In Vitro Characterization of the Bacteria-derived Hypoxia-selective Cytotoxin BE-43547.

BACKGROUND/AIM: Hypoxia-activated pro-drugs, such as TH-302, may kill hypoxic treatment-resistant tumor cells, but have failed in clinical trials. This may be related to variable levels of drug-activating reductases. Compounds such as bacteria-derived BE-43547, which target hypoxic cells independently of reductases, may be beneficial. This study characterized the in vitro potency and hypoxia selectivity of BE-43547 and TH-302. MATERIALS AND METHODS: Tumor cells were exposed to different oxygenation levels in the presence/absence of drug, and survival was quantified using total cell number (BE-43547) or clonogenic survival (BE-43547 and TH-302) assays. Half-maximal inhibitory concentration (IC50) values and the hypoxia-cytotoxicity-ratio (HCR: normoxic IC50/hypoxic IC50) were determined from dose-response curves. Finally, both drugs were tested in spheroids exposed to 20% or 0% O2 for 24 h followed by assessment of clonogenic survival. RESULTS: BE-43547 was highly potent and displayed little inter-cell line variability. Strongly enhanced cytotoxicity was observed under oxygen-restricted conditions with HCR's of ~100 and ~20 after 24 h of treatment with 0 or 0.5% O2, respectively. Reducing treatment time somewhat reduced hypoxia selectivity. Hypoxia selectivity was observed regardless of whether the drug was added before or during the hypoxic challenge. TH-302 IC50 values varied 10-fold under oxic conditions, whereas those of the anoxic-to-normoxic HCR varied from 15 to 88. Both BE-43547 and TH-302 were unable to completely sterilize anoxic incubated spheroids. CONCLUSION: BE-43547 is highly hypoxia-selective, and unlike TH-302, displayed minimal variability between cell lines, suggesting that BE-43547 targets a fundamental feature/target that is only present, or of survival importance, during hypoxia. Spheroid experiments suggested inadequate tissue penetrability, which may be overcome by designing novel drug analogs.

Forward Chemical Genetic Screen for Oxygen-Dependent Cytotoxins Uncovers New Covalent Fragments that Target GPX4.

The identification of growth inhibitory compounds with the ability to selectively target the cellular oxygenation state may be of therapeutic interest. Here, a phenotypic screen of a covalent fragment library revealed diverse compounds containing propiolamide warheads with selective toxicity for liver cancer cells in normoxic conditions. Target identification and validation through CETSA and direct pulldown experiments demonstrated that several compounds target glutathione peroxidase 4 (GPX4) and induce ferroptotic cell death. Although being an oxidative cell death mechanism, ferroptosis can be induced also under hypoxic conditions. Prompted by the selective toxicity discovered in the screen, we mapped the oxygen-dependence of several ferroptosis-inducing compounds across three different cell lines. These studies revealed combinations with notable reductions in sensitivity under hypoxic conditions. These observations are mechanistically interesting and may be relevant for the use of ferroptosis-inducers as anti-cancer agents.

Expanding the antibacterial selectivity of polyether ionophore antibiotics through diversity-focused semisynthesis.

Polyether ionophores are complex natural products capable of transporting cations across biological membranes. Many polyether ionophores possess potent antimicrobial activity and a few selected compounds have the ability to target aggressive cancer cells. Nevertheless, ionophore function is believed to be associated with idiosyncratic cellular toxicity and, consequently, human clinical development has not been pursued. Here, we demonstrate that structurally novel polyether ionophores can be efficiently constructed by recycling components of highly abundant polyethers to afford analogues with enhanced antibacterial selectivity compared to a panel of natural polyether ionophores. We used classic degradation reactions of the natural polyethers lasalocid and monensin and combined the resulting fragments with building blocks provided by total synthesis, including halogen-functionalized tetronic acids as cation-binding groups. Our results suggest that structural optimization of polyether ionophores is possible and that this area represents a potential opportunity for future methodological innovation.

Structure and Function of the Bacterial Protein Toxin Phenomycin.

Phenomycin is a bacterial mini-protein of 89 amino acids discovered more than 50 years ago with toxicity in the nanomolar regime toward mammalian cells. The protein inhibits the function of the eukaryotic ribosome in cell-free systems and appears to target translation initiation. Several fundamental questions concerning the cellular activity of phenomycin, however, have remained unanswered. In this paper, we have used morphological profiling to show that direct inhibition of translation underlies the toxicity of phenomycin in cells. We have performed studies of the cellular uptake mechanism of phenomycin, showing that endosomal escape is the toxicity-limiting step, and we have solved a solution phase high-resolution structure of the protein using NMR spectroscopy. Through bioinformatic as well as functional comparisons between phenomycin and two homologs, we have identified a peptide segment, which constitutes one of two loops in the structure that is critical for the toxicity of phenomycin.

Chemical Syntheses and Chemical Biology of Carboxyl Polyether Ionophores: Recent Highlights.

A central goal of chemical biology is to develop molecular probes that enable fundamental studies of cellular systems. In the hierarchy of bioactive molecules, the so-called ionophore class occupies an unflattering position in the lower branches, with typical labels being "non-specific" and "toxic". In fact, the mere possibility that a candidate molecule possesses "ionophore activity" typically prompts its removal from further studies; ionophores-from a chemical genetics perspective-are molecular outlaws. In stark contrast to this overall poor reputation of ionophores, synthetic chemistry owes some of its most amazing achievements to studies of ionophore natural products, in particular the carboxyl polyethers renowned for their intricate molecular structures. These compounds have for decades been academic battlegrounds where new synthetic methodology is tested and retrosynthetic tactics perfected. Herein, we review the most exciting recent advances in carboxyl polyether ionophore (CPI) synthesis and in addition discuss the burgeoning field of CPI chemical biology.

