Motexafin Gadolinium Enhances Rituximab-Induced Cytotoxicity Against B-Cell Lymphoma Cell Lines: Role of Elevated Intracellular Calcium.

Motexafin gadolinium (MGd, Xcytrin®) is a tumor selective redox active drug that is directly cytotoxic to some hematolymphoid cell lines and chronic lymphocytic leukemia (CLL) patient samples. MGd has shown single agent activity in early phase II studies of lymphoma and CLL patients. Rituximab, an anti-CD20 antibody, is used widely in the treatment of B-cell malignancies. We evaluated the effects of MGd, rituximab and the combination in HF-1, a follicular lymphoma-derived cell line. Data analysis with CalcuSyn software revealed that the combination of MGd and rituximab showed synergistic growth inhibition and cytotoxicity compared to either agent used alone. MGd/rituximab activated a caspase-dependent apoptotic pathway as demonstrated by loss of mitochondrial membrane potential and PARP cleavage. Similar results were obtained with the combination of MGd and rituximab in DHL-4 and Ramos lymphoma cell lines. Since intracellular calcium ([Ca2+]i) levels have been implicated in rituximab cytotoxicity, we explored [Ca2+]i in rituximab and MGd/rituximab treated cells. Rituximab and MGd/rituximab treated cells have increased levels of [Ca2+]i. Elevation of [Ca2+]i with thapsigargin, an agent that releases calcium from internal stores or the ionophore A23187 that transports calcium into cells, results in synergistic cytotoxicity with rituximab or the MGd/rituximab combination. These in vitro findings suggest a role for [Ca2+]i in MGd/rituximab-induced cell death and support the combined use of MGd and rituximab in the treatment of B-cell lymphoma.