[Sc(pypa)]: Thermodynamic Stability, Radiolabeling, and Biodistribution of a Prostate-Specific-Membrane-Antigen-Targeting Conjugate.

Sc is an attractive positron-emitting radionuclide for PET imaging; herein, a new complex of the Sc ion with nonmacrocyclic chelator Hpypa was synthesized and characterized with high-resolution electrospray-ionization mass spectrometry (HR-ESI-MS), as well as different nuclear magnetic resonance (NMR) spectroscopic techniques (H, C, H-C HSQC, H-C HMBC, COSY, and NOESY). In aqueous solution (pH = 7), [Sc(pypa)] presented two isomeric forms, the structures of which were predicted using density functional theory (DFT) calculation with a small energy difference of 22.4 kJ/mol, explaining their coexistence. [Sc(pypa)] was found to have superior thermodynamic stability (pM = 27.1) compared to [Sc(AAZTA)] (24.7) and [Sc(DOTA)] (23.9). In radiolabeling, [Sc][Sc(pypa)] formed efficiently at RT in 15 min over a range of pH (2-5.5), resulting in a complex that is highly stable (>99%) in mouse serum over at least six half-lives of scandium-44. Similar labeling efficiency was observed with the PSMA (prostate-specific membrane antigen)-targeting Hpypa-C7-PSMA617 at pH = 5.5 (RT, 15 min), confirming negligible disturbance from the bifunctionalization on scandium-44 scavenging. Moreover, the kinetic inertness of the radiocomplex was proved Surprisingly, the molar activity was found to have profound influence on the pharmacokinetics of the radiotracers where lower molar activity drastically reduced the background accumulations, particularly, kidney, and thus, yielded a much higher tumor-to-background contrast.