Neutralizing and protective human monoclonal antibodies recognizing the N-terminal domain of the SARS-CoV-2 spike protein.

Most human monoclonal antibodies (mAbs) neutralizing SARS-CoV-2 recognize the spike (S) protein receptor-binding domain and block virus interactions with the cellular receptor angiotensin-converting enzyme 2. We describe a panel of human mAbs binding to diverse epitopes on the N-terminal domain (NTD) of S protein from SARS-CoV-2 convalescent donors and found a minority of these possessed neutralizing activity. Two mAbs (COV2-2676 and COV2-2489) inhibited infection of authentic SARS-CoV-2 and recombinant VSV/SARS-CoV-2 viruses. We mapped their binding epitopes by alanine-scanning mutagenesis and selection of functional SARS-CoV-2 S neutralization escape variants. Mechanistic studies showed that these antibodies neutralize in part by inhibiting a post-attachment step in the infection cycle. COV2-2676 and COV2-2489 offered protection either as prophylaxis or therapy, and Fc effector functions were required for optimal protection. Thus, natural infection induces a subset of potent NTD-specific mAbs that leverage neutralizing and Fc-mediated activities to protect against SARS-CoV-2 infection using multiple functional attributes.

N-terminal domain antigenic mapping reveals a site of vulnerability for SARS-CoV-2.

The SARS-CoV-2 spike (S) glycoprotein contains an immunodominant receptor-binding domain (RBD) targeted by most neutralizing antibodies (Abs) in COVID-19 patient plasma. Little is known about neutralizing Abs binding to epitopes outside the RBD and their contribution to protection. Here, we describe 41 human monoclonal Abs (mAbs) derived from memory B cells, which recognize the SARS-CoV-2 S N-terminal domain (NTD) and show that a subset of them neutralize SARS-CoV-2 ultrapotently. We define an antigenic map of the SARS-CoV-2 NTD and identify a supersite (designated site i) recognized by all known NTD-specific neutralizing mAbs. These mAbs inhibit cell-to-cell fusion, activate effector functions, and protect Syrian hamsters from SARS-CoV-2 challenge, albeit selecting escape mutants in some animals. Indeed, several SARS-CoV-2 variants, including the B.1.1.7, B.1.351, and P.1 lineages, harbor frequent mutations within the NTD supersite, suggesting ongoing selective pressure and the importance of NTD-specific neutralizing mAbs for protective immunity and vaccine design.

In vitro and in vivo functions of SARS-CoV-2 infection-enhancing and neutralizing antibodies.

SARS-CoV-2-neutralizing antibodies (NAbs) protect against COVID-19. A concern regarding SARS-CoV-2 antibodies is whether they mediate disease enhancement. Here, we isolated NAbs against the receptor-binding domain (RBD) or the N-terminal domain (NTD) of SARS-CoV-2 spike from individuals with acute or convalescent SARS-CoV-2 or a history of SARS-CoV infection. Cryo-electron microscopy of RBD and NTD antibodies demonstrated function-specific modes of binding. Select RBD NAbs also demonstrated Fc receptor-γ (FcγR)-mediated enhancement of virus infection in vitro, while five non-neutralizing NTD antibodies mediated FcγR-independent in vitro infection enhancement. However, both types of infection-enhancing antibodies protected from SARS-CoV-2 replication in monkeys and mice. Three of 46 monkeys infused with enhancing antibodies had higher lung inflammation scores compared to controls. One monkey had alveolar edema and elevated bronchoalveolar lavage inflammatory cytokines. Thus, while in vitro antibody-enhanced infection does not necessarily herald enhanced infection in vivo, increased lung inflammation can rarely occur in SARS-CoV-2 antibody-infused macaques.

Antibodies targeting a quaternary site on SARS-CoV-2 spike glycoprotein prevent viral receptor engagement by conformational locking.

SARS-CoV-2 continues to evolve, with many variants evading clinically authorized antibodies. To isolate monoclonal antibodies (mAbs) with broadly neutralizing capacities against the virus, we screened serum samples from convalescing COVID-19 patients. We isolated two mAbs, 12-16 and 12-19, which neutralized all SARS-CoV-2 variants tested, including the XBB subvariants, and prevented infection in hamsters challenged with Omicron BA.1 intranasally. Structurally, both antibodies targeted a conserved quaternary epitope located at the interface between the N-terminal domain and subdomain 1, uncovering a site of vulnerability on SARS-CoV-2 spike. These antibodies prevented viral receptor engagement by locking the receptor-binding domain (RBD) of spike in the down conformation, revealing a mechanism of virus neutralization for non-RBD antibodies. Deep mutational scanning showed that SARS-CoV-2 could mutate to escape 12-19, but such mutations are rarely found in circulating viruses. Antibodies 12-16 and 12-19 hold promise as prophylactic agents for immunocompromised persons who do not respond robustly to COVID-19 vaccines.

