Abstract LB-317: Combination therapy with ofatumumab and bendamustine in xenograft model of chronic lymphocytic leukemia

Ofatumumab (OFA) is a fully human CD20 monoclonal antibody (mAb) recently approved for treatment of patients with fludarabine- and alemtuzumab-refractory chronic lymphocytic leukemia (CLL). OFA binds a unique membrane-proximal epitope encompassing both the small- and large-loop on the CD20 receptor. Its mechanism of action includes complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC). Combining OFA with cytotoxic chemotherapy may lead to elimination of additional tumor cells by alternate mechanisms such as apoptosis via alkylating agents. We conducted preclinical studies to determine: 1) OFA activity in B-cell lymphoma and leukemia murine xenograft models with differential CD20 expression; and 2) activity of combining OFA with bendamustine in a CLL model. CD20 expression on tumor cell lines was determined by flow cytometry, and lymphoma (Ramos, Daudi) and CLL (JVM-3) lines with differential CD20 expression were selected for in vivo animal model development. CD20-negative BC-1 lymphoma model served as a control. Tumor cells were implanted s.c. into immunodeficient 4-6 weeks old C.B-17 SCID female mice (Taconic), and tumor volumes were determined twice a week based on caliper measurements (tumor volume = width 2 × length/2). Mice were randomized and therapy was initiated when tumors reached mean volume (V) = 80 mm 3 at two weeks after tumor implantation. OFA was administered i.p. twice a week, and bendamustine was injected in a single i.v. dose on day 15 of the study. Efficacy of therapy was determined by evaluating % tumor growth inhibition (TGI=100x(1-(Vt/Vc), where Vt, Vc = mean V in the treated, or control group, respectively) and tumor growth delay (TGD=100x((Tt-Tc)/Tc) where Tt, Tv = time to end-point in the treated, or control group). CD20 expression ranged from highest to lowest on Ramos, JVM-3 and BC-1 (CD20 negative) cells. All cell lines were tumorigenic in C.B-17 SCID mice. OFA monotherapy was effective in the Ramos and JVM-3, but not in the control BC-1 s.c. xenograft model. OFA dose-response was performed and suboptimal doses of OFA (2 mg/kg i.p.) and bendamustine (50 mg/kg i.v.) were used for chemotherapy combination studies in the JVM-3 s.c. xenograft model. Combination therapy resulted in a synergistic anti-tumor activity with TGI = 96% and TGD = 42%, compared to bendamustin (TGI = 9%, TGD = 14%) or OFA alone (TGI = 16%, TGD = 14%) in the CLL model. In conclusion, single-agent therapy with OFA demonstrated significant and dose-dependent activity in preclinical animal xenograft models of human B-cell lymphoma and CLL. The combination of suboptimal OFA and bendamustine doses in the CLL model resulted in significant antitumor activity, compared exposure to either agent alone. These preclinical findings provide a rationale for clinical studies of the combination of OFA and bendamustine in patients with CLL. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-317.