The Proteolysis Targeting Chimera (PROTAC) have emerged as an important technology platform in next-generation drug discovery.Unlike conventional therapies that rely on continuous occupancy of the target protein, PROTACs leverage the ubiquitin-proteasome system (UPS) to selectively degrade target proteins within cells. PROTACs operate through an event-driven mechanism that enables catalytic protein degradation, achieving effects at sub-stoichiometric concentrations.  

A canonical PROTAC comprises three covalently linked components: a ligand that binds the POI, a ligand that recruits the E3 ligase, and a linker that bridges the two. This chimeric molecule brings together the target protein and E3 ligase to form a POI–PROTAC–E3 ternary complex. Once productively assembled, the E3 ligase transfers ubiquitin to lysine residues on the target protein, tagging it for proteasomal degradation.

Figure 1. Structure and mechanism of action of a PROTAC [1]

From Discovery to Clinical Success: The Evolution of PROTAC Technology

In 2001, the concept of PROTAC was first introduced by Craig Crews and his team at Yale University. They described the PROTAC idea and reported the first PROTAC molecule PROTAC-1, which was designed to target the methionyl aminopeptidase-2 (METAP2), as a pioneering new technology.

In 2008, the first all small-molecule-based PROTAC was developed using a ligand, nutlin-3a, to recruit MDM2 to ubiquitinate and degrade the androgen receptor (AR), marking an important step toward developing PROTACs as drug-like small molecules.

In 2019, AR-targeting PROTAC ARV-110 and ER-targeting PROTAC ARV-471, developed by Arvinas, entered clinical trials, marking the first time PROTAC candidates were evaluated in humans.

Since then, PROTAC development has progressed rapidly, with multiple candidates advancing into late-stage clinical trials. Several programs, including Vepdegestrant, BMS-986365, and BGB-16673, have entered Phase III studies.

In May 2026, the U.S. FDA approved the protein degradation therapy Veppanu (vepdegestrant) for the treatment of previously treated patients with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer. This milestone demonstrated that targeted protein degradation technology had successfully progressed from mechanism validation and clinical evaluation to regulatory approval.

Linkers: More Than a Molecular Bridge

In the early development of PROTACs, linkers were often considered simple structural components used to connect two functional ligands. However, as more clinical candidates have emerged, researchers have increasingly recognized that linkers play a critical role in determining PROTAC performance.

The activity of a PROTAC is not solely determined by the POI ligand and E3 ligase ligand. Even when the same two ligands are used, changes in linker length, structure, or attachment site can result in significant differences in degradation efficiency.

Linker Length's Impact on Ternary Complex Stability

PROTAC activity depends on the formation of a productive POI-PROTAC-E3 ligase ternary complex with the correct spatial arrangement. If the linker is too short, the two ligands may not be able to bind their respective targets simultaneously, preventing ternary complex formation. On the other hand, an excessively long linker can increase molecular flexibility, reduce the proportion of productive conformations, increase molecular weight, and negatively affect cellular permeability.

Therefore, linker design requires a careful balance between structural flexibility and complex stability.

Linker Composition's Impact on Physicochemical Properties

The chemical motifs in a linker (such as hydrophilic or hydrophobic groups) directly influence solubility, cell permeability, and metabolic stability.

Based on flexibility, linkers fall into two main categories:

Flexible linkers: Typically built from polyethylene glycol (PEG) or alkyl chains, these allow significant conformational freedom. They suit targets with uncertain spatial orientations and offer high physiological stability, simple structures, and straightforward synthesis. However, excessive flexibility can destabilize the ternary complex and reduce target binding efficiency.

Rigid linkers: Utilizing structures such as piperazine, triazole, or aromatic rings, rigid linkers constrain molecular conformation. This reduces entropic penalty and can improve binding affinity, selectivity, and metabolic stability. However, over-rigidity can cause steric clashes, leading to a loss of activity.

Linkers also differ by polarity. Introducing polar groups (such as PEG, hydroxyls, or piperazines) improves water solubility, though excessive hydrophilicity can hinder cellular uptake.

To resolve these trade-offs, researchers often design hybrid linkers—for instance, inserting flexible PEG units into rigid ring systems to balance hydrophobicity and rigidity.

Optimization of Spatial Arrangement and Attachment Sites

The specific chemical attachment points on both the warhead and the E3 ligand are equally important. Attachment sites directly affect protein-protein interactions (PPIs); subtle shifts in attachment position on the same ligand can lead to vast differences in degradation potency.

Optimizing PROTAC linkers requires identifying the most favorable vectors for structural derivation. This typically involves analyzing the solvent-exposed regions of the protein-ligand interaction interface. Introducing the linker at these positions preserves ligand binding affinity while maximizing favorable interactions between the POI and E3 ligase.

PEG-Based Linkers in PROTAC Development

PEG chains remain one of the most widely used linker types in PROTAC design.

PEG offers high water solubility, biocompatibility, and structural tuneability. By adjusting the number of PEG repeating units, researchers can precisely fine-tune linker length and optimize the spatial distance between the two ligands.

In practice, PEG linkers connect diverse POI ligands to E3 ligase ligands. Their flexibility allows functional domains to adopt optimal spatial orientations in cellular environments while improving overall drug solubility.

However, PEG linkers are not a universal solution. Excessive flexibility can increase conformational entropy, lowering the effective concentration of the productive ternary complex. As a result, hybrid designs combining PEG units with rigid elements (such as aromatic or heterocyclic rings) are increasingly used to balance flexibility and conformational stability.

Huateng Pharma: Supporting Innovation in PEG Linkers and PROTAC Research

As PROTACs, ADCs, and other targeted degradation platforms continue to advance, high-quality PEG linkers and functionalized PEG derivatives have become essential building blocks for drug discovery.

Huateng Pharma specializes in the R&D and manufacturing of PEG derivatives and functional materials, providing tailored PEG solutions for PROTACs, antibody-drug conjugates (ADCs), bioconjugation, and drug delivery systems.

With an established PEG technology platform, Huateng Pharma supplies PEG linkers with diverse chain lengths and functional groups—including amine, carboxyl, azide, alkyne, and maleimide—offering both custom synthesis and commercial supply from grams to multi-kilograms.

References:
[1]Fan G, Chen S, Zhang Q, Yu N, Shen Z, Liu Z, Guo W, Tang Z, Yang J, Liu M. Proteolysis-Targeting Chimera (PROTAC): Current Applications and Future Directions. MedComm (2020). 2025 Oct 4;6(10):e70401. doi: 10.1002/mco2.70401. Erratum in: MedComm (2020). 2025 Nov 05;6(11):e70491. doi: 10.1002/mco2.70491. PMID: 41049269; PMCID: PMC12495453.
[2] Troup RI, Fallan C, Baud MGJ. Current strategies for the design of PROTAC linkers: a critical review. Explor Target Antitumor Ther. 2020;1(5):273-312. doi: 10.37349/etat.2020.00018. Epub 2020 Oct 30. PMID: 36046485; PMCID: PMC9400730.