Synergism between fludarabine and rituximab revealed in a follicular lymphoma cell line resistant to the, cytotoxic activity of either drug alone

We have shown previously that the anti-CD20 chimaeric monoclonal antibody r ituximab exerts its effects on neoplastic B-lymphoma cell lines in part via complement-dependent cytotoxicity. In addition, membrane expression levels of complement inhibitory proteins CD55 and CD59 play a role in determining susceptibility to lysis. We have identified one t(14;18)-positive human B- cell non Hodgkin's lymphoma cell line (Karpas 422) that is resistant to rit uximab and complement and used it for subsequent studies on the possible in teraction between this novel therapeutic agent and established antineoplast ic drugs. We have exposed Karpas to several chemotherapeutic agents (doxoru bicin, idarubicin, cispIatin, taxol) for different time periods and subsequ ently exposed the cells to rituximab and human complement. The combination of these drugs with rituximab induced an additive cytotoxic effect. In cont rast, exposure to fludarabine (1 mug/ml for 48-72 h) showed a synergistic e ffect, with cell lysis increasing from 10% to 20% using fludarabine or ritu ximab and complement alone to about 70% with both cytotoxic, agents. Analys is of the mechanism for this synergistic effect showed that fludarabine dow nmodulates the membrane expression of CD55 (from 96% to 55% positive cells) without significantly altering CD20 levels. Northern analysis demonstrated that fludarabine induced a general downmodulation of steady state mRNA lev els with no change in transcription rate detected in run-off assays. The st udy of the effect of fludarabine and rituximab in six freshly isolated B-ce ll chronic lymphocytic leukaemia (B-CLL) samples showed that, in most cases , fludarabine has an additive cytotoxic activity with rituximab and complem ent. This report gives a rational support for clinical studies with combina tions of drugs, including monoclonal antibodies and fludarabine.