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. Such pathways are regulated by ERp5, which is a 
protein disulfide 
isomerase regulated using certain 
redox pathways, and those 
redox pathways are unusually sensitive to treatment using 
oxidized glutathione-based compounds. Thus, the composition of the instant invention, in some embodiments, are surprisingly effective at preventing tumor metastases. While other references have disclosed the treatment of cancers using 
oxidized glutathione-based compounds, 
no reference has suggested that signaling pathways used for 
cell migration, invasion and 
metastasis, such as the 
ErbB2, PI3K, RhoA, and AKT pathways, are highly susceptible to treatment by altering the 
redox state of the cell, e.g., by oxidized 
glutathione-based compounds. Accordingly, the use of such compositions to treat tumor metastases is surprising and could not be predicted given the teachings of the prior art.