I was particularly struck by a recent paper from the Kozmin group on the mechanism of action of bistramide A. This very interesting natural product demonstrates promising anticancer activity, but is also highly toxic when delivered to mice. Interestingly, two other members of this family of compounds known as bistramides D and K, which both lack an enone moiety that is present in bistramide A, have been shown to be much less toxic while maintaining promising levels of anticancer activity in a mouse xenograft model.
The mechanism of action of this family of natural products has been hotly debated, but a few years ago the Kozmin group identified actin as the potentially therapeutically-relevant cellular target. This hypothesis was strongly supported by this group’s recent discovery and high resolution X-ray characterization of a bistramide A/actin complex. However, the mechanistic role, if any, of the conspicuous enone moiety of bistramide A could not be determined from this structure because this portion of the crystal was highly disordered.
In their recent PNAS paper, Kozmin and coworkers harnessed the remarkable efficiency and flexibility of their previously reported total synthesis of this complex natural product to prepare a series of elegantly designed analogs that collectively revealed the criticality of this enone moiety for potent cell-based growth inhibition. Moreover, consistent with the X-ray structure, these studies demonstrated unambiguously that both the spiroketal and amide subunits of bistramide A are required for high-affinity non-covalent interactions with actin that can lead to the severing of actin filaments. Follow-up studies with mass spectroscopy and a synthesized fluorescent analog collectively demonstrated that the enhanced cell-based activity attributed to the enone is due to covalent modification of the target protein, likely via conjugate addition of a cysteine residue. Collectively, these results support a dual mode of action of bistramide A involving the severing of filamentous actin as well as covalent modification of this protein target.
Interestingly, these results reveal a potential explanation for the increased in vivo toxicity of bistramide A relative to its enone-lacking counterparts. The severing of actin filaments (which does not rely on covalent modifications) may be sufficient to inhibit the proliferation of rapidly dividing tumor cells, whereas the dose-limiting toxicity may be caused by enone-mediated covalent modifications of this ubiquitous protein target. This compelling hypothesis remains to be tested, but this paper clearly demonstrates the critical importance of fundamental understanding of small molecule function to guide the search for more effective and less toxic therapeutics. It also represents a striking demonstration of the tremendous power of an efficient and flexible total synthesis of a complex natural product to enable the execution of illuminating experiments that are otherwise simply not possible.
Marty Burke is an assistant professor in the Department of Chemistry at the University of Illinois in Urbana-Champaign. His research focuses on the synthesis and study of small molecules with the capacity to perform higher-order, protein-like functions.