One- and Two-Photon Activated Release of Oxaliplatin from a Pt(IV)-Functionalized Poly(phenylene ethynylene)

We report a water-soluble poly(phenylene ethynylene) (PPE-Pt(IV)) that is functionalized with oxidized oxaliplatin Pt(IV) units and its use for photoactivated chemotherapy. The photoactivation strategy is based on photoinduced electron transfer from the PPE backbone to oxaliplatin Pt(IV) as an electron acceptor; this process triggers the release of oxaliplatin, which is a clinically used anticancer drug. Mechanistic studies carried out using steady-state and time-resolved fluorescence spectroscopy coupled with picosecond-nanosecond transient absorption support the hypothesis that electron transfer triggers the drug release. Photoactivation is effective, producing oxaliplatin with a good chemical yield in less than 1 h of photolysis (400 nm, 5 mW cm^-2). Photorelease of oxaliplatin from PPE-Pt(IV) can also be effected with two-photon excitation by using 100 fs pulsed light at 725 nm. Cytotoxicity studies using SK-OV-3 human ovarian cancer cells demonstrate that without photoactivation PPE-Pt(IV) is not cytotoxic at concentrations up to 10 μM in polymer repeating unit (PRU) concentration. However, following a short period of 460 nm irradiation, oxaliplatin is released from PPE-Pt(IV), resulting in cytotoxicity at concentrations as low as 2.5 μM PRU.

Re-evaluation of the Fijianolide/Laulimalide Chemotype Suggests an Alternate Mechanism of Action for C-15/C-20 Analogs

Herein, we report on naturally derived microtubule stabilizers with activity against triple negative breast cancer (TNBC) cell lines, including paclitaxel, fijianolide B/laulimalide (3), fijianolide B di-acetate (4), and two new semisynthetic analogs of 3, which include fijianolide J (5) and fijianolide L (6). Similar to paclitaxel, compound 3 demonstrated classic microtubule stabilizing activity with potent (GI₅₀ = 0.7-17 nM) antiproliferative efficacy among the five molecularly distinct TNBC cell lines. Alternatively, compounds 5 or 6, generated from oxidation of C-20 or C-15 and C-20 respectively, resulted in a unique profile with reduced potency (GI₅₀ = 4-9 μM), but improved efficacy in some lines, suggesting a distinct mechanism of action. The C-15, C-20 di-acetate, and dioxo modifications on 4 and 6 resulted in compounds devoid of classic microtubule stabilizing activity in biochemical assays. While 4 also had no detectable effect on cellular microtubules, 6 promoted a reorganization of the cytoskeleton resulting in an accumulation of microtubules at the cell periphery. Compound 5, with a single C-20 oxo substitution, displayed a mixed phenotype, sharing properties of 3 and 6. These results demonstrate the importance of the C-15/C-20 chiral centers, which appear to be required for the potent microtubule stabilizing activity of this chemotype and that oxidation of these sites promotes unanticipated cytoskeletal alterations that are distinct from classic microtubule stabilization, likely through a distinct mechanism of action.

Altertoxin II, a Highly Effective and Specific Compound against Ewing Sarcoma

A screening program designed to identify natural products with selective cytotoxic effects against cell lines representing different types of pediatric solid tumors led to the identification of altertoxin II as a highly potent and selective cytotoxin against Ewing sarcoma cell lines. Altertoxin II, but not the related compounds altertoxin I and alteichin, was highly effective against every Ewing sarcoma cell line tested, with an average 25-fold selectivity for these cells as compared to cells representing other pediatric and adult cancers. Mechanism of action studies revealed that altertoxin II causes DNA double-strand breaks, a rapid DNA damage response, and cell cycle accumulation in the S phase. Our studies also demonstrate that the potent effects of altertoxin II are partially dependent on the progression through the cell cycle, because the G₁ arrest initiated by a CDK4/6 inhibitor decreased antiproliferative potency more than 10 times. Importantly, the cell-type-selective DNA-damaging effects of altertoxin II in Ewing sarcoma cells occur independently of its ability to bind directly to DNA. Ultimately, we found that altertoxin II has a dose-dependent in vivo antitumor efficacy against a Ewing sarcoma xenograft, suggesting that it has potential as a therapeutic drug lead and will be useful to identify novel targets for Ewing-sarcoma-specific therapies.

Eribulin activates the cGAS-STING pathway via the cytoplasmic accumulation of mtDNA

Microtubule-targeting agents (MTAs), including both microtubule stabilizers and destabilizers are highly effective chemotherapeutic drugs used in the treatment of solid tumors and hematologic malignancies. In addition to the shared ability of all MTAs to block cell cycle progression, growing evidence shows that different agents of this class can also have mechanistically distinct effects on nonmitotic microtubule-dependent cellular processes, including cellular signaling and transport. Herein, we test the biologic hypothesis that MTAs used in the treatment of triple-negative breast cancer (TNBC) can differentially affect innate immune signaling pathways independent of their antimitotic effects. Our data demonstrate that the microtubule destabilizer eribulin, but not the microtubule stabilizer paclitaxel, induces cGAS-STING–dependent expression of interferon-β in both myeloid and TNBC cells. Activation of the cGAS-STING pathway by eribulin was further found to be mediated by the accumulation of cytoplasmic mitochondrial DNA. Together, these findings provide mechanistic insight into how eribulin can induce innate immune signaling independent of its antimitotic or cytotoxic effects.

Efficacy of a Covalent Microtubule Stabilizer in Taxane-Resistant Ovarian Cancer Models

Ovarian cancer often has a poor clinical prognosis because of late detection, frequently after metastatic progression, as well as acquired resistance to taxane-based therapy. Herein, we evaluate a novel class of covalent microtubule stabilizers, the C-22,23-epoxytaccalonolides, for their efficacy against taxane-resistant ovarian cancer models in vitro and in vivo. Taccalonolide AF, which covalently binds β-tubulin through its C-22,23-epoxide moiety, demonstrates efficacy against taxane-resistant models and shows superior persistence in clonogenic assays after drug washout due to irreversible target engagement. In vivo, intraperitoneal administration of taccalonolide AF demonstrated efficacy against the taxane-resistant NCI/ADR-RES ovarian cancer model both as a flank xenograft, as well as in a disseminated orthotopic disease model representing localized metastasis. Taccalonolide-treated animals had a significant decrease in micrometastasis of NCI/ADR-RES cells to the spleen, as detected by quantitative RT-PCR, without any evidence of systemic toxicity. Together, these findings demonstrate that taccalonolide AF retains efficacy in taxane-resistant ovarian cancer models in vitro and in vivo and that its irreversible mechanism of microtubule stabilization has the unique potential for intraperitoneal treatment of locally disseminated taxane-resistant disease, which represents a significant unmet clinical need in the treatment of ovarian cancer patients.

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.

Yuanhuacine Is a Potent and Selective Inhibitor of the Basal-Like 2 Subtype of Triple Negative Breast Cancer with Immunogenic Potential

The heterogeneity of triple negative breast cancer (TNBC) has led to efforts to further subtype this disease with the hope of identifying new molecular liabilities and drug targets. Furthermore, the finding that TNBC is the most inherently immunogenic type of breast cancer provides the potential for effective treatment with immune checkpoint inhibitors and immune adjuvants. Thus, we devised a dual screen to identify compounds from natural product extracts with TNBC subtype selectivity that also promote the expression of cytokines associated with antitumor immunity. These efforts led to the identification of yuanhuacine (1) as a potent and highly selective inhibitor of the basal-like 2 (BL2) subtype of TNBC that also promoted an antitumor associated cytokine signature in immune cells. The mechanism of action of yuanhuacine for both phenotypes depends on activation of protein kinase C (PKC), defining a novel target for the treatment of this clinical TNBC subtype. Yuanhuacine showed potent antitumor efficacy in animals bearing BL2 tumors further demonstrating that PKC could function as a potential pharmacological target for the treatment of the BL2 subtype of TNBC.

Abstract PS17-18: Eribulin-mediated release of mitochondrial DNA activates the cGAS-STING innate immune signaling pathway

Triple-negative breast cancer (TNBC) patients have a higher response rate to immune checkpoint inhibitors as compared to patients with other types of breast cancer. This higher response rate is thought to be due to higher mutational burdens, the prevalence of PD-L1/PD-1 expression, and immune cell infiltration in TNBC. However, less than half of TNBC patients respond to these drugs as single agents, which has prompted their use in combination with chemotherapy, including microtubule targeted agents (MTAs). Multiple clinical trials now show that the combination of PD-1/PD-L1-targeted immune checkpoint inhibitors with chemotherapy, including the MTAs paclitaxel or eribulin, can improve clinical responses in both late and early-stage TNBC. Previous studies have demonstrated that MTAs can activate innate immune sensing pathways that could potentially mediate their efficacy in combination with immunotherapies, including the ability of paclitaxel to promote TLR4 signaling. However, there has not been a direct comparison of the immunogenic effects elicited by each of the five MTAs that are used to treat TNBC to inform on how agents of this class could differentially alter the tumor immune microenvironment. In the current study, we determined the concentration and time-dependent effects of paclitaxel, docetaxel, ixabepilone, eribulin, and vinorelbine on the cytokine expression profiles in both immune and TNBC cells. Our results demonstrate that the microtubule destabilizers, eribulin, and vinorelbine, but not the microtubule stabilizers, paclitaxel, docetaxel, and ixabepilone, promote upregulation of type 1 interferons (IFNα/β) and downstream interferon-stimulated-genes. A time-course analysis in the human monocytic THP-1 cell line and primary murine bone-derived macrophages found that the induction of IFNβ by eribulin occurred within 2-6 hours and was independent of mitotic arrest. A pharmacological and genetic-based assessment of the signaling pathways responsible for the eribulin-mediated expression of IFNβ revealed a dependency on the cGAS-STING cytosolic DNA-sensing innate immune signaling pathway in both immune and TNBC cells. Moreover, we demonstrate that the mechanism for the promotion of cGAS-STING-dependent interferon induction by eribulin is due to the release of mitochondrial DNA (mtDNA) into the cytoplasm. Together, these results provide evidence that different MTAs have distinct immunomodulatory properties and highlight the ability of eribulin to promote the release of mtDNA to activate the cGAS-STING pathway, which has been previously shown to enhance the efficacy of immunotherapy in TNBC. These studies were funded by Eisai.

