ALDH1A1 and cancer: In order to circumvent these inherent limitations, us and others have proposed the use of positron emission tomography (PET) as an alternative to fluorescence-based imaging.21,22 We have recently reported that [18F]N-ethyl-6-(fluoro)-N-(4-formylbenzyl)nicotinamide – [18F]1 – has excellent affinity and isozyme selectivity for ALDH1A1 in cancer cells but is rapidly oxidised to its corresponding carboxylic acid in the blood when administered at tracer doses.21 Here, we employed an enzyme-cleavable prodrug strategy to increase the blood stability of our ALDH1A1-targeted radiotracer.