Cyanobacteria Natural Products as Sources for Future Directions in Antibiotic Drug Discovery

Abstract

Cyanobacteria, an abundant source of natural products with a broad diversity of secondary metabolites, have emerged as a novel resource for the progression of synthetic analogs. Due to the rise of antibiotic resistance, there is a need for new medications and cyanobacteria-derived compounds have shown promising important alternatives for new therapeutics. These secondary metabolites are produced through nonribosomal peptide synthetase (NRPS), polyketide synthase (PKS), and mainly through mixed NRPS-PKS enzymatic systems. Current research is focused on the exploitation of cyanobacteria for the production of bioactive metabolites. Screening of cyanobacteria for pharmaceutically active compounds has received increasing attention; however, limited knowledge is available on biosynthetic mechanisms that would enhance the drug discovery process and culture-based production of desired metabolites. Overall, there is a promising outlook that cyanobacterial secondary metabolites will become alternatives for the development of new medications in a near future with enhanced pharmacological and pharmacokinetic properties.

1. Introduction

Antibiotics, the so-called “miracle drugs,” came into existence half a century ago; however, their current popularity swiftly leads to overuse. Over the last decade, it has become quite apparent that the efficiency of antibiotics is dropping due to the growth of pathogen resistance; a problem that increases as fewer new drugs become available in the market. Moreover, unraveling this resistance is not straightforward, since antibiotic resistance is actually produced in multiple ways. Considering the urgency of the issue, efforts to develop new antibiotics are being carried out by pharmaceutical companies. In this regard, natural products account for a thorough and important component of today’s pharmaceutical compendium as a fundamental source of chemical diversity. To date, several natural products have been studied, but many others still await investigation [1]. Cyanobacteria, being one of the eldest recognized creatures living on the earth with exclusive structural features, produce several bioactive compounds with varied biological activities. Moreover, cyanobacteria as photosynthetic microorganisms, which have been preserving the oxygen levels on the earth, structurally look like gram-negative bacteria. They include chlorophyll a and phycobiliproteins, as well as the photosystems II and I. The adaptation mechanisms shown by cyanobacteria allow them to survive in severe climate conditions and tolerate limiting factors, such as heat, drought, salinity, nitrogen starvation, cold, photo-oxidation, osmotic, and UV stress [2]. Additionally, cyanobacteria are able to produce biologically active natural products with known antifungal, antibacterial, anti-inflammatory, antiviral, and enzyme-inhibiting bioactivities mostly through either the nonribosomal polypeptide (NRP) or the mixed polyketide-NRP biosynthetic pathways [3]. An increasing number of cyanobacterial metabolites are found to target actin and tubulin filaments in eukaryotic cells, making them a noteworthy source of anticancer natural products. Definite bioactive compounds, for example, dolastatin-10 and curacin A, have gone through clinical trials as possible anticancer drugs [4]. Cyanobacterial bioactive products can be categorized consistently with diverse structural typologies comprising terpenes, polyketides, peptides, lipids, and alkaloids. Many structural modifications can be found in cyanobacterial compounds, especially polyketide-derived units [3]. Besides, each cyanobacterial strain produces a category of bioactive compounds, so that new drugs are being constantly discovered from these sources.

Along with all these advantageous features, cyanobacteria are also known to produce toxins, mainly neurotoxins and hepatotoxins [2, 5], which act also as activators (e.g., antillatoxin) or blockers (e.g., jamaicamide A and kalkitoxin) and in addition their possible neuroprotectant and analgesics properties, they are functional molecular to distinguish usefully channels [4, 6, 7, 8].

Patellamide and trunkamide have also clinical potential, showing moderate cytotoxicity but multi-drug resistance. Investigations about the cyanobacterial natural product and secondary metabolites have gradually adapted to the genomic revolution over the past 15 years, and the genetic characterization of these secondary metabolites has led to further investigations in the field of cyanobacterial natural product synthesis. Despite important achievements in this area, numerous pharmaceutical companies have decreased the use of natural bioactive products and drug discovery screening because of: a) difficulties associated with strain, b) troubles correlated to natural bioactive products, and c) problems with logical property rights [9, 10, 11, 12, 13, 14]. Finally, the use of compound collections prepared by combinatorial chemistry methods has been also influential.

2. Improving access to natural products

It is now evident that the chemical diversity of natural products is a better option than the variety of available synthetic compounds for drug discovery [15, 16]. Therefore, the use of natural chemical diversity in this regard is becoming increasingly frequent [11, 17]. Early publications showed that only a small number of cyanobacteria taxa were accessible for screening [9]. Now, extensive cyanobacteria collections, together with better cyanobacteria culture techniques, are providing new chemicals for use in drug discovery assays [11]. Progress is being made in the chemistry of natural products, leading to advances in synthetic methods seeking the production of compound analogs with enhanced pharmacological or pharmaceutical characteristics [18]. Another interesting feature that has made natural product “privileged” structures is their ability to be used as cores of compound (alkaloids, polyketides, terpenoids, and flavonoids) libraries produced through combinatorial techniques [19, 20].

