Targeted Delivery of a gamma-Glutamyl Transpeptidase Activatable Near-Infrared-Fluorescent Probe for Selective Cancer Imaging

The noninvasive and specific detection of cancer cells in living subjects has been essential for the success of cancer diagnoses and treatments. Herein, we report a strategy of combining an alpha(v)beta(3)-integrin-receptor-targetable ligand, c-RGD, with the gamma-glutamyl transpeptidase (GGT)-recognizable substrate, gamma-glutamate (gamma-Glu), to develop a tumor-targeting and GGT-activatable near-infrared(NIR)-fluorescent probe for the noninvasive imaging of tumors in living mice. We demonstrated that the probe's fluorescence was off initially, but when the gamma-Glu in the probe was specifically cleaved by GGT, the fluorescent product was released and could be selectively taken up by U87MG-tumor cells via alpha(v)beta(3)-receptor-mediated endocytosis. Remarkably, enhanced intracellular NIR fluorescence distributed mainly in the lysosomes was observed in the tumor cells only, showing that the probe was capable of differentiating the tumor cells from the GGT-positive, alpha(v)beta(3)-deficient normal cells. Moreover, the probe also showed a high selectivity for the real-time and noninvasive detection of GGT activity in xenograft U87MG tumors following iv administration. This study reveals the advantage of using a combination of receptor-mediated cell uptake and molecular-target-triggered activation to design molecular probes for improved cancer imaging, which could facilitate effective cancer diagnoses.