Rapid characterization of spike variants via mammalian cell surface display.

The SARS-CoV-2 spike protein is a critical component of vaccines and a target for neutralizing monoclonal antibodies (nAbs). Spike is also undergoing immunogenic selection with variants that increase infectivity and partially escape convalescent plasma. Here, we describe Spike Display, a high-throughput platform to rapidly characterize glycosylated spike ectodomains across multiple coronavirus-family proteins. We assayed ∼200 variant SARS-CoV-2 spikes for their expression, ACE2 binding, and recognition by 13 nAbs. An alanine scan of all five N-terminal domain (NTD) loops highlights a public epitope in the N1, N3, and N5 loops recognized by most NTD-binding nAbs. NTD mutations in variants of concern B.1.1.7 (alpha), B.1.351 (beta), B.1.1.28 (gamma), B.1.427/B.1.429 (epsilon), and B.1.617.2 (delta) impact spike expression and escape most NTD-targeting nAbs. Finally, B.1.351 and B.1.1.28 completely escape a potent ACE2 mimic. We anticipate that Spike Display will accelerate antigen design, deep scanning mutagenesis, and antibody epitope mapping for SARS-CoV-2 and other emerging viral threats.

Structural Insights of Transcriptionally Active, Full-Length Androgen Receptor Coactivator Complexes.

Steroid receptors activate gene transcription by recruiting coactivators to initiate transcription of their target genes. For most nuclear receptors, the ligand-dependent activation function domain-2 (AF-2) is a primary contributor to the nuclear receptor (NR) transcriptional activity. In contrast to other steroid receptors, such as ERα, the activation function of androgen receptor (AR) is largely dependent on its ligand-independent AF-1 located in its N-terminal domain (NTD). It remains unclear why AR utilizes a different AF domain from other receptors despite that NRs share similar domain organizations. Here, we present cryoelectron microscopy (cryo-EM) structures of DNA-bound full-length AR and its complex structure with key coactivators, SRC-3 and p300. AR dimerization follows a unique head-to-head and tail-to-tail manner. Unlike ERα, AR directly contacts a single SRC-3 and p300. The AR NTD is the primary site for coactivator recruitment. The structures provide a basis for understanding assembly of the AR:coactivator complex and its domain contributions for coactivator assembly and transcriptional regulation.

Rad52 Inverse Strand Exchange Drives RNA-Templated DNA Double-Strand Break Repair.

RNA can serve as a template for DNA double-strand break repair in yeast cells, and Rad52, a member of the homologous recombination pathway, emerged as an important player in this process. However, the exact mechanism of how Rad52 contributes to RNA-dependent DSB repair remained unknown. Here, we report an unanticipated activity of yeast and human Rad52: inverse strand exchange, in which Rad52 forms a complex with dsDNA and promotes strand exchange with homologous ssRNA or ssDNA. We show that in eukaryotes, inverse strand exchange between homologous dsDNA and RNA is a distinctive activity of Rad52; neither Rad51 recombinase nor the yeast Rad52 paralog Rad59 has this activity. In accord with our in vitro results, our experiments in budding yeast provide evidence that Rad52 inverse strand exchange plays an important role in RNA-templated DSB repair in vivo.

Crystal structure and biochemical analysis suggest that YjoB ATPase is a putative substrate-specific molecular chaperone.

AAA+ ATPases are ubiquitous proteins associated with most cellular processes, including DNA unwinding and protein unfolding. Their functional and structural properties are typically determined by domains and motifs added to the conserved ATPases domain. Currently, the molecular function and structure of many ATPases remain elusive. Here, we report the crystal structure and biochemical analyses of YjoB, a Bacillus subtilis AAA+ protein. The crystal structure revealed that the YjoB hexamer forms a bucket hat-shaped structure with a porous chamber. Biochemical analyses showed that YjoB prevents the aggregation of vegetative catalase KatA and gluconeogenesis-specific glyceraldehyde-3 phosphate dehydrogenase GapB but not citrate synthase, a conventional substrate. Structural and biochemical analyses further showed that the internal chamber of YjoB is necessary for inhibition of substrate aggregation. Our results suggest that YjoB, conserved in the class Bacilli, is a potential molecular chaperone acting in the starvation/stationary phases of B. subtilis growth.

