2-(3-Indolyl)acetamides and their oxazoline analogues: Anticancer SAR study

We previously studied 2-aryl-2-(3-indolyl)acetohydroxamates as potential agents against melanoma. These compounds were ineffective in a mouse melanoma xenograft model, most likely due to unfavorable metabolic properties, specifically due to glucuronidation of the N-hydroxyl of the hydoxamic moiety. In the present work, we prepared a series of analogues, 2-aryl-2-(3-indolyl)acetamides and their oxazoline derivatives, which do not contain the N-hydroxyl group. We investigated the structure-activity relationship in both series of compounds and found that the 2-naphthyl is a preferred group at C-2 of the indole in the amide series, whereas the tetralin moiety is favorable in the same location in the oxazoline series. Overall, three compounds in the amide series have GI50 values as low as 0.2-0.3 µM and the results clearly indicate that the N-hydroxyl group is not necessary for high potency in vitro.

Red-Light Activation of a Microtubule Polymerization Inhibitor via Amide Functionalization of the Ruthenium Photocage

Photoactivated chemotherapy (PACT) is a promising cancer treatment modality that kills cancer cells via photochemical uncaging of a cytotoxic drug. Most ruthenium-based photocages used for PACT are activated with blue or green light, which penetrates sub-optimally into tumor tissues. Here, we report amide functionalization as a tool to fine-tune the toxicity and excited states of a terpyridine-based ruthenium photocage. Due to conjugation of the amide group with the terpyridine π system in the excited state, the absorption of red light (630 nm) increased 8-fold, and the photosubstitution rate rose 5-fold. In vitro, red light activation triggered inhibition of tubulin polymerization, which led to apoptotic cell death both in normoxic (21 % O2 ) and hypoxic (1 % O2 ) cancer cells. In vivo, red light irradiation of tumor-bearing mice demonstrated significant tumor volume reduction (45 %) with improved biosafety, thereby demonstrating the clinical potential of this compound.

Discovery of 5-sulfonyltetrazoles as neuroblastoma differentiation agents

Previously, we developed an innovative high-content screening (HCS) approach to quantify neuroblastoma cell differentiation based on neurite outgrowth, a morphological differentiation marker of neuroblastoma cells. Here, we report the utilization of this platform to identify 1-methyl-5-(ethylsulfonyl)-1H-tetrazole (3a) as a new neuroblastoma differentiation agent using the ChemBridge DiversetTM commercial synthetic small molecule compound library. We show that this activity can be extended to a group of analogues, which can be accessed via a short two-step synthetic sequence. A new analogue, 5-(allylsulfonyl)-1-methyl-1H-tetrazole (3c) was identified in this synthetic effort as a compound that has even more pronounced differentiation and cytotoxic activities than the original hit compound 3a.

Synthesis of β-Carbolines with Electrocyclic Cyclization of 3-Nitrovinylindoles

The β-carboline motif is common in drug discovery and among numerous biologically active natural products. However, its synthetic preparation relies on multistep sequences and heavily depends on the type of substitution required in the core of the desired β-carboline target. Herein, we demonstrate that this structural motif can be accessed with the microwave-assisted electrocyclic cyclization of heterotrienic aci (alkylideneazinic acid) forms of 3-nitrovinylindoles. The reaction can start with 3-nitrovinylindoles themselves under two sets of conditions. The first one involves microwave irradiation of butanolic solutions of 3-nitrovinylindoles, whereas the second one consists of prior Boc protection of indolic nitrogen, where the protecting group cleanly comes off during the course of the reaction. Alternatively, the reaction can start with 3-nitrovinylindoles prepared in situ using various processes. Finally, the reaction may utilize indoles with β-nitrostyrenes, likely involving the intermediacy of spirocyclic oxazolines, which rearrange to similar heterotrienic systems undergoing cyclization to β-carbolines. As part of this study, several natural products, namely, alkaloids norharmane, harmane, and eudistomin N, were synthesized.

Reductive Cleavage of 4'H-Spiro[indole-3,5'-isoxazoles] En Route to 2-(1H-Indol-3-yl)acetamides with Anticancer Activities

Unusual cascade transformation involving ring opening and 1,2-alkyl shift was observed upon the reduction of 4'H-spiro[indole-3,5'-isoxazoles] or 2-(3-oxoindolin-2-yl)acetonitriles with sodium borohydride. This reaction allowed for expeditious and highly efficient preparation of 2-(1H-Indol-3-yl)acetamides with antiproliferative properties.

Medicinal chemical optimization of fluorescent pyrrole-based COX-2 probes

Recent studies have demonstrated the ability of human prostaglandin-endoperoxide synthase 2 (COX-2) to guide the formation of fluorescent pyrroles through the Paal-Knorr reaction resulting in the discovery of a central motif. This initial discovery prompted further exploration of this motif for the design of COX-2 inhibitors through the modifications of the substituents on the pyrrole core. This effort led to the discovery of a set of pyrroles whose activity was comparable to Celecoxib, an orally prescribed nonsteroidal anti-inflammatory COX-2 inhibitor. Furthermore, structure-activity relationship (SAR) data, important for the discovery of COX-2 inhibitors, has been obtained.

