2,5-diketopiperazines from the marine-derived fungus Aspergillus fumigatus YK-7

Novel Alkaloids from Aspergillus fumigatus VDL36, an Endophytic Fungus Associated with Vaccinium dunalianum

Eleven alkaloids (1-11) including seven new ones, 1-7, were isolated from the solid fermentation of Aspergillus fumigatus VDL36, an endophytic fungus isolated from the leaves of Vaccinium dunalianum Wight (Ericaceae), a perennial evergreen shrub distributed across the Southwest regions of China, Myanmar, and Vietnam. Their structures were elucidated on the basis of extensive spectroscopic methods. The isolates were evaluated for in vitro antifungal activities against five phytopathogenic fungi (Fusarium oxysporum, Coriolus versicolor, Fusarium solani, Botrytis cinerea, Fusarium graminearum). As a result, the new compounds fumigaclavine I (1), 13-ethoxycyclotryprostatin A (5), 13-dehydroxycyclotryprostatin A (6), and 12β-hydroxy-13-oxofumitremorgin C (7) exhibited antifungal activities with MIC values of 7.8-62.5 μg/mL which were comparable to the two positive controls ketoconazole (MIC = 7.8-31.25 μg/mL) and carbendazim (MIC = 1.95-7.8 μg/mL). Furthermore, compounds 1 and 5 demonstrated potent protective and curative effects against the tomato gray mold in vivo. Preliminary structure-activity relationships of the tested indole diketopiperazine alkaloids indicate that the introduction of a substituent group at position C-13 enhances their biological activities.

The potential of marine-derived piperazine alkaloids: Sources, structures and bioactivities

Marine-derived piperazine alkaloids (MDPAs) constitute a significant group of natural compounds known for their diverse structures and biological activities. Over the past five decades, substantial efforts have been devoted to isolating these alkaloids from marine sources and characterizing their chemical and bioactive profiles. To date, a total of 922 marine-derived piperazine alkaloids have been reported from various marine organisms. These compounds demonstrate a wide range of pharmacological properties, including cytotoxicity, antibacterial, antifungal, antiviral, and various other activities. Notably, among these activities, cytotoxicity emerges as the most prominent characteristic of marine-derived piperazine alkaloids. This review also summarizes the structure-activity relationship (SAR) studies associated with the cytotoxicity of these compounds. In summary, our objective is to provide an overview of the research progress concerning marine-derived piperazine alkaloids, with the aim of fostering their continued development and utilization.

Cytotoxic Compounds from Marine Fungi: Sources, Structures, and Bioactivity

Marine fungi, such as species from the Penicillium and Aspergillus genera, are prolific producers of a diversity of natural products with cytotoxic properties. These fungi have been successfully isolated and identified from various marine sources, including sponges, coral, algae, mangroves, sediment, and seawater. The cytotoxic compounds derived from marine fungi can be categorized into five distinct classes: polyketides, peptides, terpenoids and sterols, hybrids, and other miscellaneous compounds. Notably, the pre-eminent group among these compounds comprises polyketides, accounting for 307 out of 642 identified compounds. Particularly, within this collection, 23 out of the 642 compounds exhibit remarkable cytotoxic potency, with IC50 values measured at the nanomolar (nM) or nanogram per milliliter (ng/mL) levels. This review elucidates the originating fungal strains, the sources of isolation, chemical structures, and the noteworthy antitumor activity of the 642 novel natural products isolated from marine fungi. The scope of this review encompasses the period from 1991 to 2023.

2,5-Diketopiperazines (DKPs): Promising Scaffolds for Anticancer Agents

2,5-Diketopiperazine (2,5-DKP) derivatives represent a family of secondary metabolites widely produced by bacteria, fungi, plants, animals, and marine organisms. Many natural products with DKP scaffolds exhibited various pharmacological activities such as antiviral, antifungal, antibacterial, and antitumor. 2,5-DKPs are recognized as privileged structures in medicinal chemistry, and compounds that incorporate the 2,5-DKP scaffold have been extensively investigated for their anticancer properties. This review is a thorough update on the anti-cancer activity of natural and synthesized 2,5-DKPs from 1997 to 2022. We have explored various aspects of 2,5-DKPs modifications and summarized their structure-activity relationships (SARs) to gain insight into their anticancer activities. We have also highlighted the novel approaches to enhance the specificity and pharmacokinetics of 2,5-DKP-based anticancer agents.

