Abstract 5489: Utilization of cancer cell line screening and bioinformatic analyses to identify optimal developmental pathways for the novel anticancer agent BOLD-100

Cell line screening of unique compounds can provide mechanistic insights and identify optimal drug combination partners. Bioinformatics analysis of cell screen data and correlation to publicly available datasets can support identification of appropriate patient populations for subsequent preclinical and clinical development. BOLD-100 is a first-in-class ruthenium-based anticancer agent currently being tested in a Phase 1b/2a clinical trial in combination with FOLFOX in the treatment of advanced gastrointestinal cancers. BOLD-100 works by altering the unfolded protein response through selective GRP78 inhibition; and inducing reactive oxygen species which causes DNA damage and cell cycle arrest. Collectively, these result in cell death in a range of different cancer types, and in combination with many different classes of anticancer agents. To determine optimal indications for BOLD-100 development, BOLD-100 was tested against 316 cancer cell lines in 72-hour Cell Titer Glo assays with downstream bioinformatic analysis and validation experiments. Multiple cancer types showed preferential response to BOLD-100, including bladder, esophageal, pancreatic, multiple myeloma and ovarian cancers. Subtype analysis identified potential populations of increased responsiveness, including in bladder cancer where BOLD-100 had increased response in luminal and mixed subtypes, as compared to basal subtypes. Utilizing bladder cancer as a case study, subsequent combination testing of BOLD-100 in combination with fluorouracil or cisplatin demonstrated that BOLD-100 enhanced cell death across different bladder cancer cell lines through synergistic interactions with these standard-of-care agents. The pan-cancer response profile of BOLD-100 was compared against 449 other anticancer drug responses that are part of the GDSC database. BOLD-100 displayed limited correlation with existing drugs, suggesting a unique mechanism of action and clinical utility where standard-of-care agents have limited efficacy. Pharmacogenomic analysis of the cell screen data indicated potential pathways and genes of relevance to BOLD-100 response, including increased response in KRAS-mutant cancers. Collectively, BOLD-100 showed a unique sensitivity profile across a panel of over 300 cancer cell lines, identifying multiple potential indications for future development. Subsequent investigations into several cancer types of interest and drug combinations are ongoing.

Citation Format: Paromita Raha, Brian Park, Adam Carie, Jim Pankovich, Mark Bazett. Utilization of cancer cell line screening and bioinformatic analyses to identify optimal developmental pathways for the novel anticancer agent BOLD-100 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5489.

Abstract B148: Activity of the TAM kinase-targeting compound, SLC-391, is mediated by the engagement of the immune system in CT-26 syngeneic mouse model

Overexpression of TAM family receptor tyrosine kinases such as Axl has been reported in numerous types of human cancer, and found to correlate with tumor progression and prognosis, metastasis, and drug resistance. Aside from the role in cell growth and survival, TAM kinases have long been recognized for their immunosuppressive activity as well. Hence, downregulation of their activity is expected to unleash antitumor immunity in the tumor microenvironment. As one of the several TAM-targeting small-molecule inhibitors identified, SLC-391 displays a relatively strong activity against Axl, as evidenced by the reductions of phosphotransferase activity in radiometric biochemical assay and the level of Axl Y779 phosphorylation in the cell-based assay. Contradictorily, proliferation of CT-26 murine colon carcinoma cells seemed to be unaffected by the compound in the thymidine incorporation assay with an IC50 of ~10 μM. Interestingly, SLC-391 inhibited CT-26 tumor growth by 37% in a 15-day efficacy study in which the compound was administered at 50 mg/kg p.o.. In comparison, a PD-1 antibody delayed tumor growth by 27%. Tumor-infiltrating lymphocyte phenotyping revealed increases in the number of NK cells and the ratio of M1/M2-polarized macrophages in SLC-391 treatment group, followed by the rise of CD8+ T/Treg ratio and reduction in immunosuppressive myeloid cells. This is indicative of sequential engagement and stimulation of proinflammatory innate immune response and adaptive immune response. In addition, a synergistic antitumor effect was observed when the anti-PD-1 insensitive CT-26 syngeneic model was treated with a combination of SLC-391 and an anti-PD-1 antibody and the overall survival rate of the combination group was prolonged dramatically in comparison with the vehicle control group. To summarize, the antitumor activity of SLC-391 is at least in part mediated by reversing the immunosuppressive tumor microenvironment in CT-26 colon carcinoma model.

