Glucocorticoid receptor confers resistance to antiandrogens by bypassing androgen receptor blockade

The treatment of advanced prostate cancer has been transformed by novel antiandrogen therapies such as enzalutamide. Here, we identify induction of glucocorticoid receptor (GR) expression as a common feature of drug-resistant tumors in a credentialed preclinical model, a finding also confirmed in patient samples. GR substituted for the androgen receptor (AR) to activate a similar but distinguishable set of target genes and was necessary for maintenance of the resistant phenotype. The GR agonist dexamethasone was sufficient to confer enzalutamide resistance, whereas a GR antagonist restored sensitivity. Acute AR inhibition resulted in GR upregulation in a subset of prostate cancer cells due to relief of AR-mediated feedback repression of GR expression. These findings establish a mechanism of escape from AR blockade through expansion of cells primed to drive AR target genes via an alternative nuclear receptor upon drug exposure.

Development of novel metastatic prostate cancer cell lines by “organoid” in vitro culture technology

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Background: The inability to propagate patient-derived prostate cancer cells in vitro is a major impediment in the mechanistic understanding of tumorigenesis and therapeutic response. In order to generate accurate in vitro models that represent the diversity of in situ prostate cancer, we have developed a three-dimensional “organoid” system to culture metastasis samples and integrated it into our precision medicine workflow of attaining and characterizing pre-treatment biopsies. Methods: Biopsy samples of prostate cancer metastases, both soft tissue and bone, acquired at the time of therapeutic or diagnostic interventions following informed consent and institutional review board approval were obtained from two institutions. Samples were digested in Type II Collagenase (Gibco) and re-suspended in growth factor reduced Matrigel (BD), plated on plastic, and overlaid with prostate culture media (PCM). PCM consists of serum free Advanced DMEM/F12 (Gibco) with multiple growth factors optimized to propagate benign primary prostate cells. Cultures were maintained at 37°C in 5% CO2. Results: In the initial 51 samples, 15 continuous organoid cultures (29%) were established from distinct sites (9 of 32 bone, 6 of 19 soft). Tumor content of the biopsy represents a major determinant of organoid growth. Once established, organoids propagate indefinitely with different kinetics (approximately 48 hours to 1 week doubling time), and can be cryopreserved. Histological analysis shows that the organoids recapitulate the structure of the in situ cancer and genomic analysis using array CGH and whole-exome sequencing (WES) shows the presence of typical copy number alterations including TMPRSS2-ERG interstitial deletion, PTEN loss, CHD1 loss, and AR amplification. WES of two organoid/metastasis pairs shows that the growth conditions do not generate additional mutations. Conclusions: This novel tissue culture technique enables the development of new cell lines derived from metastatic deposits. This advance will facilitate research by availing new and varied cell lines, which will hopefully be more closely aligned to the spectrum of behavior of the clinical disease in comparison to the limited and problematic cell line models currently available.

"N of 1" case reports in the era of whole-genome sequencing

Prostate cancer has a range of clinical outcomes, from complete remission in response to treatment to death as a result of aggressive metastasis. Prognosis for individuals with prostate cancer is not readily predictable, and new diagnostics will be useful for treatment strategy determination. In this issue of the JCI, Haffner and colleagues use comprehensive tumor genome sequencing to investigate the origin of genetic mutations underlying a case of lethal prostate cancer. Surprisingly, the lethal clone in this individual arose from a tumor focus that is typically considered very low risk based on histology. Their report highlights the need to collect and curate "N of 1" cases to develop a database that can be used for clinical decision making.

Androgen receptor signaling regulates DNA repair in prostate cancers

We demonstrate that the androgen receptor (AR) regulates a transcriptional program of DNA repair genes that promotes prostate cancer radioresistance, providing a potential mechanism by which androgen deprivation therapy synergizes with ionizing radiation. Using a model of castration-resistant prostate cancer, we show that second-generation antiandrogen therapy results in downregulation of DNA repair genes. Next, we demonstrate that primary prostate cancers display a significant spectrum of AR transcriptional output, which correlates with expression of a set of DNA repair genes. Using RNA-seq and ChIP-seq, we define which of these DNA repair genes are both induced by androgen and represent direct AR targets. We establish that prostate cancer cells treated with ionizing radiation plus androgen demonstrate enhanced DNA repair and decreased DNA damage and furthermore that antiandrogen treatment causes increased DNA damage and decreased clonogenic survival. Finally, we demonstrate that antiandrogen treatment results in decreased classical nonhomologous end-joining.