In Vitro Effects of Fungal Phytotoxins on Cancer Cell Viability: First Insight into Structure Activity Relationship of a Potent Metabolite of Cochliobolus australiensis Radicinin

Natural compounds have always represented an important source for new drugs. Although fungi represent one such viable source, to date, no fungal metabolite has been marketed as an anticancer drug. Based on our work with phytotoxins as potential chemical scaffolds and our recent findings involving three phytopathogenic fungi, i.e., Cochliobolus australiensis, Kalmusia variispora and Hymenoscyphus fraxineus, herein, we evaluate the in vitro anti-cancer activity of the metabolites of these fungi by MTT assays on three cancer cell models harboring various resistance levels to chemotherapeutic drugs. Radicinin, a phytotoxic dihydropyranopyran-4,5-dione produced by Cochliobolus australiensis, with great potential for the biocontrol of the invasive weed buffelgrass (Cenchrus ciliaris), showed significant anticancer activity in the micromolar range. Furthermore, a SAR study was carried out using radicinin, some natural analogues and hemisynthetic derivatives prepared by synthetic methods developed as part of work aimed at the potential application of these molecules as bioherbicides. This investigation opens new avenues for the design and synthesis of novel radicinin analogues as potential anticancer agents.

Conversion of Natural Narciclasine to Its C-1 and C-6 Derivatives and Their Antitumor Activity Evaluation: Some Unusual Chemistry of Narciclasine

During the search for a general, efficient route toward the synthesis of C-1 analogues of narciclasine, natural narciclasine was protected and converted to its C-1 enol derivative using a novel semi-synthetic route. Attempted conversion of this material to its triflate in order to conduct cross-coupling at C-1 resulted in a triflate at C-6 that was successfully coupled with several functionalities. Four novel compounds were fully deprotected after seven steps and subjected to evaluation for cytotoxic activity against three cancer cell lines. Only one derivative showed moderate activity compared to that of narciclasine. Spectral and physical data are provided for all new compounds.

Design and Synthesis of C-1 Methoxycarbonyl Derivative of Narciclasine and Its Biological Activity

A 15-step chemoenzymatic total synthesis of C-1 methoxycarbonyl narciclasine (10) was accomplished. The synthesis began with the toluene dioxygenase-mediated dihydroxylation of ortho-dibromobenzene to provide the corresponding cis-dihydrodiol (12) as a single enantiomer. Further key steps included a nitroso Diels–Alder reaction and an intramolecular Heck cyclization. The C-1 homolog 10 was tested and evaluated for antiproliferative activity against natural narciclasine (1) as the positive control. Experimental and spectral data are reported for all novel compounds.

Three-component assembly of stabilized fluorescent isoindoles

The tandem addition of an amine and a thiol to an aromatic dialdehyde engages a selective three-component assembly of a fluorescent isoindole. While an attractive approach for diversity-based fluorophore discovery, isoindoles are typically unstable and present considerable challenges for their practical utility. We found that introduction of electron-withdrawing substituents into the dialdehyde component affords stable isoindole products in one step with acceptable yields and high purity.

Synthesis and biological evaluation of 10-benzyloxy-Narciclasine

A chemoenzymatic convergent synthesis of 10-benzyloxy narciclasine from bromobenzene was accomplished in 16 steps. The key transformations included toluene dioxygenase-mediated hydroxylation, nitroso Diels-Alder reaction and intramolecular Heck cyclization. The unnatural derivative of narciclasine was subjected to biological evaluation and its activity was compared to other C-10 and C-7 compounds prepared previously.

Lessons in Organic Fluorescent Probe Discovery

Fluorescent probes have gained profound use in biotechnology, drug discovery, medical diagnostics, molecular and cell biology. The development of methods for the translation of fluorophores into fluorescent probes continues to be a robust field for medicinal chemists and chemical biologists, alike. Access to new experimental designs has enabled molecular diversification and led to the identification of new approaches to probe discovery. This review provides a synopsis of the recent lessons in modern fluorescent probe discovery.

[3 + 2]-Annulation of pyridinium ylides with 1-chloro-2-nitrostyrenes unveils a tubulin polymerization inhibitor

Indolizines and pyrazolo[1,5-a]pyridines were prepared via [3 + 2]-cycloaddition of pyridinium ylides to 1-chloro-2-nitrostyrenes. The synthesized molecules were evaluated for antiproliferative activities against a BE(2)-C neuroblastoma cell line with several compounds decreasing the viability of cancer cells. Indolizine 9db showed higher potency than that of all-trans-retinoic acid, an approved cancer drug. Mechanistically, it was found to inhibit tubulin polymerization and it is thus proposed that the discovered chemistry can be exploited for the development of novel microtubule-targeting anticancer agents.

Epithelial-mesenchymal transition sensitizes breast cancer cells to cell death via the fungus-derived sesterterpenoid ophiobolin A

The epithelial-mesenchymal transition (EMT) imparts properties of cancer stem-like cells, including resistance to frequently used chemotherapies, necessitating the identification of molecules that induce cell death specifically in stem-like cells with EMT properties. Herein, we demonstrate that breast cancer cells enriched for EMT features are more sensitive to cytotoxicity induced by ophiobolin A (OpA), a sesterterpenoid natural product. Using a model of experimentally induced EMT in human mammary epithelial (HMLE) cells, we show that EMT is both necessary and sufficient for OpA sensitivity. Moreover prolonged, sub-cytotoxic exposure to OpA is sufficient to suppress EMT-imparted CSC features including sphere formation and resistance to doxorubicin. In vivo growth of CSC-rich mammary cell tumors, is suppressed by OpA treatment. These data identify a driver of EMT-driven cytotoxicity with significant potential for use either in combination with standard chemotherapy or for tumors enriched for EMT features.

