TLR7/8 Agonist Encapsulating Polymeric Nanoparticles for Cancer Immunotherapy

Abstract

The immune system is important for the prevention of cancer and formed the basis of cancer immunotherapy. That is, enhancement of the immune response for the treatment of malignant cancer cells. The field has undergone significant progress to include the use of checkpoint inhibitors, monoclonal antibodies and cytokine therapies. In addition, a cancer vaccine, composed of tumor associated antigens (TAAs) and vaccine adjuvant, is particularly promising. Effective vaccines can mobilize tumor-specific CD8 T cells to kill selectively tumor cells with cytotoxic granules and secrete IFN-ɣ that sensitize tumors to be susceptible to effector immune cells. Additionally, activated CD8 T cells become memory cells and can respond to same TAA-epitopes, which can be effective for long-term protective immunity to inhibit cancer recurrence. Activation of dendritic cells (DCs), which are the main antigen-presenting cells (APCs), is critical for T cell immunity. To an elicit tumor-specific CD8 T cell response, DCs have to process and present TAAs to CD8 T cells through the major histocompatibility complex (MHC) I. Moreover, co-stimulatory signals and pro-inflammatory cytokines are required to stimulate CD 8 T cells. However, CD8 T cell anergy and exhaustion will occur if TAA treatment is not sufficiently immunogenic to trigger DC activation. Therefore, development of immunostimulatory adjuvant that can trigger DC activation can enhance therapeutic efficacy of cancer vaccines. Imidazoquinoline-structured synthetic toll-like receptor (TLR) 7/8 agonists are strong cytokine inducers that can be a potent vaccine adjuvant. TLR7/8 ligation can activate MyD88 signaling pathways and stimulate DCs to upregulate co-stimulatory molecules and secrete pro-inflammatory cytokines and type I interferons. However, TLR7/8 agonists lack prominent efficacy in vivo due to the rapid clearance from the injection site. Following subcutaneous (SC) injection, small molecules enter the systemic circulation via blood capillaries and only small portion can reach the draining lymph nodes. Therefore, our goal was to develop a SC injectable drug carrier that can more efficiently deliver as well as prolong duration of at the site of action of TLR7/8 agonists. In this study, we fabricated poly(lactide-co-glycolide) (PLGA) nanoparticles (NPs) containing TLR7/8 agonists. Nanoparticulate delivery of TLR7/8 agonist showed enhanced DC activation and antigen-presentation compared to the soluble form of TLR7/8 agonists. When combined with peptide/tumor cell lysate-based antigens, NPs potentiated the antigen-specific CD8 T cell expansion and increased cytotoxic functions, which resulted in enhanced efficacy in both prophylactic and therapeutic tumor models. To further enhance endo/lysosomal delivery of TLR7/8 agonists in PLGA NPs, we included a sodium bicarbonate-mediated gas-generating system that is acidic pH-responsive. This approach resulted in 33-fold greater amount of TLR7/8 agonists encapsulated within the NPs. More importantly, the PLGA NP immunization elicited a stronger CD8 T cell response compared to conventional PLGA NPs, which in turn, enhanced therapeutic efficacy. As tumor microenvironment is immune suppressive, we examined whether modulation of tumor microenvironment can enhance the therapeutic efficacy of cancer vaccine. We reduced the immune suppressive cells including myeloid-derived suppressive cells (MDSCs) and regulatory T cells (Tregs) by daily oral dosing of a tyrosine kinase inhibitor (TKI), sunitinib. Additionally, we adapted an anti-PD-L1 antibody to block programmed death ligand 1 (PD-L1) expressed on tumor-associated (M2) macrophages and MDSCs that exhaust CD8 T cells, to augment the CD8 T cell activation at the tumor. In our study, combination of sunitinib and PD-L1 blockade significantly decreased the immune suppressive cell population and reduced PD-L1 expression on these cells. We also examined if nanoparticulate delivery of TLR7/8 agonist can potentiate NK cell-mediated cancer immunotherapy through its known effect on TH1 immunity. Antibody-dependent cellular cytotoxicity (ADCC) of monoclonal antibodies was found to be augmented in response to TLR7/8 agonist encapsulating NPs as a vaccine adjuvant. Overall, our studies demonstrate that PLGA NPs broaden the application of TLR7/8 agonists for improved cancer treatment. Moreover, this platform holds promise to enhance the efficacy of cancer vaccines composed of tumor associated antigens (TAAs) and vaccine adjuvant