Targeted Protein Degradation (PROTACs and Molecular Glues)

Educational note: this page reflects public evidence last checked on 2026-06-04. It does not replace a drug label, trial protocol, molecular tumor board, or clinical judgment.

TL;DR

Targeted protein degradation (TPD) uses drugs to remove disease-driving proteins instead of only inhibiting them. The best-known formats are PROTACs (bifunctional molecules that bring a target protein close to an E3 ubiquitin ligase) and molecular glues (smaller molecules that stabilize or create a target-ligase interaction). In oncology, TPD matters because some tumor dependencies involve transcription factors, scaffolds, fusion proteins, or mutant receptors that are difficult to inhibit cleanly. On May 1, 2026, FDA approved vepdegestrant (Veppanu) for adults with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer, as detected by an FDA-authorized test, after progression following at least one line of endocrine therapy. That makes TPD clinically real in one defined setting, not a general solution for cancer. Sources: [1], [2]

1. The core idea

Traditional small molecules often work by occupying an active site and blocking protein function. Degraders work differently:

text

target protein + degrader + E3 ligase
        ->
ternary complex
        ->
ubiquitination
        ->
proteasomal degradation
        ->
less target protein in the cell

This can remove enzymatic and non-enzymatic functions. That is why degraders are attractive when the problem is not just catalytic activity, but scaffolding, transcriptional regulation, protein-protein interaction, or mutant receptor signaling. Sources: [2]

2. PROTACs vs molecular glues

Feature	PROTAC	Molecular glue
Structure	Bifunctional: target ligand + linker + E3 ligand	Usually smaller, monofunctional molecule
Mechanism	Forces proximity between target and E3 ligase	Stabilizes or creates a target-ligase interface
Design	More rational, but chemically bulky	Harder to design; often found phenotypically
Examples	ER, BTK, BCL6 degraders	IMiDs, CELMoDs, RBM39 degraders
Main challenge	Oral exposure, permeability, ternary-complex geometry	Selectivity, predictability, neosubstrate biology

Both belong to the broader field of pharmacologic control of protein abundance.

3. Why oncology cares

Cancer often depends on proteins that are difficult to inhibit:

Hormone receptors - ER and androgen receptor can remain active through resistance mutations.
Transcriptional regulators - BCL6, MYC-network proteins, BRD4, and fusion oncoproteins.
Kinases with resistance mutations - degradation can remove catalytic and scaffold functions.
Epigenetic regulators - chromatin complexes often depend on non-enzymatic functions.
Mutant signaling nodes - degradation sometimes avoids the need for perfect active-site inhibition.

The therapeutic promise is not "any protein can be degraded." It is narrower: some tumor dependencies become more druggable when removal is possible.

4. Clinical maturity

Approved

Vepdegestrant (Veppanu) - FDA-approved on May 1, 2026 for adults with ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer, detected by an FDA-authorized test, after progression following at least one line of endocrine therapy. In VERITAC-2, the ESR1-mutated population had median PFS of 5.0 months with vepdegestrant versus 2.1 months with fulvestrant. Sources: [1]

Clinical-stage

Androgen receptor degraders - prostate cancer.
BTK degraders - B-cell malignancies and resistance to covalent/non-covalent BTK inhibitors.
BCL6 degraders - lymphoma.
BRD4/BET degraders - hematologic malignancies and solid tumors, with toxicity/selectivity challenges.
Molecular glues - hematologic malignancies, splicing factors, and neosubstrate discovery.

Preclinical / early translational

KRAS degraders, mutant-selective degraders, light-activated degraders, antibody-degrader conjugates, and tumor-conditional PROTACs.

5. What can go wrong

Hook effect - too much degrader can reduce ternary-complex formation.
Low permeability - many PROTACs are large and polar.
E3 ligase expression - if a tumor does not express the recruited ligase, degradation may fail.
Resistance - target mutation, altered E3 pathway, proteasome adaptation, or drug efflux.
Toxicity - degrading a target in normal tissue can be worse than partially inhibiting it.
Weak biomarker logic - target expression does not guarantee dependency or degradation.
Off-target neosubstrates - especially important for molecular glues.

Degradation is powerful because it is not subtle; that is also the safety risk.

6. What to measure

Good degrader programs need more than IC50:

DC50 - concentration causing 50% target degradation.
Dmax - maximum degradation achieved.
Kinetics - how quickly the target disappears and recovers.
Ternary-complex cooperativity - whether the degrader stabilizes target-ligase binding.
Proteomics - what else is degraded.
Functional rescue - does restoring the target reverse the phenotype?
Biomarkers - target abundance, E3 expression, mutation, and pathway shutdown.

In oncology, the clinical question is: does target degradation produce tumor control without unacceptable toxicity?

7. What technologists can build

Ternary-complex modeling that integrates protein structure, linker geometry, and E3-ligase context.
ML models trained on DC50, Dmax, and kinetics, not just binding affinity.
Proteomics pipelines for global degradation selectivity.
Biomarker dashboards combining target mutation, expression, E3-ligase status, and pathway activity.
Trial-matching tools that use mutation and prior therapy, with clinician review.
Resistance atlases mapping escape mutations and E3-pathway alterations.

8. Brazil context

FDA approval of vepdegestrant in May 2026 does not automatically mean ANVISA approval or SUS availability.
The immediate Brazil-facing relevance is molecular diagnosis: ESR1 mutation detection by ctDNA or tissue will determine who can be identified if/when access becomes available locally.
Academic opportunities include proteomics, structural modeling, selectivity studies, and real-world registries when access begins.

References

U.S. Food and Drug Administration. FDA approves vepdegestrant for ER-positive, HER2-negative, ESR1-mutated advanced or metastatic breast cancer. May 1, 2026. https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-vepdegestrant-er-positive-her2-negative-esr1-mutated-advanced-or-metastatic-breast
Zhong G, Chang X, Xie W, Zhou X. Targeted protein degradation: advances in drug discovery and clinical practice. Signal Transduct Target Ther 2024;9:308. PMID 39500878. https://doi.org/10.1038/s41392-024-02004-x
Gough SM, Flanagan JJ, Teh J, et al. Oral Estrogen Receptor PROTAC Vepdegestrant (ARV-471) Is Highly Efficacious as Monotherapy and in Combination with CDK4/6 or PI3K/mTOR Pathway Inhibitors in Preclinical ER+ Breast Cancer Models. Clin Cancer Res 2024;30:3549-3563. PMID 38819400. https://doi.org/10.1158/1078-0432.CCR-23-3465
Kumar SH, Venkatachalapathy M, Sistla R, Poongavanam V. Advances in molecular glues: exploring chemical space and design principles for targeted protein degradation. Drug Discov Today 2024;29:104205. PMID 39393773. https://doi.org/10.1016/j.drudis.2024.104205

Targeted Protein Degradation (PROTACs and Molecular Glues) ​

TL;DR ​

1. The core idea ​

2. PROTACs vs molecular glues ​

3. Why oncology cares ​

4. Clinical maturity ​

Approved ​

Clinical-stage ​

Preclinical / early translational ​

5. What can go wrong ​

6. What to measure ​

7. What technologists can build ​

8. Brazil context ​

See also ​

References ​