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

Immuno-PET is an emerging imaging tool for the selection of high potential antibodies (mAbs) for imaging and therapy. The positron emitter zirconium-89 (Zr-89) has attractive characteristics for immuno-PET with intact mAbs. Previously, we have described a multi-step procedure for stable coupling of Zr-89 to mAbs via the bifunctional chelate (BFC) tetrafluorophenol-N-succinyldesferal (TFP-N-sucDf). To enable widespread use of Zr-89-immuno-PET, we now introduce the novel BFC p-isothiocyanatobenzyl-desferrioxamine B (Df-Bz-NCS) and compare its performance in Zr-89-immuno-PET with the reference BFC TFP-N-sucDf. Three mAbs were premodified with Df-Bz-NCS and labeled with Zr-89 at different pHs to assess the reaction kinetics and robustness of the radiolabeling. Stability of both Zr-89-Df-Bz-NCS- and Zr-89-N-sucDf-conjugates was evaluated in different buffers and human serum. Comparative biodistribution and PET studies in tumor-bearing mice were undertaken. 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 37A degrees C, with < 5% Zr-89 release, and was equally stable compared to the reference conjugate when stored in the appropriate buffer at 4A degrees C. In biodistribution and imaging experiments, the novel and the reference radioimmunoconjugates showed high and similar accumulation in tumors in nude mice. The novel Df-Bz-NCS BFC allows efficient and easy preparation of optimally performing Zr-89-labeled mAbs, facilitating further exploration of Zr-89-immuno-PET as an imaging tool.