Introduction to PROTACs and Targeted Protein Degradation
What Is a PROTAC?
Why Event-Driven Degradation Differs from Occupancy-Driven Inhibition
Classical inhibitors work by occupancy. A drug molecule sits in a binding pocket, and for as long as it stays there the protein cannot do its job. The effect lasts only while the drug is bound, and getting it to bind tightly usually requires a deep, well-defined pocket.
PROTACs work differently. A single PROTAC molecule can bring one copy of the target to the ligase, trigger its ubiquitination, and then be released to repeat the cycle with another copy. Because the molecule is recycled rather than consumed, relatively low drug concentrations can drive substantial degradation, and the effect persists until the cell resynthesises the protein [2].
Two practical consequences follow. Degradation removes every function of a protein at once, including scaffolding and protein–protein interaction roles that an active-site inhibitor would leave intact. And because the protein is gone rather than blocked, degraders can overcome some resistance mutations that blunt occupancy-based drugs [2].
| Property | Small-molecule inhibitor (occupancy-driven) |
PROTAC degrader (event-driven) |
|---|---|---|
| Binding site required | Deep, well-defined, functionally important pocket | Any binding handle, including shallow or non-functional sites |
| Mechanism | Blocks activity while bound | Tags the protein for proteasomal destruction |
| Stoichiometry | Needs near-complete, sustained occupancy | Catalytic; acts at sub-stoichiometric levels |
| Scope of effect | Blocks the targeted function only | Removes all functions, including scaffolding roles |
| Duration | Lasts only while bound | Persists until the protein is resynthesised |
| Resistance | Vulnerable to some binding-site mutations | Can overcome some occupancy-based resistance |
Table 1. Degraders versus traditional small-molecule inhibitors at a glance
The Role of E3 Ligases in Targeted Protein Degradation
Every PROTAC depends on an E3 ubiquitin ligase. E3 ligases are the substrate-recognition components of the ubiquitin–proteasome system, which is the cell’s main route for regulated protein turnover. Their normal job is to select specific proteins and transfer ubiquitin tags onto them.
A PROTAC reprograms that selectivity by presenting the ligase with a new, non-natural substrate. Without a ligand that engages an E3 ligase there is no PROTAC, and as this article will show, the availability of E3-ligase-binding chemistry has become the single biggest factor shaping what targeted protein degradation can and cannot do [6].
How PROTACs Work: Mechanism of Action
The Ubiquitin–Proteasome System Explained
The ubiquitin–proteasome system is the cell’s regulated waste-disposal pathway. Proteins destined for destruction are first marked with chains of ubiquitin, a small protein tag. The tagging runs through a three-enzyme cascade: an E1 activating enzyme, an E2 conjugating enzyme, and an E3 ligase that supplies substrate specificity. Once a protein carries a sufficient polyubiquitin chain, the 26S proteasome recognises it, unfolds it, and breaks it down into peptides.
A PROTAC hijacks the final, specificity-determining step. By tethering its target to an E3 ligase, it ensures the target receives the ubiquitin marks that send it to the proteasome [1][2].
The Three Structural Components: Target Warhead, Linker, and E3 Ligand
Every PROTAC is built from three parts, and each is a design lever.
- The target warhead is the ligand that binds the protein of interest. It does not need to inhibit the target; it only needs to bind it. This is why proteins long written off as undruggable can become tractable. A shallow or functionally irrelevant binding site that would be useless for an inhibitor can still serve as a handle for a degrader.
- The E3 ligand is the moiety that recruits the ubiquitin ligase. The chemistry available here is the field’s main constraint, and it is the focus of much of this article.
- The linker connects the two ligands. Linker length, rigidity, and attachment geometry are not trivial spacers. They determine whether the target and ligase can adopt a productive orientation, and they have a strong influence on potency, selectivity, and drug-like properties [2].
Ternary Complex Formation and the Importance of Cooperativity
The decisive event in PROTAC action is the formation of a ternary complex: target protein, PROTAC, and E3 ligase bound together as one assembly. How well that complex forms, rather than how tightly the PROTAC binds either partner on its own, governs how efficiently the target is ubiquitinated.
