²²⁵Aс/²¹³Bi generator for direct synthesis of ²¹³Bi-labeled bioconjugates

²¹³Bi is a short-lived radionuclide currently trialed for alpha therapy of various oncological diseases. A serious obstacle to the wide medical use is decay losses of ²¹³Bi during a conventional synthesis of radiopharmaceuticals. In this work, we aimed to develop a two-column ²²⁵Aс/²¹³Bi generator providing the accumulation of ²¹³Bi separately from the parent ²²⁵Ac via continuous circular separation and decay of intermediate ²²¹Fr. When attaining the transient equilibrium, ²¹³Bi could be promptly extracted from the generator with an appropriate complexing agent, including chelator-protein bioconjugates.

Sorption behavior of Bi(III) ions onto the cross-linked dextran gel Sephadex G-25 was studied from solutions of hydrochloric and nitric acid, and from sodium chloride, sodium acetate and DTPA solutions. A bifunctional chelating agent p-SCN-Bn-DTPA was conjugated to an antibody Nimotuzumab specific to the epidermal growth factor receptor, and the procedure of ^207,213Bi-DTPA-Nimotuzumab synthesis in the dextran gel medium was developed. The parameters of ²²⁵Aс/²¹³Bi generator system were evaluated.

The weight distribution ratios of Bi(III) adsorbed onto the Sephadex G-25 gel were obtained. Up to 85 % of ²¹³Bi was accumulated in the second Sephadex-filled column of ²²⁵Aс/²¹³Bi generator after four-hour circulation of 0.15 M NaCl (pH 5.5) solution. Having passed the solution of DTPA-Nimotuzumab bioconjugate through the second column, a fraction of ²¹³Bi-DTPA-Nimotuzumab radioimmunoconjugate was produced with the radiochemical yield of 64 % ± 3 % (n = 6). High radionuclidic and radiochemical purity of product was achieved.

The circulating ²²⁵Aс/²¹³Bi generator provides a ²¹³Bi-labeled bioconjugate as a final product. While a conventional synthesis route including generator milking, bioconjugate labeling and size-exclusion purification takes >20 min, the duration of ²¹³Bi-DTPA-Nimotuzumab production by the method proposed in this work is reduced to 6-8 min.