Motexafin Gadolinium Enhances Rituximab-Induced Cytotoxicity Against B-Cell Lymphoma Cell Lines.

Motexafin gadolinium (MGd, Xcytrin®) is a tumor selective redox active drug that is cytotoxic to many hematolymphoid cell lines and chronic lymphocytic leukemia cells. Previous studies have shown that MGd oxidizes various cellular reducing metabolites and produces reactive oxygen species (ROS), which can induce apoptosis. Rituximab, an anti-CD20 antibody, is used widely in the treatment of B-cell malignancies; however, its precise mechanism of action is uncertain. Some studies have demonstrated that rituximab induces the generation of ROS in sensitive cells. We evaluated the effects of combining rituximab and MGd in the in vitro treatment of HF-1, a cell line derived from a patient with follicular lymphoma. Increases in both apoptosis and cell growth inhibition were seen with MGd plus rituximab compared to each agent used separately. Apoptosis was demonstrated by both annexin-V positivity and caspase-3 activity. Loss of mitochondrial membrane potential and PARP cleavage were also greater when the two drugs were used together than when used separately. Data analysis with CalcuSyn software showed that, with the combination, the Dose Reduction Index (DRI) was more than 1.5 for MGd and 2 to 6 for rituximab regarding both apoptotic effect and growth inhibition. Consistent with MGd as a redox active agent, peroxiredoxin oxidation was higher when MGd and rituximab were used together. Further experiments revealed no increase in uptake of MGd by rituximab treatment, nor a change in the amount of CD20 antigen on the surface of MGd treated cells. We investigated another B-cell lymphoma cell line, DHL-4 for similar cooperative activity of MGd and rituximab. While MGd alone has no substantial apoptotic effect on DHL-4 cells, it enhanced the activity of rituximab in inducing apoptosis. These in vitro findings support the combined use of MGd and rituximab in the treatment of B cell lymphoma.