Grassystatins A-C from marine cyanobacteria, potent cathepsin E inhibitors that reduce antigen presentation

Progress in the discovery and development of anticancer agents from marine cyanobacteria

Covering 2010-April 2024There have been tremendous new discoveries and developments since 2010 in anticancer research based on marine cyanobacteria. Marine cyanobacteria are prolific sources of anticancer natural products, including the tubulin agents dolastatins 10 and 15 which were originally isolated from a mollusk that feeds on cyanobacteria. Decades of research have culminated in the approval of six antibody-drug conjugates (ADCs) and many ongoing clinical trials. Antibody conjugation has been enabling for several natural products, particularly cyanobacterial cytotoxins. Targeting tubulin dynamics has been a major strategy, leading to the discovery of the gatorbulin scaffold, acting on a new pharmacological site. Cyanobacterial compounds with different mechanisms of action (MOA), targeting novel or validated targets in a range of organelles, also show promise as anticancer agents. Important advances include the development of compounds with novel MOA, including apratoxin and coibamide A analogues, modulating cotranslational translocation at the level of Sec61 in the endoplasmic reticulum, largazole and santacruzamate A targeting class I histone deacetylases, and proteasome inhibitors based on carmaphycins, resembling the approved drug carfilzomib. The pipeline extends with SERCA inhibitors, mitochondrial cytotoxins and membrane-targeting agents, which have not yet advanced clinically since the biology is less understood and selectivity concerns remain to be addressed. In addition, efforts have also focused on the identification of chemosensitizing and antimetastatic agents. The review covers the state of current knowledge of marine cyanobacteria as anticancer agents with a focus on the mechanism, target identification and potential for drug development. We highlight the importance of solving the supply problem through chemical synthesis as well as illuminating the biological activity and in-depth mechanistic studies to increase the value of cyanobacterial natural products to catalyze their development.

Amantamide C, an Antitrypanosomal Linear Lipopeptide from a Marine Okeania sp. Cyanobacterium

Amantamides are lipopeptides that act as selective CXC chemokine receptor 7 agonists and modulate spontaneous calcium oscillations in primary cultured neocortical neurons. We isolated a new analog of amantamides, amantamide C, from marine Okeania sp. cyanobacterium collected in Japan and established its structure based on NMR and MS/MS analyses, and degradation reactions. In addition, we evaluated the biological activity of amantamide C and revealed novel biological features of amantamide-type compounds.

Natural Products That Contain Higher Homologated Amino Acids

This review focuses on discussing natural products (NPs) that contain higher homologated amino acids (homoAAs) in the structure as well as the proposed and characterized biosynthesis of these non-proteinogenic amino acids. Homologation of amino acids includes the insertion of a methylene group into its side chain. It is not a very common modification found in NP biosynthesis as approximately 450 homoAA-containing NPs have been isolated from four bacterial phyla (Cyanobacteria, Actinomycetota, Myxococcota, and Pseudomonadota), two fungal phyla (Ascomycota and Basidiomycota), and one animal phylum (Porifera), except for a few examples. Amino acids that are found to be homologated and incorporated in the NP structures include the following ten amino acids: alanine, arginine, cysteine, isoleucine, glutamic acid, leucine, phenylalanine, proline, serine, and tyrosine, where isoleucine, leucine, phenylalanine, and tyrosine share the comparable enzymatic pathway. Other amino acids have their individual homologation pathway (arginine, proline, and glutamic acid for bacteria), likely utilize the primary metabolic pathway (alanine and glutamic acid for fungi), or have not been reported (cysteine and serine). Despite its possible high potential in the drug discovery field, the biosynthesis of homologated amino acids has a large room to explore for future combinatorial biosynthesis and metabolic engineering purpose.

A Review of the Antimicrobial Properties of Cyanobacterial Natural Products

The development of multiple-drug-resistant pathogens has prompted medical research toward the development of new and effective antimicrobial therapies. Much research into novel antibiotics has focused on bacterial and fungal compounds, and on chemical modification of existing compounds to increase their efficacy or reactivate their antimicrobial properties. In contrast, cyanobacteria have been relatively overlooked for antibiotic discovery, and much more work is required. This may be because some cyanobacterial species produce environmental toxins, leading to concerns about the safety of cyanobacterial compounds in therapy. Despite this, several cyanobacterial-derived compounds have been identified with noteworthy inhibitory activity against bacterial, fungal and protozoal growth, as well as viral replication. Additionally, many of these compounds have relatively low toxicity and are therefore relevant targets for drug development. Of particular note, several linear and heterocyclic peptides and depsipeptides with potent activity and good safety indexes have been identified and are undergoing development as antimicrobial chemotherapies. However, substantial further studies are required to identify and screen the myriad other cyanobacterial-derived compounds to evaluate their therapeutic potential. This study reviews the known phytochemistry of cyanobacteria, and where relevant, the effects of those compounds against bacterial, fungal, protozoal and viral pathogens, with the aim of highlighting gaps in the literature and focusing future studies in this field.

Structural Congeners of Izenamides Responsible for Cathepsin D Inhibition: Insights from Synthesis-Derived Elucidation

This study aimed to elucidate the structural congeners of natural izenamides A, B, and C (1-3) responsible for cathepsin D (CTSD) inhibition. Structurally modified izenamides were synthesized and biologically evaluated, and their biologically important core structures were identified. We confirmed that the natural statine (Sta) unit (3S,4S)-γ-amino-β-hydroxy acid is a requisite core structure of izenamides for inhibition of CTSD, which is closely related to the pathophysiological roles in numerous human diseases. Interestingly, the statine-incorporated izenamide C variant (7) and 18-epi-izenamide B variant (8) exhibited more potent CTSD-inhibitory activities than natural izenamides.

Marine Organisms as a Prolific Source of Bioactive Depsipeptides

Depsipeptides, an important group of polypeptides containing residues of hydroxy acids and amino acids linked together by amide and ester bonds, have potential applications in agriculture and medicine. A growing body of evidence demonstrates that marine organisms are prolific sources of depsipeptides, such as marine cyanobacteria, sponges, mollusks, microorganisms and algae. However, these substances have not yet been comprehensively summarized. In order to enrich our knowledge about marine depsipeptides, their biological sources and structural features, as well as bioactivities, are highlighted in this review after an extensive literature search and data analysis.

