Covalent inhibitors have become a key strategy in drug discovery, offering advantages such as prolonged target engagement, high potency, and selectivity. However, identifying high-quality covalent binders requires well-designed fragment libraries and robust screening methodologies to ensure efficient hit-to-lead progression.
At IRBM, we have developed a covalent fragment library that balances warhead diversity, structural flexibility, and synthetic accessibility, providing a valuable starting point for covalent hit discovery. With a collection of ~1,000 synthesized fragments and access to 7,000 additional commercial fragments, this library is optimized to explore diverse chemical space while maintaining tractability for hit optimization.
Covalent Fragment Library Design: Key Features
1. Structural and Warhead Diversity
Our covalent fragment library includes a broad range of chemically diverse warheads designed to engage nucleophilic amino acid residues, particularly Cys, Lys, and His.
By incorporating both reversible and irreversible covalent mechanisms, the library provides flexibility in screening strategies and hit optimization, allowing for greater adaptability across different target classes.
2. Optimized Physicochemical Properties
Fragments were selected based on key physicochemical parameters to ensure drug-like properties and synthetic elaboration potential:
✔ Molecular weight distribution suitable for fragment screening
✔ Optimized hydrogen bond donor (HBD) and acceptor (HBA) counts
✔ High chemical stability in PBS (pH 7.4)
✔ Moderate warhead reactivity to minimize off-target binding
3. Synthetic Accessibility & Expandability
A critical challenge in fragment-based discovery is ensuring that promising hits can be efficiently optimized into drug-like molecules. Our library is designed with:
✔ Synthesized (78%) and acquired (22%) compounds to balance diversity and accessibility
✔ Planned library expansions to target a broader range of amino acid residues
✔ Computational tools, including virtual screening and molecular visualization (e.g., t-SNE), were used to assess structural diversity and support fragment selection
Hit Identification Workflow: Screening and Validation
To ensure efficient identification and characterization of covalent binders, we employ a multi-step fragment screening workflow that includes:
1. Enzymatic Assays
✔ Evaluates whether a fragment inhibits enzymatic activity, confirming time-dependent covalent binding
2. Reactivity Assays
✔ Assess thiol and glutathione reactivity to filter out fragments with excessive intrinsic reactivity
3. Intact Mass Spectrometry
✔ Measures covalent bond formation by evaluating WT and mutant protein binding profiles
✔ Calculates apparent Kinact/KI values to rank warhead efficiency
4. Peptide Mapping
✔ Identifies specific amino acid residues modified by covalent fragments
✔ Used when intact mass analysis cannot directly determine binding sites
Supporting Covalent Drug Discovery with Medicinal Chemistry Services
Beyond providing a well-curated fragment library, IRBM’s medicinal chemistry services offer custom synthesis, SAR development, and strategic hit expansion to accelerate lead optimization.
Our team supports:
✔ Custom covalent fragment screening tailored to target class
✔ Structure-based design to guide hit evolution
✔ Hit-to-lead progression, including analog development and reactivity profiling
By combining a thoughtfully designed covalent fragment library with deep expertise in medicinal chemistry, we enable efficient discovery and optimization of covalent inhibitors—helping partners advance their programs with fewer bottlenecks.
Conclusion
Covalent drug discovery depends on both the quality of the fragment library and the strategy behind its use. A well-designed library, combined with effective screening and medicinal chemistry expertise, helps ensure that promising hits can be developed into viable leads.
At IRBM, we provide the tools, expertise, and scientific rigor necessary to accelerate covalent hit discovery while maintaining a focus on real-world tractability and lead optimization.