Targeted protein degradation (TPD) is a drug design strategy that uses small molecules such as PROteolysis TArgeting Chimeras (PROTACs), molecular glues, or related approaches to induce the selective ubiquitination and subsequent proteasomal degradation of target proteins via the ubiquitin–proteasome system or other cellular clearance pathways. Unlike traditional occupancy-driven inhibition, TPD agents catalytically trigger irreversible loss of disease-modifying proteins, enabling the removal of target proteins to provide a sustained duration of action after transient engagement. This event-driven approach is being investigated for potential therapeutic applications in oncology, neurodegeneration, and other areas.
Mechanism Small molecule drugs, compounds typically <1 kD in mass, comprise a large portion of the therapeutic market. These drugs usually operate by agonizing or antagonizing the active site on a disease-linked protein of interest, though allosteric regulation is possible. With an estimated 93% of the human proteome lacking druggable binding sites, methods have been developed to modulate protein activity through binding of any available site rather than only the active site. These drugs contain a target protein binding warhead in addition to a linker-separated active domain. This domain may recruit a second protein to the proximity, induce protease-mediated degradation, or recruit a kinase for directed phosphorylation, among other functions. These drugs expand both the mechanism of action for small molecule therapeutics and the pool of potential protein targets.
Event-driven pharmacology In the field of targeted protein degraders (TPDs), event-driven pharmacology describes a mechanism of action by which a drug exerts its biological effect not by maintaining continuous occupancy of its target, but rather by initiating an irreversible downstream event, such as the proteolytic degradation of the target protein. This contrasts with traditional occupancy-driven pharmacology, in which drug efficacy depends on sustained binding. TPDs act in a catalytic manner. Transient binding brings the target protein into proximity with an E3 ubiquitin ligase, resulting in its ubiquitination and subsequent degradation by the proteasome. Because the target protein is removed rather than merely inhibited, the pharmacological effect can persist after the TPD dissociates from the target or is cleared from the body, resulting prolonged duration of action at lower drug exposures.
Proteolysis-targeting chimeras
Proteolysis targeting chimeras (PROTACs) were first reported by Kathleen Sakamoto, Craig Crews, and Raymond Deshaies in 2001. A chimeric molecule consisting of ovalicin (a MetAP-2 small molecule inhibitor) and IκBα phosphopeptide (a recruiter of the SCFβ-TRCP E3 ligase complex) separated by a linker was constructed and shown to induce MetAP-2 degradation in in vitro cell models. Further study confirmed that E3 ligase-mediated ubiquitination and subsequent proteasome degradation was responsible for reduced MetAP-2 levels. Continued work on this system by Craig Crews and others has expanded the potential pool of E3 ligases and degradation targets with Arvinas Inc. founded in 2013 to bring PROTAC drugs to market. As of April 2023, Arvinas has one drug in Stage 3 clinical trials (vepdegestrant, an estrogen receptor degrader), and two drugs in Stage 2 clinical trials (androgen receptor degraders bavdegalutamide and luxdegalutamide) for treatment of breast and prostate cancer, respectively. Arvinas released Phase 2 clinical trial results for vepdegestrant in December, 2022. As of May 2025, PROTACs in active development that have reached at least Phase II clinical trials:
SNIPERs SNIPERs (Specific and Non-genetic IAP-dependent Protein Erasers) are chimeric small molecules that hijack the E3 ligase activity of inhibitor of apoptosis proteins (IAPs), such as cIAP1 and XIAP, to selectively induce the ubiquitin-dependent proteasomal degradation of target proteins. SNIPERs are a subclass of PROTAC degraders that specifically use IAP family ligases. Notably, SNIPERs often also degrade the IAP ligases themselves along with the intended targets, which maybe beneficial in cancer cells that overexpress IAPs.
Molecular glues
Molecular glues are small molecules that promote targeted protein degradation by stabilizing interactions between E3 ubiquitin ligases and target proteins, enabling their ubiquitin-proteasome mediated breakdown. Unlike bifunctional PROTACs, they lack a linker and directly enhance weak protein–protein interactions, leading to degradation of the target protein. Their small size enables high cell permeability and oral bioavailability. While early examples, such as thalidomide analogs recruiting cereblon, were discovered by accident, mechanistic and structural insights are now starting to drive their rational design.
Hydrophobic tag degradation Hydrophobic tag degraders contain a binding domain in addition to a linker-separated hydrophobic moiety, such as adamantyl, to induce protein degradation. An early example of a hydrophobically tagged degrader is fulvestrant, an estrogen receptor antagonist that contains a long hydrophobic side chain that induces the degradation of the estrogen receptor. Fulvestrant has inspired the development of additional selective estrogen receptor degraders (SERDs). As exposed hydrophobicity is characteristic of protein misfolding, the native cell proteasome may recognize and degrade proteins tagged with the hydrophobic moiety. Taavi Neklesa and Craig Crews first reported hydrophobic tag degradation in 2011 as a tool to probe protein function in conjunction with cognate HaloTag fusion proteins. This principle has also been further used to effectively degrade transcription factors (a traditionally difficult class to drug) and cancer-linked EZH2 in in vitro models. As of yet, no drug candidates have been publicly identified making use of this technology.
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