Post-translational backbone-acyl shift yields natural product-like peptides bearing hydroxyhydrocarbon units

Hydroxyhydrocarbon (Hhc) moieties in the backbone of peptidic natural products can exert a substantial influence on the bioactivities of the products, making Hhc units an attractive class of building blocks for de novo peptides. However, despite advances in in vitro genetic code reprogramming, the ribosomal incorporation of Hhc units remains challenging. Here we report a method for in vitro ribosomal synthesis of natural-product-like peptides bearing Hhc units. A series of azide/hydroxy acids were designed as chemical precursors of Hhc units and incorporated into the nascent peptide chain by means of genetic code reprogramming. Post-translational reduction of the azide induced an O-to-N acyl shift to rearrange the peptide backbone. This method allows for one-pot ribosomal synthesis of designer macrocycles bearing various β-, γ- and δ-type Hhc units. We also report the synthesis of a statine-containing peptidomimetic inhibitor of β-secretase 1 as a showcase example.

Lead/Drug Discovery from Natural Resources

Natural products and their derivatives have been shown to be effective drug candidates against various diseases for many years. Over a long period of time, nature has produced an abundant and prosperous source pool for novel therapeutic agents with distinctive structures. Major natural-product-based drugs approved for clinical use include anti-infectives and anticancer agents. This paper will review some natural-product-related potent anticancer, anti-HIV, antibacterial and antimalarial drugs or lead compounds mainly discovered from 2016 to 2022. Structurally typical marine bioactive products are also included. Molecular modeling, machine learning, bioinformatics and other computer-assisted techniques that are very important in narrowing down bioactive core structural scaffolds and helping to design new structures to fight against key disease-associated molecular targets based on available natural products are considered and briefly reviewed.

The toxic cosmopolitan cyanobacteria Moorena producens: insights into distribution, ecophysiology and toxicity

Moorena producens is a benthic filamentous cyanobacteria that has been widely documented for its toxicity. This cyanobacterium colonizes both temperate (37%) and tropical (63%) regions, making it a cosmopolitan cyanobacterium with a global distribution. M. producens grows across coral reefs in multiple locations but recurringly blooms in Queensland, Australia. Today, nuisance blooms of M. producens have resulted in major disruptions to recreational activities along coastal areas and are known to cause adverse effects on organism and human health upon contact or ingestion. Specifically, marine organisms such as the green turtle Chelonia mydas and hawksbill turtle Eretmochelys imbricata were fatally poisoned by M. producens after consumption of this cyanobacterium. Reports record a range of effects on human health, from pain and blistering or even death upon ingestion of contaminated seafood. Blooms of M. producens are triggered by influxes of nitrogen, phosphate and iron, from surrounding coastal runoffs or sewage effluents. Additions of these nutrients can result in an increase in growth rate by 4-16 times. Iron bioavailability also plays a crucial role in bloom formation. A total of 231 natural products from 66 groups were identified from M. producens, with the three dominant groups: malyngamides, microcolins and dolastatins. These bioactive secondary metabolites have displayed toxicities against a range of carcinoma cell lines and organisms such as brine shrimp Artemia salina and goldfish Carassius auratus. This review provides a thorough insight to the distribution, ecophysiology and toxicity of M. producens, with reports on bloom events and implications on organism and human health.

Therapeutic Potential of Marine Peptides in Prostate Cancer: Mechanistic Insights

Prostate cancer (PCa) is the leading cause of cancer death in men, and its treatment is commonly associated with severe adverse effects. Thus, new treatment modalities are required. In this context, natural compounds have been widely explored for their anti-PCa properties. Aquatic organisms contain numerous potential medications. Anticancer peptides are less toxic to normal cells and provide an efficacious treatment approach via multiple mechanisms, including altered cell viability, apoptosis, cell migration/invasion, suppression of angiogenesis and microtubule balance disturbances. This review sheds light on marine peptides as efficacious and safe therapeutic agents for PCa.

Exploring cyanobacterial diversity for sustainable biotechnology

Cyanobacteria are an evolutionarily ancient and diverse group of microorganisms. Their genetic diversity has 
allowed them to occupy and play vital roles in a wide range of ecological niches, from desert soil crusts to tropical oceans. Owing to bioprospecting efforts and the development of new platform technologies enabling their study and manipulation, our knowledge of cyanobacterial metabolism is rapidly expanding. This review explores our current understanding of the genetic and metabolic features of cyanobacteria, from the more established cyanobacterial model strains to the newly isolated/described species, particularly the fast-growing, highly productive, and genetically amenable strains, as promising chassis for renewable biotechnology. It also discusses emerging technologies for their study and manipulation, enabling researchers to harness the astounding diversity of the cyanobacterial genomic and metabolic treasure trove towards the establishment of a sustainable bioeconomy.

Microglial cathepsin E plays a role in neuroinflammation and amyloid β production in Alzheimer's disease

Regulation of neuroinflammation and β‐amyloid (Aβ) production are critical factors in the pathogenesis of Alzheimer's disease (AD). Cathepsin E (CatE), an aspartic protease, is widely studied as an inducer of growth arrest and apoptosis in several types of cancer cells. However, the function of CatE in AD is unknown. In this study, we demonstrated that the ablation of CatE in human amyloid precursor protein knock‐in mice, called APP^NL−G−F mice, significantly reduced Aβ accumulation, neuroinflammation, and cognitive impairments. Mechanistically, microglial CatE is involved in the secretion of soluble TNF‐related apoptosis‐inducing ligand, which plays an important role in microglia‐mediated NF‐κB‐dependent neuroinflammation and neuronal Aβ production by beta‐site APP cleaving enzyme 1. Furthermore, cannula‐delivered CatE inhibitors improved memory function and reduced Aβ accumulation and neuroinflammation in AD mice. Our findings reveal that CatE as a modulator of microglial activation and neurodegeneration in AD and suggest CatE as a therapeutic target for AD by targeting neuroinflammation and Aβ pathology.