Citation Format: Shenshen Lai, Rick Li, Paromita Raha, Yuxiang Hu, Jun Yan, Hong Zhang, Anthony Marotta, Zaihui Zhang. Activity of the TAM kinase-targeting compound, SLC-391, is mediated by the engagement of the immune system in CT-26 syngeneic mouse model [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 B148.

Conformational selection underpins recognition of multiple DNA sequences by proteins and consequent functional actions

Recognition of multiple functional DNA sequences by a DNA-binding protein occurs widely in nature. The physico-chemical basis of this phenomenon is not well-understood. The E. coli gal repressor, a gene regulatory protein, binds two homologous but non-identical sixteen basepair sequences in the gal operon and interacts by protein-protein interaction to regulate gene expression. The two sites have nearly equal affinities for the Gal repressor. Spectroscopic studies of the Gal repressor bound to these two different DNA sequences detected significant conformational differences between them. Comprehensive single base-substitution and binding measurements were carried out on the two sequences to understand the nature of the two protein-DNA interfaces. Magnitudes of basepair-protein interaction energy show significant variation between homologous positions of the two DNA sequences. Magnitudes of variation are such that when summed over the whole sequence they largely cancel each other out, thus producing nearly equal net affinity. Modeling suggests significant alterations in the protein-DNA interface in the two complexes, which are consistent with conformational adaptation of the protein to different DNA sequences. The functional role of the two sequences was studied by substitution of one site by the other and vice versa. In both cases, substitution reduces repression in vivo. This suggests that naturally occurring DNA sequence variations play functional roles beyond merely acting as high-affinity anchoring points. We propose that two different pre-existing conformations in the conformational ensemble of the free protein are selected by two different DNA sequences for efficient sequence read-out and the conformational difference of the bound proteins leads to different functional roles.

A phase I trial of panobinostat and epirubicin in solid tumors with a dose expansion in patients with sarcoma

BACKGROUND

Treatment options for sarcoma are limited. Histone deacetylase inhibitors increase the efficacy of topoisomerase II inhibitors by promoting access to chromatin and by down-regulating DNA repair. Thus, combined panobinostat and epirubicin therapy was evaluated to treat refractory sarcoma.

PATIENTS AND METHODS

Patients with advanced solid tumors were enrolled in a 3 + 3 dose-escalation phase I trial of panobinostat given on days 1, 3, and 5 followed by 75 mg/m(2) of epirubicin on day 5 in 21-day cycles, with a dose expansion at maximum tolerated dose (MTD) in 20 sarcoma patients. Peripheral blood mononucleocyte histone acetylation was also evaluated.

RESULTS

Forty patients received 20-60 mg panobinostat. Dose-limiting toxicities included thrombocytopenia, febrile neutropenia, and fatigue at 60 mg, defining a panobinostat MTD at 50 mg. Four responses were seen in 37 assessable patients, all after progression on prior topoisomerase II inhibitors. For those with sarcoma, 12 of 20 derived clinical benefit (1 partial response and 11 stable disease, median overall survival 8.3 months), including 8 of 14 previously progressed on topoisomerase II therapy. Treatment benefits correlated with increased histone acetylation and decreased neutrophil count on day 5.

CONCLUSIONS

Panobinostat and epirubicin treatment is well tolerated and may reverse anthracycline resistance. Changes in histone acetylation and associated decrease in neutrophil count correlated with clinical benefit and warrant investigation as predictive biomarkers.

CLINICAL TRIAL

This trial is registered at www.Clinicaltrials.gov, Identifier: NCT00878904.

Combined histone deacetylase inhibition and tamoxifen induces apoptosis in tamoxifen-resistant breast cancer models, by reversing Bcl-2 overexpression

INTRODUCTION

The emergence of hormone therapy resistance, despite continued expression of the estrogen receptor (ER), is a major challenge to curing breast cancer. Recent clinical studies suggest that epigenetic modulation by histone deacetylase (HDAC) inhibitors reverses hormone therapy resistance. However, little is known about epigenetic modulation of the ER during acquired hormone resistance. Our recent phase II study demonstrated that HDAC inhibitors re-sensitize hormone therapy-resistant tumors to the anti-estrogen tamoxifen. In this study, we sought to understand the mechanism behind the efficacy of this combination.

