The present invention concerns in vivo imaging and in particular in vivo imaging of translocator protein (TSPO, formerly known as the peripheral benzodiazepine receptor). An indole-based in vivo imaging agent is provided that overcomes problems relating to known TSPO-binding radiotracers. The present invention also provides a precursor compound useful in the synthesis of the in vivo imaging agent of the invention, as well as a method for synthesis of said precursor compound. Other aspects of the invention include a method for the synthesis of the in vivo imaging agent of the invention comprising use of the precursor compound of the invention, a kit for carrying out said method, and a cassette for carrying out an automated version of said method. In addition, the invention provides a radiopharmaceutical composition comprising the in vivo imaging agent of the invention, as well as methods for the use of said in vivo imaging agent.
