A Thiolactone Strategy for Straightforward Synthesis of Disulfide‐Linked Side‐Chain‐to‐Tail Cyclic Peptides Featuring an N‐Terminal Modification Handle

The development of straightforward and versatile peptide cyclisation methods is highly desired to meet the demand for more stable peptide‐based drugs. Herein, a new method for the synthesis of side‐chain‐to‐tail cyclic peptides with the simultaneous introduction of an N‐terminal handle, based on the introduction of an N‐terminal thiolactone building block, is described. A primary amine liberates a homocysteine analogue from the thiolactone building block, which further enables cyclisation of the peptide through disulfide‐bond formation with a C‐terminal cysteamine. Postcyclisation modification can be achieved by using small bifunctional amines. Alternatively, the synthesis of lipopeptides is demonstrated through direct thiolactone opening with long‐chain alkyl amines.     

Synthesis,Bioactivity, Docking and Molecular Dynamics Studies of Furan‐Based Peptides as 20S Proteasome Inhibitors

Proteasome inhibitors are promising compounds for a number of therapies, including cardiovascular and eye diseases, diabetes, and cancers. We previously reported a series of furan‐based peptidic inhibitors with moderate potencies against the proteasome β5 subunit, hypothesizing that the C‐terminal furyl ketone motif could form a covalent bond with the catalytic residue, threonine 1. In this context, we describe further optimizations of the furan‐based peptides, and a series of dipeptidic and tripeptidic inhibitors were designed and synthesized, aiming at improved potency and better solubility. Most of the tripeptidic inhibitors demonstrated improved potency and selectivity as β5 subunit inhibitors in both enzymatic and cellular assays, and good antineoplastic activities in various tumor cell lines were also observed. However, no inhibitory effects were observed for the dipeptidic compounds, which led us to presume that a noncovalent binding mode is adopted. Docking studies and molecular dynamics simulations were carried out to verify this presumption, with results showing that the distance between the furyl ketone motif and Thr1 is slightly too long to form covalent bond.     

Sigma‐2 Receptor Agonists as Possible Antitumor Agents in Resistant Tumors: Hints for Collateral Sensitivity

With the aim of contributing to the development of novel antitumor agents, high‐affinity σ₂ receptor agonists were developed, with 6,7‐dimethoxy‐2‐[4‐[1‐(4‐fluorophenyl)‐1H‐indol‐3‐yl]butyl]‐1,2,3,4‐tetrahydroisoquinoline ( 15 ) and 9‐[4‐(6,7‐dimethoxy‐1,2,3,4‐tetrahydroisoquinolin‐2‐yl)butyl]‐9H‐carbazole ( 25 ) showing exceptional selectivity for the σ₂ subtype. Most of the compounds displayed notable antiproliferative activity in human MCF7 breast adenocarcinoma cells, with similar activity in the corresponding doxorubicin‐resistant MCF7adr cell line. Surprisingly, a few compounds, including 25 , displayed enhanced activity in MCF7adr cells over parent cells, recalling the phenomenon of collateral sensitivity, which is under study for the treatment of drug‐resistant tumors. All of the compounds showed interaction with P‐glycoprotein (P‐gp), and 15 and 25 , with the greatest activity, were able to revert P‐gp‐mediated resistance and reestablish the antitumor effect of doxorubicin in MCF7adr cells. We therefore identified a series of σ₂ receptor agonists endowed with intriguing antitumor properties; these compounds deserve further investigation for the development of alternate strategies against multidrug‐ resistant cancers.     

A Conjugate of an Anti‐Epidermal Growth Factor Receptor (EGFR) VHH and a Cell‐Penetrating Peptide Drives Receptor Internalization and Blocks EGFR Activation

Overexpression of (mutated) receptor tyrosine kinases is a characteristic of many aggressive tumors, and induction of receptor uptake has long been recognized as a therapeutic modality. A conjugate of a synthetically produced cell‐penetrating peptide (CPP), corresponding to amino acids 38–59 of human lactoferrin, and the recombinant llama single‐domain antibody (VHH) 7D12, which binds the human epidermal growth factor receptor (EGFR), was generated by sortase A mediated transpeptidation. The conjugate blocks EGF‐mediated EGFR activation with higher efficacy than that of both modalities alone; a phenomenon that is caused by both effective receptor blockade and internalization. Thus, the VHH–CPP conjugate shows a combination of activities that implement a highly powerful new design principle to block receptor activation by its clearance from the cell surface.     