Catalytic Asymmetric [4+2]-Cycloadditions Using Tropolones: Developments, Scope, Transformations, and Bioactivity.

An organocatalyzed asymmetric [4+2]-cycloaddition between tropolones and electron-deficient dienophiles is presented. Complex and biologically interesting dihydrohomobarrelenone scaffolds are formed through a Diels-Alder reaction utilizing bifunctional Brønsted-base catalysis, affording the corresponding bridged bicyclic cycloadducts in up to quantitative yields with good enantio- (up to 92 % ee) and diastereoselectivity (up to >20:1 d.r.). The synthetic value of the obtained products is explored by downstream transformations, including photoisomerizations, and their biological relevancy by in vivo testing in MCF-7 cancer cells.

A Catalytic Oxidative Quinone Heterofunctionalization Method: Synthesis of Strongylophorine-26.

The preparation of heteroatom-substituted p-quinones is ideally performed by direct addition of a nucleophile followed by in situ reoxidation. Albeit an appealing strategy, the reactivity of the p-quinone moiety is not easily tamed and no broadly applicable method for heteroatom functionalization exists. Shown herein is that Co(OAc)2 and Mn(OAc)3 ⋅2 H2 O act as powerful catalysts for oxidative p-quinone functionalization with a collection of O, N, and S nucleophiles, using oxygen as the terminal oxidant. Preliminary mechanistic observations and the first synthesis of the cytotoxic natural product strongylophorine-26 is presented.

Synthesis of ent-BE-43547A1 reveals a potent hypoxia-selective anticancer agent and uncovers the biosynthetic origin of the APD-CLD natural products.

Tumour hypoxia is speculated to be a key driver of therapeutic resistance and metastatic dissemination. Consequently, the discovery of new potent agents that selectively target the hypoxic cell population may reveal new and untapped antitumour mechanisms. Here we demonstrate that the BE-43547 subclass of the APD-CLD (amidopentadienoate-containing cyclolipodepsipeptides) natural products possesses highly hypoxia-selective growth-inhibitory activity against pancreatic cancer cells. To enable this discovery, we have developed the first synthesis of the BE-43547-macrocyclic scaffold in 16 steps (longest linear sequence), which also allowed access to the full panel of relative stereoisomers and ultimately to the assignment of stereochemical configuration. Discrepancies between the spectroscopic signatures of the synthetic compounds with that originally reported for the BE-43547 members stimulated us to re-isolate the natural product from a BE-43547-producing microorganism during which we elucidated the biosynthetic gene clusters for the BE-43547 family as well as for all other known APD-CLDs. Our studies underline the exciting possibilities for the further development of the anticancer activities of these natural products.

Binding Sites for Acylated Trehalose Analogs of Glycolipid Ligands on an Extended Carbohydrate Recognition Domain of the Macrophage Receptor Mincle.

The macrophage receptor mincle binds to trehalose dimycolate on the surface of Mycobacterium tuberculosis Signaling initiated by this interaction leads to cytokine production, which underlies the ability of mycobacteria to evade the immune system and also to function as adjuvants. In previous work the mechanism for binding of the sugar headgroup of trehalose dimycolate to mincle has been elucidated, but the basis for enhanced binding to glycolipid ligands, in which hydrophobic substituents are attached to the 6-hydroxyl groups, has been the subject of speculation. In the work reported here, the interaction of trehalose derivatives with bovine mincle has been probed with a series of synthetic mimics of trehalose dimycolate in binding assays, in structural studies by x-ray crystallography, and by site-directed mutagenesis. Binding studies reveal that, rather than reflecting specific structural preference, the apparent affinity of mincle for ligands with hydrophobic substituents correlates with their overall size. Structural and mutagenesis analysis provides evidence for interaction of the hydrophobic substituents with multiple different portions of the surface of mincle and confirms the presence of three Ca2+-binding sites. The structure of an extended portion of the extracellular domain of mincle, beyond the minimal C-type carbohydrate recognition domain, also constrains the way the binding domains may interact on the surface of macrophages.

Total Synthesis and Biological Evaluation of Rakicidin†A and Discovery of a Simplified Bioactive Analogue.

We report a concise asymmetric synthesis of rakicidin A, a macrocyclic depsipeptide that selectively inhibits the growth of hypoxic cancer cells and stem-like leukemia cells. Key transformations include a diastereoselective organocatalytic cross-aldol reaction to build the polyketide portion of the molecule, a highly hindered ester fragment coupling reaction, an efficient Helquist-type Horner-Wadsworth-Emmons (HWE) macrocyclization, and a new DSC-mediated elimination reaction to construct the sensitive APD portion of rakicidin A. We further report the preparation of a simplified structural analogue (WY1) with dramatically enhanced hypoxia-selective activity.

The natural product brartemicin is a high affinity ligand for the carbohydrate-recognition domain of the macrophage receptor mincle.

We demonstrate that the natural product brartemicin, a newly discovered inhibitor of cancer cell invasion, is a high-affinity ligand of the carbohydrate-recognition domain (CRD) of the C-type lectin mincle. Recent studies have revealed that mincle is a key macrophage receptor for the mycobacterial virulence factor trehalose dimycolate (TDM), which is a glycolipid component of the mycobacterial cell wall. Major uncertainties, however, remain concerning the mechanism of TDM-binding and subsequent signal transduction as well as interplay of potential co-receptors. Due to the lipid nature of TDM, functional studies are difficult and soluble mincle-ligands are therefore of significant interest. Brartemicin, together with designed analogs also presented in this paper, may thus serve as useful molecular probes for future studies of mincle. Through computational studies, we further provide an insight into the probable mode of binding of brartemicin.