SIGNIFICANCE

We demonstrate that the AR regulates a network of DNA repair genes, providing a potential mechanism by which androgen deprivation synergizes with radiotherapy for prostate cancer.

Phase I study of ARN-509, a novel antiandrogen, in the treatment of castration-resistant prostate cancer

PURPOSE

ARN-509 is a novel androgen receptor (AR) antagonist for the treatment of castration-resistant prostate cancer (CRPC). ARN-509 inhibits AR nuclear translocation and AR binding to androgen response elements and, unlike bicalutamide, does not exhibit agonist properties in the context of AR overexpression. This first-in-human phase I study assessed safety, tolerability, pharmacokinetics, pharmacodynamics, and antitumor activity of ARN-509 in men with metastatic CRPC.

PATIENTS AND METHODS

Thirty patients with progressive CRPC received continuous daily oral ARN-509 at doses between 30 and 480 mg, preceded by administration of a single dose followed by a 1-week observation period with pharmacokinetic sampling. Positron emission tomography/computed tomography imaging was conducted to monitor [(18)F]fluoro-α-dihydrotestosterone (FDHT) binding to AR in tumors before and during treatment. Primary objective was to determine pharmacokinetics, safety, and recommended phase II dose.

RESULTS

Pharmacokinetics were linear and dose proportional. Prostate-specific antigen declines at 12 weeks (≥ 50% reduction from baseline) were observed in 46.7% of patients. Reduction in FDHT uptake was observed at all doses, with a plateau in response at ≥ 120-mg dose, consistent with saturation of AR binding. The most frequently reported adverse event was grade 1/2 fatigue (47%). One dose-limiting toxicity event (grade 3 abdominal pain) occurred at the 300-mg dose. Dose escalation to 480 mg did not identify a maximum-tolerated dose.

CONCLUSION

ARN-509 was safe and well tolerated, displayed dose-proportional pharmacokinetics, and demonstrated pharmacodynamic and antitumor activity across all dose levels tested. A maximum efficacious dose of 240 mg daily was selected for phase II exploration based on integration of preclinical and clinical data.

Developing standards for breakthrough therapy designation in oncology

In July 2012, Congress passed the Food and Drug Administration Safety and Innovation Act (FDASIA). The Advancing Breakthrough Therapies for Patients Act was incorporated into a Title of FDASIA to expedite clinical development of new, potential "breakthrough" drugs or treatments that show dramatic responses in early-phase studies. Using this regulatory pathway, once a promising new drug candidate is designated as a "Breakthrough Therapy", the U.S. Food and Drug Administration (FDA) and sponsor would collaborate to determine the best path forward to abbreviate the traditional three-phase approach to drug development. The breakthrough legislation requires that an FDA guidance be drafted that details specific requirements of the bill to aid FDA in implementing requirements of the Act. In this article, we have proposed criteria to define a product as a Breakthrough Therapy, and discussed critical components of the development process that would require flexibility in order to enable expedited development of a Breakthrough Therapy.

Genomic heterogeneity of circulating tumor cells in castration-resistant prostate cancer (CRPC) revealed by single-cell sequencing

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Background: Circulating tumor cells (CTC) provide an opportunity to sample multiple metastatic tumor sites through a single blood draw – a ”fluid biopsy.” NextGen DNA sequencing provides the means to obtain detailed genetic information from captured cells prior to and during treatment. Here we demonstrate the use of DNA sequencing to interrogate genome-wide copy number variations (CNV) at the single-cell level in CTC isolated from pts with CRPC. Methods: Pre- and post-treatment blood samples were obtained from pts treated at MSKCC. EpCAM+ events were collected singly and in groups by cytometric flow sorting and were subjected to DNA amplification and Illumina NextGeneration sequencing. Parallel samples were assayed using the Veridex CellSearch method to ensure the presence of malignant cells. Results: Samples with up to 50 EpCAM+ events analyzed in bulk displayed CNV patterns expected from published CRPC data. Subsequent single cell analyses showed that the method could reliably detect common genomic markers in CRPC, including AR amplification, PTEN and RB1 loss, and the TMPRSS-ERG fusion. Individual genomic CNV profiles obtained from 125 single cells isolated from 15 patients were then analyzed. Using unsupervised clustering, cells from each pt showed a closely related lineage structure, consistent with an evolution from a common ancestor. The degree of genomic heterogeneity within CTC from an individual pt was highly variable, with R2 correlation coefficients ranging from >0.92 (nearly homogeneous) to <0.75 (mixed populations). Two pts harbored separate subpopulations with both amplified AR and non-amplified AR cells and another displayed mixtures of genetic markers that changed over the course of treatment. Conclusions: The observed variation in complexity of CTC populations in CRPC pts underscores the importance of being able to sample and analyze multiple cells from an individual pt on multiple occasions and with real time analytics. Doing so is essential to understand and identify mechanisms of resistance so that they can be targeted effectively. Supported by STARR Cancer Consortium, NCI SPORE in Prostate Cancer; Department of Defense; Prostate Cancer Foundation.

