[nat/44Sc(pypa)]−: Thermodynamic Stability, Radiolabeling, and Biodistribution of a Prostate-Specific-Membrane-Antigen-Targeting Conjugate

44Sc is an attractive positron-emitting radionuclide for PET imaging; herein, a new complex of the Sc3+ ion with nonmacrocyclic chelator H4pypa was synthesized and characterized with high-resolution electrospray-ionization mass spectrometry (HR-ESI-MS), as well as different nuclear magnetic resonance (NMR) spectroscopic techniques (1H, 13C, 1H–13C HSQC, 1H–13C 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, [44Sc]­[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 H4pypa-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 in vivo. 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.