Novel irreversible covalent BTK inhibitors discovered using DNA-encoded chemistry

DNA-Encoded Library Technology─A Catalyst for Covalent Ligand Discovery

The identification of novel covalent ligands for therapeutic purposes has long depended on serendipity, with dedicated hit finding techniques emerging only in the early 2000s. Advances in chemoproteomics have enabled robust characterization of putative drugs to derisk the unique liabilities associated with covalent hit molecules, leading to a renewed interest in this targeting modality. DNA-encoded library (DEL) technology has similarly emerged over the past two decades as a highly efficient method to identify new chemical equity toward protein targets of interest. A number of commercial and academic groups have reported methods in covalent DEL synthesis and hit identification; however, it is evident that there is still much to be done to fully realize the power of this technology for covalent ligand discovery. This perspective will explore the current approaches in covalent DEL technology and reflect on the next steps to advance this field.

Covalent hits and where to find them

Covalent hits for drug discovery campaigns are neither fantastic beasts nor mythical creatures, they can be routinely identified through electrophile-first screening campaigns using a suite of different techniques. These include biophysical and biochemical methods, cellular approaches, and DNA-encoded libraries. Employing best practice, however, is critical to success. The purpose of this review is to look at state of the art covalent hit identification, how to identify hits from a covalent library and how to select compounds for medicinal chemistry programmes.

ABPP-CoDEL: Activity-Based Proteome Profiling-Guided Discovery of Tyrosine-Targeting Covalent Inhibitors from DNA-Encoded Libraries

DNA-encoded chemical library (DEL) has been extensively used for lead compound discovery for decades in academia and industry. Incorporating an electrophile warhead into DNA-encoded compounds recently permitted the discovery of covalent ligands that selectively react with a particular cysteine residue. However, noncysteine residues remain underexplored as modification sites of covalent DELs. Herein, we report the design and utility of tyrosine-targeting DELs of 67 million compounds. Proteome-wide reactivity analysis of tyrosine-reactive sulfonyl fluoride (SF) covalent probes suggested three enzymes (phosphoglycerate mutase 1, glutathione s-transferase 1, and dipeptidyl peptidase 3) as models of tyrosine-targetable proteins. Enrichment with SF-functionalized DELs led to the identification of a series of tyrosine-targeting covalent inhibitors of the model enzymes. In-depth mechanistic investigation revealed their novel modes of action and reactive ligand-accessible hotspots of the enzymes. Our strategy of combining activity-based proteome profiling and covalent DEL enrichment (ABPP-CoDEL), which generated selective covalent binders against a variety of target proteins, illustrates the potential use of this methodology in further covalent drug discovery.

Covalent fragment libraries in drug discovery-Design, synthesis, and screening methods

As the development of drugs with a covalent mode of action is becoming increasingly popular, well-validated covalent fragment-based drug discovery (FBDD) methods have been comparatively slow to keep up with the demand. In this chapter the principles of covalent fragment reactivity, library design, synthesis, and screening methods are explored in depth, focussing on literature examples with direct applications to practical covalent fragment library design and screening. Further, questions about the future of the field are explored and potential useful advances are proposed.

Small-molecule discovery through DNA-encoded libraries

The development of bioactive small molecules as probes or drug candidates requires discovery platforms that enable access to chemical diversity and can quickly reveal new ligands for a target of interest. Within the past 15 years, DNA-encoded library (DEL) technology has matured into a widely used platform for small-molecule discovery, yielding a wide variety of bioactive ligands for many therapeutically relevant targets. DELs offer many advantages compared with traditional screening methods, including efficiency of screening, easily multiplexed targets and library selections, minimized resources needed to evaluate an entire DEL and large library sizes. This Review provides accounts of recently described small molecules discovered from DELs, including their initial identification, optimization and validation of biological properties including suitability for clinical applications.

Identification of Covalent Cyclic Peptide Inhibitors in mRNA Display

Peptides have historically been underutilized for covalent inhibitor discovery, despite their unique abilities to interact with protein surfaces and interfaces. This is in part due to a lack of methods for screening and identifying covalent peptide ligands. Here, we report a method to identify covalent cyclic peptide inhibitors in mRNA display. We combine co- and post-translational library diversification strategies to create cyclic libraries with reactive dehydroalanines (Dhas), which we employ in selections against two model targets. The most potent hits exhibit low nanomolar inhibitory activities and disrupt known protein-protein interactions with their selected targets. Overall, we establish Dhas as electrophiles for covalent inhibition and showcase how separate library diversification methods can work synergistically to dispose mRNA display to novel applications like covalent inhibitor discovery.

