Structure-activity Relationship for FR901464: A Versatile Method for the Conversion and Preparation of Biologically Active Biotinylated Probes

Synthesis, growth inhibitory activity against tumor cells, and structure-activity relationship of CGK733 and its analogs

CGK733 was reported as a compound that inhibited ATM/ATR kinase activities and blocked their checkpoint signaling pathways with great selectivity. However, this paper was subsequently retracted, and the truth about the activity of CGK733 remains unclear. We synthesized various analogs of CGK733 with a modification of the carboxylic acid moiety and/or the aniline derivative moiety to accumulate knowledge of the structure-activity relationship of this compound. Growth inhibitory activity of CGK733 and novel 35 analogs against HeLa S3 cells was evaluated, and the structure-activity relationship revealed that analogs with the 2-naphthyl or 4-fluorophenyl group instead of the benzhydryl group have activity comparable to CGK733 and that the 3-nitro group on the aniline moiety significantly affects the activity.

Synthesis and antiproliferative activity of a tetrahydrofuran analog of FR901464

FR901464 is a natural product that exhibits antiproliferative activity at single-digit nanomolar concentrations in cancer cells. Its tetrahydropyran-spiroepoxide covalently binds the spliceosome. Through our medicinal chemistry campaign, we serendipitously discovered that a bromoetherification formed a tetrahydrofuran. The tetrahydrofuran analog was three orders of magnitude less potent than the corresponding tetrahydropyran analogs. This study shows the significance of the tetrahydropyran ring that presents the epoxide toward the spliceosome.

Synthesis and Conformational Analysis of FR901464-Based RNA Splicing Modulators and Their Synergism in Drug-Resistant Cancers

FR901464 is a cytotoxic natural product that binds splicing factor 3B subunit 1 (SF3B1) and PHD finger protein 5A (PHF5A), the components of the human spliceosome. The amide-containing tetrahydropyran ring binds SF3B1, and it remains unclear how the substituents on the ring contribute to the binding. Here, we synthesized meayamycin D, an analogue of FR901464, and three additional analogues to probe the conformation through methyl scanning. We discovered that the amide-containing tetrahydropyran ring assumes only one of the two possible chair conformations and that methylation of the nitrogen distorts the chair form, dramatically reducing cytotoxicity. Meayamycin D induced alternative splicing of MCL-1, showed strong synergism with venetoclax in drug-resistant lung cancer cells, and was cancer-specific over normal cells. Meayamycin D incorporates an alkyl ether and shows a long half-life in mouse plasma. The characteristics of meayamycin D may provide an approach to designing other bioactive L-shaped molecules.

Molecular Threat of Splicing Factor Mutations to Myeloid Malignancies and Potential Therapeutic Modulations

Splicing factors are frequently mutated in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). These mutations are presumed to contribute to oncogenic transformation, but the underlying mechanisms remain incompletely understood. While no specific treatment option is available for MDS/AML patients with spliceosome mutations, novel targeting strategies are actively explored, leading to clinical trials of small molecule inhibitors that target the spliceosome, DNA damage response pathway, and immune response pathway. Here, we review recent progress in mechanistic understanding of splicing factor mutations promoting disease progression and summarize potential therapeutic strategies, which, if successful, would provide clinical benefit to patients carrying splicing factor mutations.

The Role of Alternative Splicing in Cancer: Regulatory Mechanism, Therapeutic Strategy, and Bioinformatics Application

[Formula: see text] Alternative splicing (AS) can generate distinct transcripts and subsequent isoforms that play differential functions from the same pre-mRNA. Recently, increasing numbers of studies have emerged, unmasking the association between AS and cancer. In this review, we arranged AS events that are closely related to cancer progression and presented promising treatments based on AS for cancer therapy. Obtaining proliferative capacity, acquiring invasive properties, gaining angiogenic features, shifting metabolic ability, and getting immune escape inclination are all splicing events involved in biological processes. Spliceosome-targeted and antisense oligonucleotide technologies are two novel strategies that are hopeful in tumor therapy. In addition, bioinformatics applications based on AS were summarized for better prediction and elucidation of regulatory routines mingled in. Together, we aimed to provide a better understanding of complicated AS events associated with cancer biology and reveal AS a promising target of cancer treatment in the future.

