Preclinical Development of Novel PSMA-Targeting Radioligands: Modulation of Albumin-Binding Properties To Improve Prostate Cancer Therapy

Abstract

The treatment of metastatic castration-resistant prostate cancer (mCRPC) remains challenging with current treatment options. The development of more effective therapies is, therefore, urgently needed. Targeted radionuclide therapy with prostate-specific membrane antigen (PSMA)-targeting ligands has revealed promising clinical results. In an effort to optimize this concept, it was the aim of this study to design and investigate PSMA ligands comprising different types of albumin binders. PSMA-ALB-53 and PSMA-ALB-56 were designed by combining the glutamate-urea-based PSMA-binding entity, a DOTA chelator and an albumin binder based on the 4-( p-iodophenyl)-moiety or p-(tolyl)-moiety. The compounds were labeled with Lu (50 MBq/nmol) resulting in radioligands of high radiochemical purity (≥98%). Both radioligands were stable (≥98%) over 24 h in the presence of l-ascorbic acid. The uptake into PSMA-positive PC-3 PIP tumor cells in vitro was in the same range (54-58%) for both radioligands; however,Lu-PSMA-ALB-53 showed a 15-fold enhanced binding to human plasma proteins. Biodistribution studies performed in PC-3 PIP/flu tumor-bearing mice revealed high tumor uptake of Lu-PSMA-ALB-53 andLu-PSMA-ALB-56, respectively, demonstrated by equal areas under the curves (AUCs) for both radioligands. The increased retention of Lu-PSMA-ALB-53 in the blood resulted in almost 5-fold lower tumor-to-blood AUC ratios when compared toLu-PSMA-ALB-56. Kidney clearance of Lu-PSMA-ALB-56 was faster, and hence, the tumor-to-kidney AUC ratio was 3-fold higher than in the case ofLu-PSMA-ALB-53. Due to the more favorable tissue distribution profile, Lu-PSMA-ALB-56 was selected for a preclinical therapy study in PC-3 PIP tumor-bearing mice. The tumor growth delay after application ofLu-PSMA-ALB-56 and Lu-PSMA-617 applied at the same activities (2 or 5 MBq per mouse) revealed better antitumor effects in the case ofLu-PSMA-ALB-56. As a consequence, the survival of mice treated with Lu-PSMA-ALB-56 was prolonged when compared to the mice, which received the same activity ofLu-PSMA-617. Our results demonstrated the superiority of Lu-PSMA-ALB-56 overLu-PSMA-ALB-53 indicating that the p-(tolyl)-moiety was more suited as an albumin binder to optimize the tissue distribution profile. Lu-PSMA-ALB-56 was more effective to treat tumors thanLu-PSMA-617 resulting in complete tumor remission in four out of six mice. This promising results warrant further investigations to assess the potential for clinical application of Lu-PSMA-ALB-56.

Preparation of the alpha‐emitting prostate‐specific membrane antigen targeted radioligand [212Pb]Pb‐NG001 for prostate cancer

“…Radiolysis induced by free radicals can be prevented by radioprotectants, such as ascorbic acid, human serum albumin, cysteamine and glycerol. 42,43 Figure S4), which are in agreement with other studies that have examined the binding of PSMA ligands to PSMA-positive cells. [44][45][46] Gourni et al showed that [ 11 In]In-PSMA-617 to LNCaP cells yielded a k d of 5.4 nM.…”

“…Radiolysed radioligands have reduced receptor affinity, and, therefore, will lead to decreased therapeutic and diagnostic efficacy, and circulation in the body with increased radiation toxicity to organs such as liver and bone marrow. Radiolysis induced by free radicals can be prevented by radioprotectants, such as ascorbic acid, human serum albumin, cysteamine and glycerol …”

“…A good stability of [ 212 Pb]Pb‐NG001 within 48 hours in PBS and serum was demonstrated by TLC analyses (Table ). However, the affinity of the PSMA ligand for its binding site may decrease with time . PSMA ligand integrity is difficult to maintain in formulations consisting of alpha‐ or beta‐emitting radionuclides due to direct radiation damage (autoradiolysis) and indirect radiation damage (radical‐induced radiolysis).…”

“…42,43 PSMA ligand integrity is difficult to maintain in formulations consisting of alpha-or beta-emitting radionuclides due to direct radiation damage (autoradiolysis) and indirect radiation damage (radical-induced radiolysis). Radiolysed radioligands have reduced receptor affinity, 42,43 and, therefore, will lead to decreased therapeutic and diagnostic efficacy, and circulation in the body with increased radiation toxicity to organs such as liver and bone marrow. Radiolysis induced by free radicals can be prevented by radioprotectants, such as ascorbic acid, human serum albumin, cysteamine and glycerol.…”

A Single Dose of 225Ac-RPS-074 Induces a Complete Tumor Response in an LNCaP Xenograft Model

“…With this in mind, we have focused our efforts on maximizing the therapeutic window of these powerful radiopharmaceuticals. Our choice of RPS-074 for preclinical therapy studies follows the identification of the recently reported analog RPS-072 (16), which showed dramatically reduced kidney uptake and substantially increased and prolonged tumor uptake relative to several novel PSMA-targeting ligands, including PSMA-617, RPS-063 (23), CTT1403 (24), PSMA-Alb-02 (19), and PSMA-Alb-56 (25). Although the DOTA-containing RPS-072 also chelates 225 Ac 31 , radiolabeling yield was only 49%, and the 225 Ac-DOTA complex has been found to have suboptimal in vivo stability (26,27).…”