SPECT Imaging with Bifunctional Technetium-99m Nitrido Bis(thiosemicarbazonato) Complexes Tethered to Inhibitors of the Prostate-Specific Membrane Antigen

We report the design and synthesis of new conjugates (H 2 L 1–4 ) consisting of a bifunctional bis(thiosemicarbazone) chelator conjugated to prostate-specific membrane antigen pharmacophores. Radiolabeling H 2 L 1–4 with the 99m Tc nitrido core in a one-pot synthesis at 95 °C for 10 min produced the complexes [ 99m Tc][TcN( L 1–4 )] with high radiochemical purity (RCP > 95%) and excellent stability. In vitro evaluation of the [ 99m Tc]Tc PSMA complexes demonstrated high cell-surface receptor binding and internalization in cancer cells expressing PSMA (PSMA+), which was inhibited by an excess of a PSMA-specific inhibitor. The most lipophilic of the complexes, [ 99m Tc][TcN( L 2 )], also showed significant nonspecific uptake in the control cell line (PSMA−). SPECT-CT imaging of the four complexes, at 1 and 4 h postinjection and biodistribution studies at 5 h postinjection in BALB/c nude mice with both PSMA+ and PSMA– tumors, demonstrated significant differences in the receptor-specific tumor uptake and clearance pathways. [ 99m Tc][TcN( L 3 )] and the bivalent [ 99m Tc][TcN( L 4 )] showed predominantly renal clearance and the highest PSMA+ tumor uptake and retention. Conversely, [ 99m Tc][TcN( L 1 )] and [ 99m Tc][TcN( L 2 )] displayed gastrointestinal clearance and lower PSMA+ tumor uptake. The work demonstrates that bis(thiosemicarbazones) are effective bifunctional chelators for the development of 99m Tc radiopharmaceuticals targeting PSMA with the potential to be easily modified to target other diseases.