Standardized methods for the production of high specific-activity zirconium-89

Abstract

Zirconium-89 is an attractive metallo-radionuclide for use in immuno-PET due to favorable decay characteristics. Standardized methods for the routine production and isolation of high-purity and high-specific-activity (89)Zr using a small cyclotron are reported. Optimized cyclotron conditions reveal high average yields of 1.52+/-0.11 mCi/muA.h at a proton beam energy of 15 MeV and current of 15 muA using a solid, commercially available (89)Y-foil target (0.1 mm, 100% natural abundance). (89)Zr was isolated in high radionuclidic and radiochemical purity (>99.99%) as [(89)Zr]Zr-oxalate by using a solid-phase hydroxamate resin with >99.5% recovery of the radioactivity. The effective specific-activity of (89)Zr was found to be in the range 5.28-13.43 mCi/microg (470-1195 Ci/mmol) of zirconium. New methods for the facile production of [(89)Zr]Zr-chloride are reported. Radiolabeling studies using the trihydroxamate ligand desferrioxamine B (DFO) gave 100% radiochemical yields in <15 min at room temperature, and in vitro stability measurements confirmed that [(89)Zr]Zr-DFO is stable with respect to ligand dissociation in human serum for >7 days. Small-animal positron emission tomography (PET) imaging studies have demonstrated that free (89)Zr(IV) ions administered as [(89)Zr]Zr-chloride accumulate in the liver, whilst [(89)Zr]Zr-DFO is excreted rapidly via the kidneys within <20 min. These results have important implication for the analysis of immuno-PET imaging of (89)Zr-labeled monoclonal antibodies. The detailed methods described can be easily translated to other radiochemistry facilities and will facilitate the use of (89)Zr in both basic science and clinical investigations.

Figure 1

Nuclear decay scheme showing the main pathways for the decay of ⁸⁹Zr and ^89mZr to ⁸⁹ Y.[5, 47]

Figure 2

Photograph of the Memorial Sloan-Kettering Cancer Center (MSKCC) custom-made water-cooled solid-target assembly for the TR19/9 cyclotron used in the proton bombardment for the ⁸⁹Y(p,n)⁸⁹Zr transmutation reaction.

Figure 3

Measured loading capacity of the hydroxamate resin towards ⁸⁹Zr(IV) ions in 2 M HCl(aq.) solution. Binding capacities measured in triplicate for 3 separate hydroxamate resin preparations were in the range 0.13 – 0.31 (average: 0.25 ± 0.08) mmol/g. Overall yields for the two-step carboxylate-to-hydroxamate functional group conversion reactions were found to be in the range 37 – 89%, with an average of 71%.

Figure 4

Spectrum of the γ ray emissions observed from a purified sample of ⁸⁹Zr recorded 16 h after the end-of-bombardment (EOB). The spectrum is free of common and potential radioactive impurities as shown by the absence of peaks corresponding to ⁸⁸Zr (392.87(9) keV, 100%)) and ⁸⁸Y (898.042(3) and 1836.063(12) keV, with relative intensities of 93.7(3)% and 99.2(3)%, respectively) and the ⁸⁹Zr was isolated in >99.99% radionuclidic purity.[5] Two small peaks are observed at 1461 and 1713 keV assigned to the decay of ⁴⁰K (0.01% natural abundance) and ⁸⁹Zr (I_γ = 0.76%, intensity relative to the emission at 909 keV), respectively. The isomeric excited state species ^89mZr decays rapidly within 40 min. after the end of bombardment and was not observed.

Figure 5

DFO ligand titration experiment used in the determination of specific-activity of a purified ⁸⁹Zr(IV) oxalate solution.

Figure 6

Spatial resolution measurements showing: a) a static 10 min. PET image recorded by using a “Derenzo phantom” filled with 6.0 mL of ⁸⁹Zr (22.5 μCi/mL). Images were reconstructed by using filtered back-projection; b) the line-width profile and Gaussian-fit measured from a static 10 min. PET image of ⁸⁹Zr (35 μCi) in a glass microcapillary pipette (10 μm diameter, Kimble Glass Inc.). The full-width at half-maximum (FWHM) is 1.94 mm which indicates that image resolution using ⁸⁹Zr is approximately the same as the specified detector resolution of the Focus 120 microPET camera, and is comparable to the resolution obtained by using ¹⁸F or ⁶⁴Cu radionuclides.

Figure 7

Small-animal PET images of male athymic nu/nu mice injected with ⁸⁹Zr-activity via the tail vein and showing the in vivo distribution of a) [⁸⁹Zr]Zr-DFO recorded between 0 – 4 min., and b) [⁸⁹Zr]Zr-chloride 8 h post-administration.

Figure 8

Tissue-activity curves (TACs) of the heart, kidneys and bladder generated from volumes-of-interest (VOIs) drawn on the dynamic PET images of [⁸⁹Zr]Zr-DFO recorded between 0 – 60 min. post-administration.