Moreover, 89Zr is a residualising radionuclide when a mAb internalises, 89Zr-mAbs give higher tumor-to-normal tissue ratios than, e.g., the corresponding 124I-labeled mAbs resulting in better contrast on PET images. 89Zr is ideally suited for this purpose because its physical half-life of 78.4 h matches the residence time of intact mAbs in the body (typically several days). PET imaging with 89Zr-labeled monoclonal antibodies (mAbs) or other targeted vehicles (e.g., peptides, nanoparticles, and cells), collectively called 89Zr-immuno-PET, can be used for better understanding of disease targets and the in vivo behavior of targeted drugs. This makes the new octadentate DFO* chelator a candidate successor of DFO for future clinical 89Zr-immuno-PET. Zr-DFO*-trastuzumab showed superior in vitro stability and in vivo performance when compared to 89Zr-DFO-trastuzumab. about two fold for the DFO* conjugate, while it increased about two fold for the DFO conjugate. The uptake in bone decreased from 24 h to 144 h p.i. The bone uptake was significantly lower for 89Zr-DFO*-trastuzumab compared to 89Zr-DFO-trastuzumab.
In vivo uptake at 144 h post injection (p.i.) in tumors, blood, and most normal organs was similar for both conjugates, except for skin, liver, spleen, ileum, and bone. In 0.9 % NaCl 89Zr-DFO*-trastuzumab was more stable than 89Zr-DFO-trastuzumab after 72 h incubation at 2-8 ☌ 95 % and 58 % intact tracer were left, respectively, while in a histidine-sucrose buffer no difference was observed, both products were ≥ 92 % intact.
The in vivo performance of the chelators was compared in N87 tumor-bearing mice by biodistribution studies and PET imaging. In vitro stability experiments were carried out in saline, a histidine/sucrose buffer, and blood serum.
Subsequently, trastuzumab was conjugated with either DFO*- pPhe-NCS or commercial DFO- pPhe-NCS and radiolabeled with Zr-89 according to published procedures. The bifunctional chelator DFO*- pPhe-NCS was prepared from previously reported DFO* and p-phenylenediisothiocyanate. Aiming at clinical improvements, we report here on a bifunctional isothiocyanate variant of the octadentate chelator DFO* and the in vitro and in vivo comparison of its 89Zr-DFO*-mAb complex with 89Zr-DFO-mAb. The unsaturated coordination of DFO to zirconium has been suggested to result in impaired stability of the complex in vivo and consequently in unwanted bone uptake of 89Zr. This chelator provides hexadentate coordination to zirconium, leaving two coordination sites available for coordination with, e.g., water molecules, which are relatively labile ligands. All clinical 89Zr-immuno-PET studies are currently performed with the chelator desferrioxamine (DFO).
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