Chemical properties

In solution La-133 is trivalent (La³⁺); like La-135, its ionic radius closely approaches that of Ac³⁺, making La-133 a PET imaging surrogate for Ac-225-based targeted alpha therapy. La³⁺ can be efficiently chelated with the macrocyclic ligand macropa under mild conditions (room temperature, 30 min, ammonium acetate buffer pH 6), as well as with DOTA and PSMA-I&T REF.

Nuclear properties

La-133 decays with a half-life of 3.91 h, mainly by electron capture (92.9%) to Ba-133 (itself long-lived, t_1/2 = 10.55 y), with a positron-emission branch (7.1%) of 463 keV mean energy REF.

Production

La-133 is produced by proton irradiation of enriched Ba-134 carbonate targets ([¹³⁴Ba]BaCO₃) via the Ba-134(p,2n)La-133 reaction. Using a silver-backed solid target (30 mg [¹³⁴Ba]BaCO3) irradiated for 60 min at (17.9 ± 0.1) MeV and 10–15 µA beam current, activities of 1.3–1.9 GBq are obtained at end of irradiation.

Distribution

Radiochemical separations are performed at the Institute of the Radiopharmaceutical Cancer Research (HZDR, Dresden, Germany). Activity will be shipped from there to the users or PRISMAP+ biomedical research facilities in the form of [¹³³La]LaCl₃
solution.

Examples of use

• La-133 acts as a positron-emitting diagnostic surrogate for Ac-225-based targeted alpha therapy of macropa-PSMA conjugates, providing precise early PET imaging with a half-life well matched to fast tumour-targeting kinetics REF.
• In a radiohybrid approach, La-133-labelled and I-123-labelled variants of the same albumin-binding PSMA ligand showed comparable tumour uptake in LNCaP tumour-bearing mice, demonstrating that La-133 PET and I-123 SPECT can serve as complementary early- and late-stage imaging partners ahead of Ac-225 therapy REF.
• The macropa-PSMA conjugate mcp-M-PSMA has been radiolabelled with La-133 in quantitative yield (radiochemical purity ~98%) at room temperature within 15–30 min, achieving apparent molar activities up to 330 GBq/µmol REF.