Spliceostatins and Derivatives: Chemical Syntheses and Biological Properties of Potent Splicing Inhibitors

Spliceostatins and thailanstatins are intriguing natural products due to their structural features as well as their biological significance. This family of natural products has been the subject of immense synthetic interest because they exhibit very potent cytotoxicity in representative human cancer cell lines. The cytotoxic properties of these natural products are related to their ability to inhibit spliceosomes. FR901564 and spliceostatins have been shown to inhibit spliceosomes by binding to their SF3B component. Structurally, these natural products contain two highly functionalized tetrahydropyran rings with multiple stereogenic centers joined by a diene moiety and an acyclic side chain linked with an amide bond. Total syntheses of this family of natural products led to the development of useful synthetic strategies, which enabled the synthesis of potent derivatives. The spliceosome modulating properties of spliceostatins and synthetic derivatives opened the door for understanding the underlying spliceosome mechanism as well as the development of new therapies based upon small-molecule splicing modulators. This review outlines the total synthesis of spliceostatins, synthetic studies of structural derivatives, and their bioactivity.

Splicing modulators: on the way from nature to clinic

Over the course of more than two decades, natural products isolated from various microorganisms and plants have built the foundation for chemical biology research into the mechanism of pre-mRNA splicing. Hand in hand with advances in scientific methodology small molecule splicing modulators have become powerful tools for investigating, not just the splicing mechanism, but also the cellular effect of altered mRNA processing. Based on thorough structure-activity studies, synthetic analogues have moved on from scientific tool compounds to experimental drugs. With current advances in drug discovery methodology and new means of attacking targets previously thought undruggable, we can expect further advances in both research and therapeutics based on small molecule splicing modulators.

A Comprehensive Overview of Structure-Activity Relationships of Small-Molecule Splicing Modulators Targeting SF3B1 as Anticancer Agents

The pre-mRNA splicing factor SF3B1 shows recurrent mutations among hematologic malignancies and some solid tumors. In 2007, the identification of two cytotoxic natural products, which showed splicing inhibition by binding to SF3b, prompted the development of small-molecule splicing modulators of SF3B1 as therapeutics for cancer. Recent studies suggested that spliceosome-mutant cells are preferentially sensitive to pharmacologic splicing modulation; therefore, exploring the clinical utility of splicing modulator therapies in patients with spliceosome-mutant hematologic malignancies who have failed current therapies is greatly needed, as these patients have few treatment options. H3B-8800 had unique pharmacological activity and exhibited favorable data in phase I clinical trials to treat patients with advanced myeloid malignancies, indicating that further clinical trials are promising. The most established small-molecule modulators of SF3B1 can be categorized into three classes: the bicycles, the monopyranes, and the 12-membered macrolides. This review provides a comprehensive overview of the structure-activity relationships of small-molecule SF3B1 modulators, with a detailed analysis of interactions between modulators and protein binding pocket. The future strategy for splicing modulators development is also discussed.

Bacteria and bacterial anticancer agents as a promising alternative for cancer therapeutics

Cancer is the leading cause of deaths worldwide, though significant advances have occurred in its diagnosis and treatment. The development of resistance against chemotherapeutic agents, their side effects, and non-specific toxicity urge to screen for the novel anticancer agent. Hence, the development of novel anticancer agents with a new mechanism of action has become a major scientific challenge. Bacteria and bacterially produced bioactive compounds have recently emerged as a promising alternative for cancer therapeutics. Bacterial anticancer agents such as antibiotics, bacteriocins, non-ribosomal peptides, polyketides, toxins, etc. These are adopted different mechanisms of actions such as apoptosis, necrosis, reduced angiogenesis, inhibition of translation and splicing, and obstructing essential signaling pathways to kill cancer cells. Also, live tumor-targeting bacteria provided a unique therapeutic alternative for cancer treatment. This review summarizes the anticancer properties and mechanism of actions of the anticancer agents of bacterial origin and antitumor bacteria along with their possible future applications in cancer therapeutics.

