Corticotropin releasing factor (CRF) agonists with reduced amide bonds and Ser7 substitutions

The B(OH)-NH Analog Is a Surrogate for the Amide Bond (CO-NH) in Peptides: An ab Initio Study

The conformational preferences of N-methyl-methylboronamide (NMB), a B(OH)-NH analog of the amide CO-NH in natural peptides, have been investigated at the Hartree-Fock; Becke's three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Parr; and second-order Møller-Plesset levels of theory with the 6-31+G* basis set. The minima, saddle points, and rotation barriers on the potential energy surface of NMB have been located and the energy barriers estimated. Besides the global minimum, there are three local minima within 2.0 kcal mol(-)(1) of the global minimum characterized by specific ω and τ torsion values. The energy barriers for rotation about the "ω angle" are 16.4-18.8 kcal mol(-)(1) and are a consequence of the double-bond character of the B-N bond as revealed by natural bond orbitals calculations. The "ω angle" and the ω rotation barrier are nearly the same as those seen in natural peptides. The τ rotation barriers (B-O bond) are relatively low because of the single-bond character of the B-O bond. Ala-BON, the Ala-dipeptide derived from NMB, has been constructed as a model peptide to study the conformational preferences about the φ and ψ torsion angles. The study reveals a strong preference for α-helix, type-II β-turn, 2.27 ribbon, and antiparallel β-sheet conformations, and mirror images of both type-II β-turn and 2.27 ribbon motifs whose φ and ψ values fall in the "disfavored regions" of the Ramachandran map. Thus, the replacement of the carbonyl group by B-OH retains the geometry and barrier around the "ω angle" and induces a strong preference for regular secondary structure motifs and also structures with positive φ values. This makes the B(OH)-NH analog an important surrogate for the peptide bond, with the additional advantage of stability to proteolytic enzymes.

NMR structure of the first extracellular domain of corticotropin-releasing factor receptor 1 (ECD1-CRF-R1) complexed with a high affinity agonist

The corticotropin-releasing factor (CRF) peptide hormone family members coordinate endocrine, behavioral, autonomic, and metabolic responses to stress and play important roles within the cardiovascular, gastrointestinal, and central nervous systems, among others. The actions of the peptides are mediated by activation of two G-protein-coupled receptors of the B1 family, CRF receptors 1 and 2 (CRF-R1 and CRF-R2α,β). The recently reported three-dimensional structures of the first extracellular domain (ECD1) of both CRF-R1 and CRF-R2β (Pioszak, A. A., Parker, N. R., Suino-Powell, K., and Xu, H. E. (2008) J. Biol. Chem. 283, 32900-32912; Grace, C. R., Perrin, M. H., Gulyas, J., Digruccio, M. R., Cantle, J. P., Rivier, J. E., Vale, W. W., and Riek, R. (2007) Proc. Natl. Acad. Sci. U.S.A. 104, 4858-4863) complexed with peptide antagonists provided a starting point in understanding the binding between CRF ligands and receptors at a molecular level. We now report the three-dimensional NMR structure of the ECD1 of human CRF-R1 complexed with a high affinity agonist, α-helical cyclic CRF. In the structure of the complex, the C-terminal residues (23-41) of α-helical cyclic CRF bind to the ECD1 of CRF-R1 in a helical conformation mainly along the hydrophobic face of the peptide in a manner similar to that of the antagonists in their corresponding ECD1 complex structures. Unique to this study is the observation that complex formation between an agonist and the ECD1-CRF-R1 promotes the helical conformation of the N terminus of the former, important for receptor activation (Gulyas, J., Rivier, C., Perrin, M., Koerber, S. C., Sutton, S., Corrigan, A., Lahrichi, S. L., Craig, A. G., Vale, W., and Rivier, J. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 10575-10579).

Novel, potent, and radio-iodinatable somatostatin receptor 1 (sst1) selective analogues

The proposed sst(1) pharmacophore (J. Med. Chem. 2005, 48, 523-533) derived from the NMR structures of a family of mono- and dicyclic undecamers was used to design octa-, hepta-, and hexamers with high affinity and selectivity for the somatostatin sst(1) receptor. These compounds were tested for their in vitro binding properties to all five somatostatin (SRIF) receptors using receptor autoradiography; those with high SRIF receptor subtype 1 (sst(1)) affinity and selectivity were shown to be agonists when tested functionally in a luciferase reporter gene assay. Des-AA(1,4-6,10,12,13)-[DTyr(2),DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (25) was radio-iodinated ((125)I-25) and specifically labeled sst(1)-expressing cells and tissues. 3D NMR structures were calculated for des-AA(1,4-6,10,12,13)-[DPhe(2),DTrp(8),IAmp(9)]-SRIF-Thr-NH(2) (16), des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (23), and des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9),Tyr(11)]-SRIF-NH(2) (27) in DMSO. Though the analogues have the sst(1) pharmacophore residues at the previously determined distances from each other, the positioning of the aromatic residues in 16, 23, and 27 is different from that described earlier, suggesting an induced fit mechanism for sst(1) binding of these novel, less constrained sst(1)-selective family members.

