Radiosynthesis and in vivo evaluation in melanoma-bearing mice of O-(2-[18F]Fluoroethyl)-L-tyrosine as tumour tracer

Objectives: 18F-labeled amino acid O-(2-[18F]Fluoroethyl)-L-tyrosine ([18F]FET), a very promising tumour tracer, was synthesized by a different chemical route and evaluated in melanoma-bearing mice with the way of biodistribution and atuoradiography. Methods: the precursor N-tert-butyloxycarbonyl-O-(2-tosyloxyethyl)-L-tyrosine methyl ester was easily obtained from the reaction of commercially available N-(tert-butoxycarbonyl)-L-tyrosine methyl ester and 1, 2-bis(tosyloxy)ethane in anhydrous acetonitrile under the presence of potassium carbonate at 90 deg. C for 3∼4h (scheme 1). Radiosynthesis (scheme 2) of [18F]FET was started from the direct radiofluorination of the precursor with the activated complex 18F-/K2.2.2./K2CO3 in acetonitrile at 130 deg. C for 30min. Then the reaction mixture was treated with trifluoroacetic acid and sodium hydroxide to realize the hydrolysis in less 10min. The separation was carried out by solid phase extraction on Sep-pak plus silica cartridge and eluted with acetonitrile and phosphate buffer saline (PBS) in turn. Thus the final formation of [18F]FET was obtained as PBS solution and used for the following animal experiments after dilution. The animal experiments were investigated in B16 melanoma-bearing C57BL/6 mice and performed 10d after subcutaneous inoculation of the tumour cells in the right axillas when the tumours weight 50∼300mg. 1.5∼2.3MBq [18F]FET in about 100μL of PBS solution was injected into tail vein. The animals for biodistribution were killed by eye dislocation at various times post injection, and the organs of interest were rapidly dissected. The tracer uptakes in tissues were determined by gamma counter. The results were expressed as percentage of the injected dose per gram tissue (ID% /g). The animals for whole-body autoradiography were killed by the above same protocol without dissection, and then directly exposed to multisensitive imaging plates. Imaging plates were scanned by phosphor storage imaging system. Results: The radiochemical yield was with an average of 45% and radiochemical purity was more than 95%. The total synthesis time was less than 60min from EOB. [18F]FET showed very fast accumulation into the whole body with peak occurring within 10min after i.v. administration. The highest radioactivity ratios of tumour-to-brain and tumour-to-skin reached 3.4 - 5.1 and 4.1 - 2.7 at 30 - 90min p.i. Visual evaluation from autoradiography showed evident difference between tumour and other tissues. Conclusion: An efficient one-pot synthesis method of [18F]FET was established in good radiochemical yield and radiochemical purify. Higher ratios of tumour-to-non-tumour was found in our experiments which promise a high sensitivity for the detection of cerebral and peripheral tumour