PROTAC: How Degraders Work, Hook Effect and FDA Status

Short answer. A PROTAC (proteolysis-targeting chimera) is a small molecule with three parts: a ligand for a target protein, a ligand for an E3 ubiquitin ligase, and a linker. It brings the two proteins together so the target is ubiquitinated and destroyed by the proteasome, rather than just blocked. One PROTAC, vepdegestrant (VEPPANU), has been FDA approved, per its developer’s filing of 1 May 2026.

This page is general scientific information for researchers and lab staff, not medical advice.

Definition and scope

PROTAC is the best-known form of targeted protein degradation. A 2025 review in MedComm (Fan et al.) describes a canonical PROTAC as “three covalently linked components”: a ligand that binds the protein of interest (POI), a ligand that recruits an E3 ligase, and a linker joining them. The aim is to remove the protein from the system instead of occupying its active site. The same review reports that the first PROTAC entered clinical evaluation in 2019 and that more than 50 candidates were in clinical evaluation at the time of writing.

The name is also shared by an unrelated product line (a flashlight brand); this page covers only the protein-degradation technology.

How the PROTAC system works

According to Fan et al. (2025), the mechanism runs in four stages:

1. The PROTAC binds the POI and an E3 ligase at the same time, forming a POI-PROTAC-E3 ternary complex.
2. Within that complex the E3 ligase ubiquitinates the POI.
3. The ubiquitinated POI is recognised and degraded by the 26S proteasome.
4. The PROTAC is released and can be recycled for further rounds, which the review describes as catalytically erasing the protein.

This event-driven behaviour differs from conventional inhibitors, which must stay bound to block function.

The hook effect and what it means at the bench

The review (section 7.1.3) explains the hook effect: at high concentrations the PROTAC forms separate binary complexes (POI-PROTAC or PROTAC-E3) that cannot drive degradation, so activity falls as concentration rises. The review notes this complicates dose optimisation. For a bench researcher, the practical reading is that a single high dose can look like a failed degrader, so a degradation readout (for example a Western blot) should be judged across a range of concentrations rather than one. That is an editorial reading of the source, not a protocol. For blot interpretation see how to interpret Western blot results and Western blot loading controls; for curve shapes see the Hill coefficient guide.

PROTAC vs molecular glue

Both approaches harness the cell’s ubiquitin-proteasome machinery. A 2026 letter in Annals of Medicine and Surgery (Tahir et al., doi:10.1097/MS9.0000000000005002) summarises the differences:

Feature	PROTAC	Molecular glue degrader
Structure	Heterobifunctional: two ligands joined by a linker	Typically small and monovalent
Size and pharmacokinetics	Described as having poor solubility and suboptimal oral bioavailability	Generally more favourable pharmacokinetics because of smaller size
Hook effect	Described at higher concentrations	Not described in the letter
E3 ligase dependence	Relies on a limited set of well-characterised E3 ligases	Same limitation
Design	More complex and costly to design and manufacture, with greater flexibility across targets	Not described in the letter

What are the disadvantages of PROTACs?

Fan et al. (2025) list these limitations:

• Molecular weight and polarity limit oral bioavailability and tissue distribution; tissue penetration is moderate and formulation is challenging.
• The hook effect complicates dose optimisation.
• Resistance is emerging.
• There is no robust framework for choosing which targets are degradable.
• Delivery to the nucleus is difficult, and ligands for smooth or dynamic protein surfaces are hard to find.
• Most current PROTACs form ternary complexes with limited stability.

Are any PROTACs FDA approved?

Yes, one according to its developer. In a Form 8-K filed with the US SEC around 1 May 2026, Arvinas announced FDA approval of VEPPANU (vepdegestrant) and described it as the first and only FDA-approved PROTAC protein degrader. The indication is adults with ER+/HER2-, ESR1-mutated advanced or metastatic breast cancer that has progressed after at least one line of endocrine-based therapy, with the ESR1 mutation detected by an FDA-authorized test. The filing cites the Phase 3 VERITAC-2 trial, which compared vepdegestrant with fulvestrant. Treatment decisions belong with clinicians; this page does not assess efficacy or safety.

Where it matters for labs

For research groups, PROTACs matter as chemical tools: a degrader removes the protein, so its effects can differ from those of an inhibitor of the same target. Techniques that read protein level, such as blotting, and interaction methods such as co-immunoprecipitation, are the natural readouts, with controls chosen for the specific system and the primary literature for that degrader.

Common misunderstandings

• “A PROTAC is just a stronger inhibitor.” It is not; it removes the protein through ubiquitination and proteasomal degradation, and can be recycled.
• “More compound means more degradation.” The hook effect means the opposite can occur at high concentration.
• “PROTACs are still only experimental.” More than 50 candidates were in clinical evaluation per the 2025 review, and one has an FDA approval per the developer’s filing.
• “PROTACs and molecular glues are the same.” They share the ubiquitin-proteasome route but differ in structure, as the table shows.

FAQ

How does the PROTAC system work?

A PROTAC links a target protein to an E3 ubiquitin ligase, forming a ternary complex. The target is ubiquitinated and degraded by the 26S proteasome, and the PROTAC can be recycled.

What is the difference between a PROTAC and a molecular glue?

A PROTAC is a bifunctional molecule with two ligands and a linker; a molecular glue is typically a small monovalent compound. Glues are generally described as having more favourable pharmacokinetics, while PROTACs show a hook effect.

Are any PROTACs FDA approved?

Yes: vepdegestrant (VEPPANU), approved per Arvinas’s filing of 1 May 2026 for a specific ER+/HER2-, ESR1-mutated breast cancer indication.

What are the disadvantages of using PROTACs?

Size and polarity limit oral bioavailability, the hook effect complicates dosing, resistance is emerging, and target selection lacks a robust framework.

Sources

• Fan G et al. Proteolysis-Targeting Chimera (PROTAC): Current Applications and Future Directions. MedComm 2025;6(10):e70401. doi:10.1002/mco2.70401. PMC12495453. Read 2026-11-07.
• Tahir A, Khan SA, Imtiaz E, Qamar MA. Molecular glue degraders versus PROTACs: a novel paradigm in targeted protein degradation. Ann Med Surg 2026;88(6):3872-3873. doi:10.1097/MS9.0000000000005002. PMC13236241. Read 2026-11-07.
• Arvinas, Inc. Form 8-K, May 2026. SEC EDGAR. Read 2026-11-07.