STAT3 Inhibition Combined with CpG Immunostimulation Activates Antitumor Immunity to Eradicate Genetically Distinct Castration-Resistant Prostate Cancers

Purpose: Prostate cancers show remarkable resistance to emerging immunotherapies, partly due to tolerogenic STAT3 signaling in tumor-associated myeloid cells. Here, we describe a novel strategy combining STAT3 inhibition with Toll-like Receptor 9 (TLR9) stimulation to unleash immune response against prostate cancers regardless of the genetic background. Experimental Design: We developed and validated a conjugate of the STAT3 antisense oligonucleotide (ASO) tethered to immunostimulatory TLR9 agonist (CpG oligonucleotide) to improve targeting of human and mouse prostate cancer and myeloid immune cells, such as myeloid-derived suppressor cells (MDSC). Results: CpG-STAT3ASO conjugates showed improved biodistribution and potency of STAT3 knockdown in target cells in vitro and in vivo. Systemic administration of CpG-STAT3ASO (5 mg/kg) eradicated bone-localized, Ras/Myc-driven, and Ptenpc−/−Smad4pc−/−Trp53c−/− prostate tumors in the majority of treated mice. These antitumor effects were primarily immune-mediated and correlated with an increased ratio of CD8+ to regulatory T cells and reduced pSTAT3+/PD-L1+ MDSCs. Both innate and adaptive immunity contributed to systemic antitumor responses as verified by the depletion of Gr1+ myeloid cells and CD8+ and CD4+ T cells, respectively. Importantly, only the bifunctional CpG-STAT3ASO, but not control CpG oligonucleotides, STAT3ASO alone, or the coinjection of both oligonucleotides, succeeded in recruiting neutrophils and CD8+ T cells into tumors. Thus, the concurrence of TLR9 activation with STAT3 inhibition in the same cellular compartment is indispensable for overcoming tumor immune tolerance and effective antitumor immunity against prostate cancer. Conclusions: The bifunctional, immunostimulatory, and tolerance-breaking design of CpG-STAT3ASO offers a blueprint for the development of effective and safer oligonucleotide strategies for treatment of immunologically “cold” human cancers.