Doublet and Triplet Combinations of Eribulin, Fulvestrant, and Palbociclib in Preclinical Breast Cancer Models

BACKGROUND/AIM

This study investigated in vivo synergism between eribulin and palbociclib in a breast cancer patient-derived xenograft (PDX) model, with expanded scope to include fulvestrant as a third drug.

MATERIALS AND METHODS

Eribulin plus palbociclib combinations were tested in vitro in six cell lines each of estrogen receptor positive and triple-negative breast cancer, and in vivo in the OD-BRE-0192 PDX model using weekly eribulin plus 5×/week or 7×/week palbociclib (holiday or no-holiday schedules, respectively). When included as a third drug, fulvestrant was dosed weekly.

RESULTS

In vitro, combining palbociclib with eribulin led to increased eribulin IC50s in 11 of 12 cell lines, suggesting that the drugs antagonized each other due to mutual exclusion of the mitotic and G1/S cell cycle block points for eribulin and palbociclib. An in vivo study in the OD-BRE-0192 PDX model compared weekly eribulin plus either palbociclib holiday or no-holiday schedules to gauge the importance of post-palbociclib cell cycle synchronization. Results showed no advantage of holiday over no-holiday schedules, arguing that differing pharmacokinetics of the drugs were sufficient to overcome cell cycle-based mechanistic antagonism. In vivo comparisons of doublet and triplet combinations of eribulin, palbociclib, and fulvestrant showed that all three doublets were superior to individual monotherapies, and that the triplet combination was markedly superior to all three doublets, being the only group to show tumor regression in 100% of the mice.

CONCLUSION

Results show complex synergistic interactions between eribulin, fulvestrant, and palbociclib, and point to a particularly robust synergy when combining all three drugs.

Abstract P069: GDF15 contributes to the maintenance of the drug-tolerant persister state in cells responding to eribulin

A major obstacle in the fight against cancer is the development of drug resistance in response to therapy. Acquired resistance can be mediated by a small population of drug-tolerant persister cancer cells (DTPs), which are characterized by an increased capacity of adaptation to various stresses. DTPs are a small slow-cycling population of cells that tolerate cancer drugs, and most likely are responsible for generating a stable resistance. Growth differentiation factor 15 (GDF15) is a well-established marker of cellular stress. It is a member of the TGF-β superfamily, and while its physiological expression is barely detectable in most human somatic tissues, it is prominently induced under stress conditions to maintain cell and tissue homeostasis. Evidence suggests that GDF15 can be secreted in different cancer types. Nevertheless, GDF15 can have opposite effects depending on cellular context and disease stage, and further studies are needed to confirm its biological role in cancer. The working hypothesis of this study is that GDF15 plays a functional role in mediating the eribulin tolerance in DTP cells. MDA-MB-231 and MCF7 cell lines were treated for a month with an IC80 of eribulin. Initially, cells were sensitive to eribulin and approx. 85 % of the cells died after 7 days of treatment, then for about three weeks the surviving cells remained in a quiescent-DTP state. After approx. 3 weeks of treatment with a high dose of eribulin, cells started to proliferate in the presence of the drug, and we assume that at this point a stable mechanism of resistance is established. Interestingly, the analysis of the GDF15 levels during the chronic treatment showed that GDF15 expression/secretion was only detectable in the DTP state, suggesting a role of GDF15 in the maintenance of DTPs. To demonstrate the GDF15 protecting role against eribulin, we conducted loss of GDF15 function experiments, targeting both GDF15 and its receptor GFRAL. siGDF15 cells showed higher sensitivity to eribulin treatment than the siCTRL cells. We confirmed the same results targeting the GFRAL receptor, confirming an autocrine role of the secreted GDF15 in the cells that survive eribulin treatment. These results open the possibility for combining eribulin and a GDF15 antibody to specifically target DTP cells, and overcome eribulin resistance. Moreover, we envision that the circulating levels of GDF15 in metastatic breast cancer patients could be used as a pharmacodynamic biomarker during eribulin treatment. Furthermore, we are confident that GDF15 could help to better understand the onset of acquired resistance to eribulin in metastatic breast cancer patients.

Citation Format: Chiara Bellio, Marta Emperador, Esther Zamora, Violeta Serra, Cristina Saura, Bruce A. Littlefield, Josep Villanueva. GDF15 contributes to the maintenance of the drug-tolerant persister state in cells responding to eribulin [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P069.

Abstract 1399: Tumor-specific molecular changes in luminal B breast cancer patient-derived xenografts (PDXs) with acquired resistance to endocrine treatment

Worldwide, numerous preclinical agents that enter oncology clinical trials fail to demonstrate sufficient efficacy in patients to gain regulatory approval. This failure rate reflects both a poor understanding of the complexity of human cancers as well as the weak predictive value of existing preclinical models. PDXs are now widely embraced as better preserving characteristics of the original tumor and thus providing better tools for translational research. However, tumor collection for PDX generation usually coincides with initial surgery or biopsy, meaning that PDXs typically reflect the state of patient tumors at the times of collection. Unfortunately, this may not reflect changes that occur after treatment, particularly those associated with acquisition of resistance. Although analysis of changes associated with acquisition of resistance would be of great value for translational purposes, opportunities to obtain paired initial primary and later resistant samples are limited in current practice. In this study we describe establishment of a luminal breast cancer PDX model with acquired resistance to hormonotherapy. Starting from a luminal A breast cancer PDX that was both estrogen-dependent and fulvestrant-sensitive, we derived delayed outgrowth tumors under in vivo selection conditions of estrogen deprivation and/or fulvestrant treatment, thus corresponding to acquisition of resistance following hormonotherapy in the clinical setting. To understand phenotypic changes associated with resistance acquisition, resistant PDXs were analyzed for estrogen and progesterone receptor expression and subjected to whole exome sequencing, RNA sequencing, and DNA methylation analysis comparatively with the parental PDX. Intriguingly, all derived resistant PDXs showed complete loss of PR expression by immunohistochemistry regardless of selection method, whereas ER expression was only decreased in resistant PDXs that had been selected by fulvestrant treatment. Genetic analyses showed that resistant PDX tumors fell into two different transcriptomic signatures, depending on whether resistance was driven by estrogen deprivation or fulvestrant administration. Compared to the naïve parental tumor, the cluster corresponding to fulvestrant-selected PDX showed up-regulated and down-regulated pathways associated with genomic alterations related to endocrine therapy and KRAS pathways. Interestingly, the other cluster in which PDX were selected without fulvestrant showed only partial genomic alterations. Furthermore, whole exome sequencing revealed that similar driver gene mutations existed in both derived resistant and sensitive parental PDX. Finally, we confirmed pharmacological resistance to fulvestrant in the derived resistant PDX. New experiments investigating drug combinations in the context of endocrine resistance are ongoing.

Citation Format: Marc Hillairet De Boisferon, Ismahène Benzaid, Elodie Marie Dit Chatel, Nicolas Hoffmann, Bruce A. Littlefield. Tumor-specific molecular changes in luminal B breast cancer patient-derived xenografts (PDXs) with acquired resistance to endocrine treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1399.

Abstract 993: GDF15 is a candidate circulating biomarker of eribulin response in breast cancer

Microtubule-targeting agents (MTAs) constitute a diverse group of chemical compounds that bind to microtubules and affect their structure and function. Disruption of microtubules induces various cellular responses, often leading to cell cycle arrest or cell death. Eribulin is a microtubule dynamic inhibitor approved for treatment in patients with metastatic breast cancer (MBC) who had previously been administered chemotherapy. Although MTAs are widely used in the clinic, there is a research gap in the identification of pharmacodynamic (PD) biomarkers that could be used as surrogates during the course of treatment and measured non-invasively. The proteomic interrogation of the cancer secretome has emerged as a promising strategy for biomarker and drug target discovery. Secretomes have much lower complexity than plasma and tissue homogenates and are likely to be enriched with tumour derived-proteins entering into circulation. Quantitative proteomics analysis of the cancer secretome could help monitor critical aspects of cancer progression and therapeutics. The working hypothesis for this study is that eribulin induces a specific secretome that can be measured by quantitative proteomics, and could be used to identify response biomarkers linked to the action of the drug. We used three cell line models, MDA-MB-231 and HS578T (representing triple-negative BC) and MCF7 (representing luminal BC), all of them sensitive to treatment with eribulin. After a 7-day treatment with an IC70 of eribulin, we identified a subpopulation of cells with a lower proliferation rate, a slow-cycling subpopulation of cells that survived the large cell death induced by the drug. This slow-cycling population is alive in the presence of eribulin but remains in a quiescent state. The secretome profiling of slow-cycling cells revealed an oversecretion of the growth differentiation factor 15 (GDF15) when compared to parental cells and cells that after continuous exposure to eribulin start proliferating and become drug-tolerant. Interestingly, drug-tolerant cells lose the secretion of GDF15, suggesting a role for GDF15 in the response to eribulin treatment. Currently, we are validating GDF15 as a candidate circulating biomarker for the response to eribulin in patient-derived xenograft (PDX) 3D-models derived from metastatic BC patients. Furthermore, we are conducting loss of GDF15 function experiments in slow-cycling cells to understand whether the secretion of GDF15 could also contribute to the establishment and/or maintenance of the slow-cycling state in cells responding to eribulin.We envision that our work will contribute to the validation of GDF15 as a circulating biomarker that could be used to monitor eribulin's activity in the clinical setting. Furthermore, these results could also help understand the initiation of acquired resistance to eribulin.

Citation Format: Chiara Bellio, Marta Emperador, Laia Garrigós, Esther Zamora, Cristina Saura, Bruce A. Littlefield, Josep Villanueva. GDF15 is a candidate circulating biomarker of eribulin response in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 993.

Molecular Biomarkers of Response to Eribulin in Patients with Leiomyosarcoma

PURPOSE

A randomized phase III study evaluated the efficacy of eribulin versus dacarbazine in patients with advanced liposarcoma and leiomyosarcoma. Improved overall survival (OS) led to approval of eribulin for liposarcoma, but not for leiomyosarcoma.

EXPERIMENTAL DESIGN

We explored the molecular profile of 77 archival leiomyosarcoma samples from this trial to identify potential predictive biomarkers, utilizing low-coverage whole-genome and whole-exome sequencing. Tumor molecular profiles were correlated with clinical data, and disease control was defined as complete/partial response or stable disease (RECIST v1.1).

RESULTS

Overall, 111 focal copy-number alterations were observed in leiomyosarcoma. Gain of chromosome 17q12 was the most common event, present in 43 of 77 cases (56%). In the eribulin-treated group, gains of 4q26, 20p12.2, 13q13.3, 8q22.2, and 8q13.2 and loss of 1q44 had a negative impact on progression-free survival (PFS), while loss of 2p12 correlated with better prognosis. Gains of 4q22.1 and losses of 3q14.2, 2q14.1, and 11q25 had a negative impact on OS in patients with leiomyosarcoma receiving eribulin. The most commonly mutated genes were TP53 (38%), MUC16 (32%), and ATRX (17%). The presence of ATRX mutations had a negative impact on PFS in both treatment arms; however, the correlation with worse OS was observed only in the eribulin-treated patients. TP53 mutations were associated with longer PFS on eribulin.

