A plasmid DNA encoding human manganese superoxide dismutase (MnSOD) and liposomally encapsulated with potential chemoprotective activity. When administered orally and localizing in the esophagus, MnSOD-plasmid liposomes express MnSOD, which scavenges reactive oxygen species (ROS); MnSOD scavenging of ROS may result in a reduction in ROS-mediated lipid peroxidation, apoptosis, and micro-ulceration in the epithelial lining of the esophagus.