Over the past 30 years, there has been a considerable reduction in the interest by the leading pharmaceutical companies in drug discovery from natural sources. Despite this, phycologists, associated with the manufacturing industry, are exploiting this niche so that there is now a renaissance related to new improvements in spectroscopy, analytical technologies, and high-throughput screening [21]. In addition, competing technologies, such as combinatorial chemistry, have not proved to be very successful in delivering the new drug in significant numbers [22]. With the use of alternative techniques to produce analogs and derivatives of natural products, new compounds can be patented, even if the primary structure had been previously disclosed [11].

3. New approaches to the value of natural products

Multitude reasons have been suggested in regards to why natural products are such appropriate sources for drug leads, but at least one study has endeavored to quantify a connection between the drug molecules and those typically found in natural products and combinatorial chemical libraries [22]. Combinatorial libraries are synthesized in large numbers, and structures have high randomness. A multivariate evaluation of the chemical space occupied by thousands of combinatorial drug compounds compared with that of natural products revealed a good correlation between clinically approved drug molecules with natural products. This means that the structure of drugs used nowadays can be simulated by that of natural products [15]. With the progress in analytical spectroscopy, numerous clarifications are currently accessible so that the discovery of new bioactive compounds needs only a few micrograms [22]. The improvement in fractionation methods intended for isolating and purifying natural bioactive products (counter-current chromatography [20], analytical structure determination [23], etc. has led to screening natural product mixtures with timescales suitable for those expected in high-throughput screening campaigns. Complex structures can be resolved now with much less than 1 mg of the compound using the recent NMR techniques [11]. According to Quinn (developing a drug-like natural product library, 2008), it is possible to prepare a screening a library of highly diverse plant-derived compounds by pre-selecting products from the dictionary of natural products to be drug-like in their physicochemical properties. Yet, many alternative approaches are also being tested in order to enhance the speed and efficiency of drug discovery from natural products [11]. For instance, bioinformatics has been used for predicting microbes, which are able to produce new chemicals on the basis of the gene sequences encoding polyketide synthesis; this method has led to the discovery of potential antifungal and anticancer activities in some compounds [24]. Furthermore, the Metagenomics approach, which has led to the discovery of antibiotic compounds, has been recently used to achieve a broader range of synthetic cyanobacterial capabilities. This involves the collection of the entire DNA from a field cyanobacteria sample and the cloning of this DNA in host organisms, such as E. coli. Recombinant bacteria are subsequently cultured and examined for the expression of bioactive metabolites [11]. Additionally, peptide synthetase genes and polyketide synthase genes have been explored, and manipulation of biosynthetic pathways in refractory microbes, such as uncultivable, is a promising line of research. Along with the most innovative tools of genetic engineering, new approaches to metagenomic mining of environmental DNA are being popularized, so that the genetic potential of many bacteria can be explored [25]. Even though more than 200 genome projects are either already completed or still undergoing publication, there are still some striking questions on what is actually being sequenced, considering the fact that these studies are limited to cultivable microbes. The Metagenomics approach, being culture-independent, can help to solve this problem and can also help with data mining with potential interest for a broad scientific community [25]. Different techniques unite enzymatic and synthetic methods to achieve multifaceted natural bioactive products, and refining the activity of obviously occurring antibiotics [11]. Mutasynthetic techniques are useful for making the antibiotic daptomycin-associated compounds [26, 27], vancomycin analogs and anticancer compound cryptophicin have been formed using the cytochrome P450 enzymes [12]. The biosynthesis of cyanobacterial compounds supports the creation of numerous functional groups, chiefly in the gene clusters related to cyanobacterial compounds, for instance, jamaicamide A, barbamide, or curacin A [28]. Hence, undescribed enzymatic mechanisms will be revealed thanks to biochemical studies in cyanobacterial secondary metabolic pathways. From the experience in the production of pharmaceuticals from invertebrate-derived microbes, it is evident that several obstacles must be overcome before this approach becomes a conventional technology. Still, there are good reasons to be optimistic about the future [22].

