Abstract P4-04-01: Combination of intratumoral CpG with systemic anti-OX40 and anti-CTLA4 mAbs eradicates established triple negative breast tumors in mice

Background: Evidence is mounting across diverse datasets and scientific techniques implicating immune dysregulation in poor outcomes of high grade and/or triple negative breast cancers. A multimodal strategy to stimulate the immune system could be an effective therapeutic approach for breast cancer. CpG oligodeoxynucleotides are single-stranded DNA that stimulate dendritic cells and B cells by binding to Toll-like receptor 9 (TLR9). OX40 (CD134) is predominantly expressed on activated T cells and the binding of OX40 with an agonistic antibody (anti-OX40) leads to the proliferation and survival of T cells. Agonistic anti-OX40 antibodies can also reverse the suppressive effects of Treg cells. A third avenue is to target Cytotoxic T lymphocyte associated antigen (CTLA4), which plays a negative regulatory role in T cell activation and is also constitutively expressed on Treg cells. Blocking CTLA4 with an antagonistic antibody (anti-CTLA4) has been shown to enhance anti-tumor responses. In this study, we tested the combination of intratumoral (IT) CpG with systemic anti-OX40 plus anti-CTLA4 in a mouse model of triple negative breast cancer. Methods: 4T1, an aggressive, triple-negative mammary carcinoma, was implanted orthotopically into syngeneic BALB/C mice. Treatment was initiated when tumors were ∼0.5 cm in the largest dimension. Mice were divided into 8 groups. Various combinations of PBS control, CpG, taxol, and anti-OX40+anti-CTLA4 mAbs were tested (Tables 1 and 2). CpG, 100 µg, was injected IT for 5 consecutive days (days 1–5); Taxol, 16 mg/kg, was administered IP on day 1; anti-OX40 (400 µg) and anti-CTLA4 (100 µg) were given IP on days 1 and 5. Mice were sacrificed once tumors became ulcerated or reached a diameter >2 cm. Results: As shown in Table 1, CpG alone, taxol alone, or the anti-OX40 + anti-CTLA4 combination each delayed the growth of established 4T1 tumors when compared to the untreated control group. However, the majority of the mice (except one in CpG treated group) died from tumor progression. The combination of CpG + anti-OX40 + anti-CTLA4 had the best anti-tumor effects, with 4 out of 5 mice having demonstrated complete regression of their tumors (Table 2). The addition of taxol to the combination of CpG + anti-OX40 + anti-CTLA4 did not improve the anti-tumor effect, and while there was a substantial delay in growth, only one animal achieved a complete regression. Conclusion: Our study demonstrated that the combination of IT CpG with systemic anti-OX40 + anti-CTLA4 mAbs can cure mice with established triple negative breast tumors, supporting the possibilities of applying immune modulation approach in human triple negative tumors. Studies are on-going to further dissect the immunological mechanisms involved. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-04-01.