Therapeutic potential of marine peptides in cervical and ovarian cancers

Cervical and ovarian cancers contribute significantly to female morbidity and mortality worldwide. The current standard of treatment, including surgical removal, radiation therapy, and chemotherapy, offers poor outcomes. There are many side effects to traditional chemotherapeutic agents and treatment-resistant types, and often the immune response is depressed. As a result, traditional approaches have evolved to include new alternative remedies, such as natural compounds. Aquatic species provide a rich supply of possible drugs. The potential anti-cancer peptides are less toxic to normal cells and can attenuate multiple drug resistance by providing an efficacious treatment approach. The physiological effects of marine peptides are described in this review focusing on various pathways, such as apoptosis, microtubule balance disturbances, suppression of angiogenesis, cell migration/invasion, and cell viability. The review also highlights the potential role of marine peptides as safe and efficacious therapeutic agent for the treatment of cervical and ovarian cancers.

Oncogenic KRAS promotes growth of lung cancer cells expressing SLC3A2-NRG1 fusion via ADAM17-mediated shedding of NRG1

We previously found the SLC3A2-NRG1 (S-N) fusion gene in a lung adenocarcinoma specimen without known driver mutations and validated this in 59 invasive mucinous adenocarcinoma (IMA) samples. Interestingly, KRAS mutation coexisted (62.5%) in 10 out of 16 NRG1 fusions. In this study, we examined the role of mutant KRAS in regulating the S-N fusion protein in KRAS mutant (H358) and wild-type (Calu-3) cells. KRAS mutation-mediated increase in MEK1/2 and ERK1/2 activity enhanced disintegrin and metalloproteinase (ADAM)17 activity, which increased the shedding of NRG1 from the S-N fusion protein. The cleavage of NRG1 also increased the phosphorylation of ERBB2-ERBB3 heterocomplex receptors and their downstream signalling pathways, including PI3K/Akt/mTOR, even under activated KRAS mutation signalling. The concurrence of S-N fusion and KRAS mutation synergistically increased cell proliferation, colony formation, tumour growth, and the cells' resistance to EGFR kinase inhibitors more than KRAS mutation alone. Targeted inhibition of MEK1/2, and ADAM17 significantly induced apoptosis singly and when combined with each mutation singly or with chemotherapy in both the concurrent KRAS mutant and S-N fusion xenograft and lung orthotopic models. Taken together, this is the first study to report that KRAS mutation increased NRG1 cleavage from the S-N fusion protein through ADAM17, thereby enhancing the Ras/Raf/MEK/ERK and ERBB/PI3K/Akt/mTOR pathways. Moreover, the coexistence of KRAS mutant and S-N fusion in lung tumours renders them vulnerable to MEK1/2 and/or ADAM17 inhibitors, at least in part, due to their dependency on the strong positive loop between KRAS mutation and S-N fusion.

Cyanobacteria derived compounds: Emerging drugs for cancer management

The wide diversity of cyanobacterial species and their role in a variety of biological activities have been reported in the previous few years. Cyanobacteria, especially from marine sources, constitutes a major source of biologically active metabolites that have gained great attention especially due to their anticancer potential. Numerous chemically diverse metabolites from various cyanobacterial species have been recognized to inhibit the growth and progression of tumor cells through the induction of apoptosis in many different types of cancers. These metabolites activate the apoptosis in the cancer cells by different molecular mechanisms, however, the dysregulation of the mitochondrial pathway, death receptors signaling pathways, and the activation of several caspases are the crucial mechanisms that got considerable interest. The array of metabolites and the range of mechanisms involved may also help to overcome the resistance acquired by the different tumor types against the ongoing therapeutic agents. Therefore, the primary or secondary metabolites from the cyanobacteria as well as their synthetic derivates could be used to develop novel anticancer drugs alone or in combination with other chemotherapeutic agents. In this study, we have discussed the role of cyanobacterial metabolites in the induction of cytotoxicity and the potential to inhibit the growth of cancer cells through the induction of apoptosis, cell signaling alteration, oxidative damage, and mitochondrial dysfunctions. Moreover, the various metabolites produced by cyanobacteria have been summarized with their anticancer mechanisms. Furthermore, the ongoing trials and future developments for the therapeutic implications of these compounds in cancer therapy have been discussed.

Marine-Derived Compounds for the Potential Treatment of Glucocorticoid Resistance in Severe Asthma

One of the challenges to the management of severe asthma is the poor therapeutic response to treatment with glucocorticosteroids. Compounds derived from marine sources have received increasing interest in recent years due to their prominent biologically active properties for biomedical applications, as well as their sustainability and safety for drug development. Based on the pathobiological features associated with glucocorticoid resistance in severe asthma, many studies have already described many glucocorticoid resistance mechanisms as potential therapeutic targets. On the other hand, in the last decade, many studies described the potentially anti-inflammatory effects of marine-derived biologically active compounds. Analyzing the underlying anti-inflammatory mechanisms of action for these marine-derived biologically active compounds, we observed some of the targeted pathogenic molecular mechanisms similar to those described in glucocorticoid (GC) resistant asthma. This article gathers the marine-derived compounds targeting pathogenic molecular mechanism involved in GC resistant asthma and provides a basis for the development of effective marine-derived drugs.

Phormidepistatin from the Cyanobacterium UIC 10484: Assessing the Phylogenetic Distribution of the Statine Pharmacophore

A new linear lipopeptide, phormidepistatin (1), containing an epi-statine amino acid was isolated from cf. Phormidium sp. strain UIC 10484. The planar structure was elucidated by 1D and 2D NMR experimentation. The relative configuration was determined by J-based configurational analysis and the absolute configuration by advanced Marfey's analysis. Given that the statine moiety is an established pharmacophore known to inhibit aspartic proteases, phormidepistatin was evaluated against cathepsin D and displayed limited activity. With 1 containing a statine-like moiety, we sought to assess the distribution of this γ-amino acid within the phylum Cyanobacteria. In-depth MS/MS analysis identified the presence of phormidepistatin in cf. Phormidium sp. UIC 10045 and cf. Trichormus sp. UIC 10039. A structure database search identified 33 known cyanobacterial metabolites containing a statine or statine-like amino acid and, along with phormidepistatin, were grouped into 10 distinct compound classes. A phylogenetic tree was built comprising all cyanobacteria with established 16S rRNA sequences known to produce statine or statine-like-containing compound classes. This analysis suggests the incorporation of the γ-amino acid into secondary metabolites is taxonomically widespread within the phylum. Overall, it is our assessment that cyanobacteria are a potential source for statine or statine-like-containing compounds.

