Rapid Clearance of Rituximab Contributes to the Continued High Incidence of Autoimmune Complications of Chemoimmunotherapy for Chronic Lymphocytic Leukemia

Abstract 4628 Introduction: Autoimmune cytopenias (AIC) are frequently seen in patients with chronic lymphocytic leukemia (CLL). Autoimmune hemolytic anemia (AIHA) is most common, although immune thrombocytopenia (ITP) and pure red cell aplasia (PRCA) can also occur. Fludarabine (F), the most active agent used to treat CLL, has been associated with the development of AIC. Although rituximab (R) is an effective treatment for F-associated AIC, its incorporation into F-based CLL regimens has failed to consistently prevent the occurrence of AIC (incidence of 6.5% in patients treated with F, R, and cyclophosphamide (FCR) (Borthakur et al, Br J Haematol. 2007)). We hypothesized that differences in R pharmacokinetics can explain the apparent paradox of efficacy in treating but failure in preventing AIC. Patients and Methods: To determine whether the addition of R to F decreased the incidence of autoimmune complications, we compared the incidence of AIC in 2 CLL cohorts - one treated with standard dose F (25 mg/m2/d ×5 q4 wk, 1998–2004, n=21) and the other with standard dose FR (R: 375 mg/m2 q4 wk, 2005–2010, n=29). F-associated AIHA was diagnosed if the patient experienced a hemoglobin drop of ≥ 2 g/dL and had either a positive DAT or at least two of the following: absolute reticulocyte count >50K/μ L, elevated LDH, total bilirubin >1.0 mg/dL, haptoglobin <30 mg/dL. Patients with hemoglobin levels of ≥10 g/dL and either a positive DAT or at least two of the aforementioned criteria prior to the start of therapy were deemed “at-risk” of developing AIHA. We measured R serum levels during FR treatment in order to examine the CLL-specific pharmacokinetic profile of R and its potential relevance to the development of AIC. Rituximab serum levels were determined by flow cytometry, based on the binding of mAb HB43 (anti-human IgG, Fc specific) to Raji cells reacted with standards and serum samples (Beum et al, J Immunol Methods. 2004). Cycle-specific parameters of peak level, trough level, and half-life were analyzed across time. Results: Six of 21 patients (29%) in the F cohort developed AIC during therapy (2 AIHA, 3 ITP, 1 PRCA), compared to 6 of 29 patients (21%) in the FR cohort (4 AIHA, 1 autoimmune neutropenia, 1 amegakaryocytic thrombocytopenia). Four patients in the FR cohort were identified as “at-risk” for AIHA prior to therapy; 2 of these patients developed AIHA during/immediately after therapy and were treated with steroids and R, while 2 were preemptively given an additional dose of R in each of the first 2 cycles and subsequently did not develop AIHA. Within the FR cohort, median serum peak R levels of 90, 111, 135, and 173 μ g/mL were achieved in cycles 1, 2, 3, and 4, respectively. Trough R levels were undetectable in all 14 patients treated with standard dose FR at the end of cycle 1, and increased progressively over time (25%, 50%, and 71% of patients had detectable levels at the end of cycles 2, 3, and 4, respectively). By contrast, 1 of the 2 “at-risk” patients given additional R had a detectable trough level at the end of cycle 1. The serum R half-life was surprisingly short especially in cycle 1 with a median of 26 hours (range 11–73) and increased progressively in subsequent cycles (81, 133, and 194 hours in cycles 2, 3, and 4, respectively). During cycle 1, patients with heavy tumor burden (bulky lymph nodes and/or ALC>100K/μ L, n=7 patients) had a substantially shorter mean R half-life compared to those without (21 hours vs 61 hours, P=.004). Conclusion: Surprisingly, R did not significantly change the incidence of AIC in the FR cohort compared to F alone, despite its efficacy in treating this complication. Here we show that in previously untreated CLL patients, R is rapidly cleared with a half-life of only a few days in the first cycles. This effectively limits the effect of R during the initial cycles and may explain why AIC are common even with R-based chemoimmunotherapy. Furthermore, we find that the serum half-life of R is highly dependent upon tumor burden, suggesting that fixed standard dosing of R is inadequate. In patients at risk for AIC, additional doses of rituximab in the first two cycles may be able to prevent the onset of such complications. Alternatively, such patients might benefit from a non-fludarabine-containing regimen. Disclosures: Off Label Use: Rituximab was used for treatment/prevention of autoimmune hemolytic anemia.