Deciphering the chemical instability of sphaeropsidin A under physiological conditions - degradation studies and structural elucidation of the major metabolite

The fungal metabolite sphaeropsidin A (SphA) has been recognised for its promising cytotoxicity, particularly towards apoptosis- and multidrug-resistant cancers. Owing to its intriguing activity, the development of SphA as a potential anticancer agent has been pursued. However, this endeavour is compromised since SphA exhibits poor physicochemical stability under physiological conditions. Herein, SphA's instability in biological media was explored utilizing LC-MS. Notably, the degradation tendency was found to be markedly enhanced in the presence of amino acids in the cell medium utilized. Furthermore, the study investigated the presence of degradation adducts, including the identification, isolation and structural elucidation of a major degradation metabolite, (4R)-4,4',4'-trimethyl-3'-oxo-4-vinyl-4',5',6',7'-tetrahydro-3'H-spiro[cyclohexane-1,1'-isobenzofuran]-2-ene-2-carboxylic acid. Considering the reduced cytotoxic potency of aged SphA solutions, as well as that of the isolated degradation metabolite, the reported antiproliferative activity has been attributed primarily to the parent compound (SphA) and not its degradation species. The fact that SphA continues to exhibit remarkable bioactivity, despite being susceptible to degradation, motivates future research efforts to address the challenges associated with this instability impediment.

A fluorescent target-guided Paal-Knorr reaction

It has become increasingly apparent that high-diversity chemical reactions play a significant role in the discovery of bioactive small molecules. Here, we describe an expanse of this paradigm, combining a 'target-guided synthesis' concept with Paal-Knorr chemistry applied to the preparation of fluorescent ligands for human prostaglandin-endoperoxide synthase (COX-2).

Nitroalkanes as electrophiles: synthesis of triazole-fused heterocycles with neuroblastoma differentiation activity

We discovered a reaction of nitroalkanes with 2-hydrazinylquinolines, 2-hydrazinylpyridines and bis-2,4-dihydrazinylpyrimidines in polyphosphoric acid (PPA) affording 1,2,4-triazolo[4,3-a]quinolines, 1,2,4-triazolo[4,3-a]pyridines and bis[1,2,4]triazolo[4,3-a:4',3'-c]pyrimidines, respectively. The reaction expands the scope of heterocyclic annulations involving phosphorylated nitronates, believed to be the electrophilic intermediates formed from nitroalkanes in PPA. Several of the synthesized triazoles showed promising anticancer activity by inducing differentiation in neuroblastoma cancer cells. Due to the urgent need for novel differentiation agents for neuroblastoma therapy, this finding warrants further evaluation of this class of compounds against neuroblastoma.

Synergistic action of substituted indole derivatives and clinically used antibiotics against drug-resistant bacteria

Aim: The current report describes the discovery of indole derivatives that synergize with standard antibiotics. Materials & methods: The antibacterial activities were determined using an optimized time-kill method, while viability of mammalian cells was assessed using the (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Results: The synergy is observed with methicillin- and vancomycin-resistant Staphylococcus aureus bacterial strains, against which the standard antibiotics show no activities of their own. Our indole derivatives in combination with antibiotics lack toxicity toward mammalian cells, do not promote the evolution of resistance of S. aureus in comparison to clinically established antibiotics, and likely work by permeabilizing bacterial cell membranes. Conclusion: The above-mentioned findings demonstrate the potential clinical applications of our indole derivatives.

Photo-Uncaging of a Microtubule-Targeted Rigidin Analogue in Hypoxic Cancer Cells and in a Xenograft Mouse Model

Marine alkaloid rigidins are cytotoxic compounds known to kill cancer cells at nanomolar concentrations by targeting the microtubule network. Here, a rigidin analogue containing a thioether group was "caged" by coordination of its thioether group to a photosensitive ruthenium complex. In the dark, the coordinated ruthenium fragment prevented the rigidin analogue from inhibiting tubulin polymerization and reduced its toxicity in 2D cancer cell line monolayers, 3D lung cancer tumor spheroids (A549), and a lung cancer tumor xenograft (A549) in nude mice. Photochemical activation of the prodrug upon green light irradiation led to the photosubstitution of the thioether ligand by water, thereby releasing the free rigidin analogue capable of inhibiting the polymerization of tubulin. In cancer cells, such photorelease was accompanied by a drastic reduction of cell growth, not only when the cells were grown in normoxia (21% O₂) but also remarkably in hypoxic conditions (1% O₂). In vivo, low toxicity was observed at a dose of 1 mg·kg^-1 when the compound was injected intraperitoneally, and light activation of the compound in the tumor led to 30% tumor volume reduction, which represents the first demonstration of the safety and efficacy of ruthenium-based photoactivated chemotherapy compounds in a tumor xenograft.

