Abstract 1262: Targeting transcriptional addiction in anti-androgen refractory castration-resistant prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer-related mortality in men of the western world. Androgen receptor (AR) is the primary driver alteration in metastatic castration-resistant prostate cancer (mCRPC). Advanced PCa treated with first line androgen-deprivation therapy (ADT) eventually relapse in a hormone refractory or castration-resistant (CR) form. Relapsed disease is highly aggressive and poses an increased risk of morbidity and death. This refractory state of CRPC is characterized by a strong addiction to AR-mediated transcription driven by the synergy between a variety of genomic alterations and epigenetic remodeling. Despite the success of recently approved therapies targeting AR signaling, such as abiraterone and second-generation anti-androgens including enzalutamide, durable responses are limited, owing to acquired resistance. Epigenetic therapies targeting coactivators within the AR transcription machinery have shown promise as effective options for the treatment of CRPC in both its naive and Enzalutamide Resistant state (Enza-Res). Using basic proteomic and next generation genomic approaches, we have identified CDK7, as an indispensable component of the AR-transcription machinery and a novel target in CRPC. The phosphorylation of MED1 (T1457) by CDK7 leads to the formation of the chromatin bound AR-MED1 complex and is essential for its stability and onset of AR-mediated transcription. Loss of CDK7 activity either through genetic silencing or through treatment with the CDK7 inhibitor THZ1 led to de-recruitment of chromatin bound MED1, loss of AR-target gene expressions, increased apoptosis, and a drastic reduction in cell proliferation. Here, we present evidence for the efficacy of targeting CDK7 in curtailing AR signaling in the Enza-Res state. Further, we observed hyper-phosphorylation of MED1 and AR, and a concomitant decrease in the catalytic-subunit (alpha-isoform) of the protein phosphatase 2A (PP2ACα) in enzalutamide resistant cell lines and metastatic biopsies. This hyper-phosphorylated state was strongly dependent on the catalytic activity of CDK7, wherein treatment of Enza-Res cells with THZ1 led to complete loss of phosphorylation of MED1 and AR, and death in Enza-res cells. These findings point to the indispensable role of CDK7 in driving the AR oncogenic transcription program in anti-androgen refractory CRPC and its viability as a potential therapeutic target.

Citation Format: Reyaz ur Rasool, Ramakrishnan Natesan, Qu Deng, Shweta Aras, Samuel Sander Effron, Samuel Sander Effron, Erick Mitchell Velasquez, Jessica M. Posimo, Priti Lal, Donita C. Brady, Irfan A. Asangani. Targeting transcriptional addiction in anti-androgen refractory castration-resistant prostate cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1262.

CDK7 Inhibition Suppresses Castration-Resistant Prostate Cancer through MED1 Inactivation

Metastatic castration-resistant prostate cancer (CRPC) is a fatal disease, primarily resulting from the transcriptional addiction driven by androgen receptor (AR). First-line CRPC treatments typically target AR signaling, but are rapidly bypassed, resulting in only a modest survival benefit with antiandrogens. Therapeutic approaches that more effectively block the AR-transcriptional axis are urgently needed. Here, we investigated the molecular mechanism underlying the association between the transcriptional coactivator MED1 and AR as a vulnerability in AR-driven CRPC. MED1 undergoes CDK7-dependent phosphorylation at T1457 and physically engages AR at superenhancer sites, and is essential for AR-mediated transcription. In addition, a CDK7-specific inhibitor, THZ1, blunts AR-dependent neoplastic growth by blocking AR/MED1 corecruitment genome-wide, as well as reverses the hyperphosphorylated MED1-associated enzalutamide-resistant phenotype. In vivo, THZ1 induces tumor regression of AR-amplified human CRPC in a xenograft mouse model. Together, we demonstrate that CDK7 inhibition selectively targets MED1-mediated, AR-dependent oncogenic transcriptional amplification, thus representing a potential new approach for the treatment of CRPC. SIGNIFICANCE: Potent inhibition of AR signaling is critical to treat CRPC. This study uncovers a driver role for CDK7 in regulating AR-mediated transcription through phosphorylation of MED1, thus revealing a therapeutically targetable potential vulnerability in AR-addicted CRPC.See related commentary by Russo et al., p. 1490.This article is highlighted in the In This Issue feature, p. 1469.