METHODS

We generated cell lines resistant to tamoxifen, named TAMRM and TAMRT, by continuous exposure of ER-positive MCF7 and T47D cells, respectively to 4-hydroxy tamoxifen for over 12 months. HDAC inhibition, along with pharmacological and genetic manipulation of key survival pathways, including ER and Bcl-2, were used to characterize these resistant models.

RESULTS

The TAMRM cells displayed decreased sensitivity to tamoxifen, fulvestrant and estrogen deprivation. Consistent with previous models, ER expression was retained and the gene harbored no mutations. Compared to parental MCF7 cells, ER expression in TAMRM was elevated, while progesterone receptor (PGR) was lost. Sensitivity of ER to ligands was greatly reduced and classic ER response genes were suppressed. This model conveyed tamoxifen resistance through transcriptional upregulation of Bcl-2 and c-Myc, and downregulation of the cell cycle checkpoint protein p21, manifesting in accelerated growth and reduced cell death. Similar to TAMRM cells, the TAMRT cell line exhibited substantially decreased tamoxifen sensitivity, increased ER and Bcl-2 expression and significantly reduced PGR expression. Treatment with HDAC inhibitors reversed the altered transcriptional events and reestablished the sensitivity of the ER to tamoxifen resulting in substantial Bcl-2 downregulation, growth arrest and apoptosis. Selective inhibition of Bcl-2 mirrored these effects in presence of an HDAC inhibitor.

CONCLUSIONS

Our model implicates elevated ER and Bcl-2 as key drivers of anti-estrogen resistance, which can be reversed by epigenetic modulation through HDAC inhibition.

Abstract P5-09-13: HDAC inhibition re-sensitizes acquired hormone resistant cells to the cytotoxic effect of tamoxifen

Breast cancer still continues to be a major cause of cancer related deaths in women, second only to lung cancer. Administering anti-estrogens and aromatase inhibitors, both in the adjuvant and metastatic settings, in estrogen receptor (ER) positive disease, is one of the most effective treatment strategies. However, prolonged exposure to these drugs leads to the emergence of resistance in about 40% of those who initially respond. Hence, the emphasis of a multitude of studies is to understand the underlying mechanisms of this acquired resistance and implementing means to overcome it.

Recent studies have implicated epigenetic modulation of gene expression in the development of resistance to the anti-estrogen tamoxifen. A clinical trial conducted by our group has demonstrated the efficacy of combining the HDAC inhibitor vorinostat with tamoxifen in patients who had progressed on prior anti-estrogen therapies and showed a total 40% clinical benefit (19% objective response and 21% stable disease for more than 6 months). In an effort to identify patients most likely to benefit from this novel therapy, we sought to elucidate the mechanism behind the clinical efficacy of this combination.

To this end, we have generated an in vitro tamoxifen resistant cell line (TAMR) by long-term exposure of MCF7 cells to 4-hydroxy tamoxifen. Significantly reduced anti-proliferative effect of tamoxifen and other anti-estrogens in TAMR compared to the sensitive MCF7 cells demonstrates the establishment of resistance to anti-estrogens in this cell line. In TAMR cells, addition of an HDAC inhibitor reverts resistance to tamoxifen. Although estrogen receptor (ER) expression in TAMR cells appears unaltered, the classical genomic signaling of ER in these resistant cells is suppressed and unresponsive to ligands, as deduced from transcription of ER response genes and luciferase assay of an ERE-luciferase construct. However, the ER remains important, since siRNA mediated depletion of ER inhibits cell growth in TAMR cells. Treatment with the dual EGFR/Her2 kinase inhibitor or an AKT inhibitor significantly inhibits the growth of these cells only when combined with tamoxifen, indicating the importance of crosstalk between pathways. Furthermore, Akt and mTOR protein and activities are down regulated by HDAC inhibition, which are further reduced when combined with tamoxifen. SiRNA mediated depletion of ER leads to reduced Akt and mTOR activities which suggests that ER may act as a driver, possibly through its non-genomic function at the plasma membrane activating members of the growth factor signaling pathways.

Studies are ongoing to further characterize the interaction of HDAC and ER inhibition on these signaling pathways and to determine their significance to resistance.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-09-13.

