Development and clinical evaluation of ⁶⁴Cu-RAX301: A next-generation PSMA PET tracer for enhanced detection of metastatic prostate cancer

Prostate-specific membrane antigen (PSMA) PET imaging is widely used for diagnosing and staging prostate cancer, yet the currently approved ⁶⁸Ga- and ¹⁸F-labeled tracers are limited by short half-lives and high urinary excretion, which can obscure small pelvic lesions. To overcome these challenges, ⁶⁴Cu-RAX301 was developed as a high-affinity PSMA PET tracer designed for improved biochemical stability and extended imaging flexibility. The RAX301 precursor and its nat Cu-RAX301 analog were synthesized with high purity, and radiolabeling with ⁶⁴Cu was optimized to achieve high molar activity (≥2000 mCi/μmol) and radiochemical purity (>95%), and remaining stable for over 48 hours post-formulation. Surface plasmon resonance revealed sub-picomolar binding affinity (Kd < 1 pM) for both the precursor and nat Cu-RAX301, markedly stronger than PSMA-617 (22 pM). Consistent with this, cell-based assays showed two-fold higher affinity, 1.5-fold greater uptake, and four-fold higher internalization compared with ⁶⁴Cu-PSMA-617 (≈80% vs. ≈20%). In LNCaP xenograft mice, ⁶⁴Cu-RAX301 demonstrated intense and persistent tumor uptake, with tumor-to-muscle ratios exceeding one hundred at 4 hours post-injection. PET imaging in Macaca fascicularis revealed primarily renal clearance with minimal uptake in other organs. In a first-in-human study of ten patients with metastatic castration-resistant prostate cancer (mCRPC), sequential PET/CT scans were performed first with ⁶⁸Ga-PSMA-11 (5 mCi; imaging at 1 hour post-injection) and then with ⁶⁴Cu-RAX301 (5 mCi; imaging at 4 and 24 hours post-injection), separated by 1-7 days. ⁶⁴Cu-RAX301 detected all lesions identified by ⁶⁸Ga-PSMA-11 while providing higher lesion uptake and superior tumor-to-background contrast. Notably, ⁶⁴Cu-RAX301 revealed numerous additional small lesions at various sites, including pelvic lymph node metastases that were frequently missed by ⁶⁸Ga-PSMA-11 due to early bladder activity. The tracer exhibited an excellent safety profile with no significant adverse events. Collectively, these preclinical and early clinical findings demonstrate that ⁶⁴Cu-RAX301 possesses ultra-high PSMA affinity, favorable pharmacokinetics, and enhanced lesion visualization compared with ⁶⁸Ga-PSMA-11, supporting its potential as a next-generation PSMA PET imaging agent with improved sensitivity and diagnostic performance in metastatic prostate cancer. Ongoing clinical studies aim to further define its diagnostic impact and broader clinical utility.