Abstract P5-05-01: A molecularly annotated collection of breast cancer patient-derived xenograft models aligned with ongoing clinical trials built from fine needle aspiration samples throughout neoadjuvant treatment

BACKGROUND: Patient-derived xenograft (PDX) models of breast cancer replicate the diverse histologic and molecular features of patient tumors and provide a renewable source of human tumor tissue. However, collection of tissue by core needle biopsy is problematic due to patient discomfort, bleeding risk and the limited number of passes a patient can tolerate. Several studies have catalogued the maintenance of molecular features of patient tumors in PDX models of breast cancer.

METHODS: To support the neoadjuvant molecular diagnostic and drug development program in triple negative breast cancer (TNBC), a pilot study was conducted to determine if fine needle aspiration (FNA) could be used for building PDX models. Subsequently, PDX models are being established in alignment with ongoing clinical trials at MDACC. The molecular evolution of patient's tumors, matched with PDXs engrafted from their tumors, is under study throughout the neoadjuvant treatment of TNBC using RNA sequencing, whole-exome sequencing, deep sequencing of cancer genes, and histologic analyses.

RESULTS: To date, 20 established PDX models have been developed and stable PDX models continue to be generated at a rate of 2-3 per month. Several of these models are derived from serial FNAs derived from patients throughout neoadjuvant treatment. These models retain histologic and molecular features of the original patient tumors. Serial patient biopsies, matched with PDX models, have enabled measurement of the mutational and transcriptomic evolution in vivo of TNBC undergoing neoadjuvant treatment.

We have standardized the use of FNAs to generate PDX models both pre- and post-neoadjuvant therapy in the following ongoing neoadjuvant clinical trials:

1. MDACC 2014-0185 (PI Stacy Moulder, 360 patients), 'ARTEMIS: A Randomized TNBC-Enrolling trial to confirm Molecular profiling Improves Survival'

2. MDACC 2014-0045 (PI Jennifer Litton, 20+ patients), 'A pilot study of BMN673 as a neoadjuvant study in patients with a diagnosis of invasive breast cancer and a deleterious BRCA mutation'

CONCLUSION: We demonstrated that PDX models from tissue collected by FNA recapitulate the biology and clinical course of the patient's tumor. Sequencing analyses revealed that neoadjuvant chemotherapy and PDX engraftment enrich for cancer gene mutations. We observe association of the rate of successful PDX engraftment with clinical parameters such as the patient's residual cancer burden (RCB) status at the time of surgery (upon completion of neoadjuvant treatment). In addition, we observe that PDX models derived from serial patient biopsies throughout treatment are more resistant to chemotherapy treatment. These models recapitulate the variety of chemotherapy responses observed in patients with TNBC and serve as powerful tools for preclinical biomarker and discovery studies.

Citation Format: Echeverria GV, Cai S, Tu Y, McCoy A, Lau R, Redwood A, Rauch G, Adrada B, Candelaria R, Santiago L, Thompson A, Litton J, Moulder S, Symmans F, Chang JT, Piwnica-Worms H. A molecularly annotated collection of breast cancer patient-derived xenograft models aligned with ongoing clinical trials built from fine needle aspiration samples throughout neoadjuvant treatment [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-05-01.

Abstract P6-06-01: Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress

Agents that induce replication stress, such as inhibitors of Chk1 or ATR, are advancing in clinical development and are being tested for treatment of various solid tumors, including triple-negative breast cancer (TNBC). While the preclinical data are encouraging, additional studies are needed to predict with precision (i) which patients will most likely benefit from these inhibitors, (ii) the genetic and proteomic contexts in which these inhibitors will provide maximum therapeutic benefit as a single agent, or require additional sensitization via combination with a targeted- or chemotherapeutic agent, and (iii) exactly which targeted/chemotherapeutic agent will provide maximum therapeutic benefit for combination with replication stress inducers. To address these challenges in TNBC, we have attempted to gain a comprehensive understanding of how the DNA damage response pathways regulate TNBC cell survival in response to Chk1 inhibitors, by performing high throughput loss-of-function screens.

