STAT3 is persistently activated in cancer cells and in diverse tumor-associated immune cells and plays critical role for tumorigenesis and immune evasion. As a transcription factor uniting signaling from a variety of receptors and kinases, STAT3 is a highly desirable but challenging therapeutic target. An attractive strategy for inhibiting STAT3 is the use of oligodeoxynucleotides (ODNs) to prevent STAT3 DNA binding and transcriptional activation. The limiting factor in the clinical application of STAT3 ODNs is difficulty in their targeted delivery, additionally complicated by the inherent sensitivity of the immune system to nucleic acids. However, immune cells may themselves be essential targets for cancer therapy. We previously demonstrated that ligand for the intracellular receptor TLR9 (CpG ODN) allows for the uptake of oligonucleotides specifically by TLR9-positive target cells. Now, we used this approach to deliver STAT3 ODN inhibitor into variety of mouse and human target cells both in vitro and in vivo. These include normal myeloid cells or B lymphocytes and malignant cells, such as acute myeloid leukemia (AML) and B cell lymphoma. As expected, STAT3 inhibition resulted in various degrees of cytotoxic effects in tumor cells while it did not affect viability of non-malignant cells. Due to design and chemical modifications of the backbone, the CpG-STAT3 ODN has improved resistance to degradation in human serum with half-life exceeding 48 hrs. Thus, we assessed whether it is also suitable for systemic administration against blood cancers. As shown by our studies using disseminated human MV4-11 AML, repeated i.v. injections of CpG-STAT3 ODN (5 mg/kg) resulted in leukemia regression within two weeks. The antitumor efficacy of this strategy is enhanced by combined effect of STAT3-blocking/TLR9-triggering in immunocompetent mice. In syngeneic model of mouse AML, i.v. administration of CpG-STAT3 ODN over two weeks induced tumor regression in bone marrow, spleen and blood. In addition, secondary transplant experiments indicated reduced leukemia-initiating potential of AML cells in vivo treated with CpG-STAT3 ODN. Treatment with control CpG-scrambled ODN did not show significant antitumor effects in any of our tumor models. The antitumor effect was accompanied by immune cell activation and T cell infiltration into various organs. These results underscore potential of further development of this strategy for clinical application in AML and likely B cell lymphoma therapy.

Citation Format: Qifang Zhang, Ralf Buettner, Sergey Nechaev, Dayson Moreira, Agnieszka Jozwiak, Piotr Swiderski, Marcin Kortylewski. Systemic delivery of STAT3 blocking/TLR9 activating oligodeoxynucleotides induces regression of mouse and human acute myeloid leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2569. doi:10.1158/1538-7445.AM2014-2569