Androgen Receptor Ligand-binding Domain Research Articles

Identification of Oral Bioavailable Coumarin Derivatives as Potential AR Antagonists Targeting Prostate Cancer.

Androgen receptor (AR) is a crucial driver of prostate cancer (PCa), but acquired resistance to AR antagonists significantly undermines their clinical efficacy. We previously discovered coumarin derivative 1, which is capable of disrupting AR ligand-binding domain dimers, offering the potential for overcoming resistance. However, its poor oral bioavailability limited further development. In this study, comprehensive structure optimizations led to compound 4a (IC50 = 0.051 μM), which exhibited comparable AR antagonistic activity to enzalutamide (IC50 = 0.060 μM) and demonstrated excellent selectivity over other nuclear receptors in vitro. Especially, 4a showed superior efficacy against ARF876L/T877A and ARW741C mutants compared to darolutamide and enzalutamide. Moreover, 4a exhibited favorable pharmacokinetic profiles (F = 66.24%) in vivo and significant tumor growth inhibition in an LNCaP xenograft mouse model upon oral administration. These results highlight the potential of 4a as a promising oral AR antagonist for overcoming drug resistance in PCa.

Safety and clinical activity of BMS-986365 (CC-94676), a dual androgen receptor ligand-directed degrader and antagonist, in heavily pretreated patients with metastatic castration-resistant prostate cancer

Metastatic castration-resistant prostate cancer (mCRPC) that progresses on androgen receptor pathway inhibitors (ARPIs) may continue to be driven by AR signaling. BMS-986365 is an orally administered ligand-directed degrader targeting the AR via a first-in-class dual mechanism of AR degradation and antagonism. CC-94676-PCA-001 (NCT04428788) is a phase I multicenter study of BMS-986365 in patients with progressive mCRPC. Patients who progressed on androgen deprivation therapy, one or more ARPIs, and taxane chemotherapy (unless declined/ineligible) were enrolled. The study included dose escalation (part A) and expansion (part B) of BMS-986365 up to 900 mg twice daily. Primary objectives were safety and tolerability, and to define maximum tolerated dose and/or recommended phase II dose. Key secondary endpoints included decline in prostate-specific antigen ≥50% (PSA50) and radiographic progression-free survival (rPFS). Parts A and B enrolled 27 and 68 patients, respectively. In part B, the median number of prior therapies was 4 (range 2-11). The most common treatment-related adverse events were asymptomatic prolonged corrected QT interval (47%) and bradycardia (34%). Part A maximum tolerated dose was not reached and recommended phase II dose selection is ongoing. Across part B three highest doses (400-900 mg twice daily, n= 60), PSA50 was 32% (n= 19), including 50% (n= 10/20) at 900 mg; median rPFS (95% confidence interval) was 6.3 months (5.3-12.6 months), including 8.3 months (3.8-16.6 months) at 900 mg; and rPFS was longer in patients without versus with prior chemotherapy: 16.5 months (5.5 months-not evaluable) versus 5.5 months (2.7-8.3 months), respectively. Efficacy was observed in patients with mCRPC with AR ligand binding domain (LBD) WT or with AR LBD mutations. BMS-986365 was well tolerated, with a manageable safety profile, and demonstrated activity in heavily pretreated patients with mCRPC with potentially higher benefit in chemotherapy-naive patients. These data show the potential of BMS-986365 to overcome resistance to current ARPIs, regardless of AR LBD mutation status.

Prevalence and Spectrum of AR Ligand-Binding Domain Mutations Detected in Circulating-Tumor DNA Across Disease States in Men With Metastatic Castration-Resistant Prostate Cancer.