We have identified genes whose loss induces death of TNBC cells in the presence of (1) CHK1i alone, (2) chemotherapy alone or (3) CHK1i plus chemotherapy. In addition, given the role of TP53 as the most frequently mutated gene in TNBC, we also determined whether distinct vulnerabilities could be identified in TNBC cells that are p53-proficient versus p53-deficient. Thus, we have also identified the top synthetic lethal interactions that are either common to both p53-proficient and p53-deficient TNBC, or unique to p53-deficient TNBC; we are currently performing in vitro studies to validate the identified mechanisms. We anticipate these studies to be applicable to other agents that induce replication stress and cell cycle checkpoint bypass. Ongoing in vivo preclinical studies, which utilize patient-derived xenografts (PDXs) of TNBC to validate these findings are expected to impact patient selection for clinical trials, and also allow us to predict which chemotherapeutic agents will be most effective for combination with different cell-cycle checkpoint inhibitors.

Citation Format: Redwood AB, Seth S, Cai S, Piwnica-Worms H. Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-06-01.

Abstract P5-03-11: Targeted disruption of transcriptional effector GLI2 attenuates breast tumor growth and metastasis

We reported that transcriptional regulation by the major Hedgehog (HH) pathway effector GLI2 in mammary gland stromal cells coordinates a hormone-responsive niche signaling program that directs epithelial stem cell activity during puberty [Zhao, et al., Science. 2017 Apr 21;356(6335)]. HH signaling is a key growth pathway in human carcinogenesis. The first targeted therapies aimed at G-protein coupled receptor Smoothened (SMO) resulted in rapid acquired resistance, underscoring the need for developing new HH pathway-targeting therapies downstream of SMO. In BC GLI-dependent transcription may involve a non-receptor-based mechanism of GLI2 activation involving cross-talk from other signaling pathways such as TGFß54, PI3K55,56, Wnt, or NF-κB. Accordingly, we investigated the role of HH pathway and its transcriptional effectors using the MMTV-PyMT BC mouse model. Development of mammary tumors arising proceeds through neoplastic lesions ranging from carcinoma in situ (10-12 weeks) to highly invasive ductal carcinoma with high incidence of pulmonary metastasis at 16-18 weeks. Epithelial ablation of GLI2 dramatically reduced tumor progression. Cre recombinase under control of the Ck14 promoter to genetically ablate GLI2 in basal cells of the mammary gland thus dramatically attenuated metastasis despite continued (albeit reduced) formation of primary tumors. In human BC, we applied Bayesian methods to gene expression data to identify metastatic BC with HH pathway activation. Kaplan-Meier analysis demonstrated significantly worse progression-free survival in patients with HH pathway activity (Log-rank p = 0.0013). Next, we analyzed 1294 BC samples, stratified according to HH activity, using the HH probability as a continuous score ranging between 0 and 1, univariate Cox regression analysis supports the hypothesis that HH activity in BC is a risk factor for relapse after surgery, HR = 2.45 (95% CI: 1.67 – 3.61, p = 2.72e-6). There is also a significant difference in survival between HH-actives and HH-inactives (Log-rank p = 3.91e-4), suggesting a pathogenic role of GLI activation in BC progression and metastasis. Arsenic trioxide (ATO), inhibits HH pathway activity by de-stabilizing the GLI transcriptional effectors of HH signaling, likely due to arsenic displacement of zinc within the DNA-binding zinc fingers of GLI proteins. ATO treatment of BC cell lines resulted in dose-dependent cell growth inhibition (Alamar Blue) and induction of apoptosis (PARP cleavage and Annexin V expression) at clinically achievable concentrations. In NSG mice with orthotopic transplant of 3e5 SU151 human BC PDX, with strongly positive GLI-active signature, daily IP injection of 10mg/kg ATO resulted in marked tumor growth inhibition in vivo (2.75-fold smaller tumor diameter vs. PBS vehicle controls, P= 0.006; 55d vs. 17d to reach 1.5 cm criterion for ATO vs. vehicle). We conclude the major HH pathway transcriptional effector GLI2 coordinates a transcriptional program with a central role in BC growth and metastasis in a significant subset of BC, and that systemic treatment with ATO (new oral formulation in commercial development), will reduce metastatic progression and improve clinical outcomes in patients whose tumors harbor a GLI-active transcript signature.

Citation Format: Beachy P, Cai S, Ma Y, Hatakeyama J, Zhao C, Stoffels M, Verhaegh W, van de Stolpe A, Pegram M. Targeted disruption of transcriptional effector GLI2 attenuates breast tumor growth and metastasis [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-03-11.