Advances in covalent drug discovery

Covalent drugs have been used to treat diseases for more than a century, but tools that facilitate the rational design of covalent drugs have emerged more recently. The purposeful addition of reactive functional groups to existing ligands can enable potent and selective inhibition of target proteins, as demonstrated by the covalent epidermal growth factor receptor (EGFR) and Bruton's tyrosine kinase (BTK) inhibitors used to treat various cancers. Moreover, the identification of covalent ligands through 'electrophile-first' approaches has also led to the discovery of covalent drugs, such as covalent inhibitors for KRAS(G12C) and SARS-CoV-2 main protease. In particular, the discovery of KRAS(G12C) inhibitors validates the use of covalent screening technologies, which have become more powerful and widespread over the past decade. Chemoproteomics platforms have emerged to complement covalent ligand screening and assist in ligand discovery, selectivity profiling and target identification. This Review showcases covalent drug discovery milestones with emphasis on the lessons learned from these programmes and how an evolving toolbox of covalent drug discovery techniques facilitates success in this field.

PAC-FragmentDEL - photoactivated covalent capture of DNA-encoded fragments for hit discovery

We describe a novel approach for screening fragments against a protein that combines the sensitivity of DNA-encoded library technology with the ability of fragments to explore what will bind. Each of the members of the library consists of a fragment which is linked to a photoactivatable diazirine moiety. Split and pool synthesis combines each fragment with a set of linkers with the version of the library reported here containing some 70k different compounds, each with an individual DNA code. Incubation of the library with a protein sample is followed by photoactivation, washing and subsequent PCR and sequencing which allows the individual fragment hits to be identified. We illustrate how the approach allows successful hit fragment identification using only microgram quantities of material for two targets. PAK4 is a kinase for which conventional fragment screening has generated many advance leads. The as yet undrugged target, 2-epimerase, presents a more challenging active site for identification of hit compounds. In both cases, PAC-FragmentDEL identified fragments validated as hits by ligand-observed NMR measurements and crystal structure determination of off-DNA sample binding to the proteins.

Triazine-Based Covalent DNA-Encoded Libraries for Discovery of Covalent Inhibitors of Target Proteins

Since ibrutinib was approved by the FDA as an effective monotherapy for chronic lymphocytic leukemia (CLL) and multilymphoma, more and more FDA-approved covalent drugs are coming back into the market. On this occasion, the resurgence of interest in covalent drugs calls for more hit discovery techniques. However, the limited numbers of covalent libraries prevent the development of this area. Herein, we report the design of covalent DNA-encoded library (DEL) and its selection method for the discovery of covalent inhibitors for target proteins. These triazine-based covalent DELs yielded potent compounds after covalent selection against target proteins, including Bruton's Tyrosine Kinase (BTK), Janus kinase 3 (JAK3), and peptidyl-prolyl cis/trans isomerase NIMA-interacting-1 (Pin1).

DNA Compatible Oxidization and Amidation of Terminal Alkynes

Through a modified Kinugasa reaction, a novel method of amidation on terminal oligo alkyne conjugates by copper-promoted oxidation with nitrones has been developed. Unprotected bifunctional carboxylic acid-amine reagents can be transformed directly to the respective amide products under these edited Kinugasa reaction conditions. 3-Cycle DNA-encoded libraries (DELs) can be built in three steps of chemical conversion.

The protein kinase CK1: Inhibition, activation, and possible allosteric modulation

Protein kinases play a vital role in biology and deregulation of kinases is implicated in numerous diseases ranging from cancer to neurodegenerative diseases, making them a major target class for the pharmaceutical industry. However, the high degree of conservation that exists between ATP-binding sites among kinases makes it difficult for current inhibitors to be highly specific. In the context of neurodegeneration, several groups including ours, have linked different kinases such as CK1 and Alzheimer’s disease for example. Strictly CK1-isoform specific regulators do not exist and known CK1 inhibitors are inhibiting the enzymatic activity, targeting the ATP-binding site. Here we review compounds known to target CK1, as well as other inhibitory types that could benefit CK1. We introduce the DNA-encoded library (DEL) technology that might represent an interesting approach to uncover allosteric modulators instead of ATP competitors. Such a strategy, taking into account known allosteric inhibitors and mechanisms, might help designing modulators that are more specific towards a specific kinase, and in the case of CK1, toward specific isoforms.