A new sesquiterpenoid from the aconitum-derived fungus Aspergillus fumigatus M1

A new bergamotane sesquiterpenoid, fumigatanol (1), along with nine known compounds (2-10) were isolated from the Aconitum-derived fungus Aspergillus fumigatus M1. Their structures were established on the basis of extensive spectroscopic analyses, ECD experiment and NMR computational method. Antibacterial and cytotoxic activities of compound 1 were evaluated and no obvious antibacterial and cytotoxic activities were observed at concentrations of 256 μg/mL and 40.00 μM, respectively.

Indole alkaloids from marine resources: Understandings from therapeutic point of view to treat cancers

Cancer is the leading cause of mortality all over the world. Scientific investigation has demonstrated that disruptions in the process of autophagy are frequently interrelated with the emergence of cancer. Hence, scientists are seeking permanent solutions to counter the deadly disease. Indole alkaloids have been extensively studied and are acknowledged to exhibit several bioactivities. The current state of disease necessitates novel pharmacophores development. In recent decades, indole alkaloids have become increasingly significant in cancer treatment and are also used as adjuvants. A substantial amount of pharmacologically active molecules come from indole alkaloids, which are widely distributed in nature. Indole alkaloids derived from marine organisms show immense potential for therapeutic applications and seem highly effective in cancer treatment. A couple of experiments have been conducted preclinically to investigate the possibility of indole alkaloids in cancer treatment. Marine-derived indole alkaloids possess the ability to exhibit anticancer properties through diverse antiproliferative mechanisms. Certain indole alkaloids, including vincristine and vinblastine, were verified in clinical trials or are presently undergoing clinical assessments for preventing and treating cancer. Indole alkaloids from marine resources hold a significant functionality in identifying new antitumor agents. The current literature highlights recent advancements in indole alkaloids that appear to be anticancer agents and the underlying mechanisms.

An indole diketopiperazine alkaloid and a bisabolane sesquiterpenoid with unprecedented skeletons from Aspergillus fumigatus

Fumitryprostatin A (1), the first example of an indole diketopiperazine alkaloid with a tricyclic 5/6/5 skeleton characterized by a dipyrrolo[1,2-a:1',2'-d]pyrazine-5,10-dione ring system decorated with a prenylated indole moiety, and fuminoid A (2), a sesquiterpenoid with a bicyclo[3.2.1]octane ring featuring a novel carbon skeleton via the transformation of the methyl, were isolated from the fungus Aspergillus fumigatus along with six known diketopiperazine alkaloids. The structure with the absolute configuration of 1 was determined based on spectroscopic analyses and X-ray crystallographic analysis, while the configuration of 2 was assigned tentatively by ¹³C NMR data with DP4+ probability analyses and ECD calculations. A plausible biosynthetic pathway for 1 was proposed starting from L-Trp and L-Pro via normal indole diketopiperazine. Compound 1 exhibited moderate cytotoxic activity with an IC₅₀ value of 14.6 μM, while compound 8 exhibited moderate immunosuppressive activity in vitro.

The intriguing chemistry and biology of sulfur-containing natural products from marine microorganisms (1987-2020)

Natural products derived from marine microorganisms have received great attention as a potential resource of new compound entities for drug discovery. The unique marine environment brings us a large group of sulfur-containing natural products with abundant biological functionality including antitumor, antibiotic, anti-inflammatory and antiviral activities. We reviewed all the 484 sulfur-containing natural products (non-sulfated) isolated from marine microorganisms, of which 59.9% are thioethers, 29.8% are thiazole/thiazoline-containing compounds and 10.3% are sulfoxides, sulfones, thioesters and many others. A selection of 133 compounds was further discussed on their structure-activity relationships, mechanisms of action, biosynthesis, and druggability. This is the first systematic review on sulfur-containing natural products from marine microorganisms conducted from January 1987, when the first one was reported, to December 2020.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00101-2.