Overcoming mutation-based resistance to antiandrogens with rational drug design

The second-generation antiandrogen enzalutamide was recently approved for patients with castration-resistant prostate cancer. Despite its success, the duration of response is often limited. For previous antiandrogens, one mechanism of resistance is mutation of the androgen receptor (AR). To prospectively identify AR mutations that might confer resistance to enzalutamide, we performed a reporter-based mutagenesis screen and identified a novel mutation, F876L, which converted enzalutamide into an AR agonist. Ectopic expression of AR F876L rescued the growth inhibition of enzalutamide treatment. Molecular dynamics simulations performed on antiandrogen–AR complexes suggested a mechanism by which the F876L substitution alleviates antagonism through repositioning of the coactivator recruiting helix 12. This model then provided the rationale for a focused chemical screen which, based on existing antiandrogen scaffolds, identified three novel compounds that effectively antagonized AR F876L (and AR WT) to suppress the growth of prostate cancer cells resistant to enzalutamide.

DOI:http://dx.doi.org/10.7554/eLife.00499.001

Prostate cancer is the most commonly diagnosed cancer in men, and the second most lethal. All stages of prostate cancer depend upon male sex hormones, also known as androgens, to grow because these hormones bind and activate androgen receptors. A class of drugs termed ‘antiandrogens’ can effectively treat prostate cancer because they bind to androgen receptors without activating them, thereby preventing androgens from binding. However, the efficacy of even highly potent antiandrogen drugs, such as enzalutamide is short-lived in many patients, and understanding the biological mechanisms that cause drug resistance is one of the major objectives in translational prostate cancer research.

Resistance can arise through mutations of the androgen receptor that result in the receptor being activated, rather than inhibited, by antiandrogen drugs. However, no such mutations are known yet for enzalutamide, and researchers are keen to understand whether they exist and, if so, to generate new drugs for prostate cancer that overcome them. To identify mutations that may lead to resistance, Balbas et al. designed a new screening method in human prostate cancer cells and showed that androgen receptors with a specific mutation (called F876L) can be activated by enzalutamide. More comprehensive biological studies showed that prostate cancer cells harboring the mutation continued to grow when treated with the drug. Balbas et al. also showed that this mutation can arise spontaneously in human prostate cancer cells treated long term with enzalutamide.

Balbas et al. reasoned that the mutation likely altered the way enzalutamide binds to the androgen receptor, and used computer-guided structural modeling of the complex formed by the receptor and the drug to investigate how this might occur. These studies indicated that the region of the androgen receptor containing the F876L mutation comes into direct contact with the drug, and provided a structural explanation for the loss of inhibition. Because these studies showed how enzalutamide might bind to the androgen receptor, they also suggested ways in which enzalutamide could be chemically modified to restore its inhibitory activity against the mutant receptor.

Balbas et al. then designed and synthesized a set of novel compounds, which the modeling data suggested could act as inhibitors of the mutant receptor. Several of these compounds inhibited the activity of both mutant and wild-type forms of the androgen receptor, and suppressed the growth of both enzalutamide-resistant and nonresistant prostate cancer cells.

The work of Balbas et al. outlines a general screening strategy for the discovery of clinically relevant mutations in cancer genes, and shows how in silico technologies can accelerate drug discovery in the absence of a crystal structure of a protein–drug complex. It also emphasizes how understanding the manner in which a drug binds its target can stimulate rational design of improved drug candidates.

