Taccalonolide AJ Research Articles

Taccalonolide C-6 Analogues, Including Paclitaxel Hybrids, Demonstrate Improved Microtubule Polymerizing Activities.

The C-22,23-epoxy taccalonolides are microtubule stabilizers that bind covalently to β-tubulin with a high degree of specificity. We semisynthesized and performed biochemical and cellular evaluations on 20 taccalonolide analogues designed to improve target engagement. Most notably, modification of C-6 on the taccalonolide backbone with the C-13 N-acyl-β-phenylisoserine side chain of paclitaxel provided compounds with 10-fold improved potency for biochemical tubulin polymerization as compared to that of the unmodified epoxy taccalonolide AJ. Covalent docking demonstrated that the C-13 paclitaxel side chain occupied a binding pocket adjacent to the core taccalonolide pocket near the M-loop of β-tubulin. Although paclitaxel-taccalonolide hybrids demonstrated improved in vitro biochemical potency, they retained features of the taccalonolide chemotype, including a lag in tubulin polymerization and high degree of cellular persistence after drug washout associated with covalent binding. Together, these data demonstrate that C-6 modifications can improve the target engagement of this covalent class of microtubule drugs without substantively changing their mechanism of action.

Development of Taccalonolide AJ-Hydroxypropyl-β-Cyclodextrin Inclusion Complexes for Treatment of Clear Cell Renal-Cell Carcinoma.

Background: Microtubule-targeted drugs are the most effective drugs for adult patients with certain solid tumors. Taccalonolide AJ (AJ) can stabilize tubulin polymerization by covalently binding to β-tubulin, which enables it to play a role in the treatment of tumors. However, its clinical applications are largely limited by low water solubility, chemical instability in water, and a narrow therapeutic window. Clear-cell renal-cell carcinoma (cc RCC) accounts for approximately 70% of RCC cases and is prone to resistance to particularly targeted therapy drugs. Methods: we prepared a water-soluble cyclodextrin-based carrier to serve as an effective treatment for cc RCC. Results: Compared with AJ, taccalonolide AJ-hydroxypropyl-β-cyclodextrin (AJ-HP-β-CD) exhibited superior selectivity and activity toward the cc RCC cell line 786-O vs. normal kidney cells by inducing apoptosis and cell cycle arrest and inhibiting migration and invasion of tumor cells in vitro. According to acute toxicity testing, the maximum tolerated dose (MTD) of AJ-HP-β-CD was 10.71 mg/kg, which was 20 times greater than that of AJ. Assessment of weight changes showed that mouse body weight recovered over 7–8 days, and the toxicity could be greatly reduced by adjusting the injections from once every three days to once per week. In addition, we inoculated 786-O cells to generate xenografted mice to evaluate the anti-tumor activity of AJ-HP-β-CD in vivo and found that AJ-HP-β-CD had a better tumor inhibitory effect than that of docetaxel and sunitinib in terms of tumor growth and endpoint tumor weight. These results indicated that cyclodextrin inclusion greatly increased the anti-tumor therapeutic window of AJ. Conclusions: the AJ-HP-β-CD complex developed in this study may prove to be a novel tubulin stabilizer for the treatment of cc RCC. In addition, this drug delivery system may broaden the horizon in the translational study of other chemotherapeutic drugs.

Unravelling the covalent binding of zampanolide and taccalonolide AJ to a minimalist representation of a human microtubule.

Many natural products target mammalian tubulin but only a few can form a covalent bond and hence irreversibly affect microtubule function. Among them, zampanolide (ZMP) and taccalonolide AJ (TAJ) stand out, not only because they are very potent antitumor agents but also because the adducts they form with β-tubulin have been structurally characterized in atomic detail. By applying model building techniques, molecular orbital calculations, molecular dynamics simulations and hybrid QM/MM methods, we have gained insight into the 1,2- and 1,4-addition reactions of His229 and Asp226 to ZMP and TAJ, respectively, in the taxane-binding site of β-tubulin. The experimentally inaccessible precovalent complexes strongly suggest a water-mediated proton shuttle mechanism for ZMP adduct formation and a direct nucleophilic attack by the carboxylate of Asp226 on C22 of the C22R,C23R epoxide in TAJ. The M-loop, which is crucially important for interprotofilament interactions, is structured into a short helix in both types of complexes, mostly as a consequence of the fixation of the phenol ring of Tyr283 and the guanidinium of Arg284. As a side benefit, we obtained evidence supporting the existence of a commonly neglected intramolecular disulfide bond between Cys241 and Cys356 in β-tubulin that contributes to protein compactness and is absent in the βIII isotype associated with resistance to taxanes and other drugs.

