Synthesis and evaluation of novel ligands for the histamine H4 receptor based on a pyrrolo[2,3-d]pyrimidine scaffold

Starting from a known H4R ligand based on a pyrimidine skeleton, a series of novel analogues based on a pyrrolo[2,3-d]pyrimidine scaffold have been prepared. Whereas the original pyrimidine congener shows good affinity at hH4R (Ki = 0.5 μM), its lacks selectivity with a Ki value for the hH3R of 1 μM. Within the newly synthesized pyrrolo[2,3-d]pyrimidines, several congeners show Ki values of less than 1 μM at the hH4R and show a much improved selectivity profile. Therefore, these series represent an interesting starting point for the discovery of novel hH4R ligands.