The new review, authored by Shun-Ran Li, Meng-Qian Yu, and colleagues at Hangzhou Normal University and collaborating institutions, systematically surveys the explosion of dual- and multi-target degraders reported between 2023 and 2026. Its central premise is that complex diseases rarely depend on a single molecular culprit. Cancer and neurodegenerative disorders are governed by redundant and compensatory signaling networks, so when one driver protein is eliminated, others rush to fill the gap. Conventional single-target PROTACs, however powerful, can be undermined by this biological backup system. Dual PROTACs attempt to solve the problem in a single chemical entity, collapsing what would otherwise be a combination of two drugs into one compound with one set of pharmacokinetics.

The authors organize the growing catalog of dual degraders into two broad design classes. The first targets homologous proteins within the same family, where structural similarity can be exploited by a single warhead that binds both. Cyclin-dependent kinases have been a particular focus, with dual degraders reported for CDK4/6, CDK2/5, and CDK12/13, the latter exemplified by the orally bioavailable triple-negative breast cancer candidate DN1679. The BCL-2 family of apoptosis regulators has yielded dual BCL-2/BCL-xL degraders with improved anti-leukemic activity, and the field has now seen first-in-class degraders for the bromodomain proteins BAZ2A and BAZ2B as well as dual histone deacetylase degraders, including HDAC3/HDAC8 molecules that revealed new roles for histone acetylation in gene regulation.

The second, and pharmacologically more ambitious, class targets two distinct proteins sitting on different but interconnected disease pathways. Here the review catalogues an impressive roster: ERα/aromatase degraders designed to overcome endocrine-resistant breast cancer by hitting both the receptor and the enzyme that produces its activating estrogen; α-synuclein/tau degraders aimed at the protein aggregates that define Parkinson’s and Alzheimer’s disease; BET/HDAC and CBP/BRD4 degraders that combine epigenetic readers and erasers in one molecule; PI3K/mTOR degraders that suppress the entire eponymous survival pathway; and FLT3/CHK1 degraders that pair an oncogenic kinase with a checkpoint kinase to attack acute myeloid leukemia from two directions.

The technical heart of the review lies in its dissection of structure-activity relationships. Linker chemistry is where dual PROTACs live or die. Because a single molecule must accommodate two target-binding events, the linker length, attachment point, and flexibility determine not only potency but the ternary geometry between target, PROTAC, and E3 ligase that licenses ubiquitin transfer. Several case studies illustrate the point. Dual BCL-xL/BCL-w degraders were built by exploiting the bis(sulfonyl)benzene ring of the clinical inhibitor ABT-263 as a linkage vector, showing how careful warhead decoration can convert a blocker into a degrader. In the FLT3/CHK1 program, systematic linker scans revealed how marginal changes in length flipped degradation selectivity between the two kinases. The authors emphasize that hook effects, hook-like concentration dependence in which excess PROTAC saturates both binding sites separately and aborts the ternary complex, remain a persistent design trap requiring careful dose-response characterization.

E3 ligase selection emerges as another decisive variable. Most reported dual degraders recruit either the von Hippel-Lindau (VHL) ligase or cereblon (CRBN), reflecting the maturity of their ligand chemistries. CRBN-recruiting degraders derived from pomalidomide and related immunomodulatory drugs have proven especially productive for kinases and transcriptional regulators, while VHL ligands dominate among cytosolic and nuclear targets. But the review is blunt about the field’s narrow toolkit: the reliance on just two or three E3 ligases limits tissue selectivity, constrains the design space for dual targets, and contributes to off-target degradation of neo-substrates such as IKZF1 and IKZF3. Expanding the E3 ligase repertoire, including tumor-selective or tissue-specific ligases, is flagged as a priority for next-generation design.

Among the milestones the review highlights are the first dual degraders aimed at non-kinase epigenetic regulators and, strikingly, at protein aggregates themselves. Degraders capable of clearing α-synuclein and tau aggregates simultaneously represent a conceptual breakthrough for neurodegeneration, because both misfolded proteins cross-seed one another and together drive pathology in diseases such as dementia with Lewy bodies and Alzheimer’s. The authors note that these designs repurposed aggregation-binding scaffolds, such as thioflavin-derived amyloid ligands, as warheads, demonstrating that even supramolecular pathological assemblies can be brought into the reach of the ubiquitin-proteasome system. In parallel, dual GSPT1/BRD4 degraders exploit cereblon-mediated translational termination factor degradation alongside epigenetic transcriptional collapse to kill leukemia cells through mechanistically independent routes.

