Linker optimization

Linkers, as key structural units in PROTAC molecules that connect the two functional ends, not only determine the overall molecular conformation and spatial compatibility but also directly influence the stability of the ternary complex, as well as cellular permeability and degradation efficiency. In recent years, with the widespread application of click chemistry in medicinal chemistry, the incorporation of bioorthogonal reactive groups such as azides (-N₃) into PROTAC linker design has become an emerging research focus, providing an important tool for modular assembly and rapid structural optimization.

The MCE Azide PROTAC Linker Library contains 0 linkers specifically designed for targeted protein degradation molecule design and optimization. These linkers serve as efficient “click handles,” enabling rapid and highly selective covalent coupling with alkyne reaction partners, thereby facilitating modular assembly and structural diversification of PROTAC molecules. In drug development, this design not only improves the efficiency of molecular construction but also significantly accelerates the screening and optimization of lead compounds. Meanwhile, by tuning linker properties such as length, flexibility, and polarity, the ability to form ternary complexes and degradation activity can be optimized.

In PROTAC drug development, linkers are often one of the key variables determining drug-likeness and degradation efficiency. Since PROTAC systems must simultaneously satisfy target protein binding, E3 ligase recruitment, and intracellular spatial conformational matching, their structural design is essentially a multi-parameter optimization problem. Differences in linker rigidity, flexibility, and spatial extension can significantly influence the formation pathway and stability of the ternary complex, leading to substantial variations in degradation activity. Therefore, the development of linker systems with modular tunability and high structural expandability has become an important direction in PROTAC optimization.

The MCE Alkyne PROTAC Linker Library contains 0 linkers based on terminal and internal alkyne scaffolds, forming a highly derivatizable linker module system. These linkers serve as standardized building blocks for rapid assembly and iterative optimization of PROTAC molecules, and support efficient conjugation with azide-containing functional groups via click chemistry. In practical drug development, this type of structure not only facilitates the construction of diverse linker space libraries, accelerating lead compound screening, but also enables systematic tuning of molecular geometry and physicochemical properties, thereby improving ternary complex stability and targeted protein degradation efficiency.

In modern drug discovery and chemical biology research, the azide group (-N₃) is an important functional moiety that is widely used in click chemistry, biomolecular labeling, drug delivery systems, and prodrug design due to its unique reactivity and bioorthogonality.

The MCE Azide Structural Compound Library contains 100 compounds featuring -N₃ functional groups. It is designed for the construction of click chemistry reaction systems and the subsequent development of functional molecules. This library enables the rapid assembly of targeting ligands, linkers, and functional molecular modules, thereby accelerating PROTAC assembly, optimization of antibody-drug conjugate (ADC) linkers, and the development of biological labeling probes. In addition, the high reaction selectivity and excellent biocompatibility of the azide group allow it to maintain stable reactivity even in complex biological environments, improving controllability and efficiency in drug design. It serves as an indispensable molecular tool in modern medicinal chemistry and chemical biology research.

Macrocyclic compounds (≥12-atom cyclic small molecules/peptides) have unique physicochemical properties. They form preorganized conformations with high binding affinity/selectivity, target traditional small-molecule-inaccessible proteins, and bridge small-molecule drugs and biological agents. As key protein phosphorylation enzymes, kinases are linked to tumors, COPD, etc., and are critical therapeutic targets. Traditional small-molecule kinase inhibitors lack selectivity, causing off-target toxicity, low bioavailability, and acquired resistance. Macrocycles’ semi-rigid structure restricts conformations, boosts binding selectivity, optimizes pharmacokinetics, and makes macrocyclization a core kinase inhibitor optimization strategy.

Thousands of bioactive macrocycles were curated from ChEMBL. Via Transformer, macrocyclization was converted into a chemical language translation task, enabling end-to-end macrocycle generation from linear precursors with simplified inputs. Macformer achieves efficient, automated linear molecule macrocyclization via deep learning; generated macrocycles have diversity, novelty, biocompatibility, and cover broader chemical space.

MCE collected thousands of marketed/clinical kinase inhibitors, using their fragments for macrocyclization to generate derivatives. After evaluating synthetic accessibility and physicochemical properties, a million-scale virtual macrocyclic library was built for kinase-related virtual and AI-driven screening.

Macrocyclic compounds (≥12-atom cyclic small molecules/peptides) have unique physicochemical properties. They form preorganized conformations with high binding affinity/selectivity, target traditional small-molecule-inaccessible proteins, and bridge small-molecule drugs and biological agents. As key protein phosphorylation enzymes, kinases are linked to tumors, COPD, etc., and are critical therapeutic targets. Traditional small-molecule kinase inhibitors lack selectivity, causing off-target toxicity, low bioavailability, and acquired resistance. Macrocycles’ semi-rigid structure restricts conformations, boosts binding selectivity, optimizes pharmacokinetics, and makes macrocyclization a core kinase inhibitor optimization strategy.

