Full-length Androgen Receptor Research Articles

An Autophagy-Targeting Chimera Induces Degradation of Androgen Receptor Mutants and AR-v7 in Castration-Resistant Prostate Cancer.

Genetic alterations play a pivotal role in various human diseases, particularly cancer. The androgen receptor (AR) is a crucial transcription factor driving prostate cancer (PCa) progression across all stages. Current AR-targeting therapies utilize competitive AR antagonists or pathway suppressors. However, therapy resistance often emerges due to AR mutations and AR splice variants, such as AR-v7. To overcome this, we developed ATC-324, an AR degrader using the innovative protein degradation technology platform AUTOphagy-TArgeting Chimera (AUTOTAC). ATC-324 was designed to comprise enzalutamide, an AR inhibitor, as a target-binding ligand and YT 6-2, a ligand of the autophagy receptor p62/SQSTM1, as an autophagy-targeting ligand. ATC-324 induces the formation of the AR/p62 complex, leading to autophagy-lysosomal degradation of AR. Importantly, ATC-324 effectively degrades AR mutants frequently detected in PCa and co-degrades AR-v7 as a heterodimer with full-length AR. ATC-324 reduces nuclear AR levels and downregulates the target gene expression of AR and AR-v7, leading to cytotoxicity in AR-positive PCa cells. We also provide evidence of the therapeutic potential of ATC-324 in vivo as well as ex vivo bone organ culture. Moreover, ATC-324 remains potent in enzalutamide-resistant PCa cells. These results demonstrate the potential of the AUTOTAC platform to target previously considered undruggable proteins and overcome certain drug resistance mechanisms.

Targeted Delivery of AR-V7 siRNA with Bivalent PSMA Aptamers Effectively Suppresses the Growth of Enzalutamide-Resistant Prostate Cancer.

Androgen deprivation therapy has been the primary treatment strategy for advanced prostate cancer (PCa). But most patients develop castration resistance over time. For FDA-approved second-generation androgen receptor (AR) antagonists, including enzalutamide (ENZ) and abiraterone (AA), patients who initially respond to them eventually develop resistance. The key mechanism for resistance to ENZ/AA involves AR splice variants (AR-Vs) and specifically AR-V7. Current AR antagonists cannot target AR-V7 due to its lack of the C-terminal ligand-binding domain (LBD) but keeping the AR N-terminal domain (NTD) which still can activate androgen-responsive genes. Therefore, targeting the AR NTD and AR-V7 is critically important to overcome ENZ resistance. Unfortunately, AR NTD has been considered an "undruggable" target due to the difficulty in defining its three-dimensional (3D) structure. In this context, siRNA is highly suitable to address this undruggable target. However, siRNA cannot freely diffuse into cells, and a carrier is needed. In this regard, nucleic acid-based aptamers are highly suitable for cell type-specific delivery of siRNA in vivo. In this study, we have developed a serum-stable bivalent prostate-specific membrane antigen (PSMA) aptamer-AR-V7 siRNA chimera (PAP). The results show that PAP can knock down both AR-full length and AR-V7 in PSMA-expressing castration-resistant cells. It can resensitize ENZ in cell lines and PCa xenografts. ENZ combined with PAP can significantly inhibit 22Rv1 xenograft growth in mice without experiencing castration. Owing to the low toxicity, PAP has potential to offer a new antiandrogen treatment for current ENZ-resistant PCa.

Defining splicing factor requirements for androgen receptor variant synthesis in advanced prostate cancer.

Resistance to androgen receptor (AR)-targeted therapies represent a major challenge in prostate cancer (PC). A key mechanism of treatment resistance in patients who progress to castrate-resistant PC (CRPC) is the generation of alternatively spliced androgen receptor variants (AR-Vs). Unlike full-length AR (FL-AR) isoforms, AR-Vs are constitutively active and refractory to current receptor-targeting agents hence drive tumour progression. Identifying regulators of AR-V synthesis may therefore provide new therapeutic opportunities in combination with conventional AR-targeting agents. Our understanding of AR transcript splicing, and the factors that control the synthesis of AR-Vs, remains limited. While candidate-based approaches have identified a small number of AR-V splicing regulators, an unbiased analysis of splicing factors important for AR-V generation is required to fill an important knowledge gap and furnish the field with novel and tractable targets for PC treatment. To that end, we conducted a bespoke CRISPR screen to profile splicing factor requirements for AR-V synthesis. MFAP1 and CWC22 were shown to be required for the generation of AR-V mRNA transcripts and their depletion resulted in reduced AR-V protein abundance and cell proliferation in several CRPC models. Global transcriptomic analysis of MFAP1-depleted cells revealed both AR-dependent and -independent transcriptional impact, including genes associated with DDR. As such, MFAP1 downregulation sensitised PC cells to ionising radiation suggesting therapeutically targeting AR-V splicing could provide novel cellular vulnerabilities which can be exploited in CRPC. Implications: We have utilised a CRISPR screening approach to identify key regulators of pathogenic AR splicing in prostate cancer.

