Rationally Simplified Bistramide Analog Reversibly Targets Actin Polymerization and Inhibits Cancer Progression in Vitro and in Vivo

Drug discovery and optimization based on the co-crystal structure of natural product with target

Due to their structural diversities and prevalent biological activities, natural products (NPs) are momentous resources for drug discovery. Although NPs have a wide range of biological activities, many exhibit structural complexity that leads to synthetic difficulties, which combines with inefficient biological activity, toxicity, and unfavorable pharmacokinetic characteristics and ultimately imparts poor safety and efficacy outcomes. Progress in crystallization and computational techniques allow crystallography to have a seasonable influences on drug discovery. By co-crystallizing with proteins, therapeutic targets of NPs in specific diseases can be identified. By analyzing the co-crystal information, the structure-activity relationships (SARs) of NPs targeting specific proteins can be grasped. Under the guidance of co-crystal information, directional structural modification and simplification are powerful strategies for overcoming limitations of NPs, improving the success rate of NP-based drug discovery, and obtaining NP-based drugs with high selectivity, low toxicity and favorable pharmacokinetic characteristics. Here, we review the co-crystal information of a selection of NPs, focusing on the SARs of NPs reflected by co-crystal information and the modification and simplification strategies of NPs, and discuss how to apply co-crystal information in the optimization of NP-based lead compound.

Selectively inhibiting malignant melanoma migration and invasion in an engineered skin model using actin-targeting dinuclear RuII-complexes

Due to the poor prognosis of metastatic cancers, there is a clinical need for agents with anti-metastatic activity. Here we report on the anti-metastatic effect of a previously reported Ru(ii) complex [{(phen)₂Ru}₂(tpphz)]⁴⁺, 1⁴⁺, that has recently been shown to disrupt actin fiber assembly. In this study, we investigated the anti-migratory effect of ⁺1⁴⁺ and a close structural analogue⁺, 2⁴⁺, on two highly invasive, metastatic human melanoma cell lines. Laser scanning confocal imaging was used to investigate the structure of actin filament and adhesion molecule vinculin and results show disassembly of central actin filaments and focal adhesions. The effect of both compounds on actin filaments was also found to be reversible. As these results revealed that the complexes were cytostatic and produced a significant inhibitory effect on the migration of both melanoma cell lines but not human dermal fibroblasts their effect on 3D-spheroids and a tissue-engineered living skin model were also investigated. These experiments demonstrated that the compounds inhibited the growth and invasiveness of the melanoma-based spheroidal tumor model and both complexes were found to penetrate the epidermis of the skin tissue model and inhibit the invasion of melanoma cells. Taken together, the cytostatic and antimigratory effects of the complexes results in an antimetastatic effect that totally prevent invasion of malignant melanoma into skin tissue.

A Ruthenium(II) Polypyridyl Complex Disrupts Actin Cytoskeleton Assembly and Blocks Cytokinesis

The dinuclear Ru^II complex [(Ru(phen)₂)₂(tpphz)]⁴⁺ (phen=1,10‐phenanthroline, tpphz=tetrapyridophenazine) “RuRuPhen” blocks the transformation of G‐actin monomers to F‐actin filaments with no disassembly of pre‐formed F‐actin. Molecular docking studies indicate multiple RuRuPhen molecules bind to the surface of G‐actin but not the binding pockets of established actin polymerisation inhibitors. In cells, addition of RuRuPhen causes rapid disruption to actin stress fibre organisation, compromising actomyosin contractility and cell motility; due to this effect RuRuPhen interferes with late‐stage cytokinesis. Immunofluorescent microscopy reveals that RuRuPhen causes cytokinetic abscission failure by interfering with endosomal sorting complexes required for transport (ESCRT) complex recruitment.

A Ruthenium(II) Polypyridyl Complex Disrupts Actin Cytoskeleton Assembly and Blocks Cytokinesis

The dinuclear RuII complex [(Ru(phen)2)2(tpphz)]4+ (phen=1,10-phenanthroline, tpphz=tetrapyridophenazine) "RuRuPhen" blocks the transformation of G-actin monomers to F-actin filaments with no disassembly of pre-formed F-actin. Molecular docking studies indicate multiple RuRuPhen molecules bind to the surface of G-actin but not the binding pockets of established actin polymerisation inhibitors. In cells, addition of RuRuPhen causes rapid disruption to actin stress fibre organisation, compromising actomyosin contractility and cell motility; due to this effect RuRuPhen interferes with late-stage cytokinesis. Immunofluorescent microscopy reveals that RuRuPhen causes cytokinetic abscission failure by interfering with endosomal sorting complexes required for transport (ESCRT) complex recruitment.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

A New Synthetic Spiroketal: Studies on Antitumor Activity on Murine Melanoma Model In Vivo and Mechanism of Action In Vitro

BACKGROUND

In a previous study, we synthesised a new spiroketal derivative, inspired to natural products, that has shown high antiproliferative activity, potent telomerase inhibition and proapoptotic activity on several human cell lines.