Thousands of bioactive macrocycles were curated from ChEMBL. Via Transformer, macrocyclization was converted into a chemical language translation task, enabling end-to-end macrocycle generation from linear precursors with simplified inputs. Macformer achieves efficient, automated linear molecule macrocyclization via deep learning; generated macrocycles have diversity, novelty, biocompatibility, and cover broader chemical space.

MCE collected thousands of marketed/clinical kinase inhibitors, using their fragments for macrocyclization to generate derivatives. After evaluating synthetic accessibility and physicochemical properties, a million-scale virtual macrocyclic library was built for kinase-related virtual and AI-driven screening.

Macrocyclic scaffolds are increasingly valued in modern drug discovery for their exceptional activity against undruggable targets (proteases, kinases, PPIs). 2026 marks a key commercial breakthrough for oral macrocyclic peptides: enlicitide, the world’s first oral PCSK9 macrocyclic peptide, has received FDA approval. Macrocyclic candidates targeting KRAS and other classic undruggable targets have also entered clinical development, validating macrocyclization as an effective strategy to overcome druggability barriers.

Two core R&D directions lead current macrocyclic drug design: AI-driven de novo generation and structural optimization of small-molecule macrocycles, and macrocyclic peptides based on sequence design and conformational engineering. Macrocycle druggability hinges on embedded linkers, which determine cyclization efficiency, final conformation and drug-like properties. Bifunctional reaction orthogonality is the core linker selection criterion. Our linker library enables stepwise intramolecular cyclization with suppressed side reactions, accommodates varied ring sizes, and covers three key reaction systems: amide condensation, nucleophilic substitution and CuAAC click chemistry.

Built on classical macrocyclization systems, the library is processed through reaction classification, bifunctional orthogonality evaluation, novelty clustering and redundancy removal, with PROTAC long-chain and ADC cleavable linkers explicitly excluded. Featuring rigid, semi-rigid and flexible scaffolds, it is widely applicable to small-molecule macrocycle synthesis and linear peptide cyclization.

POI (Protein of Interest) refers to the target protein, namely the disease-causing protein or key functional protein that undergoes degradation or functional modulation in molecular glue-mediated processes. The Molecular Glue POI Library consists of a series of fragments that can specifically bind to different types of POIs. As key components of molecular glues, these ligands form stable interactions with target proteins, laying the foundation for molecular glues to induce the interaction between POIs and E3 ubiquitin ligases. The covered POIs include various types such as cancer-associated GSPT1, androgen receptors, and abnormally aggregated proteins linked to neurodegenerative diseases.

This fragment library can be applied to the screening and optimization of targeted protein degraders. By screening ligands with high affinity and strong selectivity for specific POIs from the library, core structures can be identified to develop novel molecular glues. For instance, optimization of ligands targeting GSPT1 has yielded molecular glue degraders with enhanced degradation activity. Since many POIs are difficult to drug due to the lack of traditional small-molecule binding pockets, some ligands in the POI Ligand Library can modulate such POIs by inducing protein-protein interactions, thereby further expanding the scope of drug discovery for undruggable targets.

MCE has compiled a POI Fragment Library comprising thousands of POI fragments with molecular weights ranging from 150 to 400. This compound library can be widely applied in Molecular Glue research and development.

Targeted Protein Degradation (TPD) is a novel and promising approach to drug development. It shows great potential for targeting proteins traditionally considered "undruggable" due to the lack of enzymatic function and absence of binding sites by tagging them for degradation or recruiting natural degradation mechanisms.

Molecular glues are a type of small-molecule degraders that primarily induce novel interactions between E3 ubiquitin ligases and target proteins, forming ternary complexes that lead to protein ubiquitination and subsequent proteasomal degradation. Compared with PROTACs, molecular glues generally have lower molecular weights, higher cell permeability, and better drug-like properties. Additionally, the design of molecular glues is relatively simple, without the requirements for complex linkers and ligand optimization. As a result, molecular glues have gradually emerged as a promising therapeutic approach for various diseases.

Multiple types of molecular glues have been reported previously. Analysis of co-crystal complex structures reveals that CRBN-related molecular glues are more versatile. Therefore, MCE researchers select active molecules related to these targets as probes for artificial intelligence (AI) screening.Subsequently, molecular docking technology was used to verify whether the screened molecules retained the key pharmacophore features. Ultimately, we obtained 317 molecular glue analogs, and these compounds serve as powerful tools for the research of molecular glues.