Citation Format: Charles Steven Fermaintt, Leila Takahashi-Ruiz, Susan L Mooberry, April L Risinger. Eribulin-mediated release of mitochondrial DNA activates the cGAS-STING innate immune signaling pathway [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS17-18.

Synthesis and Biological Evaluations of Electrophilic Steroids Inspired by the Taccalonolides

Natural products have served as inspirational scaffolds for the design and synthesis of novel antineoplastic agents. Here we present our preliminary efforts on the synthesis and biological evaluation of a new class of electrophilic steroids inspired by the naturally occurring taccalonolides. We demonstrate that these simplified analogs exhibit highly persistent antiproliferative properties similar to the taccalonolides and retain activity against resistant cancer cell lines that warrants further preclinical development.

CRISPR-Cas9 Genome-Wide Knockout Screen Identifies Mechanism of Selective Activity of Dehydrofalcarinol in Mesenchymal Stem-like Triple-Negative Breast Cancer Cells

There are no targeted therapies available for triple-negative breast cancers (TNBCs) in part because they represent a heterogeneous group of tumors with diverse oncogenic drivers. Our goal is to identify targeted therapies for subtypes of these cancers using a mechanism-blind screen of natural product extract libraries. An extract from Desmanthodium guatemalense was 4-fold more potent for cytotoxicity against MDA-MB-231 cells, which represent the mesenchymal stem-like (MSL) subtype, as compared to cells of other TNBC subtypes. Bioassay-guided fractionation led to the isolation of six polyacetylenes, and subsequent investigations of plant sources known to produce polyacetylenes yielded six additional structurally related compounds. A subset of these compounds retained selective cytotoxic effects in MSL subtype cells. Studies suggest that these selective effects do not appear to be due to PPARγ agonist activities that have previously been reported for polyacetylenes. A CRISPR-Cas9-mediated gene knockout screen was employed to identify the mechanism of selective cytotoxic activity of the most potent and selective compound, dehydrofalcarinol (1a). This genomic screen identified HSD17B11, the gene encoding the enzyme 17β-hydroxysteroid dehydrogenase type 11, as a mediator of the selective cytotoxic effects of 1a in MDA-MB-231 cells that express high levels of this protein. The Project Achilles cancer dependency database further identified a subset of Ewing sarcoma cell lines as highly dependent on HSD17B11 expression, and it was found these were also highly sensitive to 1a. This report demonstrates the value of CRISPR-Cas9 genome-wide screens to identify the mechanisms underlying the selective activities of natural products.

Correction: Eribulin rapidly inhibits TGF-β-induced Snail expression and can induce Slug expression in a Smad4-dependent manner

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Abstract 3000: Evaluating the efficacy of a novel class of covalent microtubule stabilizers in taxane resistant ovarian cancer

Ovarian cancer has few targeted therapies and high rates of metastasis. Taxane microtubule stabilizers, including paclitaxel, are a mainstay in the treatment of adult solid tumors as single agents and in combination with other cytotoxic chemotherapies, targeted therapies, and immunotherapies. The standard of care for ovarian cancer patients comprises of combination chemotherapy consisting of a taxane with a platinum agent administered intravenously or intraperitoneally. Although this approach is highly effective initially, up to 70% of ovarian cancer patients will acquire taxane resistance. Therefore, it is essential to identify drugs that retain efficacy in clinically relevant models of taxane resistance, including those with elevated P-glycoprotein (Pgp) expression. The development of a microtubule stabilizer that retains efficacy in taxane resistant ovarian cancer would be optimal since microtubule disruption is proven to be an effective target in this disease. The taccalonolides are a novel class of covalent and irreversible microtubule stabilizers that circumvent clinically relevant forms of taxane resistance in vitro and in vivo breast cancer models. However, the taccalonolides have a narrow therapeutic window and short serum half-life that would benefit from a more targeted delivery. We hypothesized that localized intraperitoneal delivery of the taccalonolides in ovarian cancer models may be an optimal strategy for the treatment of intraperitoneally disseminated disease. We have found that the taccalonolides are less susceptible than paclitaxel to Pgp-mediated drug resistance in a human ovarian cancer cell line model. Additional experiments have demonstrated that the taccalonolides retain long-term antiproliferative and cytotoxic efficacy following acute treatment of human and murine ovarian cancer cells, demonstrating superior cellular persistence as compared to other classes of microtubule stabilizing drugs. We are currently evaluating the efficacy of the taccalonolides in ovarian cancer models using localized, intraperitoneal administration in a syngeneic and orthotopic murine model, which may be optimal for delivery of this irreversible microtubule stabilizer.

Citation Format: Samantha S.M. Yee, Lin Du, April L. Risinger. Evaluating the efficacy of a novel class of covalent microtubule stabilizers in taxane resistant ovarian cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3000.

Abstract 1786: High expression of NCX-1 in triple negative breast cancer cell lines identifies a potential biomarker for sensitivity to cardiac glycosides

Triple negative breast cancers (TNBCs) represent 15-20% of all breast cancers and are defined by lack of the estrogen and progesterone receptors and HER2 gene amplification. Therefore, TNBC patients do not benefit from available therapies targeting these receptors. The significant heterogeneity of TNBC precludes the expectation that a single molecular target will be identified for this disease. Our goal is to identify compounds with selective cytotoxic effects in subsets of TNBC cells to identify new therapeutic targets for TNBC subgroups. Our studies show that the TNBC cell lines BT-549 and Hs578T are distinct from other TNBC cells in that they are highly sensitive to 9 cardiac glycosides/cardenolides isolated from Calotropis gigantea as well as digoxin. BT-549 cells are 15-times more sensitive to the isolated cardiac glycoside calotropin and 9-times more sensitive to digoxin as compared to MDA-MB-231 TNBC cells. Cardiac glycosides bind to the Na+/K+ ATPase and inhibit its activity, leading to increased intracellular Na+, which results in higher intracellular Ca2+ through reversal of the Na+/Ca2+ exchanger (NCX). Notably, among the 10 cardiac glycosides/cardenolides, there was a significant correlation between potency for inhibition of purified Na+/K+ ATPase and cytotoxic potency and selectivity for BT-549 cells. These data suggest that inhibition of Na+/K+ ATPase is critical for the selective cytotoxic effects of these compounds. BT-549 and Hs578T cells express high levels of non-specific TRPC1/4 cation channels1 and low levels of SERCA2 Ca2+ pumps2 compared to other TNBC cells, suggesting that they have impaired ability to handle intracellular Ca2+. Consistent with this hypothesis, within 3 h, concentrations of calotropin and digoxin that are selectivity cytotoxic each initiated a significant increase in intracellular Ca2+ in BT-549 cells. To test whether sensitivity to cardiac glycosides was due to higher Na+/K+ ATPase expression, message and protein levels were evaluated in 10 TNBC cell lines. The results show that there is no correlation between levels of Na+/K+ ATPase and sensitivity to cardiac glycosides. In contrast, NCX-1 expression was found to be 120 and 60-fold higher in the sensitive BT-549 and Hs578T cells, respectively, as compared to the resistant TNBC cell lines. Ongoing studies are evaluating whether genetic manipulation of NCX-1 is sufficient to modulate sensitivity to cardiac glycosides. These findings demonstrate that NCX-1 expression is associated with the selective sensitivity of a subgroup of TNBC cell lines to the cytotoxic effects of cardiac glycosides. Our results suggest that NCX-1 could be a biomarker to identify TNBC patients that could ultimately benefit from use of a cardiac glycoside for anticancer indications.

1. Grant CV, et al. Breast Cancer Res Treat. 2019;177(2):345.

2. Varga K, et al. BMC Cancer. 2018;18(1):1029.

Citation Format: Petra E. Pederson, Shengxin Cai, Chase M. Carver, Tanja Grkovic, Barry R. O'Keefe, April L. Risinger, Robert H. Cichewicz, Susan L. Mooberry. High expression of NCX-1 in triple negative breast cancer cell lines identifies a potential biomarker for sensitivity to cardiac glycosides [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1786.