CONCLUSIONS

Leiomyosarcoma has a complex genetic background, with multiple copy-number alterations and mutations affecting genes implicated in tumorigenesis. We identified several molecular changes with potential impact on survival of patients with leiomyosarcoma when treated with eribulin.

Abstract 1924: Genetic analysis of responses to eribulin versus vinorelbine and paclitaxel in 100 cancer cell lines from the cancer cell line Encyclopedia (CCLE)

Background: Eribulin mesylate, a synthetic analog of the marine sponge natural product halichondrin B, is a microtubule dynamics inhibitor that acts via both cytotoxic antimitotic mechanisms as well as non-cytotoxic effects on tumor vasculature, tumor phenotype and the tumor microenvironment. Eribulin is approved in the US for treatment of certain patients with advanced breast cancer or liposarcoma. Here we examined in vitro antiproliferative responses to eribulin versus vinorelbine and paclitaxel in 100 human cancer cell lines from the Cancer Cell Line Encyclopedia (CCLE), followed by analysis of CCLE baseline gene expression data with the goal of assessing overlapping and non-overlapping pathways and genes associated with drug response and resistance.

Methods: In vitro proliferation assays were conducted on 100 CCLE cell lines. Cancer types represented by ≥5 cell lines include breast, lung, blood, colorectum, ovary, pancreas, skin and kidney. Comparative systems-level analyses of genes, networks and pathways associated with drug response and/or resistance were performed to identify gene and pathway-level determinants of drug response that were either unique to eribulin or shared with vinorelbine and/or paclitaxel.

Results: We evaluated genes and molecular pathways driving sensitivity to eribulin versus vinorelbine and paclitaxel in a panel of 100 CCLE cell lines and identified key molecular determinants of response and resistance unique to eribulin. Pathway expression analysis revealed key pathways significantly associated with response to eribulin, including those associated with oxidative stress-induced senescence, histone deacetylation, DNA methylation and Notch signaling. The oxidative stress-induced senescence pathway was significantly enriched for genes whose pretreatment/basal mRNA expression levels were positively associated with response to eribulin but not vinorelbine or paclitaxel. Expression levels of various tubulin isoforms were also identified as positively correlated with eribulin response. Higher and lower pretreatment expression of E-cadherin and N-cadherin, respectively, were also found in cell lines sensitive to eribulin treatment.

Conclusion: Pretreatment gene expression analysis demonstrated key gene sets and molecular pathways that may drive response and resistance to eribulin and may differentiate this drug from vinorelbine and paclitaxel. Detailed analysis will be presented at the meeting.

Citation Format: Pallavi Sachdev, Roy Ronen, Janusz Dutkowski, Bruce A. Littlefield. Genetic analysis of responses to eribulin versus vinorelbine and paclitaxel in 100 cancer cell lines from the cancer cell line Encyclopedia (CCLE) [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 1924.

Abstract 5522: Evaluation of eribulin-induced alterations of the intact immune cell landscape in spleens and tumors from tumor-bearing immunocompetent mice

Eribulin, a synthetic analog of the marine sponge natural product halichondrin B, is approved in the US for certain patients with advanced breast cancer or liposarcoma. Its mechanisms of action include both cytotoxic, antimitotic effects as well as non-cytotoxic effects on tumor vasculature, phenotype and microenvironment. Known aspects of eribulin's effects on tumor phenotype, including reversal of epithelial-mesenchymal transition (EMT) and induction of cellular differentiation in various preclinical models, led us to speculate that eribulin might also alter immune homeostasis in the in vivo setting, both in tumors themselves as well as supporting immune organs such as the spleen. Here, we report on eribulin's effects on both lymphoid and myeloid lineage cellular components in both spleen and tumors from immunocompetent mice bearing subcutaneous Lewis lung carcinoma (LLC) isografts. Surprisingly, spleens of vehicle treated, LLC tumor-bearing mice are approximately twice as large spleens from non-tumor-bearing mice, with an excellent correlation (r2 = 0.92) between individual tumor size and spleen weight arguing for a direct cause and effect relationship between the presence of tumor and splenomegaly. Tumor-induced splenomegaly is associated with increased proliferation of splenic non-lymphoid (CD4-/CD8-/CD19-) cells, including both CD11b+ mature myeloid lineage cells as well as CD11b- cells. Due to high levels of P-glycoprotein (PgP) characteristic of most murine tumors, LLC tumors respond poorly to eribulin, a known PgP substrate. Thus, 3 doses of MTD eribulin (1.6 mg/kg; Q4Dx3) led to only a 30% reduction in tumor volume relative to vehicle-treated mice. Unexpectedly, however, despite this reduced tumor volume, spleens from eribulin-treated mice were twice again larger than the already-enlarged spleens from vehicle-treated tumor-bearing mice, and the clear relationship between tumor size and spleen weight was abolished. This additional eribulin-induced splenomegaly was associated with enhanced proliferation of the same CD4-/CD8-/CD19- non-lymphoid cells, CD11b+ myeloid lineage cells and CD11b- cells as in vehicle-treated, tumor-bearing mice. Evaluation of immune cell components in the associated fixed tumors from eribulin- and vehicle-treated mice is currently ongoing to assess what effect, if any, such changes in the spleen will have on the tumor immune microenvironment; these results will be presented at the meeting. In conclusion, our results point to significant and unexpected effects of eribulin treatment on the immune cell landscape in spleens and tumors from tumor-bearing immunocompetent mice.

Citation Format: Haochen Wu, Keyi Zhu, Xiaolong Tu, Diana Albu, Mary Woodall-Jappe, Bruce A. Littlefield. Evaluation of eribulin-induced alterations of the intact immune cell landscape in spleens and tumors from tumor-bearing immunocompetent mice [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 5522.

Abstract C098: Preclinical evaluation of eribulin in a veterinary clinical trial of metastatic spontaneous hemangiosarcoma in pet dogs

Eribulin is a microtubule dynamics inhibitor that is currently in clinical use for treatment of certain patients with advanced breast cancer or liposarcoma. FDA approval for both indications was based on extension of overall survival. Based on eribulin’s known vascular remodeling effects, its activity against at least one sarcoma type, and literature reports of taxane activity in angiosarcoma, we hypothesized that eribulin may show activity against angiosarcoma. Due to a paucity of preclinical cell-based, xenograft, and PDX models for human angiosarcoma, we sought other alternatives for preclinical testing in this disease. While rare in humans, angiosarcoma is common in some breeds of domestic dogs, where it is known in veterinary parlance as hemangiosarcoma (HSA). Accordingly, we undertook a small veterinary clinical trial to evaluate eribulin against spontaneous HSA in pet dogs. Canine HSA most frequently occurs in the spleen, so splenectomy is a common first-line treatment; however, non-splenic sites also occur and these are often unresectable. In keeping with eribulin’s clinical use in the metastatic setting, we selected metastatic canine HSA, which we defined as dogs with histologically confirmed HSA with either non-resectable or post-splenectomy residual disease confirmed by imaging (PET/CT, CT). Of 19 pets initially screened, 12 dogs were imaged; 6 of these had imageable disease and thus began eribulin treatment. Eribulin was administered using the human clinical schedule (d1,8Q21), with the first cycle at 0.3 mg/m2 followed by intra-patient dose escalation to 0.6 mg/m2 after determination of tolerability. Of the 6 dogs entering treatment, 5 completed cycle 1, 3 completed cycle 2, and 2 completed the full per-protocol 3 cycles of treatment. These 2 dogs survived 121 and 179 days after initial diagnosis, contrasting with the 0.9 month (~27 days) median survival time reported by Wendelburg et al. (JAVMA, 2015) for 52 post-splenectomy canine HSA patients with residual metastases. Although limited in scope, our results with spontaneous metastatic canine HSA in pet dogs provides a preclinical foundation for further exploration of eribulin in malignancies of vascular origin in both the veterinary and human settings.

Citation Format: Christina Kelly Lopes, Gerald Post, Lindsay Lambert, Benjamin Lewis, Thaddeus David Allen, Angel Patel, Bruce A Littlefield. Preclinical evaluation of eribulin in a veterinary clinical trial of metastatic spontaneous hemangiosarcoma in pet dogs [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C098. doi:10.1158/1535-7163.TARG-19-C098

Abstract A036: Identification of secretome-based eribulin biomarkers for breast cancer therapeutics

Microtubules are major components of the cytoskeleton. They are involved in a wide variety of cell functions including attribution to cell shape, motility, intracellular trafficking and mitotic spindle formation. Therefore, microtubules have gained significant interest as important targets for cancer therapy. Several microtubule-targeting agents (MTAs) are being used to treat cancer patients. Eribulin is a novel microtubule dynamic inhibitor approved for treatment of metastatic breast cancer (MBC) after two previous lines of therapy and the only cytotoxic agent in recent years to improve overall survival (OS) in heavily pretreated patients. A challenge for achieving successful management of cancer is the discovery of tumor biomarkers that represent useful surrogates during the course of the disease and therapeutic treatment, and that can be measured non-invasively. Over the last few years there has been an increasing interest in the study of cancer secretome, a sub-proteome that contains secreted and shed plasma membrane proteins. The rationale supporting this approach is that secretomes have a much lower complexity than plasma and tissue homogenates, and may be enriched with proteins that are very likely to enter the circulation. The presence of proteins linked to cancer such as growth factors and proteases in these fluids indicates that secretomes might help in monitoring critical aspects of cancer progression and therapeutic treatment. The working hypothesis for this study is that there is a secretome induced by eribulin that can be measured by quantitative proteomics, and can be used to identify response biomarkers linked to the action of the drug. We used three cell line models representing triple negative breast cancer (TNBC), MDA-MB-231 and MX1, and luminal BC, MCF7, which were already proven to be sensitive to eribulin. We generated secretomes by quantitative proteomics from these cell lines during the treatment with eribulin, and we compared them to the control secretomes from the same cell lines. The statistical analysis of the data revealed a specific secretome that could reflect the action of eribulin on tumor cells. Specifically, eribulin treatment induced the oversecretion of the same families of protein in the three different BC cell lines. In the near future, we will select candidate biomarkers for validation in samples obtained from metastatic patients at baseline and during treatment, and we will correlate their levels with the standard response evaluation by diagnostic radiologic exams. This project is the collaborative effort between the Tumor Biomarkers laboratory at VHIO, the Breast Cancer Program at Vall d’Hebron Hospital and Eisai. We are confident that identifying the eribulin-based secretome could help monitoring critical aspects of cancer therapeutic MBC treatment.

