Apoptolidins B and C: isolation, structure determination, and biological activity

Macrolides from rare actinomycetes: Structures and bioactivities

Rare actinomycetes are the sources of numerous biologically active secondary metabolites with diverse structures. Among them are macrolides, which have been shown to display several antibiotic activities. In this review, twenty-six groups of macrolides from rare actinomycetes are presented, with their bioactivities and structures of representatives from each group. It has been divided according to the classes of macrolides. The most interesting groups with a wide range of biological activities are ammocidins, bafilomycins, neomaclafungins, rosaramicins, spinosyns, and tiacumicins. Most macrolides are obtained from the genus, Micromonospora, with smaller contributions from genera such as Saccharothrix, Amycolatopsis, Nocardiopsis and Catenulispora. These macrolides display unique cytotoxic, antibacterial, antifungal, antimicrobial, insecticidal, anti-trypanosomal, antimalarial, antiprotozoal, antimycobacterial and anti-herpetic activity. Based on their noticeable bioactivities and diverse structures, macrolides from rare actinomycetes deserve to be investigated further for future applications in medicine. This work highlights the bioactivities and structures of important classes of macrolides from rare actinomycetes, which could be used in medicine in the future or which are already in the market.

Defueling the cancer: ATP synthase as an emerging target in cancer therapy

Reprogramming of cellular metabolism is a hallmark of cancer. Mitochondrial ATP synthase (MAS) produces most of the ATP that drives the cell. High expression of the MAS-composing proteins is found during cancer and is linked to a poor prognosis in glioblastoma, ovarian cancer, prostate cancer, breast cancer, and clear cell renal cell carcinoma. Cell surface-expressed ATP synthase, translocated from mitochondrion to cell membrane, involves the angiogenesis, tumorigenesis, and metastasis of cancer. ATP synthase has therefore been considered a therapeutic target. We review recent various ATP synthase inhibitors that suppress tumor growth and are being tested for the clinic.

Expeditious Asymmetric Synthesis of Polypropionates Relying on Sulfur Dioxide-Induced C–C Bond Forming Reactions

For a long time, the organic chemistry of sulfur dioxide (SO2) consisted of sulfinates that react with carbon electrophiles to generate sulfones. With alkenes and other unsaturated compounds, SO2 generates polymeric materials such as polysulfones. More recently, H-ene, sila-ene and hetero-Diels–Alder reactions of SO2 have been realized under conditions that avoid polymer formation. Sultines resulting from the hetero-Diels–Alder reactions of conjugated dienes and SO2 are formed more rapidly than the corresponding more stable sulfolenes resulting from the cheletropic additions. In the presence of a protic or Lewis acid catalyst, the sultines derived from 1-alkoxydienes are ionized into zwitterionic intermediates bearing 1-alkoxyallylic cation moieties which react with electro-rich alkenes such as enol silyl ethers and allylsilanes with high stereoselectivity. (C–C-bond formation through Umpolung induced by SO2). This produces silyl sulfinates that react with carbon electrophiles to give sulfones (one-pot four component asymmetric synthesis of sulfones), or with Cl2, generating the corresponding sulfonamides that can be reacted in situ with primary and secondary amines (one-pot four component asymmetric synthesis of sulfonamides). Alternatively, Pd-catalyzed desulfinylation generates enantiomerically pure polypropionate stereotriads in one-pot operations. The chirons so obtained are flanked by an ethyl ketone moiety on one side and by a prop-1-en-1-yl carboxylate group on the other. They are ready for two-directional chain elongations, realizing expeditious synthesis of long-chain polypropionates and polyketides. The stereotriads have also been converted into simpler polypropionates such as the cyclohexanone moiety of baconipyrone A and B, Kishi’s stereoheptad unit of rifamycin S, Nicolaou’s C1–C11-fragment and Koert’s C16–CI fragment of apoptolidin A. This has also permitted the first total synthesis of (-)-dolabriferol.

Natural products and other inhibitors of F1FO ATP synthase

F₁F_O ATP synthase is responsible for the production of >95% of all ATP synthesis within the cell. Dysregulation of its expression, activity or localization is linked to various human diseases including cancer, diabetes, and Alzheimer's and Parkinson's disease. In addition, ATP synthase is a novel and viable drug target for the development of antimicrobials as evidenced by bedaquiline, which was approved in 2012 for the treatment of tuberculosis. Historically, natural products have been a rich source of ATP synthase inhibitors that help unravel the role of F₁F_O ATP synthase in cellular bioenergetics. During the last decade, new modulators of ATP synthase have been discovered through the isolation of novel natural products as well as through a ligand-based drug design process. In addition, new data has been obtained with regards to the structure and function of ATP synthase under physiological and pathological conditions. Crystal structure studies have provided a significant insight into the rotary function of the enzyme and may provide additional opportunities to design a new generation of inhibitors. This review provides an update on recently discovered ATP synthase modulators as well as an update on existing scaffolds.

