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.

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.

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 <5% 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.

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.