Structure-activity study of oncocin: On-resin guanidinylation and incorporation of homoarginine, 4-hydroxyproline or 4,4-difluoroproline residues.

Antimicrobial peptides (AMPs) often display guanidinium functionalities, and hence robust synthetic procedures are needed to facilitate access to analogues with unnatural homologues of arginine (Arg = R). Initially, a resin-bound Arg/Pro-rich fluoren-9-yl-methyloxycarbonyl-protected fragment (Fmoc-RPRPPR) of the AMP oncocin (i.e., VDKPPYLPRPRPPRRIYNR-NH2) was employed in a comparative on-resin assessment of commercial guanidinylation reagents head-to-head with the recently studied bis-Boc-protected triazole-based reagent, 1H-triazole-1-[N,N'-bis(tert-butoxycarbonyl)]-carboxamidine, which was synthesized by a chromatography-free procedure. This reagent was found to enable quantitative conversion in solid-phase peptide synthesis (SPPS) of peptides displaying homoarginine (Har) residues and/or an N-terminal guanidinium group. SPPS was used to obtain analogues of the 18-mer oncocin with single as well as multiple Arg → Har modifications. In addition, the effect of replacement of proline (Pro) residues in oncocin was explored by incorporating single or multiple trans-4-hydroxy-l-proline (Hyp) or 4,4-difluoro-l-proline (Dfp) residues, which both affected hydrophobicity. The resulting peptide library was tested against both Gram-negative and Gram-positive bacteria. Analysis of the minimal inhibitory concentrations (MICs) showed that analogues, displaying modifications at positions 4, 5 and 12 (originally Pro residues), had retained or slightly improved antimicrobial activity. Next, an oncocin analogue with two stabilizing l-Arg → d-Arg replacements in the C-terminal part was further modified by triple-replacement of Pro by either Dfp or Hyp in positions 4, 5, and 12. The resulting analogue displaying three Pro → Dfp modifications proved to possess the best activity profile: MICs of 1-2 µg/mL against E. coli and Klebsiella pneumoniae, less than 1% hemolysis at 800 µg/mL, and an IC50 above 1280 µg/mL in HepG2 cells. Thus, incorporation of bis-fluorinated Pro residues appears to constitute a novel tool in structure-activity studies aimed at optimization of Pro-rich AMPs.

Peptide/ß-Peptoid Hybrids with Activity against Vancomycin-Resistant Enterococci: Influence of Hydrophobicity and Structural Features on Antibacterial and Hemolytic Properties.

Infections with enterococci are challenging to treat due to intrinsic resistance to several antibiotics. Especially vancomycin-resistant Enterococcus faecium and Enterococcus faecalis are of considerable concern with a limited number of efficacious therapeutics available. From an initial screening of 20 peptidomimetics, 11 stable peptide/ß-peptoid hybrids were found to have antibacterial activity against eight E. faecium and E. faecalis isolates. Microbiological characterization comprised determination of minimal inhibitory concentrations (MICs), probing of synergy with antibiotics in a checkerboard assay, time-kill studies, as well as assessment of membrane integrity. E. faecium isolates proved more susceptible than E. faecalis isolates, and no differences in susceptibility between the vancomycin-resistant (VRE) and -susceptible E. faecium isolates were observed. A test of three peptidomimetics (Ac-[hArg-ßNsce]6-NH2, Ac-[hArg-ßNsce-Lys-ßNspe]3-NH2 and Oct-[Lys-ßNspe]6-NH2) in combination with conventional antibiotics (vancomycin, gentamicin, ciprofloxacin, linezolid, rifampicin or azithromycin) revealed no synergy. The same three potent analogues were found to have a bactericidal effect with a membrane-disruptive mode of action. Peptidomimetics Ac-[hArg-ßNsce-Lys-ßNspe]3-NH2 and Oct-[Lys-ßNspe]6-NH2 with low MIC values (in the ranges 2-8 µg/mL and 4-16 µg/mL against E. faecium and E. faecalis, respectively) and displaying weak cytotoxic properties (i.e., <10% hemolysis at a ~100-fold higher concentration than their MICs; IC50 values of 73 and 41 µg/mL, respectively, against HepG2 cells) were identified as promising starting points for further optimization studies.

Peptide/ß-Peptoid Hybrids with Ultrashort PEG-Like Moieties: Effects on Hydrophobicity, Antibacterial Activity and Hemolytic Properties.

PEGylation of antimicrobial peptides as a shielding tool that increases stability toward proteolytic degradation typically leads to concomitant loss of activity, whereas incorporation of ultrashort PEG-like amino acids (sPEGs) remains essentially unexplored. Here, modification of a peptide/ß-peptoid hybrid with sPEGs was examined with respect to influence on hydrophobicity, antibacterial activity and effect on viability of mammalian cells for a set of 18 oligomers. Intriguingly, the degree of sPEG modification did not significantly affect hydrophobicity as measured by retention in reverse-phase HPLC. Antibacterial activity against both wild-type and drug-resistant strains of Escherichia coli and Acinetobacter baumannii (both Gram-negative pathogens) was retained or slightly improved (MICs in the range 2-16 µg/mL equal to 0.7-5.2 µM). All compounds in the series exhibited less than 10% hemolysis at 400 µg/mL. While the number of sPEG moieties appeared not to be clearly correlated with hemolytic activity, a trend toward slightly increased hemolytic activity was observed for analogues displaying the longest sPEGs. In contrast, within a subseries the viability of HepG2 liver cells was least affected by analogues displaying the longer sPEGs (with IC50 values of ~1280 µg/mL) as compared to most other analogues and the parent peptidomimetic (IC50 values in the range 330-800 µg/mL).