4. Activity profiling of extracts

An alternative technique to the time-consuming and expensive methods previously used for creating extensive collections of isolated and structurally characterized natural products [29] is screening the mixtures of compounds obtained from extracts of cyanobacteria strains [11]. Yet, obtaining extracts with potential biologically active novel compounds is not always simple from primary screenings. This probability can be predicted by comparing the ratio of the binding potencies at two receptor sites for a known selective ligand and for an extract by the “differential smart screens” method [30]. Furthermore, by means of a database of the usefulness of an extensive variety of identified bioactive compounds the analysis of drugs with the unknown process is imaginable. Therefore, information about previously unidentified compounds can be gained, which is precious for the antibiotic applications stated below [31]:

1. Creation of original whole-cell assays for drug screening, such as multi-patch.

2. Target identification with cDNA and quantitative real-time PCR (qRT-PCR) for confirmation of the results.

3. Revisions on mechanisms-of-action (MOA) with antibiotic-induced expression profiling.

These techniques could lead to a novel understanding of the potential effects of untested compounds (or exposure to compounds not structurally analogous and, thus, not expected to act via the same biological target) [2].

Bioinformatics and proteomics experiments are used in studies at the mRNA (transcriptome) or protein (proteome) levels, which help with the identification of DNA binding sites of transcription factors [32] and the adjustment of biological functions, respectively, in order to characterize the complex organism responses to environmental stimulates [2]. Microarrays have been used for the identification of regulon members and stimulons by many groups in the transcriptome measurement level [33, 34].

Two-dimensional gel electrophoresis in which proteins are separated according to their molecular weight and isoelectric point, is useful in most cases, but intricate protein samples can also be analyzed using the liquid chromatography-tandem MS (LC-MS/MS) in which protein and peptide combinations are supplied to a mass spectrometer (MS) from a HPLC system. Isotopic dilution strategies on a MS instrument (e.g., isotope-coded affinity tags or ICAT) can be used for a comparative quantification of protein expression. ICAT approaches were advantageous when first released but are limited by their inability to analyze more than two conditions without a large amount of multiplexing [2, 35]. Currently, a developed version of the iTRAQ approach can analyze eight different conditions simultaneously. Despite all these tools, the most useful method would involve a concurrent quantification of the expression of all the genes and proteins of interest from a biological sample.

5. Natural products as pharmacological instruments

Aside from their curative activity, natural bioactive products can operate as pharmacological instruments demonstrating novel physiological features [14]. Cyanobacteria are stubbornly obstinate to genetic manipulation, which is accessible only for a small number of strains [3]. The modularity in cyanobacterial PKS-NRPS gene clusters authorizes the heterologous expression of natural bioactive products and, thus, genetic manipulation for combinatorial biosynthesis of innovative hybrid chemical bioactive products [4]. The prosperous production of nonribosomal and ribosomal peptides in heterologous hosts permits the usage of other cyanobacterial natural bioactive products [3]. Cyanobacteria usually synthesize multiple variants of the identical natural bioactive product; this can be ascribed to a deficiency of the inactivity of the NRPS tailoring enzymes or NRPS biosynthetic pathways. The genetic basis for this modification of secondary metabolite gene clusters is probably controlled by gene duplications, gene deletions, recombination, sequential mutation followed by natural selection, and loss and gain of tailoring enzymes [36]. However, the evolutionary and adaptive importance of these processes is deficiently understood.

6. Which cyanobacteria phyla produce therapeutics?

Throughout the prior decade, several natural bacterial compounds have been described, all of which originated from five bacterial phyla: Bacteroidetes (34 compounds), cyanobacteria (220), actinobacteria (256), proteobacteria (78), firmicutes (35), and four bioactive compounds from taxonomically unknown sources [37]. The variety of cyanobacterial natural bioactive products gathers > 1100 secondary compounds recognized with composite chemical structures, stated from different genera [3]. These metabolites represent a broad range of bioactivities including some that may be related to their natural environment (antibacterial, antifungal, antiviral, and cytotoxic) [29], but others demonstrate a clear pharmaceutical interest, for example, they can be used as anticancer agents, immunomodulators, or protease inhibitors [38]. Cyanobacteria exhibit different growth forms, from unicellular to filamentous or colonial forms, and depending on their environmental conditions they may be surrounded by a mucilaginous or gelatinous sheath [29]. The PKS and NRPS genes seem to be more widespread in undifferentiated filamentous and heterocystous cyanobacterial strains. Despite the current taxonomic instability within cyanobacteria, which makes assessing the actual occurrence of natural products difficult, cyanobacterial compounds are mainly obtained from the lyngbya, symploca, microcystis, nostac, and hapalosiphon (Table 1) [3, 37].

