Stabilized Cyclopropane Analogs of the Splicing Inhibitor FD-895

Abstract

Targeting the spliceosome with small molecule inhibitors provides a new avenue to target cancer by intercepting alternate splicing pathways. Although our understanding of alternate mRNA splicing remains poorly understood, it provides an escape pathway for many cancers resistant to current therapeutics. These findings have encouraged recent academic and industrial efforts to develop natural product spliceosome inhibitors, including FD-895 (1a), pladienolide B (1b) and pladienolide D (1c), into next-generation anticancer drugs. The present study describes the application of semi-synthesis and total synthesis to reveal key structure activity relationships (SARs) for the spliceosome inhibition by 1a. This information is applied to deliver new analogs with improved stability and potent activity at inhibiting splicing in patient derived cell lines.

Coherence between Cellular Responses and in Vitro Splicing Inhibition for the Anti-tumor Drug Pladienolide B and Its Analogs

“…As noted above, absence of the acetyl group at C7 in pladienolide A reduces activity (7). Combined loss of side groups at C3, C7 and C10 abrogates bioactivity completely (29). Interestingly, herboxidiene, which has a less complex ring structure but maintains an acetyl group, inhibits splicing in vitro to the same extent as PB.…”

“…Our data indicate that the epoxide contributes to PB activity but is not absolutely required. Corroborating this observation, a recent report provides evidence that changing the epoxide of FD-985 to a cyclopropane does not drastically alter its cytotoxicity but instead appears to stabilize the compound (29). Furthermore, recent papers identified an analog of FR901464, termed spliceostatin B, and two members of the thailanstatin family of natural products that resemble FR901464 and that do not contain the epoxide (35,36).…”

“…PB is a member of a family of several pladienolide natural products from Streptomyces platensis, all of which share a complex macrolide ring structure (7,26). The other family members and a handful of synthetic analogs have varying effects on cell growth and splicing, which provides some clues as to which molecular features of PB may be important for activity (7,8,(27)(28)(29). In this study, we used an expanded series of PB analogs to directly test the requirement of PB features for in vitro splicing inhibition and effects in cells and identified both dispensable and indispensable functional groups.…”

“…However, it is notable that a hydroxyl group is also present at C16 in two other PB analogs (pladienolide D and E7107), both of which mimic the effects of PB on splicing and cells with similar potencies (8,15). Also, isomerization of the C16-C17 bond in the context of FD-895, another related natural product with an additional hydroxyl group at C17 (31), has limited effect on the cytotoxicity of the compound (29,32). More studies focusing on each position independently will be required to clear up the contribution of functional groups to PB activity at these sites.…”

“…Similarly, C10-C11 stereochemistry is not critical to activity, and it can be eliminated with the incorporation of gem-dimethyl groups. Furthermore, C18-C19 desoxy PB (4, 5) retains good potency, which indicates that stable PB analogs without labile epoxide functionality can be developed (29). Finally, because the activity of herboxidiene resembles PB in many aspects, the macrolide ring of PB may also be simplified.…”