Rituximab Enhances Radiation-Triggered Apoptosis in Non-Hodgkin's Lymphoma Cells Via Caspase-dependent and - Independent Mechanisms

Abstract

Rituximab (RTX), a chimeric human anti-CD20 monoclonal antibody, is currently employed in the treatment of malignant non-Hodgkin's lymphoma (NHL) either alone or in combination with other cytotoxic approaches. The present study examines the effects of ionizing radiation in combination with RTX on proliferation and apoptosis development in B-lymphoma RL and Raji cells. RTX was used at a concentration of 10 microg/mL 24 hours prior to irradiation at a single dose of 9 Gy. CD20 expression, cell viability, apoptosis, mitochondrial membrane potential and apoptosis-related proteins were evaluated in the treated B cells. The constitutive level of CD20 expression in RL and Raji lymphoma cells did not play an essential role in RTX-induced cell growth delay. Both lymphoma cells showed similar inhibition of cell proliferation without apoptosis development in response to RTX treatment. Exposure to ionizing radiation induced cell growth delay and apoptosis in RL cells, whereas Raji cells showed moderate radio-resistance and activation of cell growth at 24 hours after irradiation, which was accompanied by increased radiation-triggered CD20 expression. The simultaneous exposure of lymphoma cells to ionizing radiation and RTX abrogated radioresistance of Raji cells and significantly enhanced cell growth delay and apoptosis in RL cells. X-linked inhibitor of apoptosis protein (XIAP) and the inducible form of heat shock protein 70 (Hsp70) were positively modulated by RTX in combination with ionizing radiation in order to induce apoptosis. Furthermore, it was demonstrated that mitochondrial membrane potential dissipation is not an essential component to induce apoptosis-inducing factor (AIF) maturation and apoptosis. Our results show that RTX-triggered enhancement of radiation-induced apoptosis and cell growth delay is achieved by modulation of proteins involved in programmed cell death.

Pretargeted Versus Directly Targeted Radioimmunotherapy Combined with Anti-CD20 Antibody Consolidation Therapy of Non-Hodgkin Lymphoma

“…Nevertheless, several patients who received a predose of 685 mg of tositumomab before the 131 I-tositumomab imaging dose experienced tumor shrinkage even before they were administered the 131 Itositumomab therapy dose, and these patients were more likely to have a better response after the therapy dose (29,30). As the mechanism of action of anti-CD20 antibodies was examined further, the activation of apoptotic signaling of the antibody drew attention to its ability to sensitize cells to radiation (31,32). Thus, it is understandable that these predosing regimens were instituted.…”

A re-examination of radioimmunotherapy in the treatment of non-Hodgkin lymphoma: prospects for dual-targeted antibody/radioantibody therapy

“…31 Subsequent findings have indicated that an anti-CD20 IgG predose radiosensitizes the cells, thereby providing some added benefit to the combination treatment. [32][33][34][35] Parenthetically, Kapadia et al found that lower doses of rituximab radiosensitize cells, while higher doses actually appeared to be protective. 34 Both preclinical and clinical data clearly supported the value of administering a predose of the unlabeled anti-CD20 IgG to improve tumor targeting and extend the residence time of the radioimmunoconjugates in the blood; certainly, by using this approach, the radiolabeled agents were able to improve the objective response rate compared with their corresponding unlabeled antibody.…”

Sequential intranodal immunotherapy induces antitumor immunity and correlated regression of disseminated follicular lymphoma

“…Furthermore, rituximab has been shown to increase the sensitivity of lymphoma cells for external beam radiation. 45 The question can, however, be raised whether regression of lymphoma at distant sites in our study could be because of a direct systemic effect of rituximab. We believe that this is unlikely because the total dose of 30 mg of rituximab administered in lymph nodes is ;100-fold less than normally used for single-agent rituximab therapy.…”