Genome Reduction and Secondary Metabolism of the Marine Sponge-Associated Cyanobacterium Leptothoe

Sponges form symbiotic relationships with diverse and abundant microbial communities. Cyanobacteria are among the most important members of the microbial communities that are associated with sponges. Here, we performed a genus-wide comparative genomic analysis of the newly described marine benthic cyanobacterial genus Leptothoe (Synechococcales). We obtained draft genomes from Le. kymatousa TAU-MAC 1615 and Le. spongobia TAU-MAC 1115, isolated from marine sponges. We identified five additional Leptothoe genomes, host-associated or free-living, using a phylogenomic approach, and the comparison of all genomes showed that the sponge-associated strains display features of a symbiotic lifestyle. Le. kymatousa and Le. spongobia have undergone genome reduction; they harbored considerably fewer genes encoding for (i) cofactors, vitamins, prosthetic groups, pigments, proteins, and amino acid biosynthesis; (ii) DNA repair; (iii) antioxidant enzymes; and (iv) biosynthesis of capsular and extracellular polysaccharides. They have also lost several genes related to chemotaxis and motility. Eukaryotic-like proteins, such as ankyrin repeats, playing important roles in sponge-symbiont interactions, were identified in sponge-associated Leptothoe genomes. The sponge-associated Leptothoe stains harbored biosynthetic gene clusters encoding novel natural products despite genome reduction. Comparisons of the biosynthetic capacities of Leptothoe with chemically rich cyanobacteria revealed that Leptothoe is another promising marine cyanobacterium for the biosynthesis of novel natural products.

Diversity, molecular mechanisms and structure-activity relationships of marine protease inhibitors-A review

Marine habitats are well-known for their diverse life forms that are potential sources of novel bioactive compounds. Evidence from existing studies suggests that these compounds contribute significantly to the field of pharmaceuticals, nutraceuticals, and cosmeceuticals. The isolation of natural compounds from a marine environment with protease inhibitory activity has gained importance due to drug discovery potential. Despite the increasing research endeavours focusing on protease inhibitors' design and characterization, many of these compounds have failed to reach final phases of clinical trials. As a result, the search for new sources for the development of protease inhibitors remains pertinent. This review focuses on the diverse marine protease inhibitors and their structure-activity relationships. Furthermore, the potential of marine protease inhibitors in drug discovery and molecular mechanism inhibitor binding are critically discussed.

Functions of Bombyx mori cathepsin L-like in innate immune response and anti-microbial autophagy

Cathepsins belongs to the cysteine protease family, which are activated by an acidic environment. They play essential biological roles in the innate immunity and development of animals. Here, we identified a 62 kDa cathepsin L-like protease from the silkworm Bombyx mori. It contained putative conserved domains, including an I29 inhibitor domain and a peptidase C1A domain. The expression analysis revealed that cathepsin L-like was highly produced in the fat body, and 20-hydroxyecdysone (20 E) induced its expression. After challenge with three different types of heat-killed pathogens (Escherichia coli, Beauveria bassiana, and Bacillus cereus), the mRNA levels of cathepsin L-like significantly increased and displayed variable expression patterns in the immune tissues, suggesting its potential role in the innate immune response. The suppression of cathepsin L-like altered the expression of immune-related genes associated with the Toll and IMD pathway. Besides, autophagy-related genes such as Atg6, Atg8, VAMP2, Vps4, and syntaxin expression were also altered, indicating that cathepsin L-like regulates innate immunity and autophagy. Fluorescence microscopic analysis exhibited that cathepsin L-like was localized in the cytoplasm, and it was activated and dispersed throughout the cytoplasm and nucleus following the induction of anti-microbial autophagy. Altogether, our data suggest that cathepsin L-like may regulate the innate immune response and anti-microbial autophagy in the silkworm, B. mori.

Biologically active compounds from marine organisms in the strategies for combating coronaviruses

Despite the progress made in immunization and drug development, so far there are no prophylactic vaccines and effective therapies for many viral infections, including infections caused by coronaviruses. In this regard, the search for new antiviral substances continues to be relevant, and the enormous potential of marine resources are a stimulus for the study of marine compounds with antiviral activity in experiments and clinical trials. The highly pathogenic human coronaviruses-severe acute respiratory syndrome-related coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) remain a serious threat to human health. In this review, the authors hope to bring the attention of researchers to the use of biologically active substances of marine origin as potential broad-spectrum antiviral agents targeting common cellular pathways and various stages of the life cycle of different viruses, including coronaviruses. The review has been compiled using references from major databases such as Web of Science, PubMed, Scopus, Elsevier, Springer and Google Scholar (up to June 2020) and keywords such as 'coronaviruses', 'marine organisms', 'biologically active substances', 'antiviral drugs', 'SARS-CoV', 'MERS-CoV', 'SARS-CoV-2', '3CLpro', 'TMPRSS2', 'ACE2'. After obtaining all reports from the databases, the papers were carefully analysed in order to find data related to the topic of this review (98 references). Biologically active substances of marine origin, such as flavonoids, phlorotannins, alkaloids, terpenoids, peptides, lectins, polysaccharides, lipids and others substances, can affect coronaviruses at the stages of penetration and entry of the viral particle into the cell, replication of the viral nucleic acid and release of the virion from the cell; they also can act on the host's cellular targets. These natural compounds could be a vital resource in the fight against coronaviruses.

