N(G)-acylated imidazolylpropylguanidines as potent histamine H4 receptor agonists: selectivity by variation of the N(G)-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 hH(4)R, 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.