Metastatic castration-resistant prostate cancer (mCRPC) is typically treated with agents directly or indirectly targeting the androgen receptor (AR) pathway. However, such treatment is limited by resistance mechanisms, including the development of activating mutations in the AR ligand-binding domain (AR-LBD). This study evaluated a database of over 15,000 patients with advanced prostate cancer (PC) undergoing comprehensive circulating-tumor DNA analysis (Guardant360, Redwood City, CA) between 2014 and 2021, with associated clinical information from administrative claims (GuardantINFORM database). Of 15,705 patients with PC included, 54% had mCRPC at the time of their blood draw. Of those, 49% had previous treatment with an AR pathway inhibitor (ARPi). AR-LBD mutation prevalence was 15% in patients with mCRPC who were untreated with a next-generation ARPi, 22% in those after one line of ARPi therapy, and 24% in those after two lines of ARPi treatment. Next-generation ARPi treatment yielded an increase in AR L702H and T878A/S mutations after abiraterone, and an increase in AR L702H and F877L mutations after enzalutamide. AR-LBD+ patients demonstrated unique biology, including increased concurrent mutations in the cell-cycle, wingless-related integration site, homologous recombination repair, and phospho-inositide 3-kinase pathways (all P < .0005), and greater low-level (copy number <10) AR amplifications (P = .0041). AR-LBD+ patients exhibited worse overall survival (OS) relative to a matched cohort of AR-LBD- patients (50.1 v 60.7 months, unadjusted log-rank P = .013). This large database analysis demonstrates that AR-LBD mutation prevalence increases after next-generation ARPi use. AR-LBD+ tumors demonstrate unique biology (more oncogenic pathway mutations and low-level AR amplification) and reduced OS. These findings inform the development of novel therapies designed to circumvent AR-mediated therapeutic resistance.

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 6059: Discovery of ETS-007, a first-in-class degrader targeting N-terminal domain of androgen receptor derived from AR liquid-liquid phase separation inhibitor

Abstract Patients with advanced prostate cancer inevitably acquire resistance to anti-androgen therapies, and progress to lethal castration-resistant prostate cancer (CRPC). The resistance mechanisms involve AR gene amplification and overexpression, constitutively active ligand-independent splice variants such as AR-V7, and the mutations in the AR ligand-binding domain (LBD). Our previous study has shown that the AR N-terminal domain (NTD) is responsible for AR liquid-liquid phase separation (LLPS) capability, which is essential for AR/AR-V7 transcriptional activity. We conducted a compound library screening using an AR LLPS phenotypic assay and identified ET0516 as a hit to specifically inhibit AR LLPS and its downstream gene expression, indicating that inhibition of AR. LLPS through targeting its NTD is a feasible approach to overcome the resistance to current anti-androgens therapy. Here we report the identification of a novel AR degrader, ETS-007 which is originated and evolved from internal LLPS phenotypic screening, that effectively degrades both AR and AR-V7 proteins with DC50 of single-digit nM by inducing a protein-protein interaction between AR NTD and an E3 ligase complex. ETS-007 induced AR and AR-V7 polyubiquitination, followed by degradation which could be blocked by the protease inhibitor MG132. ETS-007 significantly inhibited cell proliferation in AR and/or AR-V7 positive cell lines, e.g. LNCaP and 22RV1, and downregulated the androgen responsive genes such as PSA and TMPRSS2. Compared with enzalutamide and ARV-110, ETS-007 could block transcriptional activities of AR-V7, and AR mutants in luciferase report assay. Oral administration of ETS-007 in an enzalutamide-resistant 22RV1 xenograft model led to intratumoral AR-V7 degradation and concomitant decrease in PSA protein levels, resulting in a remarkable tumor regression. Taken together, our findings provide preclinical evidence that ETS-007 as a potent AR-NTD degrader derived from our LLPS platform can effectively block the transcriptional activities of both AR and AR-V7 for the treatment of metastatic CRPC. Citation Format: Yingjie Li, Mingyue Fei, Yingke Miao, Lijian Feng, Xiaofei Fan, Jinping Li, Yaozhong Chen, Ming Xu, Wenqi Cui, Qiangang Zheng, Jidong Zhu. Discovery of ETS-007, a first-in-class degrader targeting N-terminal domain of androgen receptor derived from AR liquid-liquid phase separation inhibitor [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 6059.

Real-world outcomes of patients with metastatic castration-resistant prostate cancer (mCRPC) and tumors harboring androgen receptor (AR) ligand-binding domain (LBD) mutations.

