NG-Acylated Imidazolylpropylguanidines as Potent Histamine H4 Receptor Agonists: Selectivity by Variation of the NG-Substituent

3-(1H-Imidazol-4-yl)propylguanidine (SK&F 91486, 4) was identified as a potent partial agonist at the human histamine H-3 receptor (hH(3)R) and human histamine H-4 receptor (hH(4)R). With the aim to increase selectivity for the hH4R, the guanidine group in 4 was acylated. N-1-Acetyl-N-2-[3-(1H-imidazol-4-yl)propyl]guanidine (UR-PI288, 13) was a potent full agonist at the hH(4)R (pEC(50) = 8.31; alpha = 1.00), possessing more than 1000- and 100-fold selectivity relative to the hH(1)R and hH(2)R, respectively, and possessing only low intrinsic activity (alpha = 0.27) at the hH(3)R.