New Synthetic Strategies Toward DFO*: Enhanced Yield and Purity of a Key Chelator for ⁸⁹Zr Chemistry

Background: Zirconium-89 (⁸⁹Zr) is a key PET radionuclide and the limited in vivo stability of its clinically used ⁸⁹Zr-DFO complexes has driven the pursuit of improved chelator architectures. Among these, DFO* has attracted particular attention due to its exceptional complex stability with ⁸⁹Zr⁴⁺ and favorable pharmacokinetics of the corresponding bioconjugates in vivo. Despite these advantages, DFO*'s broader application has been hampered by significant synthetic challenges, primarily arising from its pronounced acid sensitivity. Methods: Here, we present a systematic investigation of the acid lability of DFO and DFO*-derived systems, revealing substantial degradation under acidic conditions being commonly applied during preparation and purification. These findings highlight critical limitations of conventional synthetic and purification protocols. To address this, we developed two complementary synthetic routes that consistently avoid fragmentation-inducing conditions. Results: THP/Boc- and TBDPS/Fmoc-based routes provide robust five- and six-step syntheses of DFO*, affording overall yields of 11% and 13%/6.1% and high purity (≥98%) without detectable degradation. Conclusions: By systematically investigating the acid sensitivity of DFO/DFO*-based chelators and providing practical synthetic solutions, this work enables reliable access to DFO* and advances its application in ⁸⁹Zr radiopharmaceutical development.