A selective RPL15 PROTAC degrader enhances anti-PD-1 immunotherapy in a murine melanoma tumor model

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.