Development and first-in-human study of PSMA-targeted PET tracers with improved pharmacokinetic properties.

PURPOSE: A series of new 68 Ga-labeled tracers based on [68 Ga]Ga-PSMA-617 were developed to augment the tumor-to-kidney ratio and reduce the activity accumulation in bladder, ultimately minimize radiation toxicity to the urinary system.

METHODS: We introduced quinoline group, phenylalanine and decanoic acid into different tracers to enhance their lipophilicity, strategically limiting their metabolic pathway through the urinary system. Their binding affinity onto LNCaP cells was determined through in vitro saturation assays and competition binding assays. In vivo metabolic study, PET imaging and biodistribution experiment were performed in LNCaP tumor-bearing B-NSG male mice. The most promising tracer was selected for first-in-human study.

RESULTS: Four radiotracers were synthesized with radiochemical purity (RCP) > 95% and molar activity in a range of 20.0-25.5 GBq/μmol. The binding affinities (Ki ) of TWS01, TWS02 to PSMA were in the low nanomolar range (< 10 nM), while TWS03 and TWS04 exhibited binding affinities with Ki  > 20 nM (59.42 nM for TWS03 and 37.14 nM for TWS04). All radiotracers exhibited high stability in vivo except [68 Ga]Ga-TWS03. Micro PET/CT imaging and biodistribution analysis revealed that [68 Ga]Ga-TWS02 enabled clear tumor visualization in PET images at 1.5 h post-injection, with higher tumor-to-kidney ratio (T/K, 0.93) and tumor-to-muscle ratio (T/M, 107.62) compared with [68 Ga]Ga-PSMA-617 (T/K: 0.39, T/M: 15.01) and [68 Ga]Ga-PSMA-11 (T/K: 0.15, T/M: 24.00). In first-in-human study, [68 Ga]Ga-TWS02 effectively detected PCa-associated lesions including primary and metastatic lesions, with lower accumulation in urinary system, suggesting that [68 Ga]Ga-TWS02 might be applied in the detection of bladder invasion, with minimized radiation toxicity to the urinary system.

CONCLUSION: Introduction of quinoline group, phenylalanine and decanoic acid into different tracers can modulate the binding affinity and pharmacokinetics of PSMA in vivo. [68 Ga]Ga-TWS02 showed high binding affinity to PSMA, excellent pharmacokinetic properties and clear imaging of PCa-associated lesions, making it a promising radiotracer for the clinical diagnosis of PCa. Moreover, TWS02 with a chelator DOTA could also label 177 Lu and 225 Ac, which could be used for PCa treatment without significant side effects.

TRIAL REGISTRATION: The clinical evaluation of this study was registered On October 30, 2021 at https://www.chictr.org.cn/ (No: ChiCTR2100052545).