Rituximab is an effective treatment for autoimmune cytopenias associated with chronic lymphocytic leukemia. Despite the incorporation of rituximab into fludarabine-based chemotherapy regimens, the incidence of autoimmune cytopenias has remained high. Inadequate rituximab exposure due to rapid antibody clearance may be a contributing factor. To test this hypothesis, we measured serum rituximab levels in patients treated with fludarabine and rituximab (375 mg/m(2)). All patients had undetectable rituximab trough levels by the end of cycle 1, and one-third had undetectable levels already on Day 6 of cycle 1. Although rituximab trough levels increased progressively with each cycle, only by cycle 4 did the median trough level exceed 10 ug/mL. The median half-life of rituximab during cycle 1 was 27 hours, compared to 199 hours during cycle 4 (P<0.0001). There was a significant inverse correlation between the rituximab half-life in cycle 1 and the degree of tumor burden (P=0.02). Two patients who were identified as having subclinical autoimmune hemolysis prior to therapy were given additional doses of rituximab during the initial cycles of therapy and did not develop clinically significant hemolysis. One patient who developed clinically significant hemolysis during therapy was given additional rituximab doses during cycles 3-5 and was able to successfully complete his treatment. In conclusion, rituximab is cleared so rapidly during the initial cycles of therapy for chronic lymphocytic leukemia that most patients have only transient serum levels. More frequent dosing of rituximab may be required to prevent autoimmune complications in at-risk patients (clinicaltrials.gov identifier:00001586).
Trial registration: ClinicalTrials.gov NCT00001586.