Cyanobacterial compounds (class)	Cyanobacterial strain	Biological target	Potential therapeutic uses	References
Apratoxin A	Lyngbya bouillonii	STAT3, KB, and LoVo cell lines; Cytotoxic against human tumor cell lines (0.36–0.52 nM)	Oncology, Early stage adenocarcinoma (induction of G-1 phase cell cycle arrest)	[4, 29, 37, 39]
Apratoxin D	Lyngbya sp.	Antiproliferative	Oncology	[40]
Coibamide A	Leptolyngbya.	Antiproliferative	Oncology	[41]
Curacin A-D; NRPS-PKS	Lyngbya majuscula 19 L	Colon, renal, and breast cancer cell lines. Involvement of HMG-CoA in formation of cyclopropyl ring	Oncology, Antimitotic, Inhibits microtubule assembly; Anti-inflammatory, Antiproliferative, Immunosuppressant, herbicidal	[4, 29, 37, 42, 43]
Cryptophycin	Nostoc sp.	Tubulin polymerization antiproliferative and antimitotic agents, Cytotoxicity against human tumor cell lines and human solid Tumors	Oncology, destabilization of microtubule dynamics and the induction of hyperphosphorylation of the anti-apoptotic protein B-cell leukemia/lymphoma 2 (BCl-2),triggering programed cell death	[37, 44, 45]
Largazole	Symploca sp.	Histone deacetylase	Oncology, anti-epileptics, neurological disorders, mood stabilizer	[46]
Microcystin	Microcystis aeruginosa PCC 7806,M,aeruginosa K-139; Planktothrix agardhii CYA126	Lymphocytes	Cytotoxic, inhibit membrane-bound leucine aminopeptidase; Enzyme inhibitor, cytotoxic, tumor promoter, anticancer	[47, 48, 49, 50, 51]
Hassallidins	Anabaena sp. SYKE 748A		Antifungal activity	[52]
Sulfoglycolipid	Scytonema sp.	HIV-1	Inhibit reverse transcriptase and DNA polymerases	[29]
Dolastatin-10	Symploca sp.	Binds to tubulin on rhizoxin-binding Site	Affects microtubule assembly in P388 lymphocytic leukemia cell line (NCI)	[53, 54]
Dolastatin-15	Lyngbya sp.	binds to vinca alkaloid site of tubulin	Breast cancers treatment	[55]
Jamaicamides (A-C)	L. majuscula	H-460 human lung cell carcinoma, neuro-2A-neuroblastoma cell line	Neurotoxic, cytotoxic against H-460 human lung and neuro-2a mouse neuroblastoma cell lines	[56]
Kalkitoxin	L. majuscula	Block voltage sensitive Na+ channel	Neurotoxic, Neural necrosis through N-methyl-D-aspartate Receptor mechanisms	[57]
Astaxanthin	Haematococcus pluvialis	Colon cancer cell lines	Expression decrease of cyclin D1, increase of p53 and some cyclin kinase inhibitors (p21WAF-1/CIP-1, p27)	[58]
Polysaccharide	Navicula directa	HSV1, 2, influenza A virus	Inhibition of hyaluronidase	[59, 60]
Allophycocyanin	Cryptomonads	Enterovirus 71	Inhibition of cytopathic effect, delay in synthesis of viral RNA	[61]
Hectochlorin	L. majuscula	Colon, melanoma, ovarian	Actin binding compounds,	[62]
Diadinochrome A, B, Diatoxanthin, cynthiaxanthin	Peridinium bipes	HeLa cells	Cytotoxic effect	[63]
Pheophorbide a-, b-like compounds	Dunaliella primolecta	HSV1	Inhibition of cytopathic effect	[64]

Table 1.

Current status of potential cyanobacteria therapeutics.

7. Cyanobacterial drug discovery

Systems biology can help us with the acquisition consciousness of the ways living systems function using computational power [65]. So as to study some specific facts in a definite biosynthetic pathway, some information about both the proteins in charge and the responsible gene of that event is needed. The function of linking the chemical diversity of natural bioactive products and genomes in addition to modeling and prediction by incorporating such biological information could offer considerable information for the understanding of such a complex biological system [2].

According to the Comprehensive Microbial Resource Declaration, the genome sequences of human pathogenic bacteria and non-homologous in humans, have been documented. This could be an appropriate technique for the reporting of the new drug [66], and the improvements in synthetic biology now provide a solution to cyanobacteria being stubborn to genetic manipulation, opening up cyanobacteria as a valuable source of new enzymes and novel natural bioactive products.

