Abstract: A method of measuring ?-methylacyl-CoA racemase (AMACR) activity in a sample is described comprising the steps of: (i) providing a sample; (ii) contacting said sample with a substrate compound that undergoes elimination of hydrogen and a leaving group catalysed by said AMACR to produce a product compound having an additional carbon-carbon double bond; and (iii) measuring the amount of said leaving group and/or of said product compound and/or of unreacted substrate compound. Also provided are methods of diagnosis and medical imaging, in particular for use in diagnosing prostate cancer. Also provided are substrate compounds of Formula 1 for use in the methods described herein.