Abstract GS6-06: Identifying metastatic drivers in patient-derived xenograft models of triple negative breast cancer

Metastases are responsible for the vast majority of deaths due to breast cancer. Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by high rates of metastasis and poor prognosis. We are employing patient derived xenograft (PDX) models of TNBC to identify drivers of metastasis. Tumor samples are obtained from the breast tumors of patients with TNBC and engrafted immediately into the humanized mammary fat pads of immune compromised mice. Lentiviral transduction was employed to express bioluminescent and fluorescent markers in two independent PDX models of TNBC. Using these models, we demonstrated that human breast tumors are capable of completing all stages of the metastatic cascade in mice, and metastatic lesions are observed in organs normally found in patients with metastatic breast cancer including lung, liver, bone, brain, and lymph nodes. Dynamic and reversible epithelial to mesenchymal transition (EMT) was observed as tumors metastasized to lung and were re-passaged to recipient mouse mammary glands. Lung metastases were isolated using bioluminescence imaging and lung metastasis gene expression signatures were generated. Metastasis signatures from two independent PDX models were compared to identify genes that were commonly de-regulated in lung metastases relative to corresponding mammary tumors. Comprehensive gain-of-function screens were then conducted in vivo to identify functional drivers of TNBC metastasis. Carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM5) was identified as a metastatic driver in this screen. CEACAM5 mRNA and protein levels were elevated in lung metastases relative to corresponding mammary gland tumors in mice. In addition, we demonstrated that CEACAM5 expression was upregulated in the lung metastases of breast cancer patients, and its expression inversely correlated with patient survival. Our data indicate that the metastatic function of CEACAM5 is to promote growth of breast tumors in the lung by inducing MET (mesenchymal to epithelial transition).

Citation Format: Powell E, Shao J, Picon HM, Ge Z, Echeverria GV, Peoples M, Bristow C, Cai S, Tu Y, McCoy AM, Piwnica-Worms D, Draetta G, Edwards JR, Moulder SL, Symmans WF, Heffernan TP, Liang H, Piwnica-Worms H. Identifying metastatic drivers in patient-derived xenograft models of triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr GS6-06.

Prognostic significance of tumour regression grade after neoadjuvant chemoradiotherapy for a cohort of patients with locally advanced rectal cancer: an 8-year retrospective single-institutional study

AIM

Locally advanced rectal cancer (LARC) is frequently treated with neoadjuvant chemoradiotherapy (NACRT) to reduce the risk of local recurrence and improve survival. Tumour response to NACRT is variable and may influence the prognosis after subsequent surgery. This study compared the prognostic values of tumour regression grade (TRG) and neoadjuvant pathological (ypTNM) downstaging in patients with Stage II and III rectal cancer treated with NACRT followed by curative surgery.

METHOD

This study included 185 patients with LARC treated with long-course radiotherapy (45 Gy in 25 fractions) plus 5-fluorouracil over 5 weeks between 2005 and 2013. We used multivariate analysis to assess the relationship of Dworak's five-tier TRG, ypTNM stage and ypTNM downstaging with clinicopathological factors, 5-year disease-free survival (DFS) and 5-year overall survival (OS).

RESULTS

Total regression (TRG4), good regression (TRG3), moderate regression (TRG2), minor regression (TRG1) and no regression (TRG0) were seen in 38 (20.6%), 65 (35.2%), 43 (23.2%), 28 (15.1%) and 11 (5.9%) patients, respectively. TNM downstaging following NACRT occurred in 109 (58.9%) patients. The 5-year DFS rates after NACRT for TRG0, TRG1, TRG2, TRG3 and TRG4 were 0%, 58.5%, 66.4%, 80.4% and 82.6%, respectively (P < 0.001). The ypTNM stage correlated with 5-year DFS (P = 0.004) but not 5-year OS (P = 0.075). Multivariate analysis demonstrated that TRG was related to both DFS and OS (P < 0.001).

CONCLUSION

TRG measured on a five-tier system was better than ypTNM stage for predicting outcome in patients with LARC treated with NACRT and surgery.

