Synthesis of functionalized analogs of pyochelin, a siderophore of Pseudomonas aeruginosa and Burkholderia cepacia

Antimicrobial Potency of Nor-Pyochelin Analogues and Their Cation Complexes against Multidrug-Resistant Pathogens

The opportunistic pathogen Pseudomonas aeruginosa develops increasing resistance toward even the most potent antibiotics. Like other bacteria, the pathogen produces a number of virulence factors including metallophores, which constitute an important group. Pseudomonads produce the iron-chelating metallophore (siderophore) pyochelin, which, in addition to its iron-scavenging ability, is an effector for the transcriptional regulator PchR in its FeIII-bound form (ferripyochelin). In the present study, docking studies predicted a major ferripyochelin binding site in PchR, which prompted the exploration of nor-pyochelin analogues to produce tight binding to PchR, and thereby upregulation of the pyochelin metabolism. In addition, we investigated the effects of using the analogues to bind the antimicrobial cations GaIII and InIII. Selected analogues of nor-pyochelin were synthesized, and their GaIII- and InIII-based complexes were assessed for antimicrobial activity. The results indicate that the GaIII complexes inhibit the pathogens under iron-limited conditions, while the InIII-based systems are more effective in iron-rich media. Several of the GaIII complexes were shown to be highly effective against a multidrug-resistant P. aeruginosa clinical isolate, with minimum inhibitory concentrations (MICs) of ≤1 μg/mL. Similarly, two of the InIII-based systems were particularly effective against the isolate, with an MIC of 8 μg/mL. These results show high promise in comparison with other, traditionally potent antibiotics, as the compounds generally indicated low cytotoxicity toward mammalian cells. Preliminary mechanistic investigations using pseudomonal transposon mutants suggested that the inhibitory effects of the InIII-based systems could be due to acute iron deficiency as a result of InIII-bound bacterioferritin.

Highly Selective Inhibitors of Dipeptidyl Peptidase 9 (DPP9) Derived from the Clinically Used DPP4-Inhibitor Vildagliptin

Dipeptidyl peptidase 9 (DPP9) is a proline-selective serine protease that plays a key role in NLRP1- and CARD8-mediated inflammatory cell death (pyroptosis). No selective inhibitors have hitherto been reported for the enzyme: all published molecules have grossly comparable affinities for DPP8 and 9 because of the highly similar architecture of these enzymes' active sites. Selective DPP9 inhibitors would be highly instrumental to address unanswered research questions on the enzyme's role in pyroptosis, and they could also be investigated as therapeutics for acute myeloid leukemias. Compounds presented in this manuscript (42 and 47) combine low nanomolar DPP9 affinities with unprecedented DPP9-to-DPP8 selectivity indices up to 175 and selectivity indices >1000 toward all other proline-selective proteases. To rationalize experimentally obtained data, a molecular dynamics study was performed. We also provide in vivo pharmacokinetics data for compound 42.

Design, synthesis, molecular docking, anti-quorum sensing, and anti-biofilm activity of pyochelin-zingerone conjugate

In this article, we report the chemical synthesis of pyochelin-zingerone conjugate via a hydrolysable ester linkage for drug delivery as a "Trojan Horse Strategy." It is a new therapeutic approach to combat microbial infection and to address the issue of multi drug resistance in Gram-negative, nosocomial pathogen Pseudomonas aeruginosa. Pyochelin (Pch) is a catecholate type of phenolate siderophore produced and utilized by the pathogen P. aeruginosa to assimilate iron when colonizing the vertebrate host. Zingerone, is active component present in ginger, a dietary herb known for its anti-virulent approach against P. aeruginosa. In the present study, zingerone was exploited to act as a good substitute for existing antibiotics, known to have developed resistance by most pathogens. Encouraging results were obtained by docking analysis of pyochelin-zingerone conjugate with FptA, the outer membrane receptor of pyochelin. Conjugate also showed anti-quorum sensing activity and also inhibited swimming, swarming, and twitching motilities as well as biofilm formation in vitro.

Bifunctional antimicrobial conjugates and hybrid antimicrobials

Covering: up to the end of 2016Novel antimicrobial drugs are continuously needed to counteract bacterial resistance development. An innovative molecular design strategy for novel antibiotic drugs is based on the hybridization of an antibiotic with a second functional entity. Such conjugates can be grouped into two major categories. In the first category (antimicrobial hybrids), both functional elements of the hybrid exert antimicrobial activity. Due to the dual targeting, resistance development can be significantly impaired, the pharmacokinetic properties can be superior compared to combination therapies with the single antibiotics, and the antibacterial potency is often enhanced in a synergistic manner. In the second category (antimicrobial conjugates), one functional moiety controls the accumulation of the other part of the conjugate, e.g. by mediating an active transport into the bacterial cell or blocking the efflux. This approach is mostly applied to translocate compounds across the cell envelope of Gram-negative bacteria through membrane-embedded transporters (e.g. siderophore transporters) that provide nutrition and signalling compounds to the cell. Such 'Trojan Horse' approaches can expand the antibacterial activity of compounds against Gram-negative pathogens, or offer new options for natural products that could not be developed as standalone antibiotics, e.g. due to their toxicity.