DOI:http://dx.doi.org/10.7554/eLife.00499.002

β4 Integrin signaling induces expansion of prostate tumor progenitors

The contextual signals that regulate the expansion of prostate tumor progenitor cells are poorly defined. We found that a significant fraction of advanced human prostate cancers and castration-resistant metastases express high levels of the β4 integrin, which binds to laminin-5. Targeted deletion of the signaling domain of β4 inhibited prostate tumor growth and progression in response to loss of p53 and Rb function in a mouse model of prostate cancer (PB-TAg mice). Additionally, it suppressed Pten loss-driven prostate tumorigenesis in tissue recombination experiments. We traced this defect back to an inability of signaling-defective β4 to sustain self-renewal of putative cancer stem cells in vitro and proliferation of transit-amplifying cells in vivo. Mechanistic studies indicated that mutant β4 fails to promote transactivation of ErbB2 and c-Met in prostate tumor progenitor cells and human cancer cell lines. Pharmacological inhibition of ErbB2 and c-Met reduced the ability of prostate tumor progenitor cells to undergo self-renewal in vitro. Finally, we found that β4 is often coexpressed with c-Met and ErbB2 in human prostate cancers and that combined pharmacological inhibition of these receptor tyrosine kinases exerts antitumor activity in a mouse xenograft model. These findings indicate that the β4 integrin promotes prostate tumorigenesis by amplifying ErbB2 and c-Met signaling in tumor progenitor cells.

Imaging tumor burden in the brain with 89Zr-transferrin

A noninvasive technology that indiscriminately detects tumor tissue in the brain could substantially enhance the management of primary or metastatic brain tumors. Although the documented molecular heterogeneity of diseases that initiate or eventually deposit in the brain may preclude identifying a single smoking-gun molecular biomarker, many classes of brain tumors are generally avid for transferrin. Therefore, we reasoned that applying a radiolabeled derivative of transferrin ((89)Zr-labeled transferrin) may be an effective strategy to more thoroughly identify tumor tissue in the brain, regardless of the tumor's genetic background.

METHODS

Transferrin was radiolabeled with (89)Zr, and its properties with respect to human models of glioblastoma multiforme were studied in vivo.

RESULTS

In this report, we show proof of concept that (89)Zr-labeled transferrin ((89)Zr-transferrin) localizes to genetically diverse models of glioblastoma multiforme in vivo. Moreover, we demonstrate that (89)Zr-transferrin can detect an orthotopic lesion with exceptional contrast. Finally, the tumor-to-brain contrast conferred by (89)Zr-transferrin vastly exceeded that observed with (18)F-FDG, currently the most widely used radiotracer to assess tumor burden in the brain.

CONCLUSION

The results from this study suggest that (89)Zr-transferrin could be a broadly applicable tool for identifying and monitoring tumors in the brain, with realistic potential for near-term clinical translation.

Annotating MYC status with 89Zr-transferrin imaging

A non-invasive technology that quantitatively measures the activity of oncogenic signaling pathways could broadly impact cancer diagnosis and treatment using targeted therapies. Here we describe the development of ⁸⁹Zr-desferrioxamine transferrin (⁸⁹Zr-Tf), a novel positron emission tomography (PET) radiotracer that binds the transferrin receptor 1 (TFRC, CD71) with high avidity. ⁸⁹Zr-Tf produces high contrast PET images that quantitatively reflect treatment-induced changes in MYC-regulated TFRC expression in a MYC oncogene-driven prostate cancer xenograft model. Moreover, ⁸⁹Zr-Tf imaging can detect the in situ development of prostate cancer in a transgenic MYC prostate cancer model, as well as prostatic intraepithelial neoplasia (PIN) prior to histological or anatomic evidence of invasive cancer. These preclinical data establish ⁸⁹Zr-Tf as a sensitive tool for non-invasive measurement of oncogene-driven TFRC expression in prostate, and potentially other cancers, with prospective near-term clinical application.

