Design, Synthesis, and Characterization of Cyclic Peptidomimetics of

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Design, synthesis and characterization of cyclic peptidomimetics of the inducible nitric oxide synthase (iNOS) binding epitope that disrupt the protein-protein interaction involving SPRY domain-containing suppressor of cytokine signaling box protein (SPSB) and iNOS Jitendra R. Harjani, Beow Keat Yap, Eleanor WW Leung, Andrew J. Lucke, Sandra E. Nicholson, Martin J Scanlon, David K Chalmers, Philip E. Thompson, Raymond S Norton, and Jonathan B. Baell J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.6b00386 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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Journal of Medicinal Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Journal of Medicinal Chemistry

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Design, synthesis and characterization of cyclic

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peptidomimetics of the inducible nitric oxide

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synthase (iNOS) binding epitope that disrupt the

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protein-protein interaction involving SPRY domain-

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containing suppressor of cytokine signaling box

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protein (SPSB) and iNOS

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Jitendra R. Harjani,†§ Beow Keat Yap,†§ Eleanor W. W. Leung,† Andrew Lucke,† Sandra E.

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Nicholson,‡ ¥ Martin J. Scanlon,† David K. Chalmers,† Philip E. Thompson,† Raymond S. Norton,

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†

* Jonathan B. Baell†*

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†

Medicinal Chemistry, Monash Institute of Pharmaceutical Sciences, Monash University,

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Parkville, Victoria 3052, Australia

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‡

The Walter and Eliza Hall Institute of Medical Research, Parkville, Victoria 3052, Australia

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¥

The Department of Medical Biology, University of Melbourne, Parkville, Victoria 3052,

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Australia

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§

These authors contributed equally

ACS Paragon Plus Environment

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ABSTRACT: SPSB2 knock-out mice have been found to exhibit prolonged expression of iNOS

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in macrophage cells and enhanced killing of persistent pathogens, suggesting that inhibitors of

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SPSB2-iNOS interaction have potential as novel anti-infectives. In this study, we describe the

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design, synthesis and characterization of cyclic peptidomimetic inhibitors of the SPSB2-iNOS

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interaction constrained by organic linkers to improve stability and druggability. SPR, ITC and

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binding site of SPSB2 with low nanomolar affinity (KD 29 nM), a 10-fold improvement over the

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linear peptide DINNN (KD 318 nM), and showed strong inhibition of SPSB2-iNOS interaction

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in macrophage cell lysates. This study exemplifies a novel approach to cyclize a Type II β-turn

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linear peptide and provides a foundation for future development of this group of inhibitors as

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new anti-infectives.

F NMR analyses revealed that the most potent cyclic peptidomimetic bound to the iNOS

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INTRODUCTION

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The SPRY-domain containing SOCS box protein 2 (SPSB2) modulates the lifespan of inducible

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nitric oxide synthase (iNOS) in macrophages during infection as it mediates the proteasomal

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degradation of this enzyme.1 It has been proven that the reduction of intracellular SPSB2 results

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in prolonged iNOS expression and enhanced nitric oxide (NO) production, which ultimately

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improves the killing of persistent pathogens. This suggests the potential of SPSB2-iNOS

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inhibitors as a new class of anti-infectives. X-ray crystallographic studies showed that the SPRY

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domain of SPSB2 binds to Asp184, Asn186 and Asn188 within the DINNN sequence of VASA,

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which correspond to Asp23, Asn25 and Asn27 in the N-terminal region of iNOS.1,2

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The binding affinity, KD, of the 5-residue linear peptide sequence (DINNN), as determined by

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SPR, was only 318 nM,3 about 25 times weaker than that of the 13-residue linear peptide

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sequence (KEEKDINNNVKKT) of the N-terminal region of iNOS (KD by isothermal titration

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calorimetry (ITC) ≈ 13 nM)1 suggesting that the flanking residues of DINNN linear peptide

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contributed to the low nanomolar binding between the N-terminal region of iNOS and SPSB2.

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Recently, we showed that the N-acetylated cyclic peptide c[CVDINNNC]-NH2 had a significant

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improvement in binding to SPSB2 (KD by SPR: 4.4 nM ) when compared to its linear

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counterpart.3 The disulphide-bridged cyclic peptide was also found to be stable in human plasma

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and was stable against trypsin, α-chymotrypsin and pepsin, but the presence of a disulphide

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bridge in this peptide rendered it susceptible to reduction in the cytoplasm, with a consequent

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loss of the gain in affinity afforded by cyclization.

