Synthesis, purification, and tumor cell uptake of⁶⁷Ga-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(α) and DF - folate(γ), was produced involving the α- and γ-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(γ) 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 γ-emitting radionuclide ⁶⁷Ga³⁺ and tested for uptake by cultured KB cells overexpressing the folate receptor. The cellular accumulation of ₆₇Ga-DF - folate(γ) complex was found to be time-, temperature-, and concentration-dependent. The ⁶⁷Ga-DF - folate(γ) tracer exhibited rapid uptake kinetics in cell culture with a t_1/2 of ∼3 min. The KB cell association of ⁶⁷Ga-DF - folat(γ) was competitively blocked by free folic acid, indicating that uptake of the ⁶⁷Ga-DF -folate(γ) was specifically mediated by the folate receptor. Since the folate receptor is overexpressed on the surfaces of many neoplastic cells, these results suggest that ⁶⁷Ga-DF - folate(γ) complex might be useful as a diagnostic agent for noninvasive imaging of folate receptor-positing tumors.