Cytochrome P450 Monooxygenase CYP139 Family Involved in the Synthesis of Secondary Metabolites in 824 Mycobacterial Species

Tuberculosis (TB) is one of the top infectious diseases causing numerous human deaths in the world. Despite enormous efforts, the physiology of the causative agent, Mycobacterium tuberculosis, is poorly understood. To contribute to better understanding the physiological capacity of these microbes, we have carried out extensive in silico analyses of the 1111 mycobacterial species genomes focusing on revealing the role of the orphan cytochrome P450 monooxygenase (CYP) CYP139 family. We have found that CYP139 members are present in 894 species belonging to three mycobacterial groups: M. tuberculosis complex (850-species), Mycobacterium avium complex (34-species), and non-tuberculosis mycobacteria (10-species), with all CYP139 members belonging to the subfamily “A”. CYP139 members have unique amino acid patterns at the CXG motif. Amino acid conservation analysis placed this family in the 8th among CYP families belonging to different biological domains and kingdoms. Biosynthetic gene cluster analyses have revealed that 92% of CYP139As might be associated with producing different secondary metabolites. Such enhanced secondary metabolic potentials with the involvement of CYP139A members might have provided mycobacterial species with advantageous traits in diverse niches competing with other microbial or viral agents, and might help these microbes infect hosts by interfering with the hosts’ metabolism and immune system.

Amycolatopsins A-C: antimycobacterial glycosylated polyketide macrolides from the Australian soil Amycolatopsis sp. MST-108494

A southern Australian soil isolate, Amycolatopsis sp. MST-108494, was subjected to a panel of fermentation and media optimization trials, supported by analytical chemical profiling, to detect and enhance production of a rare class of secondary metabolites. Chemical fractionation of two complementary fermentations yielded three new polyketides, identified by detailed spectroscopic analysis as the glycosylated macrolactones, amycolatopsins A (1), B (2) and C (3), closely related to the ammocidins and apoptolidins. Amycolatopsins 1 and 3 selectively inhibited growth of Mycobacterium bovis (BCG) and Mycobacterium tuberculosis (H37Rv) when compared with other Gram-positive or Gram-negative bacteria, with 3 exhibiting low levels of cytotoxicity toward mammalian cells. Thus, our data reveal promising structure activity relationship correlations where the antimycobacterial properties of amycolatopsins are enhanced by hydroxylation of the 6-Me (that is, 1 and 3), whereas mammalian cytotoxicity is decreased by hydrolysis of the disaccharide moiety (that is, 3).

Amphiphilic glycoconjugates as potential anti-cancer chemotherapeutics

Amphiphilicity is one of the desirable features in the process of drug development which improves the biological as well as the pharmacokinetics profile of bioactive molecule. Carbohydrate moieties present in anti-cancer natural products and synthetic molecules influence the amphiphilicity and hence their bioactivity. This review focuses on natural and synthetic amphiphilic anti-cancer glycoconjugates. Different classes of molecules with varying degree of amphiphilicity are covered with discussions on their structure-activity relationship and mechanism of action.

The use of fluorescently-tagged apoptolidins in cellular uptake and response studies

The apoptolidins are glycomacrolide microbial metabolites reported to be selectively cytotoxic against tumor cells. Using fluorescently tagged active derivatives we demonstrate selective uptake of these four tagged glycomacrolides in cancer cells over healthy human blood cells. We also demonstrate the utility of these five fluorescently tagged glycomacrolides in fluorescent flow cytometry to monitor cellular uptake of the six glycomacrolides and cellular response.

Succinylated Apoptolidins from Amycolatopsis sp. ICBB 8242

Two new apoptolidins, 2'-O-succinyl-apoptolidin A (11) and 3'-O-succinyl-apoptolidin A (12), were isolated from the culture broth of an Indonesian Amycolatopsis sp. ICBB 8242. These compounds inhibit the proliferation and viability of human H292 and HeLa cells. However, in contrast to apoptolidin A (1), they do not inhibit cellular respiration in H292 cells. It is proposed that apoptolidins are produced and secreted in their succinylated forms and 1 is the hydrolysis product of 11 and 12.

