Otaru Sofia, Martinmäki Tatu, Kuurne Iida, Paulus Andreas, Helariutta Kerttuli, Sarparanta Mirkka, Airaksinen Anu J
Department of Chemistry, Radiochemistry, University of Helsinki Finland.
Turku PET Centre, University of Turku Kiinamyllynkatu 4-8 FI-20520 Turku Finland
RSC Adv. 2023 Jul 26;13(32):22606-22615. doi: 10.1039/d3ra02807k. eCollection 2023 Jul 19.
Click chemistry reactions, such as the tetrazine ligation, based on the inverse-electron demand Diels-Alder (IEDDA), are chemoselective cycloaddition reactions widely used for chemical modifications and synthesis of biomolecule-based radiopharmaceuticals for positron emission tomography (PET). The reactions have potential also for pretargeted PET imaging. When used as a bioconjugation method in production of biomolecule-based radiopharmaceuticals, IEDDA-based tetrazine ligation has one significant drawback, namely the formation of a mixture comprising reduced metastable dihydropyridazines (DHPs) and oxidized cycloadducts. Conversion of the reduced DHPs to stable pyridazines requires oxidation, which is typically achieved by using oxidants or by photo-irradiated air-oxidation, both methods requiring added reagents or reaction times of several hours, not compatible with short-lived radionuclides. Here we report a mild, rapid, and catalyst-free conversion of the DHPs to pyridazines. In this study, a model peptide Tyr-octreotide (TOC) was modified with polyethylene glycol (PEG) linkers and with -cyclooctenes (TCOs) for rapid IEDDA-mediated radiolabeling. Fluorine-18-labelled alkylammoniomethyltrifluoroborate ([F]AmBF) tetrazines were conjugated to the TCO-TOC analogs at room temperature for rapid synthesis of PET imaging agent candidates. The formed DHPs were successfully converted to the oxidized form, after heating the radiolabelled bioconjugates in aqueous solution (≥95% water) at 60 °C for a minimum of 10 minutes in the presence of air, resulting in one-pot back-to-back IEDDA reaction and DHP conversion. The water content of the reaction mixture was to be found critical for the coversion. Our finding offers a straightforward method for conversion of the metastable DHPs from the IEDDA-based tetrazine ligation to stable, oxidized pyridazines. The method is especially suitable for applications requiring rapid conversion.
点击化学反应，如基于逆电子需求狄尔斯-阿尔德反应（IEDDA）的四嗪连接反应，是具有化学选择性的环加成反应，广泛用于化学修饰以及基于生物分子的正电子发射断层扫描（PET）放射性药物的合成。这些反应在预靶向PET成像方面也具有潜力。当用作基于生物分子的放射性药物生产中的生物共轭方法时，基于IEDDA的四嗪连接反应有一个显著缺点，即会形成包含还原态亚稳二氢哒嗪（DHP）和氧化环加合物的混合物。将还原态的DHP转化为稳定的哒嗪需要氧化，这通常通过使用氧化剂或光照射空气氧化来实现，这两种方法都需要添加试剂或数小时的反应时间，与短寿命放射性核素不兼容。在此，我们报告了一种将DHP温和、快速且无催化剂地转化为哒嗪的方法。在本研究中，用聚乙二醇（PEG）连接子和环辛烯（TCO）修饰了模型肽酪氨酰-奥曲肽（TOC），用于快速的IEDDA介导的放射性标记。室温下，将氟-18标记的烷基氨甲基三氟硼酸盐（[F]AmBF）四嗪与TCO-TOC类似物共轭，以快速合成PET成像剂候选物。在空气中，将放射性标记的生物共轭物在60℃的水溶液（水含量≥95%）中加热至少10分钟后，形成的DHP成功转化为氧化形式，从而实现一锅法连续进行IEDDA反应和DHP转化。发现反应混合物的水含量对转化至关重要。我们的发现提供了一种将基于IEDDA的四嗪连接反应中产生的亚稳DHP直接转化为稳定氧化哒嗪的方法。该方法特别适用于需要快速转化的应用。