A Brief Up-to-Date Overview of Amaryllidaceae Alkaloids: Phytochemical Studies of Narcissus tazetta subsp. tazetta L., Collected in Turkey

A brief up-to-date overview on the isolation, and chemical and biological characterization of new and known alkaloids from different Amaryllidaceae species, including Brunsvigia, Crinum, Cyrtanthus, Narcissus, and Nerine genera, was reported. Furthermore, the isolation and chemical characterization of alkaloids extracted from bulbs of Narcissus tazetta subsp. tazetta L. collected from Muğla, Turkey were described.

Marine-Derived Anticancer Agents: Clinical Benefits, Innovative Mechanisms, and New Targets

The role of the marine environment in the development of anticancer drugs has been widely reviewed, particularly in recent years. However, the innovation in terms of clinical benefits has not been duly emphasized, although there are important breakthroughs associated with the use of marine-derived anticancer agents that have altered the current paradigm in chemotherapy. In addition, the discovery and development of marine drugs has been extremely rewarding with significant scientific gains, such as the discovery of new anticancer mechanisms of action as well as novel molecular targets. Approximately 50 years since the approval of cytarabine, the marine-derived anticancer pharmaceutical pipeline includes four approved drugs and eighteen agents in clinical trials, six of which are in late development. Thus, the dynamic pharmaceutical pipeline consisting of approved and developmental marine-derived anticancer agents offers new hopes and new tools in the treatment of patients afflicted with previously intractable types of cancer.

Algae metabolites: from in vitro growth inhibitory effects to promising anticancer activity

Covering: 1957 to 2017 Algae constitute a heterogeneous group of eukaryotic photosynthetic organisms, mainly found in the marine environment. Algae produce numerous metabolites that help them cope with the harsh conditions of the marine environment. Because of their structural diversity and uniqueness, these molecules have recently gained a lot of interest for the identification of medicinally useful agents, including those with potential anticancer activities. In the current review, which is not a catalogue-based one, we first highlight the major biological events that lead to various types of cancer, including metastatic ones, to chemoresistance, thus to any types of current anticancer treatment relating to the use of chemotherapeutics. We then review algal metabolites for which scientific literature reports anticancer activity. Lastly, we focus on algal metabolites with promising anticancer activity based on their ability to target biological characteristics of cancer cells responsible for poor treatment outcomes. Thus, we highlight compounds that have, among others, one or more of the following characteristics: selectivity in reducing the proliferation of cancer cells over normal ones, potential for killing cancer cells through non-apoptotic signaling pathways, ability to circumvent MDR-related efflux pumps, and activity in vivo in relevant pre-clinical models.

Activity of natural and synthetic polygodial derivatives against Trypanosoma cruzi amastigotes, trypomastigotes and epimastigotes

Our laboratories have been investigating biological effects of a sesquiterpenoid polygodial and its natural and synthetic analogues. Herein, we report the evaluation of these compounds against the three forms of Trypanosoma cruzi, amastigotes, trypomastigotes and epimastigotes. Although polygodial was found to be poorly active, its natural congener epipolygodial and synthetic Wittig-derived analogues showed low micromolar potency against all three forms of the parasite. Synthetic α,β-unsaturated phosphonate 9 compared favorably with clinically approved drugs benznidazole and nifurtimox, and was effective against trypomastigotes, toward which benznidazole showed no activity.[Formula: see text].

Abstract P4-07-10: Epithelial-mesenchymal transition sensitizes breast cancer cells to paraptosis-mediated cell death via the fungus-derived sesterpenoid, Ophiobolin A

The epithelial-mesenchymal transition (EMT) enables the dissociation of cancer cells from the primary tumor by facilitating tolerance to lack of cell-adhesion, decreasing cellular division and increasing motility of individual cells, which leads to an invasive phenotype that links EMT to metastasis. Furthermore, EMT results in the acquisition of stem-cell markers and an increased ability to initiate tumor growth, supporting the concept that EMT may contribute to the development of a small, persistent sub-population of the tumor called cancer stem cells (CSCs). Cells that have undergone EMT have characteristically suppressed cell cycles, making them resistant to commonly used chemotherapies that target DNA replication or microtubule dynamics, processes essential to replicating cells. Nonspecific treatments can also rely on inducing apoptotic cell death; however, recent debates challenge the efficacy of apoptosis in solid tumors, citing high rates of acquired resistance. Utilizing a compound that induces alternative cell death, namely paraptosis, becomes attractive when these other treatments fail. Relying on an activated gene expression program, paraptosis results in the swelling of the mitochondria and endoplasmic reticulum and Apaf-1-independent alternative caspase-9 activity. Treating with paraptosis-inducing compounds such as Ophiobolin A (OpA) specifically targets otherwise-insensitive CSC and EMT cells to re-sensitize bulk tumor populations to chemotherapies. We describe EMT as a key driver of enhanced sensitivity to paraptosis-induced cell death following short-term treatment with OpA or other paraptosis-inducing compounds. Further, paraptosis selectively eliminates the CSC sub-population by reducing stem cell activity and highlights the potential of this pathway in breast cancer treatment.

Citation Format: Reisenauer K, Tao Y, Ingros A, Philip T, Evidente A, Kornienko A, Romo D, Taube J. Epithelial-mesenchymal transition sensitizes breast cancer cells to paraptosis-mediated cell death via the fungus-derived sesterpenoid, Ophiobolin A [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P4-07-10.

