A number of mechanisms of action for rituximab have been proposed,includingantibody-dependent cellular cytotoxicity, complement-mediated cytotoxicity, induction of apoptosis, recruitment of effector cells, and elaboration of cytokines (Ansell et al, abstract #377). Moreover, antibody levels can still be detected in the serum for up to 6 months after rituximab has been administered. Grillo-López and coworkers pooled data from several trials using rituximab administered alone or in combination with CHOP and noted that responses continue long after the drug has been discontinued (abstract #390). Thus, the effect of the drug is likely to be multifactorial, with some mechanisms achieving an early response, while others result in a later effect.
In a recent publication by Piro et al (Ann Oncol 10:655-661, 1999), patients with follicular/low-grade NHL received 8 weekly courses of rituximab, and the rates and duration of responses were similar to those reported with the standard 4-week schedule. These observations, taken together, provide little justification for administering maintenance therapy beyond the standard four infusions.