The invention provides compounds of formula (I) or a pharmaceutically acceptable salt or ester thereof formula (I) wherein the symbols have the meaning as defined in the description. Said compounds are inhibitors of cathepsin K and/or cathepsin S and are useful for the treatment of diseases and medical conditions in which cathepsin K and/or cathepsin S is implicated, e.g. various disorders including neuropathic pain, inflammation, rheumatoid arthritis, osteoarthritis, osteoporosis, multiple sclerosis and tumours.