Enhanced blood retention and tumor uptake of PSMA-targeting, ²²⁵Ac-labeled radioconjugates

Objectives
The prostate-specific membrane antigen (PSMA) is expressed in most cases of prostate cancer at considerably higher levels in tumor cells compared to healthy tissues [1-3] and its upregulation occurs in all stage of the disease [4, 5]. Therefore, PSMA has emerged as an attractive target for molecular imaging and especially targeted radionuclide therapy (endoradiotherapy) of metastatic castration-resistant prostate cancer (mCRPC), given the example of [177Lu]Lu-PSMA-617. We recently described the synthesis and in-depth characterization of PSMA radioligands for targeted alpha therapy with actinium-225 [6]. Our present study aimed at the design, synthesis and preclinical evaluation of albumin-binding PSMA ligands in order to optimize their tissue distribution profile and to improve their pharmacokinetic properties.

Methods
Two novel compounds were prepared by combining a macropa chelator with one or two lysine-ureido-glutamate–based PSMA-binding entities equipped with 4-(p-iodophenyl)butyrate residues. The albumin-binding properties of the 225Ac-labeled conjugates were investigated in vitro using mouse, rat and human serum. The specific interaction of both compounds with human serum albumin was confirmed by microscale thermophoresis. Cell culture-based studies regarding radioligand affinity and clonogenicity were performed on PSMA-positive LNCaP cells. The biodistribution of the radioconjugates was analyzed in LNCaP tumor-bearing mice with ensuing investigation of tissue toxicity by histological examinations.

Results
Radiolabeling of both PSMA ligands with 225Ac was achieved at up to 5 MBq/nmol with >99% radiochemical purity. In vitro assays confirmed considerable binding of the radioligands to mouse, rat and human serum proteins. Cell binding and survival studies revealed a higher cell binding affinity and an improved cell killing efficiency for the radioconjugate with two PSMA-binding entities compared to the derivative with only one targeting motif. Biodistribution studies revealed enhanced blood circulation times of the new albumin-binding PSMA ligands compared to their counterparts lacking the 4-(p-iodophenyl)butyrate residues as well as substantially higher accumulation in LNCaP tumors up to 50 %ID/g (one PSMA-binding moiety) and 150 %ID/g (two PSMA-binding moieties), respectively, after 120 h p.i. Considerable DNA damage and a massive decrease in cell proliferation was observed by histological examination of LNCaP tumors, whereas necrosis in the kidneys of the animals was not detected.

Conclusions
In this study, we demonstrate the development and evaluation of two new PSMA-targeting radioligands comprising 4-(p-iodophenyl)butyrate residues as albumin binder. The prolonged blood circulation of the novel radioligands resulted in greatly enhanced tumor uptake and retention over time. These radioconjugates have the potential to improve the efficacy of targeted alpha therapy and are promising candidates for the treatment of mCRPC.