Citation Format: Chiara Bellio, Juan M. Duran, Olga Méndez, Nathalie Meo-Evoli, Cándida Salvans, Laia Garrigós, Esther Zamora, Cristina Saura, Bruce A Littlefield, Josep Villanueva. Identification of secretome-based eribulin biomarkers for breast cancer therapeutics [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A036. doi:10.1158/1535-7163.TARG-19-A036

Abstract 4719: Synergistic anticancer activity of triple combinations of eribulin, palbociclib and fulvestrant in hormone dependent patient-derived xenograft (PDX) models of human breast cancer

Eribulin is a pharmaceutically optimized analog of the marine natural product halichondrin B. As its clinically formulated mesylate salt (Halaven®), eribulin is used for treatment of certain patients with advanced breast cancer and liposarcoma. Mechanistically, eribulin combines cytotoxic tubulin-based antimitotic effects with non-cytotoxic effects on tumor biology, including vascular remodeling, increased perfusion, mitigation of hypoxia and reversal of epithelial to mesenchymal transition (EMT). Reversal of EMT involves cell differentiation pathways that impinge on the G1/S cell cycle boundary. Since estrogenic signaling also impinges on the G1/S boundary, we asked if eribulin could combine advantageously with inhibitors of cyclin dependent kinases and hormonal agents that disrupt estrogenic signaling. Using PDX models of hormone receptor positive (HR+) breast cancer, we previously showed that combining eribulin and palbociclib is considerably more effective than either agent alone, using a “palbociclib holiday” strategy of withholding daily palbociclib doses the day before and the day of weekly eribulin doses to avoid possible cell cycle based antagonism. Here, we ask if the palbociclib holiday is strictly necessary for robust eribulin + palbociclib combination activity, and if triple combinations of eribulin, palbociclib and fulvestrant result in even better anticancer activity than doublet dosing. For the holiday/no holiday comparison, 0.125 mg/kg eribulin was dosed iv Q7Dx3, with palbociclib dosed po either at 150 mg/kg or 107 mg/kg on Q1Dx5[x3 weeks] (holiday) or Q1Dx21 (no holiday) schedules, respectively, resulting in equal palbociclib dose intensities for the 2 schedules. Results showed that synergism was seen in combination with or without palbociclib holiday, but superior results occur with holiday. Using the holiday strategy, we next investigated triple combinations of eribulin, palbociclib and fulvestrant. As single agents at the minimally effective doses selected, eribulin, palbociclib and fulvestrant led to treated/control values (T/C%) of 64%, 63% and 48%, respectively. Combining eribulin and palbociclib led to markedly superior anticancer activity (T/C 23%). Combining eribulin and fulvestrant also led to superior activity (T/C 22%), as did combining fulvestrant and palbociclib (T/C 19%). The triple combination generated the most robust activity at T/C 8%. By mouse RECIST criteria, 10%, 10% and 20% partial responses (PR) were observed for each doublet (fulvestrant/palbociclib, fulvestrant/eribulin, palbociclib/eribulin), respectively. In contrast, 90% PR was seen for the triple combination. These preclinical results support clinical exploration of eribulin, palbociclib and fulvestrant triple combinations for appropriate patients with HR+ breast cancers.

Citation Format: Marc Hillairet de Boisferon, Elodie Marie Dit Chatel, Kenichi Nomoto, Bruce A. Littlefield. Synergistic anticancer activity of triple combinations of eribulin, palbociclib and fulvestrant in hormone dependent patient-derived xenograft (PDX) models of human breast cancer [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 4719.

Differential Morphological and Biochemical Recovery from Chemotherapy-Induced Peripheral Neuropathy Following Paclitaxel, Ixabepilone, or Eribulin Treatment in Mouse Sciatic Nerves

The reversibility of chemotherapy-induced peripheral neuropathy (CIPN), a disabling and potentially permanent side effect of microtubule-targeting agents (MTAs), is becoming an increasingly important issue as treatment outcomes improve. The molecular mechanisms regulating the variability in time to onset, severity, and time to recovery from CIPN between the common MTAs paclitaxel and eribulin are unknown. Previously (Benbow et al. in Neurotox Res 29:299-313, 2016), we found that after 2 weeks of a maximum tolerated dose (MTD) in mice, paclitaxel treatment resulted in severe reductions in axon area density, higher frequency of myelin abnormalities, and increased numbers of Schwann cell nuclei in sciatic nerves. Biochemically, eribulin induced greater microtubule-stabilizing effects than paclitaxel. Here, we extended these comparative MTD studies to assess the recovery from these short-term effects of paclitaxel, eribulin, and a third MTA, ixabepilone, over the course of 6 months. Paclitaxel induced a persistent reduction in axon area density over the entire 6-month recovery period, unlike ixabepilone- or eribulin-treated animals. The abundance of myelin abnormalities rapidly declined after cessation of all drugs but recovered most slowly after paclitaxel treatment. Paclitaxel- and ixabepilone- but not eribulin-treated animals exhibited increased Schwann cell numbers during the recovery period. Tubulin composition and biochemistry rapidly returned from MTD-induced levels of α-tubulin, acetylated α-tubulin, and end-binding protein 1 to control levels following cessation of drug treatment. Taken together, sciatic nerve axons recovered more rapidly from morphological effects in eribulin- and ixabepilone-treated animals than in paclitaxel-treated animals and drug-induced increases in protein expression levels following paclitaxel and eribulin treatment were relatively transient.

Abstract B191: Synergistic anticancer activity of eribulin plus palbociclib in patient-derived xenograft (PDX) models of ER+/Her2- human breast cancer

Eribulin is a synthetic analog of the marine sponge natural product halichondrin B. Its clinical formulation is currently approved in numerous countries for treating certain patients with advanced breast cancer or advanced liposarcoma. Eribulin’s anticancer mechanisms include relatively fast antimitotic effects that lead to cell death by apoptosis, and slower complex effects on the tumor microenvironment that include increased vascular perfusion, reduced hypoxia, and changes in phenotype associated with decreased migration and invasiveness in vitro. Such changes in tumor phenotype led us to ask whether eribulin could be successfully combined with inhibitors of cyclin dependent kinases 4 and 6 (cdk 4/6), which work at the G1/S cell cycle checkpoint where phenotypic changes are typically regulated. It is now recognized that patient-derived xenograft (PDX) tumor models more faithfully recapitulate human tumor biology and drug responsiveness than standard xenograft models derived from established human cancer cell lines. Accordingly, in the current study, we used two ER+/PR+/Her2- PDX models developed from patients with luminal B breast cancers, OD-BRE-0192 and OD-BRE-0745, to test in vivo combinations of eribulin and the cdk 4/6 inhibitor palbociclib. Eribulin and palbociclib dose levels were chosen that led to only minimal effects when given alone, and were administered on Q7Dx3 and Q1Dx5[x3] schedules, respectively, with palbociclib withheld the day before and the day of weekly eribulin dosing to avoid cell cycle-based antagonism that could occur between mitotic and G1/S blockers. In the OD-BRE-0745 model only, a fourth cycle of eribulin followed by 5 days of palbociclib was administered starting 13 days after the third eribulin dose. Under these conditions, combining eribulin and palbociclib led to markedly superior anticancer activity in both models (minimum T/C values of 29% and 41%) compared to either agent alone (T/C: 55-67% and 88-98%, respectively). These preclinical PDX results support clinical exploration of eribulin and palbociclib combinations for appropriate patients with ER+/Her2- breast cancers.

Citation Format: Kenichi Nomoto, Marc H. de Boisferon, Bruce A. Littlefield. Synergistic anticancer activity of eribulin plus palbociclib in patient-derived xenograft (PDX) models of ER+/Her2- human breast cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B191.

Peripheral Neuropathy Induced by Microtubule-Targeted Chemotherapies: Insights into Acute Injury and Long-term Recovery

Chemotherapy-induced peripheral neuropathy (CIPN) is a major cause of disability in cancer survivors. CIPN investigations in preclinical model systems have focused on either behaviors or acute changes in nerve conduction velocity (NCV) and amplitude, but greater understanding of the underlying nature of axonal injury and its long-term processes is needed as cancer patients live longer. In this study, we used multiple independent endpoints to systematically characterize CIPN recovery in mice exposed to the antitubulin cancer drugs eribulin, ixabepilone, paclitaxel, or vinorelbine at MTDs. All of the drugs ablated intraepidermal nerve fibers and produced axonopathy, with a secondary disruption in myelin structure within 2 weeks of drug administration. In addition, all of the drugs reduced sensory NCV and amplitude, with greater deficits after paclitaxel and lesser deficits after ixabepilone. These effects correlated with degeneration in dorsal root ganglia (DRG) and sciatic nerve and abundance of Schwann cells. Although most injuries were fully reversible after 3-6 months after administration of eribulin, vinorelbine, and ixabepilone, we observed delayed recovery after paclitaxel that produced a more severe, pervasive, and prolonged neurotoxicity. Compared with other agents, paclitaxel also displayed a unique prolonged exposure in sciatic nerve and DRG. The most sensitive indicator of toxicity was axonopathy and secondary myelin changes accompanied by a reduction in intraepidermal nerve fiber density. Taken together, our findings suggest that intraepidermal nerve fiber density and changes in NCV and amplitude might provide measures of axonal injury to guide clinical practice.Significance: This detailed preclinical study of the long-term effects of widely used antitubulin cancer drugs on the peripheral nervous system may help guide clinical evaluations to improve personalized care in limiting neurotoxicity in cancer survivors. Cancer Res; 78(3); 817-29. ©2017 AACR.

Microtubule-Targeting Agents Eribulin and Paclitaxel Differentially Affect Neuronal Cell Bodies in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a common side effect of anticancer treatment with microtubule-targeted agents (MTAs). The frequency of severe CIPN, which can be dose limiting and even life threatening, varies widely among different MTAs. For example, paclitaxel induces a higher frequency of severe CIPN than does eribulin. Different MTAs also possess distinct mechanisms of microtubule-targeted action. Recently, we demonstrated that paclitaxel and eribulin differentially affect sciatic nerve axons, with paclitaxel inducing more pronounced neurodegenerative effects and eribulin inducing greater microtubule stabilizing biochemical effects. Here, we complement and extend these axonal studies by assessing the effects of paclitaxel and eribulin in the cell bodies of sciatic nerve axons, housed in the dorsal root ganglia (DRG). Importantly, the microtubule network in cell bodies is known to be significantly more dynamic than in axons. Paclitaxel induced activating transcription factor 3 expression, a marker of neuronal stress/injury. Paclitaxel also increased expression levels of acetylated tubulin and end binding protein 1, markers of microtubule stability and growth, respectively. These effects are hypothesized to be detrimental to the dynamic microtubule network within the cell bodies. In contrast, eribulin had no significant effect on any of these parameters in the cell bodies. Taken together, DRG cell bodies and their axons, two distinct neuronal cell compartments, contain functionally distinct microtubule networks that exhibit unique biochemical responses to different MTA treatments. We hypothesize that these distinct mechanistic actions may underlie the variability seen in the initiation, progression, persistence, and recovery from CIPN.