Microtubule-Targeting 7-Deazahypoxanthines Derived from Marine Alkaloid Rigidins: Exploration of the N3 and N9 Positions and Interaction with Multidrug-Resistance Proteins

Our laboratories have been investigating synthetic analogues of marine alkaloid rigidins that possess promising anticancer activities. These analogues, based on the 7-deazahypoxanthine skeleton, are available in one- or two-step synthetic sequences and exert cytotoxicity by disrupting microtubule dynamics in cancer cells. In the present work we extended the available structure-activity relationship (SAR) data to N3- and N9-substituted derivatives. Although N3 substitution results in loss of activity, the N9-substituted compounds retain nanomolar antiproliferative activities and the anti-tubulin mode of action of the original unsubstituted compounds. Furthermore, our results also demonstrate that multidrug-resistance (MDR) proteins do not confer resistance to both N9-unsubstituted and -substituted compounds. It was found that sublines overexpressing ABCG2, ABCC1, and ABCB1 proteins are as responsive to the rigidin analogues as their parental cell lines. Thus, the study reported herein provides further impetus to investigate the rigidin-inspired 7-deazahypoxanthines as promising anticancer agents.

Novel polygodial analogs P3 and P27: Efficacious therapeutic agents disrupting mitochondrial function in oral squamous cell carcinoma

Polygodial, a drimane sesquiterpenoid dialdehyde isolated as a pungent component of the water pepper Persicaria hydropiper, exhibits antifeedant, antimicrobial, anti-inflammatory and anticancer effects. Polygodial also activates transient receptor potential vanilloid subtype 1 (TRPV1) channels. Previously, we described the synthesis of a C₁₂-Wittig derivative of polygodial, termed P3, with significant antiproliferative effects against multiple cancer types including oral squamous cell carcinoma (OSCC). In the present study, a more potent derivative, P27, with superior anti-proliferative effects in vitro and antitumor effects in Cal-27 derived xenografts is described. Polygodial, P3, and P27 all significantly decreased OSCC tumor growth, with P27 being equipotent with polygodial and P3 being the least efficacious. However, neither analog elicited the adverse effect observed with polygodial: Profound transient inflammation. Although P3 and P27 pharmacophores are based on polygodial, novel effects on OSCC cell cycle distribution were identified and shared anticancer effects that are independent of TRPV1 activity were observed. Polygodial elicits an S-phase block, whereas P3 and P27 lead to G₂/M phase arrest. Pretreatment of OSCC cells with the TRPV1 antagonist capsazepine does not affect the antiproliferative activity of P3 or P27, indicating that TRPV1 interactions do not regulate OSCC cell proliferation. Indeed, calcium imaging studies identified that the analogs neither activate nor antagonize TRPV1. Behavioral studies using a rat model for orofacial pain confirmed that these analogs fail to induce nocifensive responses, indicating that they are non-noxious in vivo. All compounds induced a significant concentration-dependent decrease in the mitochondrial transmembrane potential and corresponding apoptosis. Considering that P27 is equipotent to polygodial with no TRPV1-associated adverse effects, P27 may serve as an efficacious novel therapy for OSCC.

Photoactivated 2,3-distyrylindoles kill multi-drug resistant bacteria

Compounds based on the 2,3-distyrylindole scaffold were found to exhibit bactericidal properties upon irradiation with white light. At the concentration of 1 μM, the lead compound 1 completely (ca. 109 CFU/mL) eradicated such Gram-positive organisms as S. aureus (MRSA, MSSA), E. faecalis (VRE), S. pyogenes and S. mutans when irradiated with white light for 2 min. At the concentration of 5 μM and in the presence of polymyxin E at non-bactericidal 1.25 μg/mL concentration, 1 also showed a 7-log to 9-log reductions in bacterial counts of such Gram-negative organisms as multi-drug resistant (MDR) A. baumannii, MDR P. aeruginosa, E. coli and Klebsiella pneumoniae (CRE: KPC and NDM-1), also when irradiated with white light for 2 min. The structure-activity relationship studies revealed that unsubstituted at benzene rings 2,3-distyrylindole 2 was most potent and gave a 5-order of magnitude eradication of a MRSA strain at the concentration of 30 nM upon irradiation with white light. Initial mechanistic experiments revealed the disruption of bacterial cell membrane, but indicated that singlet oxygen production, which is commonly associated with photodynamic therapy, may not play a role in the bactericidal effects of the 2,3-distyrylindoles.

