A key end goal of gene delivery research is to develop clinically-relevant vectors that can be used to combat elusive diseases such as AIDS. Despite promising engineering strategies, efficiency and ultimately gene modulation efficacy of nonviral vectors have been hindered by numerous in vitro and in vivo barriers that have resulted in sub-viral performance. In this perspective, we concentrate on the gene delivery barriers associated with the two most common classes of nonviral vectors, cationic-based lipids and polymers. We present the existing delivery barriers and summarize current vector-specific strategies to overcome said barriers.
With the use of contemporary tools and techniques, it has become possible to more precisely tune the biochemical mechanisms associated with using nonviral vectors for gene delivery. Consequently, nonviral vectors can incorporate numerous vector compositions and types of genetic cargo to develop diverse genetic therapies. Despite these advantages, gene delivery strategies using nonviral vectors have poorly translated into clinical success due to preclinical experimental design considerations that inadequately predict therapeutic efficacy. Furthermore, the manufacturing and distribution processes are critical considerations for clinical application that should be considered when developing therapeutic platforms. This review evaluates potential avenues towards improving the transition of gene delivery technologies from in vitro assessment to human clinical therapy.
Given the rise of antibiotic resistance and other difficult-to-treat diseases, genetic vaccination is a promising preventative approach that can be tailored and scaled according to the vector chosen for gene delivery. However, most vectors currently utilized rely on ubiquitous delivery mechanisms that ineffectively target important immune effectors such as antigen presenting cells (APCs). As such, APC targeting allows the option for tuning the direction (humoral vs cell-mediated) and strength of the resulting immune responses. In this work, we present the development and assessment of a library of mannosylated poly(beta-amino esters) (PBAEs) that represent a new class of easily synthesized APC-targeting cationic polymers. Polymeric characterization and assessment methodologies were designed to provide a more realistic physiochemical profile prior to in vivo evaluation. Gene delivery assessment in vitro showed significant improvement upon PBAE mannosylation and suggested that mannose-mediated uptake and processing influence the magnitude of gene delivery. Furthermore, mannosylated PBAEs demonstrated a strong, efficient, and safe in vivo humoral immune response without use of adjuvants when compared to genetic and protein control antigens. In summary, the gene delivery effectiveness provided by mannosylated PBAE vectors offers specificity and potency in directing APC activation and subsequent immune responses.
Well-defined cationic polylactides (CPLAs) with tertiary amine groups were synthesized by thiol-ene click functionalization of an allyl-functionalized polylactide to yield polymers with tunable charge densities. CPLAs have not previously been utilized in the context of DNA delivery. Thus, plasmid DNA (pDNA) encoding luciferase was delivered to two physiologically distinct cell lines (macrophage and fibroblast) via formation of CPLA/pDNA polyplexes by electrostatic interaction. The formulated polyplexes demonstrated high levels of transfection with low levels of cytotoxicity when compared to a positive control. Biophysical characterization of charge densities at various CPLA/pDNA weight ratios revealed a positive correlation between surface charge and gene delivery. Overall, these results help to elucidate the influence of polyplex charge and size upon the delivery of nucleic acid and support future gene delivery applications using this next-generation biomaterial.
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