Antineoplastic Agents. 603. Quinstatins: Exceptional Cancer Cell Growth Inhibitors

Discovery of the exceptionally powerful anticancer drug dolastatin 10 (1), contained in the sea hare Dolabella auricularia, opened a new frontier needed for improving human cancer treatment. Subsequently, major advances have been achieved based on results of structurally modifying this unusual natural peptide while maintaining the remarkable anticancer activity necessary for preparation of successful monoclonal antibody drug conjugates (ADC). Among the first several hundred SAR products based on dolastatin 10 our group synthesized and termed auristatins was auristatin E (2a). An anticancer activity-equivalent, desmethylaurisatin E (2b), linked to a CD30 monoclonal antibody is the very successful anticancer drug Adcetris, now approved for use in 65 countries. In the present investigation, we discovered a new subset of auristatins designated quinstatins derived from dolastatin 10 by replacing the C-terminal Doe unit with a carefully designed quinoline, which led to low or subnanomolar levels of cancer cell growth inhibition required for construction of chemically unique ADC drugs. The synthesis of quinstatins 2-8 is presented along with their cancer cell line biological data.

Characterization of Cyclodepsipeptides from the Mycelium of Isariajaponica from Vietnam

A new cyclodepsipeptide, isarin (1), was isolated from the insect pathogenic fungus Isaria japonica Yasuda, together with one known compound, beauveriolide 1 (2). The structure of this new compound was characterized using a combination of spectroscopic and spectrometric analyses.

Antiproliferative cyclodepsipeptides from the marine actinomycete Streptomyces sp. P11-23B downregulating the tumor metabolic enzymes of glycolysis, glutaminolysis, and lipogenesis

Two cyclodepsipeptides and a known cyclodepsipeptide valinomycin were isolated from a culture of the marine actinomycete Streptomyces sp. P11-23B. Their structures were established based on NMR, HRESIMS, and MS-MS spectroscopic interpretation as well as by chemical degradation. Both streptodepsipeptides P11A and P11B inhibited proliferation of different glioma cell lines, with IC₅₀ values ranging from 0.1 μM to 1.4 μM. Streptodepsipeptide P11A was found to block the cell cycle at the G₀/G₁ phase and induce apoptosis in glioma cells. Further investigation demonstrated that streptodepsipeptide P11A downregulated expression of HK2, PFKFB3, PKM2, GLS, and FASN, important tumor metabolic enzymes. Data from this study suggested that targeting multiple tumor metabolic regulators might be one anti-glioma mechanism of streptodepsipeptide P11A. A possible mechanism for this class of streptodepsipeptides is reported herein.

Identification of Cyclic Depsipeptides and Their Dedicated Synthetase from Hapsidospora irregularis

Seven cyclic depsipeptides were isolated from Hapsidospora irregularis and structurally characterized as the calcium channel blocker leualacin and six new analogues based on the NMR and HRESIMS data. These new compounds were named leualacins B-G. The absolute configurations of the amino acids and 2-hydroxyisocaproic acids were determined by recording the optical rotation values. Biological studies showed that calcium influx elicited by leualacin F in primary human lobar bronchial epithelial cells involves the TRPA1 channel. Through genome sequencing and targeted gene disruption, a noniterative nonribosomal peptide synthetase was found to be involved in the biosynthesis of leualacin. A comparison of the structures of leualacin and its analogues indicated that the A₂ and A₄ domains of the leualacin synthetase are substrate specific, while A₁, A₃, and A₅ can accept alternative precursors to yield new molecules.

