⁹⁰Nb – a potential PET nuclide: production and labeling of monoclonal antibodies

Abstract

Fast progressing immuno-PET gives reasons to develop new potential medium-long and long-lived radioisotopes. One of the promising candidates is ⁹⁰Nb. It has a half-life of 14.6 h, which allows visualizing and quantifying processes with medium and slow kinetics, such as tumor accumulation of antibodies and antibodies fragments or polymers and other nanoparticles. ⁹⁰Nb exhibits a high positron branching of 53% and an optimal energy of β⁺ emission of E_mean=0.35 MeV only. Consequently, efficient radionuclide production routes and Nb^V labeling techniques are required.

⁹⁰Nb was produced by the ⁹⁰Zr(p,n) ⁹⁰Nb nuclear reaction on natural zirconium targets. No-carrier-added (n.c.a.) ⁹⁰Nb was separated from the zirconium target via a multi-step separation procedure including extraction steps and ion-exchange chromatography. Protein labeling was exemplified using the bifunctional chelator desferrioxamine attached to the monoclonal antibody rituximab. Desferrioxamine was coupled to rituximab via two different routes, by the use of N-succinyl-desferrioxamine (N-suc-Df) and by means of the bifunctional derivative p-isothiocyanatobenzyl-desferrioxamine B (Df-Bz-NCS), respectively. Following antibody modification, labeling with ⁹⁰Nb was performed in HEPES buffer at pH 7 at room temperature. In vitro stability of the radiolabeled conjugates was tested in saline buffer at room temperature and in fetal calf serum (FCS) at 37 ºC.

The selected production route led to a high yield of 145 ± 10 MBq/μA h of ⁹⁰Nb with high radioisotopic purity of >97%. This yield may allow for large scale production of about 10 GBq ⁹⁰Nb. The separation procedure resulted in 76–81% yield. The Zr/⁹⁰Nb decontamination factor reaches 10⁷. Subsequent radiolabeling of the two different conjugates with ⁹⁰Nb gave high yields; after one hour incubation at room temperature, more than 90% of ⁹⁰Nb-Df-mAb was formed in both cases. At room temperature in aqueous solution, both ⁹⁰Nb-Df-mAb constructs were more than 99% stable over a period of 18 d.

The developed production and separation strategy provided ⁹⁰Nb with purity appropriate for radiolabeling applications. Labeling and stability studies proved the applicability of ⁹⁰Nb as a potential positron emitter for immuno-PET.