Phosphodiesterase-5 (PDE5) inhibitors (sildenafil, tadalafil, and vardenafil) are agents currently in clinical use for nonmalignant conditions. We report the use of PDE5 inhibitors as modulators of the antitumor immune response. In several mouse tumor models, PDE5 inhibition reverses tumor-induced immunosuppressive mechanisms and enables a measurable antitumor immune response to be generated that substantially delays tumor progression. In particular, sildenafil, down-regulates arginase 1 and nitric oxide synthase–2 expression, thereby reducing the suppressive machinery of CD11b+/Gr-1+ myeloid-derived suppressor cells (MDSCs) recruited by growing tumors. By removing these tumor escape mechanisms, sildenafil enhances intratumoral T cell infiltration and activation, reduces tumor outgrowth, and improves the antitumor efficacy of adoptive T cell therapy. Sildenafil also restores in vitro T cell proliferation of peripheral blood mononuclear cells from multiple myeloma and head and neck cancer patients. In light of the recent data that enzymes mediating MDSC-dependent immunosuppression in mice are active also in humans, these findings demonstrate a potentially novel use of PDE5 inhibitors as adjuncts to tumor-specific immune therapy.

Cancers display distinct patterns of organ-specific metastasis. Comparative analysis of a broad array of cell membrane molecules on a liver-metastasizing subline of B16 melanoma versus the parental B16-F0 revealed unique up-regulation of integrin α2. The direct role of integrin α2 in hepatic metastasis was shown by comparison of high versus low-expressing populations, antibody blockade, and ectopic expression. Integrin α2–mediated binding to collagen type IV (highly exposed in the liver sinusoids) and collagen type IV–dependent activation of focal adhesion kinase are both known to be important in the metastatic process. Analysis of primary colorectal cancers as well as coexisting liver and lung metastases from individual patients suggests that integrin α2 expression contributes to liver metastasis in human colorectal cancer. These findings define integrin α2 as a molecule conferring selective potential for formation of hepatic metastasis, as well as a possible target to prevent their formation.

The liver represents a major and frequently sole site of metastases for many types of cancer, particularly gastrointestinal cancers. We showed previously that coadministration of an engineered hepatic-targeting Listeria monocytogenes (LM) with a cancer vaccine enhanced the antitumor effect of vaccine-induced T cells selectively against hepatic metastases. Here, we show that administration of multiple doses of LM, in the absence of vaccine, generates therapeutic responses against hepatic metastases. LM treatment of mice bearing hepatic metastases induced tumor-specific CD8 + T-cell responses that were enhanced by depletion of regulatory T (Treg) cells by either anti-CD25 or cyclophosphamide treatment. Antitumor activity of LM further depended on natural killer (NK) cell activation but was inhibited by presence of a subset of NK T cells. These results show the utility of LM in the treatment of hepatic metastases even in the absence of vaccine administration and further suggest that blockade of Treg cells and NK T cells will enhance antitumor activity.

PurposeDespite advances in therapy for the treatment of metastatic colorectal cancer, many patients die of hepatic disease. Current immunotherapeutic strategies are likely limited by inhibitory signals from the tumor. To successfully eliminate tumor deposits within an organ, an appropriate immunologic milieu to amplify antitumor responses must be developed.MethodsWe used a murine model utilizing the CT26 colon cancer cell line to analyze primary and memory tumor-specific T-cell responses induced by an attenuated actin A and internalin B deleted immunodominant tumor-associated antigen expressing strain of Listeria monocytogenes for the treatment of metastatic colorectal cancer.ResultsTreatment of mice bearing established hepatic metastases with this L. monocytogenes strain led to the generation of a strong initial tumor-specific cytotoxic CD8+ T-cell response that successfully treated 90% of animals. Tumor antigen-specific central and effector memory T cells were also generated and protected against tumor rechallenge. These cell populations, when measured before and after tumor rechallenge, showed a marked expansion of antigen-specific effector CD8+ effector memory T cells. This strain of L. monocytogenes was able to down-modulate the expression of the immune checkpoint molecule, PD-1, within the tumor microenvironment but had variable effects on CTLA-4 expression.ConclusionsThis L. monocytogenes strain generated a highly effective antitumor T-cell response, providing a basis for the development of this vaccine platform in patients with liver metastases.Electronic supplementary materialThe online version of this article (doi:10.1245/s10434-011-2037-0) contains supplementary material, which is available to authorized users.