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In this study, we describe a set of organic linkers that were designed in silico to optimally link

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the N- and C- termini of the DINNN linear peptide. The organic linkers offer several advantages

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over amino acid linkers, including tunability of size, shape and length, in addition to resistance to


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proteolysis. Careful design of the organic linker imparts structural rigidity to the conformation

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without interfering with the ability of the cyclic peptidomimetic to bind to the protein. Organic

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linkers offer good redox stability when compared to the disulphide-based cyclic peptide.3 We

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describe the details of our design and synthesis of the building blocks that enabled us to

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synthesize the cyclic peptidomimetics using Fmoc chemistry. The interaction of these cyclic

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peptidomimetics with SPSB2 was characterized using SPR, ITC and 19F NMR experiments, and

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their ability to displace full-length iNOS from its complex with SPSB2 in a macrophage cell

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lysate was confirmed. A comparison of the binding properties presents an opportunity to

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understand how the structurally different portions in these cyclic peptidomimetics contribute to

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their binding affinities.

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RESULTS AND DISCUSSION

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Design of cyclic peptidomimetics M1, M2, M3 and M4

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The design of the cyclic peptidomimetics was based on a technique called Interactive de novo

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design,4 which we developed and which furnishes type-III mimetics,5 where the peptide

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backbone is replaced by suitable organic scaffolds. In brief, the method is based on identifying

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the vectors in a binding epitope that we wishes to match and replace with an organic scaffold

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mimetic. While there are computational approaches to such problems, we have great success

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adopting an interactive approach, where the medicinal chemist interactively builds scaffolds that

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are simultaneously both synthetically accessible as well as conformationally appropriate. We

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have applied this approach successfully to disparate, discontinuous binding epitopes where 3

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vectors are matched in an organic scaffold. In the current problem, which relates more to design

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of a molecular paperclip to furnish a cyclopeptidomimetics, we proposed to target just two

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vectors. While relatively distant residues are readily spanned, it is convenient to focus on


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constrictions and in this context observation of the DINNNNN heptapeptide revealed that the

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Asp184 amide NH was relatively close to the Asn188 C=O at only 2.685 Å (Figure 1A).

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Further, the Cα-N bond vector relationship with that of the Asn188 amide group was deemed

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suitable for mimetic design with the intention of keeping the DINNN sequence for binding to

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SPSB2.

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The crystal structure of SPSB2-VASA peptide at 1.8 Å resolution (PDB ID: 3EMW)2 was

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loaded into SYBYL®-X (version 2.0, Tripos) in order to undertake the mimetic design. All

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unnecessary residues were stripped and the Asn188 carboxamide group was flipped in silico to

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give an amide that moved closer to the Asp184 nitrogen atom. A variety of scaffolds were then

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built interactively and joined with Asp184. A preferred type III peptidomimetic is identified as

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M1 (Figure 1B). In building this scaffold and prior to minimization, it was expected that the

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torsion angles 1, 2, 3 and 4 of around 180°, 45°, -45° and -80°, respectively, had acceptably low

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strain energies. Further, the ethereal oxygen atom was deliberately chosen to facilitate synthesis

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and interact favourably with the anilide NH. This minimization led to a barely perceptible

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change in the designed conformation, with a final N-O distance of 2.01 Å.

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In this approach, initial identification of a suitably synthetically versatile and

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conformationally appropriate scaffold is always deceptively difficult, but becomes progressively

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easier once an initial scaffold has been identified. Hence it is relatively easy to envisage that

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replacement of the M1 phenyl ring with an N-methylated cisoid amide bond would also furnish

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an appropriate scaffold. The resulting acylhydrazide peptidomimetic M2 (Figure 1B) uses the

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same ethereal oxygen linkage, as part of the Asp184 incorporation. However, it contrasts with

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M1 in that it contains polar groups in the mimetic region rather than the hydrophobic phenyl

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ring. While it was not known if one might be preferred over the other, it is nevertheless a


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convenient matching pair to have. In both cases, there is the opportunity of derivatizing the

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mimetic scaffold without any conformational interference. For M1, this would be through the

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addition of other groups on the phenyl ring, while for M2 it would be the extension of the N-

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methyl group. In neither scaffold does an appropriate side chain conformation have to be

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designed; rather, this is inherited from Asp184.

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To further explore the effect of linker rigidity on the designed conformation and binding to

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SPSB2, the methylene-oxy group in M1 was substituted with an amide bond as a more rigid

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alternative scaffold, as shown in M3. We hypothesized that the benzyl group in M1 may also be

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easily replaced with a more flexible aliphatic ethylene linker as in M4. Hence M3 relates

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indirectly to the principles of Interactive de novo Design, while M4 diverges completely from

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these principles. Nonetheless, energy minimization of in silico models of both M3 and M4 did

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not result in significant changes to the overall conformation of the mimetics compared to the

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crystal structure of SPSB2-DINNN, although M4 was connected by a linker with one covalent-

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bond shorter than M1, M2 and M3. On the basis of these considerations, peptidomimetics M1,

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M2, M3 and M4 were selected for synthesis (Figure S1).

