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 (H₂L1-4) consisting of a bifunctional bis(thiosemicarbazone) chelator conjugated to prostate-specific membrane antigen pharmacophores. Radiolabeling H₂L1-4 with the 99mTc nitrido core in a one-pot synthesis at 95 °C for 10 min produced the complexes [99mTc][TcN(L1-4)] with high radiochemical purity (RCP > 95%) and excellent stability. In vitro evaluation of the [99mTc]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, [99mTc][TcN(L2)], 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. [99mTc][TcN(L3)] and the bivalent [99mTc][TcN(L4)] showed predominantly renal clearance and the highest PSMA+ tumor uptake and retention. Conversely, [99mTc][TcN(L1)] and [99mTc][TcN(L2)] displayed gastrointestinal clearance and lower PSMA+ tumor uptake. The work demonstrates that bis(thiosemicarbazones) are effective bifunctional chelators for the development of 99mTc radiopharmaceuticals targeting PSMA with the potential to be easily modified to target other diseases.