Potent and selective antiplasmodial activity of the cyanobacterial alkaloid nostocarboline and its dimers

The quaternary β-carbolinium alkaloid nostocarboline from the cyanobacterium Nostoc 78-12A and 10 bis-cationic dimeric derivatives were evaluated against four protozoan parasites and low micromolar values against Trypanosoma brucei, submicromolar values against Leishmania donovani and low nanomolar values against Plasmodium falciparum K1 were determined. Selectivity against rat myoblasts (L6 cells) was found to be up to >2500-fold.     

Synthesis and neuroprotective activity of dictyoquinazol A and analogues

A flexible and efficient synthesis of the neuroprotective alkaloid, dictyoquinazol A, is reported. Several structural analogues of the target molecule were produced, and the neuroprotective activity of this series of compounds was investigated using three different cell-based models of stroke. Several of the new compounds were found to have superior activity compared to the natural product. This work has established a new molecular scaffold that holds promise for a novel pharmaceutical treatment for stroke.     

Benzoheterocyclic amodiaquine analogues with potent antiplasmodial activity: synthesis and pharmacological evaluation

The synthesis and evaluation of antiplasmodial activity of benzothiazole, benzimidazole, benzoxazole and pyridine analogues of amodiaquine is hereby reported. Benzothiazole and benzoxazole analogues with a protonatable tertiary nitrogen atom possessed excellent activity against the W2 and K1 chloroquine resistant strains of Plasmodium falciparum, with IC(50)s ranging from 7 to 22 nM.     

Design and synthesis of plant cyclopeptide Astin C analogues and investigation of their immunosuppressive activity

To further investigate on the structure-activity relationships of immunosuppressive Astin C, seventeen analogues 1–17 were designed and synthetized via amino acid substitution strategy by the solid-phase peptide synthesis method for the first time. In comparison with Astin C (IC₅₀?=?12.6?±?3.3?μM), only compounds 2 (IC₅₀?=?38.4?±?16.2?μM), 4 (IC₅₀?=?51.8?±?12.7?μM), 5 (IC₅₀?=?65.2?±?15.6?μM), and 8 (IC₅₀?=?61.8?±?12.4?μM) exhibited immunosuppressive activity in the Lymph node cells of mice. These results showed that the Astin C analogues containing _D-amino acid residues, hydrophobic long-chain alkyl substituents, and aryl substituents performed better than those carrying hydrophilic amino acid residues and short-chain alkyl substituents. Moreover compounds 15, 16, and 17 had no immunosuppressive activity, which suggested that cis-3,4-dichlorinated proline played an important role in the immunosuppressive activity of Astin C.     

Total synthesis of zervamicin IIB and its deuterium-labelled analogues

For the first time the total synthesis of the peptaibol antibiotic zervamicin IIB is described. Synthesis of this peptaibol was achieved by the Fmoc/tert-butyl strategy in solution using a fragment condensation approach. Three fragments of zervamicin IIB were obtained by stepwise elongation with Fmoc amino acids using BOP as a coupling reagent. For the introduction of the highly sterically hindered α-aminoisobutyric acid residues BOP/DMAP activation was applied. The Fmoc group was removed by reaction with 0.1 M NaOH in dioxane/methanol/water (30/9/1, v/v/v). Peptide fragments were coupled by means of a new coupling reagent, CF₃-PyBOP. Using the strategy developed, zervamicin IIB and two analogues specifically deuterium-labelled at different positions of the glutamine-11 residue have been synthesized in 40% overall yield based on the isotopically labelled amino acid and with 98±2% of isotope enrichment. FAB mass spectroscopy, 600 MHz ¹H-NMR spectroscopy and high-performance liquid chromatography provided convincing evidence that the synthetic products, zervamicin IIB and its deuterium-labelled analogues, fully correspond to the naturally occurring zervamicin IIB. © 1997 European Peptide Society and John Wiley & Sons, Ltd.     

Total synthesis and antiangiogenic activity of cyclopentane analogues of fumagillol

Novel cyclopentane analogues of fumagillol were synthesized and their endothelial cell proliferation inhibitory activities were evaluated. The cyclopentane-fumagillol derivatives were synthesized from (-)-2,3-O-isopropylidene-D-erythronolactone via stereoselective glycolate Claisen rearrangement and intramolecular ester enolate alkylation as key steps.     