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Synthesis of building blocks required for the synthesis of M1, M2, M3 and M4

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The principle used in designing the synthesis of the cyclic peptidomimetics is illustrated with

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an example of an ethylene-based peptidomimetic M4. In the cyclic peptidomimetic M4 (Figure

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2A), only amino acid residues designated here as I2, N3, N4 and N5 (or the INNN motif) were

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connected to each other via peptide backbone. Thus, disconnection at an amide C-N bond that

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connects the INNN motif to the rest of the cyclic molecule, i.e. at the C-terminus of N5, will lead

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to a linear structure 1, which may be cyclized to M4 by a peptide coupling strategy involving the

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free terminal –NH2 and –COOH groups. This strategy requires that all other groups (such as –


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CONH2 and –COOH) in 1 that might be potentially participating in the coupling reaction or

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suffer from side reactions during the coupling conditions be suitably protected. A further

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disconnection of the INNN motif from the rest of the linear chain at an amide C-N bond at the N-

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terminus of I2 results in 2, which is an amino dicarboxylic acid that required protection of the

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malate C4 carboxylic acid group for the desired regioselective coupling. Further protection at the

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–NH2 group in 2 as –NHFmoc and at the C4 carboxylate of malate as a tert-butyl ester results in

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a target molecule 3, which is compatible with standard Fmoc peptide chemistry and therefore

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should be a good precursor for the synthesis of appropriately protected 1. The advantage of this

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approach would be that, except for 3, all other precursors and reagents required for this synthesis

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of appropriately protected 1 are commercially available. In addition to this, well optimized

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peptide coupling, deprotection, peptide cleavage and cyclization strategies commonly used in

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solid phase Fmoc peptide chemistry can be used in the synthesis of M4.

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Applying similar principles of retrosynthesis to cyclic peptidomimetics M1, M2 and M3, we

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arrive at building blocks such as 4, 5 and 6 (Figure 2B). Structurally fragmenting 4, 5 and 6

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further leads to simple chemical structures such as N-methylhydrazine, malate, anthranilamide,

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aspartate and asparagine. The simplicity of these fragments also suggests that inexpensive,

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commercially available precursors may be useful for the synthesis of the peptidomimetic

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building blocks 4, 5 and 6, which should be able to provide access to cyclic peptidomimetics

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M1, M2 and M3, respectively (Figure 2B). The synthesis of building blocks 4, 5 and 6 relied on

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the synthesis of the malate diester precursor with C1 as –COOBn and C4 as -COOtBu

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(numbering sequence identified in 2). The synthesis of this diester resulted in racemization at the

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chiral C-atom of the malate. We decided to carry through the racemic malate diester using it in

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the synthesis of 4, 5 and 6 that were racemic at the malate knowing that the final step in the


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purification of peptides generally involves purification of the peptide by separation using

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HPLC. We were expecting that isoforms of M1, M2 and M4 might be separated from their

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attempted synthesis. Detailed descriptions of synthetic procedures used for the synthesis of

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building blocks 3, 4, 5 and 6 are provided in the supporting information file.

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Synthesis and characterization of peptidomimetics M1, M2, M3 and M4 by LC-MS and

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NMR

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Utilizing the synthesized building blocks, linear peptidomimetics M1, M2, M3 and M4 were

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assembled on-resin (solid-phase peptide synthesis) using an automated Peptide Synthesizer 3

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(PS3TM) and Fmoc chemistry (Figure S1).6 Following cyclization in solution, all side-chain

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protected groups were removed with TFA in presence of scavengers. The cyclized

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peptidomimetics were purified by reverse-phase high performance liquid chromatography (RP-

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HPLC) and their identity and purity were determined by LC-MS and NMR experiments.

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The analysis of crude M1 by LC-MS indicated two closely-eluted peaks (i.e. P1 and P2) that

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had the same observed m/z and therefore the same deconvulated mass (Figure S2A). After

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separation by RP-HPLC, P1 and P2 were characterized by 1D and 2D NMR. The chemical shift

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and integration of signals in 1D NMR of both P1 and P2 indicated that they were likely to be

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isomers of M1. Both P1 and P2 exhibited similar groups of spin systems in 2D [1H, 1H]-TOCSY

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experiments7 and the backbone amide resonances of both P1 and P2 were able to be sequentially

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assigned using 2D [1H, 1H]-ROESY8 and [1H, 1H] DQF-COSY9 experiments (Table S1). To

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investigate the identity of these isomers, NOE peaks observed in the 2D ROESY NMR for P1

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and P2 were compared with the distances between atoms in the designed in silico SPSB2-bound

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model derived from the crystal structure of SPSB2-DINNN linear peptide (PDB ID: 3EMW).2 In

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principle, atoms separated through space with a distance of less than 5 Å can be observed as


