4 2 3 Research Article Received: 25 July 2008 Revised: 23 January 2009 Accepted: 18 February 2009 Published online in Wiley Interscience: 20 April 2009 (www.interscience.com) DOI 10.1002/psc.1134 Aspartic acid scaffold in bradykinin B1 antagonists J´ ozsef Husz ´ ar, a∗ Zolt´ an Tim ´ ar, a Ferenc Bog ´ ar, b Botond Penke, a,b R´ obert Kiss, c Krisztina Katalin Szalai, c ´ Eva Schmidt, c Andrea Papp c and Gy ¨ orgy Keser ˝ u c Several novel bradykinin B1 receptor (B1R) antagonists were synthesized utilizing a new aspartic acid scaff derived from the highly potent dihydroquinoxalinone scaffold published recently by researchers at Merck (H Biophys. Res. Commun. 2005, 331, 159–166). Despite the considerably limited chemical space of B1 antagon compounds still showed significant biological activity. None of the four most potent compounds showed sig on the bradykinin B2 receptor (B2R), consequently they can be considered as valuable starting points for designing potent and selective B1 antagonists. Furthermore, the synthesis of these aspartic acid derivatives is much s the original Merck compounds suggesting efficient parallel synthesis approaches during their optimization. and novel B1 antagonists into the refined B1R homology model including the second extracellular loop (EC2 importance of this loop in ligand binding. Comparative binding mode analysis revealed that our novel comp to the dihydroquinoxalinone template. Our results indicate that the rigid core of the dihydroquinoxalinone c antagonists is not crucial for maintaining B1 activity. Copyright c 2009 European Peptide Society and John Wiley & Sons, Lt Keywords: bradykinin; B1 antagonists; peptidomimetic; binding modes; docking Introduction Despite the large number of available analgesics, chronic pain is stillin the focus of pharmaceutical research and development. Although acute pain killers can be used effectively, chronic pain remains a significant unmet medical need.The most frequently used chronic medications are nonsteroidal anti-inflammatory drugs(NSAIDs)and opioid analgesics. These two classesof drugs possess, however, serious side effects [1] that make them suboptimal for long-term treatment. Kinins are oligopeptide hormones referring to two endogenous peptides:BK (amino acid sequence: RPPGFSPFR) and kallidin (Lys-BK)(amino acid sequence: KRPPGFSPR). BK and Lys-BK are produced through proteolytic cleavage of high-and low- molecular weight kininogen precursor proteins, respectively. This cleavage is realized by plasma and tissue serine proteases, called kallikrein enzymes [2,3]. The most important physiological effects of kininsare vasodilatation, increased vascular permeability, vascular and bronchial smooth muscle contraction, stimulation of sensory neurons, alteration ofion secretion ofepithelial cells,renalhomeostasis, production ofnitric oxide, release of prostaglandins and cytokines [4 –6]. Pathological overproduction of kinins was observed in several disorders, such as hyperalgesia, hypotension, sepsis,pancreatitis, brain edema,and a wide variety of immunological and inflammatory diseases (e.g. asthma, rheumatoid arthritis, and multiple sclerosis) [4]. The physiological effects of kinins are mediated by BK receptors. These are members of the G protein-coupled receptor (GPCR) superfamily. GPCRs are transmembrane (TM) receptors and consist of seven TM helices as well as a shorteighthelix thatruns parallel with the cytoplasmic surface. The highly conserved tertiary structure makes it possible to build relevant homology models of GPCRs. Until recently, these models were primarily based on the crystal structure of bovine rhodopsin (BR) first solved by Palczew et al. [7] in 2000. At the end of 2007, however, the first GPCR cry structures of the human β 2 -adrenergic receptor (hβ 2 AR) were published [8,9]. These novel GPCR crystal structures are believed to be specifically useful for developing novel GPCR homology models,especially in the case of aminergic GPCRs. For other GPCRs, however, the choice of the template (i.e. BR or h β 2 AR) should be founded on the homology between the target protein and the available templates. On the basis of the pharmacological differences, BK receptors can be classified as B1 and B2 receptors (B1R and B2R) [5,6]. Kin (BK and Lys-BK) are selective endogenous agonists of B2R. High levels of B2R are expressed in a wide variety of tissues related to acute cellular responses. B2R is primarily located on nociceptive neurons as well as in the cardiovascular system. Activators of B2R have long been known to induce acute inflammatory pain. In contrast, C-terminally truncated des-Arg carboxypeptidase metabolites of BK (des-Arg9-BK) and Lys-BK (des-Arg10-kallidin) are selective B1 ligands. B1R is only expressed in low levels unde ∗ Correspondence to: J ´ ozsef Husz ´ ar, Department of Medical Chemistry, University of Szeged, D ´ om t ´ er 8., H-6720, Szeged, Hungary. E-mail: huszarjozsef@yahoo.com a Department of Medical Chemistry, University of Szeged, D´ om t´ er 8., H-6720, Szeged, Hungary b Supramolecular and Nanostructured Materials Research Group ofthe Hungarian Academy of Sciences, University of Szeged, D ´ om t ´ er 8.,H-6720, Szeged, Hungary c Gedeon Richter Plc., Gy ¨ omr ¨ oi´ ut 19-21., H-1103, Budapest, Hungary J. Pept. Sci. 2009; 15: 423 –434 Copyrightc 2009 European Peptide Society and John Wiley & Sons, Ltd.