Preclinical Evaluation of a Lead Specific Chelator (PSC) Conjugated to Radiopeptides for 203 Pb and 212 Pb-Based Theranostics.

203 Pb and 212 Pb have emerged as promising theranostic isotopes for image-guided α-particle radionuclide therapy for cancers. Here, we report a cyclen-based Pb specific chelator (PSC) that is conjugated to tyr 3 -octreotide via a PEG 2 linker (PSC-PEG-T) targeting somatostatin receptor subtype 2 (SSTR2). PSC-PEG-T could be labeled efficiently to purified 212 Pb at 25 °C and also to 212 Bi at 80 °C. Efficient radiolabeling of mixed 212 Pb and 212 Bi in PSC-PEG-T was also observed at 80 °C. Post radiolabeling, stable Pb(II) and Bi(III) radiometal complexes in saline were observed after incubating [ 203 Pb]Pb-PSC-PEG-T for 72 h and [ 212 Bi]Bi-PSC-PEG-T for 5 h. Stable [ 212 Pb]Pb-PSC-PEG-T and progeny [ 212 Bi]Bi-PSC-PEG-T were identified after storage in saline for 24 h. In serum, stable radiometal/radiopeptide were observed after incubating [ 203 Pb]Pb-PSC-PEG-T for 55 h and [ 212 Pb]Pb-PSC-PEG-T for 24 h. In vivo biodistribution of [ 212 Pb]Pb-PSC-PEG-T in tumor-free CD-1 Elite mice and athymic mice bearing AR42J xenografts revealed rapid tumor accumulation, excellent tumor retention and fast renal clearance of both 212 Pb and 212 Bi, with no in vivo redistribution of progeny 212 Bi. Single-photon emission computed tomography (SPECT) imaging of [ 203 Pb]Pb-PSC-PEG-T and [ 212 Pb]Pb-PSC-PEG-T in mice also demonstrated comparable accumulation in AR42J xenografts and renal clearance, confirming the theranostic potential of the elementally identical 203 Pb/ 212 Pb radionuclide pair.