Synthesis and Evaluation of Diastereoisomers of 1,4,7-Triazacyclononane-1,4,7-tris-(glutaric acid) (NOTGA) for Multimeric Radiopharmaceuticals of Gallium

In the conventional synthesis of 1,4,7-tris-(glutaric acid)-1,4,7-triazacyclononane (NOTGA), four isomeric species are usually generated by the alkylation of 1,4,7-triazacyclononane with α-bromoglutaric acid diester. To estimate their biological efficacies as well as their stability and radiochemistry, the RRR/SSS and RRS/SSR NOTGA-tBu prochelators were isolated and the corresponding cyclic RGDfK (RGD) conjugates with triethylene glycol linkages were prepared. The RRR/SSS and RRS/SSR diastereomers were obtained in 69% and 17% yields, respectively. In the complexation reaction with 67GaCl3, both diastereomers provided >98% radiochemical yields at pH 5 within 10 min when the reaction was conducted at room temperature. However, the RRR/SSS diastereomer exhibited more pH-sensitive radiochemical yields between pH 3.5 to 4.5. Despite their diasteromeric nature, both 67Ga-labeled RGD-NOTGA remained stable during the apo-transferrin challenge, exhibiting similar affinity for integrin αvβ3 and biodistribution with predominant renal excretion. Similar tumor uptake was also observed in mice bearing U87MG tumor xenograft, which resulted in impressively high contrast SPECT/CT images. These findings indicate that the RGD-NOTGA conjugates of both diastereomers presented here possess equivalent biological efficacies and their combined usage would be feasible. It is worth noting that specific properties of a given biomolecule, cell expression levels of the corresponding target molecule, and presence or absence of a pharmacokinetic modifier would affect the structural differences between diastereomers on the ligand–receptor interactions and pharmacokinetics. Thus, the preparation of corresponding conjugates and evaluation of their chemical and biological performances still remains important for applying NOTGA to other biomolecules of interest using the diastereomerically pure NOTGA-tBu prochelator.