Siderophore-dependent iron uptake systems as gates for antibiotic Trojan horse strategies against Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen responsible for nosocomial infections. The prevalence of antibiotic-resistant P. aeruginosa strains is increasing, necessitating the urgent development of new strategies to improve the control of this pathogen. Its bacterial envelope constitutes of an outer and an inner membrane enclosing the periplasm. This structure plays a key role in the resistance of the pathogen, by decreasing the penetration and the biological impact of many antibiotics. However, this barrier may also be seen as the "Achilles heel" of the bacterium as some of its functions provide opportunities for breaching bacterial defenses. Siderophore-dependent iron uptake systems act as gates in the bacterial envelope and could be used in a "Trojan horse" strategy, in which the conjugation of an antibiotic to a siderophore could significantly increase the biological activity of the antibiotic, by enhancing its transport into the bacterium. In this review, we provide an overview of the various siderophore-antibiotic conjugates that have been developed for use against P. aeruginosa and show that an accurate knowledge of the structural and functional features of the proteins involved in this transmembrane transport is required for the design and synthesis of effective siderophore-antibiotic Trojan horse conjugates.

Synthesis and biological properties of conjugates between fluoroquinolones and a N3''-functionalized pyochelin

Pyochelin is a siderophore common to Pseudomonas aeruginosa and several other pathogenic bacteria. A pyochelin functionalized at the N3'' position with a propyl-amine extension was previously synthesized. In the present work we proved that this analog binds FptA, the pyochelin outer membrane receptor, and transports iron(III) efficiently into bacteria. This functionalized pyochelin seemed to be a good candidate for antibiotic vectorization in the framework of a Trojan horse prodrug strategy. In this context, conjugates between pyochelin and three fluoroquinolones (norfloxacin, ciprofloxacin and N-desmethyl-ofloxacin) were synthesized with a spacer arm that was either stable or hydrolyzable in vivo. Some pyochelin-fluoroquinolone conjugates had antibacterial activities in growth inhibition experiments on several P. aeruginosa strains. However, these activities were weaker than those of the antibiotic alone. These properties appeared to be related to both the solubility and bioavailability of conjugates and to the stability of the spacer arm used.

Chemical synthesis and biological evaluation of gallidermin-siderophore conjugates

The lantibiotic gallidermin was modified at lysine residues by regioselective attachment of derivatives of pyochelin, agrobactin and desferrioxamine B with the objective of having siderophore receptors of Gram-negative bacteria transport the antibiotic-iron chelator conjugate through the outer membrane. All of the conjugates retained activity against the Gram-positive indicator strain, Lactococcus lactis subsp. cremoris HP. However, testing of the conjugates against several Gram-negative strains yielded unexpected results. Bacteria treated with 100 μM of the conjugates complexed with Fe(3+) grew better than bacteria grown in iron-free media but worse than bacteria grown in the same media supplemented with 10 μM FeCl(3). Although these findings indicate that the conjugates are unable to inhibit the growth of Gram-negative bacteria, they indicate penetration of the outer membrane and provide structure-activity information for design of other lantibiotic conjugates. The synthetic strategy is applicable for linking biomarkers or fluorescence probes to gallidermin for studies on its localization and mode of action. As there are many lantibiotics that operate with unknown mechanisms of action, this chemical approach provides a means to modify such peptides with biomarkers for biological investigations.

Total (bio)synthesis: strategies of nature and of chemists

The biosynthetic pathways to a number of natural products have been reconstituted in vitro using purified enzymes. Many of these molecules have also been synthesized by organic chemists. Here we compare the strategies used by nature and by chemists to reveal the underlying logic and success of each total synthetic approach for some exemplary molecules with diverse biosynthetic origins.

Synthesis of carbapyochelins via diastereoselective azidation of 5-(ethoxycarbonyl)methylproline derivatives

Two configurationally stable carbon-based analogues of pyochelin have been prepared from Boc-pyroglutamic acid-tert-butyl ester in 11 and 13 steps. Introduction of the amino group was achieved by a highly diastereoselective electrophilic azidation reaction to afford novel bis-alpha-amino acid proline derivatives.

Synthesis of pyochelin-norfloxacin conjugates

Using synthetic functionalized analogues of pyochelin, a siderophore common to several pathogenic Pseudomonas and Burkholderia species, four fluoroquinolone-pyochelin conjugates were efficiently synthesized and evaluated for their biological activities.