PROTACs (Proteolysis Targeting Chimeras) are a new class of experimental drugs designed to destroy disease-causing proteins rather than simply blocking them. Traditional drugs work by attaching to a protein and interfering with its activity. PROTACs take a fundamentally different approach: they hijack the cell’s own recycling system to break the target protein down entirely.
How PROTACs Work
Every cell has a built-in protein disposal system called the ubiquitin-proteasome pathway. When a protein is no longer needed or is damaged, the cell tags it with a small molecule called ubiquitin. That tag acts as a signal: enzymes called E3 ligases attach it, and a barrel-shaped structure called the proteasome recognizes the tagged protein and shreds it into amino acids the cell can reuse.
PROTACs exploit this system by tricking the cell into tagging a disease-related protein for destruction. Each PROTAC molecule has three parts: one end binds to the target protein, the other end binds to an E3 ligase, and a chemical linker connects the two. When the PROTAC grabs both the target protein and the E3 ligase at the same time, it forces them into close contact, forming what researchers call a “ternary complex.” The E3 ligase then does what it normally does: it attaches ubiquitin tags to the target protein, marking it for the proteasome to destroy.
Once the target protein is tagged and released to the proteasome, the PROTAC molecule lets go and is free to repeat the process with another copy of the same protein. This catalytic recycling is a key distinction. A single PROTAC molecule can trigger the destruction of many protein copies over time, rather than needing to sit on one protein indefinitely.
Why This Matters Compared to Traditional Drugs
Most drugs on the market today are small-molecule inhibitors. They work through what pharmacologists call an “occupancy-driven” mechanism: the drug molecule parks itself in a protein’s active site and physically blocks its function. For this to work, you need enough drug molecules circulating in the body to occupy a large share of target proteins at any given moment. Higher doses increase effectiveness but also increase side effects and off-target interactions.
PROTACs work through an “event-driven” mechanism instead. They don’t need to stay bound to the target continuously. They just need to bring the target and the E3 ligase together long enough for tagging to occur. Because of this catalytic nature, PROTACs can theoretically work at lower doses, with longer intervals between doses, and with reduced off-target toxicity. Even a ligand with relatively weak binding affinity to the target protein can still trigger efficient degradation.
There’s another advantage. Traditional inhibitors only block one function of a protein, typically its enzymatic activity. But many disease-related proteins have multiple roles, some of which don’t depend on the active site an inhibitor occupies. By destroying the entire protein, PROTACs eliminate all of its functions at once. This is especially useful for proteins that have proven resistant to conventional inhibitors, sometimes called “undruggable” targets. Traditional inhibitors can also stabilize a protein’s structure or trigger the cell to produce more of it as compensation, gradually undermining the drug’s effectiveness. Degradation sidesteps both problems.
PROTACs can also be remarkably selective. Their selectivity comes not just from how well one end binds the target, but from the geometry of the entire ternary complex. This allows PROTACs to distinguish between very similar proteins or even different versions of the same protein in ways that occupancy-based inhibitors cannot.
Which E3 Ligases Are Used
The human body contains over 600 E3 ligases, but PROTAC development has focused heavily on just two: VHL (von Hippel-Lindau) and cereblon (CRBN). Both are widely expressed across tissues and have well-characterized binding pockets that chemists can design ligands for. The cereblon-targeting approach draws on insights from thalidomide-based compounds, which were found to redirect cereblon toward new protein targets. VHL-based PROTACs use a different class of small-molecule ligands to recruit that ligase instead.
A handful of other E3 ligases, including MDM2 and IAPs, have also been used in PROTAC design. The first-ever PROTAC, published in 2001, used an MDM2-recruiting strategy to degrade the androgen receptor. Expanding the toolkit of usable E3 ligases remains an active goal, since different ligases are expressed at different levels in different tissues, which could eventually allow researchers to fine-tune where in the body degradation occurs.
Current Clinical Progress
The most advanced PROTACs in clinical testing come from the biotech company Arvinas. ARV-471 is being evaluated in Phase 1/2 trials for estrogen receptor-positive, HER2-negative breast cancer, both as a single agent and in combination with the established drug palbociclib. It works by degrading the estrogen receptor, which fuels the growth of this cancer subtype. Rather than just blocking the receptor’s signaling (as existing hormone therapies do), ARV-471 eliminates the receptor protein from tumor cells.
Several other PROTACs are in early clinical trials targeting different cancer-driving proteins. The field is still young, and most candidates are in Phase 1 or Phase 2 testing, meaning safety and preliminary efficacy are the primary questions being answered right now.
Applications Beyond Cancer
Cancer has been the primary focus, but researchers are exploring PROTACs for neurodegenerative diseases where toxic, misfolded proteins accumulate in the brain. In Huntington’s disease, a mutant form of the huntingtin protein forms clumps that damage neurons. Lab studies have shown that PROTAC-like compounds can lower levels of mutant huntingtin in cells taken from Huntington’s patients. Similar approaches are being tested against tau and TDP-43, two proteins that aggregate in Alzheimer’s disease and amyotrophic lateral sclerosis (ALS), respectively. Other aggregation-prone proteins linked to rare neurological conditions, such as mutant ataxin-3 and mutant ataxin-7, are also being investigated as targets.
These applications are still preclinical, meaning they’ve been tested in cells and animal models but not yet in human trials. The challenge with neurodegenerative diseases is particularly steep because drugs need to cross the blood-brain barrier, and the proteins involved tend to form large, tangled aggregates that may resist the tagging process.
Challenges in Drug Development
PROTACs are large molecules. A typical small-molecule drug follows the “rule of five,” a set of guidelines predicting whether a compound can be absorbed through the gut and reach the bloodstream after being swallowed. PROTACs routinely violate these guidelines: they’re heavier, more complex, and have more chemical features that hinder absorption. This makes oral bioavailability one of the biggest hurdles in turning PROTACs into pills patients can take at home.
Researchers are tackling this through careful molecular design, using three-dimensional structural properties rather than simple two-dimensional metrics to predict which PROTACs will absorb well. Some candidates have achieved adequate oral bioavailability, but optimizing this for each new PROTAC remains a case-by-case challenge. The linker connecting the two ends of the molecule is a particularly sensitive design element: its length, flexibility, and chemical composition all influence whether the ternary complex forms properly and whether the overall molecule can survive the journey through the digestive system.
There’s also the “hook effect,” a counterintuitive problem where too high a concentration of PROTAC actually reduces its effectiveness. At very high doses, individual PROTAC molecules saturate both the target protein and the E3 ligase separately, preventing the formation of the three-way complex needed for degradation. Finding the right dosing window is therefore more nuanced than with traditional drugs, where more is generally better up to the point of toxicity.