Organofluorosilicon based ¹⁸F-radiolabeling is an efficient method for incorporating fluorine-18 into ¹⁸F-radiopharmaceuticals for positron emission tomography (PET) by ¹⁹F/¹⁸F isotopic exchange (IE). The first PET radiopharmaceutical, ¹⁸F-SiFAlin-TATE, radiolabeled with a silicon-based [¹⁸F]fluoride acceptor (SiFA), namely, a para-substituted di-tert-butyl[¹⁸F]fluorosilylbenzene, has entered clinical trials, and is paving the way for other potential [¹⁸F]SiFA-labeled radiopharmaceuticals for diagnostic use. In this study, we report the in vitro metabolism of an oxime-linked SiFA tetrazine (SiFA–Tz), a new PET-radiotracer candidate, recently evaluated for pretargeted PET imaging and macromolecule labeling. Metabolism of SiFA–Tz was studied in mouse liver microsomes (MLM) for elucidating its major biotransformation pathways. Nontargeted screening by ultrahigh performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) was utilized for detection of unknown metabolites. The oxime bond between the SiFA and Tz groups forms two geometric (E/Z) isomers, which underwent the same biotransformations, but unexpectedly with different kinetics. In total, nine proposed metabolites of SiFA–Tz from phase I and II reactions were detected, five of which were defluorinated in MLMs, elucidating the metabolic pathway leading to previously reported defluorination of [¹⁸F]SiFA–Tz in vivo. Based on the HRMS studies a biotransformation pathway is proposed: hydroxylation (+O) to tert-butyl group adjacent to the silicon, followed by oxidative defluorination (+OH/-F) cleaving the fluorine off the silicon. Interestingly, eight proposed metabolites of a reduced dihydrotetrazine analogue, SiFA–H₂Tz, from phase I and II reactions were additionally detected. To the best of our knowledge, this is the first reported comprehensive investigation of enzyme mediated metabolic pathway of tetrazines and para-substituted di-tert-butylfluorosilylbenzene fluoride acceptors, providing novel structural information on the biotransformation and fragmentation patterns of radiotracers bearing these structural motifs. By investigating the metabolism preceding defluorination, structurally optimized new SiFA compounds can be designed for expanding the portfolio of efficient ¹⁹F/¹⁸F isotopic exchange labeling probes for PET imaging.

中文翻译：

---

小鼠肝脏微粒体中生物正交PET示踪剂候选物[19F / 18F] SiFA-四嗪的代谢：UHPLC-HRMS研究了生物转化途径和脱氟。

基于有机氟硅的¹⁸ F-放射性标记是一种通过¹⁹ F / ¹⁸ F同位素交换（IE）将氟18掺入¹⁸ F-放射性药物以进行正电子发射断层扫描（PET）的有效方法。第一种PET放射性药物¹⁸ F-SiFA lin -TATE，用硅基[ ¹⁸ F]氟化物受体（SiFA）进行放射性标记，即对位取代的二叔丁基[ ¹⁸ F]氟硅烷基苯，已进入临床试验。 ，并为其他[ ¹⁸ F] SiFA标记的放射性药物用于诊断铺平了道路。在这项研究中，我们报告了体外肟连接的SiFA四嗪（SiFA–Tz）的新陈代谢（一种新型PET放射性示踪剂）最近已进行了预靶向PET成像和大分子标记的评估。在小鼠肝微粒体（MLM）中研究了SiFA–Tz的代谢，以阐明其主要的生物转化途径。通过超高效液相色谱高分辨率质谱法（UHPLC-HRMS）进行的非目标筛选被用于检测未知代谢物。SiFA和Tz基团之间的肟键形成两个几何（E / Z）异构体，它们经历了相同的生物转化，但出乎意料地具有不同的动力学。总共检测到IFA和II相反应的9种拟议的SiFA–Tz代谢产物，其中5种在MLM中被脱氟，阐明了先前报道的体内[ ¹⁸ F] SiFA–Tz脱氟的代谢途径。基于HRMS研究，提出了一种生物转化途径：羟基化（+ O）到与硅相邻的叔丁基，然后进行氧化脱氟（+ OH / -F），将氟从硅上裂解下来。有趣的是，提出了还原的二氢四嗪类似物SiFA–H _2的8种代谢物从I和II期反应还检测到Tz。据我们所知，这是首次报道的酶催化的四嗪和对位取代的二叔丁基氟甲硅烷基苯氟化物受体的代谢途径的全面研究，提供了具有这些结构基序的放射性示踪剂的生物转化和断裂模式的新结构信息。通过研究脱氟之前的代谢，可以设计结构优化的新型SiFA化合物，以扩展用于PET成像的高效¹⁹ F / ¹⁸ F同位素交换标记探针的产品范围。