Distinct patterns of dysregulated expression of enzymes involved in androgen synthesis and metabolism in metastatic prostate cancer tumors

Androgen receptor (AR) signaling persists in castration-resistant prostate carcinomas (CRPC), because of several mechanisms that include increased AR expression and intratumoral androgen metabolism. We investigated the mechanisms underlying aberrant expression of transcripts involved in androgen metabolism in CRPC. We compared gene expression profiles and DNA copy number alteration (CNA) data from 29 normal prostate tissue samples, 127 primary prostate carcinomas (PCa), and 19 metastatic PCas. Steroidogenic enzyme transcripts were evaluated by quantitative reverse transcriptase PCR in PCa cell lines and circulating tumor cells (CTC) from CRPC patients. Metastatic PCas expressed higher transcript levels for AR and several steroidogenic enzymes, including SRD5A1, SRD5A3, and AKR1C3, whereas expression of SRD5A2, CYP3A4, CYP3A5, and CYP3A7 was decreased. This aberrant expression was rarely associated with CNAs. Instead, our data suggest distinct patterns of coordinated aberrant enzyme expression. Inhibition of AR activity by itself stimulated AKR1C3 expression. The aberrant expression of the steroidogenic enzyme transcripts was detected in CTCs from CRPC patients. In conclusion, our findings identify substantial interpatient heterogeneity and distinct patterns of dysregulated expression of enzymes involved in intratumoral androgen metabolism in PCa. These steroidogenic enzymes represent targets for complete suppression of systemic and intratumoral androgen levels, an objective that is supported by the clinical efficacy of the CYP17 inhibitor abiraterone. A comprehensive AR axis-targeting approach via simultaneous, frontline enzymatic blockade, and/or transcriptional repression of several steroidogenic enzymes, in combination with GnRH analogs and potent antiandrogens, would represent a powerful future strategy for PCa management.

Converting cancer therapies into cures: lessons from infectious diseases

During the past decade, cancer drug development has shifted from a focus on cytotoxic chemotherapies to drugs that target specific molecular alterations in tumors. Although these drugs dramatically shrink tumors, the responses are temporary. Research is now focused on overcoming drug resistance, a frequent cause of treatment failure. Here we reflect on analogous challenges faced by researchers in infectious diseases. We compare and contrast the resistance mechanisms arising in cancer and infectious diseases and discuss how approaches for overcoming viral and bacterial infections, such as HIV and tuberculosis, are instructive for developing a more rational approach for cancer therapy. In particular, maximizing the effect of the initial treatment response, which often requires synergistic combination therapy, is foremost among these approaches. A remaining challenge in both fields is identifying drugs that eliminate drug-tolerant "persister" cells (infectious disease) or tumor-initiating/stem cells (cancer) to prevent late relapse and shorten treatment duration.

Modulators of prostate cancer cell proliferation and viability identified by short-hairpin RNA library screening

There is significant need to identify novel prostate cancer drug targets because current hormone therapies eventually fail, leading to a drug-resistant and fatal disease termed castration-resistant prostate cancer. To functionally identify genes that, when silenced, decrease prostate cancer cell proliferation or induce cell death in combination with antiandrogens, we employed an RNA interference-based short hairpin RNA barcode screen in LNCaP human prostate cancer cells. We identified and validated four candidate genes (AKT1, PSMC1, STRADA, and TTK) that impaired growth when silenced in androgen receptor positive prostate cancer cells and enhanced the antiproliferative effects of antiandrogens. Inhibition of AKT with a pharmacologic inhibitor also induced apoptosis when combined with antiandrogens, consistent with recent evidence for PI3K and AR pathway crosstalk in prostate cancer cells. Recovery of hairpins targeting a known prostate cancer pathway validates the utility of shRNA library screening in prostate cancer as a broad strategy to identify new candidate drug targets.

Imaging androgen receptor signaling with a radiotracer targeting free prostate-specific antigen

Despite intense efforts to develop radiotracers to detect cancers or monitor treatment response, few are widely used as a result of challenges with demonstrating clear clinical use. We reasoned that a radiotracer targeting a validated clinical biomarker could more clearly assess the advantages of imaging cancer. The virtues and shortcomings of measuring secreted prostate-specific antigen (PSA), an androgen receptor (AR) target gene, in patients with prostate cancer are well documented, making it a logical candidate for assessing whether a radiotracer can reveal new (and useful) information beyond that conferred by serum PSA. Therefore, we developed (89)Zr-labeled 5A10, a novel radiotracer that targets "free" PSA. (89)Zr-5A10 localizes in an AR-dependent manner in vivo to models of castration-resistant prostate cancer, a disease state in which serum PSA may not reflect clinical outcomes. Finally, we demonstrate that (89)Zr-5A10 can detect osseous prostate cancer lesions, a context where bone scans fail to discriminate malignant and nonmalignant signals.

