Improving GRPR-targeting peptides for radiotheranostics application: insights from chelator modifications and α-methyl-L tryptophan substitution

Abstract

Background: Targeting the gastrin-releasing peptide receptor (GRPR) is a promising approach for radionuclide therapy in prostate and breast cancers. GRPR-targeting peptides often have limited metabolic stability, which can compromise their clinical efficacy due to rapid degradation in the bloodstream, leading to reduced tumor uptake. We previously reported the GRPR-targeting peptide AU-RM26-M2 (DOTAGA-PEG2-Pip-[Sar11]RM26), which demonstrated promising pharmacokinetics in GRPR-expressing xenografts. In this study, we aimed to enhance the metabolic stability and targeting properties of AU-RM26-M2 by incorporating α-methyl-L-tryptophan (MetTrp) at position 8 in the pharmacophore, and to investigate the influence of chelator choice (DOTAGA vs. DOTA) for labeling with Lu-177, a β-emitting therapeutic nuclide. Results: Therefore, we designed two peptides: PKB2 (DOTAGA-PEG2-Pip-[MetTrp8, Sar11]RM26) and PKB3 (DOTA-PEG2-Pip-[MetTrp8, Sar11]RM26). For comparison, we also evaluated the DOTA-bearing analogue of AU-RM26-M2, PKB1 (DOTA-PEG2-Pip-[Sar11]RM26). PKB1, PKB2, and PKB3 were labeled with Lu-177, achieving high radiochemical yields (> 97%) and purities (> 93%). In PC-3 cells, [177Lu]Lu-PKB1, [177Lu]Lu-PKB2, and [177Lu]Lu-PKB3 showed affinity in the sub-nanomolar range and high specificity for GRPR, with a slow internalization rate. The radiopeptides with MetTrp8 modification had high metabolic stability against peptidases in vivo. In PC-3 xenografts, [177Lu]Lu-PKB2 and [177Lu]Lu-PKB3 demonstrated rapid background clearance and high GRPR-mediated tumor activity uptake at 2 h pi, exceeding activity uptake in the kidneys. Activity uptake in the tumor was highly retained at 24 h pi. Conclusions: This study led to the development of two metabolically stable GRPR-targeting radiopeptides, [177Lu]Lu-PKB2 and [177Lu]Lu-PKB3, with a high potential for targeted radionuclide therapy.