Epigenetic Regulation of Chromatin in Prostate Cancer

Epigenetics refers to mitotically/meiotically heritable mechanisms that regulate gene transcription without a need for changes in the DNA code. Covalent modifications of DNA, in the form of methylation, and histone post-translational modifications, in the form of acetylation and methylation, constitute the epigenetic code of a cell. Both DNA and histone modifications are highly dynamic and often work in unison to define the epigenetic state of a cell. Most epigenetic mechanisms regulate gene transcription by affecting localized/genome-wide transitions between heterochromatin and euchromatin states, thereby altering the accessibility of the transcriptional machinery and in turn, reduce/increase transcriptional output. Altered chromatin structure is associated with cancer progression, and epigenetic plasticity primarily governs the resistance of cancer cells to therapeutic agents. In this chapter, we specifically focus on regulators of histone methylation and acetylation, the two well-studied chromatin post-translational modifications, in the context of prostate cancer.

PARP-1 Expression Quantified by [18F]FluorThanatrace: A Biomarker of Response to PARP Inhibition Adjuvant to Radiation Therapy

INTRODUCTION

Poly (ADP-ribose) polymerase 1 (PARP-1) is the major target of clinical PARP inhibitors and is a potential predictive biomarker for response to therapy. Due to the limited success of PARP inhibitors as monotherapy, investigators have shifted the clinical role of PARP inhibitors to the adjuvant setting. In this study, we evaluate the radiotracer [¹⁸F]FluorThanatrace ([¹⁸F]FTT) as a marker of PARP expression in vitro and the associated biological implications of PARP-1 expression in PARP inhibitor treatment adjuvant to radiation therapy.

MATERIALS AND METHODS

SNU-251 (BRCA1-mutant) and SKOV3 (BRCA1-WT) cell lines were evaluated in vitro by using the radiotracer [¹⁸F]FTT. Pharmacological binding assays were performed at baseline and were correlated with PARP-1 protein expression measured by Western blot protein analysis. Cell viability and clonogenic assays were used to characterize in vitro cytotoxicity for treatments, including: PARP inhibitors alone, radiation alone, and PARP inhibitor adjuvant to radiation. Western blot protein analysis was used to assess response to treatment by using γH2AX to measure DNA damage and PAR to measure the catalytic inhibition of PARP.

RESULTS

[¹⁸F]FTT was capable of measuring PARP-1 protein expression in vitro and corresponded to Western blot protein analysis at baseline. The addition of a PARP inhibitor enhanced radiation effects in both cell lines; however, a greater synergy was observed in the SNU-251 cell line that expresses a BRCA1 mutation and homologous recombination deficiency. Western blot protein analysis showed that the addition of a PARP inhibitor adjuvant to radiation increases DNA damage in both cell lines and reduces PARP enzymatic activity as measured by PAR.

CONCLUSIONS

In this work, we found that PARP-1 expression positively corresponds in vitro to the response of PARP inhibitors in combination with radiation therapy in ovarian cancer.

A Radiotracer Strategy to Quantify PARP-1 Expression In Vivo Provides a Biomarker That Can Enable Patient Selection for PARP Inhibitor Therapy

Despite the availability of PARP inhibitors for cancer therapy, a biomarker to clearly stratify patients for selection of this treatment remains lacking. Here we describe a radiotracer-based method that addresses this issue, using the novel compound [(125)I] KX1: as a PARP-1-selective radiotracer that can accurately measure PARP-1 expression in vitro and in vivo The pharmacologic properties of the PARP radiotracer [(125)I] KX1: was characterized in multiple cell lines where single-agent sensitivity was correlated with [(125)I] KX1: binding to PARP-1. In vivo evaluation of [(125)I] KX1: verified in vitro results, validating PARP radiotracers to define PARP-1 enzyme expression as an in vivo biomarker. Notably, PARP-1 expression as quantified by [(125)I] KX1: correlated positively with the cytotoxic sensitivity of cell lines evaluated with PARP inhibitors. Overall, our results defined a novel technology with the potential to serve as a companion diagnostic to identify patients most likely to respond therapeutically to a PARP inhibitor. Cancer Res; 76(15); 4516-24. ©2016 AACR.