Optimized production, purification, and radiolabeling of the (203)Pb/(212)Pb theranostic pair for nuclear medicine

TRIUMF is one of the only laboratories in the world able to produce both lead-203 ((203)Pb, t(1/2) = 51.9 h) and (212)Pb (t(1/2) = 10.6 h) onsite via its 13 and 500 MeV cyclotrons, respectively. Together, (203)Pb and (212)Pb form an element-equivalent theranostic pair that potentiate image-guided, personalized cancer treatment, using (203)Pb as a single-photon emission computed tomography (SPECT) source, and (212)Pb for targeted alpha therapy. In this study, improvements to (203)Pb production were accomplished by manufacturing electroplated, silver-backed thallium (Tl) targets to improve target thermal stability, which allow for higher currents during irradiation. We implemented a novel, two-column purification method that employs selective Tl precipitation ((203)Pb only) alongside extraction and anion exchange chromatography to elute high specific activity and chemical purity (203/212)Pb in a minimal volume of dilute acid, without the need for evaporation. Optimization of the purification method translated to improvements in radiolabeling yields and apparent molar activity of lead chelators TCMC (S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane) and Crypt-OH, a derivative of a [2.2.2]-cryptand.