Sustained Accumulation of Microtubule-Binding Chemotherapy Drugs in the Peripheral Nervous System: Correlations with Time Course and Neurotoxic Severity

Chemotherapy-induced peripheral neuropathy is a dose-limiting side effect of many antineoplastic agents, but the mechanisms underlying the toxicities are unclear. At their MTDs, the microtubule-binding drugs paclitaxel and ixabepilone induce more severe neuropathy in mice relative to eribulin mesylate, paralleling their toxicity profiles in clinic. We hypothesized that the severity of their neurotoxic effects might be explained by the levels at which they accumulate in the peripheral nervous system. To test this hypothesis, we compared their pharmacokinetics and distribution in peripheral nerve tissue. After administration of a single intravenous dose, each drug was rapidly cleared from plasma but all persisted in the dorsal root ganglia (DRG) and sciatic nerve (SN) for up to 72 hours. Focusing on paclitaxel and eribulin, we performed a 2-week MTD-dosing regimen, followed by a determination of drug pharmacokinetics, tissue distribution, and multiple functional measures of peripheral nerve toxicity for 4 weeks. Consistent with the acute dosing study, both drugs persisted in peripheral nervous tissues for weeks, in contrast to their rapid clearance from plasma. Notably, although eribulin exhibited greater DRG and SN penetration than paclitaxel, the neurotoxicity observed functionally was consistently more severe with paclitaxel. Overall, our results argue that sustained exposure of microtubule-binding chemotherapeutic agents in peripheral nerve tissues cannot by itself account for their associated neurotoxicity. Cancer Res; 76(11); 3332-9. ©2016 AACR.

Abstract P1-03-09: Pegylated recombinant human hyaluronidase PH20 (PEGPH20) enhances efficacy of eribulin mesylate (HALAVEN®) in triple negative breast cancer xenografts

Hyaluronan (hyaluronic acid, HA), a glycosaminoglycan found in tissue throughout the body, overaccumulates in the tumor microenvironment (TME) of many non-hematologic malignancies, including breast cancer. HA overaccumulation in breast cancer patients correlates with tumor progression and decreased survival (Tammi 2008). Pegylated recombinant human hyaluronidase PH20 (PEGPH20), an investigational therapeutic agent entering Phase 3 clinical development in pancreatic cancer, enzymatically removes HA from the TME. In preclinical animal models, PEGPH20-mediated HA degradation is associated with remodeling of the tumor stroma, reduction of tumor pressure, expansion of tumor blood vessels and facilitated delivery of chemotherapy (Thompson 2010, Provenzano 2012, Jacobetz 2013). Accordingly, preclinical studies investigated the combination of PEGPH20 with eribulin mesylate (ERI, HALAVEN®), a microtubule dynamics inhibitor with a novel mechanism of action (Towle 2001, Jordan 2005), currently approved for treatment of certain patients with advanced breast cancer. NCr nu/nu mice were inoculated subcutaneously with human triple-negative breast cancer (TNBC) HCC1806 or HCC1806/HAS3 cells; the latter subline was engineered to accumulate high HA levels, confirmed by immunohistochemistry, via overexpression of hyaluronan synthase 3 (HAS3). When tumors reached ∼350 mm3, animals were randomly assigned to four treatments groups: vehicle, ERI (0.7 mg/kg, IV, QW), PEGPH20 (37.5 µg/kg, IV, BIW), or ERI plus PEGPH20. In the parental HCC1806 model, addition of PEGPH20 did not significantly change the antitumor effects of ERI. In contrast, combining PEGPH20 with ERI in the HCC1806/HAS3 model increased the antitumor effects of ERI by 27% (94.5% vs. 119.7% TGI, ERI alone vs. ERI+PEGPH20, respectively; p=0.05) and resulted in 6 of 7 complete tumor regressions.

In a complementary study in HCC1806/HAS3 tumors evaluating ERI pharmacokinetics with and without PEGPH20, mice were assigned to three treatments groups: ERI (0.5 mg/kg, IV), simultaneous ERI plus PEGPH20 (37.5 µg/kg, IV); or ERI plus PEGPH20 predosed 24 h prior to ERI. Animals were sacrificed at 0.5, 1, 4, 24, 48, 72 and 96 h post ERI dose, and ERI levels in tumor, muscle, plasma and liver were subsequently analyzed by liquid/liquid extraction and LC-MS/MS chromatography. Simultaneous administration of ERI and PEGPH20 increased ERI maximum tumor concentration (Cmax) slightly and approximately doubled ERI tumor exposure (AUC); whereas the 24 h pretreatment with PEGPH20 approximately doubled ERI Cmax and increased ERI AUC more than two-fold. No significant differences in plasma ERI levels were observed between groups, and no significant differences in ERI levels in liver or muscle tissue were observed between groups. Taken together, these data suggest that PEGPH20-mediated HA removal significantly increases both ERI tumor concentrations and antitumor effectiveness in an HA-high TNBC model. A clinical phase 1b/2 clinical trial is planned to evaluate PEGPH20 plus ERI in first-line HER2-negative metastatic breast cancer.

Citation Format: Bahn JD, DesJardins C, Condon KB, Fathallah A, Zimmerman S, Maneval DC, Littlefield BA, Thompson CB. Pegylated recombinant human hyaluronidase PH20 (PEGPH20) enhances efficacy of eribulin mesylate (HALAVEN®) in triple negative breast cancer xenografts. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-03-09.

Abstract 4503: Pharmacokinetics and pharmacodynamic analysis of eribulin mesylate and paclitaxel in mouse

Neuropathy arising from chemotherapy (CIPN) is a major clinical problem representing the dose-limiting side effect of many antineoplastic drugs. At their respective MTDs, we previously reported paclitaxel and ixabepilone produced more severe deficits in nerve conduction velocity, amplitude and degenerative changes in dorsal root ganglia (DRG) and sciatic nerve (SN) versus eribulin mesylate (Wozniak et al 2011). Similar trends for eribulin to cause less neuropathy have also been reported in the clinic (Cigler and Vadhat 2010, Jain and Cigler 2012, Vadhat et al 2013). The underlying reason for this differential effect remains elusive. Differences in tubulin binding may, in part, explain the differential effect (Perez et al 2009, Jordan et al 2005). Another potential explanation could reside in different pharmacokinetic (PK) and nervous tissue distribution of these agents. Toward this latter point, we conducted tissue distribution and pharmacokinetic studies following acute dosing of eribulin mesylate, paclitaxel and ixabepilone. We reported that while all three drugs rapidly cleared from plasma, they distributed into and cleared slowly (>3 days) from peripheral nervous system tissues (Wozniak et al 2014). We have now extended these studies to look out to 42 days following both acute and 2 week MTD-dosing paradigms evaluating drug pharmacokinetics in plasma, DRG and SN as well as pharmacodynamic measurements of nerve conduction and amplitude. We found plasma levels of eribulin mesylate and paclitaxel rapidly declined over 24h after infusion. In contrast, the levels of eribulin mesylate and paclitaxel persisted at above quantification levels (ranging from 5-10 ng/g) for 7 to 14 days post cessation of 2 week MTD dosing. Eribulin mesylate consistently showed greater distribution into DRG and SN over paclitaxel. When comparing the induced nerve conduction deficits, we found paclitaxel and eribulin mesylate caused reduction of nerve conduction parameters similar to our previous studies, most notably in caudal nerve amplitude (by 80.5% and 54.9% respectively). However these deficits, which were sustained for up to several weeks after cessation of dosing, were more severe with paclitaxel. The findings demonstrate that despite eribulin mesylate showing higher nervous tissue distribution than paclitaxel, eribulin mesylate caused less neurotoxicity as evidenced by milder reductions in nerve conduction and amplitude.

This work was financially supported by Eisai.

Citation Format: Krystyna Wozniak, Ying Wu, Bruce A. Littlefield, Kenichi Nomoto, Christopher DesJardins, Yanke Yu, George Lai, Larisa Reyderman, Barbara S. Slusher. Pharmacokinetics and pharmacodynamic analysis of eribulin mesylate and paclitaxel in mouse. [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 4503. doi:10.1158/1538-7445.AM2015-4503

Synergistic Anti-Tumor Activity of EZH2 Inhibitors and Glucocorticoid Receptor Agonists in Models of Germinal Center Non-Hodgkin Lymphomas

Patients with non-Hodgkin lymphoma (NHL) are treated today with a cocktail of drugs referred to as CHOP (Cyclophosphamide, Hydroxyldaunorubicin, Oncovin, and Prednisone). Subsets of patients with NHL of germinal center origin bear oncogenic mutations in the EZH2 histone methyltransferase. Clinical testing of the EZH2 inhibitor EPZ-6438 has recently begun in patients. We report here that combining EPZ-6438 with CHOP in preclinical cell culture and mouse models results in dramatic synergy for cell killing in EZH2 mutant germinal center NHL cells. Surprisingly, we observe that much of this synergy is due to Prednisolone - a glucocorticoid receptor agonist (GRag) component of CHOP. Dramatic synergy was observed when EPZ-6438 is combined with Prednisolone alone, and a similar effect was observed with Dexamethasone, another GRag. Remarkably, the anti-proliferative effect of the EPZ-6438+GRag combination extends beyond EZH2 mutant-bearing cells to more generally impact germinal center NHL. These preclinical data reveal an unanticipated biological intersection between GR-mediated gene regulation and EZH2-mediated chromatin remodeling. The data also suggest the possibility of a significant and practical benefit of combining EZH2 inhibitors and GRag that warrants further investigation in a clinical setting.

Abstract 4653: Pharmacokinetics and biodistribution of eribulin mesylate, paclitaxel, and ixabepilone in mouse

Chemotherapy-induced peripheral neurotoxicity is a major clinical problem representing the dose-limiting side effect of many antineoplastic drugs. The degree and type of neuropathy incurred is dependent on the chemotherapeutic drug, dose intensity, and cumulative dose. In an earlier study by our group, we found that paclitaxel and ixabepilone, administered to mice at their respective MTDs, produced dose dependent deficits in caudal nerve conduction velocity, caudal amplitude and digital nerve amplitudes, as well as moderate to severe degenerative pathologic changes in dorsal root ganglia (DRG) and sciatic nerve (SN). In contrast, eribulin mesylate produced no significant deleterious effects on any nerve conduction parameter measured and caused milder, less frequent effects on morphology. Similar trends for eribulin to cause less neuropathy (including later onset and resolution) in the clinic have also been reported (Cigler and Vadhat, 2010; Jain and Cigler 2012, Vadhat et al., 2013). Differences in tubulin binding may, in part, potentially explain the differential effect (Perez et al 2009; Jordan et al 2005). Another potential explanation may reside in different pharmacokinetic (PK) and peripheral nerve distribution of these agents. To elucidate this, mice received a single dose of paclitaxel, ixabepilone or eribulin mesylate (30, 3 and 1.75mg/kg respectively, based on an MTD regimen), administered IV into the caudal vein. Plasma, DRG and SN samples were obtained at 0.25, 1, 3, 6, 8, 24, 48, and 72 h post dose. Tissues were homogenized and after extraction, concentrations of eribulin mesylate, ixabepilone and paclitaxel were determined in each matrix using LC/MS/MS. PK parameters were calculated using noncompartmental analysis in WinNonLin v 5.0.3. We report that all 3 chemotherapies rapidly penetrated both SN and DRG with maximal concentrations seen at, or within 15 mins after administration. Concentrations of all 3 agents in DRG and SN then declined by 30-50% over the first 1-3h and thereafter remained relatively stable in the tissue for up to 72 h, despite clearing rapidly from plasma to reach levels below quantification, (<5 ng/ml), by 24 h. Overall, dose normalized AUC in SN was similar for eribulin mesylate and ixabepilone and higher than for paclitaxel (1927 vs 2451 vs 793 ng/h/per g tissue, respectively). The ranking for the dose normalized AUC in DRG was eribulin mesylate < paclitaxel < ixabepilone (3894 vs 7217 vs 22800 ng/h/per g tissue, respectively). These findings suggest that peripheral nerve exposure is likely not the underlying mechanism for the differential propensity of eribulin mesylate, paclitaxel and ixabepilone to produce neurotoxicity.

