Towards automated solid phase radiofluorination for dose-on-demand PET: retention of activity by solid support

On-column [18F]fluoride trapping and radiofluorination of 2-(naphthalen-1-yl)ethyl-4-methylbenzenesulfonate (C10H7(CH2)2OTs), performed on polystyrene supported phosphazene base PS-PtBu2 yielded [18F]1-(2-fluoroethyl)naphthalene ([18F]C10H7(CH2)2F) in 50% radiochemical yield but left up to 43% of activity unreacted on the resin. This activity could be eluted with Kryptofix/K2CO3 and then used for conventional radiofluorination of the same substrate, suggesting that the column-retained activity was present in the form of [18F]fluoride entrapped in polymer matrix. An approach to minimize the amount of entrapped [18F]fluoride by use of glass beads functionalized with alkylsilane-derivatized phosphazene residues was attempted but was stymied by fluorolysis/hydrolysis of the alkylsilane spacer. The results suggest that the key to high yield of on-column radiofluorination is to minimize the residual [18F]fluoride absorption in the matrix by the judicious choice of solid support