The point was made concrete by a crystal structure of the BRD4 degrader MZ1 bound to the E3 ligase VHL and a BRD4 bromodomain. It showed that the most effective degraders induce favourable, direct contacts between the target and the ligase, a phenomenon known as positive cooperativity [5]. Cooperativity helps explain why a PROTAC can degrade one member of a protein family while sparing close relatives, and why structure-guided design of the ternary complex can produce very high selectivity [5].
MZ1 and the phthalimide-based degrader dBET1 were among the first drug-like small-molecule PROTACs shown to work in cells. Both degraded BRD4, one by recruiting VHL and the other cereblon (CRBN), and together they moved the modality away from the peptide-based recruiters used in the earliest work [3][4].
E3 Ligases in PROTAC Design
What E3 Ligases Do in the Ubiquitin System
The human genome encodes more than 600 E3 ligases, each evolved to recognise particular substrates in particular cellular contexts. Some are broadly expressed. Others are restricted to specific tissues, cell states, or subcellular compartments.
In principle that diversity is useful to drug developers. A ligase with restricted expression could, if it could be recruited, enable a degrader that acts only where that ligase is present, sparing healthy tissue. In practice, almost none of this diversity has been accessed [6].
Comparing CRBN-, VHL-, IAP-, and MDM2-Based PROTACs
A handful of E3 ligases account for the overwhelming majority of PROTACs reported so far. Cereblon (CRBN) and von Hippel–Lindau (VHL) are by far the most widely used, for one reason: high-quality, drug-like small-molecule ligands exist for both.
CRBN recruitment exploits the thalidomide class of immunomodulatory drugs, whose binding to CRBN provided the chemistry behind the first practical small-molecule degraders [3]. VHL ligands came out of dedicated medicinal chemistry and underpin many of the best-characterised degraders, including the cooperative BRD4 degraders described above [4][5][7].
Two other ligases, the inhibitor-of-apoptosis proteins (IAPs) and MDM2, have also been recruited, but far less often. The result is a field that has shown considerable breadth in what it can degrade while drawing on a strikingly narrow set of ligases [6][7].
| E3 ligase | Source of the recruiting chemistry | Relative use in PROTACs | Notes |
|---|---|---|---|
| Cereblon (CRBN) | Thalidomide-class immunomodulatory drugs | Most common | Basis of the first practical small-molecule degraders and of the first approved PROTAC |
| von Hippel-Lindau (VHL) | Dedicated medicinal chemistry | Most common | Underpins many of the best-characterised, cooperative degraders |
| IAPs | IAP-binding compounds | Occasional | Recruited far less often than CRBN or VHL |
| MDM2 | MDM2 inhibitors | Rare | Recruited far less often than CRBN or VHL |
Table 2.The E3 ligases most used in targeted protein degradation.
Fewer than ten of the more than 600 human E3 ligases have been used for degradation to date [6][7].The CRBN/VHL Bottleneck: Why the Field Relies on So Few Ligases
Of more than 600 human E3 ligases, fewer than ten have been used for targeted protein degradation, and most disclosed PROTACs rely on CRBN or VHL [6]. That concentration does not mean CRBN and VHL are the right choice for every job. It reflects where ligand chemistry happens to exist.
Depending on so few ligases carries real liabilities. Resistance can emerge when tumour cells downregulate or mutate the recruited ligase. Some targets are degraded poorly by CRBN or VHL simply because a productive ternary complex cannot form. And tissue-restricted degradation, one of the modality’s more attractive theoretical advantages, stays out of reach because the relevant ligases have no ligands. Expanding the repertoire of usable E3 ligases is widely treated as one of the most important open problems in the field [6][7].
Tissue- and Disease-Selective E3 Ligases as the Next Frontier
The prize here is precision. If a ligase expressed predominantly in a particular tumour type or tissue could be recruited, a degrader built on that ligase would in principle act selectively in disease-relevant cells and leave the rest of the body largely alone. That is a built-in therapeutic window of a kind occupancy-based drugs rarely achieve [2][6].
Getting there depends on one thing above all: ligands that bind new E3 ligases. Which turns a biology problem into a hit-discovery problem – a problem Vipergen’s DNA-encoded library platform is built to solve.
The Challenge of Finding New E3 Ligase Ligands
Why Most Human E3 Ligases Remain Unliganded
If new E3 ligases are so desirable, why has the toolbox stayed so small? Several reasons.
