Ga-OncoACP3-DOTA (2.6% vs. 35.0% ID/g) at 1 hour post-injection. In an ongoing clinical trial in metastatic prostate cancer patients, increased uptake of 68 Ga-FC516 (SUV max in PET/CT) was observed at 1 to 3 hours across all lesions, with improved tumor-to-kidney ratios over time. The tracer also demonstrated low background uptake, including in salivary glands and kidneys, which was significantly lower than that of PSMA-targeted tracers. Notably, a patient with both prostate and rectal cancers showed strong uptake in the prostate tumor (SUV max 44), but no uptake in the rectal lesion, further establishing the tracer's superior specificity. No adverse events were observed. Conclusions: We have shown that 68 Ga-FC516 has high ACP3-targeting specificity, low background uptake and excellent imaging performance in preclinical and first-in-human clinical studies. These findings support the potential of ACP3 as a differentiated biomarker for prostate cancer diagnosis and patient stratification, complementing PSMA PET and addressing unmet needs in PSMA-low or PSMA-treated patient populations. The clinical validation study was approved by the Ethics Committee of the First Affiliated Hospital of Jinan University (KY-2025-254). Research Sponsor: FindCure Biosciences Co., Ltd. Clinical trial information: KY-2025-254.