RGD conjugates of the H2dedpa scaffold: synthesis, labeling and imaging with 68Ga

IntroductionThe rekindled interest in the 68Ga generator as an attractive positron emission tomography generator system has led us and others to investigate novel chelate systems for 68Ga. We have previously reported our findings with the acyclic, rapidly coordinating chelate H2dedpa and its model derivatives.MethodsIn this report, we describe the synthesis of the corresponding bifunctional chelate scaffolds (H2dp-bb-NCS and H2dp-N-NCS) as well as the radiolabeling properties, transferrin stability, binding to the target using in vitro cell models and in vivo behavior the corresponding conjugates with the αvβ3 targeting cyclic pentapeptide cRGDyK (monomeric H2RGD-1 and dimeric H2RGD-2).ResultsThe ability of the conjugated ligands to coordinate Ga isotopes within 10 min at room temperature at concentrations of 1 nmol was confirmed. Complex [67Ga(RGD-1)]+ was more stable (92% after 2 h) than [67Ga(RGD-2)]+ (73% after 2 h) in a transferrin challenge experiment. IC50 values for both conjugates (H2RGD-1 and H2RGD-2) and nonconjugated RGD were determined in a cell-based competitive binding assay with 125I-echistatin using U87MG cells, where enhanced specific binding was observed for the multivalent H2RGD-2 conjugate compared to the monovalent H2RGD-1 and nonconjugated cRGDyK. The U87MG cell line was also used to generate subcutaneous xenograft tumors on RAG2M mice, which were used to evaluate the in vivo properties of [68Ga(RGD-1)]+ and [68Ga(RGD-2)]+. After 2 h of dynamic imaging, both block and nonblock mice were sacrificed to collect select organs at the 2-h time point. Although the uptake is specific, as judged from the ratios of nonblock to block (2.36 with [67Ga(RGD-1)]+, 1.46 with [67Ga(RGD-2)]+), both conjugates display high uptake in blood.ConclusionsWe have successfully synthesized and applied the first bifunctional versions of H2dedpa for conjugation to a targeting vector and subsequent imaging of the corresponding conjugates.