Polyamine-blocking therapy reverses immunosuppression in the tumor microenvironment.

Correcting T-cell immunosuppression may unleash powerful antitumor responses; however, knowledge about the mechanisms and modifiers that may be targeted to improve therapy remains incomplete. Here, we report that polyamine elevation in cancer, a common metabolic aberration in aggressive lesions, contributes significantly to tumor immunosuppression and that a polyamine depletion strategy can exert antitumor effects that may also promote immunity. A polyamine-blocking therapy (PBT) that combines the well-characterized ornithine decarboxylase (ODC) inhibitor difluoromethylornithine (DFMO) with AMXT 1501, a novel inhibitor of the polyamine transport system, blocked tumor growth in immunocompetent mice but not in athymic nude mice lacking T cells. PBT had little effect on the proliferation of epithelial tumor cells, but it increased the number of apoptotic cells. Analysis of CD45(+) tumor immune infiltrates revealed that PBT decreased levels of Gr-1(+)CD11b(+) myeloid suppressor cells and increased CD3(+) T cells. Strikingly, in a model of neoadjuvant therapy, mice administered with PBT one week before surgical resection of engrafted mammary tumors exhibited resistance to subsequent tumor rechallenge. Collectively, our results indicate that therapies targeting polyamine metabolism do not act exclusively as antiproliferative agents, but also act strongly to prevent immune escape by the tumor. PBT may offer a general approach to heighten immune responses in cancer.

Development of polyamine transport ligands with improved metabolic stability and selectivity against specific human cancers.

Polyamine homeostasis is critical for life and is accomplished via a balance of polyamine biosynthesis, degradation, and transport. Rapidly dividing cancer cells have been shown to have high polyamine transport activity compared to normal cells, likely due to their high requirement for polyamine metabolites. The polyamine transport system (PTS) is a therapeutically relevant target, as it can provide selective drug delivery to cancer cells. This report describes the synthesis and biological evaluation of multimeric polyamine derivatives as efficient PTS ligands. Arylmethyl-polyamine derivatives were synthesized to address two important concerns in PTS drug design: (a) PTS selectivity and (b) stability to amine oxidases. N(1),N(1')-[Naphthalene-1,4-diylbis(methylene)]bis{N(4)-[4-(methylamino)butyl])butane-1,4-diamine}, 3b, was found to have an optimal balance between these parameters and demonstrated excellent targeting of melanoma (e.g., MALME-3M) and breast cancer cells (e.g., T47D) over other cancer cell lines. These results provide a method to selectively target cancers via their intrinsic need for polyamine metabolites.