Interferon-γ Is Induced in Human Peripheral Blood Immune Cells In Vitro by Sodium Stibogluconate/Interleukin-2 and Mediates Its Antitumor Activity In Vivo

Sodium stibogluconate (SSG), an inhibitor of SHP-1 that negatively regulates cytokine signaling and immunity, suppressed growth of murine Renca tumors in combination with interleukin-2 (IL-2) via a T-cell-dependent mechanism. The ability of SSG to interact with IL-2 in activating primary human immune cells was evaluated herein by assessing its induction of interferon (IFN)-gamma(+) TH1 cells in human peripheral blood in vitro. The significance of IFN-gamma(+) cells was also investigated by assessing SSG/IL-2 antitumor activity in wild-type and IFN-gamma(-/-) mice. IFN-gamma(+) cells but not IL-5(+) cells were induced markedly (9.1x) in healthy peripheral blood by SSG/IL-2 in contrast to the modest induction by SSG alone (2.1x) at its clinically achievable dose (20 mu g/mL) or by IL-2 (3.1x) at its C-max of low-dose schedule (30 IU/mL). SSG at a higher dose ( 100 mu g/mL) was less effective alone (1.5x) or in combination with IL-2 (7.8x). Peripheral IFN-gamma(+) cells were induced after 4 or 16 h treatment with SSG/IL-2 within CD4(+) and CD8(+) lymphocytes coincided with heightened CD69 expression (similar to 3-4x). SSG/IL-2 was also more effective than the single agents in inducing IFN-gamma(+) cells in the peripheral blood of melanoma patients, whose basal IFN-gamma(+) cell levels were similar to 5% of healthy controls. Renca tumor growth was inhibited by SSG/IL-2 in wild-type but not IFN-gamma(-/-) mice. These results demonstrate SSG interactions with IL-2 in vitro to activate key antitumor immune cells in peripheral blood of healthy and melanoma donors, providing further evidence for proof of concept clinical trials for effecting augmentation of IL-2 through inhibiting negative regulatory protein tyrosine phosphatases.