Camptothecin enhances I-131-rituximab-induced G1-arrest and apoptosis in Burkitt lymphoma cells

Background: Rituximab is a chimeric monoclonal antibody against CD20. It is an established immunotherapeutic agent for non-Hodgkin's lymphoma. Even though rituximab has been used in clinics for decades, only 50% of the patients respond to rituximab therapy. To enhance the in vitro effect of rituximab, it was labeled with Iodine-131 (I-131) and combined effect of I-131-rituximab and camptothecin (CPT) was studied on a tumor cell line expressing CD20. Objective: The aim is to study the magnitude of cell killing and the underlying mechanism responsible for enhancing in vitro therapeutic efficacy. Materials and Methods: Rituximab was labeled with I-131 by the iodogen method. Raji cells were pretreated with CPT (250 nM) for an hour followed by I-131-rituximab (0.37 and 3.7 MBq) and incubated for 24 h in a humidified atmosphere of CO2 incubator at 37 degrees C. Subsequently, Raji cells were harvested and thoroughly washed to carry out studies of cellular toxicity, apoptosis, cell cycle, and mitogen-activated protein kinase (MAPK) pathways. Results: Maximal inhibition of cell proliferation and enhancement of apoptotic cell death was observed in the cells treated with the combination of CPT and I-131-rituximab, compared to controls of CPT-treated and I-131-rituximab-treated cells. Raji cells undergo G1 arrest after I-131-rituximab treatment, which leads to apoptosis and was confirmed by the downregulation of bclxl protein. Expression of p38 was decreased while an increase in phosphorylation of p38 was observed in the combination treatment of CPT and I-131-rituximab. Conclusions: It was concluded from the findings that CPT enhanced I-131-rituximab-induced apoptosis, G1 cell cycle arrest and p38 MAPK phosphorylation in Raji cells.