SIGNIFICANCE

This report establishes that AR-dependent changes in PSA expression levels can be quantitatively measured at tumor lesions using a radiotracer that can be rapidly translated for human application and advances a new paradigm for radiotracer development that may more clearly highlight the unique virtues of an imaging biomarker.

Abstract IA10: Overcoming castration-resistant prostate cancer

Resistance to antiandrogen therapy is associated with increased expression of androgen receptor (AR) mRNA, AR gene amplification or AR mutation. Increased AR levels are necessary and sufficient to promote hormone-refractory growth in models and, paradoxically, alter the cellular response to classic AR antagonists such that they function as weak agonists. Therefore, second generation antiandrogens must overcome these resistance mechanisms. We searched for novel AR antagonists that might retain function in the context of increased AR expression through a cell-based screen. Using the high affinity AR agonist RU59063 as a starting point, we synthesized and screened over 200 compounds to construct a structure/activity profile that defines features of the scaffold essential for receptor binding and for maximal antagonism. We focused our further efforts on a novel compound MDV3100, which retains potent antiandrogen activity in cells expressing increased levels of AR, blocks AR function in mice and impairs the growth of LNCaP and LAPC-4 xenografts engineered to express high levels of AR, whereas bicalutamide had minimal activity. MDV3100 inhibits AR with 10-fold greater affinity than bicalutamide and functions through a novel mechanism of action that impairs nuclear translocation and DNA binding. MDV3100 produced sustained PSA responses (greater than 50% reduction) in about ∼55 percent of men with castrate-resistant prostate cancer in a 140 patient phase I-II clinical trial. In a phase III registration study of 1119 men with castration resistance, chemotherapy refractory prostate cancer, MDV3100 prolonged survival compared to best supportive care (18.4 months versus 13.6 months; hazard ratio 0.631).

The heterogeneity of clinical response to anti-androgen therapy suggests additional molecular determinants of drug resistance. By conducting an integrated genomic analysis of 214 prostate cancers, we have defined a common set of molecular lesions and pathways that may define distinct subsets of prostate cancer as well as responder/non-responder populations to antiandrogen therapy. In preclinical models we found that tumors with PTEN loss are relatively insensitive to androgen ablation therapy due to reciprocal negative feedback between the AR and PI3K signaling pathways. PTEN-mediated resistance can be overcome with combined AR + PI3K pathway inhibition. Studies of these and other molecular abnormalities reveal unique features of prostate cancer progression with implications for downstream drug development.

Citation Format: Charles L. Sawyers. Overcoming castration-resistant prostate cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr IA10.

Abstract IA4: Outcome prediction from the pattern of aberrations in prostate cancer genomes

One of the greatest challenges in prostate cancer care is distinguishing men with high-risk disease requiring aggressive therapy from those who can be managed with active surveillance. Therefore, we are pursuing an ongoing integrative genomic analysis of human primary and metastatic prostate cancers. We explore the clinical importance and pathogenicity of genomic lesions and their association with canonical features of prostate cancer biology. Previously, we found that global patterns of DNA copy number alterations (CNAs) in primary tumors were associated with disease recurrence after radical prostatectomy. Here, we will discuss our efforts to extend this discovery by validating the clinical importance of CNAs and structural rearrangements in independent cohorts, at multiple scales, and with orthogonal methods. Our principal goal is to deliver a clinical diagnosis informed by genomic abnormalities that impacts definitive treatment decisions in men diagnosed with prostate cancer.

Citation Format: Barry S. Taylor, Haley Hieronymus, Nikolaus Schultz, Charles L Sawyers, on behalf of the MSKCC Prostate Cancer Oncogenome Group. Outcome prediction from the pattern of aberrations in prostate cancer genomes [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr IA4.

Abstract IA15: Context-specific oncogenesis by ETS-family transcription factors

Recurrent translocation involving ETS-family transcription factors including ERG and ETV1, ETV4, and ETV5 occur in over half of prostate cancers, implicating them as driver oncogenes in prostate tumorigenesis. Two observations pose important questions. 1) The same ETS factors aberrantly expressed in prostate cancer can be endogenously highly expressed in other tissues to modulate normal physiology (e.g., ERG expression in endothelial cells and ETV1 expression in neurons). 2) Forced overexpression in prostate model systems such as immortalized human primary prostate epithelial cells and mouse prostate yields minimal phenotype. These observations indicate that ETS mediated oncogenesis is highly dependent on the cellular context, which is mediated by the cell/tissue type and cooperating lesions. We asked how cellular context affects the DNA binding, transcriptional output, and phenotype of ETS proteins. We compared human ETS dependent tumor cell lines from two lineages, prostate cancer and gastrointestinal stromal tumor and show that ETS binding is largely tissue specific. Further, we have generated genetically engineered mouse models that express ETS in different tissues, within different compartments of the prostate gland, and with different cooperating lesions. We show that ERG binding to genomic DNA is largely similar in PTEN intact and PTEN null mouse prostates. Yet, ERG can only mediate a transcriptional program that mediates an aggressive phenotype in PTEN null prostates. These data suggests that cellular context is critical for ETS transcription factor function.

