Kjeld Madsen

Crystal structure of the ligand-bound glucagon-like peptide-1 receptor extracellular domain.

The glucagon-like peptide-1 receptor (GLP-1R) belongs to Family B1 of the seven-transmembrane G protein-coupled receptors, and its natural agonist ligand is the peptide hormone glucagon-like peptide-1 (GLP-1). GLP-1 is involved in glucose homeostasis, and activation of GLP-1R in the plasma membrane of pancreatic β-cells potentiates glucose-dependent insulin secretion. The N-terminal extracellular domain (nGLP-1R) is an important ligand binding domain that binds GLP-1 and the homologous peptide Exendin-4 with differential affinity. Exendin-4 has a C-terminal extension of nine amino acid residues known as the “Trp cage”, which is absent in GLP-1. The Trp cage was believed to interact with nGLP-1R and thereby explain the superior affinity of Exendin-4. However, the molecular details that govern ligand binding and specificity of nGLP-1R remain undefined. Here we report the crystal structure of human nGLP-1R in complex with the antagonist Exendin-4(9–39) solved by the multiwavelength anomalous dispersion method to 2.2Å resolution. The structure reveals that Exendin-4(9–39) is an amphipathic α-helix forming both hydrophobic and hydrophilic interactions with nGLP-1R. The Trp cage of Exendin-4 is not involved in binding to nGLP-1R. The hydrophobic binding site of nGLP-1R is defined by discontinuous segments including primarily a well defined α-helix in the N terminus of nGLP-1R and a loop between two antiparallel β-strands. The structure provides for the first time detailed molecular insight into ligand binding of the human GLP-1 receptor, an established target for treatment of type 2 diabetes.

Different domains of the glucagon and glucagon-like peptide-1 receptors provide the critical determinants of ligand selectivity.

Glucagon and glucagon‐like peptide‐1 (GLP‐1) are homologous peptide hormones with important functions in glucose metabolism. The receptors for glucagon and GLP‐1 are homologous family B G‐protein coupled receptors. The GLP‐1 receptor amino‐terminal extracellular domain is a major determinant of glucagon/GLP‐1 selectivity of the GLP‐1 receptor. However, the divergent residues in glucagon and GLP‐1 that determine specificity for the GLP‐1 receptor amino‐terminal extracellular domain are not known. Less is known about how the glucagon receptor distinguishes between glucagon and GLP‐1. We analysed chimeric glucagon/GLP‐1 peptides for their ability to bind and activate the glucagon receptor, the GLP‐1 receptor and chimeric glucagon/GLP‐1 receptors. The chimeric peptide GLP‐1(7–20)/glucagon(15–29) was unable to bind and activate the glucagon receptor. Substituting the glucagon receptor core domain with the GLP‐1 receptor core domain (chimera A) completely rescued the affinity and potency of GLP‐1(7–20)/glucagon(15–29) without compromising the affinity and potency of glucagon. Substituting transmembrane segment 1 (TM1), TM6, TM7, the third extracellular loop and the intracellular carboxy‐terminus of chimera A with the corresponding glucagon receptor segments re‐established the ability to distinguish GLP‐1(7–20)/glucagon(15–29) from glucagon. Corroborant results were obtained with the opposite chimeric peptide glucagon(1–14)/GLP‐1(21–37). The results suggest that the glucagon and GLP‐1 receptor amino‐terminal extracellular domains determine specificity for the divergent residues in the glucagon and GLP‐1 carboxy‐terminals respectively. The GLP‐1 receptor core domain is not a critical determinant of glucagon/GLP‐1 selectivity. Conversely, the glucagon receptor core domain contains two or more sub‐segments which strongly determine specificity for divergent residues in the glucagon amino‐terminus.

Discovery of the Once-Weekly Glucagon-Like Peptide-1 (GLP-1) Analogue Semaglutide.

Liraglutide is an acylated glucagon-like peptide-1 (GLP-1) analogue that binds to serum albumin in vivo and is approved for once-daily treatment of diabetes as well as obesity. The aim of the present studies was to design a once weekly GLP-1 analogue by increasing albumin affinity and secure full stability against metabolic degradation. The fatty acid moiety and the linking chemistry to GLP-1 were the key features to secure high albumin affinity and GLP-1 receptor (GLP-1R) potency and in obtaining a prolonged exposure and action of the GLP-1 analogue. Semaglutide was selected as the optimal once weekly candidate. Semaglutide has two amino acid substitutions compared to human GLP-1 (Aib(8), Arg(34)) and is derivatized at lysine 26. The GLP-1R affinity of semaglutide (0.38 ± 0.06 nM) was three-fold decreased compared to liraglutide, whereas the albumin affinity was increased. The plasma half-life was 46.1 h in mini-pigs following i.v. administration, and semaglutide has an MRT of 63.6 h after s.c. dosing to mini-pigs. Semaglutide is currently in phase 3 clinical testing.

