Identification and Optimization of a Ligand-Efficient Benzoazepinone Bromodomain and Extra Terminal (BET) Family Acetyl-Lysine Mimetic into the Oral Candidate Quality Molecule I-BET432.

The bromodomain and extra terminal (BET) family of proteins are an integral part of human epigenome regulation, the dysregulation of which is implicated in multiple oncology and inflammatory diseases. Disrupting the BET family bromodomain acetyl-lysine (KAc) histone protein-protein interaction with small-molecule KAc mimetics has proven to be a disease-relevant mechanism of action, and multiple molecules are currently undergoing oncology clinical trials. This work describes an efficiency analysis of published GSK pan-BET bromodomain inhibitors, which drove a strategic choice to focus on the identification of a ligand-efficient KAc mimetic with the hypothesis that lipophilic efficiency could be drastically improved during optimization. This focus drove the discovery of the highly ligand-efficient and structurally distinct benzoazepinone KAc mimetic. Following crystallography to identify suitable growth vectors, the benzoazepinone core was optimized through an explore-exploit structure-activity relationship (SAR) approach while carefully monitoring lipophilic efficiency to deliver I-BET432 (41) as an oral candidate quality molecule.

Design, Synthesis, and Characterization of I-BET567, a Pan-Bromodomain and Extra Terminal (BET) Bromodomain Oral Candidate.

Through regulation of the epigenome, the bromodomain and extra terminal (BET) family of proteins represent important therapeutic targets for the treatment of human disease. Through mimicking the endogenous N-acetyl-lysine group and disrupting the protein-protein interaction between histone tails and the bromodomain, several small molecule pan-BET inhibitors have progressed to oncology clinical trials. This work describes the medicinal chemistry strategy and execution to deliver an orally bioavailable tetrahydroquinoline (THQ) pan-BET candidate. Critical to the success of this endeavor was a potency agnostic analysis of a data set of 1999 THQ BET inhibitors within the GSK collection which enabled identification of appropriate lipophilicity space to deliver compounds with a higher probability of desired oral candidate quality properties. SAR knowledge was leveraged via Free-Wilson analysis within this design space to identify a small group of targets which ultimately delivered I-BET567 (27), a pan-BET candidate inhibitor that demonstrated efficacy in mouse models of oncology and inflammation.

Development of an intracellular quantitative assay to measure compound binding kinetics.

Contemporary drug discovery typically quantifies the effect of a molecule on a biological target using the equilibrium-derived measurements of IC50, EC50, or KD. Kinetic descriptors of drug binding are frequently linked with the effectiveness of a molecule in modulating a disease phenotype; however, these parameters are yet to be fully adopted in early drug discovery. Nanoluciferase bioluminescence resonance energy transfer (NanoBRET) can be used to measure interactions between fluorophore-conjugated probes and luciferase fused target proteins. Here, we describe an intracellular NanoBRET competition assay that can be used to quantify cellular kinetic rates of compound binding to nanoluciferase-fused bromodomain and extra-terminal (BET) proteins. Comparative rates are generated using a cell-free NanoBRET assay and by utilizing orthogonal recombinant protein-based methodologies. A screen of known pan-BET inhibitors is used to demonstrate the value of this approach in the investigation of kinetic selectivity between closely related proteins.

Optimization of Naphthyridones into Selective TATA-Binding Protein Associated Factor 1 (TAF1) Bromodomain Inhibitors.

Bromodomain containing proteins and the acetyl-lysine binding bromodomains contained therein are increasingly attractive targets for the development of novel epigenetic therapeutics. To help validate this target class and unravel the complex associated biology, there has been a concerted effort to develop selective small molecule bromodomain inhibitors. Herein we describe the structure-based efforts and multiple challenges encountered in optimizing a naphthyridone template into selective TAF1(2) bromodomain inhibitors which, while unsuitable as chemical probes themselves, show promise for the future development of small molecules to interrogate TAF1(2) biology. Key to this work was the introduction and modulation of the basicity of a pendant amine which had a substantial impact on not only bromodomain selectivity but also cellular target engagement.

Optimization of a Series of RIPK2 PROTACs.