Today pharmaceutical industry is concentrated on prominent output screening systems, genomics tools, and bioinformatics, containing combinatorial chemistry and logical design for the recognition of new bioactive compounds [29]. Recognizing groups of secondary bioactive compounds biosynthetic gene clusters with possible therapeutic competence involvement in an initial stage, which is conducted by the chemical structure of the identified bioactive compounds in cyanobacteria strains [3]. Cyanobacterial biologically active compounds are produced through NRPS, PKS, and mixed NRPS-PKS pathways [4]. Cyanobacteria strains presentation progressive screening outcomes are then designated for proteome mining and genomic characterization in order to classify biosynthetic gene clusters responsible for proteins connected to the making of these bioactive components [2]. This is imaginable because databases of biosynthetic gene clusters and cyanobacterial chemicals have been gathered through gene libraries (http://dtp.nci.nih.gov/docs/3d_database/dis3d.html, NCBI Pubchem http://pubchem.ncbi.nlm.nih.gov/, ChemIDPlus http://chem.sis.nlm.nih.gov/chemidplus, ANTIMIC [67], and Super Natural Database http://bioinformatics.charite.de/supernatural/) [68]. As a result of the increased antibiotic resistance, available drugs are effective against only one-third of the diseases, and the identification of new biologically active compounds is thus urgently necessary [29].

8. Web-based tools and databases for drug target identification

A variety of different silico tools and databases are available for drug target determination among the identified genes in pathogens for an initial screening. DrugBank(http://insilico.charite.de/supertarget/ main.html#Home), NCBI Entrez Gene(http://www.ebi.ac.uk/msd/), TarFisDock and MATADOR (http://matador.embl.de/) could be used either by a manual searching or by BLAST search of sequenced proteins. These facilities compensate the costs of screening through very large compound collections, minimizing the pace of drug discovery by both reducing the number of compounds used in real screens and the costs of screening [2].

9. Secondary metabolites derived from Cyanobacteria strains

Natural bioactive products have been isolated from a varied diversity of strains and verified for numerous biological activities. Among these strains, cyanobacteria strains signify such a source.

Secondary metabolites derived from cyanobacteria strains were identified as a rich source of bioactive compounds [69, 70, 71]. Several bioactive compounds isolated from different cyanobacterial strains showed a varied range of chemical structures and biological activities comprising new peptides, amides, terpenes, carbohydrates, polyketides, fatty acids, alkaloids, and other organic chemicals [41, 72, 73, 74]. These compounds are regarded as good candidates for drug discovery, with functions in the industry [75, 76, 77], agriculture [19], and in pharmacy [69, 77, 78].

The cyanobacterial bioactive compounds specify useful pharmaceuticals that are problematic to produce synthetically [79]. The variety of structures found in Lyngbya majuscula is just incredible. Compounds isolated from this strain are amino acids, fatty acids, depsipeptides, pyrroles, amides, alkaloids, lactones, lipopeptides, and many others [40, 72, 80, 81]. Totally, cyanobacterial bioactive compounds show an exciting range of biological activities ranging from insecticidal, immunosuppressant, antiviral, anticancer, antimicrobial, and anti-inflammatory to proteinase-inhibiting activities which are outstanding targets of biomedical research (Table 2) [ 2, 5, 6, 7, 8, 78, 113, 114, 115, 116, 117, 118].

