School of Biomedical Engineering and Imaging Sciences, King's College London, 4th Floor Lambeth Wing, St Thomas' Hospital, London SE1 7EH, United Kingdom.
Department of Chemistry, Molecular Sciences Research Hub, Imperial College London, London W12 0BZ, United Kingdom.
Bioconjug Chem. 2022 Jul 20;33(7):1422-1436. doi: 10.1021/acs.bioconjchem.2c00284. Epub 2022 Jul 8.
Auger electron therapy exploits the cytotoxicity of low-energy electrons emitted during radioactive decay that travel very short distances (typically <1 μm). Tl, with a half-life of 73 h, emits ∼37 Auger and other secondary electrons per decay and can be tracked as its gamma emissions enable SPECT imaging. Despite the useful nuclear properties of Tl, satisfactory bifunctional chelators to incorporate it into bioconjugates for molecular targeting have not been developed. Hpypa, Hdecapa, Hneunpa-NH, and Hnoneunpa are multidentate N- and O-donor chelators that have previously been shown to have high affinity for In, Lu, and Zr. Herein, we report the synthesis and serum stability of [Tl]Tl complexes with Hpypa, Hdecapa, Hneunpa-NH, and Hnoneunpa. All ligands quickly and efficiently formed complexes with [Tl]Tl that gave simple single-peak radiochromatograms and showed greatly improved serum stability compared to DOTA and DTPA. [Tl]Tl-pypa was further characterized using nuclear magnetic resonance spectroscopy (NMR), mass spectroscopy (MS), and X-ray crystallography, showing evidence of the proton-dependent presence of a nine-coordinate complex and an eight-coordinate complex with a pendant carboxylic acid group. A prostate-specific membrane antigen (PSMA)-targeting bioconjugate of Hpypa was synthesized and radiolabeled. The uptake of [Tl]Tl-pypa-PSMA in DU145 PSMA-positive and PSMA-negative prostate cancer cells was evaluated and showed evidence of bioreductive release of Tl and cellular uptake characteristic of unchelated [Tl]TlCl. SPECT/CT imaging was used to probe the biodistribution and stability of [Tl]Tl-pypa-PSMA. In healthy animals, [Tl]Tl-pypa-PSMA did not show the myocardial uptake that is characteristic of unchelated Tl. In mice bearing DU145 PSMA-positive and PSMA-negative prostate cancer xenografts, the uptake of [Tl]Tl-pypa-PSMA in DU145 PSMA-positive tumors was higher than that in DU145 PSMA-negative tumors but insufficient for useful tumor targeting. We conclude that Hpypa and related ligands represent an advance compared to conventional radiometal chelators such as DOTA and DTPA for Tl chelation but do not resist dissociation for long periods in the biological environment due to vulnerability to reduction of Tl and subsequent release of Tl. However, this is the first report describing the incorporation of [Tl]Tl into a chelator-peptide bioconjugate and represents a significant advance in the field of Tl-based radiopharmaceuticals. The design of the next generation of chelators must include features to mitigate this susceptibility to bioreduction, which does not arise for other trivalent heavy radiometals.
Auger 电子疗法利用放射性衰变期间发射的低能电子的细胞毒性，这些电子的射程非常短（通常 <1 μm）。Tl 的半衰期为 73 小时，每次衰变会发射约 37 个 Auger 和其他二次电子，并且可以通过其伽马发射进行 SPECT 成像来追踪。尽管 Tl 具有有用的核特性，但尚未开发出令人满意的双功能螯合剂将其纳入生物缀合物以进行分子靶向。Hpypa、Hdecapa、Hneunpa-NH 和 Hnoneunpa 是多齿 N 和 O 供体螯合剂，以前已证明它们对 In、Lu 和 Zr 具有高亲和力。在此，我们报告了 Hpypa、Hdecapa、Hneunpa-NH 和 Hnoneunpa 与[Tl]Tl 配合物的合成和血清稳定性。所有配体都快速有效地与[Tl]Tl 形成配合物，得到了简单的单峰放射性色谱图，并与 DOTA 和 DTPA 相比显示出大大提高的血清稳定性。[Tl]Tl-pypa 进一步通过核磁共振波谱 (NMR)、质谱 (MS) 和 X 射线晶体学进行了表征，表明存在质子依赖性九配位配合物和带有侧挂羧酸基团的八配位配合物的证据。合成了 Hpypa 的前列腺特异性膜抗原 (PSMA) 靶向生物缀合物并进行了放射性标记。在 DU145 PSMA 阳性和 PSMA 阴性前列腺癌细胞中评估了[Tl]Tl-pypa-PSMA 的摄取，并证明了 Tl 的生物还原释放和未配位[Tl]TlCl 的细胞摄取特征。SPECT/CT 成像用于探测[Tl]Tl-pypa-PSMA 的生物分布和稳定性。在健康动物中，[Tl]Tl-pypa-PSMA 没有表现出未配位 Tl 特有的心肌摄取。在携带 DU145 PSMA 阳性和 PSMA 阴性前列腺癌异种移植瘤的小鼠中，[Tl]Tl-pypa-PSMA 在 DU145 PSMA 阳性肿瘤中的摄取高于 DU145 PSMA 阴性肿瘤，但不足以用于有用的肿瘤靶向。我们得出的结论是，与 DOTA 和 DTPA 等传统放射性金属螯合剂相比，Hpypa 和相关配体在 Tl 螯合方面代表了一项进步，但由于 Tl 的还原和随后 Tl 的释放，它们在生物环境中不能长时间抵抗解离。然而，这是第一个描述将[Tl]Tl 掺入螯合肽生物缀合物的报告，代表了 Tl 基放射性药物领域的重大进展。下一代螯合剂的设计必须包括减轻这种生物还原敏感性的特征，而对于其他三价重放射性金属则不会出现这种敏感性。