Receptor-interacting serine/threonine protein kinase 2 (RIPK2) is an important kinase of the innate immune system. Herein, we describe the optimization of a series of RIPK2 PROTACs which recruit members of the inhibitor of apoptosis (IAP) family of E3 ligases. Our PROTAC optimization strategy focused on reducing the lipophilicity of the early lead which resulted in the identification of analogues with improved solubility and increased human and rat microsomal stability. We identified a range of IAP binders that were successfully incorporated into potent RIPK2 PROTACs with attractive pharmacokinetic profiles. Compound 20 possessed the best overall profile with good solubility, potent degradation of RIPK2, and associated inhibition of TNFα release. A proof-of-concept study utilizing a slow release matrix demonstrated the feasibility of a long-acting parenteral formulation with >1 month duration. This represents an attractive alternative dosing paradigm to oral delivery, especially for chronic diseases where compliance can be challenging.

GSK973 Is an Inhibitor of the Second Bromodomains (BD2s) of the Bromodomain and Extra-Terminal (BET) Family.

Pan-BET inhibitors have shown profound efficacy in a number of in vivo preclinical models and have entered the clinic in oncology trials where adverse events have been reported. These inhibitors interact equipotently with the eight bromodomains of the BET family of proteins. To better understand the contribution of each domain to their efficacy and to improve from their safety profile, selective inhibitors are required. This Letter discloses the profile of GSK973, a highly selective inhibitor of the second bromodomains of the BET proteins that has undergone extensive preclinical in vitro and in vivo characterization.

Cellular Target Engagement Approaches to Monitor Epigenetic Reader Domain Interactions.

Malfunctions in the basic epigenetic mechanisms such as histone modifications, DNA methylation, and chromatin remodeling are implicated in a number of cancers and immunological and neurodegenerative conditions. Within GlaxoSmithKline (GSK) we have utilized a number of variations of the NanoBRET technology for the direct measurement of compound-target engagement within native cellular environments to drive high-throughput, routine structure-activity relationship (SAR) profiling across differing epigenetic targets. NanoBRET is a variation of the bioluminescence resonance energy transfer (BRET) methodology utilizing proteins of interest fused to either NanoLuc, a small, high-emission-intensity luciferase, or HaloTag, a modified dehalogenase enzyme that can be selectively labeled with a fluorophore. The combination of these two technologies has enabled the application of NanoBRET to biological systems such as epigenetic protein-protein interactions, which have previously been challenging. By synergizing target engagement assays with more complex primary cell phenotypic assays, we have been able to demonstrate compound-target selectivity profiles to enhance cellular potency and offset potential liability risks. Additionally, we have shown that in the absence of a robust, cell phenotypic assay, it is possible to utilize NanoBRET target engagement assays to aid chemistry in progressing at a higher scale than would have otherwise been achievable. The NanoBRET target engagement assays utilized have further shown an excellent correlation with more reductionist biochemical and biophysical assay systems, clearly demonstrating the possibility of using such assay systems at scale, in tandem with, or in preference to, lower-throughput cell phenotypic approaches.

Discovery of Tetrahydroquinoxalines as Bromodomain and Extra-Terminal Domain (BET) Inhibitors with Selectivity for the Second Bromodomain.

The bromodomain and extra-terminal domain (BET) family of proteins bind acetylated lysine residues on histone proteins. The four BET bromodomains-BRD2, BRD3, BRD4, and BRDT-each contain two bromodomain modules. BET bromodomain inhibition is a potential therapy for various cancers and immunoinflammatory diseases, but few reported inhibitors show selectivity within the BET family. Inhibitors with selectivity for the first or second bromodomain are desired to aid investigation of the biological function of these domains. Focused library screening identified a series of tetrahydroquinoxalines with selectivity for the second bromodomains of the BET family (BD2). Structure-guided optimization of the template improved potency, selectivity, and physicochemical properties, culminating in potent BET inhibitors with BD2 selectivity.

Prediction of intracellular exposure bridges the gap between target- and cell-based drug discovery.

Inadequate target exposure is a major cause of high attrition in drug discovery. Here, we show that a label-free method for quantifying the intracellular bioavailability (Fic) of drug molecules predicts drug access to intracellular targets and hence, pharmacological effect. We determined Fic in multiple cellular assays and cell types representing different targets from a number of therapeutic areas, including cancer, inflammation, and dementia. Both cytosolic targets and targets localized in subcellular compartments were investigated. Fic gives insights on membrane-permeable compounds in terms of cellular potency and intracellular target engagement, compared with biochemical potency measurements alone. Knowledge of the amount of drug that is locally available to bind intracellular targets provides a powerful tool for compound selection in early drug discovery.

GSK6853, a Chemical Probe for Inhibition of the BRPF1 Bromodomain.