Characterization of Macroscopic Colony-Forming Filamentous Cyanobacteria from Okinawan Coasts as Potential Sources of Bioactive Compounds

Marine macroscopic colony-forming filamentous (MMCFF) cyanobacteria are considered as prolific producers of bioactive compounds. Thus, knowledge of the diversity of MMCFF cyanobacteria as related to bioactive compound production has become very important. However, basic taxonomic studies of MMCFF cyanobacteria are lacking. Many cyanobacterial taxa are still misidentified or undescribed. In this study, a total of 32 cyanobacterial colonies from nine coastal regions of Okinawa Prefecture were investigated for a diversity assessment. A polyphasic approach including morphological and molecular studies based on 16S rRNA gene sequences was performed to characterize Okinawan MMCFF cyanobacteria. Both morphological and molecular phylogenetic results showed that MMCFF cyanobacteria from Okinawan coasts are very diverse. We found morphotypes of Lyngbya-like, Phormidium-like, and Leptolyngbya-like groups among Okinawan cyanobacterial samples. Genetically, samples were distributed in various clades in the phylogenetic tree, including within Moorena, Okeania, Caldora, Neolyngbya, Dapis, as well as several unknown clades. In addition, cytotoxic activities of three samples from Kiyan coast were tested against HeLa cells. All three crude extracts of these samples showed strong cytotoxic activity with IC₅₀ < 1 μg/ml.

Targeting eukaryotic proteases for natural products-based drug development

Covering: up to April 2020 Proteases are involved in the regulation of many physiological processes. Their overexpression and dysregulated activity are linked to diseases such as hypertension, diabetes, viral infections, blood clotting disorders, respiratory diseases, and cancer. Therefore, they represent an important class of therapeutic targets. Several protease inhibitors have reached the market and >60% of them are directly related to natural products, even when excluding synthetic natural product mimics. Historically, natural products have been a valuable and validated source of therapeutic agents, as over half of the marketed drugs across targets and diseases are inspired by natural product structures. In the past two decades the number of new protease inhibitors discovered from nature has sharply increased. Additionally, the availability of 3D structural information for proteases has permitted structure-based design and accelerated the synthesis of optimized lead structures with improved potency and selectivity profiles, resulting in some of the most-potent-in-class inhibitors. These discoveries were oftentimes maximized by in-depth biological assessments of lead inhibitors, linking them to a relevant disease state. This review will discuss some of the current and emerging drug targets and their involvement in various disease processes, highlighting selected success stories behind several FDA-approved protease inhibitors that have natural products scaffolds as well as recent selected pharmacologically well-characterized inhibitors derived from marine or terrestrial sources.

Grassystatin-derived peptides selectively inhibit cathepsin E and have low affinity to cathepsin D

Aspartic proteases are important biomarkers of human disease and interesting targets for modulation of immune response via MHC class II antigen processing inhibition. The lack of inhibitors with sufficient selectivity hampers precise analysis of the role of cathepsin E and napsin A in samples containing the ubiquitous and highly abundant homolog cathepsin D. Grassystatins from marine cyanobacteria show promising selectivity for cathepsin E but contain several ester bonds that make their synthesis cumbersome and thus limit availability of the inhibitors. Herewith, we present grassystatin-derived cathepsin E inhibitors with greatly facilitated synthesis but retained selectivity profile. We demonstrate their affinity and selectivity with both enzyme kinetic assays and streptavidin-based pull-down from cells and mouse organs. Our findings suggest that grassystatin-like inhibitors are useful tools for targeted inhibition of cathepsin E and thus provide a novel approach for cancer and immunology research.

Marine Cyanobacteria: A Source of Lead Compounds and their Clinically-Relevant Molecular Targets

The prokaryotic filamentous marine cyanobacteria are photosynthetic microbes that are found in diverse marine habitats, ranging from epiphytic to endolithic communities. Their successful colonization in nature is largely attributed to genetic diversity as well as the production of ecologically important natural products. These cyanobacterial natural products are also a source of potential drug leads for the development of therapeutic agents used in the treatment of diseases, such as cancer, parasitic infections and inflammation. Major sources of these biomedically important natural compounds are found predominately from marine cyanobacterial orders Oscillatoriales, Nostocales, Chroococcales and Synechococcales. Moreover, technological advances in genomic and metabolomics approaches, such as mass spectrometry and NMR spectroscopy, revealed that marine cyanobacteria are a treasure trove of structurally unique natural products. The high potency of a number of natural products are due to their specific interference with validated drug targets, such as proteasomes, proteases, histone deacetylases, microtubules, actin filaments and membrane receptors/channels. In this review, the chemistry and biology of selected potent cyanobacterial compounds as well as their synthetic analogues are presented based on their molecular targets. These molecules are discussed to reflect current research trends in drug discovery from marine cyanobacterial natural products.

Cathepsin E expression and activity: Role in the detection and treatment of pancreatic cancer

Cathepsin E (CTSE) is an intracellular, hydrolytic aspartic protease found to be expressed in cells of the immune and gastrointestinal systems, lymphoid tissues, erythrocytes, and cancer cells. The precise functions are not fully understood; however, various studies have investigated its numerous cell-type specific roles. CTSE expression has been shown to be a potential early biomarker for pancreatic ductal adenocarcinoma (PDAC). PDAC patients have low survival rates mostly due to the lack of early detection methods. CTSE-specific activity probes have been developed and tested to assist in tumor imaging and functional studies investigating the role of CTSE expression in PDAC tumors. Furthermore, a CTSE protease-specific, photodynamic therapy pro-drug was developed to explore its potential use to treat tumors that express CTSE. Since CTSE is expressed in pancreatic diseases that are risk factors for PDAC, such as pancreatic cysts and chronic pancreatitis, learning about its function in these disease types could assist in early PDAC detection and in understanding the biology of PDAC progression. Overall, CTSE expression and activity shows potential to detect PDAC and other pancreatic diseases. Further research is needed to fully understand its functions and potential translational applicability.

Total Syntheses of Cathepsin D Inhibitory Izenamides A, B, and C and Structural Confirmation of Izenamide B

The first total syntheses of izenamides A, B, and C, which are depsipeptides inhibitor of cathepsin D, were accomplished. In addition, the stereochemistry of izenamide B was confirmed by our syntheses. The key features of our synthetic route involve the avoidance of critical 2,5-diketopiperazine (DKP) formation and the minimization of epimerization during the coupling of amino acids for the target peptides.