Abstract P4-16-01: A novel approach to breast cancer prevention: Exploiting autologous fat grafting for the local delivery of cancer therapeutics

Background: Breast cancer remains a considerable health concern, despite major advances in the treatment and prevention of breast cancer. Inhibition of estrogen receptor signaling is one of the most effective therapies for patients with hormone receptor positive breast cancer. However, even after chemotherapy and hormonal therapy, local recurrence occurs frequently, and undesirable side effects lead to drug discontinuation in an estimated 40% of patients on adjuvant therapy.

Autologous fat transfer is an emerging therapy used in breast reconstruction to restore surgical deformities after lumpectomy, or to rebuild the breast following mastectomy. This is an attractive option for both radiated and non-radiated patients, as complication rates in both groups are exceedingly low. Fat is injected into the breast or surgical cavity to maximize its contact with local vascularized tissue, to promote re-vascularization and survival of the adipose tissue. Thus, fat cells are injected throughout the tissue surrounding the surgical margin. We hypothesize that harvested adipose tissue can be loaded with a lipophilic drug prior to its re-injection into the breast cavity and its surrounding tissue. With the release of drug from the transplanted adipose tissue over time, the local concentration of an anti-estrogen within the breast parenchyma would be increased and systemic toxicity may be minimized.

Method: Human adipose tissue was acquired from patients undergoing liposuction for autologous grafting. Tissue was washed with warm PBS, divided into 0.5 mL aliquots, and incubated in complete DMEM (10% FBS) containing increasing concentrations of 4-OH tamoxifen (0, 10, 20, or 40 uM) or fulvestrant (0, 0.1, 0.5, or 1.0 uM) for 12 hours. Following incubation, adipose tissue was washed thoroughly and co-cultured with MCF7 cells for 96 hours. Adipocyte co-cultured MCF7 were then collected and evaluated for proliferation, viability and effects on estrogen receptor signaling and compared to MCF7 directly exposed to anti-estrogens.

Results: When co-culturing tamoxifen- or fulvestrant-loaded human adipocytes with MCF7 breast cancer cells in vitro, we observed a dose-dependent reduction in cell proliferation (tamoxifen, 60% reduction; fulvestrant, 40% reduction) and viability (20%), comparable to directly drugged media. Examination of the molecular response to fulvestrant demonstrated a dose dependent down regulation of ER protein expression and activity (e.g. PgR and Cyclin D1). Currently, we are characterizing the pharmacokinetics of anti-estrogen uptake and release in both in vitro and in vivo.

Summary: Our preliminary data suggest that co-culturing human adipocytes with an anti-estrogen results in tumor growth inhibition and abrogation of estrogen receptor signaling. Human adipocytes used to fill the tumor bed cavity or mastectomy pocket could therefore be used as a vehicle to deliver anti-estrogens to residual tumor cells and prevent tumor regrowth. This method may be a novel technology to combine reconstructive surgery and anti-cancer therapy and prevention.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-16-01.

Histone deacetylase regulation of ATM-mediated DNA damage signaling

Ataxia-telangiectasia mutated (ATM) is a major regulator of the DNA damage response. ATM promotes the activation of BRCA1, CHK2, and p53 leading to the induction of response genes such as CDKN1A (p21), GADD45A, and RRM2B that promote cell-cycle arrest and DNA repair. The upregulation of these response genes may contribute to resistance of cancer cells to genotoxic therapies. Here, we show that histone deacetylases (HDAC) play a major role in mitigating the response of the ATM pathway to DNA damage. HDAC inhibition decreased ATM activation and expression, and attenuated the activation of p53 in vitro and in vivo. Select depletion of HDAC1 and HDAC2 was sufficient to modulate ATM activation, reduce GADD45A and RRM2B induction, and increase sensitivity to DNA strand breaks. The regulation of ATM by HDAC enzymes therefore suggests a vital role for HDAC1 and HDAC2 in the DNA damage response, and the potential use of the ATM pathway as a pharmacodynamic marker for combination therapies involving HDAC inhibitors.

