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.