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Fluorescence-based investigations of the dynamics and properties of cell penetrating peptide cellular uptake mechanisms


Therapeutic drug delivery to diseased tissues and organs specifically and without toxicity to the body is a huge challenge in the field of medicine and pharmaceutical chemistry. Researchers are investigating a variety of methods to overcome the various membranous biological barriers to gain entry into a diseased cell. Cell penetrating peptides (CPPs) received a lot of attention in the last two decades as a tool that could solve the drug delivery problem. Although there has been broad and continued interest in CPPs for their ability to innocuously and efficiently deliver therapeutics into individual cells, there is still much to understand about the cell entry mechanism and methods to evaluate the efficiency of entry have been lacking. To provide a thorough analysis of the CPP delivery efficiency, clarify the details of the mechanism, and characterize the pathway of cell entry, here we present an analysis of one CPP, D- arginine₉ (r₉). Using unique fluorescence based and FRET assays we show that r₉ is able to enter the cytosol and delivery a cargo at a rate and concentration that indicates that it can be potentially very useful for drug delivery. Additionally, we have discovered that intramolecular dimers of r₉ molecules can escape into the cytosol at rates 4-6x fold better than monomers. Furthermore, we discovered that endocytic vesicles containing the r₉ molecules are rapidly acidified within 28 minutes and furthermore there is indication that acidification prevents the release of these peptides into the cytosol. To provide reference to a non-peptide drug delivery technology, we compare the rate of r₉ cytosolic entry to a non-peptide guanidinium molecular transporter, Guanidinylated Neomycin using the same FRET assay. Finally, using common biochemical inhibitors of various aspects of endocytosis, we provide some evidence of the promiscuity of the endocytic routes by which r₉ may be gaining entry to the cytosol. It is our hope that these data provide some insight for future directions of the drug delivery field of study

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