77 Background: In mCRPC, AR T878 and/or H875 and L702H mutations have been associated with disease progression and poor prognosis. This study characterized real-world overall survival (rwOS), testing practices, mutation prevalence, and treatment patterns in patients with mCRPC with and without tumors with missense mutations in the AR LBD (amino acids 671–920). Methods: In this retrospective analysis of the Guardant Inform Database (Mar 11, 2014–Jun 30, 2022), men (aged ≥18 y) with mCRPC who were tested with Guardant360 (G360) next-generation sequencing and diagnosed/treated at a clinical site in the US with a first-line (1L) treatment were included. Matched rwOS was assessed in 2 subpopulations: patients with any missense mutation in AR LBD, excluding those with AR L702H alone ( AR LBD group), and patients with AR T878 and/or H875 without L702H ( AR 878/875 group) and compared with patients without any AR LBD missense mutations (control group). Patients were matched (1:5) on key prognostic variables, including age, Elixhauser Comorbidity Index weighted score, smoking status, prior novel hormonal agent (NHA) use, and 1L treatment year. Mutation prevalence, patient demographics, frequency and timing of G360 tests, and treatment patterns were assessed in patients treated from 2018 to 2022 to reflect more recent trends. Results: In the matched rwOS analysis, patients in the AR LBD group (n=275) had statistically significantly shorter 1L median rwOS compared with the control group (n=1375): 27.3 vs 47.8 months; P&lt;0.0001. Similarly, patients in the AR 878/875 group (n=154) had shortened median rwOS compared with the control group (n=770): 28.9 vs 44.4 months; P=0.0004. Of 4833 patients treated for mCRPC from 2018 to 2022, 20% had AR LBD mutations, 4% had AR 878/875 with L702H, and 6% had AR 878/875 without L702H. Most patients (59%) were first tested at some point after 1L therapy, with only 15% of patients receiving &gt;1 G360 test. Mutation prevalence was higher in patients tested in fourth-line (4L) compared with 1L, particularly for AR L702H mutations (17% vs 10%) and all AR LBD mutations (27% vs 20%). Use of NHAs was lower in 4L compared with 1L (20% vs 33%), whereas use of taxanes was higher (31% vs 22%). Conclusions: Patients with AR LBD–mutated mCRPC had notably shorter rwOS than those whose tumors did not harbor AR LBD mutations, indicating an unmet need for this population. The prevalence of AR LBD mutations was higher in later lines of therapy, but repeat testing rates were low, suggesting that AR LBD mutations may go undetected.

The Mutational and Transcriptional Landscapes of Speckle-Type POZ Protein (SPOP) and Androgen Receptor (AR) in a Single-Center pT3 Prostatectomy Cohort.

Prostate cancer (PCa) is a heterogeneous disease both clinically and genetically. According to The Cancer Genome Atlas (TCGA), the speckle‑type POZ protein (SPOP) mutant form is one of the significant core subtypes of PCa. However, the prognostic value of SPOP variations remains unknown. As a critical PCa driver and an SPOP-targeted protein, androgen receptor (AR) also plays a role in PCa initiation and progression. Thus, we aimed to analyze the mutational status of SPOP and AR with their transcriptional levels in a pathological stage 3 (pT3) prostatectomy cohort consisting of 89 Turkish PCa patients. Targeted sequence analysis and RT-qPCR were performed for SPOP and AR in the benign and malign prostate tissue samples. Our results introduced the two novel pathogenic SPOP variations, C203Y and S236R, in the BTB/POZ domain and a novel pathogenic variant in the ligand-binding domain of AR, R789W. Their predicted pathogenicities and effects on protein features were evaluated by web-based in silico analysis. The overall frequency of SPOP and AR variations for pT3 patients in our population was 3.4% (3/89) and 4.5% (4/89), respectively. The mutational results represented a possible subgroup characterized by carrying the novel variants in SPOP and AR in pT3 PCa patients. In addition to the significant clinicopathological parameters, the mutational results provide a better understanding of the molecular structure of pathologically advanced PCa in the SPOP and AR aspects.

