Polycation-mediated traditional chemo-gene therapy generally suffers from low transfection efficiency and carriers-associated severe toxicity [1]. Great efforts have been devoted to the development of multifunctional gene vectors with enhanced transfection efficiency and improved biosafety. Notable examples include Fang's versatile polycation gene carrier with tailor-made “molecular string” pendants grafted onto the polylysine backbone [2], and Zhang's reduction-sensitive polymer with pair-wised carboxyl groups for promoted chemo-immunotherapy [3]. However, the potential clinical translation of these promising materials has been hampered apparently by the carriers-related scale-up complexity and rapid cleavage and degradation of nucleic acids (e.g., plasmid DNA (pDNA) and small interfering RNA (siRNA)) in vivo caused by endonucleases [4].

Spherical nucleic acid (SNA) with modulated structures and properties is considered a promising nanoplatform for mediating genes and chemotherapeutic drug codelivery [5,6]. Based on SNA's unique dense spherical spatial arrangement structure, the rate at which the DNA strand of SNA is cleaved by endonuclease is much lower than that of linear DNA strands of the same sequence [7]. SNAs are negatively charged due to their high oligonucleotide density. They are nevertheless recognized by class A scavenger receptors through their three-dimensional structure, endocytosed by cytoplasmic membrane microcystins and rapidly internalized in almost all cell types. More importantly, the SNA structure has been reported to circumvent lysosome trafficking [8]. The high charge density of SNA affects the activity of enzymes for recognizing exogenous nucleic acids to elicit an immune response, thus further reducing the immunogenicity of SNA [9]. Zhang et al. reported an SNA system with increased nanoparticle tumor accumulation for systemic delivery of oligonucleotides and nanomaterials to target cells in vivo with low immunogenicity [10]. Mirkin's group developed a useful means for controlling the release kinetics of encapsulated cargos in the context of the polymer-SNA conjugates composed of poly(lactic‐co‐glycolic acid) nanoparticle cores [11] and further constructed a dual-antigen SNA with specific antigen arrangement for suppressing tumor growth and increasing the levels of circulating memory T cells [12]. Kumthekar et al. prepared a gold-SNA conjugate that can penetrate the blood-brain barrier and blood-tumor barrier for the treatment of glioblastoma [13].

The reported SNA-based gene/drug codelivery systems have been frequently constructed from non-degradable core domains. For example, Zhang's group developed SNA self-assemblies from polynorbornene amphiphiles with modulated paclitaxel/tyrosine kinase inhibitor phenethyl resorcinol (PR) and functional nucleic acids grafts for disease treatment [14,15]. Guo et al. fabricated SNA based on DNA-drug conjugates via grafting chemotherapeutics onto phosphorothioate oligonucleotides [16]. Bousmail et al. developed SNAs self-assembled from a polymer-DNA conjugate for further encapsulating a hydrophobic small molecule chemotherapeutic drug BKM120 [17]. Successful preparation of a fully biodegradable SNA nanoplatform is thus highly desirable for in vivo applications and potential clinical translations, but remains rarely explored likely due to the synthetic challenge.

Besides the biodegradability required for successful in vivo translations, the unsatisfactory therapeutic efficiency of traditional SNA-based codelivery systems remains to be improved. Immunotherapy has been identified in a recent decade to be a promising solution of ultimate cancer therapy, but most of the reported SNAs, to our knowledge, have shown only immunogenic cell death (ICD) properties based on the use of a single chemotherapeutic drug, which is insufficient for efficient cancer immunotherapy. It will be a useful strategy to integrate the immunomodulation properties of genes for synergistic immunogenicity enhancement. MiR122 is a liver-specific miRNA that has been reported recently to trigger a promisingly anti-tumor immune response by enhancing innate immunity [18].

For this purpose, we report herein the construction of a self-delivery SNA nanoplatform with fully degradable polycarbonate backbones to co-deliver a chemotherapeutic drug, doxorubicin (DOX) and a human liver-specific miR122 for synergistic chemo-gene therapy of hepatocellular carcinoma (HCC). Ring-opening polymerization (ROP) of a carbonate monomer was conducted to afford a well-defined biodegradable polycarbonate backbone for subsequent DOX conjugation to the pendant side chains via acidic pH-cleavage Schiff base links and miR122 incorporation to the chain termini via click coupling. The resulting amphiphilic polycarbonate-DOX-miR122 conjugate, PBis-Mpa30-DOX-miR122 can self-assemble into stabilized SNA that can further undergo efficient cellular uptake via scavenger receptor-mediated endocytosis, and realize rapidly intracellular DOX and miR122 release via the cleavage of acidic pH-sensitive Schiff base links and polycarbonate degradation, respectively to exert a synergistic apoptosis-inducing effect. Besides the desired biodegradability, another notable merit of this nanoplatform is the use of miR122 not only for gene therapy, but also for enhanced innate immune response. Together with the ICD-triggering effect of DOX, PBis-Mpa30-DOX-miR122 SNA-mediated DOX and miR122 codelivery leads to synergistic immunogenicity enhancement [19,20]. The therapeutic efficiency of this SNA nanoplatform together with the synergistic immunogenicity enhancement was evaluated comprehensively in vitro and in vivo. The main innovation point of this study is the development of a useful and reliable means toward a fully biodegradable SNA nanoplatform and an elegant integration of the natural immune-enhancing effect of genes and the ICD effect of anti-cancer drugs for efficient synergistic HCC therapy.