Search

A solid-phase strategy for the synthesis of biaryl cyclic peptides containing a side-chain to side-chain His-Tyr linkage was developed. The key step was the macrocyclization of a linear peptidyl resin incorporating a 5-bromohistidine and a 3-boronotyrosine via the formation of the biaryl bond by means of a microwave-assisted Suzuki-Miyaura reaction. This method allowed direct access to biaryl cyclic peptides containing a 3- or 5-amino acid ring and bearing the histidine residue at the N- or the C-terminus, being especially conducive for analogues in which this amino acid is located at the C-terminus. This study also served to establish a strategy for the synthesis of biaryl cyclic peptides derived from the two hemispheres of the natural biaryl bicyclic peptides aciculitins Àngel Oliveras was recipient of predoctoral fellowship from the University of Girona. This work was supported by grants AGL2009-13255-C02-02/AGR, AGL2012-39880-C02-02, AGL2015-69876-C2-2-R (MINECO/FEDER, EU) and MPCUdG2016/038

The feasibility of the solid-phase intramolecular 4(5)-arylation of a histidine residue to obtain biaryl cyclic peptides bearing a His-Phe linkage was established. The synthetic strategy involved the preparation of a linear peptidyl resin incorporating a 5-bromohistidine and a 4-boronophenylalanine, and its cyclization through the formation of the biaryl bond between the imidazole of histidine and the phenyl group of phenylalanine via a microwave-assisted Suzuki-Miyaura cross-coupling. This methodology was applied to the preparation of biaryl cyclic peptides consisting of a 3- or 5-residue ring, incorporating the His residue at the N- or the C-terminus and bearing a Leu-Leu spacer or a -NH2 group at the C-terminus. In the case of the 3-residue ring peptides, the position of the His did not influence the macrocyclization. In contrast, to obtain the 5-member ring biaryl cyclic peptides, the His residue should be located at the N-terminus. It was also observed that the Leu-Leu spacer is crucial for the intramolecular arylation. These results suggest that this approach could be useful for the preparation of a diversity of synthetic and natural biaryl cyclic peptides bearing a His-Phe linkage Iteng Ng Choi was recipient of a predoctoral fellowship from the MICINN of Spain. Àngel Oliveras was recipient of predoctoral fellowship from the University of Girona. This work was supported by grants AGL2009-13255-C02-02/AGR, AGL2012-39880-C02-02, AGL2015-69876-C2-2-R (MINECO/FEDER, EU) and MPCUdG2016/038

We have synthesized a set of bombesin derivatives with the aim of exploring their tumor targeting properties to deliver metal-based chemotherapeutics into cancer cells. Peptide QRLGNQWAVGHLL-NH2 (BN3) was selected based on its high internalization in gastrin-releasing peptide receptor (GRPR)-overexpressing PC-3 cells. Three metallopeptides were prepared by incorporating the terpyridine Pt(II) complex [PtCl(cptpy)]Cl (1) (cptpy = 4′-(4-carboxyphenyl)-2,2′:6,2″-terpyridine) at the N-terminus of BN3 or at the NƐ- or Nα-amino group of an additional Lys residue (1-BN3, Lys-1-BN3 and 1-Lys-BN3, respectively). 1-Lys-BN3 displayed the best cytotoxic activity (IC50: 19.2 ± 1.7 μM) and similar ability to intercalate into DNA than complex 1. Moreover, the polypyridine Ru(II) complex [Ru(bpy)2)(cmbpy)](PF6)2 (2) (bpy = 2,2′-bipyridine; cmbpy = 4-methyl-2,2′-bipyridine-4′-carboxylic acid), with proven activity as photosensitizer, was coupled to BN3 leading to metallopeptide 2-Lys-BN3. Upon photoactivation, 2-Lys-BN3 displayed 2.5-fold higher cytotoxicity against PC-3 cells (IC50: 7.6 ± 1.0 μM) than complex 2. To enhance the accumulation of the drugs into the cell nucleus, the nuclear localization signal (NLS) PKKKRKV was incorporated at the N-terminus of BN3. NLS-BN3 displayed higher cellular internalization along with nuclear biodistribution. Accordingly, metallopeptides 1-NLS-BN3 and 2-NLS-BN3 showed increased cytotoxicity (IC50: 12.0 ± 1.1 μM and 2.3 ± 1.1 μM). Interestingly, the phototoxic index of 2-NLS-BN3 was 8-fold higher than that of complex 2. Next, the selectivity towards cancer cells was explored using 1BR3.G fibroblasts. Higher selectivity indexes were obtained for 1-NLS-BN3 and 2-NLS-BN3 than for the unconjugated complexes. These results prove NLS-BN3 effective for targeted delivery of metallodrugs to GRPR-overexpressing cells and for enhancing the cytotoxic efficacy of metal-based photosensitizers.

The control of plant pathogens is mainly based on copper compounds and antibiotics. However, the use of these compounds has some limitations. They have a high environmental impact and the use of antibiotics is not allowed in several countries. Moreover, resistance has been developed to these pathogens. The identification of new agents able to fight plant pathogenic bacteria and fungi will represent an alternative to currently used antibiotics or pesticides. Antimicrobial peptides are widely recognized as promising candidates, however naturally occurring sequences present drawbacks that limit their development. These include susceptibility to protease degradation and low bioavailability. To overcome these problems, research has focused on the introduction of unnatural amino acids into lead peptide sequences. In particular, we have improved the biological profile of antimicrobial peptides active against plant pathogenic bacteria and fungi by incorporating triazolyl, biaryl and D-amino acids into their sequence. These modifications and their influence on the biological activity are summarized.

Carbohydrates are considered as promising templates for the display of multiple copies of antimicrobial peptides. Herein, we describe the design and synthesis of chimeric structures containing two or four copies of the antimicrobial peptides KKLFKKILKYL-NH2 (BP100) and KKLfKKILKYL-NH2 (BP143) attached to the carbohydrate template cyclodithioerythritol (cDTE) or α-D-galactopyranoside (Galp). The synthesis involved the preparation of the corresponding peptide aldehyde followed by coupling to an aminooxy-functionalized carbohydrate template. After purification, the multivalent display systems were obtained in high purities (90–98%) and in good yields (42–64%). These compounds were tested against plant and human pathogenic bacteria and screened for their cytotoxicity on eukaryotic cells. They showed lower MIC values than the parent peptides against the bacteria analyzed. In particular, the carbopeptides derived from cDTE and Galp, which contained two or four copies of BP100, respectively, were 2- to 8-fold more active than the monomeric peptide against the phytopathogenic bacteria. These results suggest that preassembling antimicrobial peptides to multimeric structures is not always associated with a significant improvement of the activity. In contrast, the carbopeptides synthesized were active against human red blood cells pointing out that peptide preassembly is critical for the hemolytic activity. Notably, peptide preassembly resulted in an enhanced bactericidal effect.