Epothilone B

Epothilone B is a 16-membered cytotoxic macrocyclic produced by the gram-negative myxobacterium Sorangium cellulosum So ce90. Like paclitaxel, it inhibits microtubule depolymerisation, leading to cell cycle arrest at the G2-M phase, resulting in apoptosis. Although this drug might mimic some of the properties of paclitaxel, it does differ in the fact that it is more effective against P-glycoprotein-expressing multiple drug resistant tumor cell lines as well as increased water solubility in comparison to paclitaxel. Due to improved water solubility, cremophors (solubilixing agents used in paclitaxel which can affect cardiac function and cause severe hypersensitivity) are no longer needed in the formulation of this anticancer drug.¹ Other undesired effects such as endotoxin-like properties where macrophages are actived to synthesize inflammatory cytokines and nitric oxide (paclitaxel)² are not observed by epothilone B.

History

Epothilones A and B were identified in 1987 as weak antifungal, cytotoxic agents, and later isolated from Sorangium cellulosum So ce90³ in 1989 by K. Gerth and co-workers.⁴ It wasn't until 1995, when the potential of epothilones as anticancer agents became apparent and comparable with that of paclitaxel.⁵ A decade later the molecular structure was determined by spectroscopy and X-ray crystallography.⁶ Today, epothilone derivatives are being produced via total synthesis and semisynthesis in hopes of finding a more potent microtubule binding analogue than that of the parent epothilones.

Biosynthesis

Epothilone B is a 16-membered polykidetide macrolactone that have a methylthiazole group connected to the macrocycle by an olefinic bond. The polyketide backbone of the molecule was synthesized by type I polyketide synthase (PKS) and the thiazole ring was derived from a cysteine incorporated by a nonribosomal peptide synthetase (NRPS). In this biosythesis, both PKS and NRPS use carrier proteins, which have been post-translationally modified by phosphopantheteine groups, to join the growing chain. PKS uses coenzyme A thioester 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 that are activated on the enzyme as aminoacyl adenylates. Unlike PKS, epimerization, N-methylation, and heterocycle formation occurs in NRPS enzyme.⁷

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 1, 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 acetate unit onto the EPOS P complex, thus initiating the formation of the thiazoline ring by intramolecular cyclodehydration.⁷

Figure 1: Formation of 2-methyl-4-carboxythiazole starter unit for epothilone biosynthesis.

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 2. 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 group was derived from the propionate extender unit, while the second methyl group was integrated by a C-methyl-transferase domain.⁷

Figure 2: Biosynthesis of Epothilone B. Epoxide formation occurs in EPOS F, which is not present in the following figure.

Total Synthsis

The total synthesis of epothilone B was first reported by S.J Danishefsky⁸, and then later by K.C Nicolaou⁹, J. Mulzer¹⁰, and D.J Carreir¹¹. However in this particular case, only the macrolactonization-based strategy will be discussed. This particular method was determined by the laboratories of K.C Nicolaou. They described the synthesis of appropriate building blocks 9, 11, and 12, derived from the retrosynthetic analysis of epothilone B (Figure 3), 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 4.  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 5.

Figure 3: Retrosynthetic analysis of epothilone B to obtain the intermediates 9, 11, and 12.⁹

Figure 4: Synthesis of the intermediates: a) keto acid, 9 b) thiazole containing fragment with phosphonium salt, 12 and c) ketone, 11.⁹

Figure 5: Total synthesis of Epothilone B and Analogues. This was obtained by coupling all the intermediates (Figure 3 and 4) together through various reactions.⁹

Mechanism of Action

Epothilone B possess the same biological effects as taxol both in vitro and in cultured cells. This is due to the fact that 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 cause cell cycle arrest at the G2-M transition phase, thus leading to cytotoxicity and eventually cell apoptosis.⁵

Clinical Trials

Epothilone B has proven to contain potent in vivo anticancer activities at tolerate dose levels in several human xenograft models.¹² As a result, epothilone B and its various analogues are currently undergoing various clinical phases (Ixabepilone (BMS-247550) and patupilone (EPO906) - phase II trials; BMS-310704 and BMS-247550 - phase I trials).

Reference

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2. Muhlradt, P.F. and Sasse, F. Epothilone B Stabilizes Microtubuli of Macrophages Like Taxol without Showing Taxol-like Endotoxin Activity. Cancer Research. 57: 3344-3346 (1997)

3. Gerth, K., Bedorf, N., Hofle, G., Irschik, H. Epothilons A and B: Antifungal and Cytotoxic Compounds from Sorangium cellulosum (Myxobacteria) Production, Physico-chemical and Biological Properties. J. Antibiotics. 49(6): 560-563 (1996)

4. Gerth, K., Steinmetz, H., Hofle, G., and Reichenbach, H. Studies on the Biosynthesis of Epothilones: The Biosynthetic Origin of the Carbon Skeleton. J. Antibiotics. 53(12): 1373-1377 (2000)

5. Bollag, DM., McQueney, PA., Zhu, J., Hensens, O., Koupal, L., Liesch, J., Goetz, M., Lazarides, E., and Woods, CM. Epothilones, a New Class of Microtubule-stabilizing Agents with a Taxol-like Mechanism of Action. Cancer Research. 55: 2325-2333(1995)

6. Hofle, G., Bedorf, N., Steinmetz, H., Schomburg, D., Gerth, K., Reichenbach, H. Epothilone A and B: Novel 16-Membered Macrolied with Cytotoxic Activity: Isolation, Crystal Structure, and Conformation In Solution. Angew. Chem., Int. Ed. Engl. 35: 1567-1569 (1996)

7 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., and Ligon, JM. The biosynthetic gene cluster for the microtubule-stabilizing agents epothilones A and B from Sorangium cellulosum So ce90. Chemistry and Biology. 7(2):97-109 (2000)

8 Su, D.S., Meng, D., Bertinato, P., Balog, A., Sorensen, E.J., Danishefsky, S.J., Zheng, Y.H., Chou, T.C., He, L., and Horwitz, S.B. Total Synthesis of (-)-Epothilone B: An Extension of the Suzuki Coupling Method and Insights into Structure -Activity Relationships of the Epothilones. Angew. Chem., Int. Ed. Engl. 36: 757-759 (1997)

9 Nicolaou, K.C., Ninkovic, S., Sarabia, F., Vourloumis, D., He, Y., Vallberg, H., Finlay, M.R.V., and Yang, Z. Total Syntheses of Epothilones A and B via Macrolactonization Based Strategy. J. Am. Chem. Soc. 119(34): 7974-7991 (1997)

10 Mulzer, J., Mantoulidis, A., and Ohler, E. Total Syntheses of Epothilones B and D. J. Org. Chem. 65(22): 7456-7467 (2000)

11 Bode, J.W. and Carreira, E.M. Stereoselevtive Syntheses of Epothilones A and B via Directed Nitrile Oxide Cycloaddition. J. Am. Chem. Soc. 123(15):3611-3612 (2001)

12 Ojima, I., Vite, G.D., and Altmann, K.H. (2001) Anticancer Agents: Frontiers in Cancer Chemotherapy. American Chemical Society, Washington, DC.