Challenges and Opportunities in Gene Delivery - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 48

2.2 Barriers to Gene Delivery
The barriers to successful DNA delivery, with or without vector, involve extracellular
barriers to DNA delivery and DNA-vector complex and consist of barriers from the
site of injection to the cellular interaction and uptake through endocytosis. This cellu-
lar uptake may be enhanced by targeting these vectors to the desired cells with the help
of ligands whose receptors are overexpressed on the target cells (as cancer cells with
overexpressed folate and transferrin receptors) [33-35] . These barriers are followed by
the intracellular barriers involving release of DNA from the DNA-carrier system from
endosome, followed by cytosolic transport to the nucleus for transgene expression.
The intracellular trafficking of DNA, followed by nuclear targeting inside the cells,
may be facilitated by using the nuclear localization signal peptide attached with DNA
or the vector [36] . Any improvement in the efficiency of any plasmid delivery step will
lead to improved transfection efficiency. Hence, it is necessary to understand the bar-
riers involved in each individual step of the DNA delivery; these can be studied sepa-
rately and improved systematically for enhancement of transgene expression.
In addition to the extracellular and intracellular barriers, biological barriers such
as immune response to the DNA sequences with central unmethylated CpG motif
and the vectors are also observed [37-39] . However, the barrier because of plasmid
immunogenecity may be resolved by optimizing the plasmid sequence [40,41] , thus
suggesting a more significant contribution of extracellular and intracellular barriers,
and the methods to resolve these barriers toward efficacious nonviral gene delivery.
Moreover, every developed product needs to be pharmaceutically produced at large
scale and marketed nationally and internationally for proper utilization and cost recovery.
The marketing of a gene-based therapeutic product requires FDA approval, and the
manufacturing company must submit the documents and scientific evidences for safe
and effective therapy. The company should also demonstrate the controlled and vali-
dated manufacturing technologies for developing the desired quality of the product.
However, the pharmacology, batch-to-batch reproducibility at large-scale production,
and safety concerns of these biological products are significantly different from con-
ventional drugs and also different from biological drugs such as recombinant proteins,
blood products, and vaccines. These concerns also act as a significant barrier that must
be resolved by the marketing company before launching the product in the market. A
complete list of the barriers to efficient DNA delivery has been tabulated in Table 2.2.
2.3 Extracellular Barriers
Extracellular barriers to local and systemic gene delivery must be overcome for effi-
cient cellular uptake and significant transgene expression for proper disease manage-
ment. Extracellular barriers to the naked DNA and DNA-vector complex delivery are
encountered from the injection site of DNA alone or as a DNA-carrier complex, until
the DNA reaches the desired cells and is endocytosized inside the cell. The extracellu-
lar barriers encountered by the DNA and DNA-vector complex are shown in Fig. 2.1 .