An agent that downregulates the activity of the anti-inflammatory cytokine human interleukin-10 (IL-10), with potential immunomodulating and antineoplastic activities. Upon administration, IL-10 immunomodulator MK-1966 blocks the activity of IL-10 and may abrogate the IL-10-induced immunosuppressive tumor microenvironment. This activates cell-mediated immunity against cancer cells, increases cytokine production, including interferon-gamma (IFN-g), decreases T regulatory cell (Treg) activity, and induces a tumor-specific cytotoxic CD8+ T-cell-mediated immune response, which enhances tumor cell death.