(+)-Discodermolide is a polyketide natural product found to stabilize microtubules. (+)-discodermolide was isolated by Gunasekera and his co-workers at the Harbor Branch Oceanographic Institute from the deep-sea sponge Discodermia dissoluta in 1990.Gunasekera, S. P.; Gunasekera, M.; Longley, R. E.; Schulte, G. K. J. Org. Chem. 1990, 55, 4912-4915. () (+)-Discodermolide was found to be a potent inhibitor of tumor cell growth in several MDR cancer cell lines. (+)-discodermolide also shows some unique characters, including a linear backbone structure, immunosuppressive properties both in vitro and in vivo,Longley, R. E.; Caddigan, D.; Harmody, D.; Gunasekera, M.; Gunasekera, S. P. Transplantation 1991, 52, 650-655.Longley, R. E.; Caddigan, D.; Harmody, D.; Gunasekera, M.; Gunasekera, S. P. Transplantation 1991, 52, 656-661. potent induction of an accelerated senescence phenotype,Klein, L.; Freeze, B. S.; Smith, A. B., III; Horwitz, S. B. Cell Cycle 2005, 4, 501–507. and synergistic antiproliferative activity in combination with paclitaxel.Longley, R. E.; Gunasekera, S. P.; Faherty, D.; McLane, J.; Dumont, F. Ann. N.Y. Acad. Sci. 1993, 696, 94–107. Discodermolide was recognized as one of the most potent natural promoters of tubulin assembly. A large number of efforts toward the total synthesis of (+)-discodermolide were directed by its interesting biological activities and extreme scarcity of natural sources (0.002% w/w from frozen marine sponge). The compound supply necessary for complete clinical trials cannot be met by harvesting, isolation, and purification. As of 2005, attempts at synthesis or semi-synthesis by fermentation have proven unsuccessful. As a result, all discodermolide used in preclinical studies and clinical trials has come from large-scale total synthesis.Mickel, S. J. et al. Org. Process Res. Dev. 2004, 8
History Discodermolide was first isolated in 1990 from the Caribbean marine sponge Discodermia dissoluta by chemist Dr. Sarath Gunasekera and biologist Dr. Ross Longley, scientists at the Harbor Branch Oceanographic Institution. The sponge contained 0.002% of discodermolide (7 mg/434 g of sponge). Since the compound is light-sensitive, the sponge must be harvested at a minimum depth of 33 meters. Discodermolide was initially found to have immunosuppressive and antifungal activities.
Structure (+)-discodermolide has a linear polypropionate backbone, punctuated by Z-olefinic linkages at C(8,9) and C(13,14), a terminal Z-diene substituent at C(21–24), 13 stereogenic centers (including four secondary hydroxyls and seven methyl substituents), a carbamate, and a fully substituted D-lactone. The relative stereochemistry was determined by X-ray crystallography. The absolute stereochemistry of (+)-discodermolide was reported by Schreiber and his co-workers in 1993.Nerenberg, J. B.; Hung, D. T.; Somers, P. K.; Schreiber, S. L. J. Am. Chem. Soc. 1993, 115, 12621–12622 Discodermolide adopts a U-shaped conformation, where the internal (Z)-alkenes act as conformational locks by minimizing allylic strain and syn-pentane interactions along the backbone.Jogalekar, A. S.; Kriel, F. H.; Shi, Q.; Cornett, B.; Cicero, D.; Snyder, J. P. J. Med. Chem. 2009, 53, 155-165 The D-lactone is held in a boat-like conformation.
Biological activities
Immunosuppressive properties Initial biological evaluation of (+)-discodermolide by the Longley group showed that it has immunosuppressive properties both in vitro and in vivo. The immunosuppression response was observed at a relatively low concentration that (+)-discodermolide was non-toxic in vitro. In both human peripheral blood leukocytes and murine splenocytes, (+)-discodermolide was found to suppress the two-way mixed lymphocyte reaction. In addition, mitogenic response of peripheral blood leukocytes was also suppressed by the (+)-discodermolide. Follow up experiments demonstrated that (+)-discodermolide also has anti-proliferative effects in several other non-lymphoid cell lines.