Abstract 965: Eribulin promotes a distinct immunogenic gene expression signature as compared with paclitaxel

Microtubule targeted agents (MTAs) are a mainstay in the treatment of breast cancer and they are often used in combination with targeted agents, including immunotherapies. Although patients with triple-negative breast cancers (TNBCs) have historically had poorer clinical outcomes due in part to a lack of targeted therapeutic options, these cancers are often more immunogenic than other types of breast cancer. Recently, the combination of the PD-L1 inhibitor atezolizumab with the taxane nab-paclitaxel was found to prolong progression-free survival in patients with metastatic TNBC. Other combinations of MTAs and immune checkpoint inhibitors are actively being evaluated in clinical trials. There is a mechanistic rationale for this combination because paclitaxel is a direct TLR4 agonist that promotes activation of the innate immune system. To date, there has not been a comprehensive comparison of the immunomodulatory effects of different MTAs on immune cells or on or TNBC cells. We hypothesized that MTAs that differentially alter interphase microtubule structure and dynamics would have distinct effects on cell-intrinsic immunogenic pathways and that these effects could impact the efficacy of these drugs in combination with immunotherapies. The effects of 5 MTAs used for the treatment of TNBC, paclitaxel, docetaxel, ixabepilone, vinorelbine, and eribulin, on the expression of antitumor cytokines and other immunomodulatory genes in myeloid and TNBC cells was evaluated. Our results show that the TLR4-mediated induction of TNFα that occurs in response to paclitaxel was not observed with either eribulin or vinorelbine. Conversely, upregulation of type 1 interferons and interferon-stimulated genes (ISGs) was observed in monocytes and in bone-marrow derived macrophages within 6 h of treatment with eribulin or vinorelbine, but not with any of the microtubule stabilizers. Eribulin-mediated induction of type 1 interferons was also observed in a subset of eribulin-treated TNBC cells and was independent of its antimitotic effects. Importantly, type I interferon signaling has been shown to enhance the efficacy of immune checkpoint inhibitors, suggesting that microtubule destabilizers could have distinct advantages as compared to microtubule stabilizers in combination with these agents. Together, these results provide evidence that MTAs have different immune modulatory properties and suggest that the specific immune signatures initiated by different MTAs need to be considered for their optimal use with targeted agents, including checkpoint inhibitors. These studies were funded by Eisai.

Citation Format: Charles Fermaintt, Susan Mooberry, April L. Risinger. Eribulin promotes a distinct immunogenic gene expression signature as compared with paclitaxel [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 965.

Triple-Negative Breast Cancer Cells Exhibit Differential Sensitivity to Cardenolides from Calotropis gigantea

Triple-negative breast cancers (TNBC) are aggressive and heterogeneous cancers that lack targeted therapies. We implemented a screening program to identify new leads for subgroups of TNBC using diverse cell lines with different molecular drivers. Through this program, we identified an extract from Calotropis gigantea that caused selective cytotoxicity in BT-549 cells as compared to four other TNBC cell lines. Bioassay-guided fractionation of the BT-549 selective extract yielded nine cardenolides responsible for the selective activity. These included eight known cardenolides and a new cardenolide glycoside. Structure-activity relationships among the cardenolides demonstrated a correlation between their relative potencies toward BT-549 cells and Na+/K+ ATPase inhibition. Calotropin, the compound with the highest degree of selectivity for BT-549 cells, increased intracellular Ca2+ in sensitive cells to a greater extent than in the resistant MDA-MB-231 cells. Further studies identified a second TNBC cell line, Hs578T, that is also highly sensitive to the cardenolides, and mechanistic studies were conducted to identify commonalities among the sensitive cell lines. Experiments showed that both cardenolide-sensitive cell lines expressed higher mRNA levels of the Na+/Ca2+ exchanger NCX1 than resistant TNBC cells. This suggests that NCX1 could be a biomarker to identify TNBC patients that might benefit from the clinical administration of a cardiac glycoside for anticancer indications.

Leucinostatins from Ophiocordyceps spp. and Purpureocillium spp. Demonstrate Selective Antiproliferative Effects in Cells Representing the Luminal Androgen Receptor Subtype of Triple Negative Breast Cancer

The structures of four leucinostatin analogues (1-4) from Ophiocordyceps spp. and Purpureocillium spp. were determined together with six known leucinostatins [leucinostatins B (5), A (6), B2 (7), A2 (8), F (9), and D (10)]. The structures of the metabolites were established using a combination of analytical methods including HRESIMS and MS/MS experiments, 1D and 2D NMR spectroscopy, chiral HPLC, and advanced Marfey's analysis of the acid hydrolysate, as well as additional empirical and chemical methods. Compounds 1-10 were evaluated for their biological effects on triple negative breast cancer (TNBC) cells. Leucinostatins 1-10 showed selective cytostatic activities in MDA-MB-453 and SUM185PE cells representing the luminal androgen receptor subtype of TNBC. This selective activity motivated further investigation into the mechanism of action of leucinostatin B (5). The results demonstrate that this peptidic fungal metabolite rapidly inhibits mTORC1 signaling in leucinostatin-sensitive TNBC cell lines, but not in leucinostatin-resistant cells. Leucinostatins have been shown to repress mitochondrial respiration through inhibition of the ATP synthase, and we demonstrated that both the mTORC1 signaling and LAR-selective activities of 5 were recapitulated by oligomycin. Thus, inhibition of the ATP synthase with either leucinostatin B or oligomycin is sufficient to selectively impede mTORC1 signaling and inhibit the growth of LAR-subtype cells.

Targeting and extending the eukaryotic druggable genome with natural products: cytoskeletal targets of natural products

Covering: 2014-2019We review recent progress on natural products that target cytoskeletal components, including microtubules, actin, intermediate filaments, and septins and highlight their demonstrated and potential utility in the treatment of human disease. The anticancer efficacy of microtubule targeted agents identified from plants, microbes, and marine organisms is well documented. We highlight new microtubule targeted agents currently in clinical evaluations for the treatment of drug resistant cancers and the accumulating evidence that the anticancer efficacy of these agents is not solely due to their antimitotic effects. Indeed, the effects of microtubule targeted agents on interphase microtubules are leading to their potential for more mechanistically guided use in cancers as well as neurological disease. The discussion of these agents as more targeted drugs also prompts a reevaluation of our thinking about natural products that target other components of the cytoskeleton. For instance, actin active natural products are largely considered chemical probes and non-selective toxins. However, studies utilizing these probes have uncovered aspects of actin biology that can be more specifically targeted to potentially treat cancer, neurological disorders, and infectious disease. Compounds that target intermediate filaments and septins are understudied, but their continued discovery and mechanistic evaluations have implications for numerous therapeutic indications.

Using the Cancer Dependency Map to Identify the Mechanism of Action of a Cytotoxic Alkenyl Derivative from the Fruit of Choerospondias axillaris

An extract prepared from the fruit of Choerospondias axillaris exhibited differential cytotoxic effects when tested in a panel of pediatric cancer cell lines [Ewing sarcoma (A-673), rhabdomyosarcoma (SJCRH30), medulloblastoma (D283), and hepatoblastoma (Hep293TT)]. Bioassay-guided fractionation led to the purification of five new hydroquinone-based metabolites, choerosponols A-E (1-5), bearing unsaturated hydrocarbon chains. The structures of the natural products were determined using a combination of 1D and 2D NMR, HRESIMS, ECD spectroscopy, and Mosher ester analyses. The purified compounds were evaluated for their antiproliferative and cytotoxic activities, revealing that 1, which contains a benzofuran moiety, exhibited over 50-fold selective antiproliferative activity against Ewing sarcoma and medulloblastoma cells with growth inhibitory (GI50) values of 0.19 and 0.07 μM, respectively. The effects of 1 were evaluated in a larger panel of cancer cell lines, and these data were used in turn to interrogate the Project Achilles cancer dependency database, leading to the identification of the MCT1 transporter as a functional target of 1. These data highlight the utility of publicly available cancer dependency databases such as Project Achilles to facilitate the identification of the mechanisms of action of compounds with selective activities among cancer cell lines, which can be a major challenge in natural products drug discovery.

Abstract P5-05-03: Eribulin treatment activates type 1 IFNs to promote a gene expression signature associated with antitumor immunity

Immune checkpoint inhibitors targeting the PD1/PD-L1 and CTLA4 signaling axes have made a significant impact in the treatment of some solid tumors, particularly melanoma and non-small-cell lung carcinoma. While triple-negative breast cancers (TNBC) typically have poorer clinical outcomes due in part to a lack of targeted therapies, they are often more immunogenic than other types of breast cancer.1 Some forms of chemotherapy, including microtubule-targeting agents, have been proposed to enhance the efficacy of immunotherapy,2 which has resulted in the evaluations of combinations of these drugs in preclinical and clinical studies. Most notably, the combination of the PD-L1 inhibitor atezolizumab with the taxane nab-paclitaxel was shown to prolong progression-free survival in patients with metastatic TNBC.3 Although it is known that paclitaxel can function as a TLR4 agonist leading to activation of the innate immune system, there has been little comparison of the immunogenic effects elicited by different clinically used MTAs for the treatment of TNBC. We evaluated the effects of 5 distinct MTAs used for the treatment of TNBC, paclitaxel, docetaxel, ixabepilone, vinorelbine, and eribulin, on the expression of antitumor cytokines and other immunomodulatory genes in myeloid and in TNBC cells. Our results show that the expression signature generated by the microtubule stabilizers, paclitaxel, docetaxel, and ixabepilone, was markedly different from the signature elicited by the microtubule destabilizers eribulin and vinorelbine. The microtubule destabilizers caused upregulation of type 1 interferons and interferon-stimulated-genes (ISGs), effects not seen with the microtubule stabilizers. A time course analysis revealed that the transcriptional upregulation of the ISG CXCL10 by eribulin occurred before the expression of interferons. Mechanistically, microtubule destabilizer-induced expression of c-Jun which could upregulate the early expression of CXCL10 through activation of the AP-1 complex. The eribulin-mediated induction of type 1 interferons is similar to the expression signature promoted by the cGAS-STING pathway that has been shown to enhance the efficacy of checkpoint inhibitors,4 suggesting that microtubule destabilizers, but not stabilizers, can promote signaling through this pathway. Together, these results provide evidence that distinct MTAs have different immune modulatory properties and suggest that the specific immune signatures initiated by the different MTAs need to be considered for their optimal use with checkpoint inhibitors. These studies were funded by Eisai.

