A comparative study on 161Tb- and 177Lu-labeled somatostatin receptor agonists and antagonists recently published, revealed an improved therapeutic efficacy of the latter when 161Tb-labeled due to a suspected, increased damage to the cell membrane. Similar outcomes are thus assumed for other transmembrane receptor antagonists, such as those targeting the gastrin-releasing peptide receptor (GRPR). Therefore, a comparative study on two GRPR antagonists, RM2 (DOTA-Pip5-D-Phe6-Gln7-Trp8-Ala9-Val10-Gly11-His12-Sta13-Leu14-NH2) and the recently introduced, metabolically more stable AMTG (DOTA-Pip5-D-Phe6-Gln7-α-Me-Trp8-Ala9-Val10-Gly11-His12-Sta13-Leu14-NH2), will be carried out with 161Tb and the commonly applied 177Lu. Due to the assumption that more severe damage will be caused to the cell membrane if a high percentage of the radiolabeled peptide is receptor-bound but not internalized, these values will be determined. Furthermore, biodistribution studies and µSPECT/CT imaging will be performed at 1, 4, 24 and 72 h post-injection in PC-3 tumor-bearing mice to investigate pharmacokinetics of RM2 and AMTG with both 161Tb and 177Lu over time. An added value of 161Tb over 177Lu will be examined via a therapy study in PC-3 tumor-bearing mice. Thereby, an improved therapeutic efficacy is anticipated for 161Tb-AMTG and 161Tb-RM2 over their 177Lu-labeled analogues, as already observed for 161Tb-labeled somatostatin receptor antagonists. Moreover, it will be interesting to see whether the improved metabolic stability of the AMTG peptide further adds some benefit to the therapeutic efficacy, particularly when 161Tb-labeled. Based on previous data on 177Lu-AMTG and the encouraging results of 161Tb-labeled somatostatin receptor antagonists, an even improved therapeutic efficacy using 161Tb-AMTG is conceivable. 161Tb-AMTG might thus become a clinically applied compound for targeted radiotherapy of GRPR-expressing malignancies, such as prostate and breast cancer, in the near future.