Using a previously reported flexible alignment model we have designed, synthesized, and evaluated a series of compounds at the human histamine H4 receptor (H4R) from which 2-(4- methyl-piperazin-1-yl)-quinoxaline (3) was identified as a new lead structure for H4R ligands. Exploration of the structure− activity relationship (SAR) of this scaffold led to the identification of 6, 7-dichloro 3-(4-methylpiperazin-1-yl) quinoxalin-2 (1 H)-one (VUF ...