Alternative Splicing as a Target for Cancer Treatment

Alternative splicing is a key mechanism determinant for gene expression in metazoan. During alternative splicing, non-coding sequences are removed to generate different mature messenger RNAs due to a combination of sequence elements and cellular factors that contribute to splicing regulation. A different combination of splicing sites, exonic or intronic sequences, mutually exclusive exons or retained introns could be selected during alternative splicing to generate different mature mRNAs that could in turn produce distinct protein products. Alternative splicing is the main source of protein diversity responsible for 90% of human gene expression, and it has recently become a hallmark for cancer with a full potential as a prognostic and therapeutic tool. Currently, more than 15,000 alternative splicing events have been associated to different aspects of cancer biology, including cell proliferation and invasion, apoptosis resistance and susceptibility to different chemotherapeutic drugs. Here, we present well established and newly discovered splicing events that occur in different cancer-related genes, their modification by several approaches and the current status of key tools developed to target alternative splicing with diagnostic and therapeutic purposes.

The synthesis and evaluation of the antiproliferative activity of deacidified GEX1A analogues

GEX1A/herboxidiene (1) is a natural product isolated from Streptomyces sp. and has been reported to target the pre-mRNA splicing process. Although 1 was shown to have antitumor activity in vivo, weight loss was observed in mice when 1 was consecutively administered. We assumed that the carboxylic acid moiety was one of the causes of this toxicity. In this study, a series of amide, carbamate and urea analogues of 1 were synthesized and their antiproliferative activity was evaluated in vitro. The synthesis of urea analogues featured Curtius rearrangement following amine treatment with the one-pot procedure from 1. Furthermore, a structure-activity relationship study of the urea analogues revealed that the pharmacologically preferable basic side chains were acceptable and that compound 9g was equipotent to parent 1. These basic urea analogues would be promising leads for the development of novel antitumor agents.

Achmatowicz Reaction and its Application in the Syntheses of Bioactive Molecules

Substituted pyranones and tetrahydropyrans are structural subunits of many bioactive natural products. Considerable efforts are devoted toward the chemical synthesis of these natural products due to their therapeutic potential as well as low natural abundance. These embedded pyranones and tetrahydropyran structural motifs have been the subject of synthetic interest over the years. While there are methods available for the syntheses of these subunits, there are issues related to regio and stereochemical outcomes, as well as versatility and compatibility of reaction conditions and functional group tolerance. The Achmatowicz reaction, an oxidative ring enlargement of furyl alcohol, was developed in the 1970s. The reaction provides a unique entry to a variety of pyranone derivatives from functionalized furanyl alcohols. These pyranones provide convenient access to substituted tetrahydropyran derivatives. This review outlines general approaches to the synthesis of tetrahydropyrans, covering general mechanistic aspects of the Achmatowicz reaction or rearrangement with an overview of the reagents utilized for the Achmatowicz reaction. The review then focuses on the synthesis of functionalized tetrahydropyrans and pyranones and their applications in the synthesis of natural products and medicinal agents.

Microbial and Natural Metabolites That Inhibit Splicing: A Powerful Alternative for Cancer Treatment

In eukaryotes, genes are frequently interrupted with noncoding sequences named introns. Alternative splicing is a nuclear mechanism by which these introns are removed and flanking coding regions named exons are joined together to generate a message that will be translated in the cytoplasm. This mechanism is catalyzed by a complex machinery known as the spliceosome, which is conformed by more than 300 proteins and ribonucleoproteins that activate and regulate the precision of gene expression when assembled. It has been proposed that several genetic diseases are related to defects in the splicing process, including cancer. For this reason, natural products that show the ability to regulate splicing have attracted enormous attention due to its potential use for cancer treatment. Some microbial metabolites have shown the ability to inhibit gene splicing and the molecular mechanism responsible for this inhibition is being studied for future applications. Here, we summarize the main types of natural products that have been characterized as splicing inhibitors, the recent advances regarding molecular and cellular effects related to these molecules, and the applications reported so far in cancer therapeutics.