Increased chromatin accessibility mediated by nuclear factor I drives transition to androgen receptor splice variant dependence in castration-resistant prostate cancer.

Androgen receptor (AR) splice variants, of which ARv7 is the most common, are increased in prostate cancer (PC) that develops resistance to androgen signaling inhibitor drugs, but the extent to which these variants drive AR activity, and whether they have novel functions or dependencies, remain to be determined. We generated a subline of VCaP PC cells (VCaP16) that is resistant to the AR inhibitor enzalutamide (ENZ) and found that AR activity was independent of the full-length AR (ARfl), despite its continued high-level expression, and was instead driven by ARv7. The ARv7 cistrome and transcriptome in VCaP16 cells mirrored that of the ARfl in VCaP cells, although ARv7 chromatin binding was weaker, and strong ARv7 binding sites correlated with higher affinity ARfl binding sites across multiple models and clinical samples. Notably, although ARv7 expression in VCaP cells increased rapidly in response to ENZ, there was a long lag before it gained chromatin binding and transcriptional activity. This lag was associated with an increase in chromatin accessibility, with the AR and nuclear factor I (NFI) motifs being most enriched at these more accessible sites. Moreover, the transcriptional effects of combined NFIB and NFIX knockdown versus ARv7 knockdown were highly correlated. These findings indicate that ARv7 can drive the AR program, but that its activity is dependent on adaptations that increase chromatin accessibility to enhance its intrinsically weak chromatin binding.

Abstract PO1-13-06: Androgen Receptor Isoform V7 (AR-V7): A Promising Biomarker for Prognosticating Breast Cancer Aggressiveness

Abstract Androgen Receptor (AR) is an emerging endocrine therapy target in Breast Cancer (BrCa) with up to 80% expression in clinical cases. AR-V7 is a constitutively activated splice variant of AR having a truncated ligand-binding domain (LBD). This confers ligand-independent transcriptional activity to AR-V7 and makes it susceptible to nonsteroidal antiandrogens such as Bicalutamide or Enzalutamide which target the LBD of AR. Higher levels of AR-V7 in metastatic prostate cancer leads to therapeutic resistance and enhanced metastasis. We assessed AR-full length (AR-FL) and AR-V7 expression in BrCa cell lines of different molecular subtypes. This was extended for a clinical correlation of AR-FL and AR-V7 in validated treatment naïve BrCa patients undergoing surgical intervention at a tertiary care hospital in India. Transcriptomic and proteomic analysis was performed using qRT-PCR and Western Blotting. Immunocytochemistry and immunohistochemistry were used to examine protein expression and localisation in cells and tissues. AR-FL and AR-V7 were variably expressed in cell lines which got elevated with Dihydrotestosterone (DHT) treatment. Cell lines stimulated with Bicalutamide or Enzalutamide showed significant downregulation in AR-FL expression. From a cohort of 82 clinical cases, more than 50% originated from the urban setting, post-menopausal stage, and in the age groups of 61 to 70 years. ER, PR, HER2, and AR were positive in 65%, 55%, 24%, and 67% respectively. We were able to correlate aggressive clinical traits and higher pathological grades with increased expression of AR-V7 in the AR-positive BrCa fraction. This was consistent with the cell line-based preclinical research. This study affiliates AR-V7 as an important prognostic biomarker for predicting breast cancer aggressiveness. It can serve as a screening marker of poor clinical outcomes and aid in appropriate therapeutic intervention. These outcomes, however, seek extensive prospective validation. Citation Format: Tryambak Pratap Srivastava, Joyeeta Talukdar, Rohit Kumar Verma, Subhradip Karmakar. Androgen Receptor Isoform V7 (AR-V7): A Promising Biomarker for Prognosticating Breast Cancer Aggressiveness [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-13-06.

Androgen receptor splice variants drive castration-resistant prostate cancer metastasis by activating distinct transcriptional programs.