OBJECTIVE

This work focused on the study of in vivo antitumor effect of this synthetic spiroketal on a murine melanoma model. In order to shed additional light on the origin of the antitumor effect, in vitro studies were performed.

METHODS

Spiroketal was administered to B16F10 melanoma mice at a dose of 5 mg/Kg body weight via intraperitoneum at alternate days for 15 days. Tumor volume measures were made every 2 days starting after 12 days from cells injection. The effects of the spiroketal on tumor growth inhibition, apoptosis induction, and cell cycle modification were investigated in vitro on B16 cells. HIF1α gene expression, the inhibition of cells migration and the changes induced in cytoskeleton conformation were evaluated.

RESULTS

Spiroketal displayed proapoptotic activity and high antitumor activity in B16 cells with nanomolar IC50. Moreover it has shown to inhibit cell migration, to strongly reduce the HIF1α expression and to induce strongly deterioration of cytoskeleton structure. A potent dose-dependent antitumor efficacy in syngenic B16/C57BL/6J murine model of melanoma was observed with the suppression of tumor growth by an average of 90% at a dose of 5 mg/kg.

CONCLUSION

The synthesized spiroketal shows high antitumor activity in the B16 cells in vitro at nM concentration and a dose-dependent antitumor efficacy in syngenic B16/C57BL/6J mice. The results suggest that this natural product inspired spiroketal may have a potential application in melanoma therapy.

Effects of Modulating Actin Dynamics on HER2 Cancer Cell Motility and Metastasis

Amplification of HER2 leads to development of HER2-positive (HER2+) cancers with high rates of metastasis compared to other cancer subtypes. The goal of this study was to probe the vulnerability of HER2+ cancer cells to a filamentous actin (F-actin) severing and capping toxin. The growth and viability of human HER2+ breast cancer (HCC1954) and ovarian cancer (SKOV3) cell lines were significantly impaired upon treatment with the marine macrolide mycalolide B (Myc B) at doses above 100 nanomolar. Further testing of Myc B in combination with the antibody-drug conjugate Trastuzumab-emtansine (T-DM1) led to improved killing of SKOV3 cells compared to either treatment alone. At sub-lethal doses, treatment of HER2+ cancer cells with Myc B resulted in rapid loss of leading edge protrusions and formation of aggresomes containing F-actin and the actin regulatory protein Cortactin. This correlated with robust inhibition of HER2+ cancer cell motility and invasion with Myc B treatment. In SKOV3 tumor xenograft assays, intratumoral injections of Myc B impaired HER2+ tumor growth and metastasis, with maximal effects observed in combination with systemic delivery of Trastuzumab. Metastasis of SKOV3 cells to the lungs following tail vein injection was also reduced by Myc B. Together, these findings provide rationale for targeting F-actin in combination with existing therapies for HER2+ cancers to reduce metastasis.

Ascidian Toxins with Potential for Drug Development

Ascidians (tunicates) are invertebrate chordates, and prolific producers of a wide variety of biologically active secondary metabolites from cyclic peptides to aromatic alkaloids. Several of these compounds have properties which make them candidates for potential new drugs to treat diseases such as cancer. Many of these natural products are not produced by the ascidians themselves, rather by their associated symbionts. This review will focus mainly on the mechanism of action of important classes of cytotoxic molecules isolated from ascidians. These toxins affect DNA transcription, protein translation, drug efflux pumps, signaling pathways and the cytoskeleton. Two ascidian compounds have already found applications in the treatment of cancer and others are being investigated for their potential in cancer, neurodegenerative and other diseases.

Iriomoteolides: novel chemical tools to study actin dynamics

Despite its promising biological profile, the cellular targets of iriomoteolide-3a, a novel 15-membered macrolide isolated from Amphidinium sp., have remained unknown. A small library of non-natural iriomoteolide-3a analogues is presented here as a result of a novel, highly convergent, catalysis-based scaffold-diversification campaign, which revealed the suitable sites for chemical editing in the original core. We provide compelling experimental evidence for actin as one of iriomoteolides' primary cellular targets, establishing the ability of these secondary metabolites to inhibit cell migration, induce severe morphological changes in cells and cause a reversible cytoplasmic retraction and reduction of F-actin fibers in a time and dose dependent manner. These results are interpreted in light of the ability of iriomoteolides to stabilize F-actin filaments. Molecular dynamics simulations provide evidence for iriomoteolide-3a binding to the barbed end of G-actin. These results showcase iriomoteolides as novel and easily tunable chemical probes for the in vitro study of actin dynamics in the context of cell motility processes including cell invasion and division.

Marine Natural Products from New Caledonia--A Review

Marine micro- and macroorganisms are well known to produce metabolites with high biotechnological potential. Nearly 40 years of systematic prospecting all around the New Caledonia archipelago and several successive research programs have uncovered new chemical leads from benthic and planktonic organisms. After species identification, biological and/or pharmaceutical analyses are performed on marine organisms to assess their bioactivities. A total of 3582 genera, 1107 families and 9372 species have been surveyed and more than 350 novel molecular structures have been identified. Along with their bioactivities that hold promise for therapeutic applications, most of these molecules are also potentially useful for cosmetics and food biotechnology. This review highlights the tremendous marine diversity in New Caledonia, and offers an outline of the vast possibilities for natural products, especially in the interest of pursuing collaborative fundamental research programs and developing local biotechnology programs.