Fluorescent probes of the apoptolidins and their utility in cellular localization studies

Apoptolidin A has been described among the top 0.1% most-cell-selective cytotoxic agents to be evaluated in the NCI 60 cell line panel. The molecular structure of apoptolidin A consists of a 20-membered macrolide with mono- and disaccharide moieties. In contrast to apoptolidin A, the aglycone (apoptolidinone) shows no cytotoxicity (>10 μM) when evaluated against several tumor cell lines. Apoptolidin H, the C27 deglycosylated analogue of apoptolidin A, displayed sub-micromolar activity against H292 lung carcinoma cells. Selective esterification of apoptolidins A and H with 5-azidopentanoic acid afforded azido-functionalized derivatives of potency equal to that of the parent macrolide. They also underwent strain-promoted alkyne-azido cycloaddition reactions to provide access to fluorescent and biotin-functionalized probes. Microscopy studies demonstrate apoptolidins A and H localize in the mitochondria of H292 human lung carcinoma cells.

Biosynthesis of the Apoptolidins in Nocardiopsis sp. FU 40

The apoptolidins are 20/21-membered macrolides produced by Nocardiopsis sp. FU40. Several members of this family are potent and remarkably selective inducers of apoptosis in cancer cell lines, likely via a distinct mitochondria associated target. To investigate the biosynthesis of this natural product, the complete genome of the apoptolidin producer Nocardiopsis sp. FU40 was sequenced and a 116 Kb region was identified containing a putative apoptolidin biosynthetic gene cluster. The apoptolidin gene cluster comprises a type I polyketide synthase, with 13 homologating modules, apparently initiated in an unprecedented fashion via transfer from a methoxymalonyl-acyl carrier protein loading module. Spanning approximately 39 open reading frames, the gene cluster was cloned into a series of overlapping cosmids and functionally validated by targeted gene disruption experiments in the producing organism. Disruption of putative PKS and P(450) genes delineated the roles of these genes in apoptolidin biosynthesis and chemical complementation studies demonstrated intact biosynthesis peripheral to the disrupted genes. This work provides insight into details of the biosynthesis of this biologically significant natural product and provides a basis for future mutasynthetic methods for the generation of non-natural apopotolidins.

Natural glycoconjugates with antitumor activity

Cancer is one of the major causes of death worldwide. As a consequence, many different therapeutic approaches, including the use of glycosides as anticancer agents, have been developed. Various glycosylated natural products exhibit high activity against a variety of microbes and human tumors. In this review we classify glycosides according to the nature of their aglycone (non-saccharidic) part. Among them, we describe anthracyclines, aureolic acids, enediyne antibiotics, macrolide and glycopeptides presenting different strengths and mechanisms of action against human cancers. In some cases, the glycosidic residue is crucial for their activity, such as in anthracycline, aureolic acid and enediyne antibiotics; in other cases, Nature has exploited glycosylation to improve solubility or pharmacokinetic properties, as in the glycopeptides. In this review we focus our attention on natural glycoconjugates with anticancer properties. The structure of several of the carbohydrate moieties found in these conjugates and their role are described. The structure–activity relationship of some of these compounds, together with the structural features of their interaction with the biological targets, are also reported. Taken together, all this information is useful for the design of new potential anti-tumor drugs.

Light-induced isomerization of apoptolidin a leads to inversion of C2-C3 double bond geometry

The isolation, characterization, and cytotoxicity against H292 cells of apoptolidin G are reported. Apoptolidin G is shown to be derived by a light-induced isomerization of the C2-C3 carbon-carbon double bond of apoptolidin A.

Apoptolidins E and F, new glycosylated macrolactones isolated from Nocardiopsis sp

Two new glycosylated macrolactones, apoptolidins E (5) and F (6), were isolated from fermentation of the actinomycete Nocardiopsis sp. and their structures assigned. Lacking the C16 and C20 oxygens of apoptolidin A (1), these macrolides are also the first members of this family to display a 4-O-methyl-l-rhamnose at C9 rather than a 6-deoxy-4-O-methyl-l-glucose.

