Background

[PubMed]

Folic acid (FA; also known as folate or vitamin B₉) is a water-soluble vitamin that is required for the synthesis and repair of cellular DNA. FA also acts as a cofactor for many biological reactions and has an important role in cell maintenance and proliferation. Although the folate receptor (FR) is expressed at low levels in normal cells, this receptor is known to be upregulated in cancers such as those of the ovary, lung, breast, brain, colon, and the hematopoietic lineage cells (1). Therefore, the FR system has been targeted with radiolabeled folate and its derivatives, such as ⁶⁷Ga-deferoxamine (DF)-folate, ¹¹¹In-diethylenetriamine pentaacetic acid-folate, ^99mTc-mercaptoacetyldiglycine-folate-methotrexate, etc., for the noninvasive detection of malignancies with single-photon emission computed tomography (2). Mathias et al. showed that ^66/67Ga-DF-folate can be used with positron emission tomography (PET) for the imaging of FR-positive cancerous tumors in mice (3). However, although these tracers could detect the FR-rich tumors, they were deemed unsuitable for imaging lesions in the abdominal regions because high levels of radioactivity were observed to accumulate in the liver and intestines of the animals (4). In addition, ^66/67Ga-labeled tracers are known to produce low-resolution images because they generate high positron energy (4.15 MeV and 1.89 MeV for ⁶⁶Ga and ⁶⁷Ga, respectively. ⁶⁷Ga-labeled agents are used for single photon emission computed tomography imaging) compared to ¹⁸F (0.64 MeV), which generates superior images and is often used to radiolabel PET imaging agents in the clinic (4). In another study, it was shown that ¹⁸F-fluorobenzylamine derivatives of folate could detect tumors that had a high expression of FR, but the radioactivity from these labeled compounds accumulated mainly at the rim of the tumor, and large amounts of the label were retained in the liver and intestines of the animals (4, 5).

In an ongoing effort to generate radiolabeled agents for the imaging of tumors that express high levels of FRs that would be superior to those developed and evaluated earlier, a ¹⁸F-fluorobenzene derivative of folic acid ([¹⁸F]-folate-1), a pyridinecarbohydrazide-folate derivative of folic acid ([¹⁸F]-folate-2), a ¹⁸F-fluorobenzene/methotrexate (MTX) conjugate of folic acid ([¹⁸F]-folate-MTX-8), and a ¹⁸F-pyridinecarbohydrazide-folate/methotrexate conjugate of folic acid ([¹⁸F]-folate-MTX-9) have been synthesized (2). The biodistribution of these tracers was investigated in healthy mice. On the basis of results obtained from these studies, [¹⁸F]-folate-1 was evaluated for the PET imaging of human KB cell line xenograft tumors (have a high expression of FR) in mice. This chapter describes the results obtained with [¹⁸F]-folate-1. Separate chapters in MICAD (http://www.micad.nih.gov/) discuss the studies performed with [¹⁸F]-folate-2 (6), [¹⁸F]-folate-MTX-8 (7), and [¹⁸F]-folate-MTX-9 (8).

Synthesis

[PubMed]

Folate-1 was synthesized with hydrazide-folate and 2,5-dioxoazolidinyl 4-fluorobenzoate in presence of triethylamine and the ¹⁸F labeling of folate-1 has been detailed elsewhere (9). The total time of synthesis was ~45 min, and the radiochemical yield and radiochemical purity of the final product were >80% (based on the initial ¹⁸F concentration) and >97% (without high-performance liquid chromatographic purification), respectively. The specific activity of the final product was reported to be >11.11 MBq/μmol (300 mCi/μmol).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Using an n-octanol-water mixture (pH 7.3), the partition coefficient of [¹⁸F]-folate-1 was determined to be 0.38 ± 0.02, indicating that the labeled compound had low lipophilicity (9).

[¹⁸F]-Folate-1 was reported to be stable in human plasma for at least 4 h at 37°C (data not presented) (9).

The FR binding affinity of [¹⁸F]-folate-1 was determined to be 13.08 ± 0.83 nM with a saturation assay using KB cell membranes (9). A cell internalization assay of [¹⁸F]-folate-1 with KB cells (using an acidic buffer, pH not mentioned) showed that 25.85 ± 0.95% of the tracer was internalized by the cells within 20 min at 37°C.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

Supplemental Information

[Disclaimers]

No information is currently available.

References

1.

Xia W., Low P.S. Folate-targeted therapies for cancer. J Med Chem. 2010;53(19):6811–24. [PubMed: 20666486]

2.

Al Jammaz I., Al-Otaibi B., Amer S., Okarvi S.M. Rapid synthesis and in vitro and in vivo evaluation of folic acid derivatives labeled with fluorine-18 for PET imaging of folate receptor-positive tumors. Nucl Med Biol. 2011;38(7):1019–28. [PubMed: 21982573]

3.

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]

4.

Sega E.I., Low P.S. Tumor detection using folate receptor-targeted imaging agents. Cancer Metastasis Rev. 2008;27(4):655–64. [PubMed: 18523731]

5.

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):1153–60. [PubMed: 16818950]

6.

Chopra, A., 2-[(4-{[(2-amino-4-oxohydropteridin-7yl)methyl]amino}phenyl) carbonylamino]-4-{N-[(4-[18F]-fluorophenyl)carbonylamino] carbamoyl}butanoic acid. Molecular Imaging and Contrast agent Database (MICAD) [database online]. National Library of Medicine, NCBI, Bethesda, MD, USA. Available from www​.micad.nih.gov, 2004 -to current. [PubMed: 22238799]

7.

Chopra, A., 2-[(4-{[(2-amino-4-oxohydropteridin-7yl)methyl]amino}phenyl) carbonylamino]-4-{N-[(2-[18F]-fluoro(4-pyridyl))carbonylamino] carbamoyl}butanoic acid. Molecular Imaging and Contrast agent Database (MICAD) [database online]. National Library of Medicine, NCBI, Bethesda, MD, USA. Available from www​.micad.nih.gov, 2004 -to current. [PubMed: 22238794]

8.

Chopra, A., 2-[18F]Fluoropyridine-4-carbohydrazide-methotrexate (9). Molecular Imaging and Contrast agent Database (MICAD) [database online]. National Library of Medicine, NCBI, Bethesda, MD, USA. Available from www​.micad.nih.gov, 2004 -to current.

9.

Jammaz I.A., Otaibi B.A., Okarvi S., Amartey J.K. Novel synthesis of [18F]-fluorobenzene and pyridinecarbohydrazide-folates as potential PET radiopharmaceuticals. Journal of Labelled Compounds and Radiopharmaceuticals. 2006;49(2):125–37.