Development of Titanium-45 for PET imaging of PSMA+ prostate cancer

Objectives: Positron Emission Tomography (PET) has become an important, non-invasive tool for diagnosis and characterization of disease. In order to synthesize effective probes for different molecular targets with varying pharmacokinetics, novel PET radionuclides with varying characteristics, such as chemistry and half-life are needed. The resulting radiopharmaceuticals can be used for imaging of various cellular targets and biological processes. Titanium-45 is a radiometal with excellent chemical and nuclear characteristics (positron branching ratio = 85%, β+_avg= 0.439 MeV, and t_1/2 = 3.08 h) for small-molecule based, targeted imaging agents. Herein, optimization of isotope production, separation, chelation and proof-of-concept synthesis and evaluation of ⁴⁵Ti-based PET probes targeting the prostate specific membrane antigen (PSMA) are presented.

Methods: Titanium-45 was produced on the University of Alabama at Birmingham (UAB) TR24 cyclotron via proton bombardment of natural scandium foils. Irradiation parameters were at 10 μA current for 30 minutes with 18 MeV protons which were attenuated with a degrader to an incident energy of 13 MeV. We developed a novel separation method, based on ion chromatography in order to separate the ⁴⁵Ti from the target material. The irradiated scandium foil was dissolved in 6M HCl, diluted to 2M final concentration and added to a column packed with 175 mg of hydroxamate resin. Following this, the target material was separated using 10 mL of 2M HCl and 10 mL MQ H₂O. ⁴⁵Ti was eluted with 3 mL of 1M citric acid. Novel PSMA targeting molecules (DFO-DUPA and DFO-KFF-DUPA) were radiolabeled with ⁴⁵Ti and further investigated. In vitro studies using PSMA+ cell lines, PC3 PIP and 22Rv1, and PSMA- cell lines, PC3 FLU and AR42J were conducted to determine in vitro specificity and integrity of the ⁴⁵Ti-labeled probes. Preliminary imaging studies with free ⁴⁵Ti in healthy mice were carried out to determine baseline uptake of the radiotracer. Images were obtained on the UAB small animal PET/CT (GNEXT Sofie Biosciences).

Results: Titanium-45 (2329 ± 111 MBq, n=12) was produced in yields suitable for preclinical studies. Our separation method resulted in a 78 ± 8% recovery yield. Radiochemical yields of up to 100% using 17 nmol of DFO-DUPA were achieved, illustrating DFO is a starting place for the development of chelators for ⁴⁵Ti. In vitro studies demonstrated statistically significant, elevated uptake in PSMA+ cell lines when compared to non-expressing cell lines.

Conclusions: Our results demonstrate that ⁴⁵Ti is a promising radioisotope that merits further development. In vivo studies are ongoing and preliminary results illustrate that ⁴⁵Ti-labeled PSMA targeting molecules can be developed and warrant further exploration as imaging agents for prostate cancer. Acknowledgments: We would like to acknowledge all contributors to this work including the University of Alabama at Birmingham, and the UAB cyclotron facility. This work was supported in part by the 2019 ERF SNMMI Predoctoral Molecular Imaging Scholar Program.