Stimuli-Responsive Macrocyclization Scaffold Allows Self-Assembly of Radioactive Tracers for Positron Emission Tomography Imaging of Enzyme Activity.

Target-enabled bioorthogonal reaction and self-assembly of a small-molecule probe into supramolecules have shown promise for molecular imaging. In this paper, we report a new stimuli-responsive bioorthogonal reaction scaffold () for controlling self-assembly by engineering the condensation reaction between 2-cyanobenzothiazole and cysteine. For probes with the scaffold, intramolecular cyclization took place soon after activation, which could efficiently outcompete free cysteine even at a low concentration and result in efficient aggregation in the target. Through integration with different enzyme-responsive substrates and an ammoniomethyl-trifluoroborate moiety (AmBF), two radioactive positron emission tomography (PET) tracers, [F] and [F], were designed, which showed high stability under physiological conditions and could produce clear PET signal in tumors to detect enzyme activity (e.g., caspase-3, γ-glutamyltranspeptidase) timely and accurately. Our results demonstrated that the scaffold could serve as a general molecular scaffold in the development of smart PET tracers for noninvasive imaging of enzyme activity, which could contribute to tumor detection and treatment efficacy evaluation.