Stressin1-A, a potent corticotropin releasing factor receptor 1 (CRF1)-selective peptide agonist

The potencies and selectivity of peptide CRF antagonists is increased through structural constraints, suggesting that the resulting ligands assume distinct conformations when interacting with CRF1 and CRF2 receptors. To develop selective CRF receptor agonists, we have scanned the sequence -Gln-Ala-His-Ser-Asn-Arg- (residues 30-35 of [DPhe12,Nle21,38]Ac-hCRF4-41) with an i-(i+3) bridge consisting of the Glui-Xaa-Xbb-Lysi+3 scaffold, where residues i=30, 31, and 32. When i=31, stressin1-A, a potent CRF1 receptor-selective agonist was generated. In vitro, stressin1-A was equipotent to h/rCRF to release ACTH. Astressin1-A showed a low nanomolar affinity for CRF1 receptor (Ki=1.7 nM) and greater than 100-fold selectivity versus CRF2 receptor (Ki=222 nM). Stressin1-A released slightly less ACTH than oCRF in adult adrenal-intact male rats, with increased duration of action. Stressin1-A, injected intraperitoneally in rats, induced fecal pellet output (a CRF1 receptor-mediated response) and did not influence gastric emptying and blood pressure (CRF2 receptor-mediated responses).

Novel sst(4)-selective somatostatin (SRIF) agonists. 1. Lead identification using a betide scan

Hypothesizing that structural constraints in somatostatin (SRIF) analogues may result in receptor selectivity, and aiming to characterize the bioactive conformation of somatostatin at each of its five receptors, we carried out an N(beta)-methylated aminoglycine (Agl) scan of the octapeptide H-c[Cys(3)-Phe(6)-Phe(7)-dTrp(8)-Lys(9)-Thr(10)-Phe(11)-Cys(14)]-OH (SRIF numbering) (ODT-8) that is potent at all SRIF receptor subtypes (sst's) but sst(1). We found that H-c[Cys-LAgl(N(beta)Me,benzoyl)-Phe-DTrp-Lys-Thr-Phe-Cys]-OH (4), H-c[Cys-Phe-LAgl(N(beta)Me,benzoyl)-Trp-Lys-Thr-Phe-Cys]-OH (6), H-c[Cys-Phe-LAgl(N(beta)Me,benzoyl)-dTrp-Lys-Thr-Phe-Cys]-OH (8), and H-c[DCys-Phe-LAgl(N(beta)Me,benzoyl)-DTrp-Lys-Thr-Phe-Cys]-OH (10) had high affinity (IC(50) = 14.3, 5.4, 5.2, and 3.4 nM, respectively) and selectivity for sst(4) (>50-fold over the other receptors). The l-configuration at positions 7 and 8 (l(7), l(8)) yields greater sst(4) selectivity than the l(7), d(8) configuration (6 versus 8). Peptides with the d(7), l(8) (7) and d(7), d(8) (9) configurations are significantly less potent at all receptors. H-c[Cys-Phe-Phe-DTrp-LAgl(betaAla)-Thr-Phe-Cys]-OH (16), H-c[Cys-Phe-Phe-DTrp-DAgl(betaAla)-Thr-Phe-Cys]-OH (17), and their N(beta)Me derivatives at position 9 (18, 19) were essentially inactive. Potent but less sst(4)-selective were members of the Agl-scan at positions 10, H-c[Cys-Phe-Phe-dTrp-Lys-lAgl(N(beta)Me,HO-Ac)-Phe-Cys]-OH (20, IC(50) = 6.5 nM), and 11, H-c[Cys-Phe-Phe-DTrp-Lys-Thr-LAgl(N(beta)Me,benzoyl)-Cys]-OH (22, IC(50) = 6.9 nM), while the d-configuration at positions 10 (21) and 11 (23) led to reduced affinity. One of our best analogues, 8, is an agonist when tested for its ability to inhibit forskolin-stimulated cAMP accumulation in sst(4)-transfected CCL39 cells (EC(50) = 1.01 nM). All Agl-containing analogues were first synthesized using unresolved Fmoc-Agl(N(beta)Me,Boc)-OH, and the diastereomers were separated using HPLC. Chiral assignment at the Agl-containing residue was subsequently done using enzymatic degradation and by de novo synthesis in the cases of H-c[Cys-Phe-DAgl(N(beta)Me,benzoyl)-DTrp-Lys-Thr-Phe-Cys]-OH (9) and H-c[DCys-Phe-DAgl(N(beta)Me,benzoyl)-DTrp-Lys-Thr-Phe-Cys]-OH (11), starting with the papain-resolved Fmoc-DAgl(Boc). These results suggested that the orientation of side chains at position 6, 7, or 11 with respect to the side chains of residues 8 and 9 may be independently responsible for sst(4) selectivity.

