Fluorescent non-imidazole histamine H 3 receptor ligands with nanomolar affinities Michael Amon, a Xavier Ligneau, b Jean-Charles Schwartz b and Holger Stark a, * a Johann Wolfgang Goethe-Universita ¨ t, ZAFES, Biozentrum, Institut fu ¨ r Pharmazeutische Chemie, 60431 Frankfurt, Germany b Bioprojet Biotech, 4 rue du Chesnay Beauregard, BP 96205, 35762 Saint Gre ´goire Cedex, France Received 2 December 2005; revised 21 December 2005; accepted 21 December 2005 Available online 24 January 2006 Abstract—x-Piperidinoalkanamine derivatives with fluorescent moieties (2-cyanoisoindol-1-yl, 7-nitrobenzofurazan-4-yl) have been synthesized starting from piperidine in three steps. The compounds display moderate to good histamine hH 3 receptor affinities with K i values ranging from 178 to 11 nM. The new compounds may act as tools for identification and understanding of the binding site on the histamine H 3 receptor. Ó 2006 Elsevier Ltd. All rights reserved. Within the four known histamine receptors, the H 3 subtype is predominantly expressed in the brain. 1 The histamine H 3 receptor has been shown to inhibit the histamine synthesis in and release from histaminergic neurons via a negative feedback loop, but also to mod- ulate the release of other neurotransmitters. 2 Actually there is great effort in the development potential of high- ly potent and selective antagonists due to their discussed importance in the treatment of various diseases, e.g., schizophrenia, Alzheimer’s disease, obesity and atten- tion-deficit hyperactivity disorder (ADHD). 1 The development of H 3 receptor antagonists opens a wide range of compounds differentiated into two main classes. Almost all potent antagonists published before 1999 were imidazole derivatives monosubstituted in the 4(5)-position (Fig. 1; A). 3 By exchange of the imidaz- ole moiety to piperidine, pyrrolidine, piperazine and related structures an additional class of non-imidazole antagonists has been described (Fig. 1; B). 1,2,4 Fluorescent compounds for G protein-coupled receptors may be useful research tools for non-radioactive binding assays or for investigations on the structural properties of these receptor–ligand interactions. 6 Such ligands may offer a multiplicity of information such as the mechanism of ligand binding, 7 localization, movement and internalisation of receptors in living cells. 8 They can give hints on the environment of receptor binding sites, because some fluorophores show excitation and emission wavelengths depending on the surrounding, lipophilicity, pH, temperature, solvent, etc. 9,10 A com- mercially available fluorescent histamine derivative (BODIPY Ò FL histamine; Molecular Probes) is able to show lysosomal localization, 11 but to our knowledge receptor binding properties have not been described. Based on the recent results on structure–activity rela- tionships 1–4,12 we have designed and prepared some nov- el non-imidazole histamine H 3 receptor ligands which possess a fluorescent chromophore in the lipophilic part of a general blueprint for antagonist structures. 1 Very recently during the preparation of this project related Bioorganic & Medicinal Chemistry Letters 16 (2006) 1938–1940 0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2005.12.084 Keywords: Histamine; Antagonist; Ligand; Fluorescence; H3; Medic- inal chemistry. * Corresponding author. Fax: +49 69 79829258; e-mail: h.stark@ pharmchem.uni-frankfurt.de Figure 1. Different structurally related lead structures for histamine H 3 receptor antagonists. 3–5