One critical mechanism through which prostate cancer (PCa) adapts to treatments targeting androgen receptor (AR) signaling is the emergence of ligand-binding domain-truncated and constitutively active AR splice variants, particularly AR-V7. While AR-V7 has been intensively studied, its ability to activate distinct biological functions compared with the full-length AR (AR-FL), and its role in regulating the metastatic progression of castration-resistant PCa (CRPC), remain unclear. Our study found that, under castrated conditions, AR-V7 strongly induced osteoblastic bone lesions, a response not observed with AR-FL overexpression. Through combined ChIP-seq, ATAC-seq, and RNA-seq analyses, we demonstrated that AR-V7 uniquely accesses the androgen-responsive elements in compact chromatin regions, activating a distinct transcription program. This program was highly enriched for genes involved in epithelial-mesenchymal transition and metastasis. Notably, we discovered that SOX9, a critical metastasis driver gene, was a direct target and downstream effector of AR-V7. Its protein expression was dramatically upregulated in AR-V7-induced bone lesions. Moreover, we found that Ser81 phosphorylation enhanced AR-V7's pro-metastasis function by selectively altering its specific transcription program. Blocking this phosphorylation with CDK9 inhibitors impaired the AR-V7-mediated metastasis program. Overall, our study has provided molecular insights into the role of AR splice variants in driving the metastatic progression of CRPC.

Manzamine A reduces androgen receptor transcription and synthesis by blocking E2F8-DNA interactions and effectively inhibits prostate tumor growth in mice.

The androgen receptor (AR) is the main driver in the development of castration-resistant prostate cancer, where the emergence of AR splice variants leads to treatment-resistant disease. Through detailed molecular studies of the marine alkaloid manzamine A (MA), we identified transcription factor E2F8 as a previously unknown regulator of AR transcription that prevents AR synthesis in prostate cancer cells. MA significantly inhibited the growth of various prostate cancer cell lines and was highly effective in inhibiting xenograft tumor growth in mice without any pathophysiological perturbations in major organs. MA suppressed the full-length AR (AR-FL), its spliced variant AR-V7, and the AR-regulated prostate-specific antigen (PSA; also known as KLK3) and human kallikrein 2 (hK2; also known as KLK2) genes. RNA sequencing (RNA-seq) analysis and protein modeling studies revealed E2F8 interactions with DNA as a potential novel target of MA, suppressing AR transcription and its synthesis. This novel mechanism of blocking AR biogenesis via E2F8 may provide an opportunity to control therapy-resistant prostate cancer over the currently used AR antagonists designed to target different parts of the AR gene.

TAS3681, an androgen receptor antagonist, prevents drug resistance driven by aberrant androgen receptor signaling in prostate cancer.

Second-generation androgen receptor (AR) signaling inhibitors (ARSIs), such as abiraterone and enzalutamide, prolong the life of patients with castration-resistant prostate cancer (CRPC). However, patients receiving ARSIs ultimately develop resistance through various complex mechanisms, including AR mutations, constitutively active AR-splice variants (AR-Vs), and AR overexpression. Here, we characterized a novel AR pure antagonist, TAS3681, which inhibits AR transcriptional activity and downregulates AR-full length (AR-FL) and AR-Vs. TAS3681 reduced the protein levels of AR-FL and AR-Vs including AR-V7 in enzalutamide-resistant cells (SAS MDV No. 3-14), in vitro and in vivo, showing strong antitumor efficacy in an AR-V7-positive xenograft model. In AR-overexpressing VCaP (prostate cancer) cells, conversely to enzalutamide, TAS3681 effectively suppressed cell proliferation and downregulated AR expression. Importantly, TAS3681 blocked the transcriptional activity of various mutant ARs, including mutations F877L/T878A and H875Y/T878A, which confer resistance to enzalutamide, and V716M and H875Y mutations, which confer resistance to darolutamide. Our results demonstrate that TAS3681 suppresses the reactivation of AR signaling, which causes resistance to ARSIs, via a newly identified mechanism of action. Therefore, TAS3681 could be a new therapeutic option for CRPC treatment.

SC912 inhibits AR-V7 activity in castration-resistant prostate cancer by targeting the androgen receptor N-terminal domain.

Androgen deprivation therapies (ADT) are the mainstay treatments for castration-resistant prostate cancer (CRPC). ADT suppresses the androgen receptor (AR) signaling by blocking androgen biosynthesis or inhibiting AR with antiandrogens that target AR's ligand-binding domain (LBD). However, the ADT's effect is short-lived, as the AR signaling inevitably arises again, which is frequently coupled with AR-V7 overexpression. AR-V7 is a truncated form of AR that lacks the LBD, thus being constitutively active in the absence of androgens and irresponsive to AR-LBD-targeting inhibitors. Though compelling evidence has tied AR-V7 to drug resistance in CRPC, pharmacological inhibition of AR-V7 is still an unmet need. Here, we discovered a small molecule, SC912, which binds to full-length AR as well as AR-V7 through AR N-terminal domain (AR-NTD). This pan-AR targeting relies on the amino acids 507-531 in the AR-NTD. SC912 also disrupted AR-V7 transcriptional activity, impaired AR-V7 nuclear localization and DNA binding. In the AR-V7 positive CRPC cells, SC912 suppressed proliferation, induced cell-cycle arrest, and apoptosis. In the AR-V7 expressing CRPC xenografts, SC912 attenuated tumor growth and antagonized intratumoral AR signaling. Together, these results suggested the therapeutic potential of SC912 for CRPC.