Cyclic Dipeptides: The Biological and Structural Landscape with Special Focus on the Anti-Cancer Proline-Based Scaffold

Cyclic dipeptides, also know as diketopiperazines (DKP), the simplest cyclic forms of peptides widespread in nature, are unsurpassed in their structural and bio-functional diversity. DKPs, especially those containing proline, due to their unique features such as, inter alia, extra-rigid conformation, high resistance to enzyme degradation, increased cell permeability, and expandable ability to bind a diverse of targets with better affinity, have emerged in the last years as biologically pre-validated platforms for the drug discovery. Recent advances have revealed their enormous potential in the development of next-generation theranostics, smart delivery systems, and biomaterials. Here, we present an updated review on the biological and structural profile of these appealing biomolecules, with a particular emphasis on those with anticancer properties, since cancers are the main cause of death all over the world. Additionally, we provide a consideration on supramolecular structuring and synthons, based on the proline-based DKP privileged scaffold, for inspiration in the design of compound libraries in search of ideal ligands, innovative self-assembled nanomaterials, and bio-functional architectures.

Biosynthetic Cyclization Catalysts for the Assembly of Peptide and Polyketide Natural Products

Many biologically active natural products are synthesized by nonribosomal peptide synthetases (NRPSs), polyketide synthases (PKSs) and their hybrids. These megasynthetases contain modules possessing distinct catalytic domains that allow for substrate initiation, chain extension, processing and termination. At the end of a module, a terminal domain, usually a thioesterase (TE), is responsible for catalyzing the release of the NRPS or PKS as a linear or cyclized product. In this review, we address the general cyclization mechanism of the TE domain, including oligomerization and the fungal C-C bond forming Claisen-like cyclases (CLCs). Additionally, we include examples of cyclization catalysts acting within or at the end of a module. Furthermore, condensation-like (CT) domains, terminal reductase (R) domains, reductase-like domains that catalyze Dieckmann condensation (RD), thioesterase-like Dieckmann cyclases, trans-acting TEs from the penicillin binding protein (PBP) enzyme family, product template (PT) domains and others will also be reviewed. The studies summarized here highlight the remarkable diversity of NRPS and PKS cyclization catalysts for the production of biologically relevant, complex cyclic natural products and related compounds.

Marine-derived fungi as a source of bioactive indole alkaloids with diversified structures

Marine-derived fungi are well known as rich sources of bioactive natural products. Growing evidences indicated that indole alkaloids, isolated from a variety of marine-derived fungi, have attracted considerable attention for their diverse, challenging structural complexity and promising bioactivities, and therefore, indole alkaloids have potential to be pharmaceutical lead compounds. Systemic compilation of the relevant literature. In this review, we demonstrated a comprehensive overview of 431 new indole alkaloids from 21 genera of marine-derived fungi with an emphasis on their structures and bioactivities, covering literatures published during 1982-2019.

Five new secondary metabolites from the fungus Phomopsis asparagi

Five new compounds, including a pair of diphenylcyclopentenone enantiomers (±)-phomopsisin A (1), a sesquiterpenoid 15-hydroxylithocarin A (2), a new diketopiperazine alkaloid prenylcyclotryprostatin A (3) and 7-hydroxy-cis-L(-)-3,6-dibenzyl-2,5-dioxopiperazine (6), along with five known compounds were isolated from the fungus Phomopsis asparagi. Their structures were elucidated on the basis of spectroscopic analyses (1D and 2D NMR), theoretical electronic circular dichroism (ECD) calculation, modified Mosher's method, and X-ray crystallography. The racemates of (±)-phomopsisin A showed inhibition on α-glucosidase with IC₅₀ of 30.07 ± 0.75 μM (positive control acarbose, 121 ± 2.7 μM).

