Trastuzumab cotreatment improves survival of mice with PC-3 prostate cancer xenografts treated with the GRPR antagonist Lu-DOTAGA-PEG -RM26.

Gastrin-releasing peptide receptors (GRPRs) are overexpressed in prostate cancer and are suitable for targeted radionuclide therapy (TRT). We optimized the bombesin-derived GRPR-antagonist PEG -RM26 for labeling with Lu and further determined the effect of treatment with Lu-labeled peptide alone or in combination with the anti-HER2 antibody trastuzumab in a murine model. The PEG -RM26 analog was coupled to NOTA, NODAGA, DOTA and DOTAGA chelators. The peptide-chelator conjugates were labeled with Lu and characterized in vitro and in vivo. A preclinical therapeutic study was performed in PC-3 xenografted mice. Mice were treated with intravenous injections (6 cycles) of (A) PBS, (B) DOTAGA-PEG -RM26, (C) Lu-DOTAGA-PEG -RM26, (D) trastuzumab or (E) Lu-DOTAGA-PEG -RM26 in combination with trastuzumab. Lu-DOTAGA-PEG -RM26 demonstrated quantitative labeling yield at high molar activity (450 GBq/μmol), high in vivo stability (5 min pi >98% of radioligand remained when coinjected with phosphoramidon), high affinity to GRPR (K = 0.4 ± 0.2 nM), and favorable biodistribution (1 hr pi tumor uptake was higher than in healthy tissues, including the kidneys). Therapy with Lu-DOTAGA-PEG -RM26 induced a significant inhibition of tumor growth. The median survival for control groups was significantly shorter than for treated groups (Group C 66 days, Group E 74 days). Trastuzumab together with radionuclide therapy significantly improved survival. No treatment-related toxicity was observed. In conclusion, based on in vitro and in vivo characterization of the four Lu-labeled PEG -RM26 analogs, we concluded that Lu-DOTAGA-PEG -RM26 was the most promising analog for TRT. Radiotherapy using Lu-DOTAGA-PEG -RM26 effectively inhibited tumor growth in vivo in a murine prostate cancer model. Anti-HER2 therapy additionally improved survival.