Taccalonolides: Microtubule-Stabilizing Steroids with in vivo Antitumor Efficacy

                  Microtubules are noncovalent polymers of a- and b- tubulin heterodimers, assembled in a filamentous tube-shaped structure. During mitosis, microtubules form the mitotic spindle that transports daughter chromosomes to opposing poles of the dividing cell. Given their important role in cell division, microtubules have been an important target for anticancer drug discovery. Microtubules must exist in a dynamic state, growing and shortening by the reversible association and dissociation of a- and b-tubulin heterodimers at both ends. Disruption of microtubule dynamics by small-molecule microtubule inhibitors prevents cell cycle progression and inhibits mitosis. Microtubule inhibitors can be stabilizing or destabilizing agents. For example, the anti-cancer drug taxol (see Figure above, boxed structure), the first microtubule stabilizer identified, operates by promoting polymerization and increasing microtubule polymer mass in cells. Destabilizing agents such as the vinca alkaloids depolymerize microtubules and decrease polymer mass. Both stabilization and destabilization disrupt microtubule dynamics, eventually resulting in apoptotic cell death. Because tumors are dependent upon continuous mitotic division in order to grow and metastasize, this pharmacological effect is particularly detrimental to various forms of cancer. Although many types of cancer respond initially to microtubule inhibitor treatment, complete remission is rarely achieved. This is often due to the emergence of multiple drug resistance (MDR), wherein tumors can effectively decrease the intracellular concentration of drug by overexpression of membrane efflux pumps such as P-glycoprotein (Pgp). Thus, new clinically efficacious microtubule stabilizers capable of evading MDR are in high demand.

            In 2003, the taccalonolides, a relatively new class of plant-derived microtubule stabilizers, were isolated from a variety of Tacca species by Susan Mooberry’s group in San Antonio. The taccalonolides are highly oxygenated hexacyclic steroid lactones that, similar to other microtubule inhibitors, increase the density of cellular microtubules, interrupt mitotic progression and, consequentially, promote apoptosis of cancer cells. Importantly, taccalonolides A and E were shown to be potent and effective antitumor agents in vivo, with the ability to circumvent multiple mechanisms of MDR. Moreover, these antimitotic steroid lactones cause microtubule-stabilizing effects through a unique binding site not shared by other known microtubule stabilizers, and via an entirely new mechanism of action, which is not currently well understood.

            The most potent of the antiproliferative taccalonolides are, without exception, epoxidized across carbons C22 and C23 on the a-face of the molecule. Treatment of naturally occurring taccalonolides that contain a C22/23-enol ester double bond with dimethyldioxirane (DMDO) cleanly installs the activity-promoting oxirane functionality, leading to dramatic increases in antiproliferative potency in HeLa cells. For example, epoxidation of the newly isolated taccalonolide T (1) results in the generation of a semisynthetic derivative (2) with a 744-fold increase in potency, relative to 1. The subnanomolar IC₅₀ value exhibited by T-epoxide (2) compares favorably with the IC₅₀ of paclitaxel (taxol) in this assay. T-epoxide was also shown to cause interphase microtubule bundling in HeLa cells, as determined by immunofluorescence visualization (images shown above). The stereoselectivity of the DMDO-mediated epoxidation may be due to steric shielding of the b-face by the axial C27-methyl substituent, highlighted in the scheme above.

Xenograft data from: Mooberry et al J. Med. Chem. 2014, 57, 6141 – 6149.

            The chemically-modified taccalonolide analog T-epoxide (2) also exhibited some in vivo antitumor efficacy in a xenograft model of breast cancer. Individual doses of 0.25 mg/kg of 2 were administered twice in the first week, with no additional doses given due to an average 10% body weight loss observed on day seven. Tumor growth was completely inhibited in the first week and significant antitumor effects were sustained for an additional week (original data from J. Med. Chem. 2014 manuscript reproduced above). In spite of an apparently narrow therapeutic window, the ability of a relatively low dose of compound 2 (total dose of 0.5 mg/kg) to produce antitumor effects highlights the exceptional potency of this exciting new microtubule-stabilizing steroid lactone.