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Review
. 2023 Jun 27;15(7):1832.
doi: 10.3390/pharmaceutics15071832.

Exosomes as an Emerging Plasmid Delivery Vehicle for Gene Therapy

Affiliations
Review

Exosomes as an Emerging Plasmid Delivery Vehicle for Gene Therapy

Margaret Wallen et al. Pharmaceutics. .

Abstract

Despite its introduction more than three decades ago, gene therapy has fallen short of its expected potential for the treatment of a broad spectrum of diseases and continues to lack widespread clinical use. The fundamental limitation in clinical translatability of this therapeutic modality has always been an effective delivery system that circumvents degradation of the therapeutic nucleic acids, ensuring they reach the intended disease target. Plasmid DNA (pDNA) for the purpose of introducing exogenous genes presents an additional challenge due to its size and potential immunogenicity. Current pDNA methods include naked pDNA accompanied by electroporation or ultrasound, liposomes, other nanoparticles, and cell-penetrating peptides, to name a few. While the topic of numerous reviews, each of these methods has its own unique set of limitations, side effects, and efficacy concerns. In this review, we highlight emerging uses of exosomes for the delivery of pDNA for gene therapy. We specifically focus on bovine milk and colostrum-derived exosomes as a nano-delivery "platform". Milk/colostrum represents an abundant, scalable, and cost-effective natural source of exosomes that can be loaded with nucleic acids for targeted delivery to a variety of tissue types in the body. These nanoparticles can be functionalized and loaded with pDNA for the exogenous expression of genes to target a wide variety of disease phenotypes, overcoming many of the limitations of current gene therapy delivery techniques.

Keywords: cancer; exosomes; gene therapy; nanoplatform; plasmid DNA delivery.

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Conflict of interest statement

Ramesh C. Gupta holds positions both at the University of Louisville and 3P Biotechnologies. The remaining other authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Lack of modulation of species-specific cytokines following exposure to bovine milk exosomes. Female Sprague–Dawley rats (5–6 weeks old) were treated by oral gavage with a single dose or daily doses for 15 days of bovine milk exosomes (25 mg/kg b. wt.). Six hours after the final dose, animals were euthanized and serum was collected for assessment of cytokine concentration using Bio-Plex cytokine TH1/TH2 assay. Animals exposed to bovine exosomes were compared to vehicle-treated controls. Cytokines assayed included those related to (A) classic inflammation pathways; (B) T-cell activation; and (C) allergic responses. Adapted with permission from Munagala et al., 2016 [133].
Figure 2
Figure 2
Abundance of folate receptors and biodistribution of exosomal formulations with and without folic acid functionalization. (A) Detection of folate receptors in normal mouse lung and in A549 subcutaneous lung tumor xenografted in nude mice. Cells and tissue protein lysates were analyzed by western blot. FR-α, folate receptor α. (B) Biodistribution of bovine colostrum exosomes and EPM, with and without FA-functionalization of exosomes. Athymic nude mice were inoculated with A549 human lung cancer cells subcutaneously. When tumors reached 300–400 cm3, AF750-labeled bovine colostrum exosomes complexed with polyethylenimine (EPM), with and without folic acid functionalization, were administered intravenously. Animals were euthanized after 4 h and select tissues were imaged ex vivo using an advanced molecular imager (AMI1000). While FA-functionalization of EPM only marginally altered accumulation in the lung, folic acid targeting of folate receptors expressed on the tumor significantly increased accumulation (ns = not significantly different, **** p < 0.0001). Warmer colors in ex vivo organ images indicate higher fluorescent signal intensity. Data were analyzed via 2-way ANOVA and post hoc comparison of column means. Adapted with permission from Munagala et al., 2021 [131].
Figure 3
Figure 3
Entrapment of plasmid DNA by the EPM. The entrapment efficiency of the EPM was assessed using plasmid DNA at concentrations of 2 μg and 10 μg per reaction. Briefly, bovine colostrum exosomes (Exo) alone, polyethylenimine (PEI) alone, or Exo + PEI (EPM) were incubated with 2 or 10 μg of pDNA (including 32P-labeled pDNA as tracer) and the resultant complex harvested by PEG 2K precipitation. After harvesting, both the pellet and the supernatant were spotted, and 32P-signal was measured by Packard Imager. Adapted by permission from Wallen et al. 2022 [167].
Figure 4
Figure 4
Delivery of plasmid DNA by the EPM. EPM loaded with pDNA deliver the coding sequence for p53 in vivo to p53-null mice following intravenous delivery. Mice were treated for 6 days (0.6 mg/kg pDNA). Tissues were harvested 24 h after the last dose and p53 levels were assessed via western blot. All tissues analyzed contained detectable p53 expression, with the highest levels being in the lung and spleen. Adapted with permission from Munagala et al., 2021 [131].

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