Modulating splicing with small molecular inhibitors of the spliceosome

Small molecule inhibitors that target components of the spliceosome have great potential as tools to probe splicing mechanism and dissect splicing regulatory networks in cells. These compounds also hold promise as drug leads for diseases in which splicing regulation plays a critical role, including many cancers. Because the spliceosome is a complicated and dynamic macromolecular machine comprised of many RNA and protein components, a variety of compounds that interfere with different aspects of spliceosome assembly is needed to probe its function. By screening chemical libraries with high-throughput splicing assays, several labs have added to the collection of splicing inhibitors, although the mechanistic insight into splicing yielded from the initial compound hits is somewhat limited so far. In contrast, SF3B1 inhibitors stand out as a great example of what can be accomplished with small molecule tools. This group of compounds were first discovered as natural products that are cytotoxic to cancer cells, and then later shown to target the core spliceosome protein SF3B1. The inhibitors have since been used to uncover details of SF3B1 mechanism in the spliceosome and its impact on gene expression in cells. Continuing structure activity relationship analysis of the compounds is also making progress in identifying chemical features key to their function, which is critical in understanding the mechanism of SF3B1 inhibition. The knowledge is also important for the design of analogs with new and useful features for both splicing researchers and clinicians hoping to exploit splicing as pressure point to target in cancer therapy. WIREs RNA 2017, 8:e1381. doi: 10.1002/wrna.1381 For further resources related to this article, please visit the WIREs website.

Design, synthesis and in vitro splicing inhibition of desmethyl and carba-derivatives of herboxidiene

Herboxidiene is a potent inhibitor of spliceosomes. It exhibits excellent anticancer activity against multiple human cancer cell lines. Herein, we describe an enantioselective synthesis of a desmethyl derivative and the corresponding carba-derivatives of herboxidiene. The synthesis involved Suzuki coupling of a vinyl iodide with boronate as the key reaction. For the synthesis of carba-derivatives, the corresponding optically active cyclohexane-1,3-dicarbonyl derivatives were synthesized using an enantioselective desymmetrization of meso-anhydride. The biological properties of these derivatives were evaluated in an in vitro splicing assay.

Generate a bioactive natural product library by mining bacterial cytochrome P450 patterns

The increased number of annotated bacterial genomes provides a vast resource for genome mining. Several bacterial natural products with epoxide groups have been identified as pre-mRNA spliceosome inhibitors and antitumor compounds through genome mining. These epoxide-containing natural products feature a common biosynthetic characteristic that cytochrome P450s (CYPs) and its patterns such as epoxidases are employed in the tailoring reactions. The tailoring enzyme patterns are essential to both biological activities and structural diversity of natural products, and can be used for enzyme pattern-based genome mining. Recent development of direct cloning, heterologous expression, manipulation of the biosynthetic pathways and the CRISPR-CAS9 system have provided molecular biology tools to turn on or pull out nascent biosynthetic gene clusters to generate a microbial natural product library. This review focuses on a library of epoxide-containing natural products and their associated CYPs, with the intention to provide strategies on diversifying the structures of CYP-catalyzed bioactive natural products. It is conceivable that a library of diversified bioactive natural products will be created by pattern-based genome mining, direct cloning and heterologous expression as well as the genomic manipulation.

Enantioselective synthesis of spliceostatin E and evaluation of biological activity

An enantioselective total synthesis of spliceostatin E has been accomplished. The δ-lactone unit A was constructed from readily available (R)-glycidyl alcohol using a ring-closing olefin metathesis as the key reaction. A cross-metathesis of ring A containing δ-lactone and the functionalized tetrahydropyran B-ring provided spliceostatin E. Our biological evaluation of synthetic spliceostatin E revealed that it does not inhibit splicing in vitro and does not impact speckle morphology in cells. Spliceostatin E was reported to possess potent antitumor activity.

Cytotoxic Spliceostatins from Burkholderia sp. and Their Semisynthetic Analogues

The spliceostatin class of natural products was reported to be potent cytotoxic agents via inhibition of the spliceosome, a key protein complex in the biosynthesis of mature mRNA. As part of an effort to discover novel leads for cancer chemotherapy, we re-examined this class of compounds from several angles, including fermentation of the producing strains, isolation and structure determination of new analogues, and semisynthetic modification. Accordingly, a group of spliceostatins were isolated from a culture broth of Burkholderia sp. FERM BP-3421, and their structures identified by analysis of spectroscopic data. Semisynthesis was performed on the major components 4 and 5 to generate ester and amide derivatives with improved in vitro potency. With their potent activity against tumor cells and unique mode of action, spliceostatins can be considered potential leads for development of cancer drugs.