1. Liu, Z., Li, M., Jiang, Z. & Wang, X. A Comprehensive Immunologic Portrait of Triple-Negative Breast Cancer. Transl Oncol 11, 311-329, (2018).

2. Emens, L. A. & Middleton, G. The interplay of immunotherapy and chemotherapy: harnessing potential synergies. Cancer Immunol Res 3, 436-443, (2015).

3. Schmid, P. et al. Atezolizumab and Nab-Paclitaxel in Advanced Triple-Negative Breast Cancer. N Engl J Med 379, 2108-2121, (2018).

4. Cheng, N. et al. A nanoparticle-incorporated STING activator enhances antitumor immunity in PD-L1-insensitive models of triple-negative breast cancer. JCI Insight 3, (2018).

Citation Format: Charles S Fermaintt, Shayne Hastings, Susan L Mooberry, April L Risinger. Eribulin treatment activates type 1 IFNs to promote a gene expression signature associated with antitumor immunity [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-05-03.

Elucidating target specificity of the taccalonolide covalent microtubule stabilizers employing a combinatorial chemical approach

The taccalonolide microtubule stabilizers covalently bind β-tubulin and overcome clinically relevant taxane resistance mechanisms. Evaluations of the target specificity and detailed drug-target interactions of taccalonolides, however, have been limited in part by their irreversible target engagement. In this study, we report the synthesis of fluorogenic taccalonolide probes that maintain the native biological properties of the potent taccalonolide, AJ. These carefully optimized, cell-permeable probes outperform commercial taxane-based probes and enable direct visualization of taccalonolides in both live and fixed cells with dramatic microtubule colocalization. The specificity of taccalonolide binding to β-tubulin is demonstrated by immunoblotting, which allows for determination of the relative contribution of key tubulin residues and taccalonolide moieties for drug-target interactions by activity-based protein profiling utilizing site-directed mutagenesis and computational modeling. This combinatorial approach provides a generally applicable strategy for investigating the binding specificity and molecular interactions of covalent binding drugs in a cellular environment.

Taccalonolide Microtubule Stabilizers

Microtubule stabilizers are a mainstay in the treatment of many solid cancers and continue to find utility in combination therapy with molecularly targeted anticancer agents and immunotherapeutics. However, innate and acquired resistance to microtubule stabilizers can limit their clinical efficacy. The taccalonolides are a unique class of microtubule stabilizers isolated from plants of Tacca that circumvent clinically relevant mechanisms of drug resistance. Although initial reports suggested that the microtubule-stabilizing activity of the taccalonolides was independent of direct tubulin binding, additional studies have identified that potent C-22, C-23 epoxidized taccalonolides covalently bind the Aspartate 226 residue of β-tubulin and that this interaction is critical for their microtubule-stabilizing activity. The taccalonolides have distinct properties as compared to other microtubule stabilizers with regard to their biochemical effects on tubulin structure and dynamics that promote distinct cellular phenotypes. Some taccalonolides have demonstrated in vivo antitumor efficacy in drug-resistant tumor models with exquisite potency and long-lasting antitumor efficacy as a result of their irreversible target engagement. The recent identification of a site on the taccalonolide scaffold that is amenable to modification has provided evidence of the specificity of the taccalonolide-tubulin interaction. This also affords an opportunity to further optimize the targeted delivery of the taccalonolides to further improve their anticancer efficacy and potential for clinical development.

Vortioxetine reverses medial prefrontal cortex-mediated cognitive deficits in male rats induced by castration as a model of androgen deprivation therapy for prostate cancer

RATIONALE

Androgen deprivation therapy (ADT) is an effective treatment for prostate cancer, but induces profound cognitive impairment. Little research has addressed mechanisms underlying these deficits or potential treatments. This is an unmet need to improve quality of life for prostate cancer survivors.

OBJECTIVES

We investigated mechanisms of cognitive impairment after ADT in rats and potential utility of the multimodal serotonin-targeting drug, vortioxetine, to improve the impairment, as vortioxetine has specific efficacy against cognitive impairment in depression.

METHODS

Male Sprague-Dawley rats were surgically castrated. Vortioxetine (28 mg/kg/day) was administered in the diet. The attentional set-shifting test was used to assess medial prefrontal cortex (mPFC) executive function. Afferent-evoked field potentials were recorded in the mPFC of anesthetized rats after stimulating the ventral hippocampus (vHipp) or medial dorsal thalamus (MDT). Gene expression changes were assessed by microarray. Effects of vortioxetine on growth of prostate cancer cells were assessed in vitro.

RESULTS

ADT impaired cognitive set shifting and attenuated responses evoked in the mPFC by the vHipp afferent, but not the MDT. Both the cognitive impairment and attenuated vHipp-evoked responses were reversed by chronic vortioxetine treatment. Preliminary investigation of gene expression in the mPFC indicates that factors involved in neuronal plasticity and synaptic transmission were down-regulated by castration and up-regulated by vortioxetine in castrated animals. Vortioxetine neither altered the growth of prostate cancer cells in vitro nor interfered with the antiproliferative effects of the androgen antagonist, enzalutamide.

CONCLUSIONS

These results suggest that vortioxetine may be useful in mitigating cognitive impairment associated with ADT for prostate cancer.

Abstract 2764A: Elucidating the pharmacological effects of covalent microtubule stabilizing taccalonolides through functional tagging

Microtubule stabilizing taxanes are highly effective in the treatment of solid tumors and continue to be used as single agents and in combination with targeted therapies and immunotherapies. However, limitations of taxane therapy include acquired and inherent drug resistance. The taccalonolides are a novel class of microtubule stabilizers that circumvent clinically relevant forms of drug resistance to the taxanes in vitro and in vivo. The taccalonolides are efficacious in taxane-resistant models due to their distinct mechanism of microtubule stabilization that involves covalent binding to β-tubulin. Pharmacological studies of the taccalonolides have been complicated by their irreversible binding to tubulin. A functional tagged taccalonolide would facilitate a greater understanding of the pharmacological, cellular, and physiological effects of this novel drug class. We have generated and characterized a series of fluorescently tagged taccalonolides that retain microtubule binding and stabilizing activities to serve as functional pharmacological taccalonolide probes. Biochemical tubulin polymerization, live cell fluorescence, immunofluorescence imaging, and the sulforhodamine B cytotoxicity assay were conducted to evaluate the microtubule binding and stabilizing effects of these taccalonolide probes. We identified stable taccalonolide probes that retain the biological activities of untagged taccalonolides and facilitate their detection in live cells and in cell lysates. The rates of taccalonolide uptake and tubulin binding in cells, tumors, and normal tissues were determined. These studies have also allowed for an evaluation of the anatomical distribution of the taccalonolides in vivo, which can inform on their potential for antitumor efficacy in distinct tumor sites. Furthermore, we have generated a cellular model to evaluate the kinetics of taccalonolide binding to β-tubulin mutants that are predicted to influence drug binding based on the crystal structure. These studies have confirmed the covalent interaction between the taccalonolides and D224 on β-tubulin and informed on other residues critical for drug binding. The generation of a functional tagged taccalonolide will continue to be a valuable tool in evaluating the pharmacokinetics of these agents and inform on drug targeting strategies that will facilitate their clinical development.

Citation Format: Samantha S. Yee, Lin Du, April L. Risinger. Elucidating the pharmacological effects of covalent microtubule stabilizing taccalonolides through functional tagging [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2764A.

Abstract 1882: Molecular context dictates the effects of eribulin on key EMT regulators: Snail and Slug

Microtubule targeting agents (MTAs) are a mainstay in the treatment of breast cancer and a growing body of evidence demonstrates that they have non-mitotic effects that contribute to their anticancer actions. Even after decades of clinical use, there is much to be learned about the mechanisms of action these drugs, and differences among them, for optimal utility. MTAs rapidly alter microtubule dynamics, often within minutes, leading to significant changes in oncogenic cellular signaling. We evaluated the early effects of eribulin on key oncogenic signaling pathways following a 2 h incubation using clinically relevant concentrations and compared the effects to those initiated by other MTAs. These studies led to the identification of novel mechanisms by which MTAs disrupt oncogenic signaling and contribute to the reversal of epithelial to mesenchymal transition (EMT), including unanticipated differences among drugs. The TGF-β-mediated expression of the Snail transcription factor is a key driver pathway of EMT in breast cancer. The effects of eribulin and other MTAs on TGF-β-induced, Smad-dependent expression of Snail were evaluated. Our results show that eribulin and vinorelbine inhibit the ability of TGF-β to promote the transcriptional induction of Snail by impeding the nuclear transport of Smad2/3 proteins in 4 triple-negative breast cancer cell lines. The effects of microtubule destabilizers contrasted with those of paclitaxel, which had no effect on the expression of Snail. This study begins to explain how microtubule disruption might contribute to the eribulin-mediated EMT reversal. Slug is another member of the Snail family of transcription factors that plays a central role in breast cancer EMT. Although Snail and Slug are often grouped together due to functional similarities, it is now understood that they regulate non-overlapping sets of genes. In contrast to our findings with Snail, eribulin and vinorelbine, but not paclitaxel or ixabepilone, induced Slug over-expression in a subset of breast cancer cell lines. We have identified molecular contexts where eribulin and vinorelbine induced Slug expression, resulting in the identification of a potential biomarker for response to microtubule destabilization. This work highlights the multifaceted nature of MTA-mediated effects on EMT-associated signaling pathways in breast cancer cells. These studies are supported by Eisai Inc.

