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 99mTcTcN(L1-4) with high radiochemical purity (RCP > 95%) and excellent stability. In vitro evaluation of the 99mTcTc 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, 99mTcTcN(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. 99mTcTcN(L3) and the bivalent 99mTcTcN(L4) showed predominantly renal clearance and the highest PSMA+ tumor uptake and retention. Conversely, 99mTcTcN(L1) and 99mTcTcN(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.