Assessment of an ¹⁸F-Labeled Phosphoramidate Peptidomimetic as a New Prostate-Specific Membrane Antigen–Targeted Imaging Agent for Prostate Cancer

Abstract

Prostate-specific membrane antigen (PSMA) is a transmembrane protein commonly found on the surface of late-stage and metastatic prostate cancer and a well-known imaging biomarker for staging and monitoring therapy. Although 111In-labeled caprop-mab pendetide is the only approved agent available for PSMA imaging, its clinical use is limited because of its slow distribution and clearance that leads to challenging image interpretation. A small-molecule approach using radiolabeled urea-based PSMA inhibitors as imaging agents has shown promise for prostate cancer imaging. The motivation of this work is to explore phosphoramidates as a new class of potent PSMA inhibitors to develop more effective prostate cancer imaging agents with improved specificity and clearance properties. Methods N-succinimidyl-4-18F-fluorobenzoate (18F-SFB) was conjugated to S-2-((2-(S-4-amino-4-carboxybutanamido)-S-2-carboxyethoxy)-hydroxyphosphorylamino)-pentanedioic acid (Phosphoramidate (1)), yielding S-2-((2-(S-4-(4-18F-fluorobenzamido)-4-carboxybutanamido)-S-2-carboxyethoxy)hydroxyphosphorylamino)-pentanedioic acid (3). In vivo studies were conducted in mice bearing either LNCaP (PSMA-positive) or PC-3 (PSMA-negative) tumors. PET images were acquired at 1 and 2 h with or without a preinjection of a nonradioactive version of the fluorophosphoramidate. Tissue distribution studies were performed at the end of the 2 h imaging sessions. Results Phosphoramidate (1) and its fluorobenzamido conjugate (2) were potent inhibitors of PSMA (inhibitory concentration of 50% [IC50], 14 and 0.68 nM, respectively). PSMA-mediated tumor accumulation was noted in the LNCaP versus the PC-3 tumor xenografts. The LNCaP tumor uptake was also blocked by the administration of nonradioactive (2) prior to imaging studies. With the exception of the kidneys, tumor-to-tissue and tumor-to-blood ratios were greater than 5:1 at 2 h. The strong kidney uptake may be due to the known PSMA expression in the mouse kidney, because significant reduction (>6-fold) in kidney activity was seen in mice injected with (2). Conclusion 18F-labeled phosphoramidate (3) is a representative of a new class of PSMA targeting peptidomimetic molecules that shows great promise as imaging agents for detecting PSMA+ prostate tumors.

PET Imaging of Macrophage Mannose Receptor–Expressing Macrophages in Tumor Stroma Using 18F-Radiolabeled Camelid Single-Domain Antibody Fragments

“…Nevertheless, dissolving 18 F-SFB in organic solvent omitted its low stability in aqueous conditions and improved conjugation yields (26)(27)(28)(29). Moreover, these conjugation yields exceeding 20% are achievable only when highly concentrated protein or peptide solutions are provided, in agreement with the conclusion of Wuest et al (30).…”

Synthesis and evaluation of constrained phosphoramidate inhibitors of prostate-specific membrane antigen

“…For a preliminary assessment of this new scaffold's in vivo performance, the 18 F-radiolabled analog [ 18 F]10a was prepared as described previously using N-succinimidyl 4-[ 18 F]fluorobenzoate ([ 18 F]SFB). 15 As shown in the PET/CT image (Fig. 2), there was significant and selective uptake of [ 18 F]10a tracer in CWR22Rv1 (PSMA+) tumor xenografts at 2 h post-injection.…”

Radiofluorinated Derivatives of 2-(Phosphonomethyl)pentanedioic Acid as Inhibitors of Prostate Specific Membrane Antigen (PSMA) for the Imaging of Prostate Cancer

“…Dimethyl rac-2-Fluoro-4-(phosphonomethyl)pentanedioic Acid (21). A mixture of 24 (34 g, 75 mmol) in aqueous 6 N hydrochloric acid (340 mL) was stirred for 6 h at 60°C.…”