Abstract 6690: A novel B7H3-targeting macrocyclic peptide conjugate efficiently delivers siRNAs to cancer cells and tumor endothelium

Abstract Short-interfering RNAs (siRNAs) can treat a wide range of human diseases from viral outbreaks to cancer, however, targeted siRNA delivery remains a significant challenge. Recently, significant efforts have focused on developing novel delivery technologies for siRNAs, targeting tumors and other extrahepatic tissues. B7-H3 (CD276), a B7 family of immune checkpoint proteins, is highly expressed in cancer cells and tumor endothelial cells (TECs). We employed an mRNA display campaign against the hB7-H3 protein and discovered a macrocyclic binding peptide, JP5. Using surface plasmon resonance, we confirmed specific binding of JP5 to both human and murine B7-H3 with a KD of approximately 770nM and 600nM, respectively. Characterization of protease stability showed that greater than 80% of the peptide remains intact after incubation with 50% serum for 24h. Further, our team conjugated JP5 to chemically stabilized siRNAs labeled with a Cy5 fluorophore to observe biodistribution of siRNAs, including cancer cells and and TECs. Mice were implanted with A431 WT or B7H3 KO cells, and upon xenograft tumor formation, mice were given PBS (control) or JP5-siRNA-Cy5 (n=5 mice/group). Gross biodistribution by IVIS imaging was found in the tumors, skin, bowel, liver and kidney, with minimal accumulation in the heart and lungs. Flow cytometry and immunofluorescence revealed that JP5 conjugation directed siRNAs to ∼80% of TECs and to 40-60% of cancer cells, with a significant reduction in B7H3 KO cells. In conclusion, the JP5 macrocycle delivers siRNAs to B7H3-expressing cancer cells and TECs in the tumor microenvironment, paving a path forward for sophisticated cancer therapeutics. Citation Format: Danyan Li, Lincy Edatt, Jillian Perry, Avery Huber, William David Green, Jarred Michael Green, Justin Milner, Albert Bowers, Chad Pecot. A novel B7H3-targeting macrocyclic peptide conjugate efficiently delivers siRNAs to cancer cells and tumor endothelium [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6690.