Theranostic Imaging Surrogates for Targeted Alpha Therapy: Progress in Production, Purification, and Applications

This article highlights recent developments of SPECT and PET diagnostic imaging surrogates for targeted alpha particle therapy (TAT) radiopharmaceuticals. It outlines the rationale for using imaging surrogates to improve diagnostic-scan accuracy and facilitate research, and the properties an imaging-surrogate candidate should possess. It evaluates the strengths and limitations of each potential imaging surrogate. Thirteen surrogates for TAT are explored: 133La, 132La, 134Ce/134La, and 226Ac for 225Ac TAT; 203Pb for 212Pb TAT; 131Ba for 223Ra and 224Ra TAT; 123I, 124I, 131I and 209At for 211At TAT; 134Ce/134La for 227Th TAT; and 155Tb and 152Tb for 149Tb TAT.

Efficient Production of the PET Radionuclide (133)La for Theranostic Purposes in Targeted Alpha Therapy Using the (134)Ba(p,2n)(133)La Reaction

Targeted Alpha Therapy is a research field of highest interest in specialized radionuclide therapy. Over the last decades, several alpha-emitting radionuclides have entered and left research topics towards their clinical translation. Especially, 225Ac provides all necessary physical and chemical properties for a successful clinical application, which has already been shown by [225Ac]Ac-PSMA-617. While PSMA-617 carries the DOTA moiety as the complexing agent, the chelator macropa as a macrocyclic alternative provides even more beneficial properties regarding labeling and complex stability in vivo. Lanthanum-133 is an excellent positron-emitting diagnostic lanthanide to radiolabel macropa-functionalized therapeutics since 133La forms a perfectly matched theranostic pair of radionuclides with the therapeutic radionuclide 225Ac, which itself can optimally be complexed by macropa as well. 133La was thus produced by cyclotron-based proton irradiation of an enriched 134Ba target. The target (30 mg of [134Ba]BaCO3) was irradiated for 60 min at 22 MeV and 10−15 µA beam current. Irradiation side products in the raw target solution were identified and quantified: 135La (0.4%), 135mBa (0.03%), 133mBa (0.01%), and 133Ba (0.0004%). The subsequent workup and anion-exchange-based product purification process took approx. 30 min and led to a total amount of (1.2−1.8) GBq (decay-corrected to end of bombardment) of 133La, formulated as [133La]LaCl3. After the complete decay of 133La, a remainder of ca. 4 kBq of long-lived 133Ba per 100 MBq of 133La was detected and rated as uncritical regarding personal dose and waste management. Subsequent radiolabeling was successfully performed with previously published macropa-derived PSMA inhibitors at a micromolar range (quantitative labeling at 1 µM) and evaluated by radio-TLC and radio-HPLC analyses. The scale-up to radioactivity amounts that are needed for clinical application purposes would be easy to achieve by increasing target mass, beam current, and irradiation time to produce 133La of high radionuclide purity (>99.5%) regarding labeling properties and side products.