Many E3 ligases lack obvious, well-formed small-molecule pockets, which makes them hard to drug by design. Others are difficult to express and purify in functional form, which frustrates conventional biochemical screening. And the historical E3 ligands were largely found by accident rather than by deliberate search. The CRBN-binding activity of thalidomide-class drugs, for example, was uncovered long after those drugs were already in clinical use [7].
Finding ligands for a chosen E3 ligase on purpose, rather than stumbling across them, calls for screening methods that can put enormous chemical diversity in front of difficult proteins.
Approaches to E3 Ligand Discovery and Their Limits
The field has tried several routes. Covalent and electrophilic fragment screening has identified reactive ligands for ligases such as DCAF16 and RNF114. Structure-based design has been applied where structural information exists. Molecular-glue discovery campaigns have surfaced compounds that recruit ligases such as DCAF15 [7].
Each approach has extended the map, and each has limits. Covalent strategies depend on a suitably positioned reactive residue. Structure-based design needs a tractable structure. Traditional high-throughput screening samples only a modest slice of chemical space and often struggles with proteins that resist purification or immobilisation [7][8].
| Approach | How it works | Main limitation |
|---|---|---|
| Covalent / fragment screening | Finds reactive fragments that bind (e.g. DCAF16, RNF114) | Needs a suitably positioned reactive residue |
| Structure-based design | Designs ligands against a known structure | Requires a tractable structure |
| Molecular-glue discovery | Identifies compounds that recruit a ligase (e.g. DCAF15) | Historically serendipitous and hard to run by design |
| Traditional high-throughput screening | Tests defined compound plates against the target | Samples a modest slice of chemical space; struggles with hard-to-purify proteins |
| DNA-encoded library (DEL) screening | Screens hundreds of millions to billions of DNA-barcoded compounds in one tube | Library fidelity and selection artefacts must be controlled |
Table 3. Approaches to E3 ligase ligand discovery
How DNA-Encoded Library (DEL) Screening Identifies Novel E3 Binders
DNA-encoded library screening changes both the scale and the logistics of ligand discovery. In a DEL, each small molecule is covalently linked to a unique DNA barcode recording its synthetic history. Because every compound’s identity is encoded in DNA, hundreds of millions to billions of distinct molecules can be pooled and screened together in a single tube, with binders identified by next-generation sequencing of the recovered barcodes.
This allows sampling of chemical space several orders of magnitude deeper than conventional high-throughput screening, at a fraction of the material cost [8]. For a problem defined by the scarcity of ligands against difficult proteins, depth of sampling is the relevant variable.
Library quality matters as much as library size. Vipergen’s YoctoReactor (yR) technology synthesises DELs through DNA self-assembly that brings building blocks into proximity within a three-dimensional DNA junction. The design produces a one-to-one correspondence between each barcode and the small molecule it encodes, which suppresses false positives and simplifies resynthesis of confirmed hits off DNA [9][10]. Screening is then performed by Vipergen’s Binder Trap Enrichment (BTE), a homogeneous, solution-phase selection method that captures binding pairs without immobilising the target on a surface, avoiding the matrix and surface artefacts that complicate many affinity-based screens [9][10].
In short, Vipergen pairs deep, high-fidelity libraries with artefact-resistant selection to address the central task of E3 ligand discovery: finding genuine, developable binders for ligases that have resisted other methods.
Degrader Discovery Against Hard and Undruggable Targets
Targeting and Degrading Intrinsically Disordered Proteins
A large share of disease-driving proteins are intrinsically disordered. They do not fold into a single stable three-dimensional structure but exist as dynamic ensembles of interconverting conformations.
Intrinsically disordered proteins and regions make up roughly a third of the human proteome and have central roles in signalling, transcription, and condensate formation. A recent computational study generated conformational ensembles for more than 28,000 disordered regions across the human proteome, which gives some sense of both their prevalence and their structural complexity [12]. Because they lack the deep, persistent pockets inhibitors require, disordered proteins have historically been labelled undruggable and remain one of the harder frontiers in drug discovery [11].
Targeted protein degradation offers a way around the impasse. A degrader does not have to occupy a functional site or lock a protein into an inactive conformation. It needs a ligand that binds the target well enough to form a productive ternary complex with an E3 ligase. A transient or shallow binding event that would be useless for inhibition can be enough to mark a disordered protein for destruction.
