Identification of C‑6 as a New Site for Linker Conjugation to the Taccalonolide Microtubule Stabilizers

• Published in: Journal of natural products (Washington, D.C.) D.C.), 2019-03, Vol.82 (3), p.583-588
• Main Authors: Du, Lin, Risinger, April L, Yee, Samantha S, Ola, Antonius R. B, Zammiello, Cynthia L, Cichewicz, Robert H, Mooberry, Susan L
• Format: Article
• Language: English
• Subjects: Cell Proliferation - drug effects; HeLa Cells; Humans; Microtubules - drug effects; Molecular Structure; Steroids - chemistry; Steroids - pharmacology; Structure-Activity Relationship; Tubulin Modulators - chemistry; Tubulin Modulators - pharmacology
• ISSN: 0163-3864; 1520-6025
• DOI: 10.1021/acs.jnatprod.8b01036

The taccalonolides are a class of microtubule stabilizers that circumvent clinically relevant forms of drug resistance due to their unique mechanism of microtubule stabilization imparted by the covalent binding of the C-22–C-23 epoxide moiety to tubulin. A taccalonolide (8) with a fluorescein group attached with a linker at C-6 was generated, and biochemical and cell-based assays showed that it bound directly to tubulin and stabilized microtubules. This pharmacological probe has allowed, for the first time, a direct visualization of a taccalonolide binding to microtubules, verifying their cellular binding site. This C-6-modified taccalonolide showed potency comparable to the untagged compound in biochemical experiments; however, its potency was lower in cellular assays, presumably due to decreased cellular permeability. These studies provide a valuable tool to facilitate the further understanding of taccalonolide pharmacology and demonstrate that C-6 is a promising site for a linker to be added to this novel class of microtubule stabilizers for targeted drug delivery.