Novel Approach to the Potential Treatment of Patients with B-Cell Lymphomas Harboring the MYD88 L265P Mutation: Combination Treatment with TLR Antagonist and Rituximab

Introduction. Patients with B-cell lymphomas harboring the oncogenic MYD88 L265P mutation have poor response to standard of care regimens such as R-CHOP (Fernandez-Rodriguez et al, Leukemia, 2014, Jun 6). These patients also have not responded well to new therapies under development, including BTK inhibitors (Wilson et al, Blood 2012; 120). The MYD88 L265P oncogenic mutation has been shown to over-activate TLR7- and TLR9-mediated signaling pathways, including NF-κB, IRAK1/4 and STAT3, which in turn promote cell survival and proliferation (Lim et al., AACR 2013, Abstract #2332). Our approach to a potential treatment of B-cell lymphomas harboring the MYD88 L265P mutation is to block TLR7- and TLR9-mediated signaling through use of a TLR antagonist. IMO-8400 is an antagonist of TLR7, TLR8, and TLR9 that inhibits cell signaling and induces apoptosis specifically in B-cell lymphoma cell lines harboring the MYD88 L265P mutation (Bhagat et al, AACR 2014, Abstract #2570). IMO-8400 is currently in clinical development for the treatment of relapsed/refractory patients with Waldenström’s macroglobulinemia (WM) and diffuse large B-cell lymphoma (DLBCL).

Methods. The current preclinical studies were designed to evaluate the combination of IMO-8400 with rituximab, which is a key component of R-CHOP. The studies were conducted in xenograft models of the activated B-cell like (ABC) DLBCL cell line OCI-Ly10 and the WM cell line MWCL-1, both positive for the MYD88 L265P mutation. For the ABC-DLBCL model, OCI-Ly10 cells were implanted s.c in NOD-SCID mice on day 0 and treatment was initiated on day 9 when mean tumor volume reached ~200 mm³. For the WM model, MWCL-1 cells were implanted s.c. in NOD-SCID mice on day 0 and treatment was initiated on day 6 when mean tumor volume reached ~250 mm³. In both models, the tumor-bearing mice were treated with PBS (control group), IMO-8400 (25 mg/kg, i.p.), rituximab (10 mg/kg, i.p.) or a combination of the two agents. IMO-8400 was administered twice per week for 3 weeks and continued further as maintenance treatment once per week. Rituximab was administered twice per week for three weeks in the ABC-DLBCL model whereas it was given only on days 3, 6, and 9 in the WM model.

Results. Treatment of tumor-bearing animals with IMO-8400 or rituximab alone and in combination was well tolerated in both the models. In the ABC-DLBCL model, tumor growth inhibition compared to PBS control was 91.3% in the combination therapy group vs. 40.8% with IMO-8400 alone (p<0.001) and 51.6% with rituximab alone (p=0.003). Inhibition of tumor growth was associated with decreases in levels of several tumor-secreted circulating human cytokines including IL-10 (p<0.05 vs control) in all the three treatment regimens. In the WM model, tumor growth inhibition was 87% in combination group vs. 47% with IMO-8400 (p<0.001) and 72% with rituximab (p<0.001). Further, tumor growth inhibition in the WM model was associated with reduction in serum IgM levels (p<0.03 vs control) in the single-agent as well as the combination treatment groups. Importantly, combination therapy resulted in complete tumor regression in more than 80% of mice in both ABC-DLBCL and WM models. Histological analysis of the residual tumors showed disappearance of malignant lymphocytes with replacement by fibrotic stroma.

Conclusions. Our data show that IMO-8400 in combination with rituximab was well tolerated in mouse xenograft models and exerted potent anti-tumor activity against B-cell lymphomas with the MYD88 L265P oncogenic mutation. These observations provide a strong rationale to evaluate IMO-8400 in combination with first-line rituximab-based therapies for the treatment of patients with B-cell lymphomas harboring the MYD88 L265P mutation.