72 As, 74 As, 77 As / Radioactive arsenic / Germanium target / Radiochemical separation / Labelled antibodies Summary. Radioarsenic labelled radiopharmaceuticals could add special features to molecular imaging with positron emission tomography (PET). For example the long physical half-lives of 72 As (T 1/2 = 26 h) and 74 As (T 1/2 = 17.8 d) in conjunction with their high positron branching rates of 88% and 29%, respectively, allow the investigation of slow physiological or metabolical processes, like the enrichment and biodistribution of monoclonal antibodies in tumour tissue or the characterization of stem cell trafficking. A method for separation and purification of no-carrier-added (nca) arsenic from irradiated metallic germanium targets based on distillation and anion exchange is developed. It finally converts the arsenic into an * As(III) synthon in PBS buffer and pH 7 suitable for labelling of proteins via As−S bond formations. The method delivers radioarsenic in high purity with separation factors of 10 6 from germanium and an overall yield from target to labelling synthon of > 40%. In a proof-ofprinciple experiment, the monoclonal antibody Bevacizumab, directed against the human VEGF receptor, was labelled with a radiochemical yield > 90% within 1 h at room temperature with nca 72/74/77 As.

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Fast progressing immuno-PET asks to explore new radionuclides. One of the promising candidates is 90 Nb. It has a half-life of 14.6 h that allows visualizing and quantifying biological processes with medium and slow kinetics, such as tumor accumulation of antibodies and antibodies fragments or drug delivery systems and nanoparticles. 90 Nb exhibits a positron branching of 53% and an average kinetic energy of emitted positrons of mean = 0.35 MeV. Currently, radionuclide production routes and Nb V labeling techniques are explored to turn this radionuclide into a useful imaging probe. However, efficient separation of 90 Nb from irradiated targets remains in challenge.Ion exchange based separation of 90 Nb from zirconium targets was investigated in systems AG 1 × 8 -HCl/H 2 O 2 and UTEVA-HCl. 95 Nb ( 1/2 = 35.0 d), 95 Zr ( 1/2 = 64.0 d) and 92m Nb ( 1/2 = 10.15 d) were chosen for studies on distribution coefficients. Separation after AG 1 × 8 anion exchange yields 99% of 90/95 Nb. Subsequent use of a solidphase extraction step on UTEVA resin further decontaminates 90/95 Nb from traces of zirconium with yields 95% of 90/95 Nb.A semi-automated separation takes one hour to obtain an overall recovery of 90/95 Nb of 90%. The amount of Zr was reduced by factor of 10 8 . The selected separation provides rapid preparation (< 1 h) of high purity 90 Nb appropriate for the synthesis of 90 Nb-radiopharmaceuticals, relevant for purposes of immuno-PET. Applying the radioniobium obtained, 90/95 Nb-labeling of a monoclonal antibody (rituximab) modified with desferrioxamine achieved labeling yields of > 90% after 1 h incubation at room temperature.

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