Recombinant expression and tryptophan-assisted analysis of human sweet taste receptor T1R3's extracellular domain in sweetener interaction studies.

The human palate can discern multiple tastes; however, it predominantly perceives five fundamental flavors: sweetness, saltiness, sourness, bitterness, and umami. Sweetness is primarily mediated through the sweet taste receptor, a membrane-bound heterodimeric structure comprising T1R2-T1R3. However, unraveling the structural and mechanistic intricacies of the sweet taste receptor has proven challenging. This study aimed to address this knowledge gap by expressing an extracellular N-terminal domain encompassing the cysteine-rich domain of human hT1R3 (hT1R3-TMD) in Escherichia coli. The expressed protein was obtained as inclusion bodies, purified by metal affinity chromatography, and refolded using the dilution-refolding method. Through rigorous analysis, we confirmed the successful refolding of hT1R3-TMD and elucidated its structural characteristics using circular dichroism spectroscopy. Notably, the refolded protein was found to exist as either a monomer or a dimer, depending on its concentration. A tryptophan fluorescence quenching assay revealed that the dissociation constants for sucrose, sucralose, and brazzein were >9500 µM, 2380 µM and 14.3 µM, respectively. Our findings highlight the utility of this E. coli expression system for producing functional hT1R3-TMD for investigations and demonstrate the efficacy of the tryptophan fluorescence quenching assay in revealing complex interactions between sweet taste receptors and various sweeteners.

A novel form of JARID2 is required for differentiation in lineage-committed cells.

Polycomb repressive complex-2 (PRC2) is a group of proteins that play an important role during development and in cell differentiation. PRC2 is a histone-modifying complex that catalyses methylation of lysine 27 of histone H3 (H3K27me3) at differentiation genes leading to their transcriptional repression. JARID2 is a co-factor of PRC2 and is important for targeting PRC2 to chromatin. Here, we show that, unlike in embryonic stem cells, in lineage-committed human cells, including human epidermal keratinocytes, JARID2 predominantly exists as a novel low molecular weight form, which lacks the N-terminal PRC2-interacting domain (ΔN-JARID2). We show that ΔN-JARID2 is a cleaved product of full-length JARID2 spanning the C-terminal conserved jumonji domains. JARID2 knockout in keratinocytes results in up-regulation of cell cycle genes and repression of many epidermal differentiation genes. Surprisingly, repression of epidermal differentiation genes in JARID2-null keratinocytes can be rescued by expression of ΔN-JARID2 suggesting that, in contrast to PRC2, ΔN-JARID2 promotes activation of differentiation genes. We propose that a switch from expression of full-length JARID2 to ΔN-JARID2 is important for the up-regulation differentiation genes.

Biophysical evaluation of the oligomerization and conformational properties of the N-terminal domain of TDP-43.

TDP-43 is an RNA-binding protein that presents four domains comprising an N-terminal region, two RNA recognition motifs and a C-terminal region. The N-terminal domain (NTD) has a relevant role in the oligomerization and splicing activity of TDP-43. In this work, we have expressed, purified and biophysically characterized the region that includes residues 1 to 102 that contains the nuclear localization signal (residues 80-102, NLS). Furthermore, we have evaluated the oligomerization equilibrium for this protein fragment. Also, we have determined changes in the tertiary structure and its stability in a broad range of pH values by means of different spectroscopic methods. Additionally, we compared this fragment with the one that lacks the NLS employing experimental and computational methods. Finally, we evaluated the motion of dimeric forms to get insights into the conformational flexibility of this TDP-43 module in an oligomeric state. Our results suggest that this domain has a conformational plasticity in the vicinity of the single tryptophan of this domain (Trp68), which is enhanced by the presence of the nuclear localization signal. All these results help to understand the molecular features of the NTD of TDP-43.

Effect of the Coat Protein N-Terminal Domain Structure on the Structure and Physicochemical Properties of Virions of Potato Virus X and Alternanthera Mosaic Virus.