Structurally diverse and bioactive alkaloids from an insect-derived fungus Neosartorya fischeri

Seven undescribed alkaloids, namely fischeramides A and B, 5,6-dimethoxycircumdatin C, 6-hydroxyacetylaszonalenin, 3-methoxyglyantrypine, 9-methoxyfumitremorgin C, and spirotryprostatin M, one undescribed natural product, namely 11-deacetyl pyripyropene A, together with nine known congeners, were isolated from the solid cultures of fungus Neosartorya fischeri, which was separated from a medicinal insect Cryptotympana atrata. Their structures were elucidated by extensive spectroscopic data, electronic circular dichroism (ECD) calculations, and single-crystal X-ray diffraction analyses. Structurally, fischeramides A and B represented a pair of rare geometric isomers of the benzodiazepinedione derivatives with a highly conjugated feature. Fischeramide A showed potential immunosuppressive activity in LPS and anti-CD3/anti-CD28 mAbs activated murine splenocytes proliferation with IC₅₀ values of 7.08 and 6.31 μM, respectively, and also showed anti-inflammatory activity against the lipopolysaccharide-induced nitric oxide production with an IC₅₀ value of 25 ± 1 μM. In addition, 5,6-dimethoxycircumdatin C showed remarkable antibacterial activity against ESBL-producing E. coli with an MIC value of 2.0 μg/mL.

Diketopiperazine Formation in Fungi Requires Dedicated Cyclization and Thiolation Domains

Cyclization of linear dipeptidyl precursors derived from nonribosomal peptide synthetases (NRPSs) into 2,5-diketopiperazines (DKPs) is a crucial step in the biosynthesis of a large number of bioactive natural products. However, the mechanism of DKP formation in fungi has remained unclear, despite extensive studies of their biosyntheses. Here we show that DKP formation en route to the fungal virulence factor gliotoxin requires a seemingly extraneous couplet of condensation (C) and thiolation (T) domains in the NRPS GliP. In vivo truncation of GliP to remove the CT couplet or just the T domain abrogated production of gliotoxin and all other gli pathway metabolites. Point mutation of conserved active sites in the C and T domains diminished cyclization activity of GliP in vitro and abolished gliotoxin biosynthesis in vivo. Verified NRPSs of other fungal DKPs terminate with similar CT domain couplets, suggesting a conserved strategy for DKP biosynthesis by fungal NRPSs.

Discovery of Bioactive Indole-Diketopiperazines from the Marine-Derived Fungus Penicillium brasilianum Aided by Genomic Information

Identification and analysis of the whole genome of the marine-derived fungus Penicillium brasilianum HBU-136 revealed the presence of an interesting biosynthetic gene cluster (BGC) for non-ribosomal peptide synthetases (NRPS), highly homologous to the BGCs of indole-diketopiperazine derivatives. With the aid of genomic analysis, eight indole-diketopiperazines (1−8), including three new compounds, spirotryprostatin G (1), and cyclotryprostatins F and G (2 and 3), were obtained by large-scale cultivation of the fungal strain HBU-136 using rice medium with 1.0% MgCl₂. The absolute configurations of 1−3 were determined by comparison of their experimental electronic circular dichroism (ECD) with calculated ECD spectra. Selective cytotoxicities were observed for compounds 1 and 4 against HL-60 cell line with the IC₅₀ values of 6.0 and 7.9 μM, respectively, whereas 2, 3, and 5 against MCF-7 cell line with the IC₅₀ values of 7.6, 10.8, and 5.1 μM, respectively.

Two New Spiro-Heterocyclic γ-Lactams from A Marine-Derived Aspergillus fumigatus Strain CUGBMF170049

Two new spiro-heterocyclic γ-lactam derivatives, cephalimysins M (1) and N (2), were isolated from the fermentation cultures of the marine-derived fungus Aspergillus fumigatus CUGBMF17018. Two known analogues, pseurotin A (3) and FD-838 (4), as well as four previously reported helvolic acid derivatives, 16-O-propionyl-16-O-deacetylhelvolic acid (5), 6-O-propionyl-6-O-deacetylhelvolic acid (6), helvolic acid (7), and 1,2-dihydrohelvolic acid (8) were also identified. One-dimensional (1D), two-dimensional (2D) NMR, HRMS, and circular dichroism spectral analysis characterized the structures of the isolated compounds.