Species of cyanobacteria	Bioactive compounds	Biological activity	Class	References
Lyngbya Lagerheimii; Phormidium tenue	Sulfolipid	Anti-HIV-1 activity	Fatty Acid (sulfo)	[82]
Lyngbya majuscula 19 L	Barbamide	Antimolluscicidal	chlorinated lipopeptide	[83, 84]
L. majuscule; L. majuscula	Antillatoxin; Antillatoxin B	Neurotoxic; Ichthyotoxic, activator of voltage-gated sodium channel	Cyclic lipopeptide	[83, 85, 86]
Synechocystis trididemni	Didemnin	Anticancer, antiviral, immunosuppressive	Lipopeptide	[87]
Cylindrospermum licheniforme	Cylindrocyclophane	Anticancer, cytotoxic	Alkaloid; Macrocycle, chloro	[43, 88, 89]
Cylindrospermopsis raciborskii	Cylindrospermopsin	Cytotoxic	Alkaloid	[90]
Prochloron didemni	Patellamide A, B, C and D	Cytotoxic, biological activity against multi-drug resistant UO-31 renal cell lines	Cyclic lipopeptide	[28, 91, 92, 93]
L. majuscula	lyngbyabellins A and B,	Cytotoxic, Anticancer, Cytoskeleton disruption	Lipopeptides	[94]
L. majuscula	Antillatoxin; Antillatoxin B	Neurotoxic; Ichthyotoxic, activator of voltage-gated sodium channel, with sodium channel-activating	Lipopeptide	[4, 83, 85, 86]
Lyngbya semiplena	Semiplenamides A-G	All displayed weak to moderate toxicity in brine shrimp assay; and 38 showed weak affinity for the rat cannabinoid CB1 receptor; showed moderate inhibitor of anandamide membrane transporter	Lipopeptide	[4]
Nostoc ellipsosporum	Cyanovirin	Anti-HIV, antiviral HIV-1 (interacts with high mannose groups of envelope glycoproteins, gp120 and blocks its interaction with target cell receptors) HIV-2; HSV-6; Measles virus; SIV; FIV	Peptide and proteins	[29, 95]
P. tenue	Monogalactopyranosyl glycerol digalactopyranosyl glycerol	Anti-HIV, anticancer	Sulfolipids	[96]
Calothrix sp.	Calothrixin	Antimalarial, anticancer Against HeLa epithelial carcinoma	Indoles	[97]
Symploca hydnoides; Symploca sp VP453	Symplostatin 1; Symplostatin 3	Against Murine colon 38 and murine mammary 16/C cell lines Against Microtubule depolymerization	analog of dolastatin-10	[54, 98]
L. majuscula	Lyngbyatoxins A-C	Cytotoxic, Ichthyotoxic, Tumor promoter, Protein Kinase C activator, Skin irritant		[2, 43]
L. majuscula	Hectochlorin	Against Colon, melanoma, ovarian and renal and lung cancer cell lines, promote actin polymerization	Lipopeptide	[4, 62]
Lyngbya majuscule	Hermitamides A and B	Ichthyotoxic, Brine shrimp toxicity, Cytotoxic		[43]
L. majuscula	Somocystinamide A	Cytotoxic against neuro-2a neuroblastoma cells (IC50 = 1.4 lg/mL), Pluripotent inhibitor of angiogenesis and tumor cell proliferation, Induces apoptosis in endothelial cells.	Lipopeptide	[4, 99]
Oscillatoria Nigroviridis	Oscillatoxin	Anticancer, Toxic general	Aromaic	[43]
Microcystis aeruginosa	Microviridin	Antibiotic, anticancer	tricyclic depsipeptides	[100, 101]
Lyngbya sp.	Kempopeptins A, B	Against colon cancer	Cyclodepsipeptides	[102]
L. majuscula	Hermitamides AeB	Against lung cancer, Potent blockers of the hNav1.2 voltage-gated sodium channel., Ichthyotoxic	Lipopeptide	[43, 103]
L. majuscula	Lyngbyatoxin A-C	Ichthyotoxic, Cytotoxic, Tumor Promoter, Protein Kinase C activator, Skin irritant		[43]
Caulerpa taxifolia; Green algae	Caulerpenyne	Cytotoxicity, anticancer, antitumor, and antiproliferative activities		[104]
L. majuscula	Carmabin A-B	Anticancer, Antiproliferative	N-Methylated Peptide	[43]
Cyanobacteria Nostoc linckia and Nostoc spongiaeforme var. tenue	Boromycin	Cytotoxicity against human epidermoid carcinoma (LoVo), human colorectal adenocarcinoma activity, potent cytotoxicity against drug-resistant murine and human solid tumors	Polyether-macrolide antibiotic	[97, 105]
L. majuscula	Majuscolamid A-D	Antifungal, Antimycotic activity		[106]
L. majuscula	Microcolin A-C	Antiproliferative, Anticancer, Cytotoxic, Immunosuppressive		[107]
L. majuscula	Malyngamide A-U	Antimicrobial, Antifeedant, Cytotoxic, Immunosuppressive	Lipopeptide	[43]
L. majuscula	Pitiamide A-B	Antifeedant	Fatty Acid Amides	[107, 108]
L. majuscula	Yanucamides A and B	Brine shrimp toxicity	Depsipeptides	[43]
Nodularia spumigena	Nodularia toxin	Enzyme inhibition	Lipopeptide	[2, 109, 110]
Aulosira fertilissima	Aulosirazole	Anticancer	Aromatic	[111]
Oscillatoria acutissima	Acutiphycin and 20,21-didehydroacutiphycin	Antineoplastic agent	Lipopeptide	[112]

Table 2.

Bioactive compounds from cyanobacteria.

10. Antiviral activity

The extension of fatal, virus-related diseases, such as HIV, has resulted in several considerable consequences. Since the only accredited therapy (HAART, highly active antiretroviral therapy) has shown toxic effects, severe induction to viral resistance, and disability to eliminate viral agents, thus the need for new and safe antiviral therapies is an urgent issue [119, 120]. Some potential antiviral compounds are described below:

10.1 Polysaccharides

Spirulan and Ca-spirulan derived from Spirulina sp. are regarded as the most notable antiviral polysaccharide compounds provided their broad-spectrum activity against HIV-1, HIV-2, H, influenza and other enveloped viruses. These compounds disable the reverse transcriptase activity of HIV-1 and prevent the attachment and fusion of virus cells with host cells. Additionally, the fusion between HIV-infected and uninfected CD4+ lymphocytes, which boosts the viral infectivity, is inhibited [29]. Their reduced anticoagulant properties make them more advantageous antiviral agents over other sulfated polysaccharides. Another interesting compound is nostoflan from Nostoc flagelliforme, an acidic polysaccharide showing potent virucidal activity against herpes simplex virus-1 [121, 122].

