Abstract: The present invention relates to novel compounds of Formula (I) wherein ‘X’ represents an amino acid group, ‘n’ is an integer between 1 and 4, ‘R1’ represents benzyl, t-butyl or 9-fluorenylmethyl and ‘R2’ represents a tetramethylmercaptoimidazole derivative or —S+R3R4, wherein R3 and R4 each independently represent lower alkyl, or a pharmaceutically and/or veterinarily acceptable derivative thereof. The present invention further relates to pharmaceutical formulations of said compound and the use thereof in the preparation of a medicament for inhibiting diseases in which transglutaminase has been implicated. Advantageously, the medicament is for treating fibrosis, scarring and/or cancer. Additionally, the invention relates to a method of inhibiting autoimmune diseases such as coeliac disease, neurodegeneration and chronic inflammatory diseases (e.g. of the joints including rheumatoid arthritis and osteoarthritis in a subject).