The ability to deliver DNA to target cells creating therapeutic effects remains an important goal in the field of gene therapy. A majority of clinical trials to overcome this issue have utilized viral vectors due to their efficiency at DNA delivery and ability to create high levels of gene expression. However, their inherent toxicity and a several clinical trials leading to patients contracting new diseases from the treatment have greatly hindered the progress of viral gene therapy. Non-viral gene delivery agents have a much better safety profile, but are also much less efficient at delivering DNA, leading to low gene expression. The reason for this low expression is the numerous barriers that must be overcome to achieve gene expression: circulation, tissue specific accumulation, internalization, release of DNA cargo, and nuclear localization. While peptides are currently being improved upon, enhancing binding and the ability to protect DNA, they are still deficient when it comes to tissue specificity. Numerous targeting methods, including the use of lectins, antibodies, aptamers, and peptides, have been designed to deliver molecules to a specific research. Research to incorporate targeting ligands onto non-viral gene delivery vectors is abundant in the literature; however, successful site specific gene delivery has not been achieved.
The somatostatin receptor 2 (SSTR2) ligand, octreotide, is a well-researched eight amino acid peptide that has extensive SAR data available. Also, the receptors have been well characterized and octreotide is used clinically in the radioscintigraphy imaging of brain tumors. While well researched, there are unexplored opportunities to utilize octreotide to enhance non-viral gene delivery vectors.
The overall scope of this thesis is to develop and synthesize non-viral gene delivery peptides conjugated to octreotide creating receptor mediated targeting of DNA polyplexes to create tissue specific accumulation. Initial experiments indicated that attachment of octreotide to the polycationic peptide WK18 does not inhibit affinity for the SSTR2 receptor. Therefore, peptides were designed and synthesized to attach octreotide onto polyacridine peptide (Acr-Lys)6. Polyplex characteristics were unchanged by the incorporation of octreotide, and exhibited very low genotoxic effects compared to the in vitro gene delivery agent PEI. Competitive binding assays suggested a stoichiometric, ligand, and temperature dependent accumulation of polyplex on SSTR2 expressing cells, but gene expression could not be achieved.
The success of (Acr-Lys)6octreotide, led to the synthesis of a di-maleimide-PEG attached to each end by (Acr-Lys4)3Acr-Lys-Cys or Cys-Gly5octreotide in attempts to create distance, and better ligand availability for the receptor, by expressing octreotide away from the polyplex. Testing of this peptide in PEGylated polyplex ad-mixtures verified that separating the DNA binding peptide from octreotide did lead to better inhibition of binding to DAOY cells in a competitive binding study. However, transfection assays with this compound showed background levels of gene expression. Although gene expression was not achieved, the synthetic strategy to create a molecule incorporating a DNA binding peptide, ligand, and PEG to create better ligand presentation to its receptor when incorporated into PEGylated polyplexes is an important step in the design of gene delivery vectors.