CRISPR Screen Reveals BRD2/4 Molecular Glue-like Degrader via Recruitment of DCAF16.

Molecular glues (MGs) are monovalent small molecules that induce an interaction between proteins (native or non-native partners) by altering the protein-protein interaction (PPI) interface toward a higher-affinity state. Enhancing the PPI between a protein and E3 ubiquitin ligase can lead to degradation of the partnering protein. Over the past decade, retrospective studies of clinical drugs identified that immunomodulatory drugs (e.g., thalidomide and analogues) and indisulam exhibit a molecular glue effect by driving the interaction between non-native substrates to CRBN and DCAF15 ligases, respectively. Ensuing reports of phenotypic screens focused on MG discovery have suggested that these molecules may be more common than initially anticipated. However, prospective discovery of MGs remains challenging. Thus, expanding the repertoire of MGs will enhance our understanding of principles for prospective design. Herein, we report the results of a CRISPR/Cas9 knockout screen of over 1000 ligases and ubiquitin proteasome system components in a BRD4 degradation assay with a JQ1-based monovalent degrader, compound 1a. We identified DCAF16, a substrate recognition component of the Cul4 ligase complex, as essential for compound activity, and we demonstrate that compound 1a drives the interaction between DCAF16 and BRD2/4 to promote target degradation. Taken together, our data suggest that compound 1a functions as an MG degrader between BRD2/4 and DCAF16 and provides a foundation for further mechanistic dissection to advance prospective MG discovery.

Controlling cellular distribution of drugs with permeability modifying moieties.

Phenotypic screening provides compounds with very limited target cellular localization data. In order to select the most appropriate target identification methods, determining if a compound acts at the cell-surface or intracellularly can be very valuable. In addition, controlling cell-permeability of targeted therapeutics such as antibody-drug conjugates (ADCs) and targeted nanoparticle formulations can reduce toxicity from extracellular release of drug in undesired tissues or direct activity in bystander cells. By incorporating highly polar, anionic moieties via short polyethylene glycol linkers into compounds with known intracellular, and cell-surface targets, we have been able to correlate the cellular activity of compounds with their subcellular site of action. For compounds with nuclear (Brd, PARP) or cytosolic (dasatinib, NAMPT) targets, addition of the permeability modifying group (small sulfonic acid, polycarboxylic acid, or a polysulfonated fluorescent dye) results in near complete loss of biological activity in cell-based assays. For cell-surface targets (H3, 5HT1A, ß2AR) significant activity was maintained for all conjugates, but the results were more nuanced in that the modifiers impacted binding/activity of the resulting conjugates. Taken together, these results demonstrate that small anionic compounds can be used to control cell-permeability independent of on-target activity and should find utility in guiding target deconvolution studies and controlling drug distribution of targeted therapeutics.

Targeting lysine specific demethylase 4A (KDM4A) tandem TUDOR domain - A fragment based approach.

The tandem TUDOR domains present in the non-catalytic C-terminal half of the KDM4A, 4B and 4C enzymes play important roles in regulating their chromatin localizations and substrate specificities. They achieve this regulatory role by binding to different tri-methylated lysine residues on histone H3 (H3-K4me3, H3-K23me3) and histone H4 (H4-K20me3) depending upon the specific chromatin environment. In this work, we have used a 2D-NMR based fragment screening approach to identify a novel fragment (1a), which binds to the KDM4A-TUDOR domain and shows modest competition with H3-K4me3 binding in biochemical as well as in vitro cell based assays. A co-crystal structure of KDM4A TUDOR domain in complex with 1a shows that the fragment binds stereo-specifically to the methyl lysine binding pocket forming a network of strong hydrogen bonds and hydrophobic interactions. We anticipate that the fragment 1a can be further developed into a novel allosteric inhibitor of the KDM4 family of enzymes through targeting their C-terminal tandem TUDOR domain.

SAR and characterization of non-substrate isoindoline urea inhibitors of nicotinamide phosphoribosyltransferase (NAMPT).

