Isopeptidic Desferrioxamine Analogues: The Role of Hydroxamate Spacing for Chelation of Zr⁴⁺

[⁸⁹Zr]Zr⁴⁺ is a radionuclide of increasing clinical relevance for PET (positron emission tomography). However, an ideal chelator for stable Zr-chelation remains to be discovered. This study describes the solid-phase synthesis of octadentate Zr-chelators based on an isopeptidic (ip) scaffold derived from the natural siderophore desferrioxamine (DFOB). Several analogues with different spacers separating the chelating hydroxamates have been prepared and converted to [⁸⁹Zr]Zr-complexes. The stability of these complexes was evaluated in human serum and in competition to excess of competing chelators. The assays revealed a beneficial effect of long hydroxamate spacing (9 atoms). Shorter spacing led to a decrease in complex stability. The most stable [⁸⁹Zr]Zr-ipDFO complex had a high stability in challenging competition experiments with a large excess of EDTA for 72 h as determined by radio TLC and LC/MS. The straightforward synthesis, high complex stability and a modular character make ipDFO derivatives promising chelators for applications in targeted PET.