ER-localized and viscosity-responsive fluorescent probe for imaging osteosarcoma microenvironment.

Osteosarcoma (OS) is a highly aggressive bone tumor with rapid progression and high metastatic potential. Current clinical diagnosis relies mainly on imaging and biopsy, which are limited in detecting early pathological abnormalities and are unsuitable for real-time or repeated monitoring. Viscosity, a key biophysical parameter of the tumor microenvironment (TME), is closely linked to metabolic activity and signal transduction; abnormal viscosity changes may serve as sensitive indicators of OS progression and microenvironmental remodeling. The endoplasmic reticulum (ER), an essential organelle involved in protein folding, lipid metabolism, calcium homeostasis, and stress responses, plays a critical role in tumor adaptation and progression. Accordingly, monitoring ER-associated viscosity may provide valuable pathological information. Herein, we developed Ost-V, an ER-localized viscosity-responsive fluorescent probe based on an alkenyl indole scaffold. Ost-V exhibited viscosity-dependent fluorescence activation, good chemical stability, broad pH tolerance, favorable photostability, and low cytotoxicity. In living cells, Ost-V produced stronger fluorescence signals in OS cells than in normal cells and sensitively responded to intracellular viscosity elevation. Co-localization imaging confirmed preferential ER accumulation. In a subcutaneous OS xenograft model, ex vivo imaging showed that Ost-V can visualize viscosity changes in OS tumor tissues with low background interference from major organs. These results indicate that Ost-V is a promising molecular imaging tool for monitoring ER-associated viscosity alterations in the OS microenvironment, offering a potential strategy for early diagnosis and progression assessment of OS.