Sulfocoumarins as dual inhibitors of human carbonic anhydrase isoforms IX/XII and of human thioredoxin reductase.

The hypothesis that sulfocoumarin acting as inhibitors of human carbonic anhydrase (CA, EC 4.2.1.1) cancer-associated isoforms hCA IX and - hCA XII is being able to also inhibit thioredoxin reductase was verified and confirmed. The dual targeting of two cancer cell defence mechanisms, i.e. hypoxia and oxidative stress, may both contribute to the observed antiproliferative profile of these compounds against many cancer cell lines. This unprecedented dual anticancer mechanism may lead to a new approach for designing innovative therapeutic agents.

Design, synthesis, and biological evaluation of novel derivatives of dithiodiglycolic acid prepared via oxidative coupling of thiols.

Human thioredoxin reductase 1 (TrxR1) is a selenocysteine-containing enzyme which plays a crucial role in regulating numerous redox signalling pathways within the cell. While its functioning is important in all cells, levels of TrxR1 expression are higher in cancer cells, possibly as an adaptation to much higher levels of reactive oxygen species and the need for more extensive DNA synthesis. This makes TrxR1 an attractive target for cancer therapy development. Inspired by the structure of disulphide compounds which have advanced through various stages of clinical development, we designed a series of dithiodiglycolic acid derivatives. These were prepared from respective thiol synthons using an iodine- or benzotriazolyl chloride-promoted oxidative disulphide bond formation. Inhibition of TrxR present in cell lysates from human neuroblastoma cells (SH-SY5Y) and rat liver cells indicated several compounds with a potential for TrxR inhibition. Some of these compounds were also tested for growth inhibition against two human cancer cell lines and normal human keratinocytes.

Polymyxins with Potent Antibacterial Activity against Colistin-Resistant Pathogens: Fine-Tuning Hydrophobicity with Unnatural Amino Acids.

In view of the increased prevalence of antimicrobial resistance among human pathogens, antibiotics against multidrug-resistant (MDR) bacteria are in urgent demand. In particular, the rapidly emerging resistance to last-resort antibiotic colistin, used for severe Gram-negative MDR infections, is critical. Here, a series of polymyxins containing unnatural amino acids were explored, and some analogues exhibited excellent antibacterial activity against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. Hydrophobicity of the compounds within this series (as measured by retention in reversed-phase analytical HPLC) exhibited a discernible correlation with their antimicrobial activity. This trend was particularly pronounced for colistin-resistant pathogens. The most active compounds demonstrated competitive activity against a panel of Gram-negative pathogens, while exhibiting low in vitro cytotoxicity. Importantly, most of these hits also retained (or even had increased) potency against colistin-susceptible strains. These findings infer that fine-tuning hydrophobicity may enable the design of polymyxin analogues with favorable activity profiles.

Development of Potent Microtubule Targeting Agent by Structural Simplification of Natural Diazonamide.

The marine metabolite diazonamide A exerts low nanomolar cytotoxicity against a range of tumor cell lines; however, its highly complex molecular architecture undermines the therapeutic potential of the natural product. We demonstrate that truncation of heteroaromatic macrocycle in natural diazonamide A, combined with the replacement of the challenging-to-synthesize tetracyclic hemiaminal subunit by oxindole moiety leads to considerably less complex analogues with improved drug-like properties and nanomolar antiproliferative potency. The structurally simplified macrocycles are accessible in 12 steps from readily available indolin-2-one and tert-leucine with excellent diastereoselectivity (99:1 dr) in the key macrocyclization step. The most potent macrocycle acts as a tubulin assembly inhibitor and exerts similar effects on A2058 cell cycle progression and induction of apoptosis as does marketed microtubule-targeting agent vinorelbine.

Atropo/Tropo Flexibility: A Tool for Design and Synthesis of Self-Adaptable Inhibitors of Carbonic Anhydrases and Their Antiproliferative Effect.

Here, we report for the first time a series of sulfonamide derivatives with scaffolds bearing flexible moieties, namely, rotamers or tropoisomers capable of adapting their geometry in the active center of enzymes thus being effective and selective carbonic anhydrase (CAs, EC 4.2.1.1) enzyme inhibitors. All compounds exhibited effective in vitro inhibition activity toward the main hCA isoforms related to cancer (i.e., hCA II, hCA IX, and hCA XII with KI values in the low nanomolar range). Three selected compounds showed a great cytotoxic effect on cancer cell lines ex vivo. X-ray crystallographic experiments assessed the binding modes of compound 35 with active centers of hCA IX and hCA XII.