10.2 Carbohydrate-binding proteins

A couple of carbohydrate-binding proteins have shown promising activity as antiviral agents. Ichthyopeptins A and B, derived from Microcystis ichthyoblabe, are potential agents against influenza virus, with an IC50 value of 12.5 mg ml–1 [123]. Cyanovirin-N and scytovirin are also potent virucidal drugs that interfere with several steps of the viral fusion process. Cyanovirin-N, for example, shows both in vitro and in vivo activity against HIV and other lentiviruses in nanomolar concentrations. These 101 amino acids long, 11 kDa polypeptide derived from Nostoc ellipsosporum is being developed as a vaginal gel for preventing sexual transmission of HIV by Cellegy Pharmaceuticals, San Francisco, CA, provided its inhibitory effects upon HIV virus-CD4 cell membrane fusion [124]. Scytovirin, on the other hand, is a 95 amino acid long, 9.7 kDa polypeptide (that includes five intra-chains disulfide bonds) derived from aqueous extracts of Scytonema varium, that is able to attach to the glycoprotein envelope of HIV (gp120, gp160, and gp41), thus making the virus inactive even in low nanomolar concentrations [125].

10.3 Sulfoglycolipids

Natural cyanobacterial sulfoglycolipids show confirmed HIV-reverse transcriptase and DNA polymerase inhibitory effects [29].

11. Antibacterial activity

If bacterial resistance strengthens, the treatment may become impossible for some diseases. Nosocomial infections such as those caused by the methicillin-resistant Staphylococcus aureus or the vancomycin-resistant enterococci, caused by multi-drug-resistant bacteria, create therapeutic problems of worldwide concern [126], hence the urgency of developing new antibiotics. Accordingly, new attempts to find antibacterial activity via screening of cyanobacterial extracts have started [127], although very few cyanobacteria-related antibacterial compounds have been detected to date. Noscomin57, from Nostoc commune [128], shows antibacterial activity against Bacillus cereus, Staphylococcus Epidermidis, and Escherichia coli. Antibacterial activity of Anabaena extracts against vancomycin-resistant S. aureus with a MIC of 32–64 mg ml-1 has been reported by [129].

12. Antiprotozoal activity

The estimations of the World Health Organization indicate that >109 people over the world suffer from tropical diseases caused by Schistosoma, Trypanosoma, Leishmania, Plasmodium, and others [130]. The unsuccessful treatment of such diseases (especially malaria) is related to the growing resistance shown by these protozoa and the slow pace of drug discovery [131, 132]. In a recent project operated by the Panamanian International Co-operative Biodiversity Group, five classes of antiprotozoal compounds were isolated from cyanobacteria. Nostocarboline, an alkaloid protease inhibitor isolated from Nostoc sp. 78-12 A, displayed activity against T. cruzi, Leishmania donovani, Trypanosoma brucei, and Plasmodium falciparum [133]. Moreover, aerucyclamide C68 isolated from Microcystis aeruginosa PCC 7806 has been also detected to be active against T. brucei.

13. Protease inhibition activity

More than 120 cyanobacterial alkaloids with various biological activities (including protease inhibition) were introduced between 2001 and 2006. Some of these compounds, such as microginins (used for the treatment of high blood pressure), aeruginosins, and cyanopeptolins (a serine inhibitor used for asthma and viral infections) are described by Jaspars and Lawton [29]. Kempopeptins are other groups of protease inhibitory products, for example, kempopeptin B (with activity against trypsin, with an IC50 of 8.4 mM), kempopeptin A (a cyclodepsipeptide derived from marine Lyngbya with activity against elastase), and chymotrypsin with an IC50 of 0.32 mM and 2.6 mM, respectively [46].

14. Immunomodulatory activity

Besides the beneficial properties of cyanobacteria, their immunomodulatory activity exhibits diverse effects on immune systems, such as the increase of phagocytic activity in macrophages, the stimulation of antibody and cytokine production and the accumulation of natural killer cells into tissues, or the activation of T and B cells [134]. For instance, the effect of Spirulina in mice was investigated by Hayashi et al., who demonstrated increased phagocytic activity and antigen production. Enhanced phagocytic and natural killer cell-mediated antitumor activities, together with increased antigen production, were also shown in chicken by Qureshi and Ali [29]. Additionally, the incremental impact of cyanobacteria extracts on 13.6-fold interferon and 3-fold interleukin (IL)-1b and -4 was observed in human blood cells. Despite Spirulina has been proved to be safe, other cyanobacteria (e.g., Microcystis sp.) produce metabolites that are cytotoxic to lymphocytes and have inhibitory effects on membrane-bound leucine amino peptidase, which is related to antigen-processing and antigen presentation response [47, 135], confirmed the immune-toxicity of microcystin that presented medical competence in the lipopolysaccharide-induced lymph proliferation in mice vaccinated with sheep T-dependent antigen red blood cells.

