Imaging of Tumor Hypoxia to Predict Treatment Sensitivity

Non-invasive detection of tumor hypoxia using radiolabeled 2-nitroimidazoles has been a major effort during the last two decades. Recent years have witnessed the introduction of several new compounds which are chemically related to [18F]fluoromisonidazole (FMISO) but show slight but distinct differences in biodistribution and metabolic clearance. Although [18F]FMISO has shown clinical potential it suffers from suboptimal oxygen dependent tissue contrast and newer agents seek to improve this essential feature. The limited data on other interesting tracers keeps the investigators busy at demonstrating the potential advantages over [18F]FMISO while efforts should start to concentrate on proving the clinical significance of such techniques in the form of outcome data from image-guided therapy modification. We review here our experiences with two hypoxia-avid agents [18F]fluoroerythronitromidazole (FETNIM) and [18F] 2-(2-nitro-1-Himidazol- 1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide (EF5) and focus on the similarities and differences of these two tracers in comparison to other radiolabeled 2-nitroimidazoles. It is recognized that only [18F]FMISO has thus far shown clinical utility and newer tracers need to be tested against this circumstance.