Spiroacetal formation through telescoped cycloaddition and carbon-hydrogen bond functionalization: total synthesis of bistramide A

Spiroacetals can be formed through a one-pot sequence of a hetero-Diels-Alder reaction, an oxidative carbon-hydrogen bond cleavage, and an acid treatment. This convergent approach expedites access to a complex molecular subunit which is present in numerous biologically active structures. The utility of the protocol is demonstrated through its application to a brief synthesis of the actin-binding cytotoxin bistramide A.

MARCKS actin-binding capacity mediates actin filament assembly during mitosis in human hepatic stellate cells

Cross-linking between the actin cytoskeleton and plasma membrane actin-binding proteins is a key interaction responsible for the mechanical properties of the mitotic cell. Little is known about the identity, the localization, and the function of actin filament-binding proteins during mitosis in human hepatic stellate cells (hHSC). The aim of the present study was to identify and analyze the cross talk between actin and myristoylated alanine-rich kinase C substrate (MARCKS), an important PKC substrate and actin filament-binding protein, during mitosis in primary hHSC. Confocal analysis and chromosomal fraction analysis of mitotic hHSC demonstrated that phosphorylated (P)-MARCKS displays distinct phase-dependent localizations, accumulates at the perichromosomal layer, and is a centrosomal protein belonging to the chromosomal cytosolic fraction. Aurora B kinase (AUBK), an important mitotic regulator, β-actin, and P-MARCKS concentrate at the cytokinetic midbody during cleavage furrow formation. This localization is critical since MARCKS-depletion in hHSC is characterized by a significant loss in cytosolic actin filaments and cortical β-actin that induces cell cycle inhibition and dislocation of AUBK. A depletion of AUBK in hHSC affects cell cycle, resulting in multinucleation. Quantitative live cell imaging demonstrates that the actin filament-binding capacity of MARCKS is key to regulate mitosis since the cell cycle inhibitory effect in MARCKS-depleted cells caused abnormal cell morphology and an aberrant cytokinesis, resulting in a significant increase in cell cycle time. These findings implicate that MARCKS, an important PKC substrate, is essential for proper cytokinesis and that MARCKS and its partner actin are key mitotic regulators during cell cycle in hHSC.

Synthesis of a platform to access bistramides and their analogues

The platform C14-C40, which can be used to prepare bistramide C and 39-oxobistramide K, was synthesized in 19 steps with an overall yield of 6.2%. Furthermore, the chemoselective reduction of the ketone at C-39 was performed giving an easy access to bistramides A, B, D, K, and L. Finally, the versatility of the synthesis of the C14-C40 fragment can allow the preparation of a large variety of stereoisomers to produce bistramide analogues.

Development of highly cytotoxic and actin-depolymerizing biotin derivatives of aplyronine A

Tied up: a PEG-linked biotin derivative of marine macrolide aplyronine A (ApA) is shown to exhibit potent cytotoxicity and cause actin disassembly in tumor cells. This method of introducing a PEG linker at the end of the aliphatic tail should offer perspectives for developing and using versatile actin-targeting molecular probes. PEG=poly(ethylene glycol).

Translating Nature's Library: The Bryostatins and Function-Oriented Synthesis

We review in part our computational, design, synthesis, and biological studies on a remarkable class of compounds and their designed analogs that have led to preclinical candidates for the treatment of cancer, a first-in-class approach to Alzheimer's disease, and a promising strategy to eradicate HIV/AIDS. Because these leads target, in part, protein kinase C (PKC) isozymes, they have therapeutic potential even beyond this striking set of therapeutic indications. This program has given rise to new synthetic methodology and represents an increasingly important direction of synthesis focused on achieving function through synthesis-informed design (function-oriented synthesis).

Design, synthesis, and evaluation of potent bryostatin analogs that modulate PKC translocation selectivity

Modern methods for the identification of therapeutic leads include chemical or virtual screening of compound libraries. Nature's library represents a vast and diverse source of leads, often exhibiting exquisite biological activities. However, the advancement of natural product leads into the clinic is often impeded by their scarcity, complexity, and nonoptimal properties or efficacy as well as the challenges associated with their synthesis or modification. Function-oriented synthesis represents a strategy to address these issues through the design of simpler and therefore synthetically more accessible analogs that incorporate the activity-determining features of the natural product leads. This study illustrates the application of this strategy to the design and synthesis of functional analogs of the bryostatin marine natural products. It is specifically directed at exploring the activity-determining role of bryostatin A-ring functionality on PKC affinity and selectivity. The resultant functional analogs, which were prepared by a flexible, modular synthetic strategy, exhibit excellent affinity to PKC and differential isoform selectivity. These and related studies provide the basic information needed for the design of simplified and thus synthetically more accessible functional analogs that target PKC isoforms, major targets of therapeutic interest.