Chemical properties

In solution, Ba-131 is divalent (Ba²⁺) and shows calcimimetic behaviour. Its chemistry and pharmacological behaviour closely resemble those of radium, making free [¹³¹Ba]Ba²⁺ a non-radiotoxic surrogate for [²²³Ra]Ra²⁺/[²²⁴Ra]Ra²⁺. Complexation of the free Ba²⁺ ion is challenging owing to the weak acceptor/donor properties of heavy alkaline-earth metals; nonetheless, the macrocyclic ligand macropa efficiently labels Ba-131 at ligand concentrations down to 10⁻⁴ M in ammonium acetate buffer, and the resulting [¹³¹Ba]Ba-macropa complex is stable in human serum for at least 3 days REF.

Nuclear properties

Ba-131 decays with a half-life of 11.5 d by 100% electron capture to Cs-131 (t_1/2 = 9.69 d), which itself decays by 100% electron capture to stable Xe-131.

Ba-131 emits γ rays at 124 keV (30%), 216 keV (20%), 371 keV (14%) and 496 keV (48%); the daughter Cs-131 emits only low-energy X rays at 29-35 keV (about 73% combined)REF.

Production

No-carrier-added Ba-131 is produced by proton irradiation of natural, monoisotopic caesium chloride targets via the Cs-133(p,3n)Ba-131 reaction. The targets are irradiated At HZDR on the TR-FLEX cyclotron with 27.5 MeV protons at 15 µA for 4 h (60 µAh). This yields (190 ± 26) MBq of Ba-131 per irradiation (saturation yield (1.2 ± 0.2) GBq/µA) REF.

Distribution

The radiochemical separation is performed at the production site and the activity will be shipped from there to the users in the form of [¹³¹Ba]Ba(NO₃)₂ solution.

Examples of use

• Ba-131 is a promising SPECT-imageable diagnostic match for Ra-223/Ra-224-based targeted alpha therapy of bone metastases (e.g., Xofigo®) and for Ra-224-based brachytherapy (DaRT), owing to the shared chemistry and bone-seeking behaviour of barium and radium.
• Small-animal SPECT/CT of free [¹³¹Ba]Ba(NO₃)₂ in mice showed rapid, specific accumulation throughout the skeleton (highest in epiphyses, vertebral bodies and the upper jaw), confirming its use as a bone-seeking imaging tracer, whereas [¹³¹Ba]Ba-macropa cleared rapidly from blood via the renal and hepatobiliary pathways with markedly (p < 0.001) lower bone uptake, demonstrating that chelation can redirect Ba-131 away from bone for use as a true radium surrogate in targeted radiopharmaceuticals REF.