Blockade of β-catenin signaling attenuates toluene diisocyanate-induced experimental asthma

BACKGROUND

Aberrant activation of β-catenin signaling by both WNT-dependent and WNT-independent pathways has been demonstrated in asthmatic airways, which is thought to contribute critically in remodeling of the airways. Yet, the exact role of β-catenin in asthma is very poorly defined. As we have previously reported abnormal expression of β-catenin in a toluene diisocyanate (TDI)-induced asthma model, in this study, we evaluated the therapeutic efficacy of two small molecules XAV-939 and ICG-001 in TDI-asthmatic male BALB/c mice, which selectively block β-catenin-mediated transcription.

METHODS

Male BALB/c mice were sensitized and challenged with TDI to generate a chemically induced asthma model. Inhibitors of β-catenin, XAV-939, and ICG-001 were respectively given to the mice through intraperitoneally injection.

RESULTS

TDI exposure led to a significantly increased activity of β-catenin, which was then confirmed by a luciferase assay in 16HBE transfected with the TOPFlash reporter plasmid. Treatment with either XAV-939 or ICG-001 effectively inhibited activation of β-catenin and downregulated mRNA expression of β-catenin-targeted genes in TDI-asthmatic mice, paralleled by dramatically attenuated TDI-induced hyperresponsiveness and inflammation of the airway, alleviated airway goblet cell metaplasia and collagen deposition, decreased Th2 inflammation, as well as lower levels of TGFβ1, VEGF, HMGB1, and IL-1β.

CONCLUSION

The results showed that β-catenin is a principal mediator of TDI-induced asthma, proposing β-catenin as a promising therapeutic target in asthma.

A Pilot Study Using Preoperative Cerebral Tissue Oxygen Saturation to Stratify Cardiovascular Risk in Major Non-Cardiac Surgery

This prospective pilot study evaluated whether low preoperative cerebral tissue oxygen saturation is associated with unfavourable outcomes after major elective non-cardiac surgery. Eighty-one patients over 60 years of age, American Society of Anesthesiologists physical status 3 or 4, were recruited. Resting cerebral tissue oxygen saturation was recorded on room air, and after oxygen supplementation, using cerebral oximetry. The primary outcome was 30-day major adverse event of combined mortality or severe morbidity, and the secondary outcome was 30-day new disability. Eleven patients (13.6%) suffered a major adverse event, and 28 patients (34.6%) experienced new disability. Room air cerebral tissue oxygen saturation was significantly different between patients who had a major adverse event, 67% (95% confidence interval [CI] 65–70) versus unaffected, 71% (95% CI 70–72; P=0.04). No statistical difference was found between patients for new disability (range 70%– 74%; P=0.73). Room air cerebral tissue oxygen saturation was significantly associated with major adverse events (odds ratio 1.36 (95% CI 1.03–1.79), P=0.03). Saturation levels ≤68% carried a positive likelihood ratio of 2.2 for death or severe morbidity, P=0.04. A definitive trial is required to confirm if cerebral oximetry can be used to stratify the cardiovascular risk of patients presenting for non-cardiac surgery.

Abstract P4-06-03: An annotated collection of pre- and post-therapy breast cancer patient-derived xenograft models built from fine needle aspiration samples aligned with ongoing clinical trials documenting response to treatment

BACKGROUND: Patient-derived xenograft (PDX) models of breast cancer replicate the diverse histologic and molecular features of patient tumors and provide a renewable source of human tumor tissue; however collection of tissue by core needle biopsy is problematic due to patient discomfort, bleeding risk and the limited number of passes a patient can tolerate. In addition, FDA guidelines caution that multiple core needle biopsies could lead to an overestimation of the true pCR rate in neoadjuvant trials.

METHODS: To support the neoadjuvant molecular diagnostic and drug development program in TNBC, a pilot study was conducted to determine if fine needle aspiration (FNA) could be used for building PDX models. Prior to engraftment, FNA samples were analysed for cell number and viability.

RESULTS: Six PDX models were successfully generated from eight individual tumor samples. These models retain histologic and molecular features of the original patient tumors as determined by immunohistochemistry, RNA expression profiling, and deep whole-exome and targeted gene sequencing. In addition, the tested PDX models recapitulate the responses to therapies across multiple chemotherapeutic agents.

