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From Inhibition to Elimination: A New Paradigm in Small Molecule Oncology

For decades, small molecule cancer drugs have been designed to inhibit the catalytic activity of oncogenic proteins. Yet many of the most important cancer drivers—such as transcription factors and scaffolding proteins—lack the deep, druggable pockets required for classical inhibition. This has left vast portions of the “cancer proteome” effectively off-limits.

Proteolysis Targeting Chimeras (PROTACs) are changing that narrative. Rather than simply blocking a protein’s function, PROTAC small molecules mark the protein for destruction by hijacking the cell’s own ubiquitin–proteasome system. This shift—from inhibition to elimination—is rapidly emerging as one of the most disruptive advances in precision oncology doi:10.1038/nrd.2019.227.

What Exactly Are PROTAC Small Molecule Drugs?

PROTACs are heterobifunctional small molecules made of three modular components:

• A ligand that binds the disease-driving target protein
• A ligand that recruits an E3 ubiquitin ligase
• A flexible chemical linker that connects these two ends

By physically bringing the target protein and E3 ligase together, PROTACs trigger ubiquitination of the target, which is then recognized and degraded by the proteasome. Crucially, PROTACs act catalytically: a single PROTAC molecule can degrade multiple copies of the target protein, enabling deep and sustained knockdown at relatively low exposures doi:10.1016/j.drudis.2021.04.008.

Why PROTACs Matter for the Future of Cancer Therapy

1. Expanding the Druggable Proteome

Many oncogenic transcription factors and signaling adaptors are considered “undruggable” by standard small molecules. PROTACs only require a binding handle, not a high-affinity active-site interaction, allowing even weak binders to be converted into potent degraders. This opens the door to targeting proteins that were previously out of reach in solid tumors and hematologic malignancies doi:10.1038/s41573-021-00279-4.

2. Overcoming On-Target Drug Resistance

Cancer cells frequently escape kinase inhibitors through point mutations in the drug-binding site or by upregulating bypass pathways. Because PROTACs remove the entire protein, they can circumvent some resistance mechanisms driven by mutated active sites. PROTACs targeting kinases such as BTK and BCR-ABL have shown activity against clinically resistant mutants in preclinical models doi:10.1038/s41573-020-0087-3.

3. Tunable Selectivity and Safety

By carefully choosing the E3 ligase ligand, linker length, and target binder, PROTACs can exploit:

• Tissue-specific E3 ligase expression to bias degradation toward tumor cells
• Cooperative binding that stabilizes ternary complexes only in defined molecular contexts
• Partial or transient degradation to fine-tune pharmacodynamic effects

These features create opportunities for wider therapeutic windows than traditional full enzymatic blockade in some oncology settings doi:10.1038/s41573-021-00279-4.

First-in-Class PROTACs in Clinical Trials

PROTACs have rapidly progressed from concept to clinic, with multiple agents now in human trials:

• ARV-110: An androgen receptor (AR) degrader for metastatic castration-resistant prostate cancer. Early-phase data show prostate-specific antigen (PSA) declines and robust AR degradation in heavily pretreated patients doi:10.1158/2159-8290.CD-20-0136.
• ARV-471: An estrogen receptor (ER) degrader for ER-positive breast cancer. Clinical studies report encouraging response rates and a favorable safety profile, including in combination with CDK4/6 inhibitors doi:10.1158/0008-5472.CAN-21-2043.

These first clinical experiences validate that PROTAC small molecules can be orally administered, systemically distributed, and pharmacologically active in human tumors.

Key Challenges in PROTAC Drug Design

Despite their promise, PROTACs present unique development hurdles:

• Physicochemical liabilities: High molecular weight and polarity can compromise oral bioavailability and cell permeability.
• Complex pharmacokinetics: Balancing systemic stability with rapid target engagement is non-trivial.
• Limited E3 ligase toolbox: Only a small fraction of the >600 human E3 ligases are well characterized or druggable.
• On-target toxicity: Pan-tissue degradation of essential proteins may lead to mechanism-based adverse events.

Next-generation PROTACs aim to address these issues through linker engineering, novel E3 ligase recruiters, and tissue-restricted degradation strategies doi:10.1038/s41573-022-00511-4.

Beyond PROTACs: Molecular Glues and the Broader TPD Landscape

PROTACs are part of a larger revolution known as targeted protein degradation (TPD). “Molecular glues” represent a complementary class of small molecules that stabilize interactions between a target protein and an E3 ligase without requiring a linker. Thalidomide analogs (IMiDs) and emerging cereblon modulators in multiple myeloma are leading examples of this concept doi:10.1038/s41573-020-0088-2.

Together, PROTACs and molecular glues are transforming small molecules into programmable degraders that can rewire protein homeostasis in cancer cells.

Conclusion: Toward a New Era of Programmable Protein Destruction

PROTAC small molecule drugs are redefining what is possible in cancer pharmacology. By converting small molecules into targeted “seek-and-destroy” agents, this technology expands the druggable proteome, offers new strategies against resistance, and enables unprecedented control over protein levels in tumors. As the toolbox of E3 ligases, linkers, and target binders continues to grow, targeted protein degradation is poised to become a central pillar of next-generation oncology.

References

• Burslem GM, Crews CM. Small-molecule modulation of protein homeostasis. Nat Rev Drug Discov. 2020;19(5):307–325. doi:10.1038/nrd.2019.227
• Sun X et al. PROTACs: great opportunities for small molecule therapeutics. Drug Discov Today. 2021;26(4):1046–1060. doi:10.1016/j.drudis.2021.04.008
• Békés M, Langley DR, Crews CM. PROTAC targeted protein degraders: the past is prologue. Nat Rev Drug Discov. 2022;21(3):181–200. doi:10.1038/s41573-021-00279-4
• Salami J et al. Androgen receptor degradation by PROTACs as a novel therapeutic strategy for prostate cancer. Cancer Discov. 2020;10(8):1100–1113. doi:10.1158/2159-8290.CD-20-0136
• Pettersson M, Crews CM. PROTACs: Protein degraders as therapeutics. Cancer Res. 2022;82(2):209–218. doi:10.1158/0008-5472.CAN-21-2043
• Oliveira AR et al. Targeted protein degradation in cancer: PROTACs, molecular glues and beyond. Nat Rev Drug Discov. 2022;21(10):815–837. doi:10.1038/s41573-022-00511-4
• Chamberlain PP, Hamann LG. Development of targeted protein degradation therapeutics. Nat Rev Drug Discov. 2019;18(10):821–838. doi:10.1038/s41573-020-0088-2