99mTc-1,4,7-Tris(carboxymethyl)-10-(4-aminoethyl)-1,4,7,10-tetraazacyclododecane-Folate

99mTc-DO3A-EA-Folate

Leung K.

Publication Details

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In vitro Rodents

Background

[PubMed]

Folic acid is a water-soluble B vitamin (1). It is essential for methylation and DNA synthesis. The primary pathway for entry of folate into cells is through a facilitated transporter, which has a low affinity for folate (Km, 1–5 μM). Some cells in the choroid plexus, kidney, lung, thyroid, spleen, placenta, and thymus also possess a receptor with higher affinity (Kd, 0.5 nM), which allows folate uptake via receptor-mediated endocytosis. Some human epithelial tumor cells were found to overexpress folate receptors (2). More than 90% of human ovarian and endometrial cancers express the high-affinity receptor, which is absent in the normal tissues. Breast, colorectal, renal, and lung carcinomas also overexpressed the high-affinity folate receptor but to a lesser frequency (20%–50%). Several 99mTc-based folate conjugates have been developed for single-photon emission computed tomography (SPECT) imaging of folate receptor expression in tumors such as 99mTc-ethylenedicysteine-folate, 99mTc-HYNIC-Folate, and 99mTc-DTPA-Folate (3). Mishra et al. (4) prepared 99mTc-1,4,7-tris(carboxymethyl)-10-(4-aminoethyl)-1,4,7,10-tetraazacyclododecane-folate (99mTc-DO3A-EA-Folate) as a new 99mTc-based folate conjugate for SPECT imaging studies.

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Synthesis

[PubMed]

99mTc was complexed efficiently (>97% yield) to DO3A-EA-folate to form 99mTc-DO3A-EA-folate, with a radiochemical purity of >97% (4). The specific activity was calculated to be ~2 μCi/μg (0.074 MBq/μg) before in vivo injection in mice. 99mTc-DO3A-EA-Folate was found to be stable in human serum for up to 24 h at 37ºC.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Receptor binding studies were performed using human oral carcinoma KB, ovarian carcinoma OAW, and malignant glioma U87MG cells with 99mTc-DO3A-EA-folate (4). The affinity binding constant (Kd) values were calculated to be 4.2 ± 0.8, 22.5 ± 0.1, and 16.7 ± 0.1 µM, respectively.

Animal Studies

Rodents

[PubMed]

Nude mice (number of mice studied not reported) were inoculated with KB tumor cells, followed by intravenous injection of 3.7 MBq (0.1 mCi) 99mTc-DO3A-EA-folate 14–17 days after tumor transplantation (4). Ex vivo biodistribution studies were performed at 1, 4, and 24 h after injection. Tumor accumulation was 5.06% injected dose/g (ID/g) at 1 h, 6.26 ± 0.40% ID/g at 4 h, and 4.06 ± 0.32% ID/g at 24 h. The organ with the highest accumulation was the liver (17.6% ID/g), followed by the kidney (15.0% ID/g), intestines (4% ID/g), and spleen (4% ID/g) at 1 h after injection, with <1% at 24 h. The blood, heart, lung, stomach, and muscle showed <2% ID/g at 1–24 h. The tumor/blood ratios were 2.3 ± 0.3 and 6.1 ± 1.0 at 1 h and 4 h after injection, respectively. There was a biphasic clearance of 99mTc-DO3A-EA-folate in the blood, with half-life values of 12 min and 140 min. Co-injection with unlabeled folate blocked the uptake of 99mTc-DO3A-EA-folate by 67%, 50%, and 60% in the tumor, kidney, and liver, respectively, at 4 h after injection.

Scintigraphic images of mice injected with 3 MBq (0.08 mCi) 99mTc-DO3A-EA-folate showed that tumors in the right hind legs could be well visualized at 1–4 h (4). The tumor/muscle ratios were estimated to be 11.8 ± 1.2 and 19.7 ± 3.3 at 1 h and 4 h, respectively. 99mTc-DO3A-EA-folate could be a useful tool for imaging tumors positive for folate receptor.

Other Non-Primate Mammals

[PubMed]

No publication is currently available.

Non-Human Primates

[PubMed]

No publication is currently available

Human Studies

[PubMed]

No publication is currently available.

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.
Ilgan S., Yang D.J., Higuchi T., Zareneyrizi F., Bayhan H., Yu D., Kim E.E., Podoloff D.A. 99mTc-ethylenedicysteine-folate: a new tumor imaging agent. Synthesis, labeling and evaluation in animals. Cancer Biother Radiopharm. 1998;13(6):427–35. [PubMed: 10851435]
4.
Mishra G., Hazari P.P., Kumar N., Mishra A.K. In vitro and in vivo evaluation of (99m)Tc-DO3A-EA-Folate for receptor-mediated targeting of folate positive tumors. J Drug Target. 2011;19(9):761–9. [PubMed: 21425922]