John A. W. Kruijtzer

Peptides and proteins as a continuing exciting source of inspiration for peptidomimetics.

Despite their enormous diversity in biological function and structure, peptides and proteins are endowed with properties that have induced and stimulated the development of peptidomimetics. Clearly, peptides can be considered as the “stem” of a phylogenetic molecular development tree from which branches of oligomeric peptidomimetics such as peptoids, peptidosulfonamides, urea peptidomimetics, as well as β‐peptides have sprouted. It is still a challenge to efficiently synthesize these oligomeric species, and study their structural and biological properties. Combining peptides and peptidomimetics led to the emergence of peptide–peptidomimetic hybrids in which one or more (proteinogenic) amino acid residues have been replaced with these mimetic residues. In scan‐like approaches, the influence of these replacements on biological activity can then be studied, to evaluate to what extent a peptide can be transformed into a peptidomimetic structure while maintaining, or even improving, its biological properties. A central issue, especially with the smaller peptides, is the lack of secondary structure. Important approaches to control secondary structure include the introduction of α,α‐disubstituted amino acids, or (di)peptidomimetic structures such as the Freidinger lactam. Apart from intra‐amino acid constraints, inter‐amino acid constraints for formation of a diversity of cyclic peptides have shaped a thick branch. Apart from the classical disulfide bridges, the repertoire has been extended to include sulfide and triazole bridges as well as the single‐, double‐ and even triple‐bond replacements, accessible by the extremely versatile ring‐closing alkene/alkyne metathesis approaches. The latter approach is now the method of choice for the secondary structure that presents the greatest challenge for structural stabilization: the α‐helix.

N-Terminal Residues of the Chemotaxis Inhibitory Protein of Staphylococcus aureus Are Essential for Blocking Formylated Peptide Receptor but Not C5a Receptor

Staphylococcus aureus excretes a factor that specifically and simultaneously acts on the C5aR and the formylated peptide receptor (FPR). This chemotaxis inhibitory protein of S. aureus (CHIPS) blocks C5a- and fMLP-induced phagocyte activation and chemotaxis. Monoclonal anti-CHIPS Abs inhibit CHIPS activity against one receptor completely without affecting the other receptor, indicating that two distinct sites are responsible for both actions. A CHIPS-derived N-terminal 6 aa peptide is capable of mimicking the anti-FPR properties of CHIPS but has no effect on the C5aR. Synthetic peptides in which the first 6 aa are substituted individually for all other naturally occurring amino acids show that the first and third residue play an important role in blocking the FPR. Using an Escherichia coli expression system, we created mutant CHIPS proteins in which these amino acids are substituted. These mutant proteins have impaired or absent FPR- but still an intact C5aR-blocking activity, indicating that the loss of the FPR-blocking activity is not caused by any structural impairment. This identifies the first and third amino acid, both a phenylalanine, to be essential for CHIPS blocking the fMLP-induced activation of phagocytes. The unique properties of CHIPS to specifically inhibit the FPR with high affinity (kd = 35.4 ± 7.7 nM) could be an important new tool to further stimulate the fundamental research on the mechanisms underlying the FPR and its role in disease processes.

Inactivation of staphylococcal phenol soluble modulins by serum lipoprotein particles.

Staphylococcus aureus virulence has been associated with the production of phenol soluble modulins (PSM). PSM are known to activate, attract and lyse neutrophils. However, the functional characterizations were generally performed in the absence of human serum. Here, we demonstrate that human serum can inhibit all the previously-described activities of PSM. We observed that serum can fully block both the cell lysis and FPR2 activation of neutrophils. We show a direct interaction between PSM and serum lipoproteins in human serum and whole blood. Subsequent analysis using purified high, low, and very low density lipoproteins (HDL, LDL, and VLDL) revealed that they indeed neutralize PSM. The lipoprotein HDL showed highest binding and antagonizing capacity for PSM. Furthermore, we show potential intracellular production of PSM by S. aureus upon phagocytosis by neutrophils, which opens a new area for exploration of the intracellular lytic capacity of PSM. Collectively, our data show that in a serum environment the function of PSM as important extracellular toxins should be reconsidered.

Solid-phase syntheses of peptoids using Fmoc-ProtectedN-substituted glycines: the synthesis of (retro)peptoids of leu-enkephalin and substance P

Solid-phase synthesis of oligomeric peptoids can be conveniently achieved by a repetitive cycle consisting of 1) the removal of the Fmoc group and 2) coupling of a N-substituted glycine derivative (a peptoid monomer). This “monomer” method allows the monitored synthesis of relatively large quantities of pure peptoids as well as the translation of in principle any peptide into the corresponding peptoid, illustrated here (below) by the solid-phase synthesis on a peptide synthesizer of the peptoid of substance P.

Rolling Loop Scan: An Approach Featuring Ring-Closing Metathesis for Generating Libraries of Peptides with Molecular Shapes Mimicking Bioactive Conformations or Local Folding of Peptides and Proteins.

Libraries of loop-containing peptides (such as the one shown schematically) can be prepared from bis-N-alkylated peptides by ring-closing metathesis. In a general solid-phase procedure the peptides are accessible by site-specific N-alkylation. Since the amino acid side chains are not involved in cyclization, they remain available for interaction with, for example, a receptor.