Abstract 6271: Novel insights into androgen receptor-variant 7 subnuclear localization and function in castration resistant prostate cancer

Abstract Prostate cancer (PC) stands as the most frequently diagnosed and second leading cause of cancer death among American men. PC mortality is predominantly attributed to castration-resistant prostate cancer (CRPC), often emerging within 2-3 years following the initiation of androgen deprivation therapy. This resistance primarily arises from sustained androgen receptor (AR) signaling, ultimately resulting in a compromised response to AR-directed therapies. AR splice variant expression is a key contributor to the persistent AR signaling in CRPC. AR-variant 7 (AR-V7) is the most clinically prevalent variant, expressed in about 75% of patients with CRPC and confers resistance to standard of care (SoC) treatments. AR-V7 differs from the full-length AR (AR-fl) because it lacks the ligand binding and hinge domains, rendering it resistant to SoC treatments. Currently, there are no selective inhibitors for AR-V7. Therefore, we sought to identify unique biological features of AR-V7, distinct from AR-fl, to exploit therapeutic options for targeting AR-V7. Previous studies have demonstrated high sequence homology, largely overlapping cistromes, and gene transactivation profiles between AR-V7 and AR-FL. Our published mechanistic studies revealed that AR-V7 utilizes a unique nuclear import pathway, not shared by AR-fl, allowing for constitutive import and activation of target genes in the nucleus. Herein, we report an additional novel subnuclear phenotype of AR-V7 in LNCAP95 and 22RV1 cells modeling CRPC. Staining with AR-V7- and AR-fl- specific antibodies revealed that unlike AR-fl, AR-V7 exhibited prominent nucleolar localization. To validate AR-V7 nucleolar localization, CRPC cells stained with fibrillarin (nucleolar marker), and subsequent fluorescence intensity analysis revealed that AR-V7 was predominantly localized in fibrillarin positive nucleoli, whereas AR-fl protein was solely localized in the nucleoplasm. This suggests a novel functional role of AR-V7 in the nucleus. To further confirm AR-V7 nucleolar localization, Actinomycin D (ActD) was used as a tool compound to disrupt nucleolar integrity. Treatment with ActD for 4 hours in CRPC cells significantly reduced the area of fibrillarin positive nucleoli and prompted AR-V7 re-distribution out of the nucleoli and into the nucleoplasm. Ongoing efforts to further investigate AR-V7 nucleolar function in CRPC cells include mutating the predicted nucleolar localization signal located in AR-V7 and identifying nucleolar-interacting proteins through proximity-labeling assays. In conclusion, we unveiled a novel nuclear compartmentalization of AR-V7 within the nucleolus, the primary hub for ribosomal biogenesis. These observations suggest that AR-V7 could regulate the high levels of protein synthesis occurring in CRPC cells, creating a window of a potential therapeutic opportunity for selective inhibition. Citation Format: Michelle K. Naidoo, Paraskevi Giannakakou. Novel insights into androgen receptor-variant 7 subnuclear localization and function in castration resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6271.

Abstract 6071: Identification of a molecular glue degrader targeting the full-length AR and AR-V7 splice variant

