New “Molecular Scissors” Pep-TACs Launched, Revolutionary Progress in Cancer Immunotherapy

In the field of cancer treatment, immune checkpoint inhibitors (such as PD-1/PD-L1 antibodies) have sparked a revolution, but challenges like resistance and tumor penetration have remained persistent. This is especially true for brain tumor patients, where the blood-brain barrier (BBB) acts as an impenetrable wall, causing over 90% of drugs to fail.

Recently, the team led by Yanfeng Gao from the School of Pharmaceutical Sciences, Sun Yat-sen University (Shenzhen), published research in Nature Communications showing that they have developed a novel platform called Pep-TACs, a covalent peptide-based lysosome-targeting degradation system for cancer immunotherapy.

Immune Checkpoint Inhibitors

In the past decade, immune checkpoint inhibitors (ICIs) have become a major breakthrough in oncology, significantly enhancing the body’s anti-tumor response by blocking tumor immune evasion mechanisms. These inhibitors have become an indispensable treatment strategy in clinical oncology. The most prominent drugs among these inhibitors are anti-PD-1/PD-L1 monoclonal antibodies. When PD-1 and PD-L1 bind, they transmit a negative regulatory signal to T-cells, preventing them from recognizing cancer cells, allowing tumor cells to escape immune detection. PD-1/PD-L1 inhibitors bind to PD-1 or PD-L1, blocking their interaction, thus restoring immune cells' ability to recognize and destroy cancer cells, preventing immune evasion (Figure 1).

Figure 1: Mechanism of action of PD-1 and PD-L1 monoclonal antibodies

Since the first CTLA-4 inhibitor, ipilimumab, was approved by the FDA in 2011, global pharmaceutical companies have introduced multiple innovative PD-1/PD-L1-targeted drugs. These formulations reshape the immune recognition mechanisms within the tumor microenvironment, successfully pushing the frontline treatment of various malignant tumors to new heights. As of the end of 2024, 24 ICIs have been approved in both the US and China, with 13 PD-1, 7 PD-L1, 1 PD-1/CTLA-4, 2 CTLA-4, and 1 LAG-3 drugs. The US has approved 13 ICIs, including 7 PD-1, 3 PD-L1, 2 CTLA-4, and 1 LAG-3 drugs; China has approved 18 ICIs , including 10 PD-1, 6 PD-L1, 1 CTLA-4, and 1 PD-1/CTLA-4 drug.

However, as clinical applications deepen, challenges like drug resistance and poor penetration of solid tumors have gradually surfaced, limiting their effectiveness (Figure 2). Drug resistance causes originally effective drugs to lose their potency, allowing tumor cells to regain the ability to grow and spread. At the same time, the difficulty of penetrating solid tumors also greatly restricts the efficacy of immune checkpoint inhibitors.

Figure 2: Resistance mechanisms of antibody therapy targeting the PD-1 pathway

Emergence and Limitations of LYTACs

Traditional targeted protein degradation technologies, such as PROTACs, mainly rely on the ubiquitin-proteasome system but cannot effectively clear extracellular and membrane-bound proteins (which account for 40% of the human genome). In July 2020, Professor Carolyn R. Bertozzi's team at Stanford University developed a lysosome-targeting chimera (LYTACs) technology, which achieves membrane protein degradation by binding the target protein to lysosomal receptors (such as M6PR) (Figure 3).

Figure 3: Mechanism of action of LYTACs

LYTAC is a bifunctional molecule with two binding domains. One end is an oligomeric sugar peptide that binds to the cell surface transmembrane receptor CI-M6PR, and the other end is an antibody or small molecule that binds to the target protein. These two binding domains are connected by a chemical linker. The CI-M6PR–LYTAC–target protein complex formed on the plasma membrane is engulfed by the cell membrane to form a vesicle. The vesicle transports the complex to the lysosome, where the target protein is degraded.