Based on this success, we have standardized the use of FNAs to generate PDX models both pre- and post-therapy in two ongoing neoadjuvant clinical trials:

1. MDACC 2014-0185 (PI Stacy Moulder, 360 patients), 'Improving outcomes in TNBC using molecular triaging and diagnostic imaging to guide neoadjuvant therapy'

2. MDACC 2014-0045 (PI Jennifer Litton, 20+ patients), 'A pilot study of BMN673 as a neoadjuvant study in patients with a diagnosis of invasive breast cancer and a deleterious BRCA mutation'

 FNA cells (x10^4)Cell viability (%)Total viable cells (x10^4)Study entry biopsy (n=67)144.5050.6544.14Post treatment biopsy (n=16)47.0732.5428.38

To date, treatment-naïve primary tumor samples from 67 patients enrolled onto these neoadjuvant trials, and 16 matched non-responsive post treatment tumor samples have been analysed for cell count and viability (table below) prior to being engrafted into the humanized mammary fat pads of NOD/SCID mice.

CONCLUSION: We have demonstrated success in using FNAs to build PDX models that recapitulate the biology and clinical course of the original tumor. In our pilot study, we successfully generated six PDX models using FNA for TNBC, including some harboring deleterious BRCA1/2 mutations. Because of the high concordance in histologic, genomic, and clinical attributes, we are now using this approach to develop a rich resource of pre- and post-treatment PDX models for the investigation of therapeutic resistance.

Citation Format: Echeverria GV, Chang JT, Cai S, Tu Y, McCoy A, Lau R, Redwood A, Kaffiabasabadi S, Rauch GM, Adrada BE, Jennifer L, Moulder SL, Symmans WF, Piwnica-Worms H. An annotated collection of pre- and post-therapy breast cancer patient-derived xenograft models built from fine needle aspiration samples aligned with ongoing clinical trials documenting response to treatment [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-06-03.

Abstract P5-06-06: Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress

This abstract was not presented at the symposium.

Abstract P6-01-08: Identifying metastatic drivers in patient derived xenograft models of triple negative breast cancer

Metastases are responsible for the vast majority of deaths due to breast cancer. Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by high rates of metastasis and poor response to chemotherapy. We are using patient derived xenograft (PDX) models of TNBC to identify drivers of TNBC metastasis. Using these models, we demonstrated that human breast tumors are capable of completing all stages of the metastatic cascade in mice, and metastatic lesions are observed in organs normally found in patients with metastatic breast cancer including lung, liver, bone, brain and lymph nodes. Lentiviral transduction was employed to express both bioluminescent and fluorescent proteins in three distinct PDX models of TNBC. In this way, metastatic lesions can be isolated using bioluminescent imaging and circulating tumor cells (CTCs) are isolated by flow cytometry. A lung metastasis gene expression signature was generated and comprehensive gain-of-function screens are being conducted in vivo to validate this signature and identify functional drivers of TNBC metastasis.

Citation Format: Powell E, Shao J, Tieu T, Peoples M, Bristow C, Manyam G, Cai S, Tu Y, Edwards JR, Heffernan TP, Piwnica-Worms D, Liang H, Piwnica-Worms H. Identifying metastatic drivers in patient derived xenograft models of triple negative breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-01-08.

A novel FOXM1 isoform, FOXM1D, promotes epithelial-mesenchymal transition and metastasis through ROCKs activation in colorectal cancer

Epithelial-mesenchymal transition (EMT) is a critical event in metastasis of colorectal cancer (CRC). Rho/ROCKs signaling has a pivotal role in orchestrating actin cytoskeleton, leading to EMT and cancer invasion. However, the underlying mechanisms for ROCKs activation are not fully understood. Here, we identified FOXM1D, a novel isoform of Forkhead box M1 (FOXM1) that has a pivotal role in ROCKs activation by directly interacting with coiled-coil region of ROCK2. FOXM1D overexpression significantly polymerizes actin assembly and impairs E-cadherin expression, resulting in EMT and metastasis in xenograft mouse model and knockdown of FOXM1D has the opposite effect. Moreover, a high FOXM1D level correlates closely with clinical CRC metastasis. FOXM1D-induced ROCKs activation could be abrogated by the ROCKs inhibitors Y-27632 and fasudil. These observations indicate that the FOXM1D-ROCK2 interaction is crucial for Rho/ROCKs signaling and provide novel insight into actin cytoskeleton regulation and therapeutic potential for CRC metastasis.