An approach to the site-selective diversification of apoptolidin A with peptide-based catalysts

We report the application of peptide-based catalysts to the site-selective modification of apoptolidin A (1), an agent that displays remarkable selectivity for inducing apoptosis in E1A-transformed cell lines. Key to the approach was the development of an assay suitable for the screening of dozens of catalysts in parallel reactions that could be conducted using only microgram quantities of the starting material. Employing this assay, catalysts (e.g., 11 and ent-11) were identified that afforded unique product distributions, distinct from the product mixtures produced when a simple catalyst (N,N-dimethyl-4-aminopyridine (10)) was employed. Preparative reactions were then carried out with the preferred catalysts so that unique, homogeneous apoptolidin analogues could be isolated and characterized. From these studies, three new apoptolidin analogues were obtained (12-14), each differing from the other in either the location of acyl group substituents or the number of acetate groups appended to the natural product scaffold. Biological evaluation of the new apoptolidin analogues was then conducted using growth inhibition assays based on the H292 human lung carcinoma cell line. The new analogues exhibited activities comparable to apoptolidin A.

Combined chemical and biosynthetic route to access a new apoptolidin congener

Glycosylation of a synthetic aglycone using precursor-directed biosynthesis is facilitated by a chemical ketosynthase "knockdown" of the apoptolidin producer Nocardiopsis sp. This synthetic approach facilitated the preparation of an unnatural disaccharide derivative of apoptolidin D that substantially restores cytotoxicity against H292 cells and deconvolutes the role of the decorating sugars in apoptolidin bioactivity.

Total synthesis of apoptolidin A

A highly convergent, enantioselective total synthesis of the potent antitumor agent apoptolidin A has been completed. The key transformations include highly selective glycosylations to attach the C27 disaccharide and the C9 6'-deoxy-l-glucose, a cross-metathesis to incorporate the C1-C10 trienoate unit, and a Yamaguchi macrolactonization to complete the macrocycle. Twelve stereocenters in the polypropionate segments and sugar units were established through diastereoselective chlorotitanium enolate aldol reactions.

Synthesis and evaluation of the cytotoxicity of apoptolidinones A and D

Apoptolidins A-D are microbial secondary metabolites shown to be selectively cytotoxic against several cancer cell lines and noncytotoxic against normal cells. Total syntheses of apoptolidinones A and D are reported. The efficient synthetic strategy leading to the apoptolidinones features construction of the common 20-membered macrolactone by an intramolecular Suzuki reaction and stereocontrolled aldol reactions establishing the C19/C20 and C22/C23 stereocenters. In contrast to apoptolidin A, the aglycones apoptolidinone A and D were shown to be noncytotoxic when evaluated against human lung cancer cells (H292).

Use of sultines in the asymmetric synthesis of polypropionate antibiotics

At low temperature and in the presence of an acid catalyst, SO2 adds to 1,3-dienes equilibrating with the corresponding 3,6-dihydro-1,2-oxathiin-2-oxides (sultines). These compounds are unstable above -60 °C and equilibrate with the more stable 2,5-dihydrothiophene 1,1-dioxides (sulfolenes). The hetero-Diels-Alder additions of SO2 are suprafacial and follow the Alder endo rule. The sultines derived from 1-oxy-substituted and 1,3-dioxy-disubstituted 1,3-dienes cannot be observed at -100 °C but are believed to be formed faster than the corresponding sulfolenes. In the presence of acid catalysts, the 6-oxy-substituted sultines equilibrate with zwitterionic species that react with electron-rich alkenes such as enoxysilanes and allylsilanes, generating β,γ-unsaturated silyl sulfinates that can be desilylated and desulfinylated to generate polypropionate fragments containing up to three contiguous stereogenic centers and an (E)-alkene unit. Alternatively, the silyl sulfinates can be reacted with electrophiles to generate polyfunctional sulfones (one-pot, four-component synthesis of sulfones), or oxidized into sulfonyl chlorides and reacted with amines, then realizing a one-pot, four-component synthesis of polyfunctional sulfonamides. Using enantiomerically enriched dienes such as 1-[(R)- or 1-(S)-phenylethyloxy]-2-methyl-(E,E)-penta-1,3-dien-3-yl isobutyrate, derived from inexpensive (R)- or (S)-1-phenylethanol, enantiomerically enriched stereotriads are obtained in one-pot operations. The latter are ready for further chain elongation. This has permitted the development of expeditious total asymmetric syntheses of important natural products of biological interest such as the baconipyrones, rifamycin S, and apoptolidin A.

Short synthesis of the C16-C28 polyketide fragment of apoptolidin A aglycone

Starting from (E,E)-1-[(1R)-(phenylethyl)oxy]-2-methylpenta-1,3-diene and triethylsilyl enol ether of butanone rapid access to Koert's advanced C10-C28 polyketide fragment of apoptolidin A is now possible.