This work was financially supported by Eisai.

Citation Format: Krystyna Wozniak, Ying Wu, Kenichi Nomoto, Bruce A. Littlefield, Christopher DesJardins, Edgar Schuck, Phil Saxton, Nancy Wong, Barbara S. Slusher. Pharmacokinetics and biodistribution of eribulin mesylate, paclitaxel, and ixabepilone in mouse. [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 4653. doi:10.1158/1538-7445.AM2014-4653

Abstract 5503: Microtubule-targeting agents induce differential myelinated axon degeneration in a mouse model of peripheral neuropathy

Microtubule-targeting agents (MTAs) have proven useful in treating many types of cancers. However, their efficacy continues to be limited by peripheral neuropathy, a painful and potentially dose-limiting side effect. While the mechanisms of MTA-induced neuropathy remain unclear, evidence suggests that peripheral nerve degeneration contributes to altered nerve activity and neuropathic pain. Recent electrophysiological studies in mice revealed that paclitaxel and ixabepilone reduced peripheral nerve conduction velocity and amplitude, while eribulin, an MTA with a lower incidence of severe peripheral neuropathy, showed no change in these parameters. Here, we sought to test the hypothesis that MTA-induced peripheral neuropathy in mice positively correlates with axon degeneration. We compared the effects of four MTAs with varying incidences of severe neuropathic symptoms (vincristine > ixabepilone = paclitaxel > eribulin). Naïve adult mice were treated at their maximum tolerated dose of each drug (or vehicle) on a Q2Dx3 schedule for 2 weeks. Sciatic nerves were extracted, fixed, sectioned, and visualized by immunofluorescence confocal microscopy. Morphological changes were quantified using Imaris software. Paclitaxel and eribulin resulted in significantly lower myelinated axon densities of 172 ± 14.5 and 264 ± 18.0 axons per 100 μm2 compared to vehicle controls of 371 ± 14.2 and 366 ± 19.3 respectively (mean ± SEM, P < 0.001). Paclitaxel and eribulin also increased the percentage of collapsed nerve fibers, which were identified by fragmented and/or contracted myelin sheaths coupled with a loss of phospho-neurofilament protein signal from the associated axons. The percent of collapsed nerve fibers was 2.2% for eribulin (vs. 0.32% vehicle; P < 0.05) and 6.6% for paclitaxel (vs. 0.21% vehicle; P<0.001). For paclitaxel, nerve fiber collapse occurred more frequently in distal than in proximal nerve sections (P < 0.01). We also observed a unique myelin “halo” morphology in eribulin treated nerves that was not found in any other condition. Surprisingly, vincristine and ixabepilone did not induce detectable nerve degeneration, even though both drugs exhibit a high incidence of severe peripheral neuropathy. Our results indicate that although significant nerve degeneration is present after two weeks of treatment with some MTAs, the extent of these changes does not correlate with the clinical incidence of severe neuropathy. Therefore, although degenerative changes in peripheral nerves may contribute to nerve dysfunction in MTA-induced peripheral neuropathy, especially in the cases of paclitaxel and eribulin, they do not appear to be the only factor underlying this condition. We conclude that additional mechanisms of MTA-induced peripheral neuropathy likely exist which may not be directly related to axonal degeneration. We gratefully thank Eisai Inc for support of this research.

Citation Format: Brett M. Cook, Sarah J. Benbow, Krystyna M. Wozniak, Barbara S. Slusher, Bruce A. Littlefield, Leslie Wilson, Stuart C. Feinstein, Mary Ann Jordan. Microtubule-targeting agents induce differential myelinated axon degeneration in a mouse model of peripheral neuropathy. [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 5503. doi:10.1158/1538-7445.AM2014-5503

Abstract 1698: Identification of drugs with eribulin combinatorial activity that kill both eribulin-sensitive and eribulin-insensitive tumor cells

Eribulin mesylate, a nontaxane microtubule dynamics inhibitor with a mechanism distinct from most other antitubulin therapeutics, is clinically used in the United States for treatment of certain patients with locally advanced or metastatic breast cancer. Eribulin has broad anticancer activity in a wide variety of preclinical cancer models; as a result, numerous clinical trials are ongoing to investigate efficacy in other non-breast tumor types. We have examined eribulin sensitivity in a panel of 25 human cancer cell lines representing a variety of tumor types albeit with a preponderance of breast and lung cancer cell lines. As expected, the cell lines vary in sensitivity to eribulin at clinically relevant concentrations. To identify combination drugs capable of increasing anticancer effects in patients already responsive to eribulin, as well as inducing de novo anticancer effects in non-responders, we performed a combinatorial high throughput screen to identify drugs that combine with eribulin to selectively kill tumor cells. Among other observations, we found that inhibitors of ErbB1/ErbB2 (lapatinib, BIBW-2992, erlotinib), MEK (E6201, trametinib), PI3K (BKM-120), mTOR (AZD 8055, everolimus), and PI3K/mTOR (BEZ 235), and a BCL2 family antagonist (ABT-263) show combinatorial activity with eribulin. In addition, antagonistic pairings with other agents, such as topoisomerase I inhibitors (topotecan hydrochloride), an HSP-90 inhibitor (17-DMAG), and gemcitabine and cytarabine, were also identified. In summary, the preclinical studies described here have identified several combination drugs that have the potential to either augment or antagonize eribulin's anticancer activity. Further elucidation of the mechanisms responsible for such interactions may be important for identifying valuable therapeutic partners for eribulin.

Citation Format: Toshimitsu Uenaka, Richard Rickles, Yasuhiro Funahashi, Ping Zhu, Jill M. Grenier, Janine Steiger, Nanding Zhao, Bruce A. Littlefield, Junji Matsui, Kenichi Nomoto. Identification of drugs with eribulin combinatorial activity that kill both eribulin-sensitive and eribulin-insensitive tumor cells. [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 1698. doi:10.1158/1538-7445.AM2014-1698

Neuropathy-inducing effects of eribulin mesylate versus paclitaxel in mice with preexisting neuropathy

Eribulin mesylate (E7389, INN:eribulin mesilate Halaven(®)) is a non-taxane microtubule dynamics inhibitor currently in clinical use for advanced breast cancer. Other microtubule-targeting agents for breast cancer, including paclitaxel and ixabepilone, display a common treatment dose-limiting toxicity of peripheral neuropathy (PN). In an earlier study, we found eribulin mesylate had a lower propensity to induce PN in mice than either paclitaxel or ixabepilone. In the current study, we compared additional PN induced by paclitaxel versus eribulin mesylate when administered to mice with preexisting paclitaxel-induced PN. Initially, paclitaxel at 0.75 × its maximum tolerated dose (MTD; 22.5 mg/kg) was given on a Q2Dx3 regimen for 2 weeks. The second chemotherapy was 0.5 MTD eribulin mesylate (0.875 mg/kg) or paclitaxel (15 mg/kg) on a similar regimen, starting 2 weeks after the first. Initial paclitaxel treatment produced significant decreases in caudal nerve conduction velocity (NCV; averaging 19.5 ± 1 and 22.2 ± 1.3 %, p < 0.001) and amplitude (averaging 53.2 ± 2.6 and 72.4 ± 2.1 %, p < 0.001) versus vehicle when measured 24 h or 2 weeks after dosing cessation, respectively. Additional 0.5 MTD paclitaxel further reduced caudal NCV and amplitude relative to immediately before initiation of the second regimen (by 11 ± 2.1 and 59.2 ± 5 %, p < 0.01, respectively). In contrast, 0.5 MTD eribulin mesylate caused no further decrease in caudal NCV. In conclusion, unlike additional paclitaxel treatment, eribulin mesylate administered to mice with preexisting paclitaxel-induced PN had limited additional deleterious effects at 6 weeks. These preclinical data suggest that eribulin mesylate may have reduced tendency to exacerbate preexisting paclitaxel-induced PN in clinical settings.

Abstract 1227: Macrophage depletion enhances the anticancer efficacy of eribulin mesylate in mice bearing syngeneic tumors

Recent reports utilizing the MMTV-PyMT spontaneous model of breast cancer established that inhibition of macrophage recruitment to the tumors resulted in increased anti-cancer activity of paclitaxel. Eribulin mesylate (Halaven®) is a non-taxane microtubule-dynamics inhibitor recently approved for third line clinical use in patients with heavily pretreated metastatic breast cancer, based upon a statistically significant increase in median overall survival (OS) compared to treatment of physician's choice. In the present study we investigated whether systemic depletion of macrophages would enhance eribulin anti-tumor efficacy. In the subcutaneous Lewis lung carcinoma (LLC) syngeneic murine tumor model, we found that macrophage depletion via liposomal clodronate enhanced eribulin inhibition of tumor growth. Current studies are investigating the mechanism(s) of this activity.

Citation Format: Diana I. Albu, Bruce A. Littlefield, Mary Woodall-Jappe. Macrophage depletion enhances the anticancer efficacy of eribulin mesylate in mice bearing syngeneic tumors. [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 1227. doi:10.1158/1538-7445.AM2013-1227

Abstract 4493: Eribulin recognizes the nucleotide states of soluble tubulin and binds with high-affinity on a site distinct from known tubulin inhibitors

Eribulin (E7389) is an antimitotic drug that inhibits microtubule assembly in vitro and in vivo by binding to tubulin. Several studies have been performed on eribulin mechanism of action; however, the mode of interaction with soluble tubulin (αβ-heterodimer) and binding site differences with vinca alkaloids (vinblastine and vincristine) and paclitaxel have not been well characterized. To understand binding differences, we performed an in vitro conformational change assays and established a surface plasmon resonance (SPR)-based direct binding assay using chemical probes and soluble tubulin. Unlike vinca alkaloids and paclitaxel, binding of eribulin with tubulin has little or no effect on conformational change of tubulin which is consistent with previous observations in the following three independent assays: 1) limited proteolysis pattern, 2) intrinsic tryptophan fluorescence and 3) exposure of hydrophobic area on the tubulin molecule. We therefore examined whether eribulin binds to the vinca or paclitaxel site of tubulin in competitive inhibition assay on SPR. Vinca alkaloids and paclitaxel failed to inhibit the binding of tubulin to immobilized eribulin chemical probe. Similarly, eribulin and paclitaxel did not inhibit the binding of tubulin to immobilized vinblastine chemical probe. Thus, the eribulin binding site on tubulin is different from that of vinca alkaloids and paclitaxel. In addition, we further investigated the binding affinities and characteristics of interaction of eribulin with GTP- or GDP-forms of tubulin, respectively. Eribulin chemical probe binds both forms of tubulin specifically with significantly higher affinity (∼40 nM) as compared with vinblastine chemical probe (∼1 μM). Interestingly, we found that eribulin chemical probe preferentially recognizes GTP- over GDP-tubulin and dissociation of GTP-form from eribulin chemical probe was slower than that of GDP-form of tubulin. In contrast, vinblastine chemical probe did not show significant difference in binding to the two nucleotide states of tubulin. This indicates that eribulin distinguishes different nucleotide state of tubulin and binds to GTP-tubulin more tightly. These characteristic features of eribulin differ from those of vinca alkaloids and paclitaxel. Overall, our results suggest that eribulin exerts its unique antimitotic activity by inhibiting microtubule assembly by binding with high affinity at a novel tubulin site at the plus-end of the growing microtubule. This may potentially relate to the mechanism of eribulin clinical efficacy and diminished neuronal toxicity.

