Abstract: The present invention generally relates to pharmacology and, in particular, to the treatment of tumors and other conditions. In some aspects, the invention is directed to the treatment of subjects having tumors or cancers that are metastatic. Surprisingly, it has been found that certain compositions comprising oxidized glutathione-based compounds are able to effectively treat such cancers by inhibiting cell migration and/or invasion processes, and thus, inhibiting tumor cell metastases. Without being bound by any theory, it is believed that such compositions are effective since the compositions are able to suppress the activation of critical signaling pathways within cells that are used for cell migration, such as the ErbB2 and/or phosphoinositide-3 kinase (PI3K) pathways, including the downstream RhoA and AKT pathways.