Mechanism of microtubule stabilization by taccalonolide AJ

As a major component of the cytoskeleton, microtubules consist of αβ-tubulin heterodimers and have been recognized as attractive targets for cancer chemotherapy. Microtubule-stabilizing agents (MSAs) promote polymerization of tubulin and stabilize the polymer, preventing depolymerization. The molecular mechanisms by which MSAs stabilize microtubules remain elusive. Here we report a 2.05 Å crystal structure of tubulin complexed with taccalonolide AJ, a newly identified taxane-site MSA. Taccalonolide AJ covalently binds to β-tubulin D226. On AJ binding, the M-loop undergoes a conformational shift to facilitate tubulin polymerization. In this tubulin–AJ complex, the E-site of tubulin is occupied by GTP rather than GDP. Biochemical analyses confirm that AJ inhibits the hydrolysis of the E-site GTP. Thus, we propose that the β-tubulin E-site is locked into a GTP-preferred status by AJ binding. Our results provide experimental evidence for the connection between MSA binding and tubulin nucleotide state, and will help design new MSAs to overcome taxane resistance.

Pharmacokinetic Analysis and in Vivo Antitumor Efficacy of Taccalonolides AF and AJ.

The taccalonolides are microtubule stabilizers that covalently bind tubulin and circumvent clinically relevant forms of resistance to other drugs of this class. Efforts are under way to identify a taccalonolide with optimal properties for clinical development. The structurally similar taccalonolides AF and AJ have comparable microtubule-stabilizing activities in vitro, but taccalonolide AF has excellent in vivo antitumor efficacy when administered systemically, while taccalonolide AJ does not elicit this activity even at maximum tolerated dose. The hypothesis that pharmacokinetic differences underlie the differential efficacies of taccalonolides AF and AJ was tested. The effects of serum on their in vivo potency, metabolism by human liver microsomes and in vivo pharmacokinetic properties were evaluated. Taccalonolides AF and AJ were found to have elimination half-lives of 44 and 8.1 min, respectively. Furthermore, taccalonolide AJ was found to have excellent and highly persistent antitumor efficacy when administered directly to the tumor, suggesting that the lack of antitumor efficacy seen with systemic administration of AJ is likely due to its short half-life in vivo. These results help define why some, but not all, taccalonolides inhibit the growth of tumors at systemically tolerable doses and prompt studies to further improve their pharmacokinetic profile and antitumor efficacy.

Abstract 657: Microtubule stabilizers inhibit the cellular transport and signaling of EGFR in breast cancer cells

Abstract Breast cancer is the second most diagnosed cancer in women and it is estimated that approximately 40,000 women die every year from this disease in this country alone. The overexpression of EGFR in breast cancer is associated with drug resistance and an aggressive nature resulting in an overall poor outcome. The EGFR signaling pathway requires internalization of the receptor, endosomal translocation and in some cases nuclear accumulation of the receptor. These processes are dependent on functional microtubules. Microtubule targeting agents (MTAs) have been used with great success in the treatment of breast cancer alone and in combination with other agents. MTAs impair microtubule dynamics, which results in the inhibition of mitosis. Microtubules are additionally important in cellular signaling events including cellular transport of the EGFR. The effects of the microtubule stabilizer taccalonolide AJ on EGFR transport and activation in breast cancer cells were compared to the effects of paclitaxel. Our results show that microtubule stabilization interrupts the cellular trafficking of EGFR, leading to the retention of the receptor in the cell periphery.. Taccalonolide AJ and paclitaxel impaired the association of EGFR with the early endosome and delayed its association with the lysosome. This disruption in cellular transport also led to decreased phosphorylation of EGFR downstream targets AKT, ERK1/2 and STAT3, with taccalonolide AJ being the more effective inhibitor. Our studies show that microtubule stabilizers also prevent the association of EGFR with the Golgi instead leading to the fragmentation of this organelle. Paclitaxel caused a higher degree of Golgi fragmentation and it was observed in nearly all cells. The inhibition of EGFR localization to the Golgi caused by taccalonolide AJ and paclitaxel suggests that microtubule stabilization interferes with the trafficking of EGFR to the nucleus. This was confirmed by our results showing that microtubule stabilization impairs the nuclear transport of EGFR in breast cancer cells. Not surprisingly, in this case, paclitaxel was much more effective at preventing this nuclear accumulation. These studies highlight the utility and versatility of microtubule stabilizers and suggest the potential for combination treatments with microtubule stabilizers and small molecule inhibitors of EGFR and EGFR dependent pathways in breast cancer. Citation Format: Cristina Coral Rohena, Nicholas F. Dybdal-Hargreaves, Susan L. Mooberry. Microtubule stabilizers inhibit the cellular transport and signaling of EGFR in breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 657. doi:10.1158/1538-7445.AM2015-657