Apoptolidin A: total synthesis and partially glycosylated analogues

The total synthesis of apoptolidin A is described employing an early glycosylation strategy. Strategic disconnections were chosen between C11-C12 (cross-coupling) and C19O-C1 (macrocyclization). The cis-selective glycosylation at C9-OH was achieved with the new SIBA protective group at O2/O3 of the L-glucose residue. Auxiliary substitutents at the 2-position of the 2-deoxy sugars were applied to form selectively the glycosidic linkages of the C27 disaccharide. The cross-coupling of the glycosylated northern half with the glycosylated southern half was achieved with CuI-thiophene carboxylate. The macrocyclization of a trihydroxy carboxylic acid produced the 20-membered macrolide selectively. H2SiF6 was suitable for the final deprotection of the silyl ethers and the conversion of the C21 methylketal into the hemiketal. The synthetic flexibility of the approach was proven by the synthesis of some glycovariants.

Lactol-directed osmylation. Stereodivergent synthesis of four C-19,20 apoptolidin diols from a single allylic hemiacetal

A synthetic approach to prepare four Apoptolidin C-19,20 diastereomeric diol derivatives was developed. Two diastereomers were obtained from the (Z)-form, which is converted to the (E)-form, followed by dihydroxylation to deliver two more diastereomers. The (E)-allylic hemiacetal and methoxyacetal showed opposite diastereoselectivity.

Apoptolidinone A: Synthesis of the Apoptolidin A Aglycone

An efficient stereocontrolled synthesis of apoptolidinone A, the aglycone of apoptolidin A is described. The synthetic strategy relies on a cross coupling between C11/C12 of a northern half (C1-C11) and a southern part (C12-C28) followed by a ring-size selective macrolactonization. Key steps for the introduction of the southern half stereocenters are a stereoselective aldol reaction, a substrate controlled dihydroxylation and a chelation-controlled Grignard/aldehyde addition. The conjugated triene of the northern half was built up successively by E-selective Wittig reactions. L-Malic acid was chosen as the chiral pool source for the C8/C9 stereocenters. The final cleavage of the silyl ethers and the conversion of the C21 methyl ketal into the hemiketal was achieved by HF.pyridine.

Correlation of F0F1-ATPase inhibition and antiproliferative activity of apoptolidin analogues

[structure: see text] Apoptolidin (1) exhibits potent and highly selective apoptosis inducing activity against sensitive cancer cell lines and is hypothesized to act by inhibition of mitochondrial F(0)F(1)-ATP synthase. A series of apoptolidin derivatives, including a new intermolecular Diels-Alder adduct, were analyzed for antiproliferative activity in E1A-transformed rat fibroblasts. Potent F(0)F(1)-ATPase inhibition was not a sufficient determinant of antiproliferative activity for several analogues, suggesting the existence of a secondary biological target or more complex mode of action for apoptolidin.

Apoptolidin: Induction of Apoptosis by a Natural Product

Apoptolidin is a natural product that selectively induces apoptosis in several cancer cell lines. Apoptosis, programmed cell death, is a biological key pathway for regulating homeostasis and morphogenesis. Apoptotic misregulations are connected with several diseases, in particular cancer. The extrinsic way to apoptosis leads through death ligands and death receptors to the activiation of the caspase cascade, which results in proteolytic degradation of the cell architecture. The intrinsic pathway transmits signals of internal cellular damage to the mitochondrion, which loses its structural integrity, and forms an apoptosome that initiates the caspase cascade. Compounds which regulate apoptosis are of high medical significance. Many natural products regulate apoptotic pathways, and apoptolidin is one of them. The known synthetic routes to apoptolidin are described and compared in this Review. Selected further natural products which regulate apoptosis are introduced briefly.

Enantioselective Synthesis of Apoptolidinone: Exploiting the Versatility of Thiazolidinethione Chiral Auxiliaries

An efficient, enantioselective synthesis of apoptolidinone has been completed, demonstrating the versatility of thiazolidinethione auxiliaries. Three propionate aldol additions and two asymmetric glycolate alkylations function to establish 8 of the 12 stereogenic carbon centers. A cross-metathesis reaction is utilized to assemble the C1-C10 trieneoate fragment and the C11-C28 polypropionate region of the molecule.

Enantioselective Synthesis of Apoptolidin Sugars

[structure: see text] The de novo synthesis of the C9 and C27 sugar subunits (2) and (3), respectively, of the potent antitumor agent, apoptolidin, has been accomplished. A titanium tetrachloride-mediated asymmetric anti glycolate aldol addition was utilized to establish the 4' and 5' stereogenic centers of each of the three monosaccharides. Elaboration of the aldol adducts efficiently provided the three sugar units. A beta-selective glycosidation completed the construction of the C27 disaccharide.