The bottleneck, again, is finding a binder at all, which returns the problem to hit discovery against a protein that may have no obvious druggable surface [11].
Screening Any Protein Class, Including Targets Without a Defined Pocket
DEL selection does not depend on a target having a classical pocket. It asks only whether any molecule in a very large library binds somewhere on the protein with useful affinity and specificity. By sampling chemical space far more deeply than conventional screens, a high-quality DEL improves the odds of finding a usable binding handle on a protein that structure-based or fragment methods would write off [8][9]. For disordered proteins and other unconventional target classes, finding a handle where none was thought to exist is the prerequisite for building a degrader at all.
In-Cell Screening Without Purified Protein
Disordered and unstable proteins pose a second, more practical obstacle: many cannot be readily produced as purified, well-behaved material for in-vitro screening.
Vipergen’s Cellular Binder Trap Enrichment (cBTE) addresses this by performing DEL selection inside living cells, which removes the requirement for a purified or immobilised target and allows screening against target classes that are otherwise difficult to handle, in a more physiologically relevant setting [10]. For a protein that refuses to behave in a test tube but is present in its native context inside a cell, in-cell selection sidesteps the problem that has blocked conventional approaches.
The same logic that makes DEL attractive for new E3 ligands, namely depth of sampling plus artefact-resistant selection with low protein requirements, applies equally to the targets on the other end of the PROTAC.
Applications and Outlook
PROTACs in Oncology and Emerging Disease Areas
Oncology has been the proving ground for targeted protein degradation, and the modality has now cleared its most important hurdle: regulatory approval.
Vepdegestrant (ARV-471), an orally administered PROTAC that degrades the estrogen receptor, was compared with the established endocrine therapy fulvestrant in the Phase 3 VERITAC-2 trial (NCT05654623) in patients with ER-positive, HER2-negative advanced breast cancer whose disease had progressed after a CDK4/6 inhibitor and endocrine therapy [13]. Progression-free survival was assessed as a primary endpoint in two populations: the intention-to-treat population and the ESR1-mutant population. In March 2025 the sponsors reported that the trial met its primary endpoint in the ESR1-mutant population, with a statistically significant improvement in progression-free survival over fulvestrant. This was the first time a PROTAC degrader had shown clinical benefit in a Phase 3 setting. The trial did not meet the endpoint in the intention-to-treat population [14].
On 1 May 2026 the FDA approved vepdegestrant (marketed as Veppanu) for adults with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer whose disease has progressed after at least one line of endocrine therapy, alongside a companion diagnostic, Guardant360 CDx, to identify ESR1 mutations [15]. It is the first PROTAC of any kind to reach approval [16].
That result moves targeted protein degradation from a promising laboratory concept to a clinically validated therapeutic strategy, and it has sharpened interest in extending degraders beyond oncology into immunology, inflammation, and neurodegeneration [2].
PROTACs vs. Traditional Small-Molecule Inhibitors
The case for degraders over inhibitors is strongest exactly where inhibitors are weakest. Inhibitors need a high-affinity, functionally important pocket, and they generally have to maintain near-complete target occupancy to be effective. Degraders need a binding handle, act catalytically, and remove the whole protein, scaffolding functions included, at sub-stoichiometric drug levels [2]. They can also address some resistance mechanisms that defeat inhibitors. (See Table 1 for a side-by-side comparison.)
None of this makes inhibitors obsolete. For many targets, a well-behaved active site is still the most direct route to a drug. But for transcription factors, scaffolding proteins, and disordered targets that have frustrated occupancy-based chemistry for decades, degradation is a real alternative [2][11].
Where Targeted Protein Degradation Is Heading
The trajectory of the field keeps pointing at the same requirement. Clinical validation will drive expansion into new targets and new diseases. That expansion will mean degrading proteins CRBN and VHL handle poorly. And that needs both new E3 ligase ligands and binding handles on previously intractable targets [6][7][11].
Both are hit-discovery problems on difficult proteins – problems Vipergen’s DEL technologies are designed to address. As the set of usable ligases grows, so does the prospect of tissue- and disease-selective degraders with wider therapeutic windows, and of degraders aimed at the disordered proteome that conventional medicinal chemistry has not been able to reach.
Frequently Asked Questions
References
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