P-FRT-RBCs showed enhanced PDT effect under hypoxic environments. a) Comparison of 1O2 generation among P-FRT-RBCs, a mixture of RBCs and free P-FRTs, and free P-FRTs, conducted in an Ar-filled cuvette. The cuvette was irradiated by a 671 nm laser (0.1 W cm −2) for up to 60 min. SOSG was used as an indicator of 1O2 production. b) HIF-1± staining on tumor samples taken from the acute hypoxia tumor model and normal mice. Green, HIF-1±. Blue, DAPI. Scale bars: 100 μm. c) H&E staining on hypoxic tumors treated by PDT-mediated by different formulations. While P-FRT-RBCs and O2-treated P-FRT-RBCs induced signifi cant damage to tumors under irradiation, CO-treated P-FRT-RBCs and P-FRTs caused minimal cancer cell death. Scale bars: 100 μm. d) TUNEL assays on hypoxic tumors treated by PDT-mediated by different formulations. Similar to observations in (c), extensive cell death was only observed in animals treated with P-FRT-RBCs and O2-treated P-FRT-RBCs. Green, TUNEL. Blue, DAPI. Scale bars: 100 μm.