Exploring the outer limits of polyplexes.

Histidine-containing polymers show promise in their transport of nucleic acids in vitro and in vivo. In addition to the pH-buffering histidine component, the polymer often contains a protonated component at physiological pH, such as lysine. These polyplexes usually accumulate in the tumor by enhanced permeability and retention, which has proved disappointing in clinical trials. We presently compare two histidine-lysine (HK) peptide polyplexes for their neuropilin-1-mediated transport of plasmids in vivo. While the polymerized HK (H2KC-48) polyplex was markedly better than the monomeric HK (H2K) polyplex in vitro, both HK polyplexes were effective in transfecting tumor xenografts over a wide range of peptide and plasmid ratios. Nevertheless, polyplexes of low peptide/DNA ratios gave higher tumor transfection and specificity than those of higher ratios. Surprisingly, there was minimal to no gel retardation of polyplexes made from these low ratios during electrophoresis. These results demonstrate that loosely packed HK polyplexes effectively transfected tumors in vivo.