Sialic acid detection and theranostic activity of phenylboronic acid-based fluorescent probe in human colorectal adenocarcinoma HT-29 cells.

A new probe, 4-(((3',6'-bis(diethylamino)-3-oxospiro[isoindoline-1,9'-xanthen]-2-yl)imino)methyl)phenyl)boronic acid (R4B) was prepared by facile condensation of 4-formylphenylboronic acid and rhodamine B hydrazide. R4B was characterized by spectroscopic methods and single crystal X-ray diffraction. The sensor R4B solution turned pink and emitted orange fluorescence only in the presence of sialic acid but remained colorless and non-fluorescent otherwise. The sugar recognition performance was investigated via UV-vis and fluorescence spectroscopic studies. Our results revealed that R4B has good affinity and selectivity for sialic acid over common monosaccharides, with a detection limit as low as 10-7 M. Furthermore, R4B selectively inhibited growth of human colorectal adenocarcinoma HT-29 (IC50 <20 µM) without significant cytotoxicity to normal human colon fibroblasts CCD-18Co. Treatment with R4B suppressed HT-29 colony formation via mitochondrial apoptosis in a time-dependent manner. Cellular imaging studies also revealed the ability of R4B as a fluorescence dye to detect intracellular sialic acid and showed mitochondria-tracking ability in HT-29 cells. In summary, R4B is a potential theranostic for the detection of intracellular sialic acid during the early incubation period, followed by induction of cancer apoptotic cell death at a later treatment point.