Targeted Inhibition of CYP11A1 in Castration-Resistant Prostate Cancer

BackgroundProstate cancer is regulated by steroid hormones, even in castration-resistant disease. ODM-208, a novel inhibitor of cytochrome P450 11A1 (which catalyzes the first step of steroid-hormone biosynthesis), was investigated in patients with heavily pretreated metastatic castration-resistant prostate cancer (mCRPC).MethodsCYPIDES is a first-in-human phase 1 (3 + 3 design) and phase 2 study. We administered ODM-208 twice daily with glucocorticoid/mineralocorticoid replacement and ongoing androgen deprivation therapy to adults with previously treated mCRPC, regardless of androgen receptor gene (AR) ligand-binding domain mutations (phase 1) and with activating AR ligand-binding domain mutations (ARmut; phase 2). Safety, pharmacokinetics, steroid-hormone pharmacodynamics, and preliminary efficacy were the key outcomes.ResultsNinety-two patients received one or more doses of ODM-208: 47 in phase 1 (20 [42.6%] with ARmut) and 45 in phase 2 (all ARmut). A dose of ODM-208 of 5 mg twice a day with dexamethasone 1 mg/fludrocortisone 0.1 mg provided a balance between decreased steroidogenesis and toxicity. Treatment-related adrenal insufficiency was the most common toxicity in phase 1 (n=17, 36.2%; necessitating ODM-208 discontinuation in one patient); this toxicity occurred in six patients (13.3%) at 5 mg twice a day in phase 2. Median circulating testosterone levels declined from 3.0 ng/dl (interquartile range, 1.3 to 6.2 ng/dl) at baseline to undetectable levels within the first week of ODM-208 5 mg twice a day treatment in 46 of 53 (87%) patients. A decrease in prostate-specific antigen levels of 50% or more occurred in 14 of 19 (73.7%) patients with ARmut and 2 of 23 (8.7%) patients with AR wild type in phase 1 and in 24 of 45 (53.3%) patients with ARmut in phase 2.ConclusionsODM-208 potently inhibited steroid-hormone biosynthesis with the expected toxicity of adrenal insufficiency. Evidence of antitumor activity was observed in this heavily pretreated mCRPC population, especially in those with ARmut. (Funded by Orion Pharma; ClinicalTrials.gov number, NCT03436485.)

SAT023 The Role Of Cysteine Residues In The Binding Of Inhibitors Of The N-Terminal Domain Of The Androgen Receptor

Abstract Disclosure: L. Deneault: None. A. Tien: None. A. Banuelos: Stock Owner; Self; ESSA. T. Tam: None. R. Andersen: Stock Owner; Self; ESSA. M.D. Sadar: Stock Owner; Self; ESSA. Background: Therapies for prostate cancer (PC) involve pharmaceutical castration by targeting the androgen receptor (AR) ligand-binding domain (LBD). These therapies are not curative with the malignancy progressing to castration-resistant PC (CRPC). Most CRPC remains driven by AR through multiple resistances, e.g. constitutively active AR splice variants that lack LBD (AR-Vs), or gain-of-function mutations in LBD. This has led to clinical development of EPI analogues (“EPI”) which target the AR N-terminal domain (NTD) to block the transcriptional activities of, AR-Vs and mutated ARs. The EPI-binding pocket involves region 341-446 amino acid residues of the NTD. AR NTD is enriched in cysteine residues with C404 suggested to be important in the binding mechanism of EPI. Free cysteine residues are able interact forming disulfide bonds, which are important in intra- and intermolecular protein-protein interactions. Here we begin to elucidate the role of cysteines in the inhibition of AR activity by EPI. Methods: Binding assays assess the role of cysteines in the covalent binding mechanism used a fluorescein-labelled EPI with fragments of recombinant AR-NTD (rNTD) with or without the addition of iodoacetamide (caps cysteines). Protein-protein interactions requiring disulfide bonds with AR examined by non-reducing SDS-PAGE and diagonal SDS-PAGE of cells treated with inhibitors. A role of specific cysteines in the activity of EPI to block AR transcriptional activity was assessed by cysteine mutants using a reporter gene construct driven by AR. Results: Capping cysteine residues on rNTD with iodoacetamide reduced the amount of covalent binding of EPI. This data suggests that cysteines are essential for covalent binding of EPI to AR-NTD. EPI bound preferentially to the oxidized form of rNTD suggesting the importance of disulfide bridges. Diagonal SDS-PAGE revealed differential alterations in intramolecular and intermolecular disulfide bonds in response to EPI compared to AR LBD inhibitors. These gels also revealed unique differences in protein-protein interactions with AR depending on the inhibitor. AR-driven reporter gene assays revealed mutations of cysteine residues in the AR- NTD did not adversely affect AR transcriptional activity in response to androgen. Conclusions: Although cysteines residing in the AR-NTD were required for covalent binding of EPI, mutation of C404 in the EPI-binding site was not essential for the efficacy of EPI to block AR transcriptional activity. There were differential patterns in protein-protein interactions with AR depending upon which domain the inhibitor targeted. Our future goal is to identify these unique proteins to provide insight into the mechanisms and develop more potent and more specific therapeutics for the treatment of CRPC. Presentation: Saturday, June 17, 2023

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.