Dual Mode of Action for Plusbacin A3 in Staphylococcus aureus

We have used C{F}, N{F}, and N{P} rotational-echo double resonance NMR to determine the location and conformation of 19F and 15N double-labeled plusbacin A3 and of double-labeled deslipo-plusbacin A3, each bound to the cell walls of whole cells of Staphyloccocus aureus grown in media containing [1-13C]glycine. The 31P is primarily in wall teichoic acid. Approximately 25% of plusbacin headgroups (the cyclic depsipeptide backbone) are in a closed conformation (N-F separation of 6 Å), while 75% are in a more open conformation (N-F separation of 12 Å). The closed headgroups have no contact with wall teichoic acid, whereas the open headgroups have a strong contact. This places the closed headgroups in hydrophobic regions of the cell wall and the open headgroups in hydrophilic regions. None of the plusbacin tails have contact with the 31P of either wall teichoic acid or the cell membrane and thus are in hydrophobic regions of the cell wall. In addition, both heads and tails of plusbacin A3 have contact with the glycyl 13C incorporated in cell-wall peptidoglycan pentaglycyl bridges and with 13C-labeled purines near the membrane surface. We interpret these results in terms of a dual mode of action for plusbacin A3: first, disruption of the peptidoglycan layer nearest to the membrane surface by closed-conformation plusbacin A3 leading to an inhibition of chain extension by transglycosylation; second, thinning and disruption of the membrane (possibly including disruption of ATP-binding cassette transporters embedded in the membrane) by open-conformation plusbacin A3, thereby leading to release of ATP to the hydrophilic regions of the cell wall and subsequent binding by plusbacin A3.

Octaminomycins A and B, Cyclic Octadepsipeptides Active against Plasmodium falciparum

Two new cyclic octadepsipeptides, octaminomycins A (1) and B (2), were isolated from a microbial metabolite fraction library of Streptomyces sp. RK85-270 based on Natural Products Plot screening. Their structures were elucidated on the basis of HRESIMS, 1D and 2D NMR spectroscopic data, and MS/MS experiments for sequence analysis. The absolute configurations of the constituent amino acid residues were determined by a combination of single-crystal X-ray diffraction and Marfey's methodology. Notably, octaminomycins A (1) and B (2) showed good in vitro antiplasmodial activity against chloroquine-sensitive as well as chloroquine-resistant strains with no cytotoxicity up to 30 μM.

Plitidepsin: design, development, and potential place in therapy

Plitidepsin is a cyclic depsipeptide that was first isolated from a Mediterranean marine tunicate (Aplidium albicans) and, at present, is manufactured by total synthesis and commercialized as Aplidin®. Its antitumor activity, observed in preclinical in vitro and in vivo studies has prompted numerous clinical trials to be conducted over the last 17 years, alone or in combination with other anticancer agents. Single-agent plitidepsin has shown limited antitumor activity and a tolerable safety profile in several malignancies, such as noncutaneous peripheral T-cell lymphoma, melanoma, and multiple myeloma. In patients with relapsed or refractory multiple myeloma, plitidepsin activity seems to be enhanced after addition of dexamethasone while remaining well tolerated, and a Phase III trial comparing plitidepsin plus dexamethasone vs dexamethasone alone is underway. Additional studies are required to better define the role of plitidepsin in combination with other active agents in these indications. Results of plitidepsin activity in other hematological malignancies or solid tumors have been disappointing so far. Further studies analyzing its mechanisms of action and potential biomarkers will help select patients who may benefit most from this drug. In this review, we critically analyze the published studies on plitidepsin in hematological malignancies and solid tumors and discuss its current role and future perspectives in treating these malignancies. We also review its design, pharmaceutical data, and mechanism of action.

Beauvericin K, a New Antifungal Beauvericin Analogue from a Marine-derived Fusarium sp

Chemical investigation of a Chinese collection of a marine-derived fungus, Fusarium sp., led to the characterization of beauvericin.K, a new analogue of beauvericin. The structure of the new compound was elucidated by detailed spectroscopic analysis, including ID and 2D NMR methods, and HRMS. Beauvericin K showed significant activity against the yeast Candida albicans with an IC50 value of 6.25 üg/mL..

Determination of the Absolute Configurations of Microtermolides A and B

Absolute configurations of the three consecutive chiral centers in the cyclic depsipeptide microtermolide A have been tentatively assigned as 2‴R, 3‴R, and 4‴R. However, on the basis of a structural comparison with vinylamycin, another depsipeptide with a unique 4-amino-2,4-pentadienolate structure, the chiral centers could also be assigned as 2‴R, 3‴R, and 4‴S. Here, the first total synthesis of microtermolide A is reported and the configurations of the three consecutive chiral centers were confirmed to be 2‴R, 3‴R, and 4‴S. A similar approach was used to determine the analogous centers in microtermolide B as 2‴R, 3‴R, and 4‴S.

