The epothilones are a new class of cytotoxic molecules identified as potential
chemotherapeuticdrugs. [Vincent T. DeVita, Jr., MD; Samuel Hellman, MD; and Steven A. Rosenberg, MD, PhD (2004) "Cancer: Principles And Practice Of Oncology" (7th Edition) Lippincott Williams & Wilkins ISBN 0-7817-4450-4] As of|September 2008, epothilones A to F have been identified and characterised. [cite journal
author = H. Spreitzer
September 15, 2008
title = Neue Wirkstoffe - Sagobepilon - eine synthetische Variation von Epothilon B als Hoffnungsträger gegen Krebs
journal = Österreichische Apothekerzeitung
issue = 19/2008
pages = 978
language = German ] Early studies in cancer cell lines and in human cancer patients indicate superior efficacy to the
taxanes. Their mechanism of action is similar, but their chemical structure is simpler. Due to their better water solubility, cremophors (solubilizing agents used for paclitaxelwhich can affect cardiac function and cause severe hypersensitivity) are not needed. [Julien, B.; Shah, S. " Antimicrob. Agents Chemother." 2002, "46", 2772.] Endotoxin-like properties known from paclitaxel, like activation of macrophages synthesizing inflammatory cytokines and nitric oxide, are not observed for epothilone B. [Muhlradt, P.F.; Sasse, F. " Cancer Research" 1997, "57",3344.]
Epothilones were originally identified as
metabolitesproduced by the myxobacterium" Sorangium cellulosum".
The structure of epothilone A was determined in
1996using x-ray crystallography. [Hofle, G.; Bedorf, N.; Steinmertz, H.; Schomburg, D.; Gerth, K.; Reichenach, H. " Angew. Chem." 1996, "35", 1567.]
Mechanism of action
The principal mechanism of the epothilone class is inhibition of
microtubulefunction. ["Epothilones: Mechanism of Action and Biologic Activity", Susan Goodin, Michael P. Kane, Eric H. Rubin, "Journal of Clinical Oncology", Vol 22, No 10 (May 15), 2004: pp. 2015-2025. ( [http://www.jco.org/cgi/content/full/22/10/2015/ Article] )] Microtubules are essential to cell division, and epothilones therefore stop cells from properly dividing. Epothilone B possess the same biological effects as taxol both "in vitro" and in cultured cells. This is because they share the same binding site, as well as binding affinity to the microtubule. Like taxol, epothilone B binds to the αβ-tubulin heterodimer subunit. Once bound, the rate of αβ-tubulin dissociation decreases, thus stabilizing the microtubules. Furthermore, epothilone B has also been shown to induce tubulin polymerization into microtubules without the presence of GTP. This is caused by formation of microtubule bundles throughout the cytoplasm. Finally, epothilone B also causes cell cycle arrest at the G2-M transition phase, thus leading to cytotoxicity and eventually cell apoptosis.Balog, D. M.; Meng, D.; Kamanecka, T.; Bertinato, P.; Su, D.-S.; Sorensen, E. J.; Danishefsky, S. J. " Angew. Chem." 1996, "108", 2976.]
Several epothilone analogs are currently undergoing clinical development for treatment of various cancers. One analog,
ixabepilone, was approved in October 2007 by the United States Food and Drug Administrationfor use in the treatment of aggressive metastatic or locally advanced breast cancer no longer responding to currently available chemotherapies [ [http://www.abcnews.go.com/Health/OnCallPlusTreatment/wireStory?id=3738535 ABC News: ABC News ] ] .
Epothilone B has proven to contain potent "in vivo" anticancer activities at tolerate dose levels in several human xenograft models. [Ojima, I.; Vite, G.D.; Altmann, K.H.; 2001 "Anticancer Agents: Frontiers in Cancer Chemotherapy." American Chemical Society, Washington, DC.] As a result, epothilone B and its various analogues are currently undergoing various clinical phases (
patupilone(EPO906) and in combination with capecitabine in metastatic breast cancer have been announced. [ [http://www.medicalnewstoday.com/medicalnews.php?newsid=72986|Medical News Today] .]
