Delivery of nucleic acid therapeutics for cancer immunotherapy

Abstract

Cancer immunotherapy has shown great potential as witnessed by an increasing number of immuno-oncology drug approvals in the past few years. Meanwhile, the field of nucleic acid therapeutics has made significant advancement. Nucleic acid therapeutics, such as plasmids, antisense oligonucleotides (ASO), small interfering RNA (siRNA) and microRNA, messenger RNA (mRNA), immunomodulatory DNA/RNA, and gene-editing guide RNA (gRNA) are attractive due to their versatile abilities to alter the expression of target endogenous genes or even synthetic genes, and modulate the immune responses. These abilities can play vital roles in the development of novel immunotherapy strategies. However, limited by the intrinsic physicochemical properties such as negative charges, hydrophilicity, as well as susceptibility to enzymatic degradation, the delivery of nucleic acid therapeutics faces multiple challenges. It is therefore pivotal to develop drug delivery systems that can carry, protect, and specifically deliver and release nucleic acid therapeutics to target tissues and cells. In this review, we attempted to summarize recent advances in nucleic acid therapeutics and the delivery systems for these therapeutics in cancer immunotherapy.

Keywords: Cancer immunotherapy; Drug delivery; Immunostimulatory nucleic acids; Nucleic acid therapeutics; Therapeutic vaccines; mRNA drug.

Fig. 1.

Milestones for the development of nucleic acid therapeutics for immunotherapy.

Fig. 2.

Tumor microenvironment and lymphoid tissues as two common tissue-level targets for the delivery of nucleic acid therapeutics in cancer immunotherapy. (A) The tumor microenvironment harbors a variety of cells including tumor cells, antigen presenting cells (DC, macrophage), lymphocytes (T cells, B cells and NK cells), and fibroblasts, which play a critical role in the development of tumor initiation, proliferation, as well as metastasis. (B) Lymphatic organs such as lymph nodes and spleen are among the desired sites for cancer therapeutics such as vaccines. DC: dendritic cells; TAM: tumor-associated macrophage; MDSC: myeloid-derived suppressor cells; NK cell: natural killer cell; APC: antigen-presenting cells; TGF-β: transforming growth factor-β; HGF: hepatocyte growth factor; MMP: matrix metalloproteinase.

Fig. 3.

Representative nanoparticulate drug delivery systems for nucleic acid therapeutics. (A) Delivery of nucleic acid therapeutics using micelles comprise of amphiphilic molecules, such as amphiphilic block polymers. Micelles contain hydrophilic regions as well as hydrophobic regions. After reaching the critical micelle concentration, micelles can be formed with nucleic acid loading in hydrophilic side via mechanisms such as electrostatic interactions or Van der Waals interactions. (B) Delivery of nucleic acid therapeutics using polymer nanoparticles that can load nucleic acids through various mechanism, most commonly using cationic polymers, which can form nanoparticles with negative nucleic acid via electrostatic interactions. (C) Delivery of nucleic acid therapeutics using lipid nanoparticles comprise of lipids such as lecithin, DOPC, and DSPE. Polyethylene glycol (PEG) modification of lipids enhance the half-life of lipid nanoparticles. DOPC: 1,2-dioleoyl-sn-glycero-3-phosphocholine; DSPE: 1,2-distearoyl-sn-glycero-3-phosphorylethanolamine. (D) Delivery of nucleic acid therapeutics using lipid calcium phosphate (CaP) nanoparticles. CaP encapsulates nucleic acids due to the co-precipitation of the phosphate on nucleic acid backbones with calcium-phosphate. Lipids make CaP nanoparticles more stable and can facilitate cellular delivery.

Fig. 4.

Schematic illustration of cGAMP delivery using liposomes. (A) The structure of liposomal cGAMP. (B) Compared to free cGAMP, liposomal cGAMP improved cellular uptake by antigen-presenting cells due to the benefit of positive charge, and after cell endocytosis, liposomal cGAMP can facilitate cGAMP to release from endosomes to further bind STING located in cytosol, triggering STING signaling pathway. Adapted by permission from [32] Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 5.

T cell-targeting mRNA nanoparticles or in vivo T cell gene editing in cancer immunotherapy. (A) mRNA nanoparticles controllably deliver mRNA to lymphocytes or hematopoietic stem cells (HSCs) by merely mixing the mRNA nanoparticles with the cells in vitro. (B) Design of targeted mRNA-carrying nanoparticles. mRNA was complexed with a positively-charged PBAE polymer, which condenses mRNA into nanocomplexes. Antibody-functionalized polyglutamic acid (PGA) was added to shield the positive charge of the PBAE-mRNA particles and confer lymphocyte-targeting. Adapted by permission from reference [54]. Copyright, 2017, Springer Nature.

Fig. 6.

Using CaP lipid nanoparticles for the delivery of relaxin-encoding pDNA for tumor immunotherapy. (A) High expression level of sigma-1 receptor (Sig-1R) on activated hepatic stellate cells (aHSCs). (B) Schematic illustration of CaP lipid nanoparticles. A Sig-1R-targeting ligand, aminoethyl anisamide (AEAA), was conjugated on liposome surface for active targeting. (C) Distribution of DiD-labeled CaP lipid nanoparticles and DiD-labeled AEAA-conjugated CaP lipid nanoparticles in the organs after injection in mice bearing liver metastatic CT26-FL3 for 13 days. (D) Relative relaxin mRNA levels after the injection of pDNA CaP lipid nanoparticles. (E) Relative relaxin expression levels after the injection of pDNA CaP lipid nanoparticles. Adapted by permission from reference [61]. Copyright 2019, Springer Nature.

Fig. 7.

PEGylated-PLGA nanoparticles co-encapsulated with anti-miRNA-21 and GEM for HCC therapy. Anti-miRNA-21 and GEM co-encapsulated nanoparticles can improve treatment efficiency in HCC cells in comparison to nanoparticles treated with nanoparticles with equal concentrations of individually loaded anti-miRNA-21 and GEM. Adapted by permission from reference [109]. Copyright 2016, American Chemical Society.