Spliceostatin hemiketal biosynthesis in Burkholderia spp. is catalyzed by an iron/α-ketoglutarate-dependent dioxygenase

Spliceostatins are potent spliceosome inhibitors biosynthesized by a hybrid nonribosomal peptide synthetase-polyketide synthase (NRPS-PKS) system of the trans-acyl transferase (AT) type. Burkholderia sp. FERM BP-3421 produces hemiketal spliceostatins, such as FR901464, as well as analogs containing a terminal carboxylic acid. We provide genetic and biochemical evidence for hemiketal biosynthesis by oxidative decarboxylation rather than the previously hypothesized Baeyer-Villiger oxidative release postulated to be catalyzed by a flavin-dependent monooxygenase (FMO) activity internal to the last module of the PKS. Inactivation of Fe(II)/α-ketoglutarate-dependent dioxygenase gene fr9P led to loss of hemiketal congeners, whereas the mutant was still able to produce all major carboxylic acid-type compounds. FMO mutants, on the other hand, produced both hemiketal and carboxylic acid analogs containing an exocyclic methylene instead of an epoxide, indicating that the FMO is involved in epoxidation rather than Baeyer-Villiger oxidation. Moreover, recombinant Fr9P enzyme was shown to catalyze hydroxylation to form β-hydroxy acids, which upon decarboxylation led to hemiketal FR901464. Finally, a third oxygenase activity encoded in the biosynthetic gene cluster, the cytochrome P450 monooxygenase Fr9R, was assigned as a 4-hydroxylase based on gene inactivation results. Identification and deletion of the gene involved in hemiketal formation allowed us to generate a strain--the dioxygenase fr9P(-) mutant--that accumulates only the carboxylic acid-type spliceostatins, which are as potent as the hemiketal analogs, when derivatized to increase cell permeability, but are chemically more stable.

Enantioselective total syntheses of FR901464 and spliceostatin A and evaluation of splicing activity of key derivatives

FR901464 (1) and spliceostatin A (2) are potent inhibitors of spliceosomes. These compounds have shown remarkable anticancer activity against multiple human cancer cell lines. Herein, we describe efficient, enantioselective syntheses of FR901464, spliceostatin A, six corresponding diastereomers and an evaluation of their splicing activity. Syntheses of spliceostatin A and FR901464 were carried out in the longest linear sequence of 9 and 10 steps, respectively. To construct the highly functionalized tetrahydropyran A-ring, we utilized CBS reduction, Achmatowicz rearrangement, Michael addition, and reductive amination as key steps. The remarkable diastereoselectivity of the Michael addition was specifically demonstrated with different substrates under various reaction conditions. The side chain B was prepared from an optically active alcohol, followed by acetylation and hydrogenation over Lindlar’s catalyst. The other densely functionalized tetrahydropyran C-ring was derived from readily available (R)-isopropylidene glyceraldehyde through a route featuring 1,2-addition, cyclic ketalization, and regioselective epoxidation. These fragments were coupled together at a late stage through amidation and cross-metathesis in a convergent manner. Six key diastereomers were then synthesized to probe the importance of specific stereochemical features of FR901464 and spliceostatin A, with respect to their in vitro splicing activity.

Pre-mRNA splicing-modulatory pharmacophores: the total synthesis of herboxidiene, a pladienolide-herboxidiene hybrid analog and related derivatives

Herboxidiene is a natural product that has previously been shown to exhibit antitumor activity by targeting the spliceosome. This activity makes herboxidiene a valuable starting point for the development of anticancer drugs. Here, we report an improved enantioselective synthesis of herboxidiene and the first report of its biologically active totally synthetic analog: 6-norherboxidiene. The synthesis of the tetrahydropyran moiety utilizes the novel application of inverse electron-demand Diels-Alder chemistry and the Ferrier-type rearrangement as key steps. We report, for the first time, cytotoxicity IC50s for synthetic herboxidiene and analogs in human tumor cell lines. We have also demonstrated that synthetic herboxidiene and its analogs can potently modulate the alternate splicing of MDM-2 pre-mRNA.

