Synthesis, purification, and tumor cell uptake of Ga-67-deferoxamine-folate, a potential radiopharmaceutical for tumor imaging

The vitamin folic acid was covalently linked to the chelating agent deferoxamine (DF) via an amide bond using a simple carbodiimide coupling reaction. A mixture of two isomers, DF-folate(a) and DF-folate(gamma), was produced involving the alpha- and gamma-carboxyl group of folic acid, respectively. These two isomers were separated by anion-exchange chromatography using a NH4HCO3 gradient. Competitive binding studies revealed that only the DF-folate(gamma) is recognized by the folate receptor on KB cells, interacting with an affinity comparable to unconjugated folic acid. The DF-folate conjugates were radiolabeled with the gamma-emitting radionuclide Ga-67(3+) and tested for uptake by cultured KB cells overexpressing the folate receptor. The cellular accumulation of Ga-67-DF-folate(gamma) complex was found to be time-, temperature-, and concentration-dependent. The Ga-67-DF-folate(gamma) tracer exhibited rapid uptake kinetics in cell culture with a t(1/2) of similar to 3 min. The KB cell association of (GaDF)-Ga-67-folate(gamma) was competitively blocked by free folic acid, indicating that uptake of the Ga-67-DF-folate(gamma) was specifically mediated by the folate receptor. Since the folate receptor is overexpressed on the surfaces of many neoplastic cells, these results suggest that Ga-67-DF-folate(gamma) complex might be useful as a diagnostic agent for noninvasive imaging of folate receptor-positing tumors.