Advances in exploring the therapeutic potential of marine natural products

Marine natural products represent novel and diverse chemotypes that serve as templates for the discovery and development of therapeutic agents with distinct mechanisms of action. These genetically encoded compounds produced by an evolutionary optimized biosynthetic machinery are usually quite complex and can be difficult to recreate in the laboratory. The isolation from the source organism results in limited amount of material; however, the development of advanced NMR technologies and dereplication strategies has enabled the structure elucidation on small scale. In order to rigorously explore the therapeutic potential of marine natural products and advance them further, the biological characterization has to keep pace with the chemical characterization. The limited marine natural product supply has been a serious challenge for thorough investigation of the biological targets. Several marine drugs have reached the markets or are in clinical trials, where those challenges have been overcome, including through the development of scalable syntheses. However, the identification of mechanisms of action of marine natural products early in the discovery process is potentially game changing, since effectively linking marine natural products to potential therapeutic applications in turn triggers motivation to tackle challenging syntheses and solve the supply problem. An increasing number of sensitive technologies and methods have been developed in recent years, some of which have been successfully applied to marine natural products, increasing the value of these compounds with respect to their biomedical utility. In this review, we discuss advances in overcoming the bottlenecks in marine natural product research, emphasizing on the development and advances of diverse target identification technologies applicable for marine natural product research.

Metabolites from Marine Microorganisms, Micro, and Macroalgae: Immense Scope for Pharmacology

Marine organisms produce a large array of natural products with relevance in drug discovery. These compounds have biological activities such as antioxidant, antibacterial, antitumor, antivirus, anticoagulant, anti-inflammatory, antihypertensive, antidiabetic, and so forth. Consequently, several of the metabolites have made it to the advanced stages of clinical trials, and a few of them are commercially available. In this review, novel information on natural products isolated from marine microorganisms, microalgae, and macroalgae are presented. Given due research impetus, these marine metabolites might emerge as a new wave of promising drugs.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Natural Products from Cyanobacteria: Focus on Beneficial Activities

Cyanobacteria are photosynthetic microorganisms that colonize diverse environments worldwide, ranging from ocean to freshwaters, soils, and extreme environments. Their adaptation capacities and the diversity of natural products that they synthesize, support cyanobacterial success in colonization of their respective ecological niches. Although cyanobacteria are well-known for their toxin production and their relative deleterious consequences, they also produce a large variety of molecules that exhibit beneficial properties with high potential in various fields (e.g., a synthetic analog of dolastatin 10 is used against Hodgkin's lymphoma). The present review focuses on the beneficial activities of cyanobacterial molecules described so far. Based on an analysis of 670 papers, it appears that more than 90 genera of cyanobacteria have been observed to produce compounds with potentially beneficial activities in which most of them belong to the orders Oscillatoriales, Nostocales, Chroococcales, and Synechococcales. The rest of the cyanobacterial orders (i.e., Pleurocapsales, Chroococcidiopsales, and Gloeobacterales) remain poorly explored in terms of their molecular diversity and relative bioactivity. The diverse cyanobacterial metabolites possessing beneficial bioactivities belong to 10 different chemical classes (alkaloids, depsipeptides, lipopeptides, macrolides/lactones, peptides, terpenes, polysaccharides, lipids, polyketides, and others) that exhibit 14 major kinds of bioactivity. However, no direct relationship between the chemical class and the respective bioactivity of these molecules has been demonstrated. We further selected and specifically described 47 molecule families according to their respective bioactivities and their potential uses in pharmacology, cosmetology, agriculture, or other specific fields of interest. With this up-to-date review, we attempt to present new perspectives for the rational discovery of novel cyanobacterial metabolites with beneficial bioactivity.

Oceans as a Source of Immunotherapy

Marine flora is taxonomically diverse, biologically active, and chemically unique. It is an excellent resource, which offers great opportunities for the discovery of new biopharmaceuticals such as immunomodulators and drugs targeting cancerous, inflammatory, microbial, and fungal diseases. The ability of some marine molecules to mediate specific inhibitory activities has been demonstrated in a range of cellular processes, including apoptosis, angiogenesis, and cell migration and adhesion. Immunomodulators have been shown to have significant therapeutic effects on immune-mediated diseases, but the search for safe and effective immunotherapies for other diseases such as sinusitis, atopic dermatitis, rheumatoid arthritis, asthma and allergies is ongoing. This review focuses on the marine-originated bioactive molecules with immunomodulatory potential, with a particular focus on the molecular mechanisms of specific agents with respect to their targets. It also addresses the commercial utilization of these compounds for possible drug improvement using metabolic engineering and genomics.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Built to bind: biosynthetic strategies for the formation of small-molecule protease inhibitors

The discovery and characterization of natural product protease inhibitors has inspired the development of numerous pharmaceutical agents.

Izenamides A and B, Statine-Containing Depsipeptides, and an Analogue from a Marine Cyanobacterium

Izenamides A, B, and C (1-3), new linear depsipeptides, were isolated from a taxonomically distinct marine cyanobacterium. Izenamides A and B contain a statine moiety [(3 S,4 S)-4-amino-3-hydroxy-6-methylheptanoic acid] and inhibited the activity of cathepsin D, an aspartic peptidase. Meanwhile, izenamides did not show growth-inhibitory activity against HeLa, HL60, or MCF-7 cells at up to 10 μM.