Splicing inhibition induces gene expression through canonical NF-κB pathway and extracellular signal-related kinase activation

Splicing, a process for mRNA maturation, is essential for correct gene expression after transcription. However, recent studies also suggest that splicing affects transcription, but its mechanism remains elusive. We previously reported that treatment with spliceostatin A (SSA), a specific splicing inhibitor targeting the splicing factor SF3b, leads to transcriptional activation of a small subset of genes. To investigate the underlying mechanism we utilized luciferase reporters driven by the Interleukin 8 (IL-8) and cytomegalovirus (CMV) promoters, as both recruit a similar set of transcription factors. We also found that SSA treatment led to increased extracellular signal-regulated protein kinase (ERK) activity and that chemical inhibition of ERK also led to decreased promoter activation. Systematic deletion studies suggested that NF-κB activation is mainly responsible for SSA-induced promoters activation.

Enantioselective syntheses of FR901464 and spliceostatin A: potent inhibitors of spliceosome

Enantioselective syntheses of FR901464 and spliceostatin A, potent spliceosome inhibitors, are described. The synthesis of FR901464 has been accomplished in a convergent manner in 10 linear steps (20 total steps). The A-tetrahydropyran ring was constructed from (R)-isopropylidene glyceraldehyde. The functionalized tetrahydropyran B-ring was synthesized utilizing a Corey-Bakshi-Shibata reduction, an Achmatowicz reaction, and a stereoselective Michael addition as the key steps. Coupling of A- and B-ring fragments was accomplished via cross-metathesis.

Genomics-guided discovery of thailanstatins A, B, and C As pre-mRNA splicing inhibitors and antiproliferative agents from Burkholderia thailandensis MSMB43

Mining the genome sequence of Burkholderia thailandensis MSMB43 revealed a cryptic biosynthetic gene cluster resembling that of FR901464 (4), a prototype spliceosome inhibitor produced by Pseudomonas sp. No. 2663. Transcriptional analysis revealed a cultivation condition in which a regulatory gene of the cryptic gene cluster is adequately expressed. Consequently, three new compounds, named thailanstatins A (1), B (2), and C (3), were isolated from the fermentation broth of B. thailandensis MSMB43. Thailanstatins are proposed to be biosynthesized by a hybrid polyketide synthase-nonribosomal peptide synthetase pathway. They differ from 4 by lacking an unstable hydroxyl group and by having an extra carboxyl moiety; those differences endow thailanstatins with a significantly greater stability than 4 as tested in phosphate buffer at pH 7.4. In vitro assays showed that thailanstatins inhibit pre-mRNA splicing as potently as 4, with half-maximal inhibitory concentrations in the single to sub-μM range. Cell culture assays indicated that thailanstatins also possess potent antiproliferative activities in representative human cancer cell lines, with half-maximal growth inhibitory concentrations in the single nM range. This work provides new chemical entities for research and development and new structure-activity information for chemical optimization of related spliceosome inhibitors.

Comparison of splicing factor 3b inhibitors in human cells

Name your splice: FR901464 analogues and herboxidiene inhibit constitutive splicing, most likely by inhibiting spliceosomal subunit SF3b. A parallel comparison of these compounds in a cell-based assay system showed meayamycin B as the most potent splicing inhibitor among these small molecules.

The development and application of small molecule modulators of SF3b as therapeutic agents for cancer

The identification of potent spliceosome modulators that demonstrate antitumor activity indicates that this complex may be a target for drug development. Several natural products have been demonstrated to bind to the SF3b1 subunit of this macromolecule and these agents modulate alternative RNA splicing. In this article we describe their biological properties, discuss the validity of the spliceosome as a therapeutic target, and propose that alteration of alternative splicing represents a viable approach for inducing tumor-selective cytotoxicity.

Synthesis of a pladienolide B analogue with the fully functionalized core structure

Starting from (R)-(-)-linalool (6), terminus differentiation and chain extension via aldol type reactions led to ketophosphonate 16 (C1-C8 building block). In a Horner-Wadsworth-Emmons reaction, 16 reacted with aldehyde 22, which contained the vicinal anti-Me-OH pattern and a vinyl iodide function, to provide the C1-C13 part of pladienolide B. After Shiina macrolactonization, reduction of the enone 26 gave the core structure 27. A Stille cross-coupling of vinyl iodide 27 with tributylphenylstannane eventually furnished analogue 30.