Citation Format: Roma Kaul, April L. Risinger, Susan L. Mooberry. Molecular context dictates the effects of eribulin on key EMT regulators: Snail and Slug [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1882.

Abstract 2143: Identification of new molecular liabilities of a subset of triple-negative breast cancers through the investigation of englerin A

Identification of new molecular liabilities of a subset of triple-negative breast cancers through the investigation of englerin A

There remains a need to identify targeted therapies for triple-negative breast cancers (TNBCs) but a major challenge has been the heterogeneity of these cancers. We initiated a screen to discover compounds that are selectively cytotoxic to cells representing distinct molecular subtypes of TNBC. The overall goal is to identify compounds with selective actions and new molecular liabilities for subtypes of TNBC. Englerin A was identified by cytotoxicity assay as a compound with a greater than 1,500-fold selectivity for BT-549 cells and 300-fold for Hs578T TNBC cells as compared to other TNBC cell lines. Interestingly, the treatment of Hs578T cells with englerin A produced a biphasic concentration response curve, which is unique among the TNBC cell lines but is seen in A-498 renal carcinoma cells where englerin A sensitivity was first noted.1 In renal cell carcinoma cell lines one strongly supported mechanism of action of englerin A is via activation of TRPC1/4/5 non-selective cation channels.2-4 In our assays, the TRPC1/4/5 antagonist Pico1455 decreases the potency of englerin A in BT-549 and Hs578T cells, but not more resistant TNBC cells, suggesting that the TRPC1/4/5 agonist activity of englerin A is likely responsible for the selective effects in the sensitive cell lines. BT-549 and Hs578T cells were found to have significantly higher expression of TRCP1 and TRPC4 subunit mRNA as compared to englerin A resistant cells. Furthermore, knockdown of TRPC4 expression in BT-549 cells decreased englerin A potency. Consistent with its function as an agonist of the TRPC1/4/5 cation channel, we found that englerin A caused a concentration-dependent increase in intracellular Ca2+ in BT-549 cells within 30 seconds of exposure, and this effect was inhibited by pre-treatment with Pico145. These studies led to the hypothesis that TNBC cells expressing high levels of TRPC4 might also be more sensitive to clinically approved classes of drugs that increase intracellular cation concentrations. Results show that BT-549 and Hs578T cells are more sensitive to digoxin than other TNBC cells. Overall, these studies suggest that a subgroup of TNBCs may be susceptible to treatments with selective targets of cation influx. Ongoing work is aimed at evaluating the sensitivity of these cell lines to other drugs that disrupt intracellular cation levels.

1. Ratnayake, R. et al. (2009).

2. Ludlow, M. J. et al. (2016).

3. Akbulut, Y. et al. (2015).

4. Carson, C. et al. (2015).

5. Rubaiy, H. N. et al. (2017).

Citation Format: Corena V. Grant, Chase M. Carver, Shayne D. Hastings, April L. Risinger, John A. Beutler, Susan L. Mooberry. Identification of new molecular liabilities of a subset of triple-negative breast cancers through the investigation of englerin A [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2143.

Triple-negative breast cancer cell line sensitivity to englerin A identifies a new, targetable subtype

PURPOSE

Triple-negative breast cancers (TNBCs) represent a heterogeneous group of tumors. The lack of targeted therapies combined with the inherently aggressive nature of TNBCs results in a higher relapse rate and poorer overall survival. We evaluated the heterogeneity of TNBC cell lines for TRPC channel expression and sensitivity to cation-disrupting drugs.

METHODS

The TRPC1/4/5 agonist englerin A was used to identify a group of TNBC cell lines sensitive to TRPC1/4/5 activation and intracellular cation disruption. Quantitative RT-PCR, the sulforhodamine B assay, pharmacological inhibition, and siRNA-mediated knockdown approaches were employed. Epifluorescence imaging was performed to measure intracellular Ca²⁺ and Na⁺ levels. Mitochondrial membrane potential changes were monitored by confocal imaging.

RESULTS

BT-549 and Hs578T cells express high levels of TRPC4 and TRPC1/4, respectively, and are exquisitely, 2000- and 430-fold, more sensitive to englerin A than other TNBC cell lines. While englerin A caused a slow Na⁺ and nominal Ca²⁺ accumulation in Hs578T cells, it elicited rapid increases in cytosolic Ca²⁺ levels that triggered mitochondrial depolarization in BT-549 cells. Interestingly, BT-549 and Hs578T cells were also more sensitive to digoxin as compared to other TNBC cell lines. Collectively, these data reveal TRPC1/4 channels as potential biomarkers of TNBC cell lines with dysfunctional mechanisms of cation homeostasis and therefore sensitivity to cardiac glycosides.

CONCLUSIONS

The sensitivity of BT-549 and Hs578T cells to englerin A and digoxin suggests a subset of TNBCs are highly susceptible to cation disruption and encourages investigation of TRPC1 and TRPC4 as potential new biomarkers of sensitivity to cardiac glycosides.

Anacolosins A-F and Corymbulosins X and Y, Clerodane Diterpenes from Anacolosa clarkii Exhibiting Cytotoxicity toward Pediatric Cancer Cell Lines

An extract of the plant Anacolosa clarkii was obtained from the NCI Natural Products Repository, and it showed cytotoxic activity toward several types of pediatric solid tumor cell lines. Bioassay-guided fractionation led to the purification of eight new clerodane diterpenes [anacolosins A-F (1-6) and corymbulosins X and Y (7 and 8)] and two known compounds (9 and 10) that contained an isozuelanin skeleton. The structures of the new natural products were determined using 1D and 2D NMR and HRESIMS data, while the relative and absolute configurations of the compounds were assessed using a combination of ¹H NMR coupling constant data, ROESY experiments, ECD (electronic circular dichroism) and VCD (vibrational circular dichroism) spectroscopy, chemical methods (including Mosher and 2-naphthacyl esterification), and chiral HPLC analyses. The purified natural products exhibited a range of cytotoxic activities against cell lines representing four pediatric cancer types (i.e., rhabdomyosarcoma, Ewing sarcoma, medulloblastoma, and hepatoblastoma) with total growth inhibitory (TGI) values in the range 0.2-4.1 μM. The rhabdomyosarcoma and medulloblastoma cell lines showed higher sensitivity to compounds 1-4, which are the first compounds reported to contain an isozuelanin skeleton and feature keto carbonyl groups at the C-6 positions. In contrast, the hepatoblastoma cell line was modestly more sensitive to 7-10, which contained a C-6 hydroxy group moiety.

In Situ Ring Contraction and Transformation of the Rhizoxin Macrocycle through an Abiotic Pathway

A Rhizopus sp. culture containing an endosymbiont partner ( Burkholderia sp.) was obtained through a citizen-science-based soil-collection program. An extract prepared from the pair of organisms exhibited strong inhibition of Ewing sarcoma cells and was selected for bioassay-guided fractionation. This led to the purification of rhizoxin (1), a potent antimitotic agent that inhibited microtubule polymerization, along with several new (2-5) and known (6) analogues of 1. The structures of 2-6 were established using a combination of NMR data analysis, while the configurations of the new stereocenters were determined using ROESY spectroscopy and comparison of GIAO-derived and experimental data for NMR chemical shift and ³ J_HH coupling values. Whereas compound 1 showed modest selectivity for Ewing sarcoma cell lines carrying the EWSR1/ FLI1 fusion gene, the other compounds were determined to be inactive. Chemically, compound 2 stands out from other rhizoxin analogues because it is the first member of this class that is reported to contain a one-carbon-smaller 15-membered macrolactone system. Through a combination of experimental and computational tests, we determined that 2 is likely formed via an acid-catalyzed Meinwald rearrangement from 1 because of the mild acidic culture environment created by the Rhizopus sp. isolate and its symbiont.

Microtubule-Targeting Drugs: More than Antimitotics

Nature has yielded numerous compounds that bind to tubulin/microtubules and disrupt microtubule function. Even with the advent of targeted therapies for cancer, natural products and their derivatives that target microtubules are some of the most effective drugs used in the treatment of solid tumors and hematological malignancies. For decades, these drugs were thought to work solely through their ability to inhibit mitosis. Accumulating evidence demonstrates that their actions are much more complex, in that they also have significant effects on microtubules in nondividing cells that inhibit a diverse range of signaling events important for carcinogenesis. The abilities of these drugs to inhibit oncogenic signaling likely underlies their efficacy, especially in solid tumors. In this review, we describe the role of microtubules in cells, the proliferation paradox of cells in culture as compared to cancers in patients, and evidence that microtubule-targeting drugs inhibit cellular signaling pathways important for tumorigenesis. The potential mechanisms behind differences in the clinical indications and efficacy of these natural-product-derived drugs are also discussed. Microtubules are an important target for structurally diverse natural products, and a fuller understanding of the mechanisms of action of these drugs will promote their optimal use.