Isolation of Coralmycins A and B, Potent Anti-Gram Negative Compounds from the Myxobacteria Corallococcus coralloides M23

Two new potent anti-Gram negative compounds, coralmycins A (1) and B (2), were isolated from cultures of the myxobacteria Corallococcus coralloides M23, together with another derivative (3) that was identified as the very recently reported cystobactamid 919-2. Their structures including the relative stereochemistry were elucidated by interpretation of spectroscopic, optical rotation, and CD data. The relative stereochemistry of 3 was revised to "S*R*" by NMR analysis. The antibacterial activity of 1 was most potent against Gram-negative pathogens, including Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumanii, and Klebsiella pneumoniae, with MICs of 0.1-4 μg/mL; these MICs were 4-10 and 40-100 times stronger than the antibacterial activities of 3 and 2, respectively. Thus, these data indicated that the β-methoxyasparagine unit and the hydroxy group of the benzoic acid unit were critical for antibacterial activity.

Stereoselective Synthesis of Southern Fragment of Hantupeptin A

The stereoselective synthesis of the southern fragment (C21-C41) of Hantupeptin A is described. The required stereochemistry of β-hydroxy-α-methyl acid unit was accomplished through the Aldol reaction using Evan's chiral auxiliary followed by the installation of the terminal alkyne with Ohira-Bestmann reagent.

Oryzamides A-E, Cyclodepsipeptides from the Sponge-Derived Fungus Nigrospora oryzae PF18

Three new cyclohexadepsipeptides, oryzamides A-C (1-3), two isolation artifacts, oryzamides D (4) and E (5), and the known congener scopularide A (6), all possessing a rare 3-hydroxy-4-methyldecanoic acid (HMDA) substructure, were isolated from the mycelial extract of the sponge-derived fungus Nigrospora oryzae PF18. Their planar structures were elucidated by spectroscopic analysis and comparison with the literature data. The absolute configurations were determined using the advanced Marfey's method and single-crystal X-ray diffraction analysis. Among them, oryzamides D (4) and E (5) were a pair of diastereomers at the sulfur atom of the l-methionine sulfoxide residue, which showcased the possible separation of a pair of methionine sulfoxide diastereomers. The X-ray crystal structure of scopularide A (6) was obtained for the first time, thereby establishing its relative and absolute configuration at C-4 of the HMDA residue. Oryzamides A-C (1-3) did not display cytotoxic, antibacterial, antiparasitic, and NF-κB inhibitory activities.

Elucidation of the Teixobactin Pharmacophore

This paper elucidates the teixobactin pharmacophore by comparing the arginine analogue of teixobactin Arg10-teixobactin to seven homologues with varying structure and stereochemistry. The roles of the guanidinium group at position 10, the stereochemistry of the macrolactone ring, and the "tail" comprising residues 1-5 are investigated. The guanidinium group is not necessary for activity; Lys10-teixobactin is more active than Arg10-teixobactin against Gram-positive bacteria in minimum inhibitory concentration (MIC) assays. The relative stereochemistry of the macrolactone ring is important. Diastereomer l-Thr8,Arg10-teixobactin is inactive, and diastereomer d-allo-Ile11,Arg10-teixobactin is less active. The macrolactone ring is critical; seco-Arg10-teixobactin is inactive. The absolute stereochemistry is not important; the enantiomer ent-Arg10-teixobactin is comparable in activity. The hydrophobic N-terminal tail is important. Truncation of residues 1-5 results in loss of activity, and replacement of residues 1-5 with a dodecanoyl group partially restores activity. These findings pave the way for developing simpler homologues of teixobactin with enhanced pharmacological properties.