The translational horizon is coming into view. The first PROTAC, vepdegestrant, has moved targeted degradation into approved oncology practice, and clinically advanced single-target degraders have validated the modality’s core principles. Dual degraders now face the harder test of converting dual potency into dual efficacy with an acceptable safety profile. The review identifies pharmacokinetics as the chief obstacle: dual PROTACs are large, often exceeding the classic rule-of-five boundaries for oral absorption, and their high polarity and molecular weight challenge permeability, metabolic stability, and brain penetration. Off-target toxicity arising from polypharmacology is a second concern, since the same warhead promiscuity that enables dual engagement can degrade unintended proteins. Strategies such as introducing one-carbon bridges to lower lipophilicity, incorporating molecular glue features into PROTAC scaffolds, and exploiting CD36-mediated endocytosis to improve cellular uptake are among the emerging countermeasures surveyed.

What the review ultimately delivers is a map. By consolidating the 2023 to 2026 literature into a coherent framework of homologous versus cross-pathway dual targets, and by extracting transferable SAR lessons on warhead choice, linker optimization, and E3 ligase pairing, the authors have given medicinal chemists a practical playbook for the next round of design. If the remaining pharmacokinetic and selectivity problems can be tamed, dual PROTACs could compress combination therapy into a single pill, offering patients with complex, drug-resistant diseases a weapon that dismantles two pillars of pathology at once. For a field that has already taught medicine to delete proteins rather than merely inhibit them, degrading disease networks two nodes at a time may be the logical, and inevitable, next act.

Subject of Research: Development of dual PROTACs that simultaneously degrade two disease-related proteins for therapeutic applications in cancer and neurodegenerative disease.

Article Title: Recent advances of dual PROTACs for potential therapeutic applications

Article References: Li, S.-R., Yu, M.-Q., Du, B.-Q., Jin, K.-J., Chen, S.-X., Hui, Z., Zhang, H., Mao, N.-D., Gao, Y., & Ye, X.-Y. (2026). Recent advances of dual PROTACs for potential therapeutic applications. Molecular Diversity. https://doi.org/10.1007/s11030-026-11734-9

Image Credits: AI Generated

DOI: 10.1007/s11030-026-11734-9

Keywords: PROTACs, targeted protein degradation, dual degraders, ubiquitin-proteasome system, E3 ubiquitin ligase, cancer therapy, neurodegenerative disease, BCL-2, BRD4, CDK inhibitors, structure-activity relationships, drug discovery

PROTACs work differently from conventional drugs that merely inhibit a protein’s activity. A typical PROTAC is a bifunctional molecule with one end that binds to a disease-associated protein and another that recruits an E3 ubiquitin ligase, part of the cell’s protein-disposal machinery. By bringing the target protein and the ligase into close proximity, the PROTAC causes the target to be tagged with ubiquitin molecules. The proteasome, a large intracellular complex that degrades ubiquitinated proteins, then dismantles the marked protein. Because a PROTAC can act catalytically—detaching after the target is destroyed and potentially engaging another copy—it may eliminate proteins rather than temporarily blocking them. That promise has attracted intense interest in oncology, where many disease-driving proteins have proved difficult to inhibit with traditional small molecules.

The same molecular features that make PROTACs powerful can also make them difficult to deliver. Many are relatively large and chemically complex, occupying what medicinal chemists call “beyond rule-of-five” space. Their size and polarity can reduce passive diffusion through the lipid bilayer of a cell membrane, while their hydrophobicity can limit water solubility and cause unfavorable distribution in the body. A PROTAC circulating in the bloodstream must reach a tumor, cross or enter cancer cells, escape destructive clearance pathways and arrive in the correct intracellular compartment before it can assemble the molecular interactions needed for protein degradation. Poor membrane permeability and inadequate tumor distribution therefore represent major barriers between promising laboratory chemistry and a practical medicine.

The researchers addressed these problems by attaching a hydrophobic PROTAC called MZ1 to a hydrophilic affibody known as Z_HER2:342. MZ1 is designed to degrade bromodomain-containing protein 4, or BRD4, while the affibody is an engineered affinity protein that recognizes human epidermal growth factor receptor 2, commonly called HER2. Affibodies are small, engineered binding proteins derived from an alpha-helical bacterial receptor domain. Unlike full-size antibodies, they are compact and can be produced and chemically modified as defined molecules. Z_HER2:342 supplies the targeting function, while MZ1 supplies the protein-degradation function. The two components were connected by a linker containing a disulfide bond, creating an amphiphilic conjugate with one water-compatible region and one water-avoiding region.

