Albumin-Binding PSMA Ligands: Optimization of the Tissue Distribution Profile

The prostate-specific membrane antigen (PSMA) has emerged as an attractive prostate cancer associated target for radiotheragnostic application using PSMA-specific radioligands. The aim of this study was to design new PSMA ligands modified with an albumin-binding moiety in order to optimize their tissue distribution profile. The compounds were prepared by conjugation of a urea-based PSMA-binding entity, a DOTA chelator, and 4-( p-iodophenyl)butyric acid using multistep solid phase synthesis. The three ligands (PSMA-ALB-02, PSMA-ALB-05, and PSMA-ALB-07) were designed with varying linker entities. Radiolabeling with ¹⁷⁷Lu was performed at a specific activity of up to 50 MBq/nmol resulting in radioligands of >98% radiochemical purity and high stability. In vitro investigations revealed high binding of all three PSMA radioligands to mouse (>64%) and human plasma proteins (>94%). Uptake and internalization into PSMA-positive PC-3 PIP tumor cells was equally high for all radioligands. Negligible accumulation was found in PSMA-negative PC-3 flu cells, indicating PSMA-specific binding of all radioligands. Biodistribution and imaging studies performed in PC-3 PIP/flu tumor-bearing mice showed enhanced blood circulation of the new radioligands when compared to the clinically employed ¹⁷⁷Lu-PSMA-617. The PC-3 PIP tumor uptake of all three radioligands was very high (76.4 ± 2.5% IA/g, 79.4 ± 11.1% IA/g, and 84.6 ± 14.2% IA/g, respectively) at 24 h post injection (p.i.) resulting in tumor-to-blood ratios of ∼176, ∼48, and ∼107, respectively, whereas uptake into PC-3 flu tumors was negligible. Kidney uptake at 24 h p.i. was lowest for ¹⁷⁷Lu-PSMA-ALB-02 (10.7 ± 0.92% IA/g), while ¹⁷⁷Lu-PSMA-ALB-05 and ¹⁷⁷Lu-PSMA-ALB-07 showed higher renal retention (23.9 ± 4.02% IA/g and 51.9 ± 6.34% IA/g, respectively). Tumor-to-background ratios calculated from values of the area under the curve (AUC) of time-dependent biodistribution data were in favor of ¹⁷⁷Lu-PSMA-ALB-02 (tumor-to-blood, 46; tumor-to-kidney, 5.9) when compared to ¹⁷⁷Lu-PSMA-ALB-05 (17 and 3.7, respectively) and ¹⁷⁷Lu-PSMA-ALB-07 (39 and 2.1, respectively). The high accumulation of the radioligands in PC-3 PIP tumors was visualized on SPECT/CT images demonstrating increasing tumor-to-kidney ratios over time. Taking all of the characteristics into account, ¹⁷⁷Lu-PSMA-ALB-02 emerged as the most promising candidate. The applied concept may be attractive for future clinical translation potentially enabling more potent and convenient prostate cancer radionuclide therapy.