Targeted Inhibition of Oncogenic microRNAs miR-21, miR-17, and miR-155 Suppresses Tumor Growth and Modulates Immune Response in Colorectal Cancer

Background and Objectives: Aggressive cancer development is characterized by rapid tumor growth and progressive immune dysfunction. Tumor-derived microRNAs (miRNAs) emerge as master regulators of both malignant transformation and immune evasion, making them promising therapeutic targets. Using the highly aggressive CT-26 peritoneal adenomatosis model, this study explored the potential of selective miRNA inhibition to simultaneously suppress tumor growth and overcome immunosuppression. Methods and Results: Our results revealed that inhibition of miR-155, miR-21, and miR-17 by methylsulfonyl phosphoramidate (mesyl) oligonucleotides exhibited markedly different therapeutic profiles. miR-155 inhibition demonstrated minimal efficacy. miR-21 suppression provided early tumor regression and prevented cancer-associated thymic atrophy, translating into extended survival. miR-17 inhibition displayed delayed but superior tumor growth inhibition, significantly reducing pathologically elevated polymorphonuclear myeloid-derived suppressor cell (MDSC) populations, and nearly doubled animal lifespan. Combination therapy targeting all three miRNAs integrated these complementary mechanisms, maintaining consistent anti-tumor efficacy across early and late stages while providing thymic protection and MDSC reduction. Importantly, therapeutic responses in vivo substantially exceeded predictions based on in vitro tumor cell proliferation and motility measurements, revealing critical contributions of systemic immunomodulation. Conclusions: These findings demonstrate that miRNA inhibition reshapes tumor–immune interactions, positioning anti-miRNA therapeutics as immunomodulatory agents for effective colorectal cancer treatment.