p-Isothiocyanatobenzyl-desferrioxamine: a new bifunctional chelate for facile radiolabeling of monoclonal antibodies with zirconium-89 for immuno-PET imaging

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Abstract
Purpose Immuno-PET is an emerging imaging tool for the selection of high potential antibodies (mAbs) for imaging and therapy. The positron emitter zirconium-89 (89Zr) has attractive characteristics for immuno-PET with intact mAbs. Previously, we have described a multi-step procedure for stable coupling of 89Zr to mAbs via the bifunctional chelate (BFC) tetrafluorophenol-N-succinyldesferal (TFP-N-sucDf). To enable widespread use of 89Zr-immuno-PET, we now introduce the novel BFC p-isothiocyanatobenzyl-desferrioxamine B (Df-Bz-NCS) and compare its performance in 89Zr-immuno-PET with the reference BFC TFP-N-sucDf. Methods Three mAbs were premodified with Df-Bz-NCS and labeled with 89Zr at different pHs to assess the reaction kinetics and robustness of the radiolabeling. Stability of both 89Zr-Df-Bz-NCS- and 89Zr-N-sucDf-conjugates was evaluated in different buffers and human serum. Comparative biodistribution and PET studies in tumor-bearing mice were undertaken. Results The selected conjugation conditions resulted in a chelate:mAb substitution ratio of about 1.5:1. Under optimal radiolabeling conditions (pH between 6.8–7.2), the radiochemical yield was >85% after 60 min incubation at room temperature, resulting in radioimmunoconjugates with preserved integrity and immunoreactivity. The new radioimmunoconjugate was very stable in serum for up to 7 days at 37°C, with 89Zr release, and was equally stable compared to the reference conjugate when stored in the appropriate buffer at 4°C. In biodistribution and imaging experiments, the novel and the reference radioimmunoconjugates showed high and similar accumulation in tumors in nude mice. Conclusions The novel Df-Bz-NCS BFC allows efficient and easy preparation of optimally performing 89Zr-labeled mAbs, facilitating further exploration of 89Zr-immuno-PET as an imaging tool.