THE HISTAMINE H-1-RECEPTOR ANTAGONIST BINDING-SITE - A STEREOSELECTIVE PHARMACOPHORIC MODEL-BASED UPON (SEMI-)RIGID H-1-ANTAGONISTS AND INCLUDING A KNOWN INTERACTION SITE ON THE RECEPTOR

A new pharmacophoric model for the H-1-antagonist binding site is derived which reveals that a simple atom to atom matching of compounds is not sufficient; in this model, interacting residues from the receptor need to be included. To obtain this model, the bioactive conformations of several (semi-)rigid classical histamine H-1-receptor antagonists have been investigated (cyproheptadine, phenindamine, triprolidine, epinastine, mequitazine, IBF28145, and mianserine). In general, these antihistamines contain two aromatic rings and a basic nitrogen atom. A previously derived pharmacophoric model with the nitrogen position fixed relative to the two aromatic rings is now found not to be suitable for describing the H-1-antagonist binding site. A procedure is described which allows for significant freedom in the position of the basic nitrogen of the histamine H-1-antagonist. The area accessible to the basic nitrogen is confined to the region accessible to its counterion on the histamine H-1-receptor, i.e., the carboxylate group of Asp(116). The basic nitrogen is assumed to form an ionic hydrogen bond with this aspartic acid which C-alpha- and C-beta-carbons are fixed with respect to the protein backbone. Via this hydrogen bond, the direction of the acidic proton of the antagonist is taken into account. Within these computational procedures, an aspartic acid is coupled to the basic nitrogen of each H-1-antagonist considered; the carboxylate group is connected to the positively charged nitrogen via geometric H-bonding restraints obtained from a thorough database search (CSD). Also to the basic nitrogen of the pharmacophore is coupled an aspartic acid (to yield our new template). In order to derive a model for the H-1-antagonist binding site, the aromatic ring systems of the antagonists and template are matched according to a previously described procedure. Subsequently, the C-alpha- and C(beta-)carbons of the aspartic acid coupled to the Hi-antagonists are matched with those of the template in a procedure which allows the antagonist and the carboxylate group to adapt their conformation (and also their relative position) in order to optimize the overlap with the template. A six-point pharmacophoric model is derived which has stereoselective features and is furthermore able to distinguish between the so-called ''cis''- and ''trans''-rings mentioned in many (Q)SAR studies on H-1-antagonists. Due to its stereoselectivity, the model is able to designate the absolute bioactive configuration of antihistamines such as phenindamine (S), epinastine (S), and IBF28145 (R). A further merit of this study is that a model is obtained which includes an amino acid from the receptor. Since this amino acid has been identified to be Asp(116), tools are now available to dock the antagonists with the aspartic acid coupled to the nitrogen in a homology model of the receptor, while matching the coupled aspartate with Asp(116) of the protein, The most likely (energetically favorable) binding site for the antagonists can then be determined by allowing for rotation around the C-alpha-C-beta bond, while leaving the position of C-alpha and C-beta unchanged. Since several homology models can be built for the protein depending on the alignment chosen, the likelihood of the binding site and especially the (dis)agreement with (Q)SAR and biological data will give clues for the most probable 3D-model (i.e., alignment) of the protein (studies in progress). The underlying approach which includes known interacting amino acids from the receptor into a pharmacophoric model is of general importance for verifying protein models with limited reliability, such as models derived for C-protein-coupled receptors from bacteriorhodopsin.