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Remediating Cancer via Splicing Modulation Mark S. Butler* Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, The University of Queensland, St. Lucia, Brisbane, 4072, Australia ABSTRACT: The identification of three distinct but structurally related classes of microbial-derived spliceosome modulators has provided an exciting opportunity for the development of mechanistically new cancer treatments. A team at UC San Diego has undertaken a SAR study on the spliceosome modulator FD-895 that focused on improving compound stability, while retaining potent antiproliferative and splicing activity. This led to the identification of a more potent and stable analog, (17S)-FD-895 (1), and a less active but extremely stable cyclopropane analog 2, which is currently undergoing preclinical evaluation. These analogs will serve as templates for next generation spliceosome modulating anticancer drugs.
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(tubulin inhibitors), paclitaxel and ixabepilone (tubulin stabilizers), everolimus (mTOR), camptothecin (topoisomerase I), doxorubicin (topoisomerase II), homoharringtonine (protein translation), and HDAC (romidepsin). Almost as important, but often downplayed, is the role of natural product chemical probes to facilitate the discovery of new biological pathways and help unravel their functions.3 One of these important new natural product probes is FD895, a polyketide with a 12-membered macrolactone. It was first isolated from a soil-derived actinomycete, Streptomyces hydroscopicus A-9561, by scientists at Taisho Pharmaceutical Co., Ltd. as described in a 1992 patent and a subsequent publication in 1994.4 FD-895 displayed subnanomolar potency against both adriamycin-resistant and adriamycin-susceptible HL-60 (human promyelocytic leukemia) cells.4 The Taisho patent appears to have been abandoned, and there were no further reports on this or any related 12-membered macrolactones until 2004 when scientists at the Japanese companies Eisai Co., Ltd. and Mercian Corp. reported the structures of seven related compounds, pladienolides A−G, from S. platensis Mer-11107.5 The pladienolides were found to inhibit hypoxiainduced vascular endothelial growth factor (VEGF) expression and manifest potent in vitro activity against cancer cell lines and had a unique National Cancer Institute (NCI) COMPARE profile.5 The most active analog, pladienolide B (Figure 1), was also found to strongly inhibit tumor growth in mouse xenograft models. Using the naturally occurring pladienolides as leads, scientists at Eisai synthesized a 7-(4-cycloheptylpiperazin-1ylcarbonyloxy) derivative of pladienolide D, E7107 (Figure 1), which entered clinical trials in 2007 (NCT00459823 and NCT00499499) but was discontinued in 2009 without explanation. The carbamate group in E7107 considerably increases solubility, while the C-16 tertiary alcohol presumably slows macrolactone hydrolysis. The most exciting development in the FD-895 and pladienolide class of anticancer agents was the discovery that the pladienolides bind to the spliceosome and modulate RNA splicing.6 The spliceosome is a multicomponent protein and
he International Agency for Research on Cancer (IARC) has estimated that 7.6 million people worldwide die each year from cancer with 4 million of these people dying prematurely between the ages of 30 and 69 years. Many of these cancers are very difficult to treat, while some cancers rapidly become resistant to approved chemotherapy drugs. As a consequence, new therapies, especially those with novel modes of action (MoA), are urgently required. The recent discovery that three classes of microbial-derived natural products (Figure 1), FD-895/pladienolide, FR901464/spliceostatin A, and GEX1A (herboxidiene), share a new MoA offers an exciting new arena for anticancer drug development. Each of these leads has potent activity when dosed at nanomolar levels against most cancer cell lines and acts by binding to the SF3b1 spliceosome subunit, thereby modulating RNA splicing. The combination of potent activity and a novel MoA has led to considerable interest in advancing splicing inhibitors into new chemotherapy drugs. A multidisciplinary team at UC San Diego led by Burkart, La Clair, and Castro1 now reports a SAR study that has identified synthetically tractable, stable FD-895 analogs, such as compound 1 (Figure 1), that exhibit enhanced stability and activity that maintain potent apoptotic and splicing activity. The key development is compound 2 (Figure 1) in which a cyclopropyl group has replaced the reactive epoxide group, which was thought to be crucial to activity of FD-895 and pladienolides but has the potential to cause nonspecific activity. These new derivatives represent the first step in the generation of druglike splicing inhibitors. Spliceosome inhibitors have great potential to treat patients with leukemia, which has frequent spliceosome mutations, especially those patients with chronic lymphocytic leukemia (CLL) that do not respond to conventional treatments such as fludarabine. Splicing modulation also has the potential to open new avenues for therapeutic treatment of a diverse set of other cancers. Natural products have been the source of many drugs, and it has been estimated that up to 50% of all drugs launched since 1940 are natural products, semisynthetic natural product derivatives, or inspired by natural product pharmacophores.2 Natural products have also played an important role in oncology with prominent examples including vinblastine © 2013 American Chemical Society
Received: August 20, 2013 Published: August 27, 2013 6573
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spectroscopic techniques and total synthesis.10 Prior to this only the planar structure of FD-895 was known.4 In this current study,1 it was found that FD-895 was not stable under neutral conditions at 37 °C and underwent hydrolysis via an allylic cation intermediate to give three ring-opened products (Figure 2). The hydrolysis products were 1000 times less active than
Figure 2. Allylic cation intermediate hydrolysis of FD-895 to give the inactive ring-opened acids.
