Structural modifications toward improved lead-203/lead-212 peptide-based image-guided alpha-particle radiopharmaceutical therapies for neuroendocrine tumors

The lead-203 (${}^{203}$Pb)/lead-212 (${}^{212}$Pb) elementally identical radionuclide pair has gained significant interest in the field of image-guided targeted alpha-particle therapy for cancer. Emerging evidence suggests that ${}^{212}$Pb-labeled peptide-based radiopharmaceuticals targeting somatostatin receptor subtype 2 (SSTR2) may provide improved effectiveness compared to beta-particle-based therapies for neuroendocrine tumors (NETs). This study aims to improve the performance of SSTR2-targeted radionuclide imaging and therapy through structural modifications to Tyr${}^3$-octreotide (TOC)-based radiopharmaceuticals. New SSTR2-targeted peptides were designed and synthesized with the goal of optimizing the incorporation of Pb isotopes through the use of a modified cyclization technique; the introduction of a Pb-specific chelator (PSC); and the insertion of polyethylene glycol (PEG) linkers. The binding affinity of the peptides and the cellular uptake of ${}^{203}$Pb-labeled peptides were evaluated using pancreatic AR42J (SSTR2+) tumor cells and the biodistribution and imaging of the ${}^{203}$Pb-labeled peptides were assessed in an AR42J tumor xenograft mouse model. A lead peptide was identified (i.e., PSC-PEG${}_2$-TOC), which was then further evaluated for efficacy in ${}^{212}$Pb therapy studies. The lead radiopeptide drug conjugate (RPDC) - [${}^{203}$Pb]Pb-PSC-PEG${}_2$-TOC - significantly improved the tumor-targeting properties, including receptor binding and tumor accumulation and retention as compared to [${}^{203}$Pb]Pb-DOTA${}^0$-Tyr${}^3$-octreotide (DOTATOC). Additionally, the modified RPDC exhibited faster renal clearance than the DOTATOC counterpart. These advantageous characteristics of [${}^{212}$Pb]Pb-PSC-PEG${}_2$-TOC resulted in a dose-dependent therapeutic effect with minimal signs of toxicity in the AR42J xenograft model. Fractionated administrations of 3.7 MBq [${}^{212}$Pb]Pb-PSC-PEG${}_2$-TOC over three doses further improved anti-tumor effectiveness, resulting in 80% survival (70% complete response) over 120 days in the mouse model. Structural modifications to chelator and linker compositions improved tumor targeting and pharmacokinetics (PK) of ${}^{203/212}$Pb peptide-based radiopharmaceuticals for NET theranostics. These findings suggest that PSC-PEG${}_2$-TOC is a promising candidate for Pb-based targeted radionuclide therapy for NETs and other types of cancers that express SSTR2.