Radiosynthesis of antitumor spliceosome modulators

A set of novel antitumor agents (the sudemycins) has recently been described that are analogs of the natural product FR901464. We report the radiosynthesis of two of these antitumor drug lead compounds, using a three step procedure: (1) ester hydrolysis, (2) Lindlar's catalyst/tritium gas to give a (S,Z)-4-acetoxypent-2-enoic acid derivative, and finally (3) amide bond formation. These labeled analogs are useful in developing a better understanding of the pharmacological properties of this new class of therapeutic lead compounds.

Structural requirements for the antiproliferative activity of pre-mRNA splicing inhibitor FR901464

FR901464, a natural product isolated from a bacterium source, activates a reporter gene, inhibits pre-mRNA splicing, and shows antitumor activity. We previously reported the development of a more potent analogue, meayamycin, through the total synthesis of FR901464. Herein, we report detailed structure-activity relationships of FR901464 that revealed the significance of the epoxide, carbon atoms in the tetrahydropyran ring, the Z geometry of the side chain, the 1,3-diene moiety, the C4-hydroxy group, and the C2''-carbonyl group. Importantly, the methyl group of the acetyl substituent was found to be inessential, leading to a new potent analogue. Additionally, partially based on in vivo data, we synthesized and evaluated potentially more metabolically stable analogues for their antiproliferative activity. These structural insights into FR901464 may contribute to the simplification of the natural product for further drug development.

Enantiopure aminopyrans by a Lewis acid promoted rearrangement of 1,2-oxazines: versatile building blocks for oligosaccharide and sugar amino acid mimetics

1,3-Dioxolanyl-substituted 1,2-oxazines, such as syn-1 and anti-1, rearrange under Lewis acidic conditions to provide bicyclic products 2-5. Subsequent reductive transformations afforded enantiopure 3-aminopyran derivatives such as 7 and 9 or their protected diastereomers 16 and 18, which can be regarded as carbohydrate mimetics. An alternative sequence of transformations including selective oxidation of the primary hydroxyl groups in 21 and 24 led to two protected beta-amino acid derivatives with carbohydrate-like backbone (sugar amino acids). Treatment of bicyclic ester 23 with samarium diiodide cleaved the N--O bond and furnished the unusual beta-lactam 27 in excellent yield. Alternatively, gamma-amino acid derivative 29 was efficiently prepared in a few steps. Fairly simple transformations gave azides 32 and 35 or alkyne 30 which are suitable substrates for the construction of oligosaccharide mimetics such as 34 by copper iodide catalyzed cycloadditions. With this report we demonstrate that enantiopure rearrangement products 2-5 are protected precursors of a variety of polyfunctionalized pyran derivatives with great potential for chemical biology.

Synthetic mRNA splicing modulator compounds with in vivo antitumor activity

We report our progress on the development of new synthetic anticancer lead compounds that modulate the splicing of mRNA. We also report the synthesis and evaluation of new biologically active ester and carbamate analogues. Further, we describe initial animal studies demonstrating the antitumor efficacy of compound 5 in vivo. Additionally, we report the enantioselective and diastereospecific synthesis of a new 1,3-dioxane series of active analogues. We confirm that compound 5 inhibits the splicing of mRNA in cell-free nuclear extracts and in a cell-based dual-reporter mRNA splicing assay. In summary, we have developed totally synthetic novel spliceosome modulators as therapeutic lead compounds for a number of highly aggressive cancers. Future efforts will be directed toward the more complete optimization of these compounds as potential human therapeutics.

Meayamycin inhibits pre-messenger RNA splicing and exhibits picomolar activity against multidrug-resistant cells

FR901464 is a potent antitumor natural product that binds to the splicing factor 3b complex and inhibits pre-mRNA splicing. Its analogue, meayamycin, is two orders of magnitude more potent as an antiproliferative agent against human breast cancer MCF-7 cells. Here, we report the picomolar antiproliferative activity of meayamycin against various cancer cell lines and multidrug-resistant cells. Time-dependence studies implied that meayamycin may form a covalent bond with its target protein(s). Meayamycin inhibited pre-mRNA splicing in HEK-293 cells but not alternative splicing in a neuronal system. Meayamycin exhibited specificity toward human lung cancer cells compared with nontumorigenic human lung fibroblasts and retained picomolar growth-inhibitory activity against multidrug-resistant cells. These data suggest that meayamycin is a useful chemical probe to study pre-mRNA splicing in live cells and is a promising lead as an anticancer agent.