Effects of the Tumor‐Vasculature‐Disrupting Agent Verubulin and Two Heteroaryl Analogues on Cancer Cells,Endothelial Cells,and Blood Vessels

Two analogues of the discontinued tumor vascular‐disrupting agent verubulin (Azixa®, MPC‐6827, 1 ) featuring benzo‐1,4‐dioxan‐6‐yl (compound 5 a ) and N‐methylindol‐5‐yl (compound 10 ) residues instead of the para‐anisyl group on the 4‐(methylamino)‐2‐methylquinazoline pharmacophore, were prepared and found to exceed the antitumor efficacy of the lead compound. They were antiproliferative with single‐digit nanomolar IC₅₀ values against a panel of nine tumor cell lines, while not affecting nonmalignant fibroblasts. Indole 10 surpassed verubulin in seven tumor cell lines including colon, breast, ovarian, and germ cell cancer cell lines. In line with docking studies indicating that compound 10 may bind the colchicine binding site of tubulin more tightly (E_bind=?9.8 kcal mol^?1) than verubulin (E_bind=?8.3 kcal mol^?1), 10 suppressed the formation of vessel‐like tubes in endothelial cells and destroyed the blood vessels in the chorioallantoic membrane of fertilized chicken eggs at nanomolar concentrations. When applied to nude mice bearing a highly vascularized 1411HP germ cell xenograft tumor, compound 10 displayed pronounced vascular‐disrupting effects that led to hemorrhages and extensive central necrosis in the tumor.     

Synthesis,SAR and Unanticipated Pharmacological Profiles of Analogues of the mGluR5 Ago‐potentiator ADX‐47273

An iterative analogue library synthesis strategy rapidly developed comprehensive SAR for the mGluR5 ago‐potentiator ADX‐47273. This effort identified key substituents in the 3‐position of oxadiazole that engendered either mGluR5 ago‐potentiation or pure mGluR5 positive allosteric modulation. The mGluR5 positive allosteric modulators identified possessed the largest fold shifts (up to 27.9‐fold) of the glutamate CRC reported to date as well as providing improved physiochemical properties.

     

Structures of Micelle‐Bound Selected Insect Neuropeptides and Analogues: Implications for Receptor Selection

Neuropeptides control essential physiological processes in insects such as water balance and muscle activity. Due to their metabolic instability and adverse physiochemical properties, insect neuropeptides are unsuited for a direct application in plant protection. As a first approximation towards the biologically active conformation, the structures of selected neuropeptides from economically important pest insects were determined by NMR spectroscopy and fluorescence measurements in a membrane‐mimicking environment. A receptor binding model is suggested for the helicokinins and discussed in connection with biological activities and membrane‐bound conformations of linear and cyclic analogues.     

Rational Design,Synthesis, and Biological Evaluation of Lactam‐Bridged Gramicidin A Analogues: Discovery of a Low‐Hemolytic Antibacterial Peptide

A linear peptide, gramicidin A (GA), folds into a β^6.3‐helix, functions as an ion channel in the cell membrane, and exerts antibacterial activity. Herein we describe the rational design, synthesis, and biological evaluation of lactam‐bridged GA analogues. The GA analogue with a 27‐membered macrolactam was found to adopt a stable β^6.3‐helical conformation and exhibits higher ion‐exchange activity than GA. Furthermore, this GA analogue retains the potent antibiotic activity of GA, but its hemolytic activity and toxicity toward mammalian cells are significantly lower than those of GA. This study thus dissociates the antibacterial and hemolytic/cytotoxic activities of GA, and charts a rational path forward for the development of new ion‐channel‐based antibiotics.     

Design,Synthesis, and Biological Evaluation of Levoglucosenone‐Derived Ras Activation Inhibitors

A panel of new potential Ras ligands was generated by decorating a tricyclic levoglucosenone‐derived scaffold with aromatic moieties. Some members of the panel show in vitro inhibitory activity toward the nucleotide exchange process on Ras and are toxic to some human cancer cell lines.

     

Design and In vitro Evaluation of Branched Peptide Conjugates: Turning Nonspecific Cytotoxic Drugs into Tumor‐Selective Agents

The use of peptide receptors as targets for tumor‐selective therapies was envisaged years ago with the findings that receptors for different endogenous regulatory peptides are overexpressed in several primary and metastatic human tumors, and can be used as tumor antigens. Branched peptides can retain or even increase, through multivalent binding, the biological activity of a peptide and are very resistant to proteolysis, thus having a markedly higher in vivo activity compared with the corresponding monomeric peptides. Oligo‐branched peptides, containing the human regulatory peptide neurotensin (NT) sequence, have been used as tumor‐specific targeting agents. These peptides are able to selectively and specifically deliver effector units, for cell imaging or killing, to tumor cells that overexpress NT receptors. Results obtained with branched NT conjugated to different functional units for tumor imaging and therapy indicate that branched peptides are promising novel multifunctional targeting molecules. This study is focused on the role of the releasing pattern of drug‐conjugated branched NT peptides. We present results obtained with oligo‐branched neurotensin peptides conjugated to 6‐mercaptopurin (6‐MP), combretastain A‐4 (CA4) and monastrol (MON). Drugs were conjugated to oligo‐branched neurotensin through different linkers, and the mode‐of‐release, together with cytotoxicity, was studied in different human cancer cell lines. The results show that branched peptides are very promising pharmacodelivery options. Among our drug‐armed branched peptides, NT4–CA4 was identified as a candidate for further development and evaluation in preclinical pharmacokinetic and pharmacodynamic studies. This peptide–drug exhibits significant activity against pancreas and prostate human cancer cells. Consequently, this derivative is of considerable interest due to the high mortality rates of pancreas neuroendocrine tumors and the high incidence of prostate cancer.     