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NOE peaks in 2D [1H, 1H]-NOESY or ROESY experiments. The inter-residue NOEs between

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assigned peaks in the 2D [1H, 1H]-ROESY spectra of both P1 and P2 in water at pH 4.8 acquired

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at a 300 ms mixing time at 10 °C were compared to each other. A strong NOE cross-peak

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between resonances at 4.22 and 4.50 ppm (Hα and Hlb) and a weaker NOE cross-peak between

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resonances at 4.22 and 4.61 ppm (Hα and Hla) was apparent in the 2D [1H, 1H]-ROESY spectra of

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P1 (Figure S3A). However, similar NOE cross-peaks between Hα and Hl* were not observed in

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the 2D [1H, 1H]-ROESY spectra of P2 (Figure S3B). When the distances between the Hα and Hl*

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in SPSB2-M1, i.e. the bound model of mimetic M1, were measured (Figure S3C), we observed

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that Hα and Hl* were spatially separated by 2.4 - 3.5 Å. This model suggested that one strong and

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one weak inter-residue NOE cross peaks are expected in the 2D [1H, 1H]-ROESY spectra of M1,

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indicating that P1 is the originally designed mimetic represented as M1 (Figure 1). As the

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racemic malate was used for the synthesis of M1, we assumed that perhaps P2 may be the epimer

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of the designed M1, although future work is warranted to confirm its identity.

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A similar observation was made for the crude product obtained when the synthesis of M2 was

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attempted. Two peaks were observed in this case and designated P3 and P4. When the purified

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products corresponding to peaks P3 and P4 of mimetic M2 were analyzed by NMR, we observed

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that the number of proton resonances in the backbone amide region of 1H NMR spectra of both

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P3 and P4 was more than the expected number of amide resonances of the designed mimetic,

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which could be due to conformational isomerism of the peptidomimetic or impurities from either

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a deletion peptide or organic impurities that co-elute with P3 and/or P4 during the purification. In

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order to investigate the possible origin of the additional amide resonances, we conducted a

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variable temperature NMR study on both P3 and P4. An increase in the temperature from 10 to

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30 °C resulted in broadening of 10 out of 12 resonances in the backbone amide region of the 1H


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NMR spectra of P3, with sharpening of 2 of the existing resonances observed after 25 °C

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(Figure S4A, left), suggesting that some amide protons may be involved in conformational

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exchange while others were rapidly exchanged with the solvent at high temperature. 2D [1H,1H]-

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TOCSY experiments on P3, however, revealed three spin systems with an A3MPT(B3)X-like

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pattern, a common pattern of the spin system for an Ile. In addition, 9 backbone amide proton

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peaks were observed, which is more than twice the expected number of amide resonances for M2

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(Figure S4B, left), suggesting that P3 may not be the designed peptidomimetic M2. In contrast,

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we were able to sequentially assign the resonances of P4 via 2D [1H,1H]-ROESY and [1H,1H]-

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DQF-COSY NMR experiments (Table S2), confirming it to be the designed mimetic M2. In

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addition, the variable-temperature NMR experiments on peak P4 indicated that the number of

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broadened peaks in the amide region with the increase in temperature matches the expected

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number of the amide resonances for M2 (Figure S4A, right). Notably, three of the resonances

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(as shown in the 2D [1H, 1H]-TOCSY experiment) belong to spin systems with an AMX pattern

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(i.e. Asn3, Asn4 and Asn5), while another resonance belongs to the spin system with the

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A3MPT(B3)X pattern (i.e. Ile) (Figure S4B, right), consistent with that of the designed mimetic

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M2. The presence of other temperature-resistant proton resonances in the NMR spectra of P4

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(including possible aromatic peaks between 7-8 ppm), suggests that the purified P4 peak also

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contains some co-eluting organic impurities from purification.

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Syntheses of mimetics M3 and M4 yielded single products. This was indicated by a single

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peak in the UV trace with a matching m/z in their LC-MS profiles (Figures S2C and S2D). In

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addition, all backbone amide proton resonances observed for the purified mimetic M3 and M4

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(Figures S5C and S5D) could be sequentially assigned via [1H, 1H]-ROESY and [1H, 1H] DQF-

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COSY with the number and characteristics of the spin systems observed by [1H, 1H]-TOCSY


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matching the predicted spin systems for each mimetic (Tables S3 and S4). These observations

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thus confirmed the identity of the products as the designed mimetic M3 and M4.

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SPR analysis of M1, M2, M3 and M4 binding to SPSB2

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The binding of M1, M2, M3 and M4 to SPSB2 was analyzed by SPR (Figure 3A). The KD

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values for M1, M2, M3 and M4 ranged from 29 to 465 nM (Table 1), with M1 bound more

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tightly to SPSB2 than M2 (p

Design, Synthesis, and Characterization of Cyclic Peptidomimetics of

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