Herein we disclose SAR studies that led to a series of isoindoline ureas which we recently reported were first-in-class, non-substrate nicotinamide phosphoribosyltransferase (NAMPT) inhibitors. Modification of the isoindoline and/or the terminal functionality of screening hit 5 provided inhibitors such as 52 and 58 with nanomolar antiproliferative activity and preclinical pharmacokinetics properties which enabled potent antitumor activity when dosed orally in mouse xenograft models. X-ray crystal structures of two inhibitors bound in the NAMPT active-site are discussed.

Turning a Substrate Peptide into a Potent Inhibitor for the Histone Methyltransferase SETD8.

SETD8 is a histone H4-K20 methyltransferase that plays an essential role in the maintenance of genomic integrity during mitosis and in DNA damage repair, making it an intriguing target for cancer research. While some small molecule inhibitors for SETD8 have been reported, the structural binding modes for these inhibitors have not been revealed. Using the complex structure of the substrate peptide bound to SETD8 as a starting point, different natural and unnatural amino acid substitutions were tested, and a potent (Ki 50 nM, IC50 0.33 µM) and selective norleucine containing peptide inhibitor has been obtained.

Glycan-Mediated, Ligand-Controlled Click Chemistry for Drug-Target Identification.

Membrane-bound proteins are important pharmaceutical drug targets, yet few strategies exist for the identification of small-molecule-targeted membrane proteins in live-cell systems. By exploiting metabolic glycan engineering of cell membrane proteins, we have developed an in situ glycan-mediated ligand-controlled click ("GLiCo-Click") chemistry methodology that enables the attachment of small-molecule chemical probes to their receptor protein through glycans on live cells. In addition to enabling receptor enrichment from cell lysates, this strategy can be used to demonstrate target receptor engagement and enables the molecular characterization of receptors.

Hybridization of complementary and homologous peptide nucleic acid oligomers to a guanine quadruplex-forming RNA.

Peptide nucleic acid (PNA) oligomers targeted to guanine quadruplex-forming RNAs can be designed in two different ways. First, complementary cytosine-rich PNAs can hybridize by the formation of Watson-Crick base pairs, resulting in hybrid PNA-RNA duplexes. Second, guanine-rich homologous PNAs can hybridize by the formation of G tetrads, resulting in hybrid PNA-RNA quadruplexes. UV thermal denaturation, circular dichroism, and fluorescence spectroscopy experiments were used to compare these two recognition modes and revealed 1:1 duplex formation for the complementary PNA and 2:1 (PNA2-RNA) quadruplex formation for the homologous PNA. Both hybrids were very stable, and hybridization was observed at low nanomolar concentrations. Hybrid quadruplex formation was equally efficient regardless of the PNA strand polarity, indicating a lack of interaction between the loop nucleobases on the PNA and RNA strands. The implications of this finding on sequence specificity as well as methods to improve affinity are also discussed.

RNA guanine quadruplex invasion by complementary and homologous PNA probes.

Guanine quadruplexes are gaining increasing attention due to their suspected roles in regulating gene expression at the transcriptional and translational levels. This paper describes the ability of short peptide nucleic acid (PNA) probes to disrupt a stable RNA quadruplex and hybridize to their target sequence. In one case, the PNA probe is complementary to the target, resulting in formation of a Watson-Crick base-paired duplex. In the second case, the PNA probe is homologous to the target and forms a hybrid quadruplex structure. The hybrid duplex is formed in a 1:1 stoichiometry, as expected based on the constraints imposed by Watson-Crick pairing. However, the hybrid quadruplex is formed in a PNA2:RNA stoichiometry, due to the ability of the short PNA to hybridize with both halves of the original RNA quadruplex.

Recent advances in the development of peptide nucleic acid as a gene-targeted drug.

Peptide nucleic acid (PNA) is a non-ionic mimic of DNA that binds to complementary DNA and RNA sequences with high affinity and selectivity. Targeting of single-stranded RNA leads to antisense effects, whereas PNAs directed toward double-stranded DNA exhibit antigene properties. Recent advances in cell uptake and in antisense and antigene effects in biological systems are summarised in this review. In addition to traditional targets, namely genomic DNA and messenger RNA, applications for PNA as a bacteriocidal antibiotic, for regulating splice site selection and as a telomerase inhibitor are described.