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

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.

Crystal Structure of the Cyclostreptin-Tubulin Adduct: Implications for Tubulin Activation by Taxane-Site Ligands

It has been proposed that one of the mechanisms of taxane-site ligand-mediated tubulin activation is modulation of the structure of a switch element (the M-loop) from a disordered form in dimeric tubulin to a folded helical structure in microtubules. Here, we used covalent taxane-site ligands, including cyclostreptin, to gain further insight into this mechanism. The crystal structure of cyclostreptin-bound tubulin reveals covalent binding to βHis229, but no stabilization of the M-loop. The capacity of cyclostreptin to induce microtubule assembly compared to other covalent taxane-site agents demonstrates that the induction of tubulin assembly is not strictly dependent on M-loop stabilization. We further demonstrate that most covalent taxane-site ligands are able to partially overcome drug resistance mediated by βIII-tubulin (βIII) overexpression in HeLa cells, and compare their activities to pironetin, an interfacial covalent inhibitor of tubulin assembly that displays invariant growth inhibition in these cells. Our findings suggest a relationship between a diminished interaction of taxane-site ligands with βIII-tubulin and βIII tubulin-mediated drug resistance. This supports the idea that overexpression of βIII increases microtubule dynamicity by counteracting the enhanced microtubule stability promoted by covalent taxane-site binding ligands.

Abstract P5-08-06: Differential effects of eribulin on key transcription factors snail and slug

Microtubule targeting agents (MTAs) are a mainstay in the treatment of breast cancer and a growing body of evidence demonstrates that they have non-mitotic effects that contribute to their anticancer actions. Even after decades of clinical use, there is much to still be learned about the mechanisms of action these drugs, and differences among them, for optimal utility. MTAs rapidly alter microtubule dynamics, often within minutes, leading to significant changes in oncogenic cellular signaling. We evaluated the early effects of eribulin on key oncogenic signaling pathways following a 2 h incubation using clinically relevant concentrations and compared the effects to those initiated by other MTAs. These studies led to the identification of novel mechanisms by which MTAs disrupt oncogenic signaling and contribute to the reversal of epithelial to mesenchymal transition (EMT), including unanticipated differences among drugs.

The TGF-β-mediated expression of the Snail transcription factor is a key driver pathway of EMT in breast cancer. The effects of eribulin and other MTAs on TGF-β-induced, Smad-dependent expression of Snail were evaluated. Our results show that eribulin and vinorelbine, but not paclitaxel or ixabepilone, inhibit the ability of TGF-β to promote the transcriptional induction of Snail by impeding the nuclear transport of Smad2/3 proteins in 4 triple-negative breast cancer cell lines. This study begins to explain how microtubule disruption might contribute to the eribulin-mediated EMT reversal observed in vitro, in vivo, and in patients.

Slug is another member of the Snail family of transcription factors that plays a central role in breast cancer EMT. Although Snail and Slug are often grouped together due to functional similarities, it is becoming increasingly clear that Slug has an independent role in regulating stemness and cancer cell survival during partial EMT. In contrast to our findings with Snail, eribulin and vinorelbine, but not paclitaxel or ixabepilone, induced Slug expression independent of TGF-β stimulation in a subset of triple-negative breast cancer cell lines. Studies are ongoing to identify the molecular pathways and consequences of eribulin and vinorelbine-induced Slug induction, which might begin to identify biomarkers of patient response to different MTAs. This work highlights the multifaceted nature of MTA-mediated effects on EMT-associated signaling pathways in breast cancer cells and prompts a reevaluation of their differential efficacy in tumors with distinct molecular profiles. These studies are supported by Eisai Inc.

Citation Format: Kaul R, Risinger AL, Mooberry SL. Differential effects of eribulin on key transcription factors snail and slug [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-08-06.

Sterically induced conformational restriction: Discovery and preclinical evaluation of novel pyrrolo[3,2-d]pyrimidines as microtubule targeting agents

The discovery, synthesis and biological evaluations of a series of nine N5-substituted-pyrrolo[3,2-d]pyrimidin-4-amines are reported. Novel compounds with microtubule depolymerizing activity were identified. Some of these compounds also circumvent clinically relevant drug resistance mechanisms (expression of P-glycoprotein and βIII tubulin). Compounds 4, 5, and 8-13 were one to two-digit nanomolar (IC₅₀) inhibitors of cancer cells in culture. Contrary to recent reports (Banerjee et al. J. Med. Chem.2018, 61, 1704-1718), the conformation of the most active compounds determined by ¹H NMR and molecular modeling are similar to that reported previously and in keeping with recently reported X-ray crystal structures. Compound 11, freely water soluble as the HCl salt, afforded statistically significant inhibition of tumor growth in three xenograft models [MDA-MB-435, MDA-MB-231 and NCI/ADR-RES] compared with controls. Compound 11 did not display indications of animal toxicity and is currently slated for further preclinical development.

Abstract 2908: Characteristics of triple negative breast cancer that result in sensitivity to englerin A

New approaches are needed to improve long term survival for patients with metastatic triple negative breast cancers (TNBCs). Patients with metastatic TNBC have a poorer 3-year survival compared to patients with metastatic HER2+ or ER/PR+ disease. Identifying new therapeutic targets of TNBC subgroups could provide opportunities for more effective treatments. We conduct a screening program to identify compounds with selective activity against a panel of TNBC cell lines, reasoning that mechanism of action studies of selective cytotoxins could identify new therapeutic targets. Englerin A was identified as a compound with highly selective actions against two TNBC cell lines: BT-549 (>1800-fold selectively) and Hs578T (>600-fold selectivity) as compared to other TNBC cell lines. Previous studies showed that englerin A is an agonist of TRPC1/4/5 non-specific cation channels and PKCθ in renal cancer cells.1–4 RT-PCR analyses showed that BT-549 and Hs578T cells express more TRPC4 than either the insensitive MDA-MB-231 TNBC cells or the A498 renal cancer cells. If BT-549 and Hs578T cells are sensitive to englerin A due to high expression of TRPC4 channels, then treatment with TRPC4 channel inhibitors should decrease sensitivity to englerin A. These studies show that the TRPC1/4/5 channel inhibitor Pico1455 caused a rightward shift in the concentration response curve of englerin A in both BT-549 and Hs578T cells. These data are consistent with TRPC1/4/5 channel activation contributing to englerin A-induced cell death of BT-549 and Hs578T cells. TRPC channels are non-specific cation channels, therefore their activation is expected to cause an influx of cations including calcium and sodium. Live cell calcium imaging showed the influx of calcium into BT-549 cells following englerin A treatment, further supporting the hypothesis that englerin A-induced cell death in these cell lines is the result of TRPC channel activation. TRPC channel activation initiates disruption of intracellular cation homeostasis, therefore we tested ouabain, a Na+/K+ATPase inhibitor, to determine if BT-549 and Hs578T cells were also sensitive to TRPC-independent methods of intracellular cation homeostasis disruption. BT-549 and Hs578T cells were 12- and 2-fold more sensitive to ouabain as compared to other TNBC cell lines, suggesting that some TNBC cells lines are particularly sensitive to disruption of intracellular cation homeostasis. Ongoing work is aimed at identifying additional factors in englerin A's mechanism of action in BT-549 and Hs578T cells. Our studies suggest that effective treatments for this subgroup of TNBC may include agents which disrupt cation homeostasis.

1.Ludlow, M. J. et al. (2016).

2.Akbulut, Y. et al. (2015).

3.Carson, C. et al. (2015).

4.Sourbier, C. et al. (2013).

5.Rubaiy, H. N. et al. (2017).

Citation Format: Corena V. Shaffer, April L. Risinger, John A. Beutler, Susan L. Mooberry. Characteristics of triple negative breast cancer that result in sensitivity to englerin A [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2908.

Abstract 2030: Eribulin disrupts TGF-β-mediated Smad2/3-dependent transcription of EMT promoting proteins in triple negative breast cancer cells

Microtubule targeting agents (MTAs) are of significant utility in the treatment of breast cancer. While decades of research have shown that these drugs cause mitotic arrest in cells by suppressing the dynamic instability of microtubules, recent evidence demonstrates that the ability of MTAs to disrupt the microtubule-dependent transport of key signaling proteins in interphase likely contributes to their anticancer actions. Ligand-mediated activation of TGF-β receptors leads to the activation of downstream pathways that induce the expression of Snail and Slug, key transcriptional-repressors that promote EMT through Smad-dependent and independent pathways. Eribulin was shown by Yoshida and colleagues to reverse epithelial-to-mesenchymal transition (EMT) in preclinical models of triple negative breast cancer (TNBC) within 7 days. We tested the hypothesis that the initial disruption of microtubule dynamics and structure by eribulin could rapidly inhibit TGF-β-dependent signaling related to EMT as compared to other clinically relevant MTAs. A panel of four TGF-β responsive triple negative breast cancer (TNBC) cells, BT-549, MDA-MB-231, HCC1937 and Hs578T, were pre-treated with clinically relevant concentrations of the MTAs eribulin, vinorelbine, paclitaxel, or ixabepilone, which caused maximum disruption of the interphase microtubule network within 2 h. These cells were then stimulated with 2 ng/mL of TGF-β ligand for 30 min - 3 h, followed by evaluation of downstream TGF-β targets. Immunofluorescence experiments showed that the microtubule destabilizers eribulin and vinorelbine inhibited the nuclear accumulation of phosphorylated-Smad 2/3 to an extent greater than the microtubule stabilizers paclitaxel and ixabepilone or the vehicle control. Further, eribulin and vinorelbine caused significant inhibition of TGF-β-induced Snail expression in all four cell lines when compared to vehicle or microtubule stabilizer-treated cells. Knock-down studies confirmed that the effects of MTAs on Snail expression are mediated by Smad 2/3. Consistent with current literature, a 7-day eribulin treatment began to reverse their mesenchymal characteristics as measured by loss of N-cadherin in two TNBC cell lines. These data support the premise that rapid, eribulin-mediated, inhibition of TGF-β signaling contributes to its ability to reverse EMT, through inhibition of Smad-2/3 dependent transcription. Ongoing studies are investigating whether eribulin differentially inhibits Smad 2/3 protein interactions with its upstream scaffold SARA that is integral for optimal signaling as compared to other MTAs. Dissecting how MTAs differentially disrupt the TGF-β-Smad-2/3 signaling pathway might facilitate a better understanding of the molecular contexts of breast cancers that might benefit from treatment with specific MTAs. These studies are supported by Eisai Inc.