1803P Phase I/II study of bavdegalutamide, a PROTAC androgen receptor (AR) degrader in metastatic castration-resistant prostate cancer (mCRPC): Radiographic progression-free survival (rPFS) in patients (pts) with AR ligand-binding domain (LBD) mutations

1803P Phase I/II study of bavdegalutamide, a PROTAC androgen receptor (AR) degrader in metastatic castration-resistant prostate cancer (mCRPC): Radiographic progression-free survival (rPFS) in patients (pts) with AR ligand-binding domain (LBD) mutations

Androgen receptor–specific CD8+ T cells with tumor cell cytotoxicity in patients with prostate cancer in Taiwan.

e17069 Background: Classical androgen receptor (AR) target is the new “tumor-specific” antigen for prostate cancer vaccine development to delay progression and castration-resistant disease. HLA-A2-restricted AR ligand-binding domain (LBD) has been demonstrated to stimulate cytotoxic T lymphocytes (CTL) in HLA-A2+ prostate cancer patients. Although the prevalence of HLA-A2 serotype and LBD-specific CD8+ T cells among Caucasians are known, their frequencies in the Taiwan patients have not been established. In this study, we performed HLA-A2 genotyping, HLA-A2+ LNCaP stimulation of autologous T cell, and CTL tumor cell cytotoxicity from 25 castration-sensitive prostate cancer patients using blood samples collected from a single hospital center. Methods: 20ml whole blood samples are collected in EDTA tube from non-metastatic, castration-sensitive prostate cancer patients. PBMCs are subjected to HLA-A2 allele sequencing, separated by CD3 magnetic bead column, and expanded and activated ex vivo using CD3/CD8 polymeric beads. Autologous T cells are then isolated by CD8 magnetic bead column, stimulated by HLA-A2+ lentivirus-transduced LNCaP (AR+), and targeted against transduced (HLA-A2+) and non-transduced (HLA-A2-) LNCaP (AR+) by Chromium-51 Release Assay. We identified and correlated their frequencies with patient’s age at diagnosis, Gleason grades, PSA level, and family history. Results: A total of 25 blood samples were tested and analyzed for HLA-A2 genotype with 44% (11/25) positive for the HLA-A2 [28% (7/25) heterozygotes, 16% (4/25) homozygotes]. Among the 11 HLA-A2+ patients, 72% (8/11) were HLA-A*02:07 allele, 100% (11/11) CD8+ T cells were successfully stimulated with HLA-A2+ transduced LNCaP (AR+) and 45% (5/11) patients demonstrated HLA-A2+ AR-specific CTL tumor cell lysis. No clinical association with patient age (54-86 y.o.), Gleason grades (6-9), PSA level (3.84-62.8 ng/ml), and family history (1/25). Conclusions: In this single hospital center study, we observed a similar prevalence of HLA-A2 alleles in Taiwan prostate cancer patient cohort with a predominant HLA-A2*02:07 allele, and higher frequencies of HLA-A2-restricted CD8+ T cell stimulation and HLA-A2+ AR-specific tumor cell cytotoxicity. Our findings support the application of AR LBD as tumor antigen for prostate cancer vaccine development for Taiwan prostate cancer patients.

Discovery of a Small-Molecule Inhibitor Targeting the Androgen Receptor N-Terminal Domain for Castration-Resistant Prostate Cancer.