The Plasma microRNA miR-1914* and -1915 Suppresses Chemoresistant in Colorectal Cancer Patients by Down-regulating NFIX

OBJECTIVE

We investigated mechanisms of colorectal cancer (CRC) chemoresistance to first-line chemotherapy (capecitabine plus oxaliplatin (XELOX)) and identified two putative chemoresistant microRNAs, miR-1914* and -1915, that are downregulated in plasma samples from patients with chemoresistant CRC.

METHODS

A number of plasma samples from CRC patients were analyzed for the levels of miR-1914* and - 1915. Effects of stable and transient expression of 2 microRNAs in human chemoresistant CRC cell lines were analyzed. Tumor formation and chemoresistance in HCT116/5-Fu/OXA that did or did not express 2 microRNAs were analyzed in mice. Nuclear factor I/X (NFIX) was predicted to target the gene of 2 miRNAs and verified in vivo and in vitro.

RESULTS

Plasma levels of miR-1914* and -1915 in chemoresistant CRC patients were different than levels in responders, and associated with clinical response. Overexpression of miR-1914* and -1915 in chemoresistant CRC cells reduced resistance to 5-FU and Oxaliplatin in vitro. The microRNAs suppressed chemoresistance in CRC tumors in mice by affecting cell growth, invasion, apoptosis and tumor suppressor function. miR-1914* and -1915 interacted with the 3'-untranslated region of NFIX and reduced NFIX its level in chemoresistant CRC cells. Overexpression of NFIX did not inhibit chemoresistant CRC cell motility and chemoresistant proteins when miR-1914* and -1915 were transfected.

CONCLUSION

Plasma miR-1914* and -1915 interact with NFIX RNA and reduce its level in chemoresistant CRC cells to first-line chemotherapy. Up-regulation of miR-1914* and -1915 decreased the chemoresistance abilities of chemoresistant CRC cells. The plasma miR-1914* and -1915 may play a role in colorectal cancer therapy and diagnosis.

Guttiferone K impedes cell cycle re-entry of quiescent prostate cancer cells via stabilization of FBXW7 and subsequent c-MYC degradation

Cell cycle re-entry by quiescent cancer cells is an important mechanism for cancer progression. While high levels of c-MYC expression are sufficient for cell cycle re-entry, the modality to block c-MYC expression, and subsequent cell cycle re-entry, is limited. Using reversible quiescence rendered by serum withdrawal or contact inhibition in PTEN(null)/p53(WT) (LNCaP) or PTEN(null)/p53(mut) (PC-3) prostate cancer cells, we have identified a compound that is able to impede cell cycle re-entry through c-MYC. Guttiferone K (GUTK) blocked resumption of DNA synthesis and preserved the cell cycle phase characteristics of quiescent cells after release from the quiescence. In vehicle-treated cells, there was a rapid increase in c-MYC protein levels upon release from the quiescence. However, this increase was inhibited in the presence of GUTK with an associated acceleration in c-MYC protein degradation. The inhibitory effect of GUTK on cell cycle re-entry was significantly reduced in cells overexpressing c-MYC. The protein level of FBXW7, a subunit of E3 ubiquitin ligase responsible for degradation of c-MYC, was reduced upon the release from the quiescence. In contrast, GUTK stabilized FBXW7 protein levels during release from the quiescence. The critical role of FBXW7 was confirmed using siRNA knockdown, which impaired the inhibitory effect of GUTK on c-MYC protein levels and cell cycle re-entry. Administration of GUTK, either in vitro prior to transplantation or in vivo, suppressed the growth of quiescent prostate cancer cell xenografts. Furthermore, elevation of FBXW7 protein levels and reduction of c-MYC protein levels were found in the xenografts of GUTK-treated compared with vehicle-treated mice. Hence, we have identified a compound that is capable of impeding cell cycle re-entry by quiescent PTEN(null)/p53(WT) and PTEN(null)/p53(mut) prostate cancer cells likely by promoting c-MYC protein degradation through stabilization of FBXW7. Its usage as a clinical modality to prevent prostate cancer progression should be further evaluated.