Abstract 5481: Differential inhibition of microtubule-dependent transport and signaling pathways important for cancer cell growth and survival

Abstract Microtubule targeting agents are some of the most valuable drugs used in the treatment of cancer. Recent studies demonstrate that the effects of these agents on interphase cells are likely central to their anticancer actions. The ability of microtubule targeting agents to interrupt multiple microtubule-dependent oncogene-driven signaling cascades might be one mechanism by which they initiate cancer cell death. The effects of diverse microtubule targeting agents on two microtubule-dependent processes essential for cancer cell survival were evaluated; TGF-β-mediated SMAD2/3 transport into the nucleus and the internalization and endocytosis of the EGFR. The ability of diverse microtubule targeting agents including paclitaxel, taccalonolide AJ, laulimalide and vinblastine that bind within different sites on tubulin/microtubules to interrupt TGF-β-mediated signaling pathways that promote cell growth, invasion and metastasis was evaluated. Using a variety of microscopy techniques, we show that microtubule stabilizers inhibit this pathway by preventing SMAD2/3 nuclear accumulation and subsequent activation of target genes. On the other hand, vinblastine caused an apparent increase in nuclear SMAD2/3 as compared to vehicle-treated cells. These effects, however, were not seen in cell lines driven by TGF-β pathways. Our results suggest that in cell lines with amplified TGF-β signaling, the microtubule regulatory component is lost. The effects of the microtubule targeting agents on EGFR internalization and endocytosis were also evaluated in A549 NSCLC cells and BT549 triple negative breast cancer cells; both of which overexpress EGFR. Taccalonolide AJ initiated distinct changes in the localization of the EGFR, impaired its internalization and endosomal sorting, leading to faster internalization and enhanced sorting to late endosomes. The data suggest that taccalonolide AJ causes premature degradation of EGFR and thus inhibited downstream signaling. Vinblastine on the other hand, did not impair the internalization of the EGFR or its degradation. Surprisingly the microtubule targeting agents had different effects on AKT phosphorylation, suggesting that they modulate EGFR dependent pathways in distinct ways. Identification of the specific signaling pathways and downstream targets interrupted by the different microtubule targeting agents might identify new rational drug combinations. This has the potential to identify patient populations that might respond better to certain microtubule targeting agents and begins to shed some light into the complex and subtly different mechanisms of action of these drugs. Citation Format: Cristina C. Rohena, Susan L. Mooberry. Differential inhibition of microtubule-dependent transport and signaling pathways important for cancer cell growth and survival. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5481. doi:10.1158/1538-7445.AM2014-5481

Hydrolysis Reactions of the Taccalonolides Reveal Structure–Activity Relationships

The taccalonolides are microtubule stabilizers isolated from plants of the genus Tacca that show potent in vivo antitumor activity and the ability to overcome multiple mechanisms of drug resistance. The most potent taccalonolide identified to date, AJ, is a semisynthetic product generated from the major plant metabolite taccalonolide A in a two-step reaction. The first step involves hydrolysis of taccalonolide A to generate taccalonolide B, and then this product is oxidized to generate an epoxide group at C-22-C-23. To generate sufficient taccalonolide AJ for in vivo antitumor efficacy studies, the hydrolysis conditions for the conversion of taccalonolide A to B were optimized. During purification of the hydrolysis products, we identified the new taccalonolide AO (1) along with taccalonolide I. When the same hydrolysis reaction was performed on a taccalonolide E-enriched fraction, four new taccalonolides, assigned as AK, AL, AM, and AN (2-5), were obtained in addition to the expected product taccalonolide N. Biological assays were performed on each of the purified taccalonolides, which allowed for increased refinement of the structure-activity relationship of this class of compounds.

Abstract 4501: Diverse microtubule stabilizers cause differential expression of mitotic kinases leading to distinct centrosomal and mitotic defects.

