EFFICIENT REGIOSELECTIVE LABELING OF THE CFC ALTERNATIVE 1,1,1,2-TETRAFLUOROETHANE (HFC-134A) WITH F-18

Efficient chemistry is described for the regioselective labelling of t he CFC alternative 1,1,1,2-tetrafluoroethane with cyclotron-produced p ositron-emitting fluorine-18 (t(1/2) = 109.7 min). [1-F-18]1,1,1,2-Tet rafluoroethane was prepared by nucleophilic addition of no-carrier-add ed [F-18]fluoride to trifluoroethylene and [2-F-18]1,1,1,2-tetrafluoro ethane by nucleophilic displacement of tosylate with [F-18]fluoride in 2,2,2-trifluoroethyl p-toluenesulphonate. Each reaction was mediated by a potassium cation-Kryptofix(R) 2.2.2 complex, with or without acet onitrile as solvent, in a sealed glassy carbon vessel. The selectiviti es were 97.2 +/- 0.4% for labelling in the 1-position by nucleophilic addition and 91.2 +/- 1.2% for labelling in the 2-position by nucleoph ilic substitution. GC separation afforded each labelled tetrafluoroeth ane in high radiochemical purity (> 99.995%) and high chemical purity (> 99.6%). Specific radioactivities of about 37 MBq (1 mCi) per mu mol were obtained. Each synthesis was fully automated to cope safely with the high initial radioactivity and delivered purified product within one physical half-life of the fluorine-18. The products are suitable f or pharmacokinetic studies in man.