Abstract P5-03-04: Identification of compounds from natural sources with selective activity against triple-negative breast cancer molecular subtypes

Triple negative breast cancers (TNBCs) lack expression of the estrogen and progesterone receptors (ER/PR) and do not have amplified HER2. While targeted therapies for ER+/PR+ and HER2-amplified breast cancers have greatly improved patient survival, there are no targeted therapies for TNBCs and no effective therapies to treat metastatic disease. There is a need to identify new therapeutic agents and molecular targets for treating TNBCs, but efforts have been limited by a lack of understanding of the subtypes of these heterogeneous diseases. However, gene expression profiling of TNBC patients recently identified 6 molecular subtypes of TNBC and representative cell lines, providing the first opportunity to identify subtype-specific leads for TNBC.

We performed high-content screening to evaluate novel libraries of extracts from Texas plants and diverse fungal cultures for antiproliferative and/or cytotoxic activity in a panel of cell lines modeling five different TNBC molecular subtypes. The aim was to identify extracts with selective activity in a single cell line. We hypothesized that extracts found to have selective activity in one of these cell lines may target a protein or cellular process critical to the growth of that subtype. We identified 11 extracts with selective activity against cell lines representing four different TNBC molecular subtypes. From a fungal culture we identified a new compound called maximiscin, which was found to have selective cytotoxic efficacy against the MDA-MB-468 cell line of the basal-like 1 subtype. From a plant extract we isolated deguelin, which had selective activity in the MDA-MB-453 cell line, a model of the luminal androgen receptor (LAR) subtype.

The molecular mechanisms of action of each compound were investigated in cell line models. Initial cell cycle studies using flow cytometry showed that maximiscin caused an accumulation of cells in G1 after 18h of treatment. Protein microarray studies indicated that maximiscin increased levels of phospho-p53, which was consistent with the observed G1 accumulation. Based on these findings, we hypothesized that maximiscin induces DNA damage and investigated the effects of maximiscin on the phosphorylation of several DNA damage response proteins. Maximiscin increased phosphorylation of Chk1, Chk2, p53 and H2A.X as soon as 2h after treatment, indicating an accumulation of DNA damage.

Previous studies have shown that LAR TNBC cells are particularly sensitive to PI3K inhibitors in vitro compared to other TNBC subtypes. The effects of deguelin on PI3K-Akt-mTORC1 signaling were evaluated in both MDA-MB-453 and MDA-MB-231 cells. Phosphorylation of both ribosomal protein S6 and 4E-BP1 were dramatically reduced in MDA-MB-453 cells 2h after deguelin treatment. Interestingly this was not observed in MDA-MB-231 cells, suggesting inhibition of mTORC1 signaling may be involved in the selective activity of deguelin in MDA-MB-453 cells. Preliminary studies suggest deguelin may also decrease androgen receptor abundance in MDA-MB-453 cells, indicating multiple molecular mechanisms may be involved in its selective effects. These results demonstrate that compounds with selective activity against TNBC subtypes can be identified from nature.

Citation Format: Robles AJ, Du L, Cai S, Risinger AL, Cichewicz RH, Mooberry SL. Identification of compounds from natural sources with selective activity against triple-negative breast cancer molecular subtypes. [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 P5-03-04.

Abstract P4-07-02: Targeted therapies for TNBC: Exploiting vulnerabilities that arise from DNA damage repair pathway dependencies

We examined the synergistic effects of DNA damage, Chk1 inhibition and poly(ADP-ribose) polymerase (PARP) inhibition in TNBC. This combinatorial targeting allows us to exploit vulnerabilities in two pathways that are often deregulated in TNBCs: DNA damage checkpoint defects due to TP53 deficiency and DNA repair defects due to alterations in homologous recombination repair (HRR). TP53 maintains genome integrity by inhibiting cells that are experiencing genotoxic stress from progressing through the cell cycle, or by inducing apoptosis or senescence. In response to DNA damage, p53 activates gene expression to arrest cells in the G1 phase of the cell cycle and to reinforce the S- and G2-checkpoints. Thus, p53-deficient cells lack a G1 checkpoint and are impaired in their ability to sustain S- and G2-checkpoints. This makes p53-deficient tumors particularly sensitive to agents that abrogate these checkpoints. Because Chk1 inhibitors abrogate both S- and G2-checkpoints, combining Chk1 inhibitors with agents that induce genotoxic stress provides a rational therapeutic strategy for killing p53-deficient TNBC.

