Combinational studies of BOLD-100/KP1339 with established chemotherapeutics in gastrointestinal multicellular tumor spheroids

BOLD-100, a ruthenium(III) prodrug also known as KP1339, is currently under clinical investigation for its suitability as a co-therapeutic anticancer drug. Unlike platinum-based chemotherapeutics, it has unique non-DNA targets, mainly GRP78, an enzyme responsible for signaling related to protein folding in the ER. Inhibition of this key mediator of the "unfolded protein response" may lead to cell death eventually. The purpose of this study was to assess in vitro the impact of BOLD-100 combinations with oxaliplatin, 5-FU, cisplatin or SN38 in multicellular tumor spheroids (MCTSs) compared to single-drug treatments. Gastric (MKN45, NCI-N87) and colorectal cancer (HCT116, HT29) cell lines were chosen, corresponding to the clinical trial in which patients with tumors of these origins are being treated. Biological effects investigated in this study include cytotoxic activity, synergism/antagonism based on Chou and Talalay's algorithm, formation of reactive oxygen species (ROS) and induction of apoptosis and necrosis. Cytotoxicity tests showed vastly different chemosensitivities in MCTSs of the same tumor origin. In both tumor entities, partially synergistic effects were revealed when BOLD-100 was combined with the drugs mentioned above. The DCFH-DA assay suggested consistent increases of ROS levels after treatment with oxaliplatin and, with restrictions, its combination with BOLD-100. Apoptosis and necrosis were induced in the spheroid models by single-drug and combined treatment, with no hints at antagonism in the combination settings. In conclusion, these findings emphasize the potential of BOLD-100 for combination therapy of gastric and colorectal cancers.