15. Anticancer activity

The urgency of brand-new anticancer medications is an important issue provided the increasing resistance against currently available drugs (such as taxanes) and the outbreak of new types of cancer subjected to chemotherapeutic treatment failure [29]. A considerable number of highly active cyanobacterial compounds target tubulin or actin filaments in eukaryotic cells and have exhibited potent antimitotic properties, which makes them a noteworthy source of potential anticancer agents [4]. Several of them have gone through Phase I and II clinical trials such as the third generation dolastatin15 and TZT-1027 (soblidotin), a synthetic derivative of dolastatin-10 [136, 137]. They generally act by blocking cell division at the M-phase by targeting tubulin with efficacy equivalent to clinical drugs, such as vinblastine, vincristine, or paclitaxel. Some of these highly cytotoxic compounds are described below in Figure 1 [138].

Figure 1.

Potential of cyanobacterial extract as anticancer activity.

15.1 Coibamide A – An anticancer agent with a novel action mechanism

Coibamide A, extracted from a Leptolyngbya strain, shows a novedous action mechanism targeting tubulin or actin filaments. Notable cytotoxical properties against breast, central nervous system, colon, and ovary cancers have been observed [41].

15.2 Cryptophycins

Cryptophycins are examples of cyanobacteria-derived tubulin-binding compounds with potent anticancer activity. Cryptophycin A was first isolated by Schwartz and co-workers in 1990 from Nostoc sp. strains ATCC 53789 and GSV224 [22]. Microtubule dynamics suppression and blocking of G2/M phases are features making this molecule a potent anti-carcinoma metabolite [29]. Cryptophycin-52 (LY 355073), a chemical analog of cryptophycin-1, was developed to improve its hydrolytic stability but produced very slight activity in the clinical trial. The second-generation analog, cryptophycins-249 and -309, show better water solubility and stability [139]. According to a study by [140], the thioesterase derived from the cryptophycin biosynthetic pathway through the macrocyclization of a series of linear synthetic forerunners generate 16-membered cyclic depsipeptides, showed significant efficiency as anticancer agents.

15.3 Largazole- a histone deacetylase inhibitor

Largazole, an ant proliferating compound with an unusual chemical scaffold, is extracted from Symploca sp. [141], and shows a considerable histone deacetylase (HDAC) inhibitory activity [142], together with a great selectivity in human mammary epithelial and fibroblastic osteosarcoma cells. The FDA ratification of HDAC inhibitor suberoylanilide hydroxamic acid as a treatment for dermal T-cell lymphomas, besides its mood stability properties and anti-epileptic characteristics, confirms this compound for cancer treatment.

15.4 Apratoxins – signal transduction inhibitors

Apratoxins, a notable class of potent cytotoxic cyclic depsipeptides, was initially isolated from a chemically rich Lyngbya boulloni strain and, according to NCI’s Developmental Therapeutics Branch, demonstrated a unique action pattern against a panel of 60 cancer cell lines [143]. Limited findings until now indicate that the induction of G1-phase cell-cycle arrest and apoptosis is how apratoxins function as anticancer agents [39]. Apratoxin A showed moderate cytotoxicity in multiple human tumor cell lines (e.g., LoVo cell lines and KB cancer cells), although this compound is acid sensitive and decomposes when exposed to the HCl present in CDCl3. Other analogs, especially apratoxin D, have been studied in order to develop a lead structure [4].

15.5 Polypeptides- Hassallidins

Polypeptides, mostly with microbial origins, have long been used for pharmaceutical applications either as antimicrobial agents or for disinfection. A group of cyclic glycosylated lipopeptide Cyanobacteria metabolites are the hassallidins A [52] and B [144], which are purified from Hassallia; these compounds are a type of comprehensive with action against human pathogenic fungi [1].

16. Conclusions

The results in this review emphasized the significance of the probable healing function of natural bioactive products purified from cyanobacteria strains, for instance, antibacterial, antitumor, protease inhibition activity, and antiviral effects, and highlighted the necessity to restart discovering natural biological sources. However, system biology for metabolite purification, characterization, and valuation in cyanobacterial bioactive compounds that have not arrived in the clinical trials so far.