A fast chemoenzymatic synthesis of [11C]-N5,N10-methylenetetrahydrofolate as a potential PET tracer for proliferating cells.
BACKGROUND
Thymidylate synthase and folate receptors are well-developed targets of cancer therapy. Discovery of a simple and fast method for the conversion of 11CH3Ito[11C]-formaldehyde (11CH2O) encouraged us to label the co-factor of this enzyme. Preliminary studies conducted on cell lines have demonstrated a preferential uptake of [11-14C]-(R)-N5,N10-methylene-5,6,7,8-tetrahydrofolate (14CH2H4folate) by cancerous cell vs. normal cells from the same organ (Saeed M., Sheff D. and Kohen A. Novel positron emission tomography tracer distinguishes normal from cancerous cells. J Biol Chem 2011;286:33872-33878), pointing out 11CH2H4folate as a positron emission tomography (PET) tracer for cancer imaging. Herein we report the synthesis of 11CH2H4folate, which may serve as a potential PET tracer.


METHODS
In a remotely controlled module, methyl iodide (11CH3I) was bubbled into a reaction vial containing trimethylamine N-oxide in N,N-Dimethylformamide (DMF) and heated to 70°C for 2 min. Formaldehyde (11CH2O) formed after the completion of reaction was then mixed with a solution of freshly prepared tetrahydrofolate (H4folate) by using a fast chemoenzymatic approach to accomplish synthesis of 11CH2H4folate. Purification of the product was carried out by loading the crude reaction mixture on a SAX cartridge, washing with water to remove unbound impurities and finally eluting with a saline solution.


RESULTS
The synthesis and purification of 11CH2H4folate were completed within 5 min. High-performance liquid chromatography analysis of the product after SAX purification indicates that more than 90% of the radioactivity that was retained on the SAX cartridge was in 11CH2H4folate, with minor (<10%) radioactivity due to unreacted 11CH2O.


CONCLUSIONS
We present a fast (∼5 min) synthesis and purification of 11CH2H4folate as a potential PET tracer. The final product is received in physiologically compatible buffer (100 mM sodium phosphate, pH 7.0 containing 500 mM NaCl) and ready for use in vivo.
Department of Chemistry, University of Iowa, IA 52242, USA.
Muhammad
M
Saeed
Timothy J
TJ
Tewson
Colbin E
CE
Erdahl
Amnon
A
Kohen
D002250:Carbon Radioisotopes; D049109:Cell Proliferation; D060326:Chemistry Techniques, Synthetic / Q000379:methods*; D005492:Folic Acid / Q000737:chemistry; D005557:Formaldehyde / Q000737:chemistry; D007700:Kinetics; D049268:Positron-Emission Tomography / Q000379:methods*; D011849:Radioactive Tracers; D013763:Tetrahydrofolates / Q000138:chemical synthesis* / Q000737:chemistry; D013940:Thymidylate Synthase / Q000378:metabolism*
D002250:Carbon Radioisotopes; D011849:Radioactive Tracers; D013763:Tetrahydrofolates; C013123:5,10-methylenetetrahydrofolic acid; D005557:Formaldehyde; D005492:Folic Acid; D013940:Thymidylate Synthase