Abstract 2852: Obinutuzumab (GA101) efficacy in chronic lymphocytic leukemia in vitro is not diminished in high risk patients.

Introduction : we and others have already demonstrated the superiority of obinutuzumab (GA101) over rituximab (RTX) in in vitro depletion assays against chronic lymphocytic leukemia (CLL) cells. It has been recently suggested that complex karyotype (3 or more chromosomal abnormalities) and recurrent somatic mutations of the TP53, NOTCH1 and SF3B1 genes, influenced treatment-free and overall survivals in CLL patients. We address the question whether these factors may also influence the efficacy of anti-CD20 directed immunotherapy. Aim: To assess in vitro pre-clinical activity of RTX and GA101 against CLL cells in freshly isolated PBMCs, and correlate depletion efficacy with modern and classical (FISH, IgVH mutational status, age, gender, Binet stage, b2-microglobulin, bulky adenopathies>5cm) prognostic parameters in 95 CLL patients. Moreover, CD20 antigen density was assessed in 24 patients. Methods: PBMCs were isolated from blood samples by Ficoll gradient centrifugation. Antibody-mediated B cell depletions were determined by enumerating trypan blue negative, flow cytometrically CD19-positive B lymphocytes after treatment with 10 μg/ml of anti-CD20 antibodies. The Quantibrite® kit was used to determine CD20 density, measured as quantity of antibodies bound per cell (CD20-ABC). The Mann-Whitney test was used to compare median depletion according to relevant clinical and biological data. Results: patients characteristics were as follows: 69% males, median age 66y (36% >70y), advanced Binet stage B/C in 39% (31.7% had NCI2008 criteria for treatment), 22.6% had lymph nodes>5cm (56% had b2-microglobulin>3.5mg/l). Molecular and cytogenetics studies were as follows: unmutated IgVH in 42.7%, del13q/tri12/del11q/del17p=25.6%/22%/20%/5.8% respectively, complex karyotype (CK) was found in 20.8% of cases, and somatic mutation of TP53/NOTCH1/SF3B1 in 11.1%/12.6/4.8% of patients. The median B-CLL depletion in freshly isolated PBMC fraction due to treatment with were 61% for GA101 vs 21.5% for RTX (n=95, p 50% in 64% of cases. As shown in Figure 1, in all categories of patients, GA101 was superior to RTX. This was particularly noticed in the patients with TP53 deletion and/or mutation (n=7). Median CD20-ABC was 8100 in 24 patients, who were categorized as high or low CD20 expression based on that median. Low CD20-ABC patients had significantly less antibody-induced B-cell depletion, 12.8% (vs 25%, p=0.37) for RTX, and 42.5% (vs 67.5%, p=0.048) for GA101. CD20-ABC was not found to be correlated with clinicobiological parameters described above. Conclusions: according to modern prognostic parameters, GA101 confirms its higher efficacy than RTX (three-fold), and deserves further combination with chemotherapy or non-chemotherapy entities in CLL management. Citation Format: Loic Ysebaert, Emilie Laprevotte, Christian Klein, Jean-Jacques Fournie, Anne Quillet-Mary. Obinutuzumab (GA101) efficacy in chronic lymphocytic leukemia in vitro is not diminished in high risk patients. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2852. doi:10.1158/1538-7445.AM2013-2852