Due to the high potency and clinical need for cancer treatments, epothilones have been the target of many total syntheses. [Luduvico, I.; Hyaric, M. L.; Almeida, M. V.; Da Silva, A. D. "Mini-Reviews in Organic Chemistry" 2006, "3", 49-75. (Review)] The first group to publish the total synthesis of epothilones was S. J. Danishefsky "et al." in 1996. [Su, D.-S.; Meng, D.; Bertinato, P.; Balog, D. M.; Sorensen, E. J.; Danishefsky, S. J.; Zheng, Y.-H.; Chou, T.-C.; He, L.; Horwitz, S. B. "
Angew. Chem. Int. Ed. Engl." 1997, "36", 757.] This total synthesis of epothilone A was achieved via an intramolecular ester enolate-aldehyde condensation. Other syntheses of epothilones have been published by Nicolaou [Yang, Z.; He, Y.; Vourloumis, D.; Vallberg, H.; Nicolaou, K. C. " Angew. Chem. Int. Ed. Engl." 1997, "36", 166.] , Schinzer [Schinzer, D.; Limberg, A.; Bauer, A.; Böhm, O. M.; Cordes, M. " Angew. Chem. Int. Ed. Engl." 1997, "36", 523.] , Mulzer [Mulzer, J.; Mantoulidis, A.; Öhler, E. " J. Org. Chem." 2000, "65", 7456-7467. (DOI|10.1021/jo0007480)] , and Carreira [Bode, J. W.; Carreira, E. M. " J. Am. Chem. Soc." 2001, "123", 3611-3612. (DOI|10.1021/ja0155635)] . In this approach, key building blocks aldehyde, glycidols, and ketoacid were constructed and coupled to olefin metathesisprecursor via an aldol reactionand then an esterificationcoupling. Grubbs' catalystwas employed to close the bis terminal olefin of the precursor compound. The resulting compounds were cis- and tran-macrocyclic isomers with distinct stereocenters. Epoxidationof cis- and trans-olefins yield epothilone A and its analogues.
The particular synthetic method determined by the laboratories of K.C NicolaouNicolaou, K.C.; Ninkovic, S.; Sarabia, F.; Vourloumis, D.; He, Y.; Vallberg, H.; Finlay, M.R.V.; Yang, Z.; "
J. Am. Chem. Soc." 1997, "119", 7974.] , described the synthesis of appropriate building blocks 9, 11, and 12, derived from the retrosynthetic analysis of epothilone B (Figure 1), both diastereoisomers and the geometrical isomers at C6-C7 and C12-C13, can be obtained to give a diverse molecular product. The synthesis of required building blocks 9, 11 and 12, were obtained in a maximum of 4 steps for each building block as seen in Figure 2. With fragments 9, 11 and 12 in hand, these intermediates can then react with one another via Wittig olefination, aldol reaction, macrolactionization, and epoxidation to give the various epothilone B as seen in Figure 3.
Epothilone B is a 16-membered
polyketide macrolactonewith a methyl thiazolegroup connected to the macrocycle by an olefinic bond. The polyketide backbone was synthesized by type I polyketide synthase(PKS) and the thiazole ring was derived from a cysteineincorporated by a nonribosomal peptide synthetase(NRPS). In this biosythesis, both PKS and NRPS use carrier proteins, which have been post-translationally modified by phosphopantheteinegroups, to join the growing chain. PKS uses coenzyme-Athioester to catalyze the reaction and modify the substrates by selectively reducing the β carbonyl to the hydroxyl(Ketoreductase, KR), the alkene(Dehydratase, DH), and the alkane(Enoyl Reductase, ER). PKS-I can also methylate the α carbon of the substrate. NRPS, on the other hand, uses amino acids activated on the enzyme as aminoacyl adenylates. Unlike PKS, epimerization, N-methylation, and heterocycle formation occurs in NRPS enzyme.Molnar, I.; Schupp, T.; Ono, M.; Zirkle, RE.; Milnamow, M.; Nowak-Thompson, B.; Engel, N.; Toupet, C.; Stratmann, A.; Cyr, DD.; Gorlach, J.; Mayo, JM.; Hu, A.; Goff, S.; Schmid, J.; Ligon, JM.; " Chemistry and Biology." 2000, "7", 97.]
Epothilone B starts with a 2-methyl-4-carboxythiazole starter unit, which was formed through the translational coupling between PKS, EPOS A (epoA) module, and NRPS, EPOS P(epoP) module. The EPOS A contains a modified β-ketoacyl-synthase (malonyl-ACP decarboxylase, KSQ), an acyltransferase (AT), an enoyl reductase (ER), and an acyl carrier protein domain (ACP). The EPOS P however, contains a heterocylization, an adenylation, an oxidase, and a thiolation domain. These domains are important because they are involved in the formation of the five-membered heterocyclic ring of the thiazole. As seen in Figure 4, the EPOS P activates the cysteine and binds the activated cysteine as an aminoacyl-S-PCP. Once the cysteine has been bound, EPOS A loads an
acetateunit onto the EPOS P complex, thus initiating the formation of the thiazoline ring by intramolecular cyclodehydration.
Once the 2-methylthiazole ring has been made, it is then transferred to the PKS EPOS B (epoB), EPOS C (epoC), EPOS D (epoD), EPOS E (epoE), and EPOS F (epoF) for subsequent elongation and modification to generate the olefinic bond, the 16-membered ring, and the epoxide, as seen in Figure 5. One important thing to note is the synthesis of the gem-dimethyl unit in module 7. These two dimethyls were not synthesized by two successive C-methylations. Instead one of the
methyl groupwas derived from the propionate extender unit, while the second methyl group was integrated by a C-methyl-transferase domain.
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