Radiochemistry and In Vivo Imaging of [⁴⁵Ti]Ti-THP-PSMA

Titanium-45 (⁴⁵Ti) is a radionuclide with excellent physical characteristics for use in positron emission tomography (PET) imaging, including a moderate half-life (3.08 h), decay by positron emission (85%), and a low mean positron energy of 0.439 MeV. However, challenges associated with titanium chemistry have led to the underdevelopment of this radionuclide for incorporation into radiopharmaceuticals. Expanding on our recent studies, which showed promising results for the complexation of ⁴⁵Ti with the tris hydroxypyridinone (THP^Me) chelator, the current work aimed to optimize the chemistry and imaging attributes of [⁴⁵Ti]Ti-THP-PSMA as a new PET radiopharmaceutical. Methods. Radiolabeling of THP-PSMA was optimized with [⁴⁵Ti]Ti-citrate at varying pHs and masses of the precursor. The stability of the radiolabeled complex was assessed in mouse serum for up to 6 h. The affinity of [⁴⁵Ti]Ti-THP-PSMA for prostate-specific membrane antigen (PSMA) was assessed using LNCaP (PSMA +) and PC3 (PSMA -) cell lines. In vivo imaging and biodistribution analysis were performed in tumor-bearing xenograft mouse models to confirm the specificity of the tumor uptake. Results. > 95% of radiolabeling was achieved with a high specific activity of 5.6 MBq/nmol under mild conditions. In vitro cell binding studies showed significant binding of the radiolabeled complex with the PSMA-expressing LNCaP cell line (11.9 ± 1.5%/mg protein-bound activity) compared to that with the nonexpressing PC3 cells (1.9 ± 0.4%/mg protein-bound activity). In vivo imaging and biodistribution studies confirmed specific uptake in LNCaP tumors (1.6 ± 0.27% ID/g) compared to that in PC3 tumors (0.39 ± 0.2% ID/g). Conclusion. This study showed a simple one-step radiolabeling method for ⁴⁵Ti with THP-PSMA under mild conditions (pH 8 and 37 °C). In vitro cell studies showed promise, but in vivo tumor xenograft studies indicated low tumor uptake. Overall, this study shows the need for more chelators for ⁴⁵Ti for the development of a PET radiopharmaceutical for cancer imaging.