Ga-labeled ODAP-Urea-based PSMA agents in prostate cancer: first-in-human imaging of an optimized agent.

Purpose: Prostate-specific membrane antigen (PSMA) is a promising target for prostate cancer imaging and therapy. The most commonly used scaffold incorporates a glutamate-urea (Glu-Urea) function. We recently developed oxalyldiaminopropionic acid-urea (ODAP-Urea) PSMA ligands in an attempt to improve upon the pharmacokinetic properties of existing agents. Here, we report the synthesis of an optimized Ga-labeled ODAP-Urea-based ligand, [Ga]Ga-P137, and first-in-human results.

Methods: Twelve ODAP-Urea-based ligands were synthesized and radiolabeled with Ga in high radiochemical yield and purity. Their PSMA inhibitory capacities were determined using the NAALADase assay. Radioligands were evaluated in mice-bearing 22Rv1 prostate tumors by microPET. Lead compound [Ga]Ga-P137 was evaluated for stability, cell uptake, and biodistribution. PET imaging of [Ga]Ga-P137 was performed in three patients head-to-head compared to [Ga]Ga-PSMA-617.

Results: Ligands were synthesized in 11.1-44.4% yield and > 95% purity. They have high affinity to PSMA (K of 0.13 to 5.47 nM). [Ga]Ga-P137 was stable and hydrophilic. [Ga]Ga-P137 showed higher uptake than [Ga]Ga-PSMA-617 in tumor-bearing mice at 6.43 ± 0.98%IA/g vs 3.41 ± 1.31%IA/g at 60-min post-injection. In human studies, the normal organ biodistribution of [Ga]Ga-P137 was grossly equivalent to that of [Ga]Ga-PSMA-617 except for within the urinary tract, in which [Ga]Ga-P137 demonstrated lower uptake.

Conclusion: The optimized ODAP-Urea-based ligand [Ga]Ga-P137 can image PSMA in xenograft models and humans, with lower bladder accumulation to the Glu-Urea-based agent, [Ga]Ga-PSMA-617, in a preliminary, first-in-human study.

Trial Registration: ClinicalTrials.gov Identifier: NCT04560725, Registered 23 September 2020. https://clinicaltrials.gov/ct2/show/NCT04560725.