SB-224289 Antagonizes the Antifungal Mechanism of the Marine Depsipeptide Papuamide A

In order to expand the repertoire of antifungal compounds a novel, high-throughput phenotypic drug screen targeting fungal phosphatidylserine (PS) synthase (Cho1p) was developed based on antagonism of the toxin papuamide A (Pap-A). Pap-A is a cyclic depsipeptide that binds to PS in the membrane of wild-type Candida albicans, and permeabilizes its plasma membrane, ultimately causing cell death. Organisms with a homozygous deletion of the CHO1 gene (cho1ΔΔ) do not produce PS and are able to survive in the presence of Pap-A. Using this phenotype (i.e. resistance to Pap-A) as an indicator of Cho1p inhibition, we screened over 5,600 small molecules for Pap-A resistance and identified SB-224289 as a positive hit. SB-224289, previously reported as a selective human 5-HT1B receptor antagonist, also confers resistance to the similar toxin theopapuamide (TPap-A), but not to other cytotoxic depsipeptides tested. Structurally similar molecules and truncated variants of SB-224289 do not confer resistance to Pap-A, suggesting that the toxin-blocking ability of SB-224289 is very specific. Further biochemical characterization revealed that SB-224289 does not inhibit Cho1p, indicating that Pap-A resistance is conferred by another undetermined mechanism. Although the mode of resistance is unclear, interaction between SB-224289 and Pap-A or TPap-A suggests this screening assay could be adapted for discovering other compounds which could antagonize the effects of other environmentally- or medically-relevant depsipeptide toxins.

Production of the Ramoplanin Activity Analogue by Double Gene Inactivation

Glycopeptides such as vancomycin and telavancin are essential for treating infections caused by Gram-positive bacteria. But the dwindling availability of new antibiotics and the emergence of resistant bacteria are making effective antibiotic treatment increasingly difficult. Ramoplanin, an inhibitor of bacterial cell wall biosynthesis, is a highly effective antibiotic against a wide range of Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus, vancomycin-intermediate resistant Clostridium difficile and vancomycin-resistant Enterococcus sp. Here, two tailoring enzyme genes in the biosynthesis of ramoplanin were deleted by double in-frame gene knockouts to produce new ramoplanin derivatives. The deschlororamoplanin A2 aglycone was purified and its structure was identified with LC-MS/MS. Deschlororamoplanin A2 aglycone and ramoplanin aglycone showed similar activity to ramoplanin A2. The results showed that α-1,2-dimannosyl disaccharide at Hpg¹¹ and chlorination at Chp¹⁷ in the ramoplanin structure are not essential for its antimicrobial activity. This work provides new precursor compounds for the semisynthetic modification of ramoplanin.

Wewakazole B, a Cytotoxic Cyanobactin from the Cyanobacterium Moorea producens Collected in the Red Sea

A mass spectrometry (MS)-guided isolation has led to the purification of a new cyanobactin, wewakazole B (1), along with the known compound curacin D from a Red Sea Moorea producens. The planar structure of 1 was elucidated using a combination of NMR and MS techniques. After ozonolysis and acid hydrolysis, the absolute configurations of the amino acid components of 1 were determined by chiral-phase LC-MS and HPLC analyses. Notably, compound 1 exhibited cytotoxic activity toward human MCF7 breast cancer cells (IC50 = 0.58 μM) and human H460 lung cancer cells (IC50 = 1.0 μM) and was also found to be inactive in a siderophore assay.

Effects of technological processes on enniatin levels in pasta

BACKGROUND

Potential human health risks posed by enniatins (ENs) require their control primarily from cereal products, creating a demand for harvesting, food processing and storage techniques capable to prevent, reduce and/or eliminate the contamination. In this study, different methodologies to pasta processing simulating traditional and industrial processes were developed in order to know the fate of the mycotoxin ENs. The levels of ENs were studied at different steps of pasta processing. The effect of the temperature during processing was evaluated in two types of pasta (white and whole-grain pasta). Mycotoxin analysis was performed by LC-MS/MS.

