Background

[PubMed]

Folic acid (folate) is a water-soluble B vitamin (1) that is essential for methylation and DNA synthesis. The primary pathway for entry of folate into cells is through the facilitated transporter, which has a low affinity for folate (Michaelis constant (K_m) = 1-5 μM). Some cells in the choroid plexus, kidney, lung, thyroid, spleen, placenta, and thymus also possess a higher affinity (dissociation constant (K_d) = 0.5 nM) receptor that allows folate uptake via receptor-mediated endocytosis. Some human epithelial tumor cells were found to overexpress folate-binding protein (2). More than 90% of human ovarian and endometrial cancers express the high-affinity receptor, which is absent in normal tissues. Breast, colorectal, renal, and lung carcinomas also overexpress the folate receptor but at lower frequencies (20-50%). Activated macrophages, but not resting macrophages, have been also found to have folate receptor (3).

Several folate-based conjugates (¹¹¹In-DTPA-folate, ^99mTc-EC-folate and [¹⁸F]FBA-folate) have been studied in tumor imaging (4-8). Deferoxamine (DF), a chelating agent, was conjugated to folic acid forming a mixture of two isomers, DF-α-folate and DF-γ-folate. Only the DF-γ-folate isomer was able to displace [³H]folic acid from its receptors with a similar IC₅₀ to folic acid (2.5 versus 2.4 nM) (9). ^68/67/66Ga-γ-DF-folate is being developed as an imaging agent for detection of folate receptors in vivo with either positron emission tomography (PET) or single photon emission computed tomography (SPECT).

Synthesis

[PubMed]

Wang et al. (9) coupled DF-γ-folate (2.25 μmol) with 118 MBq (3.2 mCi) ⁶⁷GA-(acetylacetone)₃ in buffered saline at room temperature for 24 h to yield ⁶⁷Ga-γ-DF-folate with >98% radiochemical purity. Radiochemical yields exceeded 80%. Mathias et al. (5) reported that ⁶⁶Ga-γ-DF-folate and ⁶⁸Ga-γ-DF-folate were similarly radiolabeled at 50°C for 15-30 min.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

The human nasopharyngeal carcinoma KB-31 cell line has putative folate receptors as determined by [³H]folate binding studies in cultures (9). The mean IC₅₀ values for DF-γ-folate, and folate were 25 and 24 nM, respectively. Binding of ⁶⁷Ga-γ-DF-folate to cells was rapid and reached a maximal binding of 18% and 32% at 4°C and 37°C in 30 min, respectively. Both bindings were inhibited by folic acid and DF-γ-folate but not by DF-α-folate. As controls, ⁶⁷Ga-DF and ⁶⁷Ga-citrate showed little binding to the cells.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

NIH Support

R01 CA86307, R01-CA70845

References

1.

Stanger O. Physiology of folic acid in health and disease. Curr Drug Metab. 2002;3(2):211–23. [PubMed: 12003352]

2.

Ke C.Y., Mathias C.J., Green M.A. The folate receptor as a molecular target for tumor-selective radionuclide delivery. Nucl Med Biol. 2003;30(8):811–7. [PubMed: 14698784]

3.

Nakashima-Matsushita N., Homma T., Yu S., Matsuda T., Sunahara N., Nakamura T., Tsukano M., Ratnam M., Matsuyama T. Selective expression of folate receptor beta and its possible role in methotrexate transport in synovial macrophages from patients with rheumatoid arthritis. Arthritis Rheum. 1999;42(8):1609–16. [PubMed: 10446858]

4.

Mathias C.J., Hubers D., Low P.S., Green M.A. Synthesis of [(99m)Tc]DTPA-folate and its evaluation as a folate-receptor-targeted radiopharmaceutical. Bioconjug Chem. 2000;11(2):253–7. [PubMed: 10725102]

5.

Mathias C.J., Lewis M.R., Reichert D.E., Laforest R., Sharp T.L., Lewis J.S., Yang Z.F., Waters D.J., Snyder P.W., Low P.S., Welch M.J., Green M.A. Preparation of 66Ga- and 68Ga-labeled Ga(III)-deferoxamine-folate as potential folate-receptor-targeted PET radiopharmaceuticals. Nucl Med Biol. 2003;30(7):725–31. [PubMed: 14499330]

6.

Mathias C.J., Wang S., Low P.S., Waters D.J., Green M.A. Receptor-mediated targeting of 67Ga-deferoxamine-folate to folate-receptor-positive human KB tumor xenografts. Nucl Med Biol. 1999;26(1):23–5. [PubMed: 10096497]

7.

Mathias C.J., Wang S., Waters D.J., Turek J.J., Low P.S., Green M.A. Indium-111-DTPA-folate as a potential folate-receptor-targeted radiopharmaceutical. J Nucl Med. 1998;39(9):1579–85. [PubMed: 9744347]

8.

Bettio A., Honer M., Muller C., Bruhlmeier M., Muller U., Schibli R., Groehn V., Schubiger A.P., Ametamey S.M. Synthesis and Preclinical Evaluation of a Folic Acid Derivative Labeled with 18F for PET Imaging of Folate Receptor-Positive Tumors. J Nucl Med. 2006;47(7):1153–1160. [PubMed: 16818950]

9.

Wang S., Lee R.J., Mathias C.J., Green M.A., Low P.S. Synthesis, purification, and tumor cell uptake of 67Ga-deferoxamine--folate, a potential radiopharmaceutical for tumor imaging. Bioconjug Chem. 1996;7(1):56–62. [PubMed: 8741991]

10.

Mathias C.J., Wang S., Lee R.J., Waters D.J., Low P.S., Green M.A. Tumor-selective radiopharmaceutical targeting via receptor-mediated endocytosis of gallium-67-deferoxamine-folate. J Nucl Med. 1996;37(6):1003–8. [PubMed: 8683292]