⁵⁵Co separation from deuteron-irradiated electrodeposited ⁵⁴Fe targets

Objectives ⁵⁵Co (t_1/2=17.53 h, 76% β⁺) has been labeled to several agents for imaging with PET[1-6]. The highest radionuclidic purity is obtained from the (d,n) reaction on enriched ⁵⁴Fe[7,8]. Our aim was to develop an electroplating method that allows the fabrication of thick ⁵⁴Fe targets for high power irradiations and addresses the need for recycling the valuable target material after a novel separation method based on chromatography columns that achieves high Co separation yield, Fe recovery yield and reactivity with NOTA and DOTA.

Methods 70 mg of ^natFe or ⁵⁴Fe (99.93%) powder was dissolved in 15 mL of ammonium oxalate solution. The electrolyte with pH=3 was transferred to a plating cell. 7.0 V and 100 mA/cm² was applied using a Pt anode at 1 cm from a Ag disk for >15 h. Irradiations were performed with 8 MeV deuterons. Chemical separation was carried out in HCl/EtOH medium using 4 columns: AG1-x8, C18, UTEVA and DGA, connected in a peristaltic pump driven module. ⁵⁵Co was eluted in 400μL fractions of H₂O. Radionuclidic assay employed a HPGe detector and an ion chamber for decay logging. Reactivity with NOTA and DOTA was measured by titration labeling at pH 4.5 in acetate buffer at 25°C for 1 h. MicroPET images of a Derenzo phantom were obtained for resolution evaluation.

Results 95% of the initial Fe was electroplated over 1 cm² of Ag (n=4). Maximum current was 30 μA with no mass loss during a 2 h-long irradiation. ⁵⁵Co and ⁵²Mn yields are shown in Table 1. ⁵⁵Co with >99.9% radionuclidic purity was separated in 2.5 h. The 2^nd fraction of 400μL eluate contained 82±7% (n=3) of ⁵⁵Co produced. Fe recovery yield for re-irradiation was 73±6%. Measured half-life was 17.645±0.005 h. Reactivity with NOTA and DOTA was 0.58 and 0.22 GBq/μmol. The Derenzo phantom showed resolution of 1.5 mm comparable to ¹⁸F: 1.25 mm.

Conclusions This targetry and radiochemical separation procedure makes ⁵⁵Co production economically feasible. The high reactivity renders this separated isotope useful for labeling chelator-conjugated biomolecules for PET studies.

Research Support US Department of Energy (DOE-SC0008384) Advanced Opportunity Fellowship offered by the Science and Medicine Graduate Research Scholars (SciMed GRS) Program at UW- Madison.