RESULTS

High reductions (up to 50% and 80%) were achieved during drying pasta at 45-55°C and 70-90°C, respectively. The treatments at low temperature (25°C) did not change EN levels. The effect of pasta composition did not cause a significant effect on the stability of ENs. The effect of the temperature allowed a marked mycotoxin reduction during pasta processing. Generally, ENA1 and ENB showed higher thermal stability than did ENA and ENB1 .

CONCLUSIONS

The findings from the present study suggested that pasta processing at medium-high temperatures is a potential tool to remove an important fraction of ENs from the initial durum wheat semolina.

The Halicylindramides, Farnesoid X Receptor Antagonizing Depsipeptides from a Petrosia sp. Marine Sponge Collected in Korea

Three new structurally related depsipeptides, halicylindramides F-H (1-3), and two known halicylindramides were isolated from a Petrosia sp. marine sponge collected off the shore of Youngdeok-Gun, East Sea, Republic of Korea. Their planar structures were elucidated by extensive spectroscopic data analyses including 1D and 2D NMR data as well as MS data. The absolute configurations of halicylindramides F-H (1-3) were determined by Marfey's method in combination with Edman degradation. The absolute configurations at C-4 of the dioxyindolyl alanine (Dioia) residues of halicylindramides G (2) and H (3) were determined as 4S and 4R, respectively, based on ECD spectroscopy. The C-2 configurations of Dioia in 2 and 3 were speculated to both be 2R based on the shared biogenesis of the halicylindramides. Halicylindramides F (1), A (4), and C (5) showed human farnesoid X receptor (hFXR) antagonistic activities, but did not bind directly to hFXR.

Cellular Uptake and Cytotoxic Effect of Epidermal Growth Factor Receptor Targeted and Plitidepsin Loaded Co-Polymeric Polymersomes on Colorectal Cancer Cell Lines

Encapsulating chemotherapy drugs in targeted nanodelivery systems is one of the most promising approaches to tackle cancer disease, avoiding side effects of common treatment. In the last decade, several nanocarriers with different nature have been tested, but polypeptide-based copolymers have attracted considerable attention for their biocompatibility, controlled and slow biodegradability as well as their low toxicity. In this work, we synthesized, characterized and evaluated poly(trimethylene carbonate)-bock-poly(L-glutamic acid) derived polymersomes, targeted to epidermal growth factor receptor (EGFR), loaded with plitidepsin and ultimately tested in HT29 and LS174T colorectal cancer cell lines for specificity and efficacy. Furthermore, morphology, physico-chemical properties and plitidepsin loading were carefully investigated. A thorough in vitro cytotoxicity analysis of the unloaded polymersomes was carried out for biocompatibility check, studying viability, cell membrane asymmetry and reactive oxygen species levels. Those cytotoxicity assays showed good biocompatibility for plitidepsin-unloaded polymersomes. Cellular uptake and cytotoxic effect of EGFR targeted and plitidepsin loaded polymersome indicated that colorectal cancer cell lines were.more sensitive to anti-EGFR-drug-loaded than untargeted drug-loaded polymersomes. Also, in both cell lines, the use of untargeted polymersomes greatly reduced plitidepsin cytotoxicity as well as the cellular uptake, indicating that the use of this targeted nanocarrier is a promising approach to tackle colorectal cancer disease and avoid the undesired effects of the usual treatment. Furthermore, in vivo assays support the in vitro conclusions that EGFR targeted polymersomes could be a good drug delivery system. This work provides a proof of concept for the use of encapsulated targeted drugs as future therapeutic treatments for cancer.

A Peptidyl-Transesterifying Type I Thioesterase in Salinamide Biosynthesis

Salinamide A belongs to a rare class of bicyclic depsipeptide antibiotics in which the installation of a (4-methylhexa-2,4-dienoyl)glycine handle across a hexadepsipeptide core contributes to its chemical complexity and biological properties. Herein, we report the genetic and biochemical basis for salinamide construction in the marine bacterium Streptomyces sp. CNB-091, which involves a novel intermolecular transesterification reaction catalyzed by a type I thioesterase. Heterologous expression studies revealed the central role of the nonribosomal peptide synthetase Sln9 in constructing and installing the distinctive acylglycine "basket handle" of salinamide. Biochemical characterization of the Sln9 thioesterase domain established that transesterification of the serine residue of desmethylsalinamide E with acylated glycyl thioesters yields desmethylsalinamide C.

