²²⁷Th Labeled 3,4,3-(LI-1,2-HOPO): Optimized Synthesis, Biological Distribution and Dosimetry.

Introduction: Thorium-227 (²²⁷Th) offers a longer half-life, 18.7 days, than other Targeted Alpha Therapy (TAT) agents currently being evaluated clinically such as actinium-225 (t_1/2= 9.9 days) and astatine-211 (t_1/2= 7.2 hours). This physical characteristic favors its use with biomolecules that have slower pharmacokinetics like monoclonal antibodies. Utilizing ²²⁷Th is also advantageous due to the 5 alpha emissions occurring during decay. Hydroxypyridinonate (HOPO) derivatives have been shown to be effective decorporation agents for actinides owing to their hard oxygen donor atoms and strong complexation. Therefore, utilizing these molecules as chelators for potential nuclear medicine agents should allow for radiometals to be effectively sequestered by the ligand. The biological chemistry of thorium is similar to calcium in that the unbound radiometal tends to localize in bone. Consequently, it is pivotal to establish a thorium complex that possesses high stability and thwarts bone uptake. Methods: ²²⁷Th received from Oak Ridge National Laboratory was purified from its daughters on a MP1 BioRad resin. The purified thorium was then complexed to the 3,4,3-(LI-1,2-HOPO) ligand by adding the radiometal to a 0.1 M acetate solution containing the ligand (0.08 mM) for 1 hour at 50 °C and was purified using Chelex resin column. The biological distribution of the ²²⁷Th-HOPO complex was compared to the “bare” ²²⁷Th (²²⁷Th-nitrate in 0.1 M HCl/ 5 M acetate solution) in male CD1 mice (n=3) injected with 1 μCi of either construct. At the prescribed time points (15 m, 1 h, 3 h, 6 h, 24 h, 3 d, 7 d, 14 d, 21 d, and 28 d) the organs of interest were harvested and counted using a gamma counter calibrated for ²²⁷Th. The biodistribution data was used to calculate the residence time in each organ and was then entered into the OLINDA software to produce the effective dose in rem/mCi. Terminal SPECT/CT imaging of ²²⁷Th-nitrate and ²²⁷Th-HOPO was obtained in representative mice at 20 min post injection of 15 µCi.

Results: The biodistribution results for the ²²⁷Th-nitrate species shows clearance through the kidneys (%ID/g = 26.3 ±5.3, 1-day p.i.). Also, substantial localization in the bone as expected is observed. At 15 minutes p.i. the %ID/g in the bone is 7.1 ± 3.9 and gradually increases through the 7-day timepoint, %ID/g- 28.8 ± 12.8, ending the study 28 days p.i with an accumulation of 33.6 ± 17.5 %ID/g. Conversely the ²²⁷Th-HOPO species appears to clear through the hepatobiliary tract with accumulation in the gallbladder (54.2 ± 27.7 %ID/g, 1-hour p.i.) and the intestines (29.5 ± 2.2 %ID/g, 1-hour p.i.). Most significantly the bone localization is relatively low and remains constant, approximately 1.5 %ID/g, through all timepoints. The low bone uptake is also reflected in the dosimetry where the effective dose in the bone for ²²⁷Th-nitrate and ²²⁷Th-HOPO is 1680 rem/mCi and 192 rem/mCi respectively. The effective dose of the ²²⁷Th-HOPO species is at least 1 order of magnitude lower than the nitrate species in all organs. Conclusion: We have shown that the ²²⁷Th-HOPO complex is stable in vivo and facilitates rapid elimination from the body which results in lower dose to organs, specifically the bone. The stability of the ²²⁷Th-HOPO complex suggests that it has potential for use in TAT. Acknowledgments: This work has been funded by BNL LDRD 21-00 and Seed grant under B&R code YN0100000. We would also like to thank all those who assisted with the project.