Aza vinyl sulfones: synthesis and evaluation as antiplasmodial agents

A series of novel aza vinyl sulfones were designed, synthesized in good yields and evaluated as antiplasmodial agents. Tested compounds did not show activity against papain or the Plasmodium falciparum cysteine protease falcipain-2. However, a number of the new compounds effectively inhibited the in vitro development of P. falciparum. Compounds containing a squaramide group were the most active, with IC₅₀ values between 0.95 and 4.5 μM, suggesting that these are potential lead compounds for the development of new antimalarial agents.     

A synthetic approach to novel carvotacetone and antheminone analogues with anti-tumour activity

A synthetic approach to analogues of the terpenoid natural product antheminone A is described which employs (?)-quinic acid as starting material. A key conjugate addition step proved to be unpredictable regarding its stereochemical outcome however the route allowed access to two diastereoisomeric series of compounds. The results of biological assay of the toxicity of the target compounds towards non-small-cell lung cancer cell line A549 are reported.     

Novel squaramides with in vitro liver stage antiplasmodial activity

A structure–activity relationship study was performed with ten 8-aminoquinoline-squaramides compounds active against liver stage malaria parasites, using human hepatoma cells (Huh7) infected by Plasmodium berghei parasites. In addition, their blood-schizontocidal activity was assessed against chloroquine-resistant W2 strain Plasmodium falciparum. Compound 3 was 7.3-fold more potent than the positive control primaquine against liver-stage parasites, illustrating the importance of the squarate moiety to activity.     

Total in vitro biosynthesis of the nonribosomal macrolactone peptide valinomycin

Natural products are important because of their significant pharmaceutical properties such as antiviral, antimicrobial, and anticancer activity. Recent breakthroughs in DNA sequencing reveal that a great number of cryptic natural product biosynthetic gene clusters are encoded in microbial genomes, for example, those of Streptomyces species. However, it is still challenging to access compounds from these clusters because many source organisms are uncultivable or the genes are silent during laboratory cultivation. To address this challenge, we develop an efficient cell-free platform for the rapid, in vitro total biosynthesis of the nonribosomal peptide valinomycin as a model. We achieve this goal in two ways. First, we used a cell-free protein synthesis (CFPS) system to express the entire valinomycin biosynthetic gene cluster (>19 kb) in a single-pot reaction, giving rise to approximately 37 μg/L of valinomycin after optimization. Second, we coupled CFPS with cell-free metabolic engineering system by mixing two enzyme-enriched cell lysates to perform a two-stage biosynthesis. This strategy improved valinomycin production ~5000-fold to nearly 30 mg/L. We expect that cell-free biosynthetic systems will provide a new avenue to express, discover, and characterize natural product gene clusters of interest in vitro.     

Cryptolepine analogues containing basic aminoalkyl side-chains at C-11: synthesis, antiplasmodial activity, and cytotoxicity

A series of cryptolepine derivatives has been synthesized through the incorporation of short basic side-chains in the C-11 position of the 10H-indolo[3,2-b]quinoline scaffold. Their antiplasmodial activity was evaluated in vitro against the chloroquine resistant Plasmodium falciparum W2 strain, showing IC(50) values between 22 and 184 nM, while their cytotoxicity was assessed using HUVEC cells, revealing three compounds with a selectivity ratio higher than 10. The most selective of these derivatives, 4d, with a selectivity ratio of 46, was also the least cytotoxic of the series.     

Synthesis and biological evaluation of santacruzamate A analogues for anti-proliferative and immunomodulatory activity

Santacruzamate A (SCA) is a natural product isolated from a Panamanian marine cyanobacterium, previously reported to have potent and selective histone deacetylase (HDAC) activity. To optimize the enzymatic and cellular activity, 40 SCA analogues were synthesized in a systematic exploration of the zinc-binding group (ZBG), cap terminus, and linker region. Two cap group analogues inhibited proliferation of MCF-7 breast cancer cells, with analogous increased degranulation of cytotoxic T cells (CTLs), while one cap group analogue reduced CTL degranulation, indicative of suppression of the immune response. Additional testing of these analogues resulted in reevaluation of the previously reported SCA mechanism of action. These analogues and the resulting structure–activity relationships will be of interest for future studies on cell proliferation and immune modulation.     

