Effect of Pazopanib on Myeloid-Derived Suppressor Cells and T-cell Function in Patients with Metastatic Renal Cell Carcinoma.

455 Background: Tyrosine kinase inhibitors have been shown to have an effect on T cell function. Sunitinib decreases immunosuppression in patients with metastatic renal cell carcinoma (mRCC) as seen by a reduction in myeloid derived suppressor cells (MDSC) that inhibit T cell function. The effect of pazopanib on immune function is unknown. Methods: Peripheral blood mononuclear cells from 21 patients with mRCC treated with pazopanib were thawed for cycle 1 day 1, cycle 1 day 28, cycle 2 day 28, and cycle 4 day 28. Total MDSC and neutrophilic, monocytic, and lineage-negative MDSC subsets were measured by flow cytometry. T cell response was measured by interferon-gamma production after in vitro stimulation by anti-CD3/anti-CD28 beads. Pre-treatment cycle 1 day 1 levels were compared to cycle 4 day 28. Results: In mRCC patients, MDSCs comprise 4.7 % (median, range 1.7-32.9 %) of the peripheral blood mononuclear cells pre-treatment. Neutrophilic MDSC subset is the most prevalent at 2.0 % (median, range 0.3-30.4 %). Monocytic MDSCs comprise 1.5 % (median, range 0.4-5.3 %), and lineage-negative MDSCs comprise only 0.15 % (median, range 0.01-1.03 %). We found that pazopanib does not significantly decrease the percentage of MDSCs (total or subpopulations) over time with the exception of a modest decrease in the lineage-negative MDSC subset (p = 0.08). In terms of T cell function, pre-treatment level of CD3+ interferon-gamma was found to be 10.6 % (median, range 1.3-22.3 %). In contrast, pazopanib significantly increased CD3+ interferon-gamma level over time to 18.1 % (median, range 3.0-25.0 %), p = 0.03. Conclusions: Our study shows that pazopanib does not significantly reduce MDSC levels in patients with mRCC. However, pazopanib improves T cell function over time, as seen by a significant increase in CD3+ interferon-gamma production.