The present invention relates to novel compounds of Formula I capable of inhibiting tissue transglutaminase, and uses of the same in medicine. In particular, the invention provides compounds for use in the treatment of prevention of disease and conditions such as fibrosis (e.g. cystic fibrosis), scarring, neurodegenerative diseases (e.g. Alzheimer' s disease, Huntington' s disease and Parkinson' s disease), autoimmune diseases (e.g. multiple sclerosis and coeliac disease), thrombosis, proliferative disorders (e.g. cancers), AIDS, psoriasis and inflammation (e.g. chronic inflammatory diseases).