Enzyme-Controlled Intracellular Self-Assembly of (18)F Nanoparticles for Enhanced MicroPET Imaging of Tumor

Herein, we report the development of a new "smart" radioactive probe (i.e., 1) which can undergo furin-controlled condensation and self-assembly of radioactive nanoparticles (i.e., 1-NPs) in tumor cells and its application for enhanced microPET imaging of tumors in nude mice co-injected with its cold analog (i.e., 1-Cold). Furin-controlled condensation of 1-Cold and self-assembly of its nanoparticles (i.e., 1-Cold-NPs) in vitro were validated and characterized with HPLC, mass spectra, SEM, and TEM analyses. Cell uptake studies showed that both 1 and 1-Cold have good cell permeability. TEM images of 1-Cold-treated MDA-MB-468 cells directly uncovered that the intracellular 1-Cold-NPs were at/near the location of furin (i.e., Golgi bodies). MTT results indicated that 50 µM 1-Cold did not impose cytotoxicity to MDA-MB-468 cells up to 12 hours. MicroPET imaging of MDA-MB-468 tumor-bearing mice indicated that mice co-injected with 1 and 1-Cold showed higher uptake and longer attenuation of the radioactivity in tumors than those mice only injected with same dosage of 1. Tumor uptake ratios of 1 between these two groups of mice reached the maximum of 8.2 folds at 240 min post injection. Biodistribution study indicated that the uptake ratios of 1 in kidneys between these two groups continuously increased and reached 81.9 folds at 240 min post injection, suggesting the formation of radioactive NPs (i.e., 1-NPs) in MDA-MB-468 tumors of mice co-injected with 1 and 1-Cold. And the nanoparticles were slowly digested and secreted from the tumors, accumulating in the kidneys. Our ''smart'' probe (i.e., 1), together with the strategy of co-injection, might help researchers trace the biomarkers of interest within a longer time window.