Smac mimetics (SMs) constitute a class of compounds that target the inhibitor of apoptosis proteins (IAPs) and enhance the cytotoxic activity of several drugs. In our work, we built and characterized a library of about 140 SMs and focused on SM83 due to its high affinity for the targets, cytotoxic activity and good pharmacokinetic profile. In vivo, SM83 reduced in monotherapy the primary tumor growth of two triple negative breast cancer xenografts. Furthermore, SM83 treatment, alone or in combinations with TRAIL-armed CD34+cell, resulted in the reduction of spontaneous lung metastasis formation. Mechanistically, by depleting cIAP1, SM83 affects the expression of the tumor genes and inhibits the metastasis-promoting gene Snai2, thus preventing cancer cell motility. Moreover, we tested SM83 as a standalone in ascites cancer models and described an in vivo anti-tumor effect against cancer cell lines that are intrinsically resistant to SM treatment in vitro. In the in vivo settings, SM83, in fact, triggered an inflammation event of the host, characterized by macrophage secretion of TNF, IL-1β and interferon-γ (IFNγ), and rapidly killed floating tumor cells within the ascites by a non-apoptotic mechanism. Of note, SM83 treatment caused the massive accumulation of neutrophils within the ascites and tumor nodules, which, however, was not responsible for cancer cell killing. Finally, we described the capability of SM83 to enhance the cytotoxic activity of camptothecin especially in human epithelial cells expressing oncogenic KRAS. The increased sensitivity of these premalignant cells is caused by an ERK2-dependent up-regulation of NOXA. Of note, oncogenic KRAS fails to sensitize a panel of isogenic cancer cell lines with wild type and mutated KRAS, and we demonstrated that this unresponsiveness could be reverted by concomitant inhibition of AKT. Therefore, our work suggests that the activation of AKT is capable of counterbalancing the potential pro-death stimulus triggered by oncogenic KRAS
