Monoclonal antibodies in the treatment of lymphoid malignancies

Rituximab (Rit) was the first monoclonal antibody approved for therapeutic use in cancer patients. Rit is a chimeric mouse/human monoclonal antibody, consisting of the human IgG1 and k constant Fc region, and a mouse variable Fab region specific against the B-cell antigen CD20. Rit exerts its antilymphoma activity through many different mechanisms. Binding of antibody to CD20 antigen, provokes apoptosis through downstream signals that lead to caspase-3 activation. Complement activation by the Fc portion of the antibody results in complement-dependent cytotoxicity. However, the most effective mechanism of action seems to be antigen-dependent cellular cytotoxicity. Effector cytotoxic cells such as natural killer cells (NK) are activated after binding to the Fc portion of the anti-CD20 molecule. Activated NK cells kill the coated lymphoma cells with the use of granzyme-perforin system. More recently, pre-clinical data support the concept that Rituximab can provoke a vaccination-like effect. Finally in-vitro experiments and clinical trials have shown that co-administration of the antibody with cytotoxics confers a strong synergistic effect. The relative contribution of these mechanisms in vivo and in different lymphoma subtypes is not well known and remains to be further evaluated. Among the different histological groups, follicular lymphoma (FL) has been proven to be the most sensitive to Rit when used as a single agent, with overall response rates of 80% and 50% in untreated and previously treated patients, respectively. Moreover, Rit in combination with chemotherapy is superior to chemotherapy alone in terms of response rate and event-free survival, while early data indicate a significant prolongation in overall survival as well. Similarly, the addition of Rit to standard chemotherapy improves the disease-free and overall survival of patients with diffuse large B-cell lymphoma. There is no doubt that Rit represents one of the greatest achievements of biotechnology engineering. However, we need to understand better the mechanisms of its action as well as the mechanisms of resistance to Rit, in order to design more effective treatment modalities.