The amino acid sequences of the coat proteins (CPs) of the potexviruses potato virus X (PVX) and alternanthera mosaic virus (AltMV) share ~40% identity. The N-terminal domains of these proteins differ in the amino acid sequence and the presence of the N-terminal fragment of 28 residues (ΔN peptide) in the PVX CP. Here, we determined the effect of the N-terminal domain on the structure and physicochemical properties of PVX and AltMV virions. The circular dichroism spectra of these viruses differed significantly, and the melting point of PVX virions was 10-12°C higher than that of AltMV virions. Alignment of the existing high-resolution 3D structures of the potexviral CPs showed that the RMSD value between the Cα-atoms was the largest for the N-terminal domains of the two compared models. Based on the computer modeling, the ΔN peptide of the PVX CP is fully disordered. According to the synchrotron small-angle X-ray scattering (SAXS) data, the structure of CPs from the PVX and AltMV virions differ; in particular, the PVX CP has a larger portion of crystalline regions and, therefore, is more ordered. Based on the SAXS data, the diameters of the PVX and AltMV virions and helix parameters in solution were calculated. The influence of the conformation of the PVX CP N-terminal domain and its position relative to the virion surface on the virion structure was investigated. Presumably, an increased thermal stability of PVX virions vs. AltMV is provided by the extended N-terminal domain (ΔN peptide, 28 amino acid residues), which forms additional contacts between the adjacent CP subunits in the PVX virion.

Glycyrrhizin as a promising kryptonite against SARS-CoV-2: Clinical, experimental, and theoretical evidences.

COVID-19 is the most devastating disease in recent times affecting most people globally. The higher rate of transmissibility and mutations of SARS-CoV-2 along with the lack of potential therapeutics has made it a global crisis. Potential molecules from natural sources could be a fruitful remedy to combat COVID-19. This systematic review highlights the detailed therapeutic implication of naturally occurring glycyrrhizin and its related derivatives against COVID-19. Glycyrrhizin has already been established for blocking different biomolecular targets related to the SARS-CoV-2 replication cycle. In this article, several experimental and theoretical evidences of glycyrrhizin and related derivatives have been discussed in detail to evaluate their potential as a promising therapeutic strategy against COVID-19. Moreover, the implication of glycyrrhizin in traditional Chinese medicines for alleviating the symptoms of COVID-19 has been reviewed. The potential role of glycyrrhizin and related compounds in affecting various stages of the SARS-CoV-2 life cycle has also been discussed in detail. Derivatization of glycyrrhizin for designing potential lead compounds along with combination therapy with other anti-SARS-CoV-2 agents followed by extensive evaluation may assist in the formulation of novel anti-coronaviral therapy for better treatment to combat COVID-19.

Tricyclic Diterpenoids Selectively Suppress Androgen Receptor-Positive Prostate Cancer Cells.

Androgen receptor (AR) is a viable therapeutic target for lethal castration-resistant prostate cancer (CRPC), because the continued progression of CRPC is mainly driven by the reactivation of AR transcriptional activity. The current FDA-approved AR antagonists binding to ligand binding domain (LBD) become ineffective in CRPC with AR gene amplification, LBD mutation, and the evolution of LBD-truncated AR splice variants. Encouraged by the fact that tricyclic aromatic diterpenoid QW07 has recently been established as a potential N-terminal AR antagonist, this study aims to explore the structure-activity relationship of tricyclic diterpenoids and their potential to suppress AR-positive cell proliferation. Dehydroabietylamine, abietic acid, dehydroabietic acid, and their derivatives were selected, since they have a similar core structure as QW07. Twenty diterpenoids were prepared for the evaluation of their antiproliferative potency on AR-positive prostate cancer cell models (LNCaP and 22Rv1) using AR-null cell models (PC-3 and DU145) as comparisons. Our data indicated that six tricyclic diterpenoids possess greater potency than enzalutamide (FDA-approved AR antagonist) towards LNCaP and 22Rv1 AR-positive cells, and four diterpenoids are more potent than enzalutamide against 22Rv1 AR-positive cells. The optimal derivative possesses greater potency (IC50 = 0.27 µM) and selectivity than QW07 towards AR-positive 22Rv1 cells.

A repeated triple lysine motif anchors complexes containing bone sialoprotein and the type XI collagen A1 chain involved in bone mineralization.