Citation Format: Roma Kaul, April L. Risinger, Susan L. Mooberry. Eribulin disrupts TGF-β-mediated Smad2/3-dependent transcription of EMT promoting proteins in triple negative breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2030.

Abstract 2804: Identifying an optimal site for linker conjugation of the taccalonolides, novel microtubule stabilizing drugs

The taccalonolides are a structurally distinct class of microtubule stabilizers purified from the roots and rhizomes of plants of the genus Tacca. The covalent binding of taccalonolides bearing a C-22,23 epoxide to β-tubulin generates a distinct profile of microtubule stabilization that results in a high level of cellular persistence and the ability to circumvent multiple mechanisms of clinically relevant drug resistance, including expression of P-glycoprotein. The biological effects of 121 structurally distinct taccalonolides, including 23 isolated natural products and 98 semi-synthetic derivatives have been evaluated. These compounds range in potency from 0.6 nM to over 50 µM, allowing for detailed structure activity relationship studies.

Some of the most potent taccalonolide derivatives include semi-synthetic modifications at C-6, C-7, or C-15 in combination with C-22,23 epoxidation. While these compounds did not have an acceptable therapeutic window for in vivo antitumor activity by systemic administration, they demonstrated highly effective and exquisitely persistent antitumor activity in a drug-resistant tumor model when delivered directly into the tumor. Additional studies were undertaken to see whether large chemical moieties could be added at these sites to facilitate targeted delivery of these compounds. In contrast to C-7 and C-15 derivatives, which are cleaved, C-6 modifications are stable and retain microtubule stabilizing and cytotoxic activities. In particular, fluorescent C-6 taccalonolide conjugates allowed, for the first time, direct visualization of these compounds in live cells, which co-localized with bundled microtubules. Further probing of the functionality at taccalonolide C-6 will allow for more detailed mechanistic studies of drug transport and distribution as well as optimization of a linker that could be used for antibody-based drug delivery.

Citation Format: April L. Risinger, Lin Du, Antonius Ola, Robert H. Cichewicz, Susan L. Mooberry. Identifying an optimal site for linker conjugation of the taccalonolides, novel microtubule stabilizing drugs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2804.

Taccalonolide Microtubule Stabilizers Generated Using Semisynthesis Define the Effects of Mono Acyloxy Moieties at C-7 or C-15 and Disubstitutions at C-7 and C-25

The taccalonolides are a unique class of microtubule stabilizers isolated from Tacca spp. that have efficacy against drug-resistant tumors. Our previous studies have demonstrated that a C-15 acetoxy taccalonolide, AF, has superior in vivo antitumor efficacy compared to AJ, which bears a C-15 hydroxy group. With the goal of further improving the in vivo efficacy of this class of compounds, we semisynthesized and tested the biological activities of 28 new taccalonolides with monosubstitutions at C-7 or C-15 or disubstitutions at C-7 and C-25, covering a comprehensive range of substituents from formic acid to anthraquinone-2-carbonyl chloride. The resulting taccalonolide analogues with diverse C-7/C-15/C-25 modifications exhibited IC₅₀ values from 2.4 nM to >20 μM, allowing for extensive in vitro structure-activity evaluations. This semisynthetic strategy was unable to provide a taccalonolide with improved therapeutic window due to hydrolysis of substituents at C-7 or C-15 regardless of size or steric bulk. However, two of the most potent new taccalonolides, bearing isovalerate modifications at C-7 or C-15, demonstrated potent and highly persistent antitumor activity in a drug-resistant xenograft model when administered intratumorally. This study demonstrates that targeted delivery of the taccalonolides to the tumor could be an effective, long-lasting approach to treat drug-resistant tumors.

Abstract P2-13-01: Differential efficacies of DNA damaging agents in basal-like TNBC subtypes

New approaches are needed to improve long term survival for patients with metastatic triple negative breast cancer (TNBC). Women with metastatic TNBC have a worse 3-year survival compared to women with metastatic HER2+ or ER/PR+ disease.1 Identifying the molecular vulnerabilities of TNBC subtypes could provide for optimal treatment strategies. Gene expression analyses of TNBC patient samples by Lehmann and Bauer, et al. identified 6 subtypes of TNBC.2 Based on the analysis of laser dissected tumor samples, the number of tumor subtypes was reduced from 6 to 4.3 Both analyses identified two types of basal-like cancers, basal-like 1 (BL1) and basal-like 2 (BL2).2,3 The BL1 subtype is defined by the amplified expression of genes involved in cell division and DNA damage response, and RNA polymerase.2 BL2 cancers express high levels of genes involved in growth factor signaling, glycolysis, and gluconeogenesis.2 They show that cell lines representative of the BL1 and BL2 subtypes of TNBC are more sensitive to cisplatin than cell lines representing the other TNBC subtypes. The BL1 or BL2 categorization, however, failed to predict sensitivity to olaparib.2 BL1 and BL2 representative cell lines responded differently when treated with veliparib; BL1 cells were more sensitive than BL2 cells.2 We investigated the response of BL1 and BL2 cells to a variety of DNA damaging agents to determine if the BL1 or BL2 subtype was predictive of sensitivity to a particular drug. DNA damaging agents, including alkylating agents, anti-metabolites, topoisomerase inhibitors, and PARP inhibitors were tested in cells resenting the BL1 and BL2 subtypes. HCC1806 cells (BL2) were found to be selectively sensitive to gemcitabine when compared to HCC1937 cells (BL1). The panel of drugs and cell lines was expanded to further understand the sensitivity of BL1 and BL2 subtypes to a variety of DNA damaging agents. Our goal is to provide preclinical evidence for the informed use of specific DNA damaging agents in the treatment of patients with BL1 or BL2 subtypes of TNBCs.

1. Li, X. et al. Breast Cancer Res. Treat. 161, 279–287 (2017).

2. Lehmann, B. D. et al. J. Clin. Invest. 121, 2750–2767 (2011).

3. Lehmann, B. D. et al. PLoS One 11, e0157368 (2016).

Citation Format: Shaffer CV, Robles AJ, Risinger AL, Mooberry SL. Differential efficacies of DNA damaging agents in basal-like TNBC subtypes [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-13-01.

Regulation of E-cadherin localization by microtubule targeting agents: rapid promotion of cortical E-cadherin through p130Cas/Src inhibition by eribulin

Microtubule targeting agents (MTAs) are some of the most effective anticancer drugs used to treat a wide variety of adult and pediatric cancers. Building evidence suggests that these drugs inhibit interphase signaling events and that this contributes to their anticancer actions. The effects of diverse MTAs were evaluated following a 2 hour incubation with clinically relevant concentrations to test the hypothesis that these drugs rapidly and differentially disrupt epithelial-to-mesenchymal transition (EMT)-related signaling. The MTAs rapidly promoted the cortical localization of internal pools of E-cadherin in HCC1937 breast cancer cells, with the most robust effects observed with the microtubule destabilizers eribulin and vinorelbine. Cortical E-cadherin localization was also promoted by the Src kinase inhibitor dasatinib or by siRNA-mediated depletion of the p130Cas scaffold. Mechanistic studies demonstrate that eribulin disrupts the interaction between p130Cas and Src, leading to decreased cortical Src phosphorylation that precedes the accumulation of cortical E-cadherin. These results suggest that microtubules can be actively co-opted by cancer cells to inhibit cortical E-cadherin localization, a hallmark of EMT, and provide a direct link between the initial disruption of the microtubule network and reversal of EMT phenotypes demonstrated by eribulin in long-term studies.

Pharmacological augmentation of nicotinamide phosphoribosyltransferase (NAMPT) protects against paclitaxel-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) arises from collateral damage to peripheral afferent sensory neurons by anticancer pharmacotherapy, leading to debilitating neuropathic pain. No effective treatment for CIPN exists, short of dose-reduction which worsens cancer prognosis. Here, we report that stimulation of nicotinamide phosphoribosyltransferase (NAMPT) produced robust neuroprotection in an aggressive CIPN model utilizing the frontline anticancer drug, paclitaxel (PTX). Daily treatment of rats with the first-in-class NAMPT stimulator, P7C3-A20, prevented behavioral and histologic indicators of peripheral neuropathy, stimulated tissue NAD recovery, improved general health, and abolished attrition produced by a near maximum-tolerated dose of PTX. Inhibition of NAMPT blocked P7C3-A20-mediated neuroprotection, whereas supplementation with the NAMPT substrate, nicotinamide, potentiated a subthreshold dose of P7C3-A20 to full efficacy. Importantly, P7C3-A20 blocked PTX-induced allodynia in tumored mice without reducing antitumoral efficacy. These findings identify enhancement of NAMPT activity as a promising new therapeutic strategy to protect against anticancer drug-induced peripheral neurotoxicity.

