Low-dose fludarabine increases rituximab cytotoxicity in B-CLL cells by triggering caspases activation in vitro

Rituximab maintenance therapy provides a significant benefit in patients with indolent B-cell non-Hodgkin lymphoma (NHL). Based on its efficacy in improving response to chemotherapy, the anti-CD20 antibody is currently under evaluation as maintenance therapy also in patients with B-CLL. We evaluated rituximab-mediated cytotoxicity in 10 B-CLL cases pretreated in vitro with non-cytotoxic concentrations of fludarabine. This combination induced a synergic cytotoxic effect in 8 out of 10 patients at a mean level of 26.15 +/- 13.9%, compared to 8.05 +/- 5.3% cytotoxicity observed with rituximab alone. Consistent with the viability assay, we found an increased caspase-3 activity together with activation of caspase-9 in B-CLL cells sensitive to sequential non-cytotoxic fludarabine and rituximab exposure. Non-cytotoxic fludarabine concentrations may sensitize B-CLL cells to rituximab-mediated cytotoxicity via caspase-3 activation.