While details remain unclear, initiation of woven bone mineralization is believed to be mediated by collagen and potentially nucleated by bone sialoprotein (BSP). Interestingly, our recent publication showed that BSP and type XI collagen form complexes in mineralizing osteoblastic cultures. To learn more, we examined the protein composition of extracellular sites of de novo hydroxyapatite deposition which were enriched in BSP and Col11a1 containing an alternatively spliced "6b" exonal sequence. An alternate splice variant "6a" sequence was not similarly co-localized. BSP and Col11a1 co-purify upon ion-exchange chromatography or immunoprecipitation. Binding of the Col11a1 "6b" exonal sequence to bone sialoprotein was demonstrated with overlapping peptides. Peptide 3, containing three unique lysine-triplet sequences, displayed the greatest binding to osteoblastic cultures; peptides containing fewer lysine triplet motifs or derived from the "6a" exon yielded dramatically lower binding. Similar results were obtained with 6-carboxyfluorescein (FAM)-conjugated peptides and western blots containing extracts from osteoblastic cultures. Mass spectroscopic mapping demonstrated that FAM-peptide 3 bound to 90 kDa BSP and its 18 to 60 kDa fragments, as well as to 110 kDa nucleolin. In osteoblastic cultures, FAM-peptide 3 localized to biomineralization foci (site of BSP) and to nucleoli (site of nucleolin). In bone sections, biotin-labeled peptide 3 bound to sites of new bone formation which were co-labeled with anti-BSP antibodies. These results establish the fluorescent peptide 3 conjugate as the first nonantibody-based method to identify BSP on western blots and in/on cells. Further examination of the "6b" splice variant interactions will likely reveal new insights into bone mineralization during development.

N-Terminus-Mediated Solution Structure of Dimerization Domain of PRC1.

Microtubule-associated proteins (MAPs) are essential for the accurate division of a cell into two daughter cells. These proteins target specific microtubules to be incorporated into the spindle midzone, which comprises a special array of microtubules that initiate cytokinesis during anaphase. A representative member of the MAPs is Protein Regulator of Cytokinesis 1 (PRC1), which self-multimerizes to cross-link microtubules, the malfunction of which might result in cancerous cells. The importance of PRC1 multimerization makes it a popular target for structural studies. The available crystal structure of PRC1 has low resolution (>3 Å) and accuracy, limiting a better understanding of the structure-related functions of PRC1. Therefore, we used NMR spectroscopy to better determine the structure of the dimerization domain of PRC1. The NMR structure shows that the PRC1 N terminus is crucial to the overall structure integrity, but the crystal structure bespeaks otherwise. We systematically addressed the role of the N terminus by generating a series of mutants in which N-terminal residues methionine (Met1) and arginine (Arg2) were either deleted, extended or substituted with other rationally selected amino acids. Each mutant was subsequently analyzed by NMR spectroscopy or fluorescence thermal shift assays for its structural or thermal stability; we found that N-terminal perturbations indeed affected the overall protein structure and that the solution structure better reflects the conformation of PRC1 under solution conditions. These results reveal that the structure of PRC1 is governed by its N terminus through hydrophobic interactions with other core residues, such hitherto unidentified N-terminal conformations might shed light on the structure−function relationships of PRC1 or other proteins. Therefore, our study is of major importance in terms of identifying a novel structural feature and can further the progress of protein folding and protein engineering.

The soluble cytoplasmic N-terminal domain of the FocA channel gates bidirectional formate translocation.

FocA belongs to the pentameric FNT (formate-nitrite transporter) superfamily of anion channels, translocating formate bidirectionally across the cytoplasmic membrane of Escherichia coli and other microorganisms. While the membrane-integral core of FocA shares considerable amino acid sequence conservation with other FNT family members, the soluble cytoplasmic N-terminal domain does not. To analyze the potential biochemical function of FocA's N-terminal domain in vivo, we constructed truncation derivatives and amino acid-exchange variants, and determined their ability to translocate formate across the membrane of E. coli cells by monitoring intracellular formate levels using a formate-sensitive reporter system. Analysis of strains synthesizing these FocA variants provided insights into formate efflux. Strains lacking the ability to generate formate intracellularly allowed us to determine whether these variants could import formate or its toxic chemical analog hypophosphite. Our findings reveal that the N-terminal domain of FocA is crucial for bidirectional FocA-dependent permeation of formate across the membrane. Moreover, we show that an amino acid sequence motif and secondary structural features of the flexible N-terminal domain are important for formate translocation, and efflux/influx is influenced by pyruvate formate-lyase. The soluble N-terminal domain is, therefore, essential for bidirectional formate translocation by FocA, suggesting a "gate-keeper" function controlling anion accessibility.