Unique amalgamation of primary and secondary structural elements transform peptaibols into potent bioactive cell-penetrating peptides

Mass-spectrometry-based metabolomics and molecular phylogeny data were used to identify a metabolically prolific strain of Tolypocladium that was obtained from a deep-water Great Lakes sediment sample. An investigation of the isolate's secondary metabolome resulted in the purification of a 22-mer peptaibol, gichigamin A (1). This peptidic natural product exhibited an amino acid sequence including several β-alanines that occurred in a repeating ααβ motif, causing the compound to adopt a unique right-handed 311 helical structure. The unusual secondary structure of 1 was confirmed by spectroscopic approaches including solution NMR, electronic circular dichroism (ECD), and single-crystal X-ray diffraction analyses. Artificial and cell-based membrane permeability assays provided evidence that the unusual combination of structural features in gichigamins conferred on them an ability to penetrate the outer membranes of mammalian cells. Compound 1 exhibited potent in vitro cytotoxicity (GI50 0.55 ± 0.04 µM) and in vivo antitumor effects in a MIA PaCa-2 xenograft mouse model. While the primary mechanism of cytotoxicity for 1 was consistent with ion leakage, we found that it was also able to directly depolarize mitochondria. Semisynthetic modification of 1 provided several analogs, including a C-terminus-linked coumarin derivative (22) that exhibited appreciably increased potency (GI50 5.4 ± 0.1 nM), but lacked ion leakage capabilities associated with a majority of naturally occurring peptaibols such as alamethicin. Compound 22 was found to enter intact cells and induced cell death in a process that was preceded by mitochondrial depolarization.

A Stereocontrolled Annulation of the Taccalonolide Epoxy Lactone onto the Molecular Framework of trans-Androsterone

A robust and scalable route to the taccalonolide skeleton starting from trans-androsterone is presented. The synthesis features a cyclic hydroboration carbonylation reaction, which effectively establishes the trans-hydrindane DE ring junction in a remarkable annulation reaction, as well as a Claisen rearrangement and a catalytic Ullmann-type cyclization. This work is part of a larger effort to uncover new clinical candidates from the taccalonolide class of anticancer agents through advances in chemical synthesis.

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 P4-04-04: Eribulin impairs TGF-β type I receptor localization and signaling in BT-549 cells

Microtubule targeting agents (MTAs) continue to be some of the most valuable drugs used to treat breast cancer. While decades of research have shown that these drugs cause mitotic arrest in cells by suppressing the dynamic instability of microtubules, recent evidence demonstrates that the ability of MTAs to disrupt microtubule-dependent transport of key signaling components, including proteins and microRNAs, in interphase cells likely contributes to their anticancer actions. TGF-β signaling is a driver of oncogenesis and epithelial-to-mesenchymal transition (EMT) that involves multiple protein trafficking and intracellular signaling events. Ligand mediated activation of the cell surface TGF-β receptors leads to downstream signaling in the canonical and the non-canonical pathways that collectively lead to the expression of proteins implicated in EMT, including Snail and Slug. Additionally, TGF-β type 1 receptor (TGF-βR1) undergoes constant cycling from the plasma membrane to the cytosol, a microtubule-dependent process. We tested the hypothesis that a short-term treatment of breast cancer cells with eribulin or 3 other clinically relevant MTAs would differentially disrupt interphase microtubules and alter the transport and downstream signaling of TGF-βR1. BT-549 cells were treated for 2 h with concentrations of MTAs that cause comparable disruption of the interphase microtubule network; 100 nM was used for the destabilizers, eribulin or vinorelbine and 1 µm was used for the stabilizers, paclitaxel or ixabepilone. The results show that TGF-βR1 was extensively localized along the stabilizer-induced microtubule bundles, a phenomenon not observed with destabilizer-treated cells. The downstream consequences were further assessed. The expression of Snail and Slug were evaluated in cells pretreated with MTAs followed by TGF-β stimulation. Eribulin and vinorelbine significantly inhibited the TGF-β-induced expression of Snail and Slug while stabilizers did not alter their expression levels. We hypothesize that eribulin and vinorelbine are inhibiting the non-canonical TGF-β signaling pathway since they had no effect on the localization and expression of Smad2/3, proteins involved in the canonical pathway. Eribulin induced inhibition of TGF-β signaling is consistent with previous studies that show that a 7 day treatment reversed TGF-β mediated EMT in breast cancer cells1. Data from patients shows that eribulin decreases the plasma concentration of TGF-β within 7 days of treatment initiation2. Our data suggest that eribulin rapidly inhibits non-canonical TGF-β signaling indicating that this is a potential mechanism for the eribulin-mediated EMT reversal. This information, together with previously published reports, suggest that eribulin has multiple effects leading to inhibition of TGF-β signaling. These studies begin to shed light into the diverse mechanisms of action of MTAs.

1. Yoshida T, et al., Brit J Cancer 110(6): 1497-505, 2014

2. Ueda S, et al., Brit J Cancer, 2016

This work is funded by Eisai Inc.

Citation Format: Kaul R, Risinger AL, Mooberry SL. Eribulin impairs TGF-β type I receptor localization and signaling in BT-549 cells [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-04-04.

Janus Compounds, 5-Chloro-N⁴-methyl-N⁴-aryl-9H-pyrimido[4,5-b]indole-2,4-diamines, Cause Both Microtubule Depolymerizing and Stabilizing Effects

While evaluating a large library of compounds designed to inhibit microtubule polymerization, we identified four compounds that have unique effects on microtubules. These compounds cause mixed effects reminiscent of both microtubule depolymerizers and stabilizers. Immunofluorescence evaluations showed that each compound initially caused microtubule depolymerization and, surprisingly, with higher concentrations, microtubule bundles were also observed. There were subtle differences in the propensity to cause these competing effects among the compounds with a continuum of stabilizing and destabilizing effects. Tubulin polymerization experiments confirmed the differential effects and, while each of the compounds increased the initial rate of tubulin polymerization at high concentrations, total tubulin polymer was not enhanced at equilibrium, likely because of the dueling depolymerization effects. Modeling studies predict that the compounds bind to tubulin within the colchicine site and confirm that there are differences in their potential interactions that might underlie their distinct effects on microtubules. Due to their dual properties of microtubule stabilization and destabilization, we propose the name Janus for these compounds after the two-faced Roman god. The identification of synthetically tractable, small molecules that elicit microtubule stabilizing effects is a significant finding with the potential to identify new mechanisms of microtubule stabilization.

Drosophila Ringmaker regulates microtubule stabilization and axonal extension during embryonic development

Axonal growth and targeting are fundamental to the organization of the nervous system, and require active engagement of the cytoskeleton. Polymerization and stabilization of axonal microtubules is central to axonal growth and maturation of neuronal connectivity. Studies have suggested that members of the tubulin polymerization promoting protein (TPPP, also known as P25α) family are involved in cellular process extension. However, no in vivo knockout data exists regarding its role in axonal growth during development. Here, we report the characterization of Ringmaker (Ringer; CG45057), the only Drosophila homolog of long p25α proteins. Immunohistochemical analyses indicate that Ringer expression is dynamically regulated in the embryonic central nervous system (CNS). ringer-null mutants show cell misplacement, and errors in axonal extension and targeting. Ultrastructural examination of ringer mutants revealed defective microtubule morphology and organization. Primary neuronal cultures of ringer mutants exhibit defective axonal extension, and Ringer expression in cells induced microtubule stabilization and bundling into rings. In vitro assays showed that Ringer directly affects tubulin, and promotes microtubule bundling and polymerization. Together, our studies uncover an essential function of Ringer in axonal extension and targeting through proper microtubule organization.

Identification of Compounds with Efficacy against Malaria Parasites from Common North American Plants

Some of the most valuable antimalarial compounds, including quinine and artemisinin, originated from plants. While these drugs have served important roles over many years for the treatment of malaria, drug resistance has become a widespread problem. Therefore, a critical need exists to identify new compounds that have efficacy against drug-resistant malaria strains. In the current study, extracts prepared from plants readily obtained from local sources were screened for activity against Plasmodium falciparum. Bioassay-guided fractionation was used to identify 18 compounds from five plant species. These compounds included eight lupane triterpenes (1-8), four kaempferol 3-O-rhamnosides (10-13), four kaempferol 3-O-glucosides (14-17), and the known compounds amentoflavone and knipholone. These compounds were tested for their efficacy against multi-drug-resistant malaria parasites and counterscreened against HeLa cells to measure their antimalarial selectivity. Most notably, one of the new lupane triterpenes (3) isolated from the supercritical extract of Buxus sempervirens, the common boxwood, showed activity against both drug-sensitive and -resistant malaria strains at a concentration that was 75-fold more selective for the drug-resistant malaria parasites as compared to HeLa cells. This study demonstrates that new antimalarial compounds with efficacy against drug-resistant strains can be identified from native and introduced plant species in the United States, which traditionally have received scant investigation compared to more heavily explored tropical and semitropical botanical resources from around the world.