We chose to use a geminal diacetate to protect the aldehyde on [ 18 F]1 to create [ 18 F](4-((N-ethyl-6-fluoronicotinamido)methyl)phenyl)methylene diacetate [ 18 F]2. This protecting group was chosen as it is stable to acid and mildly basic conditions, but importantly is susceptible to enzymatic cleavage via esterase activity to intracellularly liberate the free aldehyde. ALDH1A1 is known to exhibit esterase activity and we envisaged that following passive diffusion into the cell, the diacetate would be cleaved to furnish [ 18 F]1. ALDH-mediated oxidation of [ 18 F]1 to the carboxylate [ 18 F]3 would subsequently result in intracellular trapping of the radiolabelled product as a result of its negative charge.