Efficacy of histone deacetylase and estrogen receptor inhibition in breast cancer cells due to concerted down regulation of Akt

Hormonal therapy resistance remains a considerable barrier in the treatment of breast cancer. Activation of the Akt-PI3K-mTOR pathway plays an important role in hormonal therapy resistance. Our recent preclinical and clinical studies showed that the addition of a histone deacetylase inhibitor re-sensitized hormonal therapy resistant breast cancer to tamoxifen. As histone deacetylases are key regulators of Akt, we evaluated the effect of combined treatment with the histone deacetylase inhibitor PCI-24781 and tamoxifen on Akt in breast cancer cells. We demonstrate that while both histone deacetylase and estrogen receptor inhibition down regulate AKT mRNA and protein, their concerted effort results in down regulation of AKT activity with induction of cell death. Histone deacetylase inhibition exerts its effect on AKT mRNA through an estrogen receptor-dependent mechanism, primarily down regulating the most abundant isoform AKT1. Although siRNA depletion of AKT modestly induces cell death, when combined with an anti-estrogen, cytotoxicity is significantly enhanced. Thus, histone deacetylase regulation of AKT mRNA is a key mediator of this therapeutic combination and may represent a novel biomarker for predicting response to this regimen.

A phase I trial of the histone deacetylase inhibitor panobinostat (LBH589) and epirubicin in patients with solid tumor malignancies

3058

Background: Preclinical and clinical data suggest pre-exposure of cancer cells to a histone deacetylase inhibitor (HDACi) potentiates topoisomerase inhibitors. HDACi-induced histone acetylation and chromatin modulation facilitates DNA access and target recruitment for topo II inhibitors. In vitro data further suggest effective inhibition of HDAC2 is necessary for enhanced epirubicin-induced apoptosis. Methods: This phase I trial explores the safety, tolerability, and maximum tolerated dose (MTD) of escalating doses of panobinostat given orally on days 1, 3, and 5 followed by epirubicin administered intravenously at 75 mg/m2 on day 5 in 21-day cycles. Histone acetylation and HDAC2 expression are evaluated in pre- and post-treatment peripheral blood mononuclear cells (PBMCs) in all patients and in tumor cells of 16 patients treated at the MTD. Results: 36 patients have enrolled [10M/26F, median age 47 years (22-80)] in 5 panobinostat cohorts: 20, 30, 40, 50, 60 mg. Tumor types include melanoma (n=6), breast (n=6), sarcoma (n=16), ovarian (n=2), lung (n=2), and one each of neuroblastoma, pancreatic, testicular, and colon cancer. Prior to enrollment, patients received a median of 3 (0-8) prior chemotherapy regimens and 40% had anthracyclines. Dose-limiting toxicities (DLTs) included 1/3 grade 3 fatigue and 1/3 grade 4 thrombocytopenia at 60 mg of panobinostat, 1/6 patient experienced grade 3 atrial fibrillation at 50 mg, defining 50 mg panobinostat as the MTD. Non-dose–limiting grade 3/4 hematological toxicities include neutropenia (n=19, 53%), febrile neutropenia (n=6, 17%), thrombocytopenia (n=6, 17%), and anemia (n=4, 11%). Of 34 evaluable patients, 5 had partial responses and 14 had stable disease in anthracycline-refractory sarcomas (4) and Her2neu positive breast cancer (2), and small cell lung cancer. Correlative studies demonstrate increased H4 acetylation in PBMCs on day 3 and 5 suggesting sufficient histone deacetylase inhibition. Conclusions: Sequence-specific combination of panobinostat and epirubicin shows early activity without potentiating epirubicin toxicity. Dose expansion in anthracycline-pretreated sarcoma patients is ongoing.

High-affinity quasi-specific sites in the genome: how the DNA-binding proteins cope with them

Many prokaryotic transcription factors home in on one or a few target sites in the presence of a huge number of nonspecific sites. Our analysis of λ-repressor in the Escherichia coli genome based on single basepair substitution experiments shows the presence of hundreds of sites having binding energy within 3 Kcal/mole of the O(R)1 binding energy, and thousands of sites with binding energy above the nonspecific binding energy. The effect of such sites on DNA-based processes has not been fully explored. The presence of such sites dramatically lowers the occupation probability of the specific site far more than if the genome were composed of nonspecific sites only. Our Brownian dynamics studies show that the presence of quasi-specific sites results in very significant kinetic effects as well. In contrast to λ-repressor, the E. coli genome has orders of magnitude lower quasi-specific sites for GalR, an integral transcription factor, thus causing little competition for the specific site. We propose that GalR and perhaps repressors of the same family have evolved binding modes that lead to much smaller numbers of quasi-specific sites to remove the untoward effects of genomic DNA.