Abstract Prostate cancer is the most common cancer in males and the fifth leading cause of cancer mortality worldwide. A variety of therapeutic approaches exist including steroid synthesis inhibitors and next-generation ligand binding domain (LBD) antagonists [e.g., enzalutamide (Enza)]. However, these approaches are only effective against tumors bearing the full-length androgen receptor (AR-FL). In the course of their disease, many patients acquire androgen receptor splice variant 7 (AR-V7), a mutant form of the receptor which lacks the LBD and is constitutively activated. To date, no effective therapy has been demonstrated for patients with metastatic prostate cancer bearing the AR-V7 signature. We have recently developed a high throughput platform that allows for the identification of ligands that can bind to a wide variety of protein targets. Using this approach, we screened a diverse chemical library of 100,000 small molecules to identify compounds that are bound to AR-V7. One such molecule, AR-600, directly bound to AR-V7 and moreover, induced proteasomal-mediated targeted protein degradation of both AR-V7 and AR-FL. In prostate cancer cell lines expressing AR-FL, AR-600 induced a gene expression profile similar to cells treated with Enza including a marked reduction in KLK3 (PSA) expression. Moreover, AR-600 inhibited the growth of 22RV1 cells (IC50=23 nM), a cell line that predominantly expresses AR-V7, as well as C4-2 cells that express predominantly full-length AR (IC50=51 nM). In contrast, the compound did not affect the growth of a wide variety of non-tumorigenic cells, non-prostate tumor cell lines or PC3 cells, a prostate cell line that exhibits AR-independent cell growth (IC50>100uM). Preliminary exploration of the mechanism of action of AR-600 (M.W. ~300 Daltons) demonstrates the compound works as a molecular glue. These results thereby identify a small molecule glue degrader that can specifically induce the targeted protein degradation of the AR-V7 splice variant. Given the strong association of this isoform with treatment resistance, this approach would appear to provide an attractive strategy for patients with metastatic castration resistant prostate cancer. Citation Format: Yuan Liu, Mads B. Larsen, Bo Lin, Irene A. Cardenal, Jason Kennerdell, Toren Finkel, Bill B. Chen. Identification of a molecular glue degrader targeting the full-length AR and AR-V7 splice variant [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6071.

Analysis of the responsiveness to antiandrogens in multiple breast cancer cell lines.

Antiandrogens were originally developed as therapeutic agents for prostate cancer but are also expected to be effective for breast cancer. However, the role of androgen signaling in breast cancer has long been controversial due to the limited number of experimental models. Our study aimed to comprehensively investigate the efficacy of antiandrogens on breast cancer. In the present study, a total of 18 breast cancer cell lines were treated with the agonist or antagonists of the androgen receptor (AR). Among the 18 cell lines tested, only T-47D cells proliferated in an androgen-dependent manner, while the other cell lines were almost irresponsive to AR stimulation. On the other hand, treatment with AR antagonists at relatively high doses suppressed the proliferation of not only T-47D cells but also some other cell lines including AR-low/negative cells. In addition, expression of the full-length AR and constitutively active AR splice variants, AR-V7 and ARV567es, was not correlated with sensitivity to AR antagonists. These data suggest that the antiproliferative effect of AR antagonists is AR-independent in some cases. Consistently, proliferation of AR-knockout BT-549 cells was inhibited by AR antagonists. Identification of biomarkers would be necessary to determine which breast cancer patients will benefit from these drugs.

Treatments Targeting the Androgen Receptor and Its Splice Variants in Breast Cancer.

Breast cancer is a major cause of death worldwide. The complexity of endocrine regulation in breast cancer may allow the cancer cells to escape from a particular treatment and result in resistant and aggressive disease. These breast cancers usually have fewer treatment options. Targeted therapies for cancer patients may offer fewer adverse side effects because of specificity compared to conventional chemotherapy. Signaling pathways of nuclear receptors, such as the estrogen receptor (ER), have been intensively studied and used as therapeutic targets. Recently, the role of the androgen receptor (AR) in breast cancer is gaining greater attention as a therapeutic target and as a prognostic biomarker. The expression of constitutively active truncated AR splice variants in breast cancer is a possible mechanism contributing to treatment resistance. Therefore, targeting both the full-length AR and AR variants, either through the activation or suppression of AR function, depending on the status of the ER, progesterone receptor, or human epidermal growth factor receptor 2, may provide additional treatment options. Studies targeting AR in combination with other treatment strategies are ongoing in clinical trials. The determination of the status of nuclear receptors to classify and identify patient subgroups will facilitate optimized and targeted combination therapies.

Development of a Double-Stapled Peptide Stabilizing Both α-Helix and β-Sheet Structures for Degrading Transcription Factor AR-V7.

Peptide drugs offer distinct advantages in therapeutics; however, their limited stability and membrane penetration abilities hinder their widespread application. One strategy to overcome these challenges is the hydrocarbon peptide stapling technique, which addresses issues such as poor conformational stability, weak proteolytic resistance, and limited membrane permeability. Nonetheless, while peptide stapling has successfully stabilized α-helical peptides, it has shown limited applicability for most β-sheet peptide motifs. In this study, we present the design of a novel double-stapled peptide capable of simultaneously stabilizing both α-helix and β-sheet structures. Our designed double-stapled peptide, named DSARTC, specifically targets the androgen receptor (AR) DNA binding domain and MDM2 as E3 ligase. Serving as a peptide-based PROTAC (proteolysis-targeting chimera), DSARTC exhibits the ability to degrade both the full-length AR and AR-V7. Molecular dynamics simulations and circular dichroism analysis validate the successful constraint of both secondary structures, demonstrating that DSARTC is a "first-in-class" heterogeneous-conformational double-stapled peptide drug candidate. Compared to its linear counterpart, DSARTC displays enhanced stability and an improved cell penetration ability. In an enzalutamide-resistant prostate cancer animal model, DSARTC effectively inhibits tumor growth and reduces the levels of both AR and AR-V7 proteins. These results highlight the potential of DSARTC as a more potent and specific peptide PROTAC for AR-V7. Furthermore, our findings provide a promising strategy for expanding the design of staple peptide-based PROTAC drugs, targeting a wide range of "undruggable" transcription factors.

