Synthesis of [^99mTc]DTPA-folate and its evaluation as a folate-receptor-targeted radiopharmaceutical

A DTPA-folate conjugate was radiolabeled with ^99mTc by stannous chloride reduction of [^99mTc]sodium pertechnetate in an aqueous solution of DTPA-folate. The radiochemical purity of the product consistently exceeded 97%, as assessed by thin-layer chromatography employing conditions analogous to those for radiochemical quality control of the radiopharmaceutical [^99mTc]DTPA. HPLC demonstrated that the radiolabeled product resulted from the intact DTPA-folate conjugate and not unconjugated DTPA. The ability of [^99mTc]DTPA-folate to target folate receptors in vivo was assessed in biodistribution studies with athymic mice bearing subcutaneous folate-receptor-positive human KB cell tumors. As an internal control, previously studied [¹¹¹In]DTPA-folate was coinjected with the [^99mTc]DTPA-folate, along with varying amounts of DTPA-folate (0.38 mg/kg, 1.6 mg/kg, or 14 mg/kg). At each DTPA-folate dose, [^99mTc]DTPA-folate exhibited tumor uptake comparable to that of the coadministered [¹¹¹In]DTPA-folate, with radiotracer levels declining at the higher DTPA-folate doses due to competitive receptor binding of the unlabeled conjugate. Tumor uptake of both tracers was also competitively blocked by preadministered folic acid dihydrate (2.9 mg/kg). Tumor-to-background tissue contrast obtained with [^99mTc]DTPA-folate was generally similar to that obtained with [¹¹¹In]DTPA-folate. The ^99mTc-labeled DTPA-folate conjugate may have utility as a targeted radiopharmaceutical for imaging neoplastic tissues known to overexpress the folate receptor.