Design and synthesis of biologically active peptides: a 'tail' of amino acids can modulate activity of synthetic cyclic peptides

In earlier work, we synthesized a cyclic 9-amino acid peptide (AFPep, cyclo[EKTOVNOGN]) and showed it to be useful for prevention and therapy of breast cancer. In an effort to explore the structure–function relationships of AFPep, we have designed analogs that bear a short ‘tail’ (one or two amino acids) attached to the cyclic peptide distal to its pharmacophore. Analogs that bore a tail of either one or two amino acids, either of which had a hydrophilic moiety in the side chain (e.g., cyclo[EKTOVNOGN]FS) exhibited greatly diminished biological activity (inhibition of estrogen-stimulated uterine growth) relative to AFPep. Analogs that bore a tail of either one or two amino acids which had hydrophobic (aliphatic or aromatic) side chains (e.g., cyclo[EKTOVNOGN]FI) retained (or had enhanced) growth inhibition activity. Combining in the same biological assay a hydrophilic-tailed analog with either AFPep or a hydrophobic-tailed analog resulted in decreased activity relative to that for AFPep or for the hydrophobic-tailed analog alone, suggesting that hydrophilic-tailed analogs are binding to a biologically active receptor. An analog with a disrupted pharmacophore (cyclo[EKTOVGOGN]) exhibited little or no growth inhibition activity. An analog with a hydrophilic tail and a disrupted pharmacophore (cyclo[EKTOVGOGN]FS) exhibited no growth inhibition activity of its own and did not affect the activity of a hydrophobic-tailed analog, but enhanced the growth inhibition activity of AFPep. These results are discussed in the context of a two-receptor model for binding of AFPep and ring-and-tail analogs. We suggest that tails on cyclic peptides may comprise a useful method to enhance diversity of peptide design and specificity of ligand–receptor interactions.     

Artemisinin-quinoline hybrid-dimers: synthesis and in vitro antiplasmodial activity

Novel artemisinin-quinoline hybrid-dimers were synthesized from dihydroartemisinin and different aminoquinolines at elevated temperatures (90-110°C). All compounds were obtained as the β-isomers and were tested against both chloroquine sensitive and resistant strains of Plasmodium falciparum. Hybrid-dimer 8 showed the highest antiplasmodial activity, inheriting the optimum chain length of three carbon atoms.     

Synthesis and biological activity of adipokinetic hormone analogues modified at the C-terminus

A series of Locusta adipokinetic hormone I (AKH-I), C-terminal threonine residue using a combination of solid- and liquid-phase methodology and evaluated in Locusta migratoria, in a lipid mobilization assay in vivo and an acetate uptake assay in vitro. Modifications at Thr¹⁰ of AKH-I involved replacement of its C-terminal amide by the groups -OH, -OCH₃, -NHCH₃, -N(CH₃)₂, and -NHC₆H₅; the last three groups were also applied to the amide of AKH-I-[Thr(Brl)¹⁰]. The methyl ester, monomethyl, and dimethyl analogues were all of lower activity than the parent in the lipid mobilization assay, but lost less than two orders of potency. In the acetate uptake assay, again the methyl ester analogue showed the greatest retention of biological activity of all modified peptides. A cyclic analogue, cyclo (PLNFTPNWGT), was active in both assay, but only at very high concentrations. Almost all analogues were more active in the acetate uptake assay than in the lipid assay, but unusually, AKH-I-NHCH₃ and AKH-I-N(CH₃)₂, together with cyclo (PLNFTPNWGT), were more active in the lipid mobilization assay. In addition, the acid AKH-I analogue did not suffer as large a loss in potency in the lipid mobilization assay as in the acetate uptake assay, although it was less potent in the former. The relative potencies of these two methyl analogues contrast with those for AKH AKH-I-[Thr(Bzl)¹⁰]-NHCH₃ and AKH-I-[Thr(Bzl)¹⁰]-N(CH₃)₂, which, together with both phenyl analogues, were significantly more active in the acetate uptake assay. We conclude that the acetate uptake assay has a greater preference for a hydrophobic C-terminus, compared with the lipid mobilization assay.     

The synthesis and evaluation of thymoquinone analogues as anti-ovarian cancer and antimalarial agents

Thymoquinone (TQ), 2-isopropyl-5-methyl-1,4-benzoquinone, a natural product isolated from Nigella sativa L., has previously been demonstrated to exhibit antiproliferative activity in vitro against a range of cancers as well as the human malarial parasite Plasmodium falciparum. We describe here the synthesis of a series of analogues of TQ that explore the potential for nitrogen-substitution to this scaffold, or reduction to a hydroquinone scaffold, in increasing the potency of this antiproliferative activity against ovarian cancer cell lines and P. falciparum. In addition, alkyl or halogen-substituted analogues were commercially sourced and tested in parallel. Several TQ analogues with improved potency against ovarian cancer cells and P. falciparum were found, although this increase is suggested to be moderate. Key aspects of the structure activity relationship that could be further explored are highlighted.     