Abstract B61: Targeting the ATM pathway through histone deacetylase inhibition

Ataxia telangiectasia mutated (ATM) is a major regulator of DNA double strand break repair. It is a member of the phosphatidyl inositol 3' kinase-related (PIKK) family of serine/threonine kinases, whose activation is promoted by the MRN complex (MRE11, Rad50, NBS1) in the presence of DNA damage. Mutations in ATM lead to increased susceptibility to developing lymphoid and breast cancers. Cells with mutated ATM often demonstrate enhanced chromatin decondensation, increased genomic instability, and greater sensitivity to DNA damaging agents. Interestingly, pharmacological inhibition of histone deacetylases (HDACs) often results in many of these same traits. Inhibitors of histone deacetylases (HDACs) have been demonstrated to potentiate the effects of various DNA damaging therapies through regulation of chromatin structure and down-regulating DNA repair. The contribution of HDAC proteins to DNA repair have yet to be fully understood. Therefore, we examined the effect of HDACs on DNA damage signaling and repair.

Treatment of cancer cells with HDAC inhibitors led to a significant decrease in ATM mRNA and protein levels. This caused attenuated induction of the DNA damage response signaling downstream of ATM in the presence of DNA damage. HDAC inhibition led ultimately to reduced DNA repair after therapeutic induction of DNA damage, and decreased overall cell survival. Select depletion of HDAC1 and HDAC2 was sufficient to reduce ATM expression, and potentiate the effects of DNA damaging agents. These results were recapitulated in vivo and demonstrated that pharmacological inhibition of HDACs significantly reduced the activation of the ATM pathway in response to chemotherapeutic induction of DNA damage. Specifically, HDAC inhibition significantly reduced the levels of ATM and down-stream stabilized p53 after treatment of tumors with therapeutic DNA damaging agents. Taken together, these results suggest a contribution of histone deacetylase enzymes in regulating the ATM pathway in response to DNA damage.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B61.

Abstract B108: The novel HDAC inhibitor PCI-24781 synergizes with tamoxifen in ER-positive breast cancer by concertedly downregulating AKT1 expression

Breast cancer continues to be the most frequently diagnosed malignancy in women, and is second only to lung cancer as the cause of cancer death. While aromatase inhibitors are more commonly used in postmenopausal women, tamoxifen remains the only approved hormonal therapy for premenopausal patients presenting with estrogen receptor (ER)-positive disease. However, the effectiveness of tamoxifen is limited, as many tumors develop resistance. As such, substantial effort has been made to improve and extend the effectiveness of tamoxifen. To this end, we have demonstrated that histone deacetylase (HDAC) inhibitors potentiate the anti-tumor activity of tamoxifen in preclinical models, and in an early phase trial durable responses were achieved in a heavily pre-treated patient cohort. Mechanistic studies have revealed that HDAC inhibition directs breast cancer cells into mitochondrial-mediated apoptosis in part by altering the balance of apoptotic gatekeepers (e.g. Bax, Bak, and Bcl-2). In the current study, we extend our mechanistic understanding of this combination in ER-positive cells using the novel hydroxamic acid type HDAC inhibitor PCI-24781. Early clinical evaluation suggests that PCI-24781 is a potent HDAC inhibitor and is well tolerated at doses exceeding those needed for preclinical synergy. Here we demonstrate that PCI-24781 and tamoxifen act individually to additively abrogate AKT cell signaling by down regulating AKT1 mRNA and protein expression. Furthermore, PCI-24781 represses ESR1 transcription and thus exerts its effect on AKT1 expression in part through the regulation of ER expression. Attenuated AKT signaling manifests in elevated GSK3α/β phosphorylation, Bim, and p21 and a reduction in cyclin D. Cells exhibit an induction of an array of pro-apoptotic drivers, mitochondrial release of cytochrome C, caspase activation, and cell death. These results suggest that HDACs regulate the response of AKT to tamoxifen and that the cooperative effects of HDAC inhibitors and tamoxifen lead to enhance cell death. Therefore, early introduction of an HDAC inhibitor into hormonal therapy in patients with ER-positive breast cancer may counter the emergence of tamoxifen resistance.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B108.

