Radiolabeling and biological evaluation of dextran-folate conjugate for tumor scintigraphy

Folic acid derivatives have been shown to be selectively taken up by tumor cells via folate receptors, which are over expressed on tumor cells. The aim of the present study was to evaluate dextran-folate conjugate for targeted tumor scintigraphy. High molecular weight and non-immunogenic macromolecules can not be excreted from kidney and remain in systemic circulation for prolonged time period. Further, they can't accumulate in other organs/tissues due to lower vascular permeability. However, they can accumulate in tumor cells because leaky nature of tumor vasculature. So the conjugation of anticancer agents with such kind of macromolecules results in self targeted system and this novel concept is known as Enhanced Permeability Retention (EPR).

Dextran-Folic acid (dextran molecular weight 40000) conjugate was synthesized following the established procedure. Both folic acid and dextran-folate conjugate were labeled with Tc99m by direct labeling method using stannous chloride as reducing agent. Various labeling parameters (pH, stannous ion concentration and incubation time) were optimized to achieve the maximum labeling efficiency (>98%). In vitro stability of the labeled complexes was checked and the complexes were found to be stable for 24h. The blood clearance kinetics was studied in New Zealand albino rabbits and scintigraphy was performed in EAT tumor bearing mice.

It was observed that folic acid cleared very rapidly from the systemic circulation, while the dextran-folate conjugate remained in the circulation for longer time duration. A significant radioactivity was recorded in blood after 24h following the intravenous injection of dextran-folate. Scintigraphic images of the animals after 30 min of intravenous injection showed a significant accumulation of both the formulations at tumor site. A significant activity was also observed in kidney and bladder in case of pure folic acid that indicates the rapid clearance of the same. Delayed imaging (after 6h and 24h) were also performed that showed the better retention of dextran-folate conjugate at tumor site as compared to pure folic acid.

The study signifies the potential of dextran-folate conjugate as a tumor imaging agent. The approach can be extended to conjugation of other anticancer moieties with such types of polymers for improving cancer chemotherapy using anticancer drugs and such studies are under progress in our laboratory.