Abstract Microtubule stabilizers are some of the most useful and successful drugs used to treat adult solid tumors. However, the molecular events responsible for their antimitotic actions are not yet fully understood. In this study we evaluated the mitotic defects and signaling events initiated by three structurally and biologically different microtubule stabilizers; taccalonolide AJ, laulimalide/fijianolide B and paclitaxel. These microtubule stabilizers cause the formation of aberrant, but morphologically distinct mitotic spindles leading to the hypothesis that they initiate distinct mitotic signaling events. Each microtubule stabilizer caused different patterns of expression of key mitotic signaling proteins. Taccalonolide AJ caused centrosome separation failure to a much greater extent than paclitaxel or laulimalide. Taccalonolide AJ is also unique in that it was the only stabilizer to cause centrosome disjunction failure. These observations were consistent with the different defects in expression and activation of Plk1 and Eg5 caused by each stabilizer. Localization studies revealed that TPX2 and Aurora A are associated with each spindle aster formed by each stabilizer, which suggests a common mechanism of aster formation. However, taccalonolide AJ was the only stabilizer evaluated that caused pericentrin accumulation on every spindle aster. Pericentrin's localization to every spindle aster could facilitate the maintenance and stability of the highly focused asters formed by taccalonolide AJ. Laulimalide and paclitaxel caused completely different patterns of expression and activation of these proteins, as well as phenotypically different spindle phenotypes. Elucidating how chemically distinct microtubule stabilizers disturb mitotic signaling could identify key proteins involved in controlling sensitivity and resistance to the antimitotic actions of these agents. Citation Format: Cristina C. Rohena, Jiangnan Peng, Tyler A. Johnson, Phillip Crews, Susan L. Mooberry. Diverse microtubule stabilizers cause differential expression of mitotic kinases leading to distinct centrosomal and mitotic defects. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4501. doi:10.1158/1538-7445.AM2013-4501

Chemically diverse microtubule stabilizing agents initiate distinct mitotic defects and dysregulated expression of key mitotic kinases

Microtubule stabilizers are some of the most successful drugs used in the treatment of adult solid tumors and yet the molecular events responsible for their antimitotic actions are not well defined. The mitotic events initiated by three structurally and biologically diverse microtubule stabilizers; taccalonolide AJ, laulimalide/fijianolide B and paclitaxel were studied. These microtubule stabilizers cause the formation of aberrant, but structurally distinct mitotic spindles leading to the hypothesis that they differentially affect mitotic signaling. Each microtubule stabilizer initiated different patterns of expression of key mitotic signaling proteins. Taccalonolide AJ causes centrosome separation and disjunction failure to a much greater extent than paclitaxel or laulimalide, which is consistent with the distinct defects in expression and activation of Plk1 and Eg5 caused by each stabilizer. Localization studies revealed that TPX2 and Aurora A are associated with each spindle aster formed by each stabilizer. This suggests a common mechanism of aster formation. However, taccalonolide AJ also causes pericentrin accumulation on every spindle aster. The presence of pericentrin at every spindle aster initiated by taccalonolide AJ might facilitate the maintenance and stability of the highly focused asters formed by this stabilizer. Laulimalide and paclitaxel cause completely different patterns of expression and activation of these proteins, as well as phenotypically different spindle phenotypes. Delineating how diverse microtubule stabilizers affect mitotic signaling pathways could identify key proteins involved in modulating sensitivity and resistance to the antimitotic actions of these compounds.

Potent Taccalonolides, AF and AJ, Inform Significant Structure–Activity Relationships and Tubulin as the Binding Site of These Microtubule Stabilizers

The taccalonolides are a class of microtubule stabilizing agents isolated from plants of the genus Tacca. In efforts to define their structure-activity relationships, we isolated five new taccalonolides, AC-AF and H2, from one fraction of an ethanol extract of Tacca plantaginea. The structures were elucidated using a combination of spectroscopic methods, including 1D and 2D NMR and HR-ESI-MS. Taccalonolide AJ, an epoxidation product of taccalonolide B, was generated by semisynthesis. Five of these taccalonolides demonstrated cellular microtubule-stabilizing activities and antiproliferative actions against cancer cells, with taccalonolide AJ exhibiting the highest potency with an IC(50) value of 4.2 nM. The range of potencies of these compounds, from 4.2 nM to >50 μM, for the first time provides the opportunity to identify specific structural moieties crucial for potent biological activities as well as those that impede optimal cellular effects. In mechanistic assays, taccalonolides AF and AJ stimulated the polymerization of purified tubulin, an activity that had not previously been observed for taccalonolides A and B, providing the first evidence that this class of microtubule stabilizers can interact directly with tubulin/microtubules. Taccalonolides AF and AJ were able to enhance tubulin polymerization to the same extent as paclitaxel but exhibited a distinct kinetic profile, suggesting a distinct binding mode or the possibility of a new binding site. The potencies of taccalonolides AF and AJ and their direct interaction with tubulin, together with the previous excellent in vivo antitumor activity of this class, reveal the potential of the taccalonolides as new anticancer agents.