Design,synthesis and biological evaluation of curcumin analogues as novel LSD1 inhibitors

Histone lysine-specific demethylase 1 (LSD1) was the first discovered histone demethylase. Inactivating LSD1 or downregulating its expression inhibits cancer-cell development, and thus, it is an attractive molecular target for the development of novel cancer therapeutics. In this study, we worked on the structural optimization of natural products and identified 30 novel LSD1 inhibitors. Utilizing a structure-based drug design strategy, we designed and synthesized a series of curcumin analogues that were shown to be potent LSD1 inhibitors in the enzyme assay. Compound WB07 displayed the most potent LSD1 inhibitory activity, with an IC₅₀ value of 0.8 μM. Moreover, WA20 showed an anticlonogenic effect on A549 cells with an IC₅₀ value of 4.4 μM. Molecular docking simulations were also carried out, and the results indicated that the inhibitors bound to the protein active site located around the key residues of Asp555 and Asp556. These findings suggested that compounds WA20 and WB07 are the first curcumin analogue-based LSD1 inhibitors with remarkable A549 suppressive activity, providing a novel scaffold for the development of LSD1 inhibitors.     

Design,synthesis and evaluation of antimicrobial activity of N-terminal modified Leucocin A analogues

Class IIa bacteriocins are potent antimicrobial peptides produced by lactic acid bacteria to destroy competing microorganisms. The N-terminal domain of these peptides consists of a conserved YGNGV sequence and a disulphide bond. The YGNGV motif is essential for activity, whereas, the two cysteines involved in the disulphide bond can be replaced with hydrophobic residues. The C-terminal region has variable sequences, and folds into a conserved amphipathic α-helical structure. To elucidate the structure–activity relationship in the N-terminal domain of these peptides, three analogues (1–3) of a class IIa bacteriocin, Leucocin A (LeuA), were designed and synthesized by replacing the N-terminal β-sheet residues of the native peptide with shorter β-turn motifs. Such replacement abolished the antibacterial activity in the analogues, however, analogue 1 was able to competitively inhibit the activity of native LeuA. Native LeuA (37-mer) was synthesized using native chemical ligation method in high yield. Solution conformation study using circular dichroism spectroscopy and molecular dynamics simulations suggested that the C-terminal region of analogue 1 adopts helical folding as found in LeuA, while the N-terminal region did not fold into β-sheet conformation. These structure–activity studies highlight the role of proper folding and complete sequence in the activity of class IIa bacteriocins.     

Synthesis and cytotoxicity of pyranonaphthoquinone natural product analogues under bioreductive conditions

We have synthesised a focused library of derivatives of natural products containing the pyranonaphthoquinone moiety including the first report of such a scaffold with an appended tetrazole functionality. Examples include kalafungin derivatives as well as analogues of nanaomycin and eleutherin. These compounds were assessed for cytotoxic activation by breast cancer cell lines engineered to express the prototypic human one- and two-electron quinone bioreductive enzymes, NADPH: cytochrome P450 oxidoreductase (POR) and NAD(P)H: quinoneoxidoreductase 1 (NQO1; DT-diaphorase), respectively. Several compounds were observed to be cytotoxic at sub-micromolar level and a pattern of increased aerobic potency was observed in cells over expressing POR. A subset of analogues was assessed under anoxic conditions, where cytotoxicity was reduced, implicating redox cycling as a major mechanism of toxicity. The substrate specificity for reductive enzymes is relevant to the future design of bioreductive prodrugs to treat cancer.     

Design,synthesis, and SAR of embelin analogues as the inhibitors of PAI-1 (plasminogen activator inhibitor-1)

The natural product embelin was found to have PAI-1 inhibitory activity with the IC₅₀ value of 4.94 μM. Based on the structure of embelin, a series of analogues were designed, synthesized, and evaluated for their ability to inhibit PAI-1. The SAR study on these compounds disclosed that the inhibitory potency largely depended on the hydroxyl groups at C2 and C5, and the length of the alkyl chains at C3 and C6. Compound 11 displayed the best PAI-1 inhibitory potency with the IC₅₀ value of 0.18 μM.