Marked increase in tumor transfection with a truncated branched polymer

Background We previously determined that polyplexes formed by linear H2K peptides were more effective in transfecting tumors in vivo than polyplexes formed by branched H2K4b‐20 peptides. Based on trypsin digest and salt displacement studies, the linear H2K polyplexes were less stable than the branched H2K4b‐20 polyplexes. Because binding and release of the polymer and DNA from the H2K4b‐20 polyplex may account for the ineffectiveness, we investigated whether four‐branched histidine‐lysine (HK) peptides with varying numbers of amino acids in their branches would be more effective in their ability to increase gene expression in tumors in vivo. Methods Linear and branched peptides with multiple ‐KHHK‐ motifs were synthesized by solid‐phase synthesis. The branched H2K4b‐20, ‐18, ‐14 and 12 peptides had 20, 18, 14 and 12 amino acids in their branches, respectively. These peptides were examined for their ability to carry luciferase‐expressing plasmids to human breast cancer xenografts in a mouse model. With gel retardation and in vivo transfection, the incorporation of a targeting ligand and an endosomal lysis peptide into these polyplexes was also examined. A blocking antibody was pre‐injected prior to the polyplexes to determine the role of neuropilin 1 in the uptake of these polyplexes by the tumor. The size of the polyplexes was measured by dynamic light scattering. Results Of the four negative surface‐charge polyplexes formed by the branched carriers, the H2K4b‐14 polyplex was determined to be the most effective plasmid delivery platform to tumors. The incorporation of a targeting ligand and an endosomal lysis peptide into H2K4b‐14 polyplexes further enhanced their ability to transfect tumors in vivo. Furthermore, after pre‐injecting tumor‐bearing mice with a blocking antibody to the neuropilin‐1 receptor (NRP‐1), there was a marked reduction of tumor gene expression with the modified H2K4b‐14 polyplexes, suggesting that NRP‐1 mediated their transport into the tumor. Conclusions The present study established that branched peptides intermediate in length were very efficient in delivering plasmids to tumors in vivo. To take advantage of increased expression of αvβ3 integrins and the neuropilin receptor‐1 (NRP‐1) transport system in tumors including their vessels, several histidine‐lysine polyplexes were designed to target these receptors and carry luciferase‐expressing plasmids to tumors. After initially targeting integrins and subsequently the NRP‐1 receptor