The gastrin-releasing peptide receptor (GRPR) has been extensively studied as a biomolecular target for peptide-based radiotheranostics. However, the lack of metabolic stability and the rapid clearance of peptide radioligands, including radiolabeled GRPR-antagonists, often impede clinical application. Aiming at circumventing these drawbacks, we have designed three new GRPR-antagonist radioligands using [99mTc]Tc-DB15 ([99mTc]Tc-N4-AMA-DIG-DPhe-Gln-Trp-Ala-Val-Sar-His-Leu-NHEt; AMA: p-aminomethylaniline; DIG: diglycolate) as a motif, due to its high GRPR-affinity and stability to neprilysin (NEP). The new analogues carry the DOTAGA-chelator (1,4,7,10-tetraazacyclododecane-1-glutaric acid-4,7,10-triacetic acid) through different linkers at the N-terminus to allow for labeling with the theranostic radionuclide pair In-111/Lu-177. After labeling with In-111 the following radioligands were evaluated: (i) [111In]In-AU-SAR-M1 ([111In]In-DOTAGA-AMA-DIG-DPhe-Gln-Trp-Ala-Val-Sar-His-Leu-NHEt), (ii) [111In]In-AU-SAR-M2 ([111In]In-[DOTAGA-Arg]AU-SAR-M1) and (iii) [111In]In-AU-SAR-M3 ([111In]In-[DOTAGA-DArg]AU-SAR-M1). These radioligands were compared in a series of in vitro assays using prostate adenocarcinoma PC-3 cells and in murine models. They all displayed high and GRPR-specific uptake in PC-3 cells. Analysis of mice blood collected 5 min post-injection (pi) revealed similar or even higher metabolic stability of the new radioligands compared with [99mTc]Tc-DB15. The stability could be further increased when the mice were treated with Entresto® to in situ induce NEP-inhibition. In PC-3 xenograft-bearing mice, [111In]In-AU-SAR-M1 displayed the most favourable biodistribution profile, combining a good tumor retention with the highest tumor-to-organ ratios, with the kidneys as the dose-limiting organ. These findings strongly point at AU-SAR-M1 as a promising radiotherapeutic candidate when labeled with Lu-177, or other medically appealing therapeutic radiometals, especially when combined with in situ NEP-inhibition. To this goal further investigations are currently pursued.

中文翻译：

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具有改善的代谢稳定性的新型 GRPR 拮抗剂在肿瘤学放射治疗中的临床前评估

胃泌素释放肽受体（GRPR）作为肽放射治疗的生物分子靶点已被广泛研究。然而，肽放射性配体（包括放射性标记的 GRPR 拮抗剂）缺乏代谢稳定性和快速清除，常常阻碍临床应用。为了克服这些缺点，我们使用[99mTc]Tc-DB15（[99mTc]Tc-N4-AMA-DIG-DPhe-Gln-Trp-Ala-Val-Sar-His-Leu-）设计了三种新的GRPR拮抗剂放射性配体NHEt；AMA：对氨基甲基苯胺；DIG：二甘醇酸盐）作为基序，因为其对脑啡肽酶 (NEP) 具有高 GRPR 亲和力和稳定性。新类似物通过 N 末端的不同接头携带 DOTAGA 螯合剂（1,4,7,10-四氮杂环十二烷-1-戊二酸-4,7,10-三乙酸），以便用治疗诊断放射性核素对进行标记In-111/Lu-177。用 In-111 标记后，评估以下放射性配体： (i) [111In]In-AU-SAR-M1 ([111In]In-DOTAGA-AMA-DIG-DPhe-Gln-Trp-Ala-Val-Sar-His -Leu-NHEt)、(ii) [111In]In-AU-SAR-M2 ([111In]In-[DOTAGA-Arg]AU-SAR-M1) 和 (iii) [111In]In-AU-SAR-M3 ([111In]In-[DOTAGA-DArg]AU-SAR-M1)。使用前列腺腺癌 PC-3 细胞和小鼠模型在一系列体外测定中对这些放射性配体进行了比较。它们都在 PC-3 细胞中表现出高的 GRPR 特异性摄取。对注射后 5 分钟 (pi) 收集的小鼠血液的分析显示，与 [99mTc]Tc-DB15 相比，新放射性配体具有相似甚至更高的代谢稳定性。当用 Entresto® 处理小鼠以原位诱导 NEP 抑制时，稳定性可以进一步提高。在携带 PC-3 异种移植物的小鼠中，[111In]In-AU-SAR-M1 显示出最有利的生物分布特征，将良好的肿瘤保留与最高的肿瘤与器官比率相结合，其中肾脏作为剂量限制器官。这些发现强烈表明，当用 Lu-177 或其他医学上有吸引力的治疗性放射性金属标记时，特别是与原位 NEP 抑制相结合时，AU-SAR-M1 是一种有前途的放射治疗候选药物。为了实现这一目标，目前正在进行进一步的调查。