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Updated: Jan 10, 2026
Experimental Melanoma Immunotherapy Model Using Tumor Vaccination with a Hematopoietic Cytokine
Published on: February 24, 2023
A selective RPL15 PROTAC degrader enhances anti-PD-1 immunotherapy in a murine melanoma tumor model
Runa Takahashi1, Kazuki Yamamoto2, Hikaru Toya3
1Department of Immunology, Graduate School of Pharmaceutical Sciences, Hokkaido University, Hokkaido, Japan.
Abstract:
Damage-associated molecular patterns (DAMPs) are secreted from damaged or dying cells and activate innate immune signaling via pattern-recognition receptors such as Toll-like receptors and cGAS. We previously showed that topotecan, a chemotherapeutic drug and topoisomerase I (TOP1) inhibitor, binds to ribosomal protein RPL15 and induces the secretion of DAMPs from cancer cells, which activate cGAS-STING signaling in dendritic cells. RPL15-knockdown B16-F10 melanoma tumors were sensitized to anti-PD-1 antibody, suggesting that RPL15 inhibition may have the potential to improve immune checkpoint inhibitor efficacy. However, topotecan and its derivatives, including SN-38, are highly cytotoxic because of their TOP1 inhibitory activity. Here, we synthesized SN-38-conjugated pomalidomide (SN38-PROTAC) and showed that SN38-PROTAC induced ubiquitin-mediated degradation of RPL15, but not TOP1. SN38-PROTAC treatment induced DAMP secretion from cancer cells, which activated cGAS-STING signaling in dendritic cells. The cytotoxicity of SN38-PROTAC in MCF7 cells was 100-fold lower than SN-38. SN38-PROTAC treatment increased the CTL/Treg ratio in tumors and sensitized B16-F10 tumors to anti-PD-1 antibody in a mouse model. The enhanced antitumor effects of SN38-PROTAC and anti-PD-1 antibody combination were abolished in STING-deficient mice. Our results indicate that SN38-PROTAC, which induces RPL15 degradation, has the potential to enhance ICI efficacy in PD-1-resistant cancer with low cytotoxicity.
Insights
SN38-PROTAC targets ribosomal protein RPL15 for degradation, inducing damage-associated molecular patterns (DAMPs) and enhancing anti-PD-1 therapy efficacy. This novel approach shows potential for treating immune checkpoint inhibitor-resistant cancers with reduced toxicity.
Area of Science:
- Immunology
- Oncology
- Drug Discovery
Background:
- Damage-associated molecular patterns (DAMPs) activate innate immunity via pattern-recognition receptors.
- Topotecan, a TOP1 inhibitor, induces DAMP secretion by binding RPL15, enhancing anti-PD-1 efficacy in preclinical models.
- Current TOP1 inhibitors like SN-38 exhibit high cytotoxicity.
Purpose of the Study:
- To develop a novel SN-38 derivative with reduced cytotoxicity but retained DAMP-inducing properties.
- To investigate the potential of SN38-PROTAC to enhance immune checkpoint inhibitor (ICI) efficacy.
- To elucidate the mechanism of SN38-PROTAC-mediated immune activation.
Main Methods:
- Synthesis of SN-38-conjugated pomalidomide (SN38-PROTAC).
- Assessment of SN38-PROTAC-induced RPL15 degradation and cytotoxicity in cancer cells.
- Evaluation of DAMP secretion and cGAS-STING signaling activation in dendritic cells.
- Testing the efficacy of SN38-PROTAC in combination with anti-PD-1 antibody in B16-F10 melanoma mouse models, including STING-deficient controls.
Main Results:
- SN38-PROTAC induced ubiquitin-mediated degradation of RPL15, not TOP1.
- SN38-PROTAC exhibited 100-fold lower cytotoxicity than SN-38 in MCF7 cells.
- SN38-PROTAC treatment stimulated DAMP secretion, activated cGAS-STING signaling, increased the CTL/Treg ratio in tumors, and sensitized B16-F10 tumors to anti-PD-1 therapy.
- Combination therapy efficacy was dependent on STING signaling.
Conclusions:
- SN38-PROTAC effectively induces RPL15 degradation and DAMP release with significantly reduced cytotoxicity.
- SN38-PROTAC demonstrates potential as a strategy to enhance immune checkpoint inhibitor efficacy in PD-1-resistant cancers.
- Targeting RPL15 degradation offers a promising therapeutic avenue for overcoming resistance to immunotherapy.