The BRPF (Bromodomain and PHD Finger-containing) protein family are important scaffolding proteins for assembly of MYST histone acetyltransferase complexes. A selective benzimidazolone BRPF1 inhibitor showing micromolar activity in a cellular target engagement assay was recently described. Herein, we report the optimization of this series leading to the identification of a superior BRPF1 inhibitor suitable for in vivo studies.

Direct Measurement of Intracellular Compound Concentration by RapidFire Mass Spectrometry Offers Insights into Cell Permeability.

One of the key challenges facing early stage drug discovery is understanding the commonly observed difference between the activity of compounds in biochemical assays and cellular assays. Traditionally, indirect or estimated cell permeability measurements such as estimations from logP or artificial membrane permeability are used to explain the differences. The missing link is a direct measurement of intracellular compound concentration in whole cells. This can, in some circumstances, be estimated from the cellular activity, but this may also be problematic if cellular activity is weak or absent. Advances in sensitivity and throughput of analytical techniques have enabled us to develop a high-throughput assay for the measurement of intracellular compound concentration for routine use to support lead optimization. The assay uses a RapidFire-MS based readout of compound concentration in HeLa cells following incubation of cells with test compound. The initial assay validation was performed by ultra-high performance liquid chromatography tandem mass spectrometry, and the assay was subsequently transferred to RapidFire tandem mass spectrometry. Further miniaturization and optimization were performed to streamline the process, increase sample throughput, and reduce cycle time. This optimization has delivered a semi-automated platform with the potential of production scale compound profiling up to 100 compounds per day.

1,3-Dimethyl Benzimidazolones Are Potent, Selective Inhibitors of the BRPF1 Bromodomain.

The BRPF (bromodomain and PHD finger-containing) protein family are important scaffolding proteins for assembly of MYST histone acetyltransferase complexes. Here, we report the discovery, binding mode, and structure-activity relationship (SAR) of the first potent, selective series of inhibitors of the BRPF1 bromodomain.

5-{2-[4-(2-methyl-5-quinolinyl)-1-piperazinyl]ethyl}-2(1H)-quinolinones and 3,4-dihydro-2(1H)-quinolinones: dual-acting 5-HT1 receptor antagonists and serotonin reuptake inhibitors. Part 3.

5-{2-[4-(2-Methyl-5-quinolinyl)-1-piperazinyl]ethyl}-2(1H)-quinolinones and 3,4-dihydro-2(1H)-quinolinones have been identified with different combinations of 5-HT(1) autoreceptor antagonist and hSerT potencies and excellent rat PK profiles. The availability of tool compounds with a range of profiles at targets known to play a key role in the control of synaptic 5-HT levels will allow exploration of different pharmacological profiles in a range of animal behavioral and disease models.

Design and synthesis of novel tricyclic benzoxazines as potent 5-HT(1A/B/D) receptor antagonists leading to the discovery of 6-{2-[4-(2-methyl-5-quinolinyl)-1-piperazinyl]ethyl}-4H-imidazo[5,1-c][1,4]benzoxazine-3-carboxamide (GSK588045).

Bioisoteric replacement of the metabolically labile N-methyl amide group of a series of benzoxazinones with small heterocyclic rings has led to novel series of fused tricyclic benzoxazines which are potent 5-HT(1A/B/D) receptor antagonists with and without concomitant human serotonin transporter (hSerT) activity. Optimizing against multiple parameters in parallel identified 6-{2-[4-(2-methyl-5-quinolinyl)-1-piperazinyl]ethyl}-4H-imidazo[5,1-c][1,4]benzoxazine-3-carboxamide (GSK588045) as a potent 5-HT(1A/B/D) receptor antagonist with a high degree of selectivity over human ether-a-go-go related gene (hERG) potassium channels, favorable pharmacokinetics, and excellent activity in vivo in rodent pharmacodynamic (PD) models. On the basis of its outstanding overall profile, this compound was progressed as a clinical candidate with the ultimate aim to assess its potential as a faster acting antidepressant/anxiolytic with reduced side-effect burden.

8-[2-(4-Aryl-1-piperazinyl)ethyl]-2H-1,4-benzoxazin-3(4H)-ones: dual-acting 5-HT1 receptor antagonists and serotonin reuptake inhibitors--part II.

8-[2-(4-Aryl-1-piperazinyl)ethyl]-2H-1,4-benzoxazin-3(4H)-ones have been identified as highly potent 5-HT(1A/B/D) receptor antagonists with and without additional SerT activity and a high degree of selectivity over hERG potassium channels. Modulation of the different target activities gave compounds with a range of profiles suitable for further in vivo characterization.