Aromatic Core Extension in the Series of N‐Cyclic Bay‐Substituted Perylene G‐Quadruplex Ligands: Increased Telomere Damage,Antitumor Activity,and Strong Selectivity for Neoplastic over Healthy Cells

Based on previous work on both perylene and coronene derivatives as G‐quadruplex binders, a novel chimeric compound was designed: N,N′‐bis[2‐(1‐piperidino)‐ethyl]‐1‐(1‐piperidinyl)‐6‐[2‐(1‐piperidino)‐ethyl]‐benzo[ghi]perylene‐3,4:9,10‐tetracarboxylic diimide (EMICORON), having one piperidinyl group bound to the perylene bay area (positions 1, 12 and 6, 7 of the aromatic core), sufficient to guarantee good selectivity, and an extended aromatic core able to increase the stacking interactions with the terminal tetrad of the G‐quadruplex. The obtained “chimera” molecule, EMICORON, rapidly triggers extensive DNA damage of telomeres, associated with the delocalization of telomeric protein protection of telomeres 1 (POT1), and efficiently limits the growth of both telomerase‐positive and ‐negative tumor cells. Notably, the biological effects of EMICORON are more potent than those of the previously described perylene derivative (PPL3C), and more interestingly, EMICORON appears to be detrimental to transformed and tumor cells, while normal fibroblasts expressing telomerase remain unaffected. These results identify a new promising G‐quadruplex ligand, structurally and biologically similar on one side to coronene and on the other side to a bay‐monosubstituted perylene, that warrants further studies.     

Discovery of Novel,Potent, and Specific Cell‐Death Inducers in the Jurkat Acute Lymphoblastic Leukemia Cell Line

The limited clinical efficacy of many cancer therapeutics has initiated intense research efforts toward the discovery of novel chemical entities in this field. In this study, 31 hit candidates were selected from nearly 800 000 database compounds in a ligand‐based virtual screening campaign. In turn, three of these hits were found to have (sub)micromolar potencies in proliferation assays with the Jurkat acute lymphatic leukemic cell line. In this assay, the three hits were found to exhibit higher potency than clinically tested cell‐death inducers (GDC‐0152, AT‐406, and birinapant). Importantly, antiproliferative activity toward non‐cancer peripheral blood mononuclear cells (PBMCs) was found to be marginal. Further biological characterization demonstrated the cell‐death‐inducing properties of these compounds. Biological testing of hit congeners excluded a nonspecific, toxic effect of the novel structures. Altogether, these findings may have profound relevance for the development of clinical candidates in tumor therapy.     

(−)‐Tarchonanthuslactone: Design of New Analogues,Evaluation of their Antiproliferative Activity on Cancer Cell Lines,and Preliminary Mechanistic Studies

Natural products containing the α,β‐unsaturated δ‐lactone skeleton have been shown to possess a variety of biological activities. The natural product (?)‐tarchonanthuslactone ( 1 ) possessing this privileged scaffold is a popular synthetic target, but its biological activity remains underexplored. Herein, the total syntheses of dihydropyran‐2‐ones modeled on the structure of 1 were undertaken. These compounds were obtained in overall yields of 17–21 % based on the Keck asymmetric allylation reaction and were evaluated in vitro against eight different cultured human tumor cell lines. We further conducted initial investigation into the mechanism of action of selected analogues. Dihydropyran‐2‐one 8 [(S,E)‐(6‐oxo‐3,6‐dihydro‐2H‐pyran‐2‐yl)methyl 3‐(3,4‐dihydroxyphenyl)acrylate], a simplified analogue of (?)‐tarchonanthuslactone ( 1 ) bearing an additional electrophilic site and a catechol system, was the most cytotoxic and selective compound against six of the eight cancer cell lines analyzed, including the pancreatic cancer cell line. Preliminary studies on the mechanism of action of compound 8 on pancreatic cancer demonstrated that apoptotic cell death takes place mediated by an increase in the level of reactive oxygen species. It appears as though compound 8 , possessing two Michael acceptors and a catechol system, may be a promising scaffold for the selective killing of cancer cells, and thus, it deserves further investigation to determine its potential for cancer therapy.     

Disulfide Bonds versus Trp⋅⋅⋅Trp Pairs in Irregular β‐Hairpins: NMR Structure of Vammin Loop 3‐Derived Peptides as a Case Study

Where a noncovalent interaction is better than a covalent bond : The most stabilising cross‐strand pairs were incorporated into an irregular β‐hairpin, loop 3 of vammin. ¹H and ¹³C NMR conformational analyses of these designed peptides indicated that an edge‐to‐face Trp???Trp interaction leads to a β‐hairpin that is more stable than a disulfide bond.