An inhibitor of a glutaminyl peptide cyclotransferase, and use thereof for the treatment and/or prevention of a disease or disorder selected from the group consisting of inflammatory diseases selected from a. neurodegenerative diseases, e.g. mild cognitive impairment (MCI), Alzheimer's disease, neurodegeneration in Down Syndrome, Familial British Dementia, Familial Danish Dementia, multiple sclerosis, b. chronic and acute inflammations, e.g. rheumatoid arthritis, atherosclerosis, restenosis, pancreatitis, c. fibrosis, e.g. lung fibrosis, liver fibrosis, renal fibrosis, d. cancer, e.g. cancer/hemangioendothelioma proliferation, gastric carcinomas, e. metabolic diseases, e.g. hypertension, f. and other inflammatory diseases, e.g. neuropathic pain, graft rejection/graft failure/graft vasculopathy, HIV infections/AIDS, gestosis, tuberous sclerosis. Additionally disclosed are a respective diagnostic method, assay and kit.