Polygodial analog induces apoptosis in LNCaP prostate cancer cells

Prostate cancer (PCa) is the second leading cause of death in American men. The chemotherapeutic treatment strategies are generally not effective and can lead to side effects. Hence, there is an urgent need to identify novel chemotherapeutic agents. The aim of this study was to synthesize and evaluate the therapeutic effects of a synthetic analog of polygodial (PG), a pungent constituent abundantly present in mountain pepper, water pepper and dorrigo pepper, on LNCaP PCa cell line and its anti-cancer mechanisms in a preclinical study. We evaluated the anti-cancer potential of the PG analog namely DRP-27 using various assays such as cell viability by MTT assay, anchorage independent growth by soft agar assay, reactive oxygen species generation by 2',7'-dichlorofluorescein probe-based fluorescence assay, and apoptosis by Annexin-V and TUNEL assays respectively. Western blot analysis was performed to identify the molecular mechanism of DRP-27-induced cell death. Our results showed that DRP-27 significantly inhibited LNCaP cell proliferation in a dose-dependent manner at 48 h treatment in vitro. In addition, DRP-27 potently inhibited anchorage-independent growth of these cells. Flow cytometry, Annexin-V and TUNEL assays confirmed that DRP-27 induces apoptosis in LNCaP cells. DRP-27 also induced the activation of intracellular reactive oxygen species. Western blot analysis revealed that DRP-27 downregulated the expression of survivin, while activating Bax and DNA damage marker pH₂AX in LNCaP cells. In conclusion, our study suggests that DRP-27 might be an effective anti-cancer agent for PCa.

Synthetic analogues of the montanine-type alkaloids with activity against apoptosis-resistant cancer cells

In a search of small molecules active against apoptosis-resistant cancer cells, a skeletal rearrangement of alkaloid haemanthamine was utilized to generate a series of compounds possessing the alkaloid montanine ring system. The synthesized compounds were found to inhibit proliferation of cancer cells resistant to apoptosis at micromolar concentrations. Selected compounds were also active against patient-derived glioblastoma cells expressing stem-cell markers. This is the first report describing the preparation of synthetic analogues of the montanine-type alkaloids with antiproliferative activity. The compounds prepared in the current investigation appear to be a useful starting point for the development of agents to fight cancers with apoptosis resistance, and thus, associated with poor prognoses.

The Amaryllidaceae Alkaloid Haemanthamine Binds the Eukaryotic Ribosome to Repress Cancer Cell Growth

Alkaloids isolated from the Amaryllidaceae plants have potential as therapeutics for treating human diseases. Haemanthamine has been studied as a novel anticancer agent due to its ability to overcome cancer cell resistance to apoptosis. Biochemical experiments have suggested that hemanthamine targets the ribosome. However, a structural characterization of its mechanism has been missing. Here we present the 3.1 Å resolution X-ray structure of haemanthamine bound to the Saccharomyces cerevisiae 80S ribosome. This structure reveals that haemanthamine targets the A-site cleft on the large ribosomal subunit rearranging rRNA to halt the elongation phase of translation. Furthermore, we provide evidence that haemanthamine and other Amaryllidaceae alkaloids also inhibit specifically ribosome biogenesis, triggering nucleolar stress response and leading to p53 stabilization in cancer cells. Together with a computer-aided interpretation of existing structure-activity relationships of Amaryllidaceae alkaloids congeners, we provide a rationale for designing molecules with enhanced potencies and reduced toxicities.

The Rigidins: Isolation, Bioactivity, and Total Synthesis-Novel Pyrrolo[2,3-d]Pyrimidine Analogues Using Multicomponent Reactions

Rigidins (2-6) are pyrrolopyrimidine alkaloids isolated from marine tunicates. Since their isolation, refinement of their total syntheses, and biochemical evaluation, interest toward this pyrrolo[2,3-d]pyrimidine scaffold as a medicinal candidate has been triggered. The derivatization of these natural products has led to the discovery of a novel range of 7-deazahypoxanthines, which exhibit extremely potent anticancer activity in human cancer cell lines. A major breakthrough toward the synthesis of rigidin and various rigidin analogues has been the application of multicomponent reactions (MCRs). The rapid assembly of molecular diversity and flexibility displayed by MCRs makes it an attractive strategy for the preparation of rigidin-inspired small molecules. Furthermore, a number of rigidin-like 7-deazaxanthine compounds have been reported in the literature and the popularity of implementing MCRs to construct these 7-deazaxanthines is highlighted here. It is our hope that the synthetic methods described in this chapter will result in the further generation of rigidin-inspired compounds that will move on from being "hits" into "leads" in the medicinal chemistry drug discovery pipeline and potentially into anticancer therapeutics.

A nitroalkane-based approach to one-pot three-component synthesis of isocryptolepine and its analogs with potent anti-cancer activities

A second generation polyphosphoric acid-mediated one-pot three-component synthesis of indoloquinoline scaffold is developed. This improved version of the process involves electrophilically activated nitroalkanes for the installation of strategic C–C and C–N bonds and ring C assembly. This modification allows the elimination of unnecessary solvent change operations and all steps are carried out in a true, uninterrupted one-pot manner. A further improvement involves the possibility to install an ortho-amino group in situ. A synthetic application of this method is showcased by the concise synthesis of an isocryptolepine alkaloid and its synthetic analogs with potent anticancer activities.

An improved one-pot three-component synthesis involving electrophilically activated nitroalkanes allowed for efficient preparation of indoloquinolines with potent anticancer activities.

Irreversible Protein Labeling by Paal-Knorr Conjugation

The application of new chemical reactions in a biological context has advanced bioconjugation methods for both fundamental research and commercial arenas. Recent adaptations of reactions such as Huisgen 1,3-dipolar or Diels-Alder cycloadditions have enabled the labeling of specific residues in biomolecules by the attachment of molecules carrying azides, alkynes, or strained alkenes. Although these are fundamental tools, there is a need for the discovery of reactions that can label native proteins. We report herein the adaptation of the Paal-Knorr reaction to label lysine residues in proteins via pyrrole linkages.