Secondary metabolites from Aspergillus fumigatus, an endophytic fungus from the liverwort Heteroscyphus tener (Steph.) Schiffn

Three new metabolites, asperfumigatin (1), isochaetominine (10), and 8'-O-methylasterric acid (21), together with nineteen known compounds, were obtained from the culture of Aspergillus fumigatus, an endophytic fungus from the Chinese liverwort Heteroscyphus tener (Steph.) Schiffn. Their structures were established by extensive analysis of the spectroscopic data. The absolute configurations of 1 and 10 were determined by analysis of their respective CD spectra. Cytotoxicity of these isolates against four human cancer cell lines was also determined.

Terpenoids from the Marine-Derived Fungus Aspergillus fumigatus YK-7

Two new β-bergamotane sesquiterpenoids, E-β-trans-5,8,11-trihydroxybergamot-9-ene (1) and β-trans-2β,5,15-trihydroxybergamot-10-ene (2), were isolated from the marine-derived fungus Aspergillusfumigatus YK-7, along with three known terpenoids 3–5. Their structures were determined by spectroscopic methods (1D and 2D NMR, HR-ESI-MS). Antiproliferative effects on human leukemic monocyte lymphoma U937 and human prostate cancer PC-3 cell lines were measured in vitro. Compound 4 exhibited potent activity against the U937 cell line with an IC₅₀ value of 4.2 μM.

Marine Indole Alkaloids

Marine indole alkaloids comprise a large and steadily growing group of secondary metabolites. Their diverse biological activities make many compounds of this class attractive starting points for pharmaceutical development. Several marine-derived indoles were found to possess cytotoxic, antineoplastic, antibacterial and antimicrobial activities, in addition to the action on human enzymes and receptors. The newly isolated indole alkaloids of marine origin since the last comprehensive review in 2003 are reported, and biological aspects will be discussed.

Marine natural products as breast cancer resistance protein inhibitors

Breast cancer resistance protein (BCRP) is a protein belonging to the ATP-binding cassette (ABC) transporter superfamily that has clinical relevance due to its multi-drug resistance properties in cancer. BCRP can be associated with clinical cancer drug resistance, in particular acute myelogenous or acute lymphocytic leukemias. The overexpression of BCRP contributes to the resistance of several chemotherapeutic drugs, such as topotecan, methotrexate, mitoxantrone, doxorubicin and daunorubicin. The Food and Drugs Administration has already recognized that BCRP is clinically one of the most important drug transporters, mainly because it leads to a reduction of clinical efficacy of various anticancer drugs through its ATP-dependent drug efflux pump function as well as its apparent participation in drug resistance. This review article aims to summarize the different research findings on marine natural products with BCRP inhibiting activity. In this sense, the potential modulation of physiological targets of BCRP by natural or synthetic compounds offers a great possibility for the discovery of new drugs and valuable research tools to recognize the function of the complex ABC-transporters.

A new cyclic hexapeptide and a new isocoumarin derivative from the marine sponge-associated fungus Aspergillus similanensis KUFA 0013

A new isocoumarin derivative, similanpyrone C (1), a new cyclohexapeptide, similanamide (2), and a new pyripyropene derivative, named pyripyropene T (3) were isolated from the ethyl acetate extract of the culture of the marine sponge-associated fungus Aspergillus similanensis KUFA 0013. The structures of the compounds were established based on 1D and 2D NMR spectral analysis, and in the case of compound 2 the stereochemistry of its amino acid constituents was determined by chiral HPLC analysis of the hydrolysate by co-injection with the d and l amino acids standards. Compounds 2 and 3 were evaluated for their in vitro growth inhibitory activity against MCF-7 (breast adenocarcinoma), NCI-H460 (non-small cell lung cancer) and A373 (melanoma) cell lines, as well as antibacterial activity against reference strains and the environmental multidrug-resistant isolates (MRS and VRE). Only compound 2 exhibited weak activity against the three cancer cell lines, and neither of them showed antibacterial activity.