Absolute Stereochemistry of the β-Hydroxy Acid Unit in Hantupeptins and Trungapeptins

The β-hydroxy/amino acid unit is a common structural feature of many bioactive marine cyanobacterial depsipeptides. In this study, the absolute stereochemistry of the β-hydroxy acid moieties in hantupeptins and trungapeptins were determined through their synthesis and HPLC analysis of the Mosher ester derivatives. Synthesis of two3-hydroxy-2-methyloctanoic acid (Hmoa) stereoisomers, (2S,3R)-Hmoa and (2S,3S)-Hmoa, were achieved using diastereoselective asymmetric method and the retention times of all four Hmoa isomers were established indirectly by RPLC-MS analysis of their Mosher ester derivative standards. Based on the retention times of the standards, the absolute configuration of the Hmoa unit in hantupeptin C (3) and trungapeptin C (6) was assigned as (2R,3S)- and (2S,3R)-Hmoa, respectively. The use of the Mosher's reagents, coupled with HPLC analysis, provided a viable alternative to the absolute stereochemical determination of β-hydroxy acid units in depsipeptides.

Isolation and Structure of Kurahyne B and Total Synthesis of the Kurahynes

Kurahyne B (2), a new analogue of kurahyne (1), was isolated from the marine cyanobacterium Okeania sp. Its gross structure was elucidated based on spectroscopic analyses, and the absolute configuration was established by total synthesis. Kurahyne B (2) inhibited the growth of both HeLa and HL60 cells, with IC50 values of 8.1 and 9.0 μM, respectively. The growth-inhibitory activity of kurahyne B was the same as kurahyne (1). In parallel, the first total synthesis of kurahyne (1) was also achieved.

Arthroamide, a Cyclic Depsipeptide with Quorum Sensing Inhibitory Activity from Arthrobacter sp

Nonfilamentous actinobacteria have been less studied as secondary metabolite producers than their filamentous counterparts such as Streptomyces. From our collection of nonfilamentous actinobacteria isolated from sandstone, an Arthrobacter strain was found to produce a new cyclic peptide arthroamide (1) together with the known compound turnagainolide A (2). These compounds inhibited the quorum sensing signaling of Staphylococcus aureus in the submicromolar to micromolar range.

Synthesis and Antiproliferative Activity Evaluation of the Disulfide-Containing Cyclic Peptide Thiochondrilline C and Derivatives

Thiochondrilline C (4) was previously isolated from Verrucisispora sp. and reported to have moderate cytotoxicity against human lung adenocarcinoma cells. Herein, we report the synthesis of thiochondrilline C by N-terminal peptide extension, oxidative disulfide bond formation, and heterocycle installation as key steps. Antiproliferative activities for the prepared natural product and several derivatives against the NCI 60 cancer cell line panel are also described. Derivative 22 was identified as a moderately potent antiproliferative agent (50% growth inhibition (GI50) = 0.2-12.2 μM) with leukemia (average GI50 = 1.8 ± 0.1 μM) and colon (average GI50 = 2.4 ± 0.3 μM) cells being most sensitive.

Two New Cyclic Depsipeptides from the Endophytic Fungus Fusarium sp

Two new cyclic depsipeptides, W493 C (1) and D (2), along with two known derivatives W493 A (3) and B (4) were obtained from the endophytic fungus Fusarium sp. isolated from the Mangrove plant Ceriops tagal. The structures of the new compounds were determined on the basis of one- and two dimensional NMR and high-resolution mass spectroscopic data. The absolute configurations of the amino acid residues of 1 and 2 were confirmed by application of Marfey's method. W493 A (3) and B (4) exhibited moderate activity against the fungus Cladosporium cladosporiodes and weak antitumor activity against the human ovarian cancer cell line A2780.