Homologous and heterologous serological response to the N-terminal domain of SARS-CoV-2 in humans and mice.

The increasing numbers of infected cases of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) poses serious threats to public health and the global economy. Most SARS-CoV-2 neutralizing antibodies target the receptor binding domain (RBD) and some the N-terminal domain (NTD) of the spike protein, which is the major antigen of SARS-CoV-2. While the antibody response to RBD has been extensively characterized, the antigenicity and immunogenicity of the NTD protein are less well studied. Using 227 plasma samples from COVID-19 patients, we showed that SARS-CoV-2 NTD-specific antibodies could be induced during infection. As compared to the results of SARS-CoV-2 RBD, the serological response of SARS-CoV-2 NTD is less cross-reactive with SARS-CoV, a pandemic strain that was identified in 2003. Furthermore, neutralizing antibodies are rarely elicited in a mice model when NTD is used as an immunogen. We subsequently demonstrate that NTD has an altered antigenicity when expressed alone. Overall, our results suggest that while NTD offers a supplementary strategy for serology testing, it may not be suitable as an immunogen for vaccine development.

Machine learning identifies molecular regulators and therapeutics for targeting SARS-CoV2-induced cytokine release.

Although 15-20% of COVID-19 patients experience hyper-inflammation induced by massive cytokine production, cellular triggers of this process and strategies to target them remain poorly understood. Here, we show that the N-terminal domain (NTD) of the SARS-CoV-2 spike protein substantially induces multiple inflammatory molecules in myeloid cells and human PBMCs. Using a combination of phenotypic screening with machine learning-based modeling, we identified and experimentally validated several protein kinases, including JAK1, EPHA7, IRAK1, MAPK12, and MAP3K8, as essential downstream mediators of NTD-induced cytokine production, implicating the role of multiple signaling pathways in cytokine release. Further, we found several FDA-approved drugs, including ponatinib, and cobimetinib as potent inhibitors of the NTD-mediated cytokine release. Treatment with ponatinib outperforms other drugs, including dexamethasone and baricitinib, inhibiting all cytokines in response to the NTD from SARS-CoV-2 and emerging variants. Finally, ponatinib treatment inhibits lipopolysaccharide-mediated cytokine release in myeloid cells in vitro and lung inflammation mouse model. Together, we propose that agents targeting multiple kinases required for SARS-CoV-2-mediated cytokine release, such as ponatinib, may represent an attractive therapeutic option for treating moderate to severe COVID-19.

A phase 1 study to assess the safety, pharmacokinetics, and anti-tumor activity of the androgen receptor n-terminal domain inhibitor epi-506 in patients with metastatic castration-resistant prostate cancer.

BACKGROUND: EPI-506 is the first of a new class of drugs targeting the N-terminal domain (NTD) of the androgen receptor (AR), potentially overcoming known resistance mechanisms to androgen receptor pathway inhibitors (ARPIs) among men with metastatic castration resistant prostate cancer (mCRPC). METHODS: Patients with mCRPC who had progressed on prior ARPI were enrolled in this phase 1 open-label, adaptive 3 + 3 dose escalation study. The primary outcome was safety and tolerability of oral EPI-506. Secondary objectives included determination of the maximal tolerated dose (MTD), pharmacokinetic profile, and antitumor efficacy. RESULTS: 28 mCRPC patients were enrolled into 7 dose cohorts of EPI-506 ranging from 80-3600 mg given once daily and 1800 mg given twice daily. Six DLTs occurred in 4 patients; Grade 4 elevated amylase; Grade 3 abdominal pain; Grade 3 elevated ALT and Grade 3 elevated AST; Grade 2 nausea and Grade 1 vomiting which resulted in study drug intake of < 75% of the expected dose during the DLT assessment period. The most common drug-related adverse events included diarrhea, nausea and fatigue. Six patients had a PSA decline not meeting PSA response criteria. The study was terminated prior to reaching the MTD due to poor oral bioavailability. CONCLUSIONS: This phase 1 trial established the safety of EPI-506 and provides proof of concept for targeting the AR NTD. Next generation compounds with improved bioavailability and potency are in clinical development.