**1. Introduction**

A novel approach to overcome cell membrane impermeability and to deliver a large variety of particles and macromolecules into cells has been recently emerged, which is called cell-penetrating peptides (CPPs), also known as protein transduction domains (PTDs) [1, 2]. CPPs are generally short (up to 30 amino acids in length) water-soluble, cationic, and/or amphipathic peptides which make them promising vectors for therapeutic delivery, leading to a considerable amount of research focused on the intracellular delivery of drugs [3–5]. There are two principal types of CPPs that have been utilized for this purpose: (i) cationic CPPs, composed of short sequence of amino acids (arginine, lysine, and histidine). The indicated amino acids give the cationic charge to the peptide and permit its interaction with anionic motifs on the plasma membrane by a receptor-independent mechanism. (ii) amphipathic peptides, which have lipophilic and hydrophilic tails that are responsible for a direct peptide translocation mechanism across the plasma membrane [6].

The most important characteristic of CPPs is that they are able to translocate the plasma membrane at low micromolar concentrations in vivo and in vitro without using any receptors and without causing any significant membrane damage [7, 8]. Other benefits of using CPPs for therapeutic delivery are the absence of toxicity as compared to other cytoplasmic delivery devices, such as liposomes, polymers, etc. [6]. The mechanism for the CPP-facilitated cellular uptake remains not clear and depends on cargo and cellular type [9]. Due to its high density of basic amino acid residues (Arg and Lys), the large charge at physiological pH excludes the passive diffusion of CPPs across the lipid bilayer. Furthermore, it seems that classical uptake mechanisms such as protein-based receptors and transporters are not involved. On the contrary, endocytosis was shown as a common uptake mechanism, but is controversial at the same time. For example, in a number of reports, CPP

uptake was not inhibited at 4°C or in the presence of inhibitors of endocytosis; in contrast, a capture of CPPs in the endocytotic vesicles was observed when soluble heparin sulfate was added [9, 10]. Many other studies indicate that aggregation of the cell surface glycosaminoglycan heparan sulfate (HS) is an important element in the uptake mechanism [2]. The challenge of the strategy using CPPs should take into consideration the size, stability, nonspecific versus specific associations, and potency versus toxicity that all play an important role for the selection of delivery systems [5].