Superiority of sandwich ELISA over competitive RIA for the estimation of ANP-270, an analogue of human atrial natriuretic factor

ANP-270 is a 26 amino acid analogue of naturally occurring atrial natriuretic factor (ANF) which it was anticipated would be of value for the treatment of congestive heart failure and acute renal failure. Two sensitive assays--a radioimmunoassay (RIA) and a sandwich enzyme linked immunosorbent assay (ELISA)--were developed and validated for use in clinical investigations. The RIA utilized a single C terminal monoclonal antibody whereas two monoclonal antibodies directed against different epitopes were used for the ELISA. The two assays were comparable with respect to sensitivity and precision, but assay results obtained on samples from normal volunteers dosed intravenously with ANP-270 differed widely. Thus, in one volunteer the elimination half-life was estimated to be 123 min using RIA results but 6 min using the ELISA results. By reversed phase liquid chromatographic fractionation of plasma extracts followed by RIA and ELISA, these discrepancies were shown to be due to fragments of ANP-270 cross-reacting in the RIA but not in the ELISA. Consequently, the sandwich ELISA was the method of choice for estimating this compound in plasma.

Aminomethylthiophene-2-carboxylic acids as dipeptide mimetic in new growth hormone secretagogues

Abstract 3-Aminomethylbenzoic acid is a well established dipeptide mimetic. Herein, aminomethylthiophene-2-carboxylic acids 1) have been synthesized as analogues of 3-aminomethylbenzoic acid. Their use as a dipeptide-mimetic at the N -terminal of novel growth hormone secretagogues is described.

Demonstration of the strength of focused combinatorial libraries in SAR optimisation of growth hormone secretagogues

Abstract Aseries of 96 growth hormone secretagogues, derived from ipamorelin are described. The compounds are prepared as a 6 × 4 × 4 member library on solid support using a PAL resin. The compounds are all acylated dipeptides, based on two aromatic amino acids and a free amino N-terminal. All compounds are characterised by HPLC, LC-MS and their ability to release GH in a pituitary cell based assay. The most potent compounds show EC50 values at 1 nM and are full agonists. We demonstrate the strength of focused combinatorial libraries and confirm the pitfall in broad SAR exploration by giving examples where selected fragments obviously show poor receptor interaction except in very defined structural arrangements.

Synthesis and in vitro characterization of new growth hormone secretagogues derived from ipamorelin with dipeptidomimetic N-terminals

The structural requirements for N-terminal features for the minimal structure of growth hormone secretagogues derived from ipamorelin are investigated. It is found, that incorporation of nonpolar peptidomimetic amino acids at the N-terminal can replace the Aib-His moiety and lead to compounds with high in vitro potency with respect to their growth hormone secretagogue properties. New unnatural amino acids with double bonds, ether-linkages, and 1,3-phenylene-moieties in the backbone proved to be valuable dipeptidomimetics. Using them, growth hormone secretagogues with high potencies were obtained.

Solid phase synthesis of ψ[CH(CN)NH] pseudopeptides. Application to the synthesis of analogues of neurotensin [NT(8–13)].

Abstract The introduction of the cyanomethyleneamino [CH(CN)NH] group, a new type of peptide bond surrogate, under solid phase peptide synthesis conditions has been studied using both Boc and Fmoc strategies. The pseudohexapeptides H-Arg-Pro-Tyrψ[CH(CN)NH]Ile-Leu-OH ( 1 ) and H-Arg-Arg-Pro-Tyr-Ileψ[CH(CN)NH]Leu-OH ( 2 ), analogues of the C -terminal hexapeptide of neurotensin [NT(8–13)], were successfully obtained via Fmoc synthesis.

Constrained C-terminal hexapeptide neurotensin analogues containing a 3-oxoindolizidine skeleton

In order to enforce different spatial orientations in the C-terminal hexapeptide of neurotensin (NT8−13) and to gain information about the importance of the 10–11 peptide bond for binding to NT receptors, the Pro10-Tyr11 fragment has been replaced with (2R,8S,8aR)-, (2S,8S,8aR)-, (2S,8S,8aS)-, (2S,8R,8aS)- and (2R,8R,8aS)-8-amino-2-benzyl-3-oxoindolizidine-2-carboxylic acid. Molecular dynamics calculations and energy minimization studies have shown that, contrarily to the Pro-Tyr moiety, none of these indolizidines display a tendency to adopt type I and III β-turns, but those having (8S,8aR) or (8R,8aS) stereochemistry essentially adopt extended conformations and the (8S,8aS) stereoisomer prefers a nonstandard folding. The four diastereomeric NT8−13 analogues incorporating (8S,8aR) or (8R,8aS) indolizidines displayed binding affinities for the brain NT receptor similar to that of [Ala11]-NT8−13 and only five- to ninefold lower than that of the corresponding analogue, [Phe11]NT8−13. Although this slight decrease could be attributed to differences in conformational behavior between these constrained NT8−13 analogues and [Phe11]NT8−13 or NT8−13, it is not clear whether the β-turn around Pro10-AA11 (AA=Phe, Tyr) is conserved upon receptor binding. An excessive restriction in the motions of the aromatic side chain, imposed by the highly steric constraint of the indolizidine moiety, emerges as an alternative explanation. The findings reported here demonstrate the possibility of replacing the Pro10-Tyr11 dipeptide in NT8−13 with a non-peptide residue without affecting considerably the affinity for brain NT receptors.

Design, synthesis and biological activity of cyclic GHRP-6 analogues

Growth Hormone (GH) secretion is normally stimulated by GHRH. In 1984 C.Y. Bowers and F.A. Momany synthesized the hexa-peptide Growth Hormone Releasing Peptide (GHRP-6) and discovered its ability to stimulate GH release in various species [1]. With the aim to identify the bioactive conformation of GHRP-6, a series of constrained monocyclic analogs were prepared. The cyclizations involved the termini and/or the side chains and, in some of the analogs, a reduced peptide bond was also introduced.