Covalent modification of biological targets with natural products through Paal-Knorr pyrrole formation

Covering: up to June 2017Natural products and endogenous metabolites engage specific targets within tissues and cells through complex mechanisms. This review examines the extent to which natural systems have adopted the Paal-Knorr reaction to engage nucleophilic amine groups within biological targets. Current understanding of this mode of reactivity is limited by only a few examples of this reaction in a biological context. This highlight is intended to stimulate the scientific community to identify potential research directions and applications of the Paal-Knorr reaction in native and engineered biological systems.

Marine Mollusk-Derived Agents with Antiproliferative Activity as Promising Anticancer Agents to Overcome Chemotherapy Resistance

The chemical investigation of marine mollusks has led to the isolation of a wide variety of bioactive metabolites, which evolved in marine organisms as favorable adaptations to survive in different environments. Most of them are derived from food sources, but they can be also biosynthesized de novo by the mollusks themselves, or produced by symbionts. Consequently, the isolated compounds cannot be strictly considered as "chemotaxonomic markers" for the different molluscan species. However, the chemical investigation of this phylum has provided many compounds of interest as potential anticancer drugs that assume particular importance in the light of the growing literature on cancer biology and chemotherapy. The current review highlights the diversity of chemical structures, mechanisms of action, and, most importantly, the potential of mollusk-derived metabolites as anticancer agents, including those biosynthesized by mollusks and those of dietary origin. After the discussion of dolastatins and kahalalides, compounds previously studied in clinical trials, the review covers potentially promising anticancer agents, which are grouped based on their structural type and include terpenes, steroids, peptides, polyketides and nitrogen-containing compounds. The "promise" of a mollusk-derived natural product as an anticancer agent is evaluated on the basis of its ability to target biological characteristics of cancer cells responsible for poor treatment outcomes. These characteristics include high antiproliferative potency against cancer cells in vitro, preferential inhibition of the proliferation of cancer cells over normal ones, mechanism of action via nonapoptotic signaling pathways, circumvention of multidrug resistance phenotype, and high activity in vivo, among others. The review also includes sections on the targeted delivery of mollusk-derived anticancer agents and solutions to their procurement in quantity.

Data in support of a harmine-derived beta-carboline in vitro effects in cancer cells through protein synthesis

A harmine-derived beta-carboline, CM16, inhibits cancer cells growth through its effects on protein synthesis, as described in “A harmine-derived beta-carboline displays anti-cancer effects in vitro by targeting protein synthesis” (Carvalho et al., 2017)[1]. This data article provides accompanying data on CM16 cytostatic evaluation in cancer cells as well as data related to its effects on transcription and translation. After confirming the cytostatic effect of CM16, we investigated its ability to arrest the cell cycle in the glioma Hs683 and SKMEL-28 melanoma cell lines but no modification was evidenced. According to the global protein synthesis inhibition induced by CM16 [1], transcription phase, a step prior to mRNA translation, evaluated by labelled nucleotide incorporation assay was not shown to be affected under CM16 treatment in the two cell lines. By contrast, mRNA translation and particularly the initiation step were shown to be targeted by CM16 in [1]. To further decipher those effects, we established herein a list of main actors in the protein synthesis process according to literature survey for comparative analysis of cell lines displaying different sensitivity levels to CM16. Finally, one of these proteins, PERK, a kinase regulating eIF2-α phosphorylation and thereby activity, was evaluated under treatment with CM16 in a cell-free system.

A harmine-derived beta-carboline displays anti-cancer effects in vitro by targeting protein synthesis

Growing evidence indicates that protein synthesis is deregulated in cancer onset and progression and targeting this process might be a selective way to combat cancers. While harmine is known to inhibit DYRK1A and intercalate into the DNA, tri-substitution was shown previously to modify its activity profile in favor of protein synthesis inhibition. In this study, we thus evaluated the optimized derivative CM16 in vitro anti-cancer effects unfolding its protein synthesis inhibition activity. Indeed, the growth inhibitory profile of CM16 in the NCI 60-cancer-cell-line-panel correlated with those of other compounds described as protein synthesis inhibitors. Accordingly, CM16 decreased in a time- and concentration-dependent manner the translation of neosynthesized proteins in vitro while it did not affect mRNA transcription. CM16 rapidly penetrated into the cell in the perinuclear region of the endoplasmic reticulum where it appears to target translation initiation as highlighted by ribosomal disorganization. More precisely, we found that the mRNA expression levels of the initiation factors EIF1AX, EIF3E and EIF3H differ when comparing resistant or sensitive cell models to CM16. Additionally, CM16 induced eIF2α phosphorylation. Those effects could explain, at least partly, the CM16 cytostatic anti-cancer effects observed in vitro while neither cell cycle arrest nor DNA intercalation could be demonstrated. Therefore, targeting protein synthesis initiation with CM16 could represent a new promising alternative to current cancer therapies due to the specific alterations of the translation machinery in cancer cells as recently evidenced with respect to EIF1AX and eIF3 complex, the potential targets identified in this present study.

