Purpose: Receptors for vitamin folic acid are frequently overexpressed on epithelial cancer cells, especially ovarian cancer cells. In this study, we examined whether this expression might be exploited to specifically deliver antisense oligodeoxynucleotides (ODN) to tumor cells.
Methods: A conjugate was prepared by directly coupling folic acid to the 3' terminus of an anti-c-fos ODN and its cellular uptake and tumor inhibitory effect were evaluated using FD2008 cells that overexpress folate receptors.
Results: When a phosphorothioate (PS)/phosphodiester (PO) chimeric ODN was conjugated with folic acid, its uptake by FD2008 cells was increased by about 8-fold (P < 0.01). In contrast, conjugation of folate to the ODN did not increase its uptake by CHO cells that lack the expression of FBP (P > 0.05). Furthermore, the increase in the uptake of conjugated ODN by FD2008 cells could be blocked by adding an excess amount of folic acid. The PS/PO antisense ODN had some inhibitory effect on the growth of FD2008 cells. However, its activity was significantly increased following conjugation with folic acid (P < 0.01). ODN of scrambled sequences with and without conjugation with folic acid failed to inhibit the growth of FD2008 cells. Finally, the antisense effect of the conjugated ODN on FD2008 cells was inhibited by an excess amount of free folic acid, suggesting that the sequence-dependent effect of folate-antisense ODN conjugate was mediated by folate binding protein.
Conclusions: Direct derivatization of ODN with folate significantly improves their targeting efficiency to tumor cells in vitro. The folate-conjugated ODN, due to their small size and possibly efficient extravasation at tumor site, has the potential for treating solid tumors that overexpress folate receptors.