A novel polyamine blockade therapy activates an anti-tumor immune response

Most tumors maintain elevated levels of polyamines to support their growth and survival. This study explores the anti-tumor effect of polyamine starvation via both inhibiting polyamine biosynthesis and blocking the upregulated import of polyamines into the tumor. We demonstrate that polyamine blockade therapy (PBT) co-treatment with both DFMO and a novel polyamine transport inhibitor, Trimer PTI, significantly inhibits tumor growth more than treatment with DFMO or the Trimer PTI alone. The anti-tumor effect of PBT was lost in mice where CD4⁺ and CD8⁺ T cells were antibody depleted, implying that PBT stimulates an anti-tumor immune effect that is T-cell dependent. The PBT anti-tumor effect was accompanied by an increase in granzyme B⁺, IFN-γ⁺ CD8⁺ T-cells and a decrease in immunosuppressive tumor infiltrating cells including Gr-1⁺CD11b⁺ myeloid derived suppressor cells (MDSCs), CD4⁺CD25⁺ Tregs, and CD206⁺F4/80⁺ M2 macrophages. Stimulation with tumor-specific peptides elicited elevated antigen-specific IFN-γ secretion in splenocytes from PBT-treated mice, indicating that PBT treatment stimulates the activation of T-cells in a tumor-specific manner. These data show that combined treatment with both DFMO and the Trimer PTI not only deprives polyamine-addicted tumor cells of polyamines, but also relieves polyamine-mediated immunosuppression in the tumor microenvironment, thus allowing the activation of tumoricidal T-cells.