The current mainstay therapeutic strategy for advanced prostate cancer is to suppress androgen receptor (AR) signaling. However, castration-resistant prostate cancer (CRPC) invariably arises with restored AR signaling activity. To date, the AR ligand-binding domain (LBD) is the only targeted region for all clinically available AR signaling antagonists, such as enzalutamide (ENZ). Major resistance mechanisms have been uncovered to sustain the AR signaling in CRPC despite these treatments, including AR amplification, AR LBD mutants, and the emergence of AR splice variants (AR-Vs) such as AR-V7. AR-V7 is a constitutively active truncated form of AR that lacks the LBD; thus, it can not be inhibited by AR LBD-targeting drugs. Therefore, an approach to inhibit AR through the regions outside of LBD is urgently needed. In this study, we have discovered a novel small molecule SC428, which directly binds to the AR N-terminal domain (NTD) and exhibits pan-AR inhibitory effect. SC428 potently decreased the transactivation of AR-V7, ARv567es, as well as full-length AR (AR-FL) and its LBD mutants. SC428 substantially suppressed androgen-stimulated AR-FL nuclear translocation, chromatin binding, and AR-regulated gene transcription. Moreover, SC428 also significantly attenuated AR-V7-mediated AR signaling that does not rely on androgen, hampered AR-V7 nuclear localization, and disrupted AR-V7 homodimerization. SC428 inhibited in vitro proliferation and in vivo tumor growth of cells that expressed a high level of AR-V7 and were unresponsive to ENZ treatment. Together, these results indicated the potential therapeutic benefits of AR-NTD targeting for overcoming drug resistance in CRPC.

Abstract LB048: AR-targeted linked-read sequencing reveals complex AR structural variants in CRPC tumors

Abstract Inhibition of the androgen receptor (AR) transcription factor is the mainstay therapeutic strategy for systemic prostate cancer. However, development of a resistance phenotype termed castration resistant prostate cancer (CRPC) enables disease progression that underlies virtually all prostate cancer mortality. In the majority of CRPC, AR is transcriptionally re-activated in part due to accumulation of numerous alterations in the AR gene. These genomic alterations include missense mutations in exons that encode the ligand binding domain of AR, amplification of the AR gene body and/or an upstream transcriptional enhancer, and AR structural variants (SVs) resulting from inaccurate repair of DNA double strand breaks. In CRPC patient samples, these AR genomic alterations co-occur within tumors, which makes elucidating the fitness contribution of each alteration difficult. While short-read DNA sequencing has allowed for characterization of these events, they are underpowered to preserve linkage information. To address this challenge, we performed AR-targeted linked-read DNA-sequencing of CRPC cell line and patient derived xenograft (PDX) models with the goal of resolving sub-clonal and complex AR SVs. This approach confirmed the heterogeneity of AR SVs in these PDXs, including phased AR SVs that can be traced back to a single high molecular weight DNA molecule. These phased AR SVs were consistently found to co-occur in CRPC specimens with amplification of AR. Our results define a new class of AR SVs where multiple rearrangements co-occur on amplified DNA molecules to yield a complex array of AR gene structures in CRPC. Citation Format: Andrej Zivanovic, Sarah A. Munro, Jeffrey T. Miller, Yingming Li, Courtney N. Passow, Scott M. Dehm. AR-targeted linked-read sequencing reveals complex AR structural variants in CRPC tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB048.

A partially open conformation of an androgen receptor ligand-binding domain with drug-resistance mutations.

Mutations in the androgen receptor (AR) ligand-binding domain (LBD) can cause resistance to drugs used to treat prostate cancer. Commonly found mutations include L702H, W742C, H875Y, F877L and T878A, while the F877L mutation can convert second-generation antagonists such as enzalutamide and apalutamide into agonists. However, pruxelutamide, another second-generation AR antagonist, has no agonist activity with the F877L and F877L/T878A mutants and instead maintains its inhibitory activity against them. Here, it is shown that the quadruple mutation L702H/H875Y/F877L/T878A increases the soluble expression of AR LBD in complex with pruxelutamide in Escherichia coli. The crystal structure of the quadruple mutant in complex with the agonist dihydrotestosterone (DHT) reveals a partially open conformation of the AR LBD due to conformational changes in the loop connecting helices H11 and H12 (the H11-H12 loop) and Leu881. This partially open conformation creates a larger ligand-binding site for AR. Additional structural studies suggest that both the L702H and F877L mutations are important for conformational changes. This structural variability in the AR LBD could affect ligand binding as well as the resistance to antagonists.