Discovery, Synthesis, Pharmacological Profiling, and Biological Characterization of Brintonamides A-E, Novel Dual Protease and GPCR Modulators from a Marine Cyanobacterium

Five novel modified linear peptides named brintonamides A-E (1-5) were discovered from a marine cyanobacterial sample collected from Brinton Channel, Florida Keys. The total synthesis of 1-5 in addition to two other structurally related analogues (6 and 7) was achieved, which provided more material to allow rigorous biological evaluation and SAR studies. Compounds were subjected to cancer-focused phenotypic cell viability and migration assays and orthogonal target-based pharmacological screening platforms to identify their protease and GPCR modulatory activity profiles. The cancer related serine protease kallikrein 7 (KLK7) was inhibited to similar extents with an IC50 near 20 μM by both representative members 1 and 4, which differed in the presence or lack of the N-terminal unit. In contrast to the biochemical protease profiling study, clear SAR was observed in the functional GPCR screens, where five GPCRs in antagonist mode (CCR10, OXTR, SSTR3, TACR2) and agonist mode (CXCR7) were modulated by compounds 1-7 to varying extents. Chemokine receptor type 10 (CCR10) was potently modulated by brintonamide D (4) with an IC50 of 0.44 μM. We performed in silico modeling to understand the structural basis underlying the differences in the antagonistic activity among brintonamides toward CCR10. Because of the significance of KLK7 and CCR10 in cancer progression and metastasis, we demonstrated the ability of brintonamide D (4) at 10 μM to significantly target downstream cellular substrates of KLK7 (Dsg-2 and E-cad) in vitro and to inhibit CCL27-induced CCR10-mediated proliferation and the migration of highly invasive breast cancer cells.

Novel therapeutic features of disulfiram against hepatocellular carcinoma cells with inhibitory effects on a disintegrin and metalloproteinase 10

Our previous genome-wide association study identified the anti-tumor ligand MHC class I polypeptide-related sequence A (MICA) as a susceptibility gene for hepatitis C virus-induced hepatocellular carcinoma (HCC). We subsequently proved that pharmacological restoration of membrane-bound MICA in HCC cells boosted natural killer cell-mediated anti-cancer effects, confirming that a MICA sheddase, a disintegrin and metalloproteinase 10 (ADAM10), is a therapeutic target. We here searched for approved drugs with inhibitory effects on ADAM10 in vitro, and the anti-alcoholism agent, disulfiram, was identified. Disulfiram elevated membrane-bound MICA levels and reduced production of soluble MICA, an immunological decoy, while simultaneously not having unfavorable off-target effects on natural killer cells and normal human hepatocytes. Functional analyses indicated a mode of non-zinc-binding inhibition of ADAM10 by disulfiram, which also suppressed HCC cell migration. These effects of disulfiram against HCC are expected to further the development of novel therapeutic regimens.

Grassystatins D-F, Potent Aspartic Protease Inhibitors from Marine Cyanobacteria as Potential Antimetastatic Agents Targeting Invasive Breast Cancer

Three new modified peptides named grassystatins D-F (1-3) were discovered from a marine cyanobacterium from Guam. Their structures were elucidated using NMR spectroscopy and mass spectrometry. The hallmark structural feature in the peptides is a statine unit, which contributes to their aspartic protease inhibitory activity preferentially targeting cathepsins D and E. Grassystatin F (3) was the most potent analogue, with IC50 values of 50 and 0.5 nM against cathepsins D and E, respectively. The acidic tumor microenvironment is known to increase the activation of some of the lysosomal proteases associated with tumor metastasis such as cathepsins. Because cathepsin D is a biomarker in aggressive forms of breast cancer and linked to poor prognosis, the effects of cathepsin D inhibition by 1 and 3 on the downstream cellular substrates cystatin C and PAI-1 were investigated. Furthermore, the functional relevance of targeting cathepsin D substrates was evaluated by examining the effect of 1 and 3 on the migration of MDA-MD-231 cells. Grassystatin F (3) inhibited the cleavage of cystatin C and PAI-1, the activities of their downstream targets cysteine cathepsins and tPA, and the migration of the highly aggressive triple negative breast cancer cells, phenocopying the effect of siRNA-mediated knockdown of cathepsin D.

New Peptides Isolated from Marine Cyanobacteria, an Overview over the Past Decade

Marine cyanobacteria are significant sources of structurally diverse marine natural products with broad biological activities. In the past 10 years, excellent progress has been made in the discovery of marine cyanobacteria-derived peptides with diverse chemical structures. Most of these peptides exhibit strong pharmacological activities, such as neurotoxicity and cytotoxicity. In the present review, we summarized peptides isolated from marine cyanobacteria since 2007.

Glutamate-dependent ectodomain shedding of neuregulin-1 type II precursors in rat forebrain neurons

The neurotrophic factor neuregulin 1 (NRG1) regulates neuronal development, glial differentiation, and excitatory synapse maturation. NRG1 is synthesized as a membrane-anchored precursor and is then liberated by proteolytic processing or exocytosis. Mature NRG1 then binds to its receptors expressed by neighboring neurons or glial cells. However, the molecular mechanisms that govern this process in the nervous system are not defined in detail. Here we prepared neuron-enriched and glia-enriched cultures from embryonic rat neocortex to investigate the role of neurotransmitters that regulate the liberation/release of NRG1 from the membrane of neurons or glial cells. Using a two-site enzyme immunoassay to detect soluble NRG1, we show that, of various neurotransmitters, glutamate was the most potent inducer of NRG1 release in neuron-enriched cultures. NRG1 release in glia-enriched cultures was relatively limited. Furthermore, among glutamate receptor agonists, N-Methyl-D-Aspartate (NMDA) and kainate (KA), but not AMPA or tACPD, mimicked the effects of glutamate. Similar findings were acquired from analysis of the hippocampus of rats with KA-induced seizures. To evaluate the contribution of members of a disintegrin and metalloproteinase (ADAM) families to NRG1 release, we transfected primary cultures of neurons with cDNA vectors encoding NRG1 types I, II, or III precursors, each tagged with the alkaline phosphatase reporter. Analysis of alkaline phosphatase activity revealed that the NRG1 type II precursor was subjected to tumor necrosis factor-α-converting enzyme (TACE) / a Disintegrin And Metalloproteinase 17 (ADAM17) -dependent ectodomain shedding in a protein kinase C-dependent manner. These results suggest that glutamatergic neurotransmission positively regulates the ectodomain shedding of NRG1 type II precursors and liberates the active NRG1 domain in an activity-dependent manner.

