Preclinical Evaluation of a ¹⁷⁷ Lu-Labeled Gastrin-Releasing Peptide Receptor Antagonist and Prostate Cancer Treatment with Monotherapy and in Combination with Everolimus

This study evaluates the potential of a ¹⁷⁷Lu-labeled GRPR-targeting antagonist as a radiotherapeutic agent for tumors expressing the gastrin-releasing peptide receptor (GRPR). The therapeutic effect of the radioligand was investigated both as a monotherapy and in combination with the mTOR inhibitor everolimus. The GRPR antagonist, LF1 (AAZTA⁵-Pip-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂), was synthesized using the chelator AAZTA⁵ linked via a 4-amino-1-carboxymethylpiperidine (Pip) spacer and radiolabeled with lutetium-177. The preclinical evaluation included assessments of binding kinetics, blood and organ clearance, plasma protein binding, and metabolic stability. SPECT/CT imaging and biodistribution studies were performed in mice bearing PC3 xenograft tumors. To assess its therapeutic efficacy, PC-3-mice were treated with [¹⁷⁷Lu]-Lu-LF1 either alone or following everolimus pretreatment. [¹⁷⁷Lu]-Lu-LF1 showed high binding affinity (K_d = 0.12 ± 0.01 nM) and favorable pharmacokinetics, including rapid blood clearance and low plasma protein binding (2-3% at 5 and 15 min p.i.). Although subject to enzymatic degradation, the radioligand demonstrated high, sustained, and specific tumor uptake (42 ± 5.0% IA/g at 1 h and 3.9 ± 1.1% IA/g at 144 h p.i.). Pancreatic uptake cleared quickly, allowing for high-contrast SPECT/CT imaging. Therapeutically, tumors treated with 93 MBq of [¹⁷⁷Lu]-Lu-LF1 grew more slowly than those treated with 41 MBq. The combination of everolimus and [¹⁷⁷Lu]-Lu-LF1 resulted in significant tumor growth inhibition, compared to the relevant monotherapies with either [¹⁷⁷Lu]-Lu-LF1 or everolimus. [¹⁷⁷Lu]-Lu-LF1 shows promise as a therapeutic radioligand for GRPR-positive prostate cancer, offering high tumor uptake and rapid clearance from nontarget tissues. Mice bearing PC3 xenograft tumors were well tolerated and demonstrated enhanced therapeutic efficacy when combined with everolimus.