Protein Destruction as Medicine: How PROTACs Are Changing Cancer Therapy

Edited and Approved by Stephen C. Rose, PhD, MS

Most cancer drugs are built around a familiar idea: find a troublesome protein and block it. That can work brilliantly, but only when the protein has a pocket a drug can reach and when the cancer does not simply make more protein, alter the pocket, or route around the blockade. PROTACs take a more radical approach. Instead of occupying a cancer-driving protein, they arrange for the cell to destroy it.

A 2026 review in Biomolecules surveys this fast-moving field, from molecular design to clinical trials [1]. Its publication was followed by a historic development: the first regulatory approval of a PROTAC. That milestone turns targeted protein degradation from an elegant laboratory concept into a clinically validated form of medicine. It does not mean PROTACs can treat every cancer, but it does mean that purposeful protein disposal can benefit a carefully selected group of patients.

A molecular matchmaker for the cellular trash system

A PROTAC—short for proteolysis-targeting chimera—is one molecule with two binding ends joined by a linker. One end recognizes the protein of interest. The other recruits an E3 ubiquitin ligase, part of the cell's protein-quality-control system. When all three components come together, the ligase attaches small ubiquitin tags to the target. Those tags direct the protein to the proteasome, a molecular machine that dismantles proteins into reusable pieces.

The first published PROTAC experiment, in 2001, tethered a target protein to an E3-ligase complex and showed that the target could be ubiquitinated and degraded [2]. Early constructs were bulky and peptide-based. A major advance came when researchers developed smaller, cell-permeable PROTACs that reduced target proteins by more than 90% at nanomolar concentrations and worked in mouse tumor models [3].

This is called event-driven pharmacology. A conventional inhibitor usually has to remain attached to keep a protein quiet. A PROTAC only needs to create a productive encounter long enough to trigger destruction. It may then disengage and repeat the process with another copy of the target. Removing the whole protein can erase not only its enzyme activity but also its structural and signaling roles—functions that an active-site inhibitor may leave untouched.

Why cancer researchers are interested

Cancer cells depend on abnormal proteins to grow, avoid death, repair damage, and resist treatment. Some of those proteins lack the deep chemical pockets that ordinary drugs need. Others have several jobs, so blocking one function is not enough. In principle, degradation expands the set of druggable targets because a PROTAC needs a reliable point of attachment, not necessarily a pocket that controls the protein's activity.

The technology may also create selectivity through biology, not just binding chemistry. BCL-XL, for example, helps some cancer cells survive, but platelets also depend on it. Direct BCL-XL inhibitors can therefore cause dangerous thrombocytopenia. Researchers built the degrader DT2216 to recruit the VHL ligase, which is scarce in platelets. In cells and mice, DT2216 destroyed BCL-XL more effectively in tumors while largely sparing platelets [4]. The important principle is that the identity and tissue distribution of the recruited ligase can help decide where degradation occurs.

The first decisive human test

The clearest clinical evidence comes from vepdegestrant, an oral PROTAC that degrades the estrogen receptor. In the phase 3 VERITAC-2 trial, 624 people with previously treated estrogen-receptor-positive, HER2-negative advanced breast cancer were assigned to vepdegestrant or fulvestrant. Among the 270 participants whose tumors carried an ESR1 mutation, median progression-free survival was 5.0 months with vepdegestrant and 2.1 months with fulvestrant. The hazard ratio was 0.58, representing a substantial reduction in the risk of progression or death during the study period [5].

The result was more modest in the full trial population: median progression-free survival was 3.8 versus 3.6 months, and the difference did not meet the conventional threshold for statistical significance. That contrast is essential. The drug was not a universal winner for everyone with this form of breast cancer; its strongest benefit appeared in a biomarker-defined subgroup. Grade 3 or higher adverse events occurred in 23.4% of participants receiving vepdegestrant and 17.6% receiving fulvestrant, while treatment discontinuation due to adverse events remained uncommon.

On May 1, 2026, the U.S. Food and Drug Administration approved vepdegestrant for adults with ESR1-mutated, estrogen-receptor-positive, HER2-negative advanced or metastatic breast cancer after at least one line of endocrine therapy. It was the first approved PROTAC [6]. This is genuine proof of concept, but the indication is narrow, biomarker testing is required, and overall-survival data were still immature at approval.

Other targets are earlier in the journey

The rest of the clinical pipeline should be judged by its actual phase of evidence. In a first-in-human study of DT2216, 20 people with heavily pretreated solid tumors received escalating intravenous doses. The drug produced rapid, sustained BCL-XL degradation in blood cells at the selected phase 2 dose. Four patients had stable disease, but no objective tumor responses were reported. One person developed grade 4 thrombocytopenia that resolved quickly, and transient platelet declines remained the main drug-related concern [7]. This shows target engagement and helps define dosing; it does not yet establish anticancer efficacy.

