The profiles of human and primate [3H]Nalpha-methylhistamine binding differ from that of rodents.

Characterization of the histamine H3 receptor in rodent species has been extensive but limited characterization has been done with primate or human tissue. We have characterized the binding of [3H]Nalpha-methylhistamine to cynomolgus monkey and human brain membranes to determine whether there are any significant differences among species' pharmacology. In monkey, [3H]Nalpha-methylhistamine bound, in a guanine nucleotide-sensitive fashion, to an apparently homogeneous class of sites at equilibrium (K(D) = 1.4 nM, Bmax = 34 fmol/mg protein). The profile of binding was broadly similar to that of rodents, with a couple of significant differences. Most notably, the potency of the histamine H3-receptor-specific antagonist thioperamide (Ki = 240 nM) was substantially less than reported for rodents and under assay conditions that yield a two-site curve fit in rodents only a single class of thioperamide binding sites was detected in monkey. Burimamide, however, yielded a two-site curve fit (KiH = 6.7 nM, KiL = 1100 nM) independent of the presence of sodium in the assay, as it does in rodents. Characterization of the human brain histamine H3 receptor showed that it was similar to the monkey and not rodent receptor. Our findings indicate that differences between primate and rodent histamine H3 receptors of potentially serious importance for the discovery of antagonists active in humans do exist.

1980 Award in Medicinal Chemistry: Medicinal chemistry and dynamic structure-activity analysis in the discovery of drugs acting at histamine H2 receptors.

Applications of physical organic chemistry and dynamic structure--activity analysis are illustrated by studies made during the development of specific drugs acting as antagonists of histamine at H2 receptors. Imidazolylalkylguanidines, isothioureas, and carboxamidines were found to be partial agonists, and attempts were made to remove the agonist component by lengthening the side chain and replacing the strongly basic group in these molecules by nonbasic groups. This approach furnished thioureas and cyanoguanidines as competitive antagonist, e.g., burimamide and cimetidine. Thoughts about molecular interactions led to the discovery of other nonbasic chemical groups for antagonist structures, in particular 1,1-diamino-2-nitroethenes and 2-amino-pyrimidine-4-ones (isocytosines). Such moieties are incorporated, respectively, in the recently described antagonist drugs ranitidine and oxmetidine.

Localization of histamine and histamine H2-receptor antagonists in the gastric mucosa.

Histamine stimulates acid secretion by the parietal cell and this secretion is inhibited by the histamine H2-receptor antagonists. Whole body autoradiography showed that radioactivity from 14C-histamine was localized in the artery walls of the stomach and in the muscularis mucosae, but that the level in the fundic mucosa was the same as the blood. When the H2-receptor antagonists burimamide, metiamide and cimetidine were labelled with 35S, 14C or 3H and dosed to rats, whole body autoradiography showed that the stomach was predominantly labelled in the glandular mucosa from 5 to 120 min after administration. Microautoradiography in the rat and dog after intravenous injection of [3H]metiamide or [3H]cimetidine demonstrated an uptake of tritium in the parietal cell cytoplasm that was 3- to 4-times greater than that found in adjacent peptic cells or areas of muscularis mucosa. The preferential labelling persisted at a low level up to 6h after injection in the rat. The localization of radioactivity from the H2-antagonists in the parietal cell cytoplasm correlates well with their pharmacological activity in preventing acid secretion from this cell.

In vitro anaphylaxis in guinea-pig skin: amplification by burimamide.

The effects of burimamide, an H2-antihistamine, on the anaphylactic reaction in the skin of ovalbumin-sensitized guinea pigs were studied in vitro. Burimamide enhanced the concentration of histamine in the supernatant fraction of antigen-challenged sensitized guinea-pig skin in a dose-related way, but did not alter the concentration of residual histamine in the skin after antigen challenge. The enhanced histamine concentration in the supernatant was not due to increased histamine synthesis by the target cells during the reaction because the increase was not inhibited by a histidine decarboxylase inhibitor, Brocresine. In further experiments it was shown that guinea-pig skin possesses potent histamine degrading enzyme activity which is inhibited by burimamide. We suggest that inhibition of these degrading enzymes leads to the increase in histamine concentration in the presence of burimamide.