Epigenetic modulation: a novel therapeutic target for overcoming hormonal therapy resistance

For more than four decades, modulation of estrogen receptor activity with antiestrogens has been a successful strategy for the treatment of breast cancer. However, therapeutic resistance limits this approach. Patients whose tumors lack estrogen receptors are not candidates for antiestrogens. Furthermore, roughly half that do express estrogen receptors fail to respond. Together, these tumors are considered to be de novo resistant. For those with tumors that do respond, most will eventually acquire resistance. As such, the underlying mechanisms of both de novo and acquired resistance have been the subject of considerable research, so that new therapeutic targets might be discovered and developed. From this work, epigenetic regulation of gene expression has emerged as a major contributor to both forms of resistance. In this article, we present our current understanding of the mechanisms that contribute to antiestrogen resistance, focusing on epigenetic regulation, and examine the approaches being used that target epigenetic machinery to overcome resistance both in the laboratory and in the clinic.

Abstract 2614: Inhibition of class I histone deacetylases attenuates anthracycline induced activation of the ATM pathway

Breast cancer is one of the most common cancers to afflict women in the world. Anthracycline-based regimens, including doxorubicin and epirubicin (epidoxorubicin), are common therapies for patients with breast cancer. Doxorubicin and epirubicin are inhibitors of topoisomerase II, which cause double strand DNA breaks. Following anthracycline-induced damage, DNA damage response pathways are activated in order to initiate cell cycle arrest, DNA repair, and/or cell death. The recent introduction of PARP inhibitors highlights the clinical relevance of targeting DNA repair pathways in treating this disease.

Recently, two histone deacetylase (HDAC) inhibitors have been approved by the Federal Drug Administration for the treatment of cutaneous T-cell lymphoma: vorinostat and romidepsin. HDAC inhibitors reduce the repair of DNA, and are being used in combination with DNA damaging chemotherapeutic agents. The mechanisms regulating the HDAC inhibitor-mediated reduction of DNA repair in breast cancer cells are not well established. Understanding these mechanisms will identify key therapeutic targets for reducing DNA repair, define the role of HDACs in DNA repair, and allow for the enrichment of patients most susceptible to HDAC inhibitor-chemotherapy combined treatment.

Previously, our group has demonstrated an increase in DNA decondensation which permits greater access of anthracyclines to DNA after therapeutic inhibition of HDACs. New results demonstrate that phosphorylation of p53 at Serine 15 is decreased in the presence of epirubicin after pre-treatment with vorinostat in a time- and concentration-dependent manner. Total p53 protein levels are also decreased, while mRNA levels remain unchanged after combined treatment. Treatment of breast cancer cells with valproic acid or entinostat also attenuates phosphorylation of p53 after epirubicin treatment, suggesting the involvement of Class I HDACs in the activation of p53. Phosphorylation of DNA repair kinase ATM is also decreased following DNA damage in the presence of clinically relevant concentrations of vorinostat, and corresponds with a decrease in activation of downstream factors including CHK1, CHK2, and BRCA1. These results demonstrate that inhibition of Class I HDACs in breast cancer cells delays DNA damaging response signaling through p53 modification after anthracycline administration; suggesting a possible mechanism for synergy between HDAC inhibitors and DNA damaging agents.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2614. doi:10.1158/1538-7445.AM2011-2614

Abstract 1738: Histone deacetylase inhibitor mediated reversal of acquired endocrine therapy resistance in breast cancer cell lines

Although nearly 70% of breast cancers are estrogen receptor (ER) positive, endocrine therapy using the selective ER modulator tamoxifen or the selective ER down regulator fulvestrant have proved to be beneficial in less than 50% of these patients. Moreover, most ER positive patients that initially respond to tamoxifen eventually acquire resistance. Numerous studies have focused on determining the mechanism of anti-estrogen resistance which involve cellular changes such as variation in uptake/metabolism of tamoxifen, expression of altered ER proteins, ligand-independent ER activation, loss of ER expression and/or variation in recruitment of cofactors and altered cross-talk with other growth factor signaling pathways.

Recently, an alternative strategy to modulate ER signaling has been the use of histone deacetylase (HDAC) inhibitors. Preclinical studies have shown that with the addition of HDAC inhibitors, the anti-tumor activity of tamoxifen is potentiated. In these cells, apoptotic cell death is induced, which cannot be rescued by estrogen, indicating the potential to reverse hormone therapy resistance, a strategy currently being evaluated in several clinical trials.

