RESUMO
The synthesis and potential H1 receptor antagonistic activity of two novel series of condensed 2-arylaminoethylpyrimidin-4(3H)-ones (4, 5) and 4-amino-2-arylaminoethyl pyrimidines (6) have been reported. All the novel compounds were found to antagonize histamine in a competitive and reversible manner. When tested on guinea-pig ileum, compounds exhibited H1-antagonistic activity, (pA2 values) in the range of 8.6 to 9.7. Some of the lead compounds were evaluated by an in vivo method and were found to protect the guinea pigs against the histamine induced asphyxic shock at the doses comparable to or lower than those of the standard drugs, cetirizine (CAS 83881-51-0) and terfenadine (CAS 50679-08-8). The pA2 acetylcholine values of some of the lead compounds reflect about 1000-fold selectivity for histamine (H1) receptors. The 4-aminopyrimidines (6) were found to be more selective than their 4-one analogs (4, 5). In the radioligand binding study, one of the lead compounds, 6e, was found to bind reversibly at the histamine H1 receptor with the K1 value of 1.3 mumol/l and IC50 of 3.8 mumol/l. The lead compounds were found to have negligible sedative potential when tested in vivo. An indirect type of molecular modeling approach, using temelastine (CAS 86181-42-2) as the standard ligand, indicates that the potent activity of 4, 5 and 6 may be due to the increased spacer chain length between the pyrimidine nucleus and the side-chain aromatic ring.