Figure 1. Structures of the bacterially derived spliceosome inhibitors FD-895, pladienolide B, pladienolide D, FR901464, GEX1A (herboxidiene), the semisynthetic clinical candidate E7107 and next generation leads 1 and 2 (blue color) disclosed in this study.
FD-895 and so represent a major roadblock for the development of FD-895 as a drug itself. More stable derivatives will be required for drug development. In their previous work,10 the authors showed that (17S)-FD-895 (1) was more active than FD-895 and in this study demonstrate that this stereochemical change also increased the stability of (17S)-FD-895 (1) compared to FD-895. The final piece in the puzzle was to remove the epoxide group because of its potential to undergo undesirable on-target or off-target covalent interactions that could result in toxicity issues. By use of the synthetic route established to synthesize FD-895, the (17S)-cyclopropyl analog 2 without the epoxide was found to be even more stable than 1 while retaining activity in the 150 nM range. The therapeutic potential of these new analogs was strengthened by demonstrating spliceosome modulation against a range of patient-derived, drug-resistant CLL samples. In summary, the UC San Diego team has identified a series of stereoisomers of FD-895 with enhanced activity, which is exemplified by 1, and defined a new stable class of analogs, represented by 2, wherein the C18−C19 epoxide group has been replaced with a cyclopropyl group to avoid possible toxic liabilities. These new analogs are synthetically accessible in gram quantities, and the cyclopropyl-containing analog 2 is currently being evaluated in preclinical studies.1 Cancer researchers will be eagerly awaiting further news of this
RNA complex that mediates pre-mRNA splicing, which is an integral process in mammalian cells. Its structure and biological potential as an anticancer target has recently been reviewed in detail.7 The SAP130 protein in the SF3b1 subunit was identified as the target of the pladienolides by using a combination of tritiated, biotinylated, photoaffinity, and fluorescent probes and by comparison of pladienolide resistant and sensitive cell lines.6 At the same time, the natural product FR901464 and its more stable methyl ester derivative, spliceostatin A, were also found to bind to the SF3b1 subunit.8 Derivatives of FR901464 such as the meayamycins, sudemycins, and newly reported naturally occurring analogs, the thailanstatins, also display similar activity profiles. GEX1A (herboxidiene), which shares a similar subunit epoxyalkyl to FD-895, was shown to bind to the SAP155 protein in the SF3b1 subunit.9 There are common structural motifs between the three classes of spliceosome inhibitors (Figure 1), which is suggestive of overlapping binding sites; however, further work is required to pinpoint their binding sites and fully elucidate the downstream events associated with this unique MoA.5 In 2012, Burkart, La Clair, and co-workers determined the absolute configuration of FD-895 using a combination of 6574
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(10) Villa, R.; Mandel, A. L.; Jones, B. D.; La, C. J. J.; Burkart, M. D. Structure of FD-895 revealed through total synthesis. Org. Lett. 2012, 14, 5396−5399.
exciting, new class of anticancer agents. The hope is that these next generation spliceosome modulators will revolutionize the treatment of leukemia and other complex cancers. Regardless of their therapeutic outcome, the spliceosome modulating classes of natural products enter a long line of natural product chemical probes that have significantly contributed to our understanding of key biological pathways.
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AUTHOR INFORMATION
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*Phone: +61 7 3346 2992. E-mail:
[email protected].
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REFERENCES
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