A new triazacyclononane-based ligand for trivalent ⁶⁸Ga tracers of high stability for positron emission tomography

Objectives Gallium-68 is one of the emerging radionuclides in molecular imaging due to its availability from a generator. It forms stable complexes with 1,4,7-triazacyclononane-1,4,7- triacetic acid (NOTA). Since multivalent peptide agents exhibit higher avidity to their targets than their monovalent counterparts, we designed and synthesized a new NOTA based bifunctional ligand, 1,4,7-triazacyclononane-1,4,7-triglutaric acid (NOTGA). ⁶⁷Ga labeling efficiency and stability of the ⁶⁷Ga complex were compared with a previously reported ligand, 1,4,7-triazacyclononane-1,4,7-tris[methyl(2-carboxyethyl)phosphinic acid (PrP9).

Methods α-Bromo glutaric acid 1-tert-butyl ester 5-benzyl ester was synthesized and reacted with 1,4,7-triazacyclononane. After removing the benzyl groups, phenethylamine (PEA) or cRGDfK peptide was conjugated. PrP9 was also conjugated to PEA. To characterize ⁶⁷Ga-labeled compounds, non-radioactive Ga complexes were synthesized and characterized. The stability of HPLC purified ⁶⁷Ga-labeled compounds was estimated in the presence of human apo-transferrin. The biodistribution of ⁶⁷Ga-NOTGA-RGD₃ in mice were determind.

Results Both NOTGA-PEA and PrP9-PEA quantitatively provided ⁶⁷Ga-labeled compounds within 15 min at 100 °C, pH 3 with the ligand concentration of 10 μM. The HPLC retention times of each ⁶⁷Ga-labeled compound were identical to those of their non-radioactive complexes. However, PrP9 generated two complexes, which is attributable to two enantiomers. ⁶⁷Ga-NOTGA-PEA remained stable in the presence of human apo-transferrin. ⁶⁷Ga-NOTGA-RGD₃ showed the rapid clearance from blood and the majority of the radioactivity was excreted via kidney.

Conclusions NOTGA would constitute a useful ligand for developing ⁶⁸Ga-labeled trivalent probes of high urinary excretion without impairing the inherent chelating ability of NOTA