Abstract Purpose: HER3 is frequently overexpressed in a range of solid tumors and contributes to resistance against both targeted and chemotherapies. To address this, we developed a triple-function therapeutic approach using HER3-targeted HPK nanocapsid (NC) to deliver FOXC1-specific siRNA, thereby silencing this master transcriptional regulator of tumor aggressiveness and metastasis. The siRNA was synthesized with T7 polymerase to incorporate a 5′-triphosphate (5′-ppp) moiety, which serves as an activator of cellular immune responses. This strategy enables targeted and protected delivery of siRNA cargo, while simultaneously achieving FOXC1 silencing and tumor-intrinsic activation of cellular Interferon response. Methods: HPK proteins were engineered to self-assemble into virus-like NCs capable of encapsulating siRNA. We evaluated the properties, stability, and morphology of the nanoparticles. HPK design incorporates the adenovirus penton base protein for membrane penetration and neuregulin for HER3-targeted binding. T7-transcribed FOXC1 siRNAs (HSiFox-T7) were packaged into HPK NCs and evaluated in HER3+ tumor cell lines and mouse models. We assessed cytokine production, cell viability, interferon role in cell cytotoxicity, FOXC1 knockdown, migration, immune cell infiltration, and in vivo therapeutic efficacy in both HER3+ immunodeficient melanoma and immunocompetent triple-negative breast tumor (TNBC) models. Results: The HPK and T7-transcribed siRNA formed stable and smaller nanoparticles compared to synthetic siRNAs. In vitro, T7-derived 5′-ppp siRNAs induced greater cytotoxicity than synthetic controls mainly via activated type I interferon response. HSiFox-T7 achieved robust FOXC1 knockdown, significantly reducing cell migration and viability. In vivo, systemic administration of HSiFox-T7 suppressed tumor growth and increased type I interferon response in immunocompromised HER3+ melanoma models. It also suppressed FOXC1, reduced metastatic lung burden, and increased tumor apoptosis with minimal off-target toxicity while protecting siRNA content in the circulation of immunocompetent TNBC mice model. Treatment enhanced infiltration of NK cells and CD8+ T cells into both primary tumors and metastases, indicating strong immunostimulatory effects. Conclusions: HPK NCs represent a multifunctional platform that combines HER3-targeted delivery, master gene silencing, and immune activation. The HSiFox-T7 system effectively suppressed HER3+ tumor growth and metastasis while reprogramming the tumor microenvironment toward immune responsiveness. This triple-action strategy overcomes major barriers in siRNA therapeutics including protecting nucleic acid content in circulation and avoiding off-target effects while offering translational potential for treating aggressive cancers such as TNBC and melanoma. Citation Format: Amirhesam Babajani, Felix Alonso-Valenteen, James Teh, Nelyda Gonzalez, Simoun Mikhael, Michelle Wong, Xiaojiang Cui, Ravinder Abrol, Lali K. Medina-Kauwe. HER3-targeted nano-bioparticles deliver 5′-triphosphate-modified FOXC1 siRNA to induce tumor regression and immune activation in HER3+ cancers abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1772.
Babajani et al. (Fri,) studied this question.