Rapamycin treatment induces the selective shutdown of initially functional tumor vessels. The effect of a 3-day rapamycin treatment (1.5 mg/kg/d) schedule on the functional capillary density (FCD) of normal and CT-26 tumor (TU) tissue (day 11 after tumor implantation) are shown. Intravital microscopy views of (A) peritumoral vessels, (B) a normal tumor vessel network in control mice, and (C) significant loss of FCD in the tumor of a rapamycin-treated animal. (D) A computer-assisted quantitative analysis of blood vessels demonstrates a significant loss of FCD after rapamycin treatment in the peripheral and central tumor region compared to untreated tumors or compared to peritumoral tissue. □ indicates saline control mice; ▦, mice treated with 1.5 mg/kg/d rapamycin. Data shown are mean values ± SEM of 7 mice per group. ^*P < .05 versus saline control. Total magnification in panels A-C, 630×, 20× Leitz Wetzlar long-distance objective lens, numerical aperture 0.32. Images were acquired with Image J software.