Antiproliferative and antimitotic properties (+)-Discodermolide is a highly potent antiproliferative agent. (+)-Discodermolide treated murine Do11.10T hybridoma cells could not proceed normal cell cycling. In untreated controls, 68% of cells were found at G1 phase, and 31% were found at S phase, and less than 1% was found at the G2/M phase. However, after 3 hours (+)-discodermolide treatment, 52% were found at G1 phase, 40% at S phase, and 58% at G2 and M phase. This result indicated that (+)-discodermolide blocks the cell cycle at G2 and M phase. This inhibition effect was also found to be reversible. Cells resume normal cycling within 48 hours after removal of (+)-discodermolide from the cell culture medium. (+)-Discodermolide arrests cell cycle via binding and stabilization of the microtubule network. Hyper-stabilization of the mitotic spindle causes cell cycle arresting at G2 and M phase and eventually leads to cell death by apoptosis. At 10μM concentration, (+)-discodermolide is able to promote microtubules assembly without microtubule-associated proteins and GTP. With microtubule-associated protein and GTP presented, 10μM (+)-discodermolide is able to initiate tubulin polymerization at 0 °C.ter Haar, E.; Kowalski, R. J.; Hamel, E.; Lin, C. M.; Longley, R. E.; Gunasekera, S. P.; Rosenkranz, H. S.; Day, B. W. Biochemistry 1996, 35, 243–250.Hung, D. T.; Chen, J.; Schreiber, S. L. Chem. Biol. 1996, 3, 287–293.
Potent Inducer of accelerated cell senescence (+)-Discodermolide is able to induce the senescence phenotype. (+)-Discodermolide treated Hela, MDA-MB-231, HCT-116, and A549 cells exhibited moderated to high levels of β-galactosidase activity in all four cell lines. The β-galactosidase activity is one of the hallmarks of senescence.Dimri, G.; Lee, X.; Basile, G.; Acosta, M.; Scott, G.; Roskelley, C.; Medrano, E.; Linskens, M.; Rubelj, I.; Pereira-Smith, O.; Peacocke, M.; Campisi, J. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 9363–9367. Other common features include a cessation of proliferation and an increased cytoplasmic area. (+)-Discodermolide also induced up-regulation and activation in three proteins (p66Shc, Erk1, and Erk2).
Neuroprotective agent (+)-Discodermolide has also been found to be neuroprotective in recent Alzheimer research.Forman, M. S.; Trojanowski, J. Q.; Lee, V. M.-Y. Nat. Med. 2004, 10, 1055–1063 The microtubule-stabilizing feature of (+)-discodermolide was used to restore neuron functions that have been disrupted by the amyloid induced sequestration. Tau protein is known to stabilize the microtubule network in healthy neurons.Zhang, B.; Maiti, A.; Shively, S.; Lakhani, F.; McDonald- Jones, G.; Bruce, J.; Lee, E. B.; Xie, S. X.; Joyce, S.; Li, C.; Toleikis, P. M.; Lee, V. M.-Y.; Trojanowski, J. Q. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 227–231. It served as the “railroad track” upon which actin, tubulin, mitochondria, neurotransmitter-related enzymes and vesicles carrying messenger proteins are delivered. The presence of Amyloid in the cell could lead to tau protein aggregation and microtubule numbers reduction. In transgenic mouse model for human tauopathy, (+)-discodermolide stabilizes microtubules and restores fast axonal transport in cells, offsetting the loss of function caused by aggregation of tau protein. The motor-impaired mice exhibited a full restoration of normal movement with (+)-discodermolide treatment.