Citation Format: Yu Chen, Shipra Shukla, Ping Chi, Brett Carver, Charles L. Sawyers. Context-specific oncogenesis by ETS-family transcription factors [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr IA15.

Abstract C16: Targeting AR in castration-resistant prostate cancer: Development of ARN-509 and second-generation antiandrogen resistance models

The androgen receptor (AR) plays a central role in the development and progression of prostate cancer. Recent studies demonstrate that AR remains essential in the majority of castration resistant prostate cancer (CRPC) after classical androgen ablation therapies have failed. The clinical efficacies of MDV3100 and abiraterone acetate, both of which target the AR pathway in the castrate resistant setting, support these findings. ARN-509 is a 2nd generation competitive AR antagonist that, unlike bicalutamide, maintains full antagonist activity in preclinical CRPC models. ARN-509 does not robustly induce AR nuclear localization or DNA binding. However, ARN-509 displays maximal efficacy in the LNCaP/AR xenograft model of CRPC at lower dose and steady state plasma concentrations compared to MDV3100, suggesting potential for higher therapeutic index and ability to deliver the maximally efficacious dose in man. To date, ARN-509 has shown promising antitumor activity in mCRPC patients enrolled in a Phase 1 study. Given that approximately 50% of CRPC patients have suboptimal response to MDV3100 and abiraterone acetate as well as the observation that resistance eventually develops in patients who initially respond to therapy, we sought to determine whether AR remains a viable therapeutic target in the MDV3100 and ARN-509 resistant setting. To this end, we generated several MDV3100 and ARN-509 resistant derivatives of the LNCaP and LNCaP/AR cell lines. While work is underway to determine the molecular mechanisms of resistance, a subset of cell lines does not require androgens for growth in vitro. These androgen independent derivatives express AR at levels comparable to LNCaP/AR (approximately 3X LNCaP) or 2-3 fold LNCaP-AR. When representative lines are injected into castrated mice, they demonstrate a decreased latency of tumor formation compared to the parental cell line both in the presence and absence of ARN-509. Importantly, in all lines tested, small interfering RNA mediated reduction in AR levels dramatically impaired the ability of the androgen independent resistant cell lines to proliferate in the absence of androgens. These data support the hypothesis that AR remains a viable therapeutic target for second generation anti-androgen resistant prostate cancer and is the first step toward establishing a platform to screen for next generation anti-androgens.

Citation Format: James D. Joseph, Anna Aparicio, Josh Kaufman, Jackie Julien, Celine Bonnefous, Nicholas D. Smith, Peter Rix, Michael E. Jung, Charles L. Sawyers, Richard A. Heyman, Jeffrey H. Hager, Nicola J. Clegg, John Sensintaffar, Nhin Lu, Kate Grillot, Eric Bischoff, Gang Shao, Jing Qian, Beatrice Darimont. Targeting AR in castration-resistant prostate cancer: Development of ARN-509 and second-generation antiandrogen resistance models [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr C16.

Abstract PR3: The snoRNP assembly factor SHQ1 is a novel prostate cancer tumor suppressor gene

Approximately 50% of prostate tumors harbor the TMPRSS2-ERG translocation, but precisely how this mutation contributes to prostate cancer initiation and progression is unclear. In an effort to identify other causative mutations involved in prostate cancer, we previously performed an integrated genomic analysis of &lt; 200 primary and advanced human prostate cancer samples and cell lines, including assessment of genomic copy-number alterations, mRNA expression, and focused exon sequencing. We identified a recurrent genomic loss, a focal region of chromosome 3p14.1-p13, which was significantly associated with TMPRSS2-ERG translocation. Comparison of copy-number and mRNA expression data implicated at least three genes in this region (FOXP1, RYBP, and SHQ1) as potential cooperative tumor suppressors, which may function in concert with TMPRSS2-ERG translocation.

