Synthesis and in vitro/in vivo evaluation of 99mTc-labeled folate conjugates for folate receptor imaging.

INTRODUCTION

Folate receptor (FR) is a potential molecular target for radionuclide imaging since it is overexpressed in many human epithelial tumor cells. In this study, a novel folate conjugate was synthesized and labeled with (99m)Tc using different coligands. In vitro and in vivo evaluations of these complexes have been done to explore the effect of coligands on the stable, affinity and pharmacokinetic properties.

METHODS

A novel folate conjugate, HYNIC-NHHN-FA, was synthesized and characterized. This conjugate was radiolabeled with (99m)Tc using tricine, tricine/diphenylphosphinobenzene-3-sulfonic acid sodium (TPPMS) and tricine/trisodium triphenylphosphine-3,3',3''-trisulfonate (TPPTS) as coligands, respectively. The complexes were purified by high-pressure liquid chromatography (HPLC). In vitro and in vivo evaluations were performed with FR-positive KB cells, normal Kunming mice and athymic nude mice bearing KB tumors.

RESULTS

Labeling with (99m)Tc using different coligands resulted in three complexes, (99m)Tc (HYNIC-NHHN-FA)(tricine), 5, (99m)Tc (HYNIC-NHHN-FA)(tricine/TPPMS), 6 and (99m)Tc (HYNIC-NHHN-FA)(tricine/TPPTS), 7. Complex 5 showed at least two isomers and was unstable after being purified by HPLC. Complexes 6 and 7 displayed high stability and similar affinity to FR in vitro. Biodistribution results in athymic nude mice bearing KB tumor showed that complex 7 had a high uptake in FR-positive tumor (9.79±1.66%ID/g at 4 h postinjection), and the results of blockade studies confirmed the specific accumulation of the radiotracer in vivo. However, complex 6 showed a low tumor uptake due to its fast excretion via the gastrointestinal tract.

CONCLUSION

The modification of the coligands can significantly alter the pharmacokinetic properties of the corresponding (99m)Tc-HYNIC complexes. (99m)Tc (HYNIC-NHHN-FA)(tricine/TPPTS), 7 could be a promising radiotracer for FR imaging.