Potent natural promoter of tubulin assembly Discodermolide competes with paclitaxelMartello, L. A.; McDaid, H. M.; Regl, D. L.; Yang, C.-P. H.; Meng, D.; Pettus, T. R. R.; Kaufman, M. D.; Arimoto, H.; Danishefsky, S. J.; Smith, A. B., III; Horwitz, S. B. Clin. Cancer Res. 2000, 6, 1978–1987 for microtubule binding, but with higher affinity and is also effective in paclitaxel- and in epothilone-resistant cancer cells. Discodermolide also seems to demonstrate a remarkably consistent 3D molecular conformation in the solid-state, in solution and when bound to tubulin; molecules with the conformational flexibility of discodermolide usually present very different conformations in different environments.
Biosynthesis Many marine-derived sponges cannot be cultured out of their natural environment. Finding the genes responsible for the biosynthesis of a sponge derived polyketides is a difficult task to accomplish because of the sponges’ colonial nature. Scientists are not yet able to culture the sponges; therefore, the genes for the biosynthesis of (+)-discodermolide have not yet been discovered.Shaw, S. J.; Zhang, D.; Sundermann, K. F.; Myles, D. C. Fragment Assembly: An Alternative Approach to Generating Complex Polyketides. Synthetic Commun. 2005, 35, 1735-1743.
Total syntheses An intense effort has been made towards the total synthesis of (+)-discodermolide in order to meet the growing interest of studying its clinical profile. More than a dozen of syntheses have been published by different research groups. After decade of work, discodermolide syntheses have become more convergent and more practical. All of the total syntheses approaches started with the construction of three major fragments of roughly equivalent complexity, each of which contains the methyl-hydroxyl-methyl triad of contiguous centers that matches the stereogenicity of discodermolide target. Here are three examples of the retrosynthetic analyses of (+)-discodermolide:
Total syntheses of unnatural antipode (-)-discodermolide
The Schreiber synthesis of (-)-discodermolide
In 1993, Schreiber and his co-workers reported the first total synthesis of the unnatural antipode (-)-discodermolide and determined the absolute stereochemistry of the natural product. The Schreiber team recognized three fragments of roughly equal complexity that are separated by olefinic units in discodermolide. The two starting materials, homoallylic alcohols, are both readily derived from 3-hydroxy-2-methylpropionate.
The trisubstituted (Z)-alkene of first fragment was generated by using the Still-Gennari reagent. Gilbert's reagent was then used to homologate it to an acetylene. Goekjian and Kishi method was then used to get the desired fragment, iodoacetylene. The same homoallylic alcohol was converted to diol in four steps. After oxidation to keto-aldehyde, it was homologated to dienes by a palladium-catalyzed coupling with vinylzinc bromide to generated the second fragment. The six-membered ring subunit was transformed from an acetal, which was obtained by an intramolecular 1,4-addition of a presumed hemiacetal intermediate.
The Schreiber team found that the NiCl2 and CrCl2 promoted addition of alkynyl iodide to aldehyde effectively. The reaction provides a 2:1 mixture of desired product and its isomer. However, the undesired isomer can be recycled to the desired epimer in three steps, including Swern oxidation and Corey's asymmetric reduction. Pd catalyzed partial hydrogenation of the alkyne to establish the Z-configuration at C(8)–C(9). The pivaloyloxy group was then converted to a bromide for the final coupling. A stereoselective enolate alkylation between the allylic bromide with the lithium enolate derived from the second fragment provided a 3:1 mixture of isomers and completed the backbone assembly of (-)-discodermolide. During the final conversion, the NaBH4 reduction of ketone provided a separable 2.5:1 mixture of isomers. The Schreiber synthesis of (-)-discodermolide has an overall yield of 3.2% with a longest linear sequence of 24 steps and 36 total steps. In 1996, the Schreiber group published the total synthesis of natural discodermolide with similar synthetic strategy (4.3% overall yield, 36 steps, 24 steps longest linear sequence.
The first-generation Smith synthesis of (-)-discodermolide