Figure 4: Identification of cellular determinants of LYTAC-mediated membrane protein degradation

In October 2023, Professor Carolyn R. Bertozzi's team published further research in the journal Science, revealing more detailed regulatory mechanisms of LYTACs (Figure 4). The study found that a large portion of the cell surface CI-M6PR is occupied by glycoproteins modified with endogenous mannose-6-phosphate (M6P), meaning that LYTACs have nowhere to bind. By inhibiting the biosynthesis of M6P, the proportion of unoccupied receptors on the cell surface increases, which in turn enhances the internalization of the CI-M6PR-LYTAC-target protein complex. However, LYTACs face three major fatal flaws:

• Synthesis Difficulty: LYTACs are antibody-small molecule conjugates, and their synthesis process is complex, which increases research and production costs, limiting their large-scale production and clinical application.
• Poor Tumor Penetration: Antibody-based LYTACs have difficulty penetrating solid tumors. Solid tumors have complex structures and microenvironments, including dense extracellular matrices and abnormal blood vessel systems, which obstruct LYTACs from entering the tumor and impacting their degradation effects on intracellular target proteins.
• Poor Blood-Brain Barrier Penetration: The blood-brain barrier is a specialized structure designed to protect the brain and strictly limits many substances from entering the brain. LYTACs are unable to cross the blood-brain barrier, significantly limiting their effectiveness in treating brain diseases, such as brain tumors, where they cannot effectively target brain proteins.

These various limitations present significant challenges for LYTACs in clinical research and applications, hindering their further development and promotion.

Pep-TACs: A Precision “Molecular Scissors”

To address the shortcomings of existing membrane protein degradation technologies, Yanfeng Gao's team at Sun Yat-sen University took a different approach. They discovered that transferrin receptor (TFRC) is highly expressed in both tumor cells and blood-brain barrier endothelial cells, with efficient circulation characteristics. Using this as a breakthrough point, the team designed a covalent peptide degradation platform based on TFRC called Pep-TACs, aimed at cancer immunotherapy, offering a new direction for solving membrane protein degradation and cancer treatment challenges.

The researchers introduced a flexible aromatic sulfonyl fluoride group and conjugated a structurally stable and high-affinity D-type peptide, DT7, which specifically targets TFRC, with a covalent peptide, OPBP1, that targets PD-L1, forming a chimeric peptide. They found that the chimeric peptide formed by linking DT7 and OPBP1 could strongly bind to TFRC and PD-L1, inducing tumor cells with overexpressed TFRC to undergo endocytosis. To enhance the flexibility of the aromatic sulfonyl fluoride group (ASF), a long flexible side chain was introduced with D-lysine (k) to modify ASF, and a series of Pep-TACs capable of covalently binding to PD-L1, such as f12x, s11x, y10x, and v9x, were synthesized (Figure 5).

Figure 5: Design optimization and functional verification of pep-tac

Pep-TACs promote target protein degradation through a TFRC-mediated lysosome-targeted endocytosis mechanism. Experiments show that they can significantly reduce PD-L1 expression on tumor cells, dendritic cells, and macrophages, enhancing T-cell function and macrophage-mediated tumor phagocytosis. In various tumor models, Pep-TACs trigger significant anti-tumor immune responses, can cross the blood-brain barrier (Figure 6), inhibit brain tumor growth, extend mouse survival, and induce tumor-specific immune memory.

Figure 6: In vivo distribution and blood-brain barrier translocation of PEP-Tac

Conclusion

The birth of Pep-TACs marks the entry of targeted protein degradation technology into the “membrane protein era.” Compared to traditional LYTACs, Pep-TACs offer unique advantages, such as being unaffected by endogenous ligand competition, being able to target various cancers, and crossing the blood-brain barrier. They provide a more efficient and specific tool for membrane protein degradation, advancing the progress of this technology.

Reference

[1]Tang Q, Chen Y, Li X, et al. The role of PD-1/PD-L1 and application of immune-checkpoint inhibitors in human cancers. Front Immunol. 2022;13:964442.

[2]Vesely MD, Zhang T, Chen L. Resistance Mechanisms to Anti-PD Cancer Immunotherapy. Annu Rev Immunol. 2022;40:45-74.

[3]Banik SM, Pedram K, Wisnovsky S, Ahn G, Riley NM, Bertozzi CR. Lysosome-targeting chimaeras for degradation of extracellular proteins.Nature. 2020;584(7820):291-297.

[4]Ahn G, Riley NM, Kamber RA, et al. Elucidating the cellular determinants of targeted membrane protein degradation by lysosome-targeting chimeras.Science. 2023;382(6668):eadf6249.

[5]Xiao Y, He Z, Li W, et al. A covalent peptide-based lysosome-targeting protein degradation platform for cancer immunotherapy. Nat Commun. 2025;16(1):1388.