Background <p>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.</p> Results <p>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 (&gt; 97%) and purities (&gt; 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&#xa0;h pi, exceeding activity uptake in the kidneys. Activity uptake in the tumor was highly retained at 24&#xa0;h pi.</p> Conclusions <p>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.</p>