Toward synthesis of alpha-alkyl amino glycines (A3G), new amino acid surrogates

A general method giving access to protected alpha-alkyl amino glycines (A3G) 4 from the previously described precursor alpha-isopropylthioglycine 1 is described. In the presence of N-bromosuccinimide, displacement of the thiol by a large variety of amines afforded the corresponding racemic amino acid mimics. The efficiency of the reaction was strongly dependent on the protective groups of the nucleophile used in the condensation.

Differential responsiveness of CRF receptor subtypes to N-terminal truncation of peptidic ligands

The role of the N-terminal domains of corticotropin-releasing factor (CRF) and CRF-like peptides in receptor subtype selectivity, ligand affinity and biological potency was investigated. Therefore, human CRF(12-41), human URP(12-38) and antisauvagine-30 (aSvg) were N-terminally prolonged by consecutive addition of one or two amino acids. The peptides obtained were tested for their binding affinities to rat CRF1 and murine CRF(2beta) receptor, and their capability to stimulate cAMP-release by HEK cells producing either receptor. It was observed that human CRF N-terminally truncated by eight residues was bound with high affinity to CRF2 receptor (Ki=5.4nM), whereas affinity for CRF1 receptor was decreased (Ki=250 nM). A similar shift of affinity was found with sauvagine (Svg) analogs. Truncation of human URP analogs did not affect their preference for CRF(2beta) receptor, but reduced their affinity. Changes in affinity were positively correlated with changes in potency. These results indicated that CRF1 receptor was more stringent in its structural requirements for ligands to exhibit high affinity binding than CRF(2beta) receptor.

Therapeutic potential of CRF receptor antagonists: a gut-brain perspective

Activation of the corticotropin-releasing factor (CRF) family of neuropeptide receptors in the brain and periphery appears to mediate stress-related changes in a variety of physiological and functional domains. Comparative pharmacology of CRF receptor agonists suggests that CRF, urocortin, sauvagine and urotensin consistently mimic, and conversely peptide CRF receptor antagonists lessen, the functional consequences of stressor exposure. Together with the development of novel non-peptide CRF receptor antagonists, a growing number of CRF receptor selective ligands are available to elucidate the neurobiology and physiological role of CRF systems. The present review considers available preclinical evidence as well as results from one Phase II clinical trial which address the hypothesis that CRF receptor antagonists may represent a new option for pharmacotherapy of stress-related disorders.

Corticotropin releasing hormone: therapeutic implications and medicinal chemistry developments

Corticotropin releasing hormone (CRH, sometimes known as CRF) is an endogenous 41 amino acid peptide that has been implicated in the onset of pregnancy, the 'fight or flight' response, in addition to a large number of physiological disorders. Recently, medicinal chemists have developed a number of potent and selective compounds that show promise in a vast array of therapeutic uses. Herein we review the current status of research.

Corticotropin-releasing factor antagonists, binding-protein and receptors: implications for central nervous system disorders

Corticotrophin-releasing factor (CRF; interchangeable with corticotrophin-releasing hormone, CRH) is a neurohormone family of peptides which implements endocrine, physiological and behavioural responses to stressor exposure. Built-in biological diversity and selectivity of CRF system function is provided by multiple endogenous ligands and receptors which are heterogeneously distributed in both brain and peripheral tissues across species. At present, there are at least five distinct targets for CRF with unique cDNA sequences, pharmacology and localization. These fall into three distinct classes, encoded by three different genes and have been termed the CRF1 and CRF2 receptors and the CRF-binding protein. Significant gains in knowledge about the physiological role of CRF binding sites in brain have emerged recently due to the proliferation of novel, high-affinity, receptor-selective pharmacological tools as well as multiple knock-out and knock-in mutant mouse models. These results support a role for CRF binding sites in co-ordinating stress reactivity, emotionality and energy balance over the life-span of the organism.