The field of targeted protein degradation, one of the most rapidly evolving areas in modern pharmacology, has just taken a significant step forward. Researchers have unveiled a new class of degrader molecules, termed macropinocytosis-mediated recyclable lysosome-targeting chimeras, or McR-TACs, that promise to overcome two of the most stubborn limitations that have held back the clinical translation of this technology. Unlike conventional lysosome-targeting chimeras, which depend on specific cell-surface receptors to ferry their targets to the lysosome and which are consumed in the process, the new molecules work in a receptor-independent fashion and, remarkably, recycle themselves after each round of degradation. The study, published in Nature Biotechnology, demonstrates that McR-TACs can durably eliminate both cell membrane proteins and extracellular proteins in mouse models of triple-negative breast cancer, suppressing tumor growth through the sustained removal of biologically important disease drivers.
To appreciate why this development matters, it helps to understand the logic of targeted protein degradation. Small-molecule inhibitors block the activity of a protein, but degraders eliminate the protein itself, which can be far more effective when the target protein has scaffolding roles, non-enzymatic functions, or the ability to rebound after inhibition. Intracellular protein degradation has been revolutionized by proteolysis-targeting chimeras, or PROTACs, which hijack the ubiquitin-proteasome system to tag unwanted intracellular proteins for destruction. But an estimated 40 percent of the proteome, including most receptors, ligands, and secreted factors implicated in cancer, inflammation, and fibrosis, resides outside the cell or on its surface, beyond the reach of the proteasome. For these targets, the lysosome is the natural destination, and a family of technologies collectively known as lysosome-targeting chimeras, or LYTACs, was conceived to exploit it.
The original LYTAC concept is elegant in principle. A bifunctional molecule carries one binding arm directed at a protein of interest and another directed at a cell-surface receptor that constitutively traffics to the lysosome, such as the cation-independent mannose-6-phosphate receptor or the asialoglycoprotein receptor. By physically linking the target protein to such a receptor, the chimera tricks the cell’s endocytic machinery into internalizing the target along with the receptor. Once in the acidic environment of the lysosome, both the target protein and the chimera are degraded. This mechanism, however, carries two fundamental weaknesses that have limited the technology’s reach. First, degradation is absolutely dependent on the expression level of the shuttling receptor, which varies enormously between cell types, tissues, and disease states, and can itself be saturated or downregulated. Second, because the receptor and the chimera are destroyed along with the cargo, each round of degradation consumes degrading machinery and drug molecules alike, imposing a stoichiometric burden that demands high and repeated dosing and risks depletion of the very receptors the cell needs for its own housekeeping.
The new study set out to solve both problems simultaneously by asking a deceptively simple question: could a degrader exploit the cell’s own bulk transport pathways instead of a dedicated receptor, and could it survive the journey it asks the cell to make? The answer came in the form of a chimera built from a polyzwitterion, a synthetic polymer bearing a balanced arrangement of positive and negative charges, conjugated to a ligand that binds the protein of interest. Polyzwitterions occupy an unusual position in polymer science: their net-neutral charge surfaces resist nonspecific protein adsorption, yet certain architectures interact productively with the plasma membrane in ways that can induce membrane ruffling. The researchers harnessed this property deliberately, showing that their polyzwitterion-ligand conjugates trigger macropinocytosis, a form of endocytosis in which the cell engulfs large gulps of extracellular fluid and whatever solutes it contains, forming large vesicles called macropinosomes. Crucially, macropinocytosis is a receptor-independent process. The cell does not need to recognize a specific receptor-ligand pair; it simply drinks in the surrounding medium, and the chimera rides along with its bound target protein.
Once inside the cell, the chemistry of the endocytic pathway takes over. As endosomes acidify, dropping to a pH of around 5 to 6 in late endosomes and below 5 in lysosomes, the bond between the polyzwitterion carrier and the ligand for the protein of interest cleaves. This pH-responsive dissociation is the linchpin of the design. The target protein, released within the endolysosomal system, proceeds to the lysosome and is degraded by the acidic hydrolases waiting there. The chimera itself, however, is not condemned to the same fate. The researchers demonstrated that the liberated polyzwitterion is routed through the endoplasmic reticulum-Golgi transcytosis pathway, a cellular recycling route normally used to shuttle cargo across the cell from one membrane domain to another. Through this pathway, the intact chimera is exocytosed back into the extracellular space, ready to bind another molecule of its target protein and initiate another round of capture, internalization, release, and degradation. In effect, each McR-TAC molecule operates as a catalytic degrader, turning over many copies of its target rather than being consumed stoichiometrically with each one.