A hotspot for posttranslational modifications on the androgen receptor dimer interface drives pathology and anti-androgen resistance

Mutations of the androgen receptor (AR) associated with prostate cancer and androgen insensitivity syndrome may profoundly influence its structure, protein interaction network, and binding to chromatin, resulting in altered transcription signatures and drug responses. Current structural information fails to explain the effect of pathological mutations on AR structure-function relationship. Here, we have thoroughly studied the effects of selected mutations that span the complete dimer interface of AR ligand-binding domain (AR-LBD) using x-ray crystallography in combination with in vitro, in silico, and cell-based assays. We show that these variants alter AR-dependent transcription and responses to anti-androgens by inducing a previously undescribed allosteric switch in the AR-LBD that increases exposure of a major methylation target, Arg761. We also corroborate the relevance of residues Arg761 and Tyr764 for AR dimerization and function. Together, our results reveal allosteric coupling of AR dimerization and posttranslational modifications as a disease mechanism with implications for precision medicine.

Study of novel androgen receptor V770 variant in androgen insensitivity syndrome patients reveals the transitional state of the androgen receptor ligand binding domain homodimer.

We report the discovery of the androgen receptor missense mutation V770D, that was found in two sisters suffering from complete androgen insensitivity. Experimental validation of AR V770 variants demonstrated that AR V770D was transcriptionally inactive due to the inability to dimerize and a reduced ligand binding affinity. The more conservative AR V770A variant showed a dimerization defect at low levels of DHT with a partial recovery of the transcriptional activity and of the receptor's ability to dimerize when increasing the DHT levels. With V770 located outside of the proposed LBD dimerization interface of the AR LBD homodimer crystal structure, the effects of the V770A mutation on AR dimerization were unexpected. We therefore explored whether the AR LBD dimerization interface would be better described by an alternative dimerization mode based on available human homodimeric LBD crystal structures of other nuclear receptors. Superposition of the monomeric AR LBD in the homodimeric crystal structures of GR, PR, ER, CAR, TRβ, and HNF-4α showed that the GR-like LBD dimer model was energetically the most stable. Moreover, V770 was a key energy residue in the GR-like LBD dimer while it was not involved in the stabilization of the AR LBD homodimer according to the crystal structure. Additionally, the observation that 4 AIS mutations impacted the stability of the AR LBD dimer while 16 mutations affected the GR-like LBD dimer, suggested that the AR LBD dimer crystal is a snapshot of a breathing AR LBD homodimer that can transition into the GR-like LBD dimer model.

Androgen receptor (AR) mutations in men with metastatic castration-resistant prostate cancer (mCRPC): Incidence and natural history.

221 Background: AR ligand binding domain (LBD) activating somatic point mutation is a known mechanism of resistance to androgen-receptor signaling inhibitor (ARSI) treatment in mCRPC, leading to a persistent addiction to steroid hormones. Two early phase trials investigating CYP11A1 inhibitors - CYPIDES (ODM208, NCT03436485) and STESIDES (ODM209, NCT03878823) - recruited mCRPC patients with and without pre-specified AR LBD mutation. In CYPIDES Part 1, AR LBD mutations (ARm) strongly predicted for PSA decline with ODM-208 (Fizazi, ASCO GU 2021). Here, we evaluated clinical and genomic data associated with ARm in patients with mCRPC. Methods: We reviewed clinical and molecular data from patients with mCRPC who progressed after ≥1 ARSI and ≥1 line of taxane-based chemotherapy or were ineligible to chemotherapy, pre-screened in CYPIDES and STESIDES trials at our center. Circulating tumor DNA (ctDNA) was tested for AR LBD mutation (Guardant360 CDx and Sysmex OncoBEAM) and for genomic analysis (FoundationOne Liquid CDx, STING trial, NCT04932525) to screen for actionable targets. Results: From March 2020 to January 2022, 272 men were screened. Median age was 62y. Overall, 212 (78%), 202 (74%), and 240 (88%) patients had received previous abiraterone, enzalutamide, and docetaxel respectively. Median PSA at screening was 59 ng/mL (IQR 11; 246). ARm was found in 69 patients (25%). The most frequent mutations were L702H (15%), T878A (12%), H875Y (7%), V716M (2%), F877L (2%), W742C (1%), globally consistent with previous reports. Associations of ARm with relevant baseline characteristics of patients are summarized. Duration of enzalutamide exposure was correlated with the number of AR mutations per patient (p = 0.006) whereas no correlation was found for abiraterone exposure duration (p=0.71). In 173 (64%) patients with available genomic testing, no alterations were found to be mutually exclusive with ARm, including in DNA repair alteration. Conclusions: AR LBD mutations were detected in ctDNA in 25% of men with advanced mCRPC treated with at least one ARSI. Their incidence was associated with a longer duration of treatment with enzalutamide and a longer time from CRPC and prostate cancer diagnosis. [Table: see text]

Real-world assessment of AR-LBD mutations in metastatic castration-resistant prostate cancer.