Single dish gradient screening of small molecule localization

Understanding trafficking in cells and tissues is one of the most critical steps in exploring the mechanisms and modes of action (MOAs) of a small molecule. Typically, deciphering the role of concentration presents one of the most difficult challenges associated with this task. Herein, we present a practical solution to this problem by developing concentration gradients within single dishes of cells. We demonstrate the method by evaluating fluorescently-labelled probes developed from two classes of natural products that have been identified as potential anti-cancer leads by STORM super-resolution microscopy.

Sesterterpenoids with Anticancer Activity

Terpenes have received a great deal of attention in the scientific literature due to complex, synthetically challenging structures and diverse biological activities associated with this class of natural products. Based on the number of C5 isoprene units they are generated from, terpenes are classified as hemi- (C5), mono- (C10), sesqui- (C15), di- (C20), sester- (C25), tri (C30), and tetraterpenes (C40). Among these, sesterterpenes and their derivatives known as sesterterpenoids, are ubiquitous secondary metabolites in fungi, marine organisms, and plants. Their structural diversity encompasses carbotricyclic ophiobolanes, polycyclic anthracenones, polycyclic furan-2-ones, polycyclic hydroquinones, among many other carbon skeletons. Furthermore, many of them possess promising biological activities including cytotoxicity and the associated potential as anticancer agents. This review discusses the natural sources that produce sesterterpenoids, provides sesterterpenoid names and their chemical structures, biological properties with the focus on anticancer activities and literature references associated with these metabolites. A critical summary of the potential of various sesterterpenoids as anticancer agents concludes the review.

Marine Invertebrate Metabolites with Anticancer Activities: Solutions to the "Supply Problem"

Marine invertebrates provide a rich source of metabolites with anticancer activities and several marine-derived agents have been approved for the treatment of cancer. However, the limited supply of promising anticancer metabolites from their natural sources is a major hurdle to their preclinical and clinical development. Thus, the lack of a sustainable large-scale supply has been an important challenge facing chemists and biologists involved in marine-based drug discovery. In the current review we describe the main strategies aimed to overcome the supply problem. These include: marine invertebrate aquaculture, invertebrate and symbiont cell culture, culture-independent strategies, total chemical synthesis, semi-synthesis, and a number of hybrid strategies. We provide examples illustrating the application of these strategies for the supply of marine invertebrate-derived anticancer agents. Finally, we encourage the scientific community to develop scalable methods to obtain selected metabolites, which in the authors' opinion should be pursued due to their most promising anticancer activities.

Synthesis and in vitro growth inhibitory activity of novel silyl- and trityl-modified nucleosides

Seventeen silyl- and trityl-modified (5'-O- and 3',5'-di-O-) nucleosides were synthesized with the aim of investigating the in vitro antiproliferative activities of these nucleoside derivatives. A subset of the compounds was evaluated at a fixed concentration of 100μM against a small panel of tumor cell lines (HL-60, K-562, Jurkat, Caco-2 and HT-29). The entire set was also tested at varying concentrations against two human glioma lines (U373 and Hs683) to obtain GI50 values, with the best results being values of ∼25μM.

Isatin derivatives with activity against apoptosis-resistant cancer cells

In a search of small molecules active against apoptosis-resistant cancer cells, a series of isatin-based heterocyclic compounds were synthesized and found to inhibit proliferation of cancer cell lines resistant to apoptosis. The synthesis of these compounds involved a condensation of commercially available, active methylene heterocycles with isatin proceeding in moderate to excellent yields. The heterocyclic scaffolds prepared in the current investigation appear to be a useful starting point for the development of agents to fight cancers with apoptosis resistance, and thus, associated with dismal prognoses.

Higginsianins A and B, Two Diterpenoid α-Pyrones Produced by Colletotrichum higginsianum, with in Vitro Cytostatic Activity

Two new diterpenoid α-pyrones, named higginsianins A (1) and B (2), were isolated from the mycelium of the fungus Colletotrichum higginsianum grown in liquid culture. They were characterized as 3-[5a,9b-dimethyl-7-methylene-2-(2-methylpropenyl)dodecahydronaphtho[2,1-b]furan-6-ylmethyl]-4-hydroxy-5,6-dimethylpyran-2-one and 4-hydroxy-3-[6-hydroxy-5,8a-dimethyl-2-methylene-5-(4-methylpent-3-enyl)decahydronaphthalen-1-ylmethyl]-5,6-dimethylpyran-2-one, respectively, by using NMR, HRESIMS, and chemical methods. The structure and relative configuration of higginsianin A (1) were confirmed by X-ray diffractometric analysis, while its absolute configuration was assigned by electronic circular dichroism (ECD) experiments and calculations using a solid-state ECD/TDDFT method. The relative and absolute configuration of higginsianin B (2), which did not afford crystals suitable for X-ray analysis, were determined by NMR analysis and by ECD in comparison with higginsianin A. 1 and 2 were the C-8 epimers of subglutinol A and diterpenoid BR-050, respectively. The evaluation of 1 and 2 for antiproliferative activity against a panel of six cancer cell lines revealed that the IC50 values, obtained with cells reported to be sensitive to pro-apoptotic stimuli, are by more than 1 order of magnitude lower than their apoptosis-resistant counterparts (1 vs >80 μM). Finally, three hemisynthetic derivatives of 1 were prepared and evaluated for antiproliferative activity. Two of these possessed IC50 values and differential sensitivity profiles similar to those of 1.