Maternal obesity impacts fetal liver androgen signalling in a sex-specific manner

BackgroundMaternal obesity (MO) increases fetal androgen concentrations, the prevalence of macrosomia, and predisposes offspring to metabolic dysfunction in later life, especially males. These risks may be, in part, the result of increased liver-specific androgen signalling pathway activity in utero. Androgen signalling activity can be suppressed by androgen metabolism via cytochrome P450 (CYP) isoenzymes (CYP2B6, CYP3A) or through inhibition of the full-length androgen receptor (AR-FL) via the antagonistic isoform, AR-45. We hypothesised MO impairs CYP enzyme activity and AR-45 expression in male fetal livers, thereby enhancing activity of androgen signalling pathways. MethodsNine months prior to pregnancy, nulliparous female baboons were assigned to either ad libitum control or high fat diet. At 165 day (d) gestation (term, 180 d) fetal liver was collected (n = 6/sex/group). CYP activity was quantified using functional assays; subcellular AR expression was measured using Western blot. ResultsCYP2B6 and CYP3A activity, and nuclear expression of AR-45, was reduced in MO males only. Nuclear AR-45 expression was inversely related with fetal body weight of MO males only. ConclusionsReduced CYP2B6 and CYP3A activity in conjunction with decreased nuclear AR-45 expression may enhance liver androgen signalling in males from MO pregnancies, thereby increasing the risk of macrosomia, as well as metabolic dysfunction in later life.

Abstract C085: OPN-6602, a dual EP300/CBP bromodomain inhibitor modulates androgen-driven transcription, including MYC, in mCRPC

Abstract Metastatic castration-resistant prostate cancer (mCRPC) remains an urgent unmet medical need despite existing therapies that target androgen receptor (AR) signaling. Disease progression is commonly a result of acquired resistance to existing anti-androgen therapies. Biological models have revealed potential resistance mechanisms of CRPC through the reactivation of the AR pathway. Transcriptional coactivators CREB-binding protein (CBP) and E1A binding protein (EP300) are high value targets in mCRPC as they mediate androgen-dependent and -independent activity of the AR pathway. EP300 and CBP proteins harbor both histone acetyltransferase (HAT)- and bromo-domains, and the HAT activity of EP300 regulates AR function. EP300 bromodomain inhibition partially blocks its HAT activity, reducing AR-mediated gene expression and tumor growth. OPN-6602 and OPN-6742 are potent, selective, and orally active small molecule dual EP300/CBP bromodomain inhibitors which abrogate persistent AR signaling in CRPC by modulating H3K27 acetylation and the recruitment of EP300, AR full length (AR-FL), and AR variant 7 (AR-v7) to AR response elements. OPN-6602 and OPN-6742 obstruct the EP300 bromodomain leading to downregulation of the AR pathway and consequent anti-tumor effects in AR-dependent CRPC. OPN-6602 and OPN-6742 are highly selective (>100-fold) against other bromodomains based on a screening panel of 40 bromodomains. Biochemical screens using AlphaScreen technology show these compounds to be equipotent against EP300 and CBP. OPN-6602 is particularly potent by Surface Plasmon Resonance analysis, displaying a KD of 0.87 nM, with a fast association rate (1.5 E+06/Ms) and a slow dissociation rate (1.3 E-03/s). Consequently, OPN-6602 displays a noteworthy residence time of 13 minutes. Both compounds show an AR-selective mode of action with anti-proliferative activity in AR-dependent prostate cancer cell lines (e.g. VCaP, LNCaP, and 22Rv-1) but are inactive against AR-negative prostate cancer cell lines (e.g. DU145). VCaP and mCRPC patient-derived xenograft (PDX) RNA profiling provides evidence of a marked, dose-dependent, pharmacodynamic response indicative of EP300/CBP inhibition. In the VCaP xenograft model, OPN-6602 and OPN-6742 show dose-dependent tumor growth inhibition and corresponding pharmacodynamic responses with prostate-specific antigen level decreases and decreased H3K27 acetylation. In the QR (AR-v7+) PDX model of CRPC (post-androgen deprivation therapy), a synergistic effect was observed between OPN-6602 and enzalutamide. This combination further modulates the expression of AR-driven transcripts such as MYC compared to single agent treatments. The pharmacokinetic profiles of OPN-6602 and OPN-6742 are favorable for oral single daily dose administration due to a high Cmax and short half-life. Due to its higher potency, OPN-6602 has been chosen as clinical candidate and a first-in-human study will start in 2024. Citation Format: Bernice Matusow, Wayne Spevak, Chao Zhang, Yan Ma, Rafe Shellooe, Peipei Li, James Tsai, Pan- Yu Chen, Parmveer Singh, Jackie Walling, Christine Nichols, Jason Halladay, Kerry Inokuchi, Gaston Habets, Gideon Bollag. OPN-6602, a dual EP300/CBP bromodomain inhibitor modulates androgen-driven transcription, including MYC, in mCRPC [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr C085.

