Preparation of 18F labeled amino acid O-(2-[18F]fluoroethyl)-L-tyrosine using indirect and direct labeling methods

Abstract

18 F-labeled amino acid O-(2 -[ 18 F]fluoroethyl) -L-tyrosine ([ 18 F]FET) has been used as an in vivo positron emission tomography (PET) tracer for tumor metabolitic imaging. Here, t he synthesis of [ 18 F]FET using two different methods is reported. The first Method (I) for the [ 18 F]FET synthesis was an indirect labeling route , which was a two -step reaction consisting of fluorination of 1,2 -bis(tosyloxy)ethane and fluoroalkylation of un protected L -tyrosine. The second Method (II) was a direct labeling route which was the direct nucleophilic radiofluorination of the protected precursor N-BOC -(O-(2 -tosyloxyethyl)) -L-tyrosine methyl ester, followed by a rapid removal of the protecting group . For the first method, the radiochemical yield was about 45% at the end of synthesis (EOS), and the radiochemical purity was over 97%. The radiochemical yield in the second method was 40% (EOS) on an average, and the radiochemical purity was over 96%. Mic rowave heating was also introduced into the synthesis of [ 18 F]FET. For Method I, each of two steps could be completed within 2 –3 minutes under microwave conditions and the radiochemical yields were 85% and 95%, respectively, which was reasonably high and r eproducible. For Method II, the highest labeling efficiency in the radiofluorinated step was 37% in the synthesis time of 3 minutes under microwave conditions.