Antitumor compounds based on a natural product consensus pharmacophore

We report the design and highly enantioselective synthesis of a potent analogue of the spliceosome inhibitor FR901464, based on a non-natural product scaffold. The design of this compound was facilitated by a pharmacophore hypothesis that assumed key interaction types that are common to FR901464 and an otherwise unrelated natural product (pladienolide). The synthesis allows for the preparation of numerous novel analogues. We present results on the in vitro activity for this compound against several tumor cell lines.

Inhibition of splicing and nuclear retention of pre-mRNA by spliceostatin A in fission yeast

Nuclear retention of pre-mRNAs is tightly regulated by several security mechanisms that prevent pre-mRNA export into the cytoplasm. Recently, spliceostatin A, a methylated derivative of a potent antitumor microbial metabolite FR901464, was found to cause pre-mRNA accumulation and translation in mammalian cells. Here we report that spliceostatin A also inhibits splicing and nuclear retention of pre-mRNA in a fission yeast strain that lacks the multidrug resistance protein Pmd1. As observed in mammalian cells, spliceostatin A is bound to components of the SF3b complex in the spliceosome. Furthermore, overexpression of nup211, a homolog of Saccharomyces cerevisiae MLP1, suppresses translation of pre-mRNAs accumulated by spliceostatin A. These results suggest that the SF3b complex has a conserved role in pre-mRNA retention, which is independent of the Mlp1 function.

Spliceostatin A targets SF3b and inhibits both splicing and nuclear retention of pre-mRNA

The removal of intervening sequences from transcripts is catalyzed by the spliceosome, a multicomponent complex that assembles on the newly synthesized pre-mRNA. Pre-mRNA translation in the cytoplasm leads to the generation of aberrant proteins that are potentially harmful. Therefore, tight control to prevent undesired pre-mRNA export from the nucleus and its subsequent translation is an essential requirement for reliable gene expression. Here, we show that the natural product FR901464 (1) and its methylated derivative, spliceostatin A (2), inhibit in vitro splicing and promote pre-mRNA accumulation by binding to SF3b, a subcomplex of the U2 small nuclear ribonucleoprotein in the spliceosome. Importantly, treatment of cells with these compounds resulted in leakage of pre-mRNA to the cytoplasm, where it was translated. Knockdown of SF3b by small interfering RNA induced phenotypes similar to those seen with spliceostatin A treatment. Thus, the inhibition of pre-mRNA splicing during early steps involving SF3b allows unspliced mRNA leakage and translation.

Total syntheses, fragmentation studies, and antitumor/antiproliferative activities of FR901464 and its low picomolar analogue

FR901464 is a potent anticancer natural product that lowers the mRNA levels of oncogenes and tumor suppressor genes. In this article, we report a convergent enantioselective synthesis of FR901464, which was accomplished in 13 linear steps. Central to the synthetic approach was the diene-ene cross olefin metathesis reaction to generate the C6-C7 olefin without the use of protecting groups as the final step. Additional key reactions include a Zr/Ag-promoted alkynylation to set the C4 stereocenter, a mild and chemoselective Red-Al reduction, a reagent-controlled stereoselective Mislow-Evans-type [2,3]-sigmatropic rearrangement to install the C5 stereocenter, a Carreira asymmetric alkynylation to generate the C4' stereocenter, and a highly efficient ring-closing metathesis-allylic oxidation sequence to form an unsaturated lactone. The decomposition pathways of FR901464's right fragment were studied under physiologically relevant conditions. Facile epoxide opening by beta-elimination gave two enones, one of which could undergo dehydration via its hemiketal to form a furan. To prevent this decomposition pathway, a right fragment was rationally designed and synthesized. This analogue was 12 times more stable than the right fragment of the natural product. Using this more stable right fragment analogue, an FR901464 analogue, meayamycin, was prepared in 13 linear steps. The inhibitions of human breast cancer MCF-7 cell proliferation by synthetic FR901464 and meayamycin were studied, and the GI50 values for these compounds were determined to be 1.1 nM and 10 pM, respectively. Thus, meayamycin is among the most potent anticancer small molecules that do not bind to either DNA or microtubule.