204 Background: Treatment of men with metastatic castration-resistant prostate cancer (mCRPC) using drugs targeting the AR pathway is hampered by development of resistance including activating AR ligand-binding domain (LBD) point mutations. AR LBD mutations are found in approximately 10-20% of patients (pts) with mCRPC progressing on AR pathway inhibition (ARPi) therapy and may be associated with AR activation by alternative steroid hormones. We utilized the Guardant INFORM real-world database to explore the detection rate and clinical features associated with AR-LBD mutations in men with mCRPC. Methods: This observational study was conducted in a nationally representative clinical-genomic database of 200,000+ advanced cancer pts undergoing comprehensive ctDNA analysis (Guardant360, Redwood City, CA) between 2014 and 2021, with associated clinical information from administrative claims. ctDNA samples of men with mCRPC were assessed for presence or absence of an activating AR-LBD mutation. The incidence of co-occurring genetic alterations was also investigated. Clinical outcomes were studied for matched cohorts comparing pts following first line (1L) or second line (2L) mCRPC therapy with or without AR-LBD mutations. Results: Of 15,705 prostate cancer pts included in the database, 8420 (53.6%) had mCRPC at the time of sample collection. 4122 (49.0%) of these mCRPC pts had prior treatment with an ARPi, of which 854 (21%) had an AR-LBD mutation. AR-LBD mutation prevalence was 15% in mCRPC pts untreated with an ARPi, 23% in those after 1 line, and 24% in those after 2 lines of ARPi treatment. AR L702H and T878A/S showed the greatest increase in prevalence post abiraterone, whereas AR L702H, T878S and F877L had the greatest increase in prevalence post enzalutamide. Prevalence of AR L702H and H875Y doubled (from 8.8% to 17.5%) for pts that received chemotherapy post ARPi, while AR F877L incidence declined by half (3.5% to 1.7%). Compared to pts without AR-LBD mutations, AR-LBD+ mCRPC pts were enriched for co-alterations in APC, CTNNB1, PTEN, BRCA2, and ARID1A. Rates of TP53 and RB1 alterations were comparable in the AR-LBD+ and ARLBD- cohorts. Matched mCRPC pts had similar time-to-treatment-discontinuation (TTD) and time-to-next-treatment (TTNT) between AR-LBD+ and AR-LBD- mCRPC pts in the 1L and 2L settings; however, AR-LBD+ pts exhibited worse overall survival from 1L mCRPC therapy (50.1 vs 60.7 months, log-rank P=0.013). Conclusions: In this large database, the overall detection rate of activating AR-LBD mutations in men with mCRPC undergoing ctDNA analysis was 21%, and increased with prior use of 2nd generation ARPi (abiraterone and enzalutamide). Men with AR-LBD mutations were more likely to harbor other alterations relevant to tumor progression. These findings inform on the ctDNA prevalence and impact of AR-LBD mutations in a real-world dataset of mCRPC.

Selective androgen receptor degrader (SARD) to overcome antiandrogen resistance in castration-resistant prostate cancer.

In patients with castration-resistant prostate cancer (CRPC), clinical resistances such as androgen receptor (AR) mutation, AR overexpression, and AR splice variants (ARVs) limit the effectiveness of second-generation antiandrogens (SGAs). Several strategies have been implemented to develop novel antiandrogens to circumvent the occurring resistance. Here, we found and identified a bifunctional small molecule Z15, which is both an effective AR antagonist and a selective AR degrader. Z15 could directly interact with the ligand-binding domain (LBD) and activation function-1 region of AR, and promote AR degradation through the proteasome pathway. In vitro and in vivo studies showed that Z15 efficiently suppressed AR, AR mutants and ARVs transcription activity, downregulated mRNA and protein levels of AR downstream target genes, thereby overcoming AR LBD mutations, AR amplification, and ARVs-induced SGAs resistance in CRPC. In conclusion, our data illustrate the synergistic importance of AR antagonism and degradation in advanced prostate cancer treatment.