When placed in water, the conjugates spontaneously organized into nanoparticles, a process known as self-assembly. Amphiphilic molecules can form nanoscale structures because their hydrophilic and hydrophobic sections seek different environments: the water-compatible affibody portions remain exposed to the surrounding liquid, while the hydrophobic MZ1 portions cluster away from it. This arrangement allows the drug molecules to be carried in a compact, water-dispersible form without requiring a separate polymeric carrier or lipid shell. The resulting formulation, called the Z_HER2:342-MZ1 affibody-PROTAC conjugate nanomedicine, was intended to solve two delivery problems at once—keeping MZ1 dispersed in the bloodstream and displaying the HER2-binding affibody on the nanoparticle surface.

The targeting mechanism depends on the abundance of HER2 on the surface of selected cancer cells. HER2 is a receptor tyrosine kinase involved in signaling pathways that regulate proliferation, survival and differentiation. In some breast, ovarian and other cancers, the receptor is produced at unusually high levels, creating a molecular marker that can distinguish malignant cells from many normal tissues. According to the study, the nanoparticles used HER2 receptor-mediated endocytosis to gain entry into cancer cells. In this process, binding at the cell surface triggers the membrane to fold inward and form an intracellular vesicle containing the bound material. The researchers reported effective accumulation and internalization of the conjugate in HER2-positive cancer cells in vitro, consistent with the idea that affibody-mediated recognition improved delivery beyond what free MZ1 could achieve.

The disulfide linker was designed to respond to the reducing conditions inside cells. Glutathione, or GSH, is a major intracellular antioxidant and is generally present at higher concentrations within cells than in the extracellular space or bloodstream. Its thiol group can participate in reduction reactions that cleave disulfide bonds. In the proposed system, intracellular GSH breaks the linker connecting the affibody and MZ1, releasing the PROTAC after the nanoparticle has been internalized. This is a form of chemically triggered release: the carrier remains comparatively stable during circulation but becomes labile in a cellular environment rich in reducing agents. Once liberated, MZ1 can interact with BRD4 and recruit the ubiquitin-proteasome system, converting the delivery event into targeted destruction of an intracellular protein.

BRD4 belongs to the bromodomain and extraterminal, or BET, family of epigenetic reader proteins. Rather than acting as a conventional DNA-binding transcription factor, BRD4 recognizes acetylated lysine residues on histones and other proteins, helping organize transcriptional machinery at active genes. It is particularly associated with regulatory regions such as enhancers and super-enhancers, where it can support expression programs that sustain cancer-cell proliferation and survival. Degrading BRD4 can therefore disrupt multiple transcriptional networks at once. The study reported that the released MZ1 produced BRD4 deficiency and subsequently induced apoptosis, the regulated form of cell death. This mechanism is distinct from simply slowing an enzyme: it removes an entire protein platform that cancer cells may depend on for maintaining gene expression.

The researchers then evaluated the conjugate in vivo after administration through the tail vein in mice carrying HER2-positive SKOV-3 tumors. Intravenous delivery places the formulation directly into the circulation, where its size, surface properties and targeting ligand influence how long it remains in the blood and where it accumulates. The study reported outstanding tumor-specific targeting, increased drug accumulation, enhanced BRD4 degradation and improved antitumor efficacy compared with relevant controls. These findings suggest that the nanoparticles retained their targeting function in the complex environment of an animal and that sufficient MZ1 reached tumor cells to engage its intracellular mechanism. The results also support the value of combining receptor-mediated uptake with a redox-sensitive release step, rather than relying solely on passive nanoparticle accumulation in tumors.

The work does not yet establish whether the platform is safe or effective in people. Mouse tumors do not reproduce the full biological diversity of human cancers, and HER2 expression can vary between tumors and even between cells within the same tumor. The distribution, metabolism and elimination of affibody-based nanoparticles will also need to be characterized in detail, as will possible immune responses, off-target BRD4 degradation and toxicity in healthy tissues. In addition, a clinical formulation would have to meet demanding requirements for manufacturing consistency, stability and dose control. Even so, the study points toward a versatile strategy: a compact targeting protein, a cleavable chemical linker and a self-assembling PROTAC payload are integrated into one molecule that builds its own nanomedicine. If the design can be optimized and validated in more advanced models, it could help turn targeted protein degradation from a promising intracellular concept into a more precise way of delivering cancer therapy.