PDE-5 Inhibition Decreases Cancer Tumor Burden through Immunomodulation

Myeloid-derived suppressor cells (MDSC) are CD14-/CD11b+/Gr1+ cells that have been shown to suppress T-cell anti-tumor activity. They do so namely by using arginase and iNOS to deplete the microenvironment of L-arginine and producing unopposed NO, respectively. Sildenafil, a PDE5 inhibitor, has been shown to decrease tumor burden and increase T-cell tumor infiltration, though its mechanism is poorly understood. This study aimed at determining the mechanisms by which Sildenafil affected MDSC suppression mechanisms on cytotoxic T-cells and investigate the effects on invasive 4T1 cancer cells both in vitro and in vivo.

In our in vitro model, lymphoid cells were obtained from 4T1 tumor bearing BalbC mice 14 days after exposure and extracted using a magnetic bead separation technique, then expanded for 6 days using ionomicin, bryostatin and IL-2 pulsed for 24 hours followed by IL-7 and IL-15 in RPMI and FBS media. MDSCs were obtained from the spleens of 4T1 exposed mice. The sildenafil-exposed MDSCs showed a significant decrease in arginase activity. Additionally, when cultured in sildenafil and media, 4T1 cells showed a 50% reduction in growth. In this same model, T-cells had a 13% increase in growth with a therapeutic dose of sildenafil.

In vivo studies were performed using BalbC mice injected with 4T1 cells and treated with sildenafil injections as well as PBS control injections. Spleens were harvested at day 14 and purified using flow cytometry to GR1 and CD11b antigens. Sildenafil-treated mice showed a 43% decrease in tumor burden and a significant decrease in arginase activity, though iNOS levels showed no significant difference between groups. The MDSC pathway remains a promising area of study in allowing activated T-cells to decrease tumor and the use of sildenafil has proven effective in reducing MDSC activity through decreases in arginase activity.