Optimization of Covalent MKK7 Inhibitors via Crude Nanomole-Scale Libraries

High-throughput nanomole-scale synthesis allows for late-stage functionalization (LSF) of compounds in an efficient and economical manner. Here, we demonstrated that copper-catalyzed azide-alkyne cycloaddition could be used for the LSF of covalent kinase inhibitors at the nanoscale, enabling the synthesis of hundreds of compounds that did not require purification for biological assay screening, thus reducing experimental time drastically. We generated crude libraries of inhibitors for the kinase MKK7, derived from two different parental precursors, and analyzed them via the high-throughput In-Cell Western assay. Select inhibitors were resynthesized, validated via conventional biological and biochemical methods such as western blots and liquid chromatography-mass spectrometry (LC-MS) labeling, and successfully co-crystallized. Two of these compounds showed over 20-fold increased inhibitory activity compared to the parental compound. This study demonstrates that high-throughput LSF of covalent inhibitors at the nanomole-scale level can be an auspicious approach in improving the properties of lead chemical matter.

Strategies for developing DNA-encoded libraries beyond binding assays

DNA-encoded chemical libraries (DELs) have emerged as a powerful technology in drug discovery. The wide adoption of DELs in the pharmaceutical industry and the rapid advancements of DEL-compatible chemistry have further fuelled its development and applications. In general, a DEL has been considered as a massive binding assay to identify physical binders for individual protein targets. However, recent innovations demonstrate the capability of DELs to operate in the complex milieu of biological systems. In this Perspective, we discuss the recent progress in using DNA-encoded chemical libraries to interrogate complex biological targets and their potential to identify structures that elicit function or possess other useful properties. Future breakthroughs in these aspects are expected to catapult DEL to become a momentous technology platform not only for drug discovery but also to explore fundamental biology.

DNA Conjugates as Tool Compounds for DEL Selections

DEL selections are designed to discover novel small molecule compounds attached to DNA that bind to a target protein. A known small molecule or peptide ligand that binds to the target protein can be conjugated to DNA to mimic compounds contained in DEL libraries. The conjugate can be used as a ligand in preselection binding assays to validate a target protein and optimize selection methods and serve as a positive control in selection experiments. In this chapter, the design, synthesis, and use of DNA conjugate tool compounds are discussed. As an example, I describe the design of a DNA conjugate of a ligand to the CCR5 receptor and its use to optimize selection conditions and as a spike-in positive control in a DEL selection experiment. These methods are broadly applicable to both soluble protein targets and to targets that are displayed on the cell surface and to various types of compounds that can be conjugated to DNA.

Chemical Probes for Understudied Kinases: Challenges and Opportunities

Over 20 years after the approval of the first-in-class protein kinase inhibitor imatinib, the biological function of a significant fraction of the human kinome remains poorly understood while most research continues to be focused on few well-validated targets. Given the strong genetic evidence for involvement of many kinases in health and disease, the understudied fraction of the kinome holds a large and unexplored potential for future therapies. Specific chemical probes are indispensable tools to interrogate biology enabling proper preclinical validation of novel kinase targets. In this Perspective, we highlight recent case studies illustrating the development of high-quality chemical probes for less-studied kinases and their application in target validation. We spotlight emerging techniques and approaches employed in the generation of chemical probes for protein kinases and beyond and discuss the associated challenges and opportunities.

Combinatorial technology revitalized by DNA-encoding

Combinatorial chemistry invented nearly 40 years ago was welcomed with enthusiasm in the drug research community. The method offered access to a practically unlimited number of new compounds. The new compounds however are mixtures, and methods had to be developed for the identification of the bioactive components. This was one of the reasons why the method could not providethe expected cornucopia of new drugs. Among the different screening methods, two approaches seem to offer the best results. One of them is based on the intrinsic property of the combinatorial split and pool solid-phase synthesis: One compound forms on each bead of the solid support. Different methods have been developed to encode the beads and identify the structure of compounds formed on them. The most important method applies DNA oligomers for encoding. As a second approach in screening, DNA-encoded combinatorial libraries are synthesized omitting the solid support and the mixtures are screened in solution using affinity binding methods. Libraries containing billions and even trillions of components are synthesized and successfully tested, which led to the identification of a significant number of new leads.