Loss of HRR increases dependence of cells on a class of enzymes called PARPs, and Chk1 has also been shown to be important for efficient HRR. Thus, by interfering with HRR, Chk1 inhibitors are predicted to sensitize TNBC cells to PARP inhibitors. We tested the hypotheses that by impairing HRR, Chk1 inhibitors will sensitize TNBCs to PARP inhibition, and that therapies that combine Chk1 inhibitors with PARP inhibitors will be effective at killing TNBCs because they will simultaneously induce checkpoint bypass and block DNA repair. We generated a set of isogenic TNBC cell lines that are p53-proficient (p53WT) or p53-deficient (p53KD), and evaluated their sensitivity to Chk1 inhibitors (LY2606368) and DNA damaging agents (cisplatin). Loss of p53 conferred a dramatic increase in sensitivity to treatment with cisplatin + LY2606368. Surprisingly, inhibition of PARP1 (BMN673) did not increase sensitivity to Chk1 inhibitor ± cisplatin. To determine why Chk1 inhibition did not sensitize cells to PARP inhibition, we evaluated the effect of Chk1 inhibition on the ability of cells to recruit HRR proteins to sites of DNA damage. In line with CHK1 regulating HRR, Chk1 inhibition was associated with an inability of Rad51 to localize to sites of DNA double strand breaks. Interestingly, we also found that upstream of Rad51, there was a significant alteration in the formation of phopho-RPA2 foci in cells treated with the Chk1 inhibitor. On-going studies are evaluating whether there are changes in the kinetics of formation and/or resolution of Rad51 and phospho-RPA2 foci in response to Chk1 inhibition.

Citation Format: Redwood AB, Cai S, Piwnica-Worms H. Targeted therapies for TNBC: Exploiting vulnerabilities that arise from DNA damage repair pathway dependencies. [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 P4-07-02.

Abstract P5-04-18: Extracts derived from fungi and plants demonstrate specificity for subtyptes of triple negative breast cancer

New effective therapies are needed for patients with triple negative breast cancers (TNBC). The identification by Lehmann and Bauer1 of distinct subtypes of TNBC and representative cell lines that are driven by different defects and signaling pathways provided the opportunity, for the first time, to screen for selective activities against these subtypes of TNBC. Using this knowledge, we initiated a screen of diverse natural product extract libraries with the goal of identifying extracts selective for subtypes of TNBC. The compounds with this selective activity will then be purified using bioassay-guided fractionation. Drugs derived from plants and fungi have provided some of the most important pharmaceuticals used today, including numerous anticancer agents.2 Natural products occupy a biologically validated chemical space that does not overlap with compounds found in most synthetic chemical libraries.3 Additionally, there are differences in chemical space between plant and fungal-derived compounds4 and different compound classes are expected to be isolated from these two sources. A total of 1,953 extracts of fungi collected from diverse environments, including Great Lakes sediments and 2,200 plant extracts from tropical environments have been screened for selective cytotoxic activities against cell lines representing 5 subtypes of TNBC. These subtypes are the basal-like 1 and 2 (BL1, BL2), mesenchymal (M), mesenchymal stem-like (MSL) and luminal androgen receptor (LAR). The initial screening using one concentration, 2 µg/ml for fungal extracts and 20 µg/ml for plant extracts, identified many extracts with selective activity against the TNBC subtypes. Detailed dose response curves were then generated with these extracts in each of the TNBC cell lines. A total of 4 fungal extracts and 7 plant extracts with selective cytotoxic activities were identified with selectivity up to 100-fold for 3 of the extracts. Bioassay-guided fractionation is ongoing to identify the active constituents. These results demonstrate that natural product extracts can yield selective actions against TNBC subtypes. We expect that these plant and fungal extracts will yield compounds that target molecular drivers specific to the TNBC subtypes. It is our expectation that compounds with selective, targeted activities will continue to be isolated from these extract collections.

1. Lehman BD and Bauer JA et al. J Clin Invest: 121, 2750-2767, 2011.

2. Newman DJ and Cragg GM. J Nat Prod: 75, 311-335, 2012.

3. Feher M and Schmidt JM. J Chem Inf Comput Sci: 43, 218-227, 2003.

4. El-Elimat, T et al. ACS Med Chem Lett: 3, 645-649, 2012.

Citation Format: Shaffer CV, Cai S, Perez A, Risinger AL, Du L, O'Keefe BR, Cichewicz RH, Mooberry SL. Extracts derived from fungi and plants demonstrate specificity for subtyptes of triple negative breast cancer. [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 P5-04-18.