Selective inhibitors of fibroblast activation protein (FAP) with a xanthine scaffold†
Fibroblast activation protein (FAP) is a serine protease that is selectively expressed in many diseases involving activated stroma, including cancer, arthritis and hepatic and pulmonary fibrosis. FAP is closely related to dipeptidyl peptidase IV (DPPIV), of which many inhibitors are known and several are marketed as drugs. One of these is the xanthine derivative linagliptin. In a broad literature screen amongst reported DPPIV inhibitors, linagliptin was the only druglike compound identified that possessed significant FAP potency. Hence, this compound served as a starting point for a SAR study that aimed to identify structural determinants that selectively increase FAP-potency of linagliptin analogues. By investigating the influence of the substitution pattern on N1, N7 and C8 of the xanthine scaffold, we managed to decouple DPPIV and FAP potency and identified the first selective xanthine-based FAP inhibitors with low micromolar potency. Furthermore, these compounds are the only known FAP-inhibitors that do not rely on a warhead functionality to obtain potencies in this range.