A manganese-based non-peptidyl mimetic of the human mitochondrial manganese superoxide dismutase (MnSOD), with potential antioxidant and chemo/radioprotective activities. Upon administration, imisopasem manganese mimics the activity of MnSOD and scavenges reactive oxygen species (ROS), such as superoxide anion, which prevents oxygen free radical damage to macromolecules such as DNA. This reduces ROS-mediated lipid peroxidation, prevents apoptosis and protects against oxygen free radical-induced toxicity in normal tissues.