An update on 2,5-diketopiperazines from marine organisms

2,5-Diketopiperazines (2,5-DKPs) are an important category of structurally diverse cyclic dipeptides with prominent biological properties. These 2,5-DKPs have been obtained from a variety of natural resources, including marine organisms. Because of the increasing numbers and biological importance of these compounds, this review covers 90 marine originated 2,5-DKPs that were reported from 2009 to the first half-year of 2014. The review will focus on the structure characterizations, biological properties and proposed biosynthetic processes of these compounds.

Marine natural products

This review covers the literature published in 2012 for marine natural products, with 1035 citations (673 for the period January to December 2012) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1241 for 2012), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Potential allelopathic indole diketopiperazines produced by the plant endophytic Aspergillus fumigatus using the one strain-many compounds method

On the basis of the OSMAC (one strain-many compounds) strategy, 14 indole diketopiperazine (DKP) alkaloids, including spirotryprostatins (1-3), tryprostatins (4-6), and cyclotryprostatins (7-14), were isolated from the endophyte Aspergillus fumigatus associated with Melia azedarach L. Their structures were identified by nuclear magnetic resonance and electrospray ionization mass spectrometry data. All the indole DKPs were evaluated for plant growth regulation using the lettuce (Lactuca sativa) seedling growth bioassay, which showed the plant growth influence of the seedling. Among these compounds tested, a tryprostatin-type compound, brevianamide F (6), was identified as a new type of natural potential plant growth inhibitor with a response index (RI) higher than that of the positive control glyphosate, a broad-spectrum systemic herbicide. 6 can also inhibit turnip (Raphanus sativus) shoot and root elongation with RIs of -0.76 and -0.70, respectively, at 120 ppm, and it strongly inhibits amaranth (Amaranthus mangostanus) seedling growth with a high RI of -0.9 at 40 ppm. The structure-allelopathic activity relationship analysis of these isolated alkaloids indicates that tryprostatin-type alkaloids without the C5 prenyl and methoxy group give the most potent inhibition of seedling growth. Brevianamide F (6) could be used to develop a natural eco-friendly herbicide.

Anticancer and antifungal compounds from Aspergillus, Penicillium and other filamentous fungi

This review covers important anticancer and antifungal compounds reported from filamentous fungi and in particular from Aspergillus, Penicillium and Talaromyces. The taxonomy of these fungi is not trivial, so a focus of this review has been to report the correct identity of the producing organisms based on substantial previous in-house chemotaxonomic studies.

Solid-phase synthesis of diverse spiroisoxazolinodiketopiperazines

A convenient, efficient protocol to prepare diverse spiroisoxazolino-diketopiperazines via a parallel solid-supported synthesis was developed. The key steps are (1) a coupling reaction of an amino acid; (2) tosylation with concomitant β-elimination to form an α, β-unsaturated ester; (3) a 1,3-dipolar cycloaddition with an oxime to form isoxazoline rings; and (4) cyclic cleavage to release the product from the resin. All reaction steps and workup procedures were modified to allow the use of automated or semiautomated equipment. A 100-member demonstration library with two diversity sites was prepared in good purity and acceptable overall yields.

The diketopiperazine-fused tetrahydro-β-carboline scaffold as a model peptidomimetic with an unusual α-turn secondary structure

Aiming at restricting the conformational freedom of tryptophan-containing peptide ligands, we designed a THBC (tetrahydro-β-carboline)-DKP (diketopiperazine)-based peptidomimetic scaffold capable of arranging in an unusual α-turn conformation. The synthesis is based on a diastereoselective Pictet–Spengler condensation to give the THBC core, followed by an intramolecular lactamization to complete the tetracyclic THBC-DKP fused ring system. The presence of conformers bearing the intramolecular thirteen-membered hydrogen bond that characterizes the α-turn structure is confirmed by ¹H NMR conformational studies. To the best of our knowledge, this scaffold represents one of the rare examples of a designed constrained α-turn mimic.