γ-Glutamyltranspeptidase (GGT) is a cell -membrane-associated enzyme which has been recognized as a promising biomarker for the diagnosis of many malignant tumors. Herein, we rationally designed a fluorine-18 labeled small-molecule probe, [ 18 F]γ-Glu-Cys(StBu)-PPG(CBT)-AmBF 3 ( 18 F-1G), by applying a biocompatible CBT-Cys condensation reaction and ingeniously decorating it with a GGT-recognizable substrate, γ-glutamate (γ-Glu), for enhancing PET imaging to detect GGT level of tumors in living nude mice. The probe had exceptional stability at physiological conditions, but could be efficiently cleaved by GGT, followed by a reduction-triggered self-assembly and formation of nanoparticles (NPs) progressively that could be directly observed by transmission electron microscopy (TEM). In in vitro cell experiments, 18 F-1G showed GGT-targeted uptake contrast of 2.7-fold to that of 18 F-1 for the detection of intracellular GGT level. Moreover, the higher uptake in GGT overexpressed HCT116 tumor cells (∼4-fold) compared to GGT-deficient L929 normal cells demonstrated that 18 F-1G was also capable of distinguishing some tumor cells from normal cells. In vivo PET imaging revealed enhanced and durable radioactive signal in tumor regions after 18 F-1G coinjecting with 1G, thus allowing real-time detection of endogenous GGT level with high sensitivity and noninvasive effect. We anticipated that our probe could serve as a new tool to investigate GGT-related diseases in the near future.

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 (SF) for controlling in situ self-assembly by engineering the condensation reaction between 2-cyanobenzothiazole and cysteine. For probes with the SF 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 (AmBF3), two radioactive positron emission tomography (PET) tracers, [18F]SF-DEVD and [18F]SF-Glu, 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 SF 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.

A PET imaging probe with optimized pharmacokinetics was designed for real-time monitoring of the activity of caspase-3 in tumors.