Citation Format: Kishan Lal Agarwala, Kenji Kubara, Koji Sagane, Boris M. Seletsky, Bruce A. Littlefield. Eribulin recognizes the nucleotide states of soluble tubulin and binds with high-affinity on a site distinct from known tubulin inhibitors. [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 4493. doi:10.1158/1538-7445.AM2013-4493

Abstract 899: Characterization and establishment of a drug-resistant human lung cancer model

Introduction In 2012, lung cancer is expected to be the leading cause of cancer death in the United States, accounting for about 28% of all cancer deaths. Eribulin mesylate (Halaven®) is a microtubule-dynamics inhibitor approved for third line clinical use in patients with heavily pretreated metastatic breast cancer based upon statistically significant increase in median overall survival (OS) compared to treatment of physician's choice.

Methods H522-T1 human non-small cell lung cancer subcutaneous xenograft tumors were established in nude mice that were subsequently treated with a sub-optimal dose of eribulin (half MTD). Two to three days after final drug administration, residual tumors were harvested and implanted again into naïve nude mice. These tumors were treated again with the same regimen and the residual tumors were again collected and re-implanted into naïve nude mice for another cycle of selection. A cell line named H522-T5er was established from the 4th cycle of residual tumor.

Results In vitro, H522-T5er showed strikingly increased resistance to eribulin compared to the parental cell line (82-fold). In vivo, H522-T5er xenograft tumors also showed high resistance to eribulin dosed at MTD. Further flow cytometry analysis identified enrichment of a CD44+/CD24- subpopulation of H522-T5er cells, suggesting stem/progenitor cell properties of H522-T5er. Expression profiling analysis of parental tumors and drug-selected residual tumors, as well as H522-T1 and H522-T5er cells were conducted. A total of 71 genes with more than 2-fold changes were identified in the comparison between parental tumor and drug-selected residual tumors, while 187 genes showed more than 2-fold changes between H522-T1 and H522-T5er cells. Twelve genes were identified in both comparison and only 5 of them were changed in the same direction. Protein levels of several drug pumps reported to be frequently involved in drug resistance were also determined by Western blot analysis. No significant increase of Pgp (1.3-fold) was identified in H522-T5er cells. Both BCRP and LRP showed about a 2-fold increase in H522-T5er cells, while a slight decrease of MRP1 (20%) was observed in H522-T5er cells.

Conclusions An eribulin-resistant cell line, not characterized by significant increase of Pgp expression, has been generated. It provides a valuable tool to develop and test agents which can target residual tumor after eribulin treatment.

Citation Format: Kuan-Chun Huang, Judith Oestreicher, Natalie Twine, Winnie Lee, Xingfeng Bao, Sergei Agoulnik, Bruce A. Littlefield, Mary Woodall-Jappe, Kenichi Nomoto. Characterization and establishment of a drug-resistant human lung cancer model. [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 899. doi:10.1158/1538-7445.AM2013-899

Broad spectrum preclinical antitumor activity of eribulin (Halaven(R)): optimal effectiveness under intermittent dosing conditions

BACKGROUND

Eribulin is a pharmaceutically and structurally optimized analog of the marine sponge natural product halichondrin B. Its salt form, eribulin mesylate (Halaven®) is clinically used in the United States, the European Union, and Japan for the treatment of heavily pretreated patients with metastatic breast cancer, who previously received an anthracycline and a taxane. Early preclinical studies of this new inhibitor of microtubule dynamics showed high antitumor potency towards several human cancer types in vitro and in vivo. Here we extend those early studies by examining the effects of eribulin against a wider spectrum of human tumor xenografts in vivo, and by directly comparing the in vivo effectiveness of different dosing administration schedules.

MATERIALS AND METHODS

In single-schedule studies, in vivo activity of eribulin against HT-1080 fibrosarcoma, U251 glioblastoma, SR-475 head and neck cancer, SK-LMS-1 leiomyosarcoma, NCI-H322M and NCI-H522 non-small cell lung cancer (NSCLC), PANC-1 pancreatic cancer, and NCI-H82 small cell lung cancer (SCLC) xenografts was examined at dose levels of 0.19-4.0 mg/kg using q2d×3(×3), q4d×3, q4d×4, and q7d×2 schedules. Administration schedule dependence was evaluated by directly comparing q1d×5, q2d×3(×3), q4d×3, and q7d×3 schedules in the MDA-MB-435 breast cancer xenograft model, using conditions of equivalent total dosing over the course of the experiment.

RESULTS

In single-schedule studies, maximum tolerated dose (MTD) values (or maximal 'at or below MTD' values) ranged from 0.8-1.7 mg/kg. In vivo antitumor responses at these dosing levels included tumor growth inhibition, stasis, and regression; several studies showing regression also yielded long-term tumor-free survivors. Effectiveness of eribulin showed model-to-model variability that appeared to be unrelated to dose level or administration schedule, suggesting that characterization of models with differing eribulin sensitivities may reveal potential biomarker strategies. Results of the dose schedule comparison study in the MDA-MB-435 model suggested the following order of effectiveness and tolerability: q2d×3(×3)>q4d×3≈q7d×3>> q1d×5. Moderately intermittent dosing thus shows optimal preclinical effectiveness, in good agreement with the approved intermittent clinical schedule for eribulin (days 1 and 8 of a 21-day cycle).

CONCLUSION

The current results show that eribulin has broad spectrum preclinical antitumor activity against a wide variety of human cancer types, and indicate that maximum effectiveness and optimal tolerability are obtained using moderately intermittent dosing schedules.

Heavy metal and pesticide content in commonly prescribed individual raw Chinese Herbal Medicines

Heavy metal and pesticide contamination has previously been reported in Chinese Herbal Medicines (CHMs), in some cases at potentially toxic levels. This study was conducted to determine general patterns and toxicological significance of heavy metal and pesticide contamination in a broad sample of raw CHMs. Three-hundred-thirty-four samples representing 126 species of CHMs were collected throughout China and examined for arsenic, cadmium, chromium, lead, and mercury. Of the total, 294 samples representing 112 species were also tested for 162 pesticides. At least 1 metal was detected in all 334 samples (100%) and 115 samples (34%) had detectable levels of all metals. Forty-two different pesticides were detected in 108 samples (36.7%), with 1 to 9 pesticides per sample. Contaminant levels were compared to toxicological reference values in the context of different exposure scenarios. According to a likely scenario of CHM consumption, only 3 samples (1%) with heavy metals and 14 samples (5%) with pesticides were found with concentrations that could contribute to elevated background levels of contaminant exposure. According to the most conservative scenario of CHM consumption, 231 samples (69%) with heavy metals and 81 samples (28%) with pesticides had contaminants that could contribute to elevated levels of exposure. Wild collected plants had higher contaminant levels than cultivated samples. Cadmium, chromium, lead, and chlorpyrifos contamination showed weak correlations with geographic location. Based on our assumptions of the likely mode of consumption of raw CHMs, the vast majority (95%) of the 334 samples in this study contained levels of heavy metals or pesticides that would be of negligible concern. However, given the number of samples with detectable contaminants and the range between the more likely and more conservative scenarios of contaminant exposure, more research and monitoring of heavy metals (especially cadmium and chromium) and pesticide residues (especially chlorpyrifos) in raw CHMs are advised.

In vitro and in vivo anticancer activity of (+)-spongistatin 1

The marine natural product (+)-spongistatin 1 is an extremely potent growth inhibitory agent having activity against a wide variety of cancer cell lines, while exhibiting low cytotoxicity against quiescent human fibroblasts. Consistent with a microtubule-targeting mechanism of action, (+)-spongistatin 1 causes mitotic arrest in DU145 human prostate cancer cells. More importantly, (+)-spongistatin 1 exhibits significant in vivo antitumor activity in the LOX-IMVI human melanoma xenograft model. (+)-Spongistatin 1 is, thus, an important class of microtubule targeting anticancer agent that warrants further investigation.

Effective antibody therapy induces host-protective antitumor immunity that is augmented by TLR4 agonist treatment

Toll-like receptors are potent activators of the innate immune system and generate signals leading to the initiation of the adaptive immune response that can be utilized for therapeutic purposes. We tested the hypothesis that combined treatment with a Toll-like receptor agonist and an antitumor monoclonal antibody is effective and induces host-protective antitumor immunity. C57BL/6 human mutated HER2 (hmHER2) transgenic mice that constitutively express kinase-deficient human HER2 under control of the CMV promoter were established. These mice demonstrate immunological tolerance to D5-HER2, a syngeneic human HER2-expressing melanoma cell line. This human HER2-tolerant model offers the potential to serve as a preclinical model to test both antibody therapy and the immunization potential of human HER2-targeted therapeutics. Here, we show that E6020, a Toll-like receptor-4 (TLR4) agonist effectively boosted the antitumor efficacy of the monoclonal antibody trastuzumab in immunodeficient C57BL/6 SCID mice as well as in C57BL/6 hmHER2 transgenic mice. E6020 and trastuzumab co-treatment resulted in significantly greater inhibition of tumor growth than was observed with either agent individually. Furthermore, mice treated with the combination of trastuzumab and the TLR4 agonist were protected against rechallenge with human HER2-transfected tumor cells in hmHER2 transgenic mouse strains. These findings suggest that combined treatment with trastuzumab and a TLR4 agonist not only promotes direct antitumor effects but also induces a host-protective human HER2-directed adaptive immune response, indicative of a memory response. These data provide an immunological rationale for testing TLR4 agonists in combination with antibody therapy in patients with cancer.

Comparison of neuropathy-inducing effects of eribulin mesylate, paclitaxel, and ixabepilone in mice

Chemotherapy-induced neurotoxicity is a significant problem associated with successful treatment of many cancers. Tubulin is a well-established target of antineoplastic therapy; however, tubulin-targeting agents, such as paclitaxel and the newer epothilones, induce significant neurotoxicity. Eribulin mesylate, a novel microtubule-targeting analogue of the marine natural product halichondrin B, has recently shown antineoplastic activity, with relatively low incidence and severity of neuropathy, in metastatic breast cancer patients. The mechanism of chemotherapy-induced neuropathy is not well understood. One of the main underlying reasons is incomplete characterization of pathology of peripheral nerves from treated subjects, either from patients or preclinically from animals. The current study was conducted to directly compare, in mice, the neuropathy-inducing propensity of three drugs: paclitaxel, ixabepilone, and eribulin mesylate. Because these drugs have different potencies and pharmacokinetics, we compared them on the basis of a maximum tolerated dose (MTD). Effects of each drug on caudal and digital nerve conduction velocity, nerve amplitude, and sciatic nerve and dorsal root ganglion morphology at 0.25 × MTD, 0.5 × MTD, 0.75 × MTD, and MTD were compared. Paclitaxel and ixabepilone, at their respective MTDs, produced significant deficits in caudal nerve conduction velocity, caudal amplitude and digital nerve amplitudes, as well as moderate to severe degenerative pathologic changes in dorsal root ganglia and sciatic nerve. In contrast, eribulin mesylate produced no significant deleterious effects on any nerve conduction parameter measured and caused milder, less frequent effects on morphology. Overall, our findings indicate that eribulin mesylate induces less neuropathy in mice than paclitaxel or ixabepilone at equivalent MTD-based doses.