Synthesis and Structure Revision of Symplocin A

Symplocin A, a linear peptide possessing N-terminal N,N-dimethylisoleucine, statine, and valic acid residues, has been synthesized for the first time employing our previously established 'one-pot intramolecular tandem protocol'. Moreover, the stereochemistry of natural symplocin A was unambiguously revised through the confirmation by 1D NMR, 2D NMR, and HPLC comparisons with authentic natural product.

Trichophycin A, a Cytotoxic Linear Polyketide Isolated from a Trichodesmium thiebautii Bloom

In an effort to isolate and characterize bioactive secondary metabolites from Trichodesmium thiebautii blooms, collected cyanobacteria biomass was subjected to bioassay-guided extraction and fractionation using the human colon cancer cell line HCT-116, resulting in the isolation and subsequent structure characterization of a linear polyketide trichophycin A (1). The planar structure of 1 was completed using 1D and 2D NMR spectroscopy and high-resolution electrospray ionization mass spectrometry (HRESIMS). Trichophycin A was moderately toxic against the murine neuroblastoma cell line Neuro-2A (EC50: 6.5 μM) and HCT-116 cells (EC50: 11.7 μM). Trichophycin A was significantly more cytotoxic than the previously isolated polyketides trichotoxin A and trichotoxin B. These cytotoxicity observations suggest that toxicity may be related to the polyol character of these polyketide compounds.

Tasiamide F, a potent inhibitor of cathepsins D and E from a marine cyanobacterium

In search of novel protease inhibitors with therapeutic potential, our efforts exploring the marine cyanobacterium Lyngbya sp. have led to the discovery of tasiamide F (1), which is an analogue of tasiamide B (2). The structure was elucidated using a combination of NMR spectroscopy and mass spectrometry. The key structural feature in 1 is the presence of the Phe-derived statine core, which contributes to its aspartic protease inhibitory activity. The antiproteolytic activity of 1 and 2 was evaluated in vitro against cathepsins D and E, and BACE1. Tasiamide F (1) displayed IC50 values of 57nM, 23nM, and 0.69μM, respectively, indicating greater selectivity for cathepsins over BACE1 compared with tasiamide B (2). Molecular docking experiments were carried out for compounds 1 and 2 against cathepsins D and E to rationalize their activity towards these proteases. The dysregulated activities of cathepsins D and E have been implicated in cancer and modulation of immune responses, respectively, and these proteases represent potential therapeutic targets.

Diastereoselective Synthesis and Conformational Analysis of (2R)- and (2S)-Fluorostatines: An Approach Based on Organocatalytic Fluorination of a Chiral Aldehyde

Stereoselectively fluorinated analogues of the amino acid statine have been efficiently synthesized. The key step is an organocatalytic electrophilic fluorination of a chiral β-oxygenated aldehyde, which provided a test of both diastereoselectivity and chemoselectivity. The target statine analogues were found to adopt unique conformations influenced by the fluorine gauche effect, rendering them potentially valuable building blocks for incorporation into bioactive peptides.

Natural Products as Cathepsin Inhibitors

Cathepsins are proteases found in all animals as well as other organisms. There are approximately a dozen members of this family, which are distinguished by their structure, their catalytic mechanism, and which proteins they cleave. Most of the members become activated at the low pH found in lysosomes. Cathepsins have been identified as therapeutic targets in the search for new drugs against a number of human pathologies, including cancer, Alzheimer's, and osteoporosis.

A number of natural products have been reported as selective inhibitors of some cathepsins. Chemical structure of natural products as inhibitors of cathepsins can be very diverse. Some peptidic natural products are inhibitors of the cysteine protease cathepsins such as E-64 isolated from Aspergillus, which is a cathepsin B inhibitor, or more recently the marine cyanobacterial metabolite gallinamide A which is a selective inhibitor of human cathepsin L. Also amino acid derivatives have been reported as inhibitors of cathepsin A.

Other natural products include chalcone natural products possessing cytotoxic activities against prostate cancer cells and inhibiting cysteine cathepsins in vitro, antipain and its analogues isolated from Streptomyces as inhibitors of cathepsin K, and natural biflavones as novel inhibitors of cathepsins B and K.

In this review we will report the most representative examples of natural products as inhibitors of cathepsins, especially the ones reported during the last decade.

Biological targets and mechanisms of action of natural products from marine cyanobacteria

Marine cyanobacteria are an ancient group of organisms and prolific producers of bioactive secondary metabolites. These compounds are presumably optimized by evolution over billions of years to exert high affinity for their intended biological target in the ecologically relevant organism but likely also possess activity in different biological contexts such as human cells. Screening of marine cyanobacterial extracts for bioactive natural products has largely focused on cancer cell viability; however, diversification of the screening platform led to the characterization of many new bioactive compounds. Targets of compounds have oftentimes been elusive if the compounds were discovered through phenotypic assays. Over the past few years, technology has advanced to determine mechanism of action (MOA) and targets through reverse chemical genetic and proteomic approaches, which has been applied to certain cyanobacterial compounds and will be discussed in this review. Some cyanobacterial molecules are the most-potent-in-class inhibitors and therefore may become valuable tools for chemical biology to probe protein function but also be templates for novel drugs, assuming in vitro potency translates into cellular and in vivo activity. Our review will focus on compounds for which the direct targets have been deciphered or which were found to target a novel pathway, and link them to disease states where target modulation may be beneficial.

Recent developments in therapeutic applications of Cyanobacteria

The cyanobacteria (blue-green algae) are photosynthetic prokaryotes having applications in human health with numerous biological activities and as a dietary supplement. It is used as a food supplement because of its richness in nutrients and digestibility. Many cyanobacteria (Microcystis sp, Anabaena sp, Nostoc sp, Oscillatoria sp., etc.) produce a great variety of secondary metabolites with potent biological activities. Cyanobacteria produce biologically active and chemically diverse compounds belonging to cyclic peptides, lipopeptides, fatty acid amides, alkaloids and saccharides. More than 50% of the marine cyanobacteria are potentially exploitable for extracting bioactive substances which are effective in killing cancer cells by inducing apoptotic death. Their role as anti-viral, anti-tumor, antimicrobial, anti-HIV and a food additive have also been well established. However, such products are at different stages of clinical trials and only a few compounds have reached to the market.