Development of controlled drug delivery by means of Tz-triggered decaging of TCO-drug conjugates or vice versa (click-to-release) makes this chemistry even more attractive. Recently, we performed a systematic study of the relationship between the physicochemical properties of Tzs and their in vivo click performance in a colon tumor model. This study revealed that only hydrophilic Tzs with fast click kinetics were suitable for pretargeted imaging. Such Tzs are unsuitable for imaging of brain targets as their hydrophilicity prevents any reasonable BBB penetration. We hypothesized, however, that pretargeted imaging within the brain is possible as other blood supplying and excreting tissues are spatially separated from the brain. This should decrease background and possibly allow to image targets within the brain.