Novel second generation analogs of eribulin. Part I: Compounds containing a lipophilic C32 side chain overcome P-glycoprotein susceptibility

Eribulin mesylate (Halaven™), a totally synthetic analog of the marine polyether macrolide halichondrin B, has recently been approved in the United States as a treatment for breast cancer. It is also currently under regulatory review in Japan and the European Union. Our continuing medicinal chemistry efforts on this scaffold have focused on oral bioavailability, brain penetration and efficacy against multidrug resistant (MDR) tumors by lowering the susceptibility of these compounds to P-glycoprotein (P-gp)-mediated drug efflux. Replacement of the 1,2-amino alcohol C32 side chain of eribulin with fragments neutral at physiologic pH led to the identification of analogs with significantly lower P-gp susceptibility. The analogs maintained low- to sub-nM potency in vitro against both sensitive and MDR cell lines. Within this series, increasing lipophilicity generally led to decreased P-gp susceptibility. In addition to potency in cell culture, these compounds showed in vivo activity in mouse xenograft models.

Eribulin induces irreversible mitotic blockade: implications of cell-based pharmacodynamics for in vivo efficacy under intermittent dosing conditions

Eribulin (E7389), a mechanistically unique microtubule inhibitor in phase III clinical trials for cancer, exhibits superior efficacy in vivo relative to the more potent compound ER-076349, a fact not explained by different pharmacokinetic properties. A cell-based pharmacodynamic explanation was suggested by observations that mitotic blockade induced by eribulin, but not ER-076349, is irreversible as measured by a flow cytometric mitotic block reversibility assay employing full dose/response treatment. Cell viability 5 days after drug washout established relationships between mitotic block reversibility and long-term cell survival. Similar results occurred in U937, Jurkat, HL-60, and HeLa cells, ruling out cell type-specific effects. Studies with other tubulin agents suggest that mitotic block reversibility is a quantifiable, compound-specific characteristic of antimitotic agents in general. Bcl-2 phosphorylation patterns parallel eribulin and ER-076349 mitotic block reversibility patterns, suggesting persistent Bcl-2 phosphorylation contributes to long-term cell-viability loss after eribulin's irreversible blockade. Drug uptake and washout/retention studies show that [3H]eribulin accumulates to lower intracellular levels than [3H]ER-076349, yet is retained longer and at higher levels. Similar findings occurred with irreversible vincristine and reversible vinblastine, pointing to persistent cellular retention as a component of irreversibility. Our results suggest that eribulin's in vivo superiority derives from its ability to induce irreversible mitotic blockade, which appears related to persistent drug retention and sustained Bcl-2 phosphorylation. More broadly, our results suggest that compound-specific reversibility characteristics of antimitotic agents contribute to interactions between cell-based pharmacodynamics and in vivo pharmacokinetics that define antitumor efficacy under intermittent dosing conditions.

Developing a library of authenticated Traditional Chinese Medicinal (TCM) plants for systematic biological evaluation--rationale, methods and preliminary results from a Sino-American collaboration

While the popularity of and expenditures for herbal therapies (aka "ethnomedicines") have increased globally in recent years, their efficacy, safety, mechanisms of action, potential as novel therapeutic agents, cost-effectiveness, or lack thereof, remain poorly defined and controversial. Moreover, published clinical trials evaluating the efficacy of herbal therapies have rightfully been criticized, post hoc, for their lack of quality assurance and reproducibility of study materials, as well as a lack of demonstration of plausible mechanisms and dosing effects. In short, clinical botanical investigations have suffered from the lack of a cohesive research strategy which draws on the expertise of all relevant specialties. With this as background, US and Chinese co-investigators with expertise in Traditional Chinese Medicine (TCM), botany, chemistry and drug discovery, have jointly established a prototype library consisting of 202 authenticated medicinal plant and fungal species that collectively represent the therapeutic content of the majority of all commonly prescribed TCM herbal prescriptions. Currently housed at Harvard University, the library consists of duplicate or triplicate kilogram quantities of each authenticated and processed species, as well as "detanninized" extracts and sub-fractions of each mother extract. Each species has been collected at 2-3 sites, each separated geographically by hundreds of miles, with precise GPS documentation, and authenticated visually and chemically prior to testing for heavy metals and/or pesticides contamination. An explicit decision process has been developed whereby samples with the least contamination were selected to undergo ethanol extraction and HPLC sub-fractionation in preparation for high throughput screening across a broad array of biological targets including cancer biology targets. As envisioned, the subfractions in this artisan collection of authenticated medicinal plants will be tested for biological activity individually and in combinations (i.e., "complex mixtures") consistent with traditional ethnomedical practice. This manuscript summarizes the rationale, methods and preliminary "proof of principle" for the establishment of this prototype, authenticated medicinal plant library. It is hoped that these methods will foster scientific discoveries with therapeutic potential and enhance efforts to systematically evaluate commonly used herbal therapies worldwide.

Eribulin binds at microtubule ends to a single site on tubulin to suppress dynamic instability

Eribulin mesylate (E7389), a synthetic analogue of the marine natural product halichondrin B, is in phase III clinical trials for the treatment of cancer. Eribulin targets microtubules, suppressing dynamic instability at microtubule plus ends through an inhibition of microtubule growth with little or no effect on shortening [Jordan, M. A., et al. (2005) Mol. Cancer Ther. 4, 1086-1095]. Using [(3)H]eribulin, we found that eribulin binds soluble tubulin at a single site; however, this binding is complex with an overall K(d) of 46 microM, but also showing a real or apparent very high affinity (K(d) = 0.4 microM) for a subset of 25% of the tubulin. Eribulin also binds microtubules with a maximum stoichiometry of 14.7 +/- 1.3 molecules per microtubule (K(d) = 3.5 microM), strongly suggesting the presence of a relatively high-affinity binding site at microtubule ends. At 100 nM, the concentration that inhibits microtubule plus end growth by 50%, we found that one molecule of eribulin is bound per two microtubules, indicating that the binding of a single eribulin molecule at a microtubule end can potently inhibit its growth. Eribulin does not suppress dynamic instability at microtubule minus ends. Preincubation of microtubules with 2 or 4 microM vinblastine induced additional lower-affinity eribulin binding sites, most likely at splayed microtubule ends. Overall, our results indicate that eribulin binds with high affinity to microtubule plus ends and thereby suppresses dynamic instability.

Abstract LB-324: A toll-like receptor 4 agonist enhances the efficacy of trastuzumab therapy and promotes adaptive immunity and long-term protection against a human ErbB-2 (HER2)-transfected syngeneic tumor in a human HER2 transgenic mouse model

Antibodies against cancer-relevant targets can recruit Fc receptor-bearing myeloid and natural killer cells to mediate antibody dependent cellular cytotoxicity (ADCC). Toll-like receptors are potent activators of the innate immune system and generate signals leading to the initiation of an adaptive immune response that can be utilized for therapeutic purposes. We tested the efficacy of E6020, a TLR4 agonist, administered in combination with the clinically approved anti-HER2 antibody trastuzumab. A four-week course of therapy with trastuzumab and E6020 protected against the outgrowth of syngeneic D5 melanoma cells transfected with the human tumor antigen HER2 in immunodeficient C57BL/6 SCID mice, in immunocompetent wild-type C57BL/6 mice as well as in human HER2-tolerant C57BL/6 hmHER2 transgenic mice. In each tested strain of mice, the addition of E6020 to trastuzumab significantly increased the proportion of animals that survived initial tumor challenge compared to trastuzumab treatment alone (83% vs. 33% in C57BL/6 hmHER2 transgenic mice; p &lt; 0.05). To determine if apparently cured mice had been effectively immunized against human HER2, the transgenic mice were inoculated subcutaneously with 103 D5-HER2 cells and then received a 4-week course of trastuzumab+E6020, or trastuzumab alone. Animals that survived the primary tumor challenge were followed for at least 120 days and then rechallenged with 5 × 103 D5-HER2 or unmodified D5 cells and followed for a minimum of 60 additional days. Mice that had been initially treated with the combination of trastuzumab and E6020 demonstrated superior protection (22/30; 73%) to those treated with trastuzumab alone (16/36; 44%; p &lt; 0.05) against rechallenge with D5-HER2, but no protection was seen against D5 challenge. Both CD4 and CD8 T-cells were required for full protection against rechallenge. These findings suggest that trastuzumab therapy can induce a host-protective adaptive immune response. Moreover, combined treatment with trastuzumab and a TLR4 agonist enhances both the direct anti-tumor effects of trastuzumab and a host-protective human HER2-directed adaptive immune response.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-324.

Tubulin-based antimitotic mechanism of E7974, a novel analogue of the marine sponge natural product hemiasterlin

E7974 is a synthetic analogue of the marine sponge natural product hemiasterlin. Here, we show that E7974, such as parental hemiasterlin, acts via a tubulin-based antimitotic mechanism. E7974 inhibits polymerization of purified tubulin in vitro with IC(50) values similar to those of vinblastine. In cultured human cancer cells, E7974 induces G(2)-M arrest and marked disruption of mitotic spindle formation characteristic of tubulin-targeted anticancer drugs. Extensive hypodiploid cell populations are seen in E7974-treated cells, indicating initiation of apoptosis after prolonged G(2)-M blockage. Consistent with this observation, E7974 induces caspase-3 activation and poly ADP ribose polymerase cleavage, typical biochemical markers of apoptosis. Only a short cellular exposure to E7974 is sufficient to induce maximum mitotic arrest, suggesting that E7974's antitumor effects in vivo may persist even after blood levels of the drug decrease after drug administration. Interactions of E7974 with purified tubulin were investigated using two synthetic tritiated photoaffinity analogues incorporating a benzophenone photoaffinity moiety at two different positions of the E7974 scaffold. Both analogues preferentially photolabeled alpha-tubulin, although minor binding to beta-tubulin was also detected. E7974 thus seems to share a unique, predominantly alpha-tubulin-targeted mechanism with other hemiasterlin-based compounds, suggesting that, unlike many tubulin-targeted natural products and related drugs, the hemiasterlins evolved to mainly target alpha-tubulin, not beta-tubulin subunits.

