Pladienolide

Pladienolide D Macrolide E7107 (171) Oncology Rinds with Phase I Eisai 857-860... 

Sakai T, Sameshima T, Matsufuji M, Kawamura N, Dobashi K, Mizui Y. (2004) Pladienolides, new substances from culture of Streptomyces platensis Mer-11107.1. Taxonomy, fermentation, isolation and screening. J Antibiot (Tokyo) 57 173-179. 

Figure 3.14 Structures of pladienolide B, pladienolide D, FD-895, and fluorescent and affinity pladienolide B probes. 

In mid-2007, the stereochemistry of pladienolides B and D was determined using a combination of NMR methods and degradation experiments.187 Coupling constants from the H-NMR data as well as correlations from 2D experiments provided a first-level approximation of the stereochemistry. Further derivatisation experiments were performed to confirm the relative stereochemistry within pladienolide B. The absolute stereochemistry was determined via a modified Mosher method that compared an C3,C21-bis-(R)-MPTA ester to an C3,C21-bis-(S)-MPTA ester. The structural assignment has since been validated by chemical synthesis.187 189... 

While the identification of SF3b was an impressive start, there is some concern over the considerable (645-fold) loss of activity in the Eisai probes prepared at C7. While these probes were used to identify the splicing factor SF3b, it is possible the lack in activity of Eisai s probe molecules failed to provide a complete description of the targets of the pladienolides. In addition, the mode of action of pladienolide B has yet to be directly... 

Discovery Pladienolides are 12-membered macrohdes isolated as an antitumor substance by cell-based hypoxia-induced gene expression assay from Streptomyces platensis Mer-11107 [1]. The structures of pladienolide B and D were determined as 1 and 2, respectively [2,3]. Total synthesis of pladienolide B [4] and biosynthetic gene cluster [5] is also reported. 

Biochemical Studies Pladienolide B is the most potent inhibitor for hypoxia-induced VEGF expression and proliferation of the U251 cancer cell line [6], COMPARE analyses using a 39-cell line panel showed that pladienolide B has strong antitumor activities in vitro with a novel mechanism of action. 

Further, pladienolide B inhibited tumor growth extensively in a xenograft model series. 

Identification of Molecular Target To isolate the binding protein of pladienolide, [ H]-labeled (3), fluorescence-tagged (4) and photoaffinity/biotin (PB)-tagged (5) pladienolide B were synthesized and treated with HeLa cells [7], The [ H] probe... 

Sakai, T Asai, N., Okuda, A., Kawamura, N., Mizui, Y. (2004). Pladienolides, new substances from cnltnre of Streptomyces platensis Mer-11107 ... 

Machida, K., Arisawa, A., Takeda, S., et al. (2008). Organization of the biosynthetic gene clnster for the polyketide antimmor macrolide, pladienolide, in Streptomyces platensis Mer-11107. Biosci. Biotechnol Biochem., 72, 2946-2952. 

Kotake, Y Sagane, K Owa, T et al. (2007). Splicing factor SF3b as a target of the antitumor natural product pladienolide. Nat. Chem. Biol, 3, 570-575. 

Before the discovery of spliceostatin A (SSA) and pladienolide B, the spliceo-some had not been seriously considered as a potential drug target [3,4]. Naturally, identification of the first inhibitors allowed further dissection of the splicing apparatus and investigation of the cellular consequences of splicing dysfunction. 

In 2007, two papers were published back to back, each reporting the identification of a small-molecule inhibitor of the spliceosome [3,4]. Since the initial description of SSA and pladienolide B, a number of further splicing inhibitors have been discovered and opened a new and active field of investigation (Figure 22.2). Besides novelty, these molecules have proved to be potent probes into cellular processes, while also holding clinical promise. 

Figure 22.2 Ovennew of the splicing inhibitors discussed. FR901464 and derivatives are listed on the left, pladienolide and relatives on the right with the artificial inhibitor containing elements of both FR901464 and pladienolide, sudemycin in the middle. Herboxidiene/GEXl A and isoginkgetin are at the bottom.

As for pladienolide, a related Streptomyces natural product, herboxidiene (also called GEXIA) had been identified previously and proved amenable to full synthesis [34-36]. 

Both SSA and pladienolide present a bona fide example for target identification in chemical biology. Both molecules proved amenable to derivatization, without losing too much activity or going off-target. 

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