Abstract A052: Discovery and characterization of a p300-selective degrader demonstrates potent anti-tumor activity in preclinical models of prostate cancer

Abstract Strategies targeting androgen receptor (AR) in the treatment of prostate cancer (PrCa) have shown clear clinical benefit. However, a variety of AR-signaling bypass mechanisms ultimately result in progression to metastatic castrate resistant prostate cancer (mCRPC), and therapies for these patients remain an urgent unmet need. The histone acetyltransferase paralogs p300 and CBP, key coactivators of AR, are currently under investigation as attractive therapeutic targets to abrogate aberrant AR signaling but face a challenging clinical toxicity profile in hematopoietic cells. We postulate that p300 serves as a key regulator of AR signaling, and by sparing CBP we should retain potency against AR-driven PrCa while providing a larger safety margin. We discovered potent heterobifunctional degraders with demonstrated selectivity for p300 over CBP. Our tool compound exhibits degradation of p300 within 2 hours (DC50 = 7.5nM, Dmax = 79%) with minimal impact on CBP at 48 h (DC50 >1uM, Dmax = 5%). The AR/ARv7 driven VCaP PrCa cell line demonstrated exceptional sensitivity to this p300 degrader in proliferation assays (gIC50 = 0.8 nM) comparable to the dual p300/CBP inhibitor, CCS-1477, or the AR degrader, ARV-110 and undergoes a cytotoxic response. Profiled against known AR target genes, our degrader was able to suppress KLK2, KLK3 and FKBP5 transcript levels akin to CCS-1477 in both LNCaP and VCaP PrCa cell lines, supporting the hypothesis that p300, not CBP, acts as the primary coactivator of AR signaling. In addition, our compound led to greater downregulation of full-length AR, ARv7 and cMyc compared to CCS-1477. Subsequent assessment in an in vivo model (VCaP CDX) demonstrated tumor growth inhibition at the highest dose tested (88%; p=2.6e-4). Lastly, we profiled our p300 degrader in human bone marrow derived hematopoietic progenitor colony-forming assays where we observed a significant reduction in toxicity (IC50 = 3.9uM) compared to either CCS-1477 (IC50 = 121nM) or a dual degrader tool compound (IC50 = 16nM), supporting the hypothesis that a p300-specific mechanism will offer an improved therapeutic index. Treatment with our p300-specific degrader resulted in a potent anti-tumor effect on PrCa cells both in vitro and in vivo, comparing favorably to both dual p300/CBP inhibitor or AR degrader strategies. Furthermore, selective degradation of p300 has the potential to minimize toxicity to hematopoietic cells commonly observed with dual targeting of CBP and p300. Taken together, these results support the role p300 may play in driving AR-mediated signaling in PrCa and demonstrate that a p300-specific degrader may serve as an effective therapeutic modality in this disease setting. Citation Format: Mike Russell, Cassandra L Lowenstein, Xuqing Zhang, Jeremy Roach, Jianing Song, Rakesh Nagilla, Peter Orth, Matt Tudor, Qiaolin Deng, Zhihua Sui, Corey Strickland, Larry J Jolivette, E Scott Priestley, Helai P Mohammad. Discovery and characterization of a p300-selective degrader demonstrates potent anti-tumor activity in preclinical models of prostate cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A052.

Differences in Androgen Receptor Expression in Human Heart Tissue in Various Types of Cardiomyopathy and in Aortic Valve Stenosis.