Our laboratory is interested in exploring the potential of HDAC inhibition for the reversal of anti-estrogen resistance in ER-positive breast cancer. To this end, we have generated several MCF-7 cell lines resistant to various levels of tamoxifen; up to 10 micromolar. Our preliminary data demonstrate the addition of an HDAC inhibitor to tamoxifen reverses resistance. Significant effects on proliferation and viability are observed in these cells with clinically relevant concentrations of pan-HDAC inhibitors. Furthermore, with the addition of an HDAC inhibitor to tamoxifen, colony formation is attenuated. Initial characterization of these resistant cells indicated a several fold higher growth rate than the parental cells but a similar growth rate in presence and absence of the ligand estrogen, suggesting the possibility of loss of estrogen-dependence. Both the parental and resistant cells, in presence of estrogen, show a stabilization of the estrogen receptor protein by tamoxifen, which is not altered by the HDACi, suggesting an effect that goes beyond modulation of ER protein level. Tamoxifen or the combination in the resistant cells do not decrease Cyclin D1 protein level as is seen in parental cells, suggesting the involvement of pathways not involving ER.

Our previous data using siRNA against HDAC2 indicate sensitization of ER positive breast cancer cells to tamoxifen-mediated apoptosis suggesting the importance of HDAC2. We plan to study the importance of ER, in resistant cells with varied ER status, in mediating the hormone therapy response and to identify the potential involvement of other signaling pathways, using both HDAC inhibitors and selectively depleting HDAC2.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1738. doi:10.1158/1538-7445.AM2011-1738

Abstract 2631: The HDAC inhibitor PCI-24781 potentiates the anti-tumor activity of tamoxifen in ER-positive breast cancer cells

The anti-estrogen tamoxifen remains the treatment of choice for patients with estrogen receptor (ER)-positive breast cancer. Although initially very effective, patients responsive to tamoxifen will eventually develop resistance and progress. Thus, the development of new therapies to combat breast cancer is needed.

An emerging strategy to modulate ER signaling has been to employ histone deacetylase (HDAC) inhibitors. Preclinical studies from our and other laboratories have shown that HDAC inhibitors potentiate the anti-tumor activity of tamoxifen by promoting apoptosis. We have further shown that select depletion of HDAC2 is sufficient to elicit synergistic cell death when combined with tamoxifen, indicating it as a relevant target for this combined therapy. In a recent phase II clinical trial conducted by our laboratory, durable responses were observed in patients with metastatic ER-positive breast cancer when the HDAC inhibitor vorinostat was co-administered with tamoxifen. To evaluate HDAC inhibitor activity, histone acetylation in surrogate peripheral blood mononuclear cells was measured in patients pre- and post-treatment. This analysis indicated that increased histone acetylation correlated with patient response. Increased acetylation was observed in only 58% of patients, suggesting that greater HDAC inhibition would lead to the enrichment of patients likely to benefit. Furthermore, patient response correlated with baseline expression of HDAC2. Therefore, a more potent inhibitor of HDAC2 may provide greater benefit in patients when combined with tamoxifen.

PCI-24781 is a hydroxamic acid inhibitor that targets class I and II HDACs. Preclinical data demonstrates anti-tumor activity against a variety of hematological and solid tumor malignancies when administered at nanomolar concentrations. Early phase clinical evaluation has shown that PCI-24781 is tolerable at these concentrations. As PCI-24781 is 8 times more potent against HDAC2 compared to vorinostat (IC50: 19 nM vs 164 nM, respectively), we sought to evaluate the effectiveness of combining PCI-24781 with tamoxifen against breast cancer. At clinically feasible doses, we found that PCI-24781 potentiates the anti-tumor activity of tamoxifen in various ER-positive breast cancer cell lines (e.g. MCF-7, BT474, and MDA-361). This combination induces cell death by activating the apoptotic program, as measured by PARP cleavage and TUNEL staining. Furthermore, growth is inhibited by attenuating ER signaling, illustrated by down-regulation of ER protein and its pro-growth response genes (e.g. PR, Cyclin D, and Myc). Therefore, early preclinical evaluation demonstrates that PCI-24781 enhances the effectiveness of tamoxifen in vitro and may represent a more potent combination for combating breast cancer. Mouse xenograft studies are on going to determine whether this benefit translates in vivo.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2631. doi:10.1158/1538-7445.AM2011-2631