M40419 (now GC4419) is a Mn(II)-containing pentaazamacrocyclic selective superoxide dismutase mimetic. It is a first-in-class, small molecule enzyme mimetic that converts superoxide to hydrogen peroxide and molecular oxygen. GC4419 is currently being evaluated in an ongoing randomized Phase 2 clinical trial to assess its effect on the incidence, severity and duration of severe oral mucositis (OM) when given to patients with squamous cell cancers of the head and neck in combination with radiation and chemotherapy. GC4419 has received Breakthrough Therapy and Fast Track designations from the U.S. Food and Drug Administration.