Collagen degradation and neutrophilic infiltration: a vicious circle in inflammatory bowel disease

Objective Proline–glycine–proline (PGP) has been shown to have chemotactic effects on neutrophils via CXCR2 in several lung diseases. PGP is derived from collagen by the combined action of matrix metalloproteinase (MMP) 8 and/or MMP9 and prolyl endopeptidase (PE). We investigated the role of PGP in inflammatory bowel disease (IBD). Design In intestinal tissue from patients with IBD and mice with dextran sodium sulfate (DSS)-induced colitis, MMP8, MMP9 and PE were evaluated by ELISA, immunoblot and immunohistochemistry. Peripheral blood polymorphonuclear cell (PMN) supernatants were also analysed accordingly and incubated with collagen to assess PGP generation ex vivo. PGP levels were measured by mass spectrometry, and PGP neutralisation was achieved with a PGP antagonist and PGP antibodies. Results In the intestine of patients with IBD, MMP8 and MMP9 levels were elevated, while PE was expressed at similar levels to control tissue. PGP levels were increased in intestinal tissue of patients with IBD. Similar results were obtained in intestine from DSS-treated mice. PMN supernatants from patients with IBD were far more capable of generating PGP from collagen ex vivo than healthy controls. Furthermore, PGP neutralisation during DSS-induced colitis led to a significant reduction in neutrophil infiltration in the intestine. Conclusions The proteolytic cascade that generates PGP from collagen, as well as the tripeptide itself, is present in the intestine of patients with IBD and mice with DSS-induced colitis. PGP neutralisation in DSS-treated mice showed the importance of PGP-guided neutrophilic infiltration in the intestine and indicates a vicious circle in neutrophilic inflammation in IBD.

αvβ3 integrin-targeting of intraperitoneally growing tumors with a radiolabeled RGD peptide

Ovarian cancer is the fourth most common cause of cancer deaths among females in the western world after cancer of the breast, colon and lung. The inability to control the disease within the peritoneal cavity is the major cause of treatment failure in patients with ovarian cancer. The majority of ovarian carcinomas express the αvβ3 integrin. Here we studied the tumor targeting potential of an 111In‐labeled cyclic RGD peptide in athymic BALB/c mice with intraperitoneally (i.p.) growing NIH:OVCAR‐3 human ovarian carcinoma tumors. The cyclic RGD peptide, c(RGDfK)E, was synthesized, conjugated with DOTA and radiolabeled with 111In. The targeting potential of 111In‐DOTA‐E‐c(RGDfK) was studied in athymic mice with i.p. growing NIH:OVCAR‐3 xenografts and the optimal dose of this compound was determined (0.01 μg up to 10 μg). The biodistribution at optimal peptide dose was determined at various time points (0.5 up to 72 hr). Furthermore, the therapeutic potential of 177Lu‐DOTA‐E‐c(RGDfK) was studied in this model. Two hours after i.p. administration, 111In‐DOTA‐E‐c(RGDfK) showed high and specific uptake in the i.p. growing tumors. Optimal uptake in the i.p. growing tumors was observed at a 0.03–0.1 μg dose range. Tumor uptake of 111In‐DOTA‐E‐c(RGDfK) peaked 4 hr p.i. [(38.8 ± 2.7)% ID/g], gradually decreasing at later time points [(24.0 ± 4.1)% ID/g at 48 hr p.i.]. Intraperitoneal growth of OVCAR‐3 could be significantly delayed by injecting 37 MBq 177Lu‐labeled peptide i.p. Radiolabeled DOTA‐E‐c(RGDfK) is suitable for targeting of i.p. growing tumors and potentially can be used for peptide receptor radionuclide therapy of these tumors.

Combinatorial chemistry of hydantoins

Access to combinatorial chemistry of hydantoins is provided by convenient and versatile methods for the solid phase synthesis of libraries of 3,5-, 1,3- and 1,3,5-substituted hydantoins. The preparation of trisubstituted hydantoins features a Mitsunobu reaction for introduction of the substituent on N-1.

Peptoid–Peptide Hybrids That Bind Syk SH2 Domains Involved in Signal Transduction

Peptoid–peptide hybrids are oligomeric peptidomimetics that contain one or more N‐substituted glycine residues. In these hybrids, the side chains of one or several amino acids are “shifted” from the α‐carbon atom to the amide nitrogen atom. A library of phosphorylated peptoid–peptide hybrids derived from the sequence pTyr‐Glu‐Thr‐Leu was synthesized and tested for binding to the tandem SH2 domain of the protein tyrosine kinase Syk. A considerable influence of the side chain position was observed. Compounds 19–21, 24, and 25 comprising a peptoid NpTyr and/or NGlu residue did not show any binding. Compounds 22, 23, and 26 containing an NhThr (hThr=homothreonine) and/or NLeu peptoid residue showed binding with IC50 values that were only five to eight times higher than that of the tetrapeptide lead compound 18. These data show that side chain shifting is possible with retention of binding capacity, but only at the two C‐terminal residues of the tetramer. This method of a peptoid scan using peptoid–peptide hybrids appears to be very useful to explore to what extent a peptide sequence can be transformed into a peptoid while retaining its affinity.

Glucocorticoid‐Loaded Core‐Cross‐Linked Polymeric Micelles with Tailorable Release Kinetics for Targeted Therapy of Rheumatoid Arthritis

Polymerizable and hydrolytically cleavable dexamethasone (DEX, red dot in picture) derivatives were covalently entrapped in core-cross-linked polymeric micelles that were prepared from a thermosensitive block copolymer (yellow and gray building block). By varying the oxidation degree of the thioether in the drug linker, the release rate of DEX could be controlled. The DEX-loaded micelles were used for efficient treatment of inflammatory arthritis in two animal models.