A separate phase 1 study tested HP518, an oral degrader of the androgen receptor, in 22 people with metastatic castration-resistant prostate cancer. Most adverse events were grade 1 or 2. Two participants had partial radiographic responses and three achieved a reduction of at least 50% in prostate-specific antigen [8]. Those signals are encouraging, but the trial was small, uncontrolled, and designed mainly to assess safety and dose behavior. Larger comparative studies are needed.

The engineering problems are part of the biology

A PROTAC must enter the right cells, bind two partners, form the correct three-part complex, expose usable sites for ubiquitin tagging, and persist long enough to work without accumulating where it should not. Its two binding ends and linker often make it larger than a typical pill, creating challenges for solubility, membrane passage, oral absorption, and distribution. A longer linker is not automatically better, and a stronger binder is not automatically a better degrader. The geometry of the entire complex matters.

Dose can behave strangely as well. At very high concentrations, separate two-part complexes may form instead of the productive three-part complex, reducing degradation—a phenomenon called the hook effect. Off-target degradation could remove a protein rather than merely suppress it, making careful proteomic testing important. Manufacturing may also be difficult because these molecules can have complex chemistry and multiple three-dimensional forms.

Cancer can evolve around protein destruction, just as it evolves around inhibition. In laboratory cancer cells exposed chronically to BET-targeting PROTACs, resistance arose through genomic damage to components of the recruited E3-ligase machinery [9]. Other routes may include altered target binding, increased drug export, or changes in the ubiquitin-proteasome system. A PROTAC does not abolish evolution; it changes the selective pressure and therefore the likely escape routes.

What this means for longevity

The direct connection to longevity is cancer control. Cancer incidence rises sharply with age, so treatments that produce longer, better-quality survival in older adults can contribute to healthy longevity. PROTACs may eventually help by overcoming specific resistance mechanisms or by reaching disease-driving proteins that have resisted conventional drug design. Researchers are also studying targeted degradation outside oncology, but those applications remain separate questions requiring their own trials.

It would be premature to describe PROTACs as anti-aging drugs or as a general way to clean damaged proteins from the body. They are programmed agents aimed at selected targets in selected cells. Their promise lies in precision, not indiscriminate cellular housekeeping.

The bottom line

PROTACs have crossed an important boundary. The mechanism is experimentally established, and vepdegestrant has demonstrated meaningful benefit and earned approval for a defined breast-cancer population. Beyond that success, most candidates remain in early trials or preclinical development. The field now has to prove that its ingenious chemistry can repeatedly deliver useful tumor control, manageable toxicity, practical dosing, and durable benefit.

The conceptual shift is nevertheless profound. Medicine is no longer limited to blocking a harmful protein's active site. It can sometimes hand that protein to the cell's own disposal machinery and let biology remove the problem. The first success is not the end of the story; it is the first rigorous demonstration that protein destruction can be a drug.

References

[1] Faryal B, Ashraf W, Abdurakhimova L, et al. Targeted Protein Degradation in Cancer: PROTACs, New Targets, and Clinical Mechanisms. Biomolecules. 2026;16(2):325. doi:10.3390/biom16020325.

[2] Sakamoto KM, Kim KB, Kumagai A, et al. Protacs: chimeric molecules that target proteins to the Skp1-Cullin-F box complex for ubiquitination and degradation. Proc Natl Acad Sci U S A. 2001;98(15):8554-8559. doi:10.1073/pnas.141230798.

[3] Bondeson DP, Mares A, Smith IED, et al. Catalytic in vivo protein knockdown by small-molecule PROTACs. Nat Chem Biol. 2015;11(8):611-617. doi:10.1038/nchembio.1858.

[4] Khan S, Zhang X, Lv D, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25(12):1938-1947. doi:10.1038/s41591-019-0668-z.

[5] Campone M, De Laurentiis M, Jhaveri K, et al. Vepdegestrant, a PROTAC Estrogen Receptor Degrader, in Advanced Breast Cancer. N Engl J Med. 2025;393(6):556-568. doi:10.1056/NEJMoa2505725.

[6] Approval of First PROTAC Opens New Era for Targeted Protein Degradation. Cancer Discov. 2026;16(7):OF1. doi:10.1158/2159-8290.CD-NW2026-0054.

[7] Mahadevan D, Barve M, Mahalingam D, et al. First in human phase 1 study of DT2216, a selective BCL-xL degrader, in patients with relapsed/refractory solid malignancies. J Hematol Oncol. 2025;18(1):98. doi:10.1186/s13045-025-01753-8.

[8] Azad AA, Gurney H, Underhill C, et al. Phase 1 study of HP518, a PROTAC AR degrader in patients with mCRPC: results on safety, pharmacokinetics, and anti-tumor activity. Invest New Drugs. 2025;43(2):435-445. doi:10.1007/s10637-025-01533-8.

[9] Zhang L, Riley-Gillis B, Vijay P, Shen Y. Acquired Resistance to BET-PROTACs (Proteolysis-Targeting Chimeras) Caused by Genomic Alterations in Core Components of E3 Ligase Complexes. Mol Cancer Ther. 2019;18(7):1302-1311. doi:10.1158/1535-7163.MCT-18-1129.