Background: Sex-specific differences in heart disease outcomes are influenced by the levels of the steroid hormones, estrogen and testosterone. While the roles of estrogen receptors in cardiac disease are well-studied in animals and humans, respective research on androgen receptors (AR) is limited. Here we investigate AR protein and mRNA expression in human myocardium of various cardiac diseases. Methods: AR expression was analyzed by western blotting in myocardium from human non-failing hearts (NF, n = 6) and patients with aortic stenosis (AS, n = 6), hypertrophic cardiomyopathy (HCM, n = 7), dilated cardiomyopathy (DCM, n = 7), and ischemic cardiomyopathy (ICM, n = 7). Using an AR45-specific antibody, a subsequent western blot assessed samples from male and female patients with HCM (n = 10) and DCM (n = 10). The same sample set was probed for full-length AR and AR45 mRNA expression. Immunohistochemistry (IHC) localized AR in myocardium from HCM and AS hearts. Results: Full-length AR was notably enriched in AS and HCM hearts compared to ICM, DCM, and NF. Similarly, AR45 was more abundant in HCM than in DCM. In contrast to the pattern observed for AR protein, full-length AR mRNA levels were lower in HCM compared to DCM, with no discernible difference for the AR45 isoform. Although gender differences in AR expression were not detected in western blots or qRT-PCR, IHC showed stronger nuclear AR signals in males than in females. Conclusions: Our findings indicate disease-specific regulation of AR mRNA and/or AR protein in cardiac hypertrophy, underscoring a potential role in this cardiac pathology.

Characterization of allosteric modulators that disrupt androgen receptor co-activator protein-protein interactions to alter transactivation-Drug leads for metastatic castration resistant prostate cancer.

Three series of compounds were prioritized from a high content screening campaign that identified molecules that blocked dihydrotestosterone (DHT) induced formation of Androgen Receptor (AR) protein-protein interactions (PPIs) with the Transcriptional Intermediary Factor 2 (TIF2) coactivator and also disrupted preformed AR-TIF2 PPI complexes; the hydrobenzo-oxazepins (S1), thiadiazol-5-piperidine-carboxamides (S2), and phenyl-methyl-indoles (S3). Compounds from these series inhibited AR PPIs with TIF2 and SRC-1, another p160 coactivator, in mammalian 2-hybrid assays and blocked transcriptional activation in reporter assays driven by full length AR or AR-V7 splice variants. Compounds inhibited the growth of five prostate cancer cell lines, with many exhibiting differential cytotoxicity towards AR positive cell lines. Representative compounds from the 3 series substantially reduced both endogenous and DHT-enhanced expression and secretion of the prostate specific antigen (PSA) cancer biomarker in the C4-2 castration resistant prostate cancer (CRPC) cell line. The comparatively weak activities of series compounds in the H3-DHT and/or TIF2 box 3 LXXLL-peptide binding assays to the recombinant ligand binding domain of AR suggest that direct antagonism at the orthosteric ligand binding site or AF-2 surface respectively are unlikely mechanisms of action. Cellular enhanced thermal stability assays (CETSA) indicated that compounds engaged AR and reduced the maximum efficacy and right shifted the EC50 of DHT-enhanced AR thermal stabilization consistent with the effects of negative allosteric modulators. Molecular docking of potent representative hits from each series to AR structures suggest that S1-1 and S2-6 engage a novel binding pocket (BP-1) adjacent to the orthosteric ligand binding site, while S3-11 occupies the AR binding function 3 (BF-3) allosteric pocket. Hit binding poses indicate spaces and residues adjacent to the BP-1 and BF-3 pockets that will be exploited in future medicinal chemistry optimization studies. Small molecule allosteric modulators that prevent/disrupt AR PPIs with coactivators like TIF2 to alter transcriptional activation in the presence of orthosteric agonists might evade the resistance mechanisms to existing prostate cancer drugs and provide novel starting points for medicinal chemistry lead optimization and future development into therapies for metastatic CRPC.

Spermine is a natural suppressor of AR signaling in castration-resistant prostate cancer

In castration-resistant prostate cancer (CRPC), clinical response to androgen receptor (AR) antagonists is limited mainly due to AR-variants expression and restored AR signaling. The metabolite spermine is most abundant in prostate and it decreases as prostate cancer progresses, but its functions remain poorly understood. Here, we show spermine inhibits full-length androgen receptor (AR-FL) and androgen receptor splice variant 7 (AR-V7) signaling and suppresses CRPC cell proliferation by directly binding and inhibiting protein arginine methyltransferase PRMT1. Spermine reduces H4R3me2a modification at the AR locus and suppresses AR binding as well as H3K27ac modification levels at AR target genes. Spermine supplementation restrains CRPC growth invivo. PRMT1 inhibition also suppresses AR-FL and AR-V7 signaling and reduces CRPC growth. Collectively, we demonstrate spermine as an anticancer metabolite by inhibiting PRMT1 to transcriptionally inhibit AR-FL and AR-V7 signaling in CRPC, and we indicate spermine and PRMT1 inhibition as powerful strategies overcoming limitations of current AR-based therapies in CRPC.