Synthesis and evaluation of a new bifunctional NETA chelate for molecular targeted radiotherapy using⁹⁰Y or¹⁷⁷Lu

Introduction: Therapeutic potential of β-emitting cytotoxic radionuclides ⁹⁰Y and ¹⁷⁷Lu has been demonstrated in numerous preclinical and clinical trials. A bifunctional chelate that can effectively complex with the radioisotopes is a critical component for molecular targeted radiotherapy ⁹⁰Y and ¹⁷⁷Lu. A new bifunctional chelate 5p-C-NETA with a relatively long alkyl spacer between the chelating backbone and the functional unit for conjugation to a tumor targeting moiety was synthesized. 5p-C-NETA was conjugated to a model targeting moiety, a cyclic Arg-Gly-Asp-D-Tyr-Lys (RGDyK) peptide binding integrin α_vβ₃ protein overexpressed on various cancers. 5p-C-NETA was conjugated to c(RGDyK) peptide and evaluated for potential use in molecular targeted radiotherapy of ⁹⁰Y and ¹⁷⁷Lu. Methods: 5p-C-NETA conjugated with c(RGDyK) was evaluated in vitro for radiolabeling, serum stability, binding affinity, and the result of the in vitro studies of 5p-C-NETA-c(RGDyK) was compared to that of 3p-C-NETA-c(RGDyK). ¹⁷⁷Lu-5p-C-NETA-c(RGDyK) was further evaluated for in vivo biodistribution using gliobastoma bearing mice. Result: The new chelate rapidly and tightly bound to a cytotoxic radioisotope for cancer therapy, ⁹⁰Y or ¹⁷⁷Lu with excellent radiolabeling efficiency and maximum specific activity under mild condition (> 99%, RT, < 1 min). ⁹⁰Y- and ¹⁷⁷Lu-radiolabeled complexes of the new chelator remained stable in human serum without any loss of the radiolanthanide for 14 days. Introduction of the tumor targeting RGD moiety to the new chelator made little impact on complexation kinetics and stability with ⁹⁰Y or ¹⁷⁷Lu. ¹⁷⁷Lu-radiolabeled 5p-C-NETA-c(RGDyK) conjugate was shown to target tumors in mice and produced a favorable in vivo stability profile. Conclusion: The results of in vitro and in vivo evaluation suggest that 5p-C-NETA is an effective bifunctional chelate of ⁹⁰Y and ¹⁷⁷Lu that can be applied for generation of versatile molecular targeted radiopharmaceuticals