Active Combination Therapy of Bortezomib (Velcade) and Ibritumomab Tiuxetan (Zevalin) in an In Vivo Diffuse Large B-Cell Lymphoma Model.

Chemotherapy-resistant diffuse large B-cell lymphomas (DLBCL) pose an unresolved clinical problem and most patients die of their disease within months. The goal of this study was to test a novel, non-chemotherapy containing regimen for Non Hodgkin’s lymphomas, including DLBCL, by combining the proteasome inhibitor bortezomib (PS-341, VELCADE^®) with radio-immunotherapy using ibritumomab tiuxetan (Zevalin™). Previously, we have shown that the combination of bortezomib and rituximab is an active and at least additive regimen in an in vitro and in vivo DLBCL model. These data supported an ongoing clinical trial using this combination in patients with low grade NHL. In addition, Pervan et al. identified the proteasome as a novel, sensitive target for ionizing radiation (Mol Cancer Res. 2005 (7):381–90), providing further rationale for this combination study. The combination treatment was evaluated in vivo in a SCID diffuse large B-cell lymphoma model. Bortezomib treatment did not alter the CD20 expression levels of SUDHL-16 cells in vitro. MMT assays showed an additive effect of bortezomib and gamma radiation on SUDHL-16 cell proliferation. In preliminary experiments, we identified the maximal tolerated dose of ibritumomab tiuxetan in our mouse model as 50 μCi/mouse. 2x10⁶ SUDHL-16 cells injected s.q. in the flank resulted in local tumor growth within 4 weeks. When tumors became palpable, animals were stratified in 4 different treatment groups of 12 mice each. Mice received i.v. injections of either diluent, single agent bortezomib, single agent ibritumomab tiuxetan, or the combination of both. Ibritumomab tiuxetan was injected once i.v. at a dose of 20 μCi /mouse. Bortezomib was given i.v. as single dose of 0.01 mg/mouse. The combined treatment group received i.v. bortezomib as a radio-sensitizer 3 hrs prior to the injection of ibritumomab tiuxetan. All treatment schedules delayed tumor growth compared with controls. After 16 days of treatment, bortezomib reduced tumor progression by 52%, ibritumomab tiuxetan by 69%, and the combination treatment by 91%. Selected mice were sacrificed at day two and their tumors studied for potential immediate therapeutic effects. The Y90 content of tumors and normal tissues was measured and demonstrated targeting of Y90 to the xenograft. The tumors of the control and treatment groups did not differ by microscopic morphological appearance. In situ Tunel assays did not reveal increased apoptosis at treatment day two. In summary, bortezomib as a radio-sensitizer in combination with ibritumomab tiuxetan is an active and at least additive regimen in an in vivo DLBCL model.