Metallic radionuclides conjugated to biological vectors via an appropriate chelator are employed in nuclear medicine for the diagnosis (imaging) and radiotherapy of diseases. For the application of radiolabeled antibodies using positron emission tomography (immunoPET), zirconium-89 has gained increasing interest over the last decades as its physical properties (t_1/2 = 78.4 h, 22.6% β⁺ decay) match well with the slow pharmacokinetics of antibodies (t_biol. = days to weeks) allowing for late time point imaging. The most commonly used chelator for ⁸⁹Zr in this context is desferrioxamine (DFO). However, it has been shown in preclinical studies that the hexadentate DFO ligand does not provide ⁸⁹Zr-complexes of sufficient stability in vivo and unspecific uptake of the osteophilic radiometal in bones is observed. For clinical applications, this might be of concern not only because of an unnecessary dose to the patient but also an increased background signal. As a consequence, next generation chelators based on hydroxamate scaffolds for more stable coordination of ⁸⁹Zr have been developed by different research groups. In this review, we describe the progress in this research field until end of 2020, including promising examples of new candidates of chelators currently in advanced stages for clinical translation that outrun the performance of the current gold standard DFO.

Background: Actinium-225 (²²⁵Ac) has gained interest in nuclear medicine for use in targeted alpha therapy (TAT) for the treatment of cancer. However, the number of suitable chelators for the stable complexation of ²²⁵Ac³⁺ is limited. The promising physical properties of ²²⁵Ac result in an increased demand for the radioisotope that is not matched by its current supply. To expand the possibilities for the development of ²²⁵Ac-based TAT therapeutics, a new hydroxamate-based chelator, DFO*¹², is described. We report the DFT-guided design of dodecadentate DFO*¹² and an efficient and convenient automated solid-phase synthesis for its preparation. To address the limited availability of ²²⁵Ac, a small-scale ²²⁹Th/²²⁵Ac generator was constructed in-house to provide [²²⁵Ac]AcCl₃ for research. Methods: DFT calculations were performed in ORCA 5.0.1 using the BP86 functional with empirical dispersion correction D3 and Becke–Johnson damping (D3BJ). The monomer synthesis over three steps enabled the solid-phase synthesis of DFO*¹². The small-scale ²²⁹Th/²²⁵Ac generator was realized by extracting ²²⁹Th from aged ²³³U material. Radiolabeling of DFO*¹² with ²²⁵Ac was performed in 1 M TRIS pH 8.5 or 1.5 M NaOAc pH 4.5 for 30 min at 37 °C. Results: DFT calculations directed the design of a dodecadentate chelator. The automated synthesis of the chelator DFO*¹² and the development of a small-scale ²²⁹Th/²²⁵Ac generator allowed for the radiolabeling of DFO*¹² with ²²⁵Ac quantitatively at 37 °C within 30 min. The complex [²²⁵Ac]Ac-DFO*¹² indicated good stability in different media for 20 h. Conclusions: The novel hydroxamate-based dodecadentate chelator DFO*¹², together with the developed ²²⁹Th/²²⁵Ac generator, provide new opportunities for ²²⁵Ac research for future radiopharmaceutical development and applications in TAT.

Bioorthogonal bond-cleavage reactions are powerful tools for investigating biological dynamics and advancing therapeutic strategies. Primed by click reactivity, the multi-step cascades that lead to bioorthogonal release have nevertheless been challenging to navigate. For the widely recognized tetrazine-triggered cleavage of trans-cyclooctenes (TCO), the complexities of post-click tautomerization are the key barrier to achieving next-level molecular tools. Strategies to anchor directing functionalities to tetrazines (Tz) accelerate tautomerization, but their scope has been limited by interlocking impacts on Tz click reactivity and stability. Here we shift the tautomerization driving force from the Tz to the TCO, introducing iTCO, a click-cleavable linker that unlocks remarkably efficient release with highly reactive tetrazines, regardless of click orientation. Across biological systems of rising complexity—from media to in vivo—iTCO enables fast and complete click-triggered cleavage at single-digit micromolar concentrations in seconds, driving release performance to levels of bioorthogonal efficiency previously achieved only in ligations.

Arene–tetrazine cycloadditions involving benzene as the dienophile represent an exceptionally rare class of Diels–Alder reactions. In this study, we computationally investigate the origin of the intrinsic reactivity of simple aromatic systems toward tetrazines and identify the factors that enable successful cycloaddition. Our results show that only highly activated tetrazines can overcome the unfavorable orbital overlap associated with the compact π-system of benzene. Energy decomposition analysis reveals that reactivity is governed by a combination of frontier molecular orbital interactions and charge-control effects. Substituent effects strongly influence the activation barrier, particularly when they promote a shift toward a more asynchronous and polar transition state. These findings provide mechanistic insight into a rare mode of arene reactivity and clarify the electronic requirements for enabling tetrazine Diels–Alder reactions with aromatic dienophiles.

Theoretical enzyme models (“theozymes”) offer a reduced yet chemically insightful view of enzymatic active sites, enabling the study of catalytic mechanisms at the quantum chemical level. In this work, we present a methodological proof-of-concept applying energy decomposition analysis to theozyme models to investigate the electronic contributions of individual amino acid residues to transition state stabilization. Using a Diels–Alder cycloaddition as a model reaction, we selectively removed key active-site residues and analyzed the resulting changes in activation barriers and energy components. While some residues exhibited only minor electronic effects, others proved counterproductive by introducing destabilizing hydrogen bonds in the context of normal electron-demand reactivity. The analysis also revealed limitations in cases where structural inconsistencies between reactant complexes and transition states hinder interpretability. Despite these constraints, the study demonstrates that energy decomposition analysis applied to theozymes can provide mechanistic insight into local electronic effects and offers a promising approach for guiding rational modifications in enzyme design.

Accurate 3D structure generation from SMILES is essential for data-driven chemistry but often fails for strained ring systems. We introduce strainedSMILES2xyz, a Python workflow that improves conformer generation by relaxing RDKit constraints, exploring stereoisomer variants, and correcting errors using force-field refinement. Benchmarking on strained and unstrained rings shows that it outperforms existing tools, generating correct geometries in nearly all cases. The workflow is available as a Python package and Jupyter notebook.