The functional consequences of this design were validated in biologically demanding settings. The team targeted programmed cell death ligand 1, better known as PD-L1, the transmembrane protein through which many tumors suppress antitumor T-cell immunity and which is a cornerstone target of the checkpoint inhibitor class of cancer immunotherapies. They also targeted macrophage migration inhibitory factor, or MIF, a secreted pro-inflammatory cytokine that promotes tumor progression, immune evasion, and metastasis. Both targets were chosen in part because they represent the two categories of proteins that LYTACs are meant to address: a cell-surface membrane protein and a soluble extracellular protein. In a triple-negative breast cancer mouse model, one of the most aggressive and difficult-to-treat breast cancer subtypes, McR-TACs directed against PD-L1 and MIF achieved durable depletion of both proteins from tumor tissue. The degradation was sustained rather than transient, a direct benefit of the recycling mechanism, and the effect translated into a significant inhibition of tumor growth in the treated animals.
The implications of receptor-independent, recyclable degradation extend well beyond these two targets. Receptor-dependent LYTACs are inherently tissue-restricted by receptor expression, which can be an advantage for targeted therapy but a severe limitation when the target cell does not express sufficient receptor or when the receptor is saturated by endogenous ligands. By invoking macropinocytosis, a process that many cells can perform, and whose activity is in fact frequently upregulated in cancer cells as part of their nutrient-scavenging metabolism, McR-TACs sidestep this dependency altogether. This may prove especially valuable in oncology, where the very cells a degrader must enter are often the most macropinocytically active cells in the body. The stoichiometric advantage is equally consequential. A degrader that recycles can achieve the same degree of target knockdown at lower administered doses and with longer effective duration of action, reducing manufacturing burden, cost, and the frequency of administration, all of which are decisive factors in whether a biological drug can reach the clinic.
The study also contributes a conceptual lesson that resonates beyond the specific chemistry involved: the cell’s natural transport pathways constitute a rich, largely untapped pharmacological toolbox. Rather than forcing cells to use artificial routes, McR-TACs recruit physiological processes, macropinocytosis for entry, endosomal acidification for release, and ER-Golgi transcytosis for exit, in a sequence that mirrors how the cell already moves material through its compartments. The authors suggest that this strategy of leveraging natural transport pathways for recyclable protein degradation could be adapted broadly, with different target-binding ligands grafted onto the recyclable polyzwitterion platform to address a wide range of membrane and extracellular proteins implicated in human disease.
Significant work remains before such molecules could approach human trials. The pharmacokinetics, immunogenicity, biodistribution, and long-term safety of polyzwitterion carriers must be characterized, and the efficiency of macropinocytosis induction will need to be validated across a broader spectrum of tissues and cell types. Dosing, formulation, and potential off-target degradation of bystander proteins drawn into macropinosomes all require careful scrutiny. Nevertheless, the demonstration that a single bifunctional molecule can repeatedly shepherd proteins to their destruction while returning intact for another cycle marks a conceptual milestone. If the recyclable paradigm generalizes, it could reshape the design logic of extracellular protein therapeutics much as catalytic turnover reshaped small-molecule drug discovery, turning what was once a stoichiometric war of attrition against disease proteins into a genuinely catalytic one.
Subject of Research: Development of macropinocytosis-mediated recyclable LYTACs (McR-TACs), receptor-independent, self-recycling chimeras for degradation of cell membrane and extracellular proteins
Subject of Research: Medicine
Article Title: Macropinocytosis-mediated recyclable LYTACs (McR-TACs) for receptor-independent protein degradation
Article References: Liu, P., Li, Y., Ma, T., You, Y., Chen, Y., Cai, M. Y., & Hu, Q. (2026). Macropinocytosis-mediated recyclable LYTACs (McR-TACs) for receptor-independent protein degradation. Nature Biotechnology. https://doi.org/10.1038/s41587-026-03302-1
Image Credits: AI Generated
DOI: 10.1038/s41587-026-03302-1
Keywords: lysosome-targeting chimeras, LYTACs, McR-TACs, macropinocytosis, protein degradation, polyzwitterion, receptor-independent, PD-L1, macrophage migration inhibitory factor, triple-negative breast cancer, transcytosis, targeted therapy
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Drew Townsend. (September 8, 2026). Recyclable McR-TACs enable receptor-independent degradation of extracellular proteins. Scienmag. https://scienmag.com/recyclable-mcr-tacs-enable-receptor-independent-degradation-of-extracellular-proteins/
Drew Townsend. “Recyclable McR-TACs enable receptor-independent degradation of extracellular proteins.” Scienmag, 8 September 2026, https://scienmag.com/recyclable-mcr-tacs-enable-receptor-independent-degradation-of-extracellular-proteins/. Accessed 8 September 2026.
Drew Townsend. “Recyclable McR-TACs enable receptor-independent degradation of extracellular proteins.” Scienmag. September 8, 2026. https://scienmag.com/recyclable-mcr-tacs-enable-receptor-independent-degradation-of-extracellular-proteins/
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