Structure-activity, theoretical, and X-ray studies on the intramolecular interactions in a series of novel histamine H2 receptor antagonists.

The furan ring of the histamine H2 receptor antagonist 3-amino-4-[[2-[[[5-[(dimethylamino)methyl]-2-furanyl]-methyl] thio]ethyl]amino]-1,2,5-thiadiazole 1-oxide (1a) was replaced by thiophene, pyridine, benzene, and pyrrole. The relative receptor affinities of these analogues were estimated by in vitro and in vivo techniques. A theoretical model for the stacking interaction, observed by single crystal X-ray analysis of 1a, was developed, and the ability to enter into this type of interaction was estimated. The X-ray analysis of the pyridine analogue of 1a revealed no intramolecular stacking interaction. The theoretical studies were evaluated in light of the observed receptor affinities, and the relevance of the solid-state geometry of 1a to the receptor-bound geometry was assessed. It is suggested that the stacked geometry found in the X-ray structure of 1a does not represent a conformation that is relevant to that bound at the histamine H2 receptor.

Conformational requirements for histamine H2-receptor inhibitors: a structure-activity study of phenylene analogues related to cimetidine and tiotidine.

Two series of compounds related to cimetidine and tiotidine were synthesized as part of a study to evaluate the importance of conformational parameters in binding at histamine H2 receptors. The flexible methylthioethyl connecting chain was replaced by a conformationally restricting phenylene unit. These compounds were evaluated for antagonism of the dimaprit-stimulated chronotropic response in the guinea pig atrium and inhibition of histamine stimulated secretion of gastric acid in the dog. In both series, biological activity is markedly dependent on the m-phenylene regioisomers. Histamine H2-receptor activity is retained in both series; however, in the tiotidine series, gastric antisecretory activity is significantly improved. Regardless of the end group, N-cyanoguanidine 1,1-diamino-2-nitroethene or 3,4-diamino-1,2,5-thiadiazole 1-oxide, each 3 3-(2-guanidino-4-thiazolyl)phenyl analogue was ca. 8 and 90 times more potent intravenously than tiotidine and cimetidine, respectively. The electronic influences of the phenylene unit on biological activity were also evaluated. It was concluded that the geometric constraints imposed by the m-phenylene connecting element were more important than electronic factors in binding events at the histamine H2 receptor.

Zebrafish fgf3 and fgf8 encode redundant functions required for otic placode induction.

Members of the fibroblast growth factor (FGF) family of peptide ligands have been implicated in otic placode induction in several vertebrate species. Here, we have functionally analyzed the roles of fgf3 and fgf8 in zebrafish otic development. The role of fgf8 was assessed by analyzing acerebellar (ace) mutants. fgf3 function was disrupted by injecting embryos with antisense morpholino oligomers (MO) specifically designed to block translation of fgf3 transcripts. Disruption of either fgf3 or fgf8 causes moderate reduction in the size of the otic vesicle. Injection of fgf3-MO into ace/ace mutants causes much more severe reduction or complete loss of otic tissue. Moreover, preplacode cells fail to express pax8 and pax2.1, indicating disruption of early stages of otic induction in fgf3-depleted ace/ace mutants. Both fgf3 and fgf8 are normally expressed in the germring by 50% epiboly and are induced in the primordium of rhombomere 4 by 80% epibloy. In addition, fgf3 is expressed during the latter half of gastrulation in the prechordal plate and paraxial cephalic mesendoderm, tissues that either pass beneath or persist near the prospective otic ectoderm. Conditions that alter the pattern of expression of fgf3 and/or fgf8 cause corresponding changes in otic induction. Loss of maternal and zygotic one-eyed pinhead (oep) does not alter expression of fgf3 or fgf8 in the hindbrain, but ablates mesendodermal sources of fgf signaling and delays otic induction by several hours. Conversely, treatment of wild-type embryos with retinoic acid greatly expands the periotic domains of expression of fgf3, fgf8, and pax8 and leads to formation of supernumerary and ectopic otic vesicles. These data support the hypothesis that fgf3 and fgf8 cooperate during the latter half of gastrulation to induce differentiation of otic placodes.

Identification and SAR for a selective, nonpeptidyl thrombin inhibitor.

A novel, nonpeptidyl thrombin inhibitor, L-636,619 (1), was identified via topological similarity searching over the Merck Corporate Sample Database. X-ray crystallographic studies determined the geometry for ligand binding to the enzyme. Chemical modification of the P1 and P3 segments of the ligand resulted in enhanced potency and improvement in the chemical stability of the lead. Analog 9 proved to be the most interesting lead from this structurally novel series.