Potent in vitro and in vivo anticancer activities of des-methyl, des-amino pateamine A, a synthetic analogue of marine natural product pateamine A

We report here that des-methyl, des-amino pateamine A (DMDA-PatA), a structurally simplified analogue of the marine natural product pateamine A, has potent antiproliferative activity against a wide variety of human cancer cell lines while showing relatively low cytotoxicity against nonproliferating, quiescent human fibroblasts. DMDA-PatA retains almost full in vitro potency in P-glycoprotein-overexpressing MES-SA/Dx5-Rx1 human uterine sarcoma cells that are significantly resistant to paclitaxel, suggesting that DMDA-PatA is not a substrate for P-glycoprotein-mediated drug efflux. Treatment of proliferating cells with DMDA-PatA leads to rapid shutdown of DNA synthesis in the S phase of the cell cycle. Cell-free studies show that DMDA-PatA directly inhibits DNA polymerases α and γ in vitro albeit at concentrations considerably higher than those that inhibit cell proliferation. DMDA-PatA shows potent anticancer activity in several human cancer xenograft models in nude mice, including significant regressions observed in the LOX and MDA-MB-435 melanoma models. DMDA-PatA thus represents a promising natural product-based anticancer agent that warrants further investigation.

Inhibition of centromere dynamics by eribulin (E7389) during mitotic metaphase

Eribulin (E7389), a synthetic analogue of halichondrin B in phase III clinical trials for breast cancer, binds to tubulin and microtubules. At low concentrations, it suppresses the growth phase of microtubule dynamic instability in interphase cells, arrests mitosis, and induces apoptosis, suggesting that suppression of spindle microtubule dynamics induces mitotic arrest. To further test this hypothesis, we measured the effects of eribulin on dynamics of centromeres and their attached kinetochore microtubules by time-lapse confocal microscopy in living mitotic U-2 OS human osteosarcoma cells. Green fluorescent protein-labeled centromere-binding protein B marked centromeres and kinetochore-microtubule plus-ends. In control cells, sister chromatid centromere pairs alternated under tension between increasing and decreasing separation (stretching and relaxing). Eribulin suppressed centromere dynamics at concentrations that arrest mitosis. At 60 nmol/L eribulin (2 x mitotic IC(50)), the relaxation rate was suppressed 21%, the time spent paused increased 67%, and dynamicity decreased 35% (but without reduction in mean centromere separation), indicating that eribulin decreased normal microtubule-dependent spindle tension at the kinetochores, preventing the signal for mitotic checkpoint passage. We also examined a more potent, but in tumors less efficacious antiproliferative halichondrin derivative, ER-076349. At 2 x IC(50) (4 nmol/L), mitotic arrest also occurred in concert with suppressed centromere dynamics. Although media IC(50) values differed 15-fold between the two compounds, the intracellular concentrations were similar, indicating more extensive relative uptake of ER-076349 into cells compared with eribulin. The strong correlation between suppression of kinetochore-microtubule dynamics and mitotic arrest indicates that the primary mechanism by which eribulin blocks mitosis is suppression of spindle microtubule dynamics.

In vitro and in vivo anticancer activities of synthetic (-)-laulimalide, a marine natural product microtubule stabilizing agent

Laulimalide is a cytotoxic natural product isolated from marine sponges. It is structurally distinct from taxanes. However, like paclitaxel, laulimalide binds to tubulin and enhances microtubule assembly and stabilization. It exhibits potent inhibition of cellular proliferation with IC50 values in the low nM range against numerous cancer cell lines. In contrast to paclitaxel, however, laulimalide is also very potent against multidrug-resistant (MDR) cancer cell lines which overexpress P-glycoprotein (PgP). It has unique structural and biological properties, and attempts at synthesis have attracted considerable effort in recent years, resulting in more than ten published total syntheses. Despite this extensive attention, there have been no reported in vivo evaluations of laulimalide to date, probably due to the structural complexity of laulimalide and the scarcity of natural material. In our studies to explore the therapeutic potential of laulimalide, a total synthesis capable of producing gram quantities of laulimalide was designed, which enabled both in vitro and in vivo evaluation. Our in vitro results with synthetic material confirmed the previous reports that laulimalide is a mitotic blocker that can inhibit the growth of a variety of both non-MDR and MDR human cancer cell lines. However, despite demonstrating promise in cell-based and pharmacokinetic studies, laulimalide exhibited only minimal tumor growth inhibition in vivo and was accompanied by severe toxicity and mortality. The unfavorable efficacy to toxicity ratio in vivo suggests that laulimalide may have limited value for development as a new anticancer therapeutic agent.

Preferential inducibility of CYP1A1 and CYP1A2 by TCDD: Differential regulation in primary human hepatocytes versus transformed human cells

Cytochrome P4501A1 (CYP1A1) induction, a marker of aryl hydrocarbon (Ah) receptor activation, has been associated with carcinogenicity of the environmental agent 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Consistently, we show that TCDD treatment led to induction of CYP1A1 in responsive human cancer cell lines including HepG2, LS174T, and MCF-7, as determined by Western blotting and CYP1A form-selective R-warfarin 6- and 8-hydroxylation. TCDD, however, preferably induced CYP1A2, not CYP1A1, in primary human hepatocytes. Such CYP1A form-preferred induction at the protein level was apparently uncorrelated with non-preferred mRNA induction in any cells studied. Moreover, while both genes were up-regulated by TCDD in primary hepatocytes and HepG2 cells, the induction of CYP1A1 and CYP1A2 at the mRNA level was distinguishable, indicated by the marked differences in activation kinetics and the response to the protein synthesis inhibitors, anisomycin and cycloheximide. Furthermore, formation of total benzo(a)pyrene (BaP)-DNA adducts was not altered following BaP exposure in TCDD-treated primary hepatocytes, whereas significantly elevated, in a CYP1A1-dependent manner, in the treated HepG2 cells. Taken together, our findings, demonstrating the complexities of TCDD-associated human Ah receptor function and differential regulations of CYP 1A enzymes, suggest clearly the need for caution when extrapolating data obtained in cell-based models.

The primary antimitotic mechanism of action of the synthetic halichondrin E7389 is suppression of microtubule growth

E7389, which is in phase I and II clinical trials, is a synthetic macrocyclic ketone analogue of the marine sponge natural product halichondrin B. Whereas its mechanism of action has not been fully elucidated, its main target seems to be tubulin and/or the microtubules responsible for the construction and proper function of the mitotic spindle. Like most microtubule-targeted antitumor drugs, it inhibits tumor cell proliferation in association with G(2)-M arrest. It binds to tubulin and inhibits microtubule polymerization. We examined the mechanism of action of E7389 with purified microtubules and in living cells and found that, unlike antimitotic drugs including vinblastine and paclitaxel that suppress both the shortening and growth phases of microtubule dynamic instability, E7389 seems to work by an end-poisoning mechanism that results predominantly in inhibition of microtubule growth, but not shortening, in association with sequestration of tubulin into aggregates. In living MCF7 cells at the concentration that half-maximally blocked cell proliferation and mitosis (1 nmol/L), E7389 did not affect the shortening events of microtubule dynamic instability nor the catastrophe or rescue frequencies, but it significantly suppressed the rate and extent of microtubule growth. Vinblastine, but not E7389, inhibited the dilution-induced microtubule disassembly rate. The results suggest that, at its lowest effective concentrations, E7389 may suppress mitosis by directly binding to microtubule ends as unliganded E7389 or by competition of E7389-induced tubulin aggregates with unliganded soluble tubulin for addition to growing microtubule ends. The result is formation of abnormal mitotic spindles that cannot pass the metaphase/anaphase checkpoint.

Macrocyclic ketone analogues of halichondrin B

Structurally simplified macrocyclic ketone analogues of halichondrin B were prepared by total synthesis and found to retain the potent cell growth inhibitory activity in vitro, stability in mouse serum, and in vivo efficacy of the natural product.

Structurally simplified macrolactone analogues of halichondrin B

A structurally simplified macrolactone analogue of halichondrin B was identified that retains the potent cell growth inhibitory activity of the natural product in vitro.

Induction of Morphological and Biochemical Apoptosis following Prolonged Mitotic Blockage by Halichondrin B Macrocyclic Ketone Analog E7389

E7389, a macrocyclic ketone analog of the marine natural product halichondrin B, currently is undergoing clinical trials for cancer. This fully synthetic agent exerts its highly potent in vitro and in vivo anticancer effects via tubulin-based antimitotic mechanisms, which are similar or identical to those of parental halichondrin B. In an attempt to understand the impressive potency of E7389 in animal models of human cancer, its ability to induce apoptosis following prolonged mitotic blockage was evaluated. Treatment of U937 human histiocytic lymphoma cells with E7389 led to time-dependent collection of cells in the G2-M phase of the cell cycle, beginning as early as 2 h and becoming maximal by 12 h. Increased numbers of hypodiploid events were seen beginning at 12 h, suggesting initiation of apoptosis after prolonged E7389-induced mitotic blockage. The identity of hypodiploid events as apoptotic cells under these conditions was confirmed by two additional morphologic criteria: green to orange/yellow shifts on acridine orange/ethidium bromide staining, and cell surface annexin V binding as assessed by flow cytometry. Several biochemical correlates of apoptosis also were seen following E7389 treatment, including phosphorylation of the antiapoptotic protein Bcl-2, cytochrome c release from mitochondria, proteolytic activation of caspase-3 and -9, and cleavage of the caspase-3 substrate poly(ADP-ribose) polymerase (PARP). In LNCaP human prostate cancer cells, treatment with E7389 also led to generation of hypodiploid cells, activation of caspase-3 and -9, and appearance of cleaved PARP, indicating that E7389 can activate cellular apoptosis pathways under anchorage-independent and -dependent cell culture conditions. These results show that prolonged mitotic blockage by E7389 can lead to apoptotic cell death of human cancer cells in vitro and can provide a mechanistic basis for the significant in vivo anticancer efficacy of E7389.

Synthesis and biological evaluation of (−)-laulimalide analogues

Analogues of the marine natural product (-)-laulimalide were prepared by total synthesis and evaluated in vitro for anticancer activity.

Anti-angiogenesis Effects of Borrelidin are Mediated through Distinct Pathways: Threonyl-tRNA Synthetase and Caspases are Independently Involved in Suppression of Proliferation and Induction of Apoptosis in Endothelial Cells

Borrelidin, an antibiotic with anti-angiogenic activity, not only suppresses new capillary tube formation, but also collapses formed capillary tubes in a rat aorta culture model. Since it selectively inhibits threonyl-tRNA synthetase, we examined the effect of threonine on its anti-angiogenic activity. We found that a high concentration of threonine (1 mM) attenuated the ability of borrelidin to inhibit both capillary tube formation in the rat aorta culture model and human umbilical vein endothelial cells (HUVEC) proliferation, yet did not affect the ability of borrelidin to collapse formed capillary tubes or to induce apoptosis in HUVEC. Borrelidin activated caspase-3 and -8, and inhibitors of both caspase-3 and -8 suppressed borrelidin-induced apoptosis in HUVEC. Taken together, these data suggest that the anti-angiogenic effects of borrelidin are mediated through at least two mechanisms, i.e. one threonine-dependent and the other threonine-independent, and borrelidin induces apoptosis in endothelial cells via the caspase-8/-3 pathway.