Some chemical aspects of histamine H2-receptor antagonists

Certain chemical properties, which may determine the biological actions of the recently discovered histamine H2-receptor antagonists burimamide and metiamide, are identified, partly by considering the derivation of these antagonists. Examples are given of attempts to design antagonists using histamine as starting point. A partial agonist was eventually obtained through modifying the side chain of histamine but retaining the imidazole ring. Further developments led to the synthesis of uncharged thioureido analogues and to the discovery of the antagonist, burimamide. Consideration of the relative concentration of imidazole tautomers led to the replacement of a methylene group (-CH2-) with an isosteric thioether (-S-) link in the side chain, and incorporation of a methyl group in the imidazole ring; these changes afforded metiamide, an orally active antagonist. These developments emphasize that the imidazole ring appears to have a special importance at H2 receptors. Burimamide and metiamide are hydrophilic molecules that resemble histamine in having an imidazole ring but differ in the side chain which, though polar, is uncharged. By contrast, the H1-receptor antihistaminic drugs are lipophilic molecules; their resemblance to histamine is in having a positively charged ammonium side chain. These substantial chemical differences between the respective antagonists probably determine their selectivity in distinguishing between the two types of histamine receptor. Furthermore, the very low lipophilicities of these H2-receptor antagonists probably account for the lack of central nervous system and local anesthetic effects normally associated with the use of antihistaminic drugs.