Albumin-Binding PSMA Ligands: Implications for Expanding the Therapeutic Window

Abstract

Despite significant gains in the treatment of metastatic castrationresistant prostate cancer by radioligands targeting prostate-specific membrane antigen (PSMA), 30% of patients never respond to therapy. One possible explanation is insufficient dose delivery to the tumor because of suboptimal pharmacokinetics. We have recently described RPS-063, a trifunctional ligand targeting PSMA with high uptake in LNCaP xenograft tumors but also in kidneys. We aimed to use structural modifications to increase the tumor-to-kidney ratio through increased albumin binding and tumor uptake and reduction of kidney activity. Methods: Four structurally related trifunctional PSMA-targeting small molecules were prepared by either varying the albumin-binding group or inserting a polyethylene glycol 8 linker into a common structure. The compounds were ranked by PSMA affinity and albumin affinity and were radiolabeled with 68 Ga and 177 Lu. Tissue kinetics were determined in male BALB/C nu/nu mice bearing LNCaP xenograft tumors. Results: Each of the compounds binds PSMA with a half-maximal inhibitory concentration of no more than 10 nM. The albumin-binding group had a minimal effect on PSMA affinity but changed albumin affinity by an order of magnitude. However, the addition of a polyethylene glycol 8 spacer weakened affinity for albumin in each case. Increased affinity for albumin corresponded with delayed blood clearance and modified uptake kinetics in the tumor and kidney. Uptake of 177 Lu-RPS-072 (34.9 ± 2.4 %ID/g) and 177 Lu-RPS-077 (27.4 ± 0.6 %ID/g) increased up to 24 h after injection, and washout by 96 h was not significant. As a result, the area under the curve (AUC) in the tumor was in the following order: 177 Lu-RPS-072 . 177 Lu-RPS-077 . 177 Lu-RPS-063 . 177 Lu-RPS-071. Increased linker length corresponded to more rapid clearance from kidneys. Consequently, the ratio of tumor AUC and kidney AUC was 4.7 ± 0.3 for 177 Lu-RPS-072. Conclusion: The tumor AUC and tumor-to-kidney ratio of 177 Lu-RPS-072 are significantly enhanced compared with any small molecule investigated in a LNCaP xenograft model to date. In comparison to other PSMAtargeting radioligands that have been evaluated in a PC3-PIP model, activity in kidneys is reduced and activity in tumors compares favorably when the different PSMA expression levels in LNCaP and PC3-PIP cells are considered. RPS-072 therefore exhibits an increased therapeutic index, shows the potential to increase the dose delivered to tumors, and is a highly promising candidate for targeted radioligand therapy.

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

“…The IC 50 of RPS-074 was determined to be 12.0 6 3.4 nM, a value consistent with the reported PSMA affinities of structurally analogous trifunctional ligands such as RPS-072 (16).…”

“…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).…”

“…Briefly, RPS-074 was generated from the coupling of di-tert-butyl (((S)-1-(tert-butoxy)-6-(3-(3-ethynylphenyl)ureido)-1-oxohexan-2-yl)carbamoyl)-L-glutamate (14), macropa-isothiocyanate (15), and N 6 -(4-(4-iodophenyl)butanoyl)-Llysine by 2 linkers with 3 and 8 PEG units, respectively. RPS-072 was synthesized following a previously published method (16).…”

SARS-CoV-2 Bound Human Serum Albumin and Systemic Septic Shock

“…Serum albumin ( 43 ) functions as a significant regulator of plasma oncotic pressure and a transporter of ligands ( 44 – 46 ). It has a half-life in the body of about 20 days and decreases by 10–15% after 50 years of age ( 47 ).…”

Preclinical Evaluation of the GRPR-Targeting Antagonist RM26 Conjugated to the Albumin-Binding Domain for GRPR-Targeting Therapy of Cancer

“…The activity concentration in blood 1 h pi was as high as 32 ± 4% ID/g and decreased only 3.5-fold at 24 h pi. Such extension would not be possible for the relatively small ABD-RM26 conjugate (7 kDa) without interaction with albumin in blood circulation [ 35 , 36 , 37 ]. High activity concentration in blood contributed to elevated activity uptake in non-targeted organs (see Figure 8 ).…”