Step forward with Bi-213: Cyclen-based phosphonates show improved radiolabeling and stability

Objectives Bi-213 is an attractive alpha-emitter for targeted radiotherapy of cancer. It is commonly introduced into small peptides via a DOTA complex, and into antibodies via CHX-A-DTPA. We investigated Bi-213-labeling efficiency, kinetics and stability of phosphonic and phosphinic acid analogs of DOTA.

Methods Bi-213 (≈4 mCi) was eluted from a ²²⁵Ac/²¹³Bi generator (Institute for Transuranium Elements, Karlsruhe, Germany) as anionic species (²¹³BiI₄^-/²¹³BiI₅^2-). Labeling of CHX-A-DTPA, DOTA, its phosphonic analog DOTP, monoethylester DOTP-OEt, phosphinic analogs DOTPH and DOTPI was done at r.t. and pH 5.5 (1 M aq. NaOAc). The formation of Bi-213 complexes as well as their stability in PBS, HCl (pH 1), plasma, DTPA (0.1 M, aq., pH 7.5) and in the presence of Zn²⁺ (50 mM), was evaluated by radio-TLC.

Results DOTP followed by DOTPH outperformed CHX-A-DTPA in terms of labeling efficiency. Somewhat lower were the radiochemical yields (RCY) obtained with DOTP-OEt and DOTPI. Labeling of DOTA resulted only in 60 % incorporated activity, even at c = 100 µM. DOTP was labeled to ≈ 90 % already at 1 µM concentration, whereas similar RCY required c = 100 µM of DOTPH and CHX-A-DTPA. The labeling kinetics in 10 µM ligand solutions followed the order: CHX-A-DTPA ≈ DOTP > > DOTPH > DOTPI > DOTP-OEt >> DOTA. Highest ²¹³Bi-complex kinetic stability was observed for phosphonates, followed by phosphinates, DOTA and less stable CHX-A-DTPA.

Conclusions DOTA-analog phosphonate chelators show excellent ²¹³Bi complexation properties. Particularly, the cyclen-based tetraphosphonic acid ligand DOTP was ²¹³Bi-labeled with higher efficiency than DOTA and CHX-A-DTPA, and showed higher kinetic inertness. Therefore, bifunctional analogues of DOTP-like ligands are highly recommendable for future development of Bi-213 radiopharmaceuticals.

Research Support Financial support by DFG (SFB 824, projects Z1/B5) is gratefully acknowledged.