TE1PA as Innovating Chelator for ⁶⁴Cu Immuno-TEP Imaging: A Comparative in Vivo Study with DOTA/NOTA by Conjugation on 9E7.4 mAb in a Syngeneic Multiple Myeloma Model

Following the successful synthesis of a C-functionalized version of the TE1PA ligand, a monopicolinate cyclam, we looked to demonstrate its in vivo properties versus DOTA and NOTA, after conjugation on the 9E7.4 rat antibody, an IgG_2a against CD138 murine, which has relevant properties for multiple myeloma targeting. For each ligand, different conjugation approaches had been considered to select the most appropriate for the comparative study. The p-SCN-Bn-TE1PA, NHS-DOTA, and p-SCN-Bn-NOTA were finally chosen for conjugation and radiolabeling tests. For in vivo comparison, we used a model of subcutaneous grafted mice with 5T33 tumor cells. In vitro tests and immuno-PET study highlighted ⁶⁴Cu-9E7.4-p-SCN-Bn-NOTA as the least attractive. Further competitive biodistribution and hepatic metabolic studies at 2, 24, and 48 h post-injection (100 μg radiolabeled with 10 MBq of ⁶⁴Cu) were then performed with the ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA and ⁶⁴Cu-9E7.4-NHS-DOTA. Results show a better in vivo resistance of ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA to transchelation compared to ⁶⁴Cu-9E7.4-NHS-DOTA, especially at later times. This was confirmed with ⁶⁴Cu-9E7.4-p-SCN-Bn-NOTA at 48 h PI. ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA also demonstrated an excellent hepatic clearance. ⁶⁴Cu-9E7.4-p-SCN-Bn-TE1PA displayed an overall superiority compared to ⁶⁴Cu-9E7.4-NHS-DOTA and ⁶⁴Cu-9E7.4-p-SCN-Bn-NOTA in terms of in vivo stability, reinforcing the usefulness of the p-SCN-Bn-TE1PA ligand for ⁶⁴Cu immuno-PET imaging.