IR 780 is a heptamethine cyanine fluorescent probe for in vivo imaging of tumor cells. It displays excitation/emission maxima of 780/799 nm, respectively. IR 780 is transported into tumor cells, an effect that can be blocked by the organic anion transporting polypeptide (OATP) inhibitor sulfobromophthalein (bromosulfalein; ), and targeted to the mitochondria. It has an additive effect on the increase in reactive oxygen species (ROS) production induced by sonodynamic therapy (SDT) in 4T1 breast cancer cells when used at a concentration of 16 µM. IR 780 (0.2 mg/kg, i.v.) preferentially accumulates in tumor tissue in MCF-7, HeLa, and MG-63 mouse xenograft models and in a syngeneic mouse model of chemically induced lung tumors, and its fluorescence is detectable for up to 20 days. It also has an additive effect on the SDT-induced reduction in tumor growth in an 4T1 murine mammary carcinoma model. IR 780 has been incorporated into heparin-folic acid-IR 780 nanoparticles for use in photothermal therapy.