A preclinical study on the influence of linkers in [68Ga]Ga-NOTA-X-RM26 radiotracers for PET imaging of GRPR expression.

Background: Gastrin-releasing peptide receptor (GRPR) is overexpressed in several cancers, including prostate and breast, making it an attractive target for radiopharmaceutical development. Studies on GRPR-targeting radioligands highlight the critical role of the spacer region between the GRPR-recognition motif and radiolabeled moiety, which can significantly influence peptide pharmacokinetics and pharmacodynamics. Herein, we investigated the impact of structurally restricted spacers on the performance of RM26-based radioligands.

Results: Three novel radioligands were designed to each bear a NOTA chelator via different spacers composed of N-acetyl-lysine followed by either o-ethyltoluene (oET), o-methylanisole (oMA), or m-methylanisole (mMA) motifs. The peptides were successfully labeled with Ga-68, achieving high radiochemical yield, purity, and molar activity. The resulting [⁶⁸Ga]-labeled peptides demonstrated high and GRPR-specific binding to prostate cancer PC-3 cells, antagonistic behavior, and the IC₅₀ values to GRPR were in the single-digit nanomolar range. Biodistribution studies at 2 h post-injection in PC-3 xenograft-bearing mice revealed high, GRPR-mediated tumor uptake for all three radioligands. In addition, high hepatobiliary excretion with elevated uptake in the liver and the gastrointestinal tract and pronounced pancreatic uptake were observed.

Conclusions: Among the three radioligands, the peptide bearing the N-acetyl-lysine-oET spacer exhibited the fastest background clearance and better PET imaging of prostate cancer xenografts. The incorporation of conformationally restricted spacers is a promising strategy for developing tracers with high GRPR binding and good imaging properties, but further optimization is necessary to reduce uptake in healthy tissues.