In addition to the genomic loss of SHQ1, we identified point mutations in both SHQ1 and its interacting partner DKC1/dyskerin in primary prostate tumors, leading us to focus on this SHQ1-dyskerin pathway as a potential tumor-suppressive mechanism. SHQ1 is a critical assembly factor for H/ACA-class snoRNA-containing snoRNPs (small nucleolar ribonucleoproteins), of which the core component is the RNA-modifying enzyme DKC1/dyskerin. Downstream targets of dyskerin-containing snoRNPs include the ribosome, splicesome, and telomerase RNPs. DKC1/dyskerin is mutated in the human syndrome dyskeratosis congenita (DC), a disease also caused by mutations in the telomerase complex, which results in bone marrow failure and increased incidence of various neoplasias. We found that, in both human prostate cancer cell lines and mouse fibroblasts in vitro, knockdown of SHQ1 led to increased growth and partial transformation, as evidenced by loss of anchorage-dependence. Additionally, loss of either SHQ1 or dyskerin in vitro led to a global impairment of snoRNA levels, confirming that snoRNA maturation is a major downstream target of the SHQ1-dyskerin pathway in prostate cancer cells. Strikingly, in a mouse model of prostate regeneration by sub-renal capsule implantation, SHQ1-loss in conjunction with ERG expression, but not SHQ1-loss alone, led to development of prostate intraepithelial neoplasia and a low incidence of invasive cancer. Finally, in an in vitro interaction assay, prostate cancer-derived mutations in either SHQ1 or dyskerin impaired their association, to a degree similar to that seen with mutations in dyskerin found in DC. These data, along with the identification of point mutations in both SHQ1 and DKC1/dyskerin in other human cancers, strongly implicate SHQ1 as a novel prostate cancer tumor suppressor gene, potentially acting via disruption of snoRNA maturation.

This abstract is also presented as Poster C60.

Citation Format: Phillip J. Iaquinta, Chee Wai Chua, Rosario Machado-Pinilla, Haley Hieronymus, John Wongvipat, U. Thomas Meier, Michael Shen, Charles L. Sawyers. The snoRNP assembly factor SHQ1 is a novel prostate cancer tumor suppressor gene [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr PR3.

Abstract IA22: Molecular Imaging of androgen receptor signalling in CRPC

Androgen receptor (AR) plays a key role in development of castrate-resistant prostate cancer (CRPC), the lethal form of the disease. For this reason we have developed Molecular Imaging (MI) methods to study AR, both in laboratory and clinic. For this work we have used a radiotracer that is highly specific for AR, a flourine-18 analog of dihydrotestoterone, (FDHT) the most abundant androgen at the tissue level in human cancers. In the laboratory for example, we have used FDHT to document AR binding of novel drugs and to determine their relative affinity and the number of AR receptor sites in human prostate cancer cell lines and xenografts. In the clinic we have imaged patients with CRPC to determine the impact of therapies on AR expression. During the course of this work we have developed quantitative kinetic models based on PET imaging and have shown the FDHT binding and uptake into cancers is AR dependent, and that the metabolites of FDHT do not bind appreciably to AR. We have compared FDHT and FDG in CRPC patients and have determined that there are an average of 17 tumors per patient (n=107), and the 90% of metabolically active tumors for FDG or FDHT express AR, but that there is a group of lesions that do not but are detected by metabolism of FDG. We have shown that drugs which target AR displace FDHT completely when used in full pharmacologic doses. So far the data is consistent with the view that this type of displacement is a necessary condition for tumor response, but clearly some patients whose AR binding is completely blocked have lesions which continue to progress through unknown mechanisms. We believe that molecular imaging using antibodies labeled with PET emitters will add greatly to our knowledge about CRPC, and studies in man with 89Zr-DFO J591, and antibody targeting PSMA are just beginning. The impact of AR signaling on PSMA expression has been shown in the laboratory to be down regulation, and we intend to explore the use of antibodies to provide a pharmacodynamic biomarker of AR signaling in man during therapy with AR blockers in the clinic.

Citation Format: Steven M. Larson, Joe Fox, Michael Morris, Michael Evans, Jason Lewis, John Humm, Charles L. Sawyers, Howard I. Scher. Molecular Imaging of androgen receptor signaling in CRPC [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr IA22.