6-[2-(4-Aryl-1-piperazinyl)ethyl]-2H-1,4-benzoxazin-3(4H)-ones: dual-acting 5-HT1 receptor antagonists and serotonin reuptake inhibitors.

Investigation of a series 6-[2-(4-aryl-1-piperazinyl)ethyl]-2H-1,4-benzoxazin-3(4H)-ones has led to the discovery of potent 5-HT(1A/1B/1D) receptor antagonists with and without additional SerT affinity. Modulation of the different target activities gave compounds with a range of profiles suitable for further in vivo characterization.

Discovery of potent, orally bioavailable, selective 5-HT1A/B/D receptor antagonists.

5-HT1 receptor antagonists have been discovered with good selectivity over the 5-HT transporter. This is the first report of highly potent, selective ligands for the 5-HT1A/B/D receptors with low intrinsic activity, which represent a useful set of molecules for further understanding the roles of the 5-HT1 receptor subtypes and providing new approaches for the treatment of depression.

SB-649915, a novel, potent 5-HT1A and 5-HT1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue.

An increase in brain 5-HT levels is thought to be the key mechanism of action which results in an antidepressant response. It has been proven that selective serotonin re-uptake inhibitors are effective antidepressants but the delay to therapeutic onset of these agents is thought to be due to the time required for 5-HT1A, and possibly 5-HT1B, autoreceptor desensitisation. Therefore an agent incorporating 5-HT re-uptake inhibition coupled with 5-HT1A and/or 5-HT1B autoreceptor antagonism may provide a fast acting clinical agent. The current studies describe the in vitro profile of SB-649915 (6-[(1-{2-[(2-methylquinolin-5-yl)oxy]ethyl}piperidin-4-yl)methyl]-2H-1,4-benzoxazin-3(4H)-one), a novel compound which has high affinity for human recombinant 5-HT1A, 5-HT1B and 5-HT1D receptors (pKi values of 8.6, 8.0, 8.8, respectively) and the human recombinant 5-HT transporter (pKi value of 9.3). SB-649915 also displays high affinity for rat, guinea pig, mouse and marmoset native tissue 5-HT1A, 5-HT1B and 5-HT1D receptors and rat native tissue 5-HT transporters (pKi values>or=7.5). In functional [35S]GTPgammaS binding studies, SB-649915 (up to 1 microM) does not display intrinsic activity in HEK293 cells expressing human recombinant 5-HT1A receptors but acts as a partial agonist at human recombinant 5-HT1B and 5-HT1D receptors with intrinsic activity values of 0.3 and 0.7, respectively, as compared to the full agonist 5-HT. From Schild analysis, SB-649915 caused a concentration-dependent, rightward shift of 5-HT-induced stimulation of basal [35S]GTPgammaS binding in cells expressing human recombinant 5-HT1A or 5-HT1B receptors to yield pA2 values of 9.0 and 7.9, respectively. In electrophysiological studies in rat dorsal raphe nucleus, SB-649915 did not affect the cell firing rate up to 1 microM but attenuated (+)8-hydroxy-2-(di-n-propylamino) tetralin-induced inhibition of cell firing with an apparent pKb value of 9.5. SB-649915 (1 microM) significantly attenuated exogenous 5-HT-induced inhibition of electrically-stimulated [3H]5-HT release from guinea pig cortex. In studies designed to enhance endogenous 5-HT levels, and therefore increase tone at 5-HT1B autoreceptors, SB-649915 significantly potentiated [3H]5-HT release at 100 and 1000 nM. In LLCPK cells expressing human recombinant 5-HT transporters and in rat cortical synaptosomes, SB-649915 inhibited [3H]5-HT re-uptake with pIC50 values of 7.9 and 9.7, respectively. In summary, SB-649915 is a novel, potent 5-HT1A/1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue systems and represents a novel mechanism that could offer fast acting antidepressant action.

8-Piperazinyl-2,3-dihydropyrrolo[3,2-g]isoquinolines: potent, selective, orally bioavailable 5-HT1 receptor ligands.

The novel 8-piperazinyl-2,3-dihydropyrroloisoquinoline template was synthesized in nine steps. The template was N-substituted to give a series of compounds showing binding to human cloned 5-HT1A, 5-HT1B and 5-HT1D receptors with pKi's greater than 9 and selectivities up to 1000-fold against other serotonin, dopamine and adrenergic receptors. Several compounds were shown to possess weak partial agonist activity in cloned receptors, which translated to antagonism in in vitro studies.