Development of oxathiino[6,5-b]pyridine 2,2-dioxide derivatives as selective inhibitors of tumor-related carbonic anhydrases IX and XII.

Oxathiino[6,5-b]pyridine 2,2-dioxides are identified as a new class of isoform-selective nanomolar inhibitors of tumor associated human carbonic anhydrases (hCA) IX and XII. At the same time they do not inhibit or poorly inhibit cytosolic isoforms hCA I and II. Oxathiino[6,5-b]pyridine 2,2-dioxides exhibited good antiproliferative properties on tumor cell lines MCF-7 (Human breast adenocarcinoma), A549 (human lung (alveolar) adenocarcinoma) and HeLa (epithelioid cervix carcinoma).

Alternating Cationic-Hydrophobic Peptide/Peptoid Hybrids: Influence of Hydrophobicity on Antibacterial Activity and Cell Selectivity.

The influence of hydrophobicity on antibacterial activity versus the effect on the viability of mammalian cells for peptide/peptoid hybrids was examined for oligomers based on the cationic Lys-like peptoid residue combined with each of 28 hydrophobic amino acids in an alternating sequence. Their relative hydrophobicity was correlated to activity against both Gram-negative and Gram-positive species, human red blood cells, and HepG2 cells. This identified hydrophobic side chains that confer potent antibacterial activity (e. g., MICs of 2-8 µg/mL against E. coli) and low toxicity toward mammalian cells (<10 % hemolysis at 400 µg/mL and IC50 >800 µg/mL for HepG2 viability). Most peptidomimetics retained activity against drug-resistant strains. These findings corroborate the hypothesis that for related peptidomimetics two hydrophobicity thresholds may be identified: i) it should exceed a certain level in order to confer antibacterial activity, and ii) there is an upper limit, beyond which cell selectivity is lost. It is envisioned that once identified for a given subclass of peptide-like antibacterials such thresholds can guide further optimisation.

Novel electrophilic amides amenable by the Ugi reaction perturb thioredoxin system via thioredoxin reductase 1 (TrxR1) inhibition: Identification of DVD-445 as a new lead compound for anticancer therapy.

A series of peptidomimetic compounds incorporating an electrophilic moiety was synthesized using the Ugi reaction. These compounds (termed the Ugi Michael acceptors or UMAs) were designed to target the selenocysteine catalytic residue of thioredoxin reductase 1 (TrxR1), a promising cancer target. The compounds were assessed for their potential to inhibit TrxR1 using human neuroblastoma (SH-SY5Y) cell lysate. Based on this initial screening, six compounds were selected for testing against recombinant rat TrxR1 and in the insulin assay to reveal low-micromolar to submicromolar potency of these inhibitors. The same frontrunner compounds were evaluated for their ability to exert antiproliferative activity and induce cell death and this activity was compared to the UMA effects on the levels of reactive oxygen and nitrogen species (RONS). Collectively, the UMA compounds class presented itself as a rich source of leads for TrxR1 inhibitor discovery for anticancer application. Compound 7 (DVD-445) was nominated a lead for further optimization.

Fragment-Based Discovery of 2-Aminoquinazolin-4(3H)-ones As Novel Class Nonpeptidomimetic Inhibitors of the Plasmepsins I, II, and IV.

2-Aminoquinazolin-4(3H)-ones were identified as a novel class of malaria digestive vacuole plasmepsin inhibitors by using NMR-based fragment screening against Plm II. Initial fragment hit optimization led to a submicromolar inhibitor, which was cocrystallized with Plm II to produce an X-ray structure of the complex. The structure showed that 2-aminoquinazolin-4(3H)-ones bind to the open flap conformation of the enzyme and provided clues to target the flap pocket. Further improvement in potency was achieved via introduction of hydrophobic substituents occupying the flap pocket. Most of the 2-aminoquinazolin-4(3H)-one based inhibitors show a similar activity against digestive Plms I, II, and IV and >10-fold selectivity versus CatD, although varying the flap pocket substituent led to one Plm IV selective inhibitor. In cell-based assays, the compounds show growth inhibition of Plasmodium falciparum 3D7 with IC50 ∼ 1 µM. Together, these results suggest 2-aminoquinazolin-4(3H)-ones as perspective leads for future development of an antimalarial agent.

Plasmepsin inhibitory activity and structure-guided optimization of a potent hydroxyethylamine-based antimalarial hit.

Antimalarial hit 1 SR (TCMDC-134674) identified in a GlaxoSmithKline cell based screening campaign was evaluated for inhibitory activity against the digestive vacuole plasmepsins (Plm I, II, and IV). It was found to be a potent Plm IV inhibitor with no selectivity over Cathepsin D. A cocrystal structure of 1 SR bound to Plm II was solved, providing structural insight for the design of more potent and selective analogues. Structure-guided optimization led to the identification of structurally simplified analogues 17 and 18 as low nanomolar inhibitors of both, plasmepsin Plm IV activity and P. falciparum growth in erythrocytes.