Labeling proteins with ⁸⁹Zr separated from large yttrium bulks

Objectives ⁸⁹Zr (T_½=78.4 h, β⁺=22.7%) is an upcoming attractive PET radionuclide, easily accessible with hospital cyclotrons. The goal was to investigate if ⁸⁹Zr produced in relatively large yttrium bulks can be isolated and used for labeling of protein e.g. trastuzumab conjugated with the bifunctional chelate p-isothiocyanatobenzyl-desferrioxamine (Df-Bz-NCS).

Methods Yttrium foils (50x20x0.64 mm³, 2.8 g, ~350 USD) were direct-cooled with water and irradiated with 45 µA, 12.8 MeV protons (MC 17 Scanditronix) and remotely dropped in a lead pig after irradiation. The foils were dissolved in 6 M HCl and diluted to <2 M. ⁸⁹Zr was isolated with a hydroxamate resin. The monoclonal antibody trastuzumab was conjugated with the chelator Df-Bz-NCS. ⁸⁹Zr-labeling of Df-trastuzumab was made with a slightly modified protocol [1]. Radiolabeling efficiency was determined with ITLC. Purification was made with a PD-10 column. In vitro stability of the labeled conjugate was assessed in PBS. The binding capability of ⁸⁹Zr-Df-trastuzumab was studied in vitro using HER2-expressing SKBR-3 cells.

Results Typically ⁸⁹Zr productions were about 2.1 GBq (58 mCi) for 1 h irradiation. More than 90% of the produced ⁸⁹Zr was recovered as [⁸⁹Zr]Zr-oxalate. The Df-trastuzumab conjugate gave an overall labeling yield of 90% and a radiochemical purity >95%. The ⁸⁹Zr-Df-trastuzumab was stable (99% intact) in PBS for at least 3 weeks and retained its binding specificity to HER2 receptors in vitro.

Conclusions Thick foils allow high ⁸⁹Zr activity productions and provide mechanical stability which is used in the remote handling of the activated foils. The results show the feasibility of using hospital cyclotrons for high activity production of ⁸⁹Zr for high efficiency protein labeling.

Research Support Swedish Cancer Foundation, Swedish Research Council, Berta Kamprad Foundation, Gunnar Nilsson Cancer Foundation and Medical faculty ALF grants. [1] Vosjan M.J.W.D., et al., Nature.Protocols 2010;5(4):739-4