Although correcting mechanisms of T cell immunosuppression may unleash powerful antitumor responses, there remains incomplete knowledge about mechanisms and modifiers that may be targeted to improve therapy. 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) combining the well-characterized ODC inhibitor difluoromethylornithine (DFMO) with AMXT1501, 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 PBT one week before surgical resection of engrafted mammary tumors exhibited resistance to subsequent tumor re-challenge. Collectively, our results indicate that therapies targeting polyamine metabolism do not act exclusively as anti-proliferative agents, but also act strongly to prevent immune escape by the tumor. PBT may offer a general approach to heighten immune responses in cancer.
- Received August 8, 2013.
- Revision received September 16, 2013.
- Accepted September 29, 2013.
- Copyright © 2013, American Association for Cancer Research.