Targeted Protein Degradation: How PROTACs and Molecular Glues Are Drugging the Undruggable
Targeted protein degradation is reshaping how researchers think about drugging the undruggable. Rather than blocking a protein’s activity through continuous occupancy of an active site, degraders induce the removal of the entire target protein from the cell.This approach unlocks new opportunities for tackling disease drivers that have resisted conventional inhibitors.
How degraders work
Degraders fall into two major classes: bifunctional degraders and molecular glues. Bifunctional degraders, often called PROTACs (proteolysis-targeting chimeras), physically link a ligand for the protein of interest to a ligand that recruits an E3 ubiquitin ligase. Bringing the ligase and target together tags the protein with ubiquitin, sending it to the proteasome for degradation.
Molecular glues are smaller molecules that enhance or create protein–protein interactions between a target and an E3 ligase, driving ubiquitination without a large linker.
Advantages over traditional inhibitors
– Broader target space: Degraders can remove scaffolding proteins, transcription factors, and other non-enzymatic targets that lack classic active sites.
– Catalytic mode of action: One degrader molecule can trigger the destruction of multiple target molecules, potentially lowering required doses.
– Overcoming resistance: Degradation can bypass some mechanisms of resistance to inhibitors, such as mutations that alter binding pockets.
– Temporal control: Degradation enables transient or sustained protein knockdown depending on compound properties and dosing, allowing new biological experiments and therapeutic strategies.
Key challenges
– Selectivity and off-target effects: Recruiting ubiquitous ligases risks degrading unintended proteins.
Carefully optimizing target engagement and ligase choice is essential.
– Pharmacokinetics and cell permeability: Bifunctional molecules are often larger and more polar than typical small molecules, creating hurdles for oral bioavailability, tissue penetration, and brain delivery.
– E3 ligase dependency: Most degraders exploit a handful of well-characterized E3 ligases, limiting tissue selectivity and raising competition for ligase usage.
– Resistance mechanisms: Cells can downregulate or mutate components of the ubiquitin–proteasome system, or alter target expression to evade degradation.
Strategies advancing translational success
– Structure-guided design and proteomics-driven screening help optimize ternary complex formation between the degrader, target, and ligase to maximize potency and selectivity.
– Linker engineering—varying length, rigidity, and attachment geometry—fine-tunes degrader-induced proximity and influences degradation profiles.
– Discovery of new E3 ligases and ligands broadens tissue selectivity and reduces reliance on a small set of ligases, enabling more precise therapeutic windows.
– Development of orally bioavailable and brain-penetrant chemotypes addresses delivery limitations, expanding indications to central nervous system disorders.
– Biomarker development, including quantitative proteomics and targeted assays, enables early readouts of target engagement and degradation in preclinical and clinical studies.
Therapeutic opportunities
Degraders show promise across oncology, immunology, and neurodegeneration.
Oncology programs target oncogenic drivers and resistance mechanisms; neurodegeneration efforts focus on reducing aggregate-prone proteins and modulating pathogenic pathways. Combination approaches pairing degraders with immune modulators or pathway inhibitors also hold potential to amplify efficacy.
What to watch in drug discovery research
Researchers are exploring tissue-selective degrading strategies, targeted delivery systems, and next-generation ligases to improve safety and potency. Advances in chemical biology, proteomics, and translational pharmacology are informing smarter degrader design and more predictive preclinical models.
As the toolkit of degraders expands, opportunities to address historically intractable targets are increasing, making targeted protein degradation one of the most impactful areas of modern drug discovery research.