Shana Kelley of the University of Toronto in Canada reveals how cell-penetrating peptides deliver the goods when it comes to biology and medicine

A universal platform to deliver bioactive agents into cells is an attractive goal for myriad applications. Although several useful drug delivery vectors have emerged in recent years, including liposomes, viral vectors, dendrimers, and polymer-based nanoparticles, each has been limited by some feature: difficult synthetic procedures, toxicity, low stability, or a restricted cargo range. The ideal cellular transporter would be low cost, non-toxic, and could transport any cargo across the cellular membrane. Such a transporter would be of enormous value to deliver drugs, imaging agents, and biomolecules such as proteins and DNA - but could such a versatile transporter exist?

The discovery of a cell-permeable class of molecules in the 1990s aroused immediate interest. The finding that simple biomolecules - small synthetically-accessible peptides - could traverse the cell membrane hinted that a versatile transporter could finally be on the horizon. Since this first discovery, research on these cell-penetrating peptides (CPPs) has been conducted at the interface of the chemical, biological, and medical communities with important contributions from each of these disciplines. Hundreds of studies have been completed, uncovering important aspects of CPPs' chemical features, interactions with membranes, how they enter into cells, and their scope as delivery vectors.

Cell-penetrating peptides can carry many different cargoes into a cell

Shortly after the first CPPs - penetratin from Drosophila melanogaster and Tat from HIV - were discovered, several more cell-penetrating sequences were identified from natural sources. This was closely followed by the development of synthetic sequences incorporating features of natural CPPs found to be critical for uptake into cells, specifically, positive charge and guanidinium headgroups. Not only were peptide sequences developed using the principles learned from natural CPPs, but other chemical scaffolds were also adapted to have cell-penetrating capacity, including beta-peptides, carbamates, and dendrimers. Studies elucidating the CPP counterion role in crossing the lipid bilayer were also critical in providing insight into the chemical modifications that would mediate cellular uptake.

What is perhaps most exciting about CPPs is the wide variety of cargoes that these short peptide sequences can deliver by mediating their uptake across the cell membrane. Using both covalent and non-covalent linkages, CPPs have transported macromolecules such as proteins, antibodies, and nucleic acids, therapeutics ranging in size and chemical character, and imaging agents, including quantum dots and magnetic particles - CPPs' potential as a universal delivery agent is evident. Moreover, their ability to preserve biological activity for therapeutic cargoes makes the CPP delivery approach truly practical.

An intriguing future direction for CPPs is the potential to achieve targeted delivery into specific organelles within cells. Imaging studies using fluorophore-labelled CPPs have shown that they locate predominantly in the nucleus and cytoplasm, but recent studies have shown that short, synthetic, cell-permeable peptide sequences can be engineered to locate within mitochondria. Critical chemical parameters modulate this localisation. Given mitochondria have roles in many cellular processes and their connection with the treatment of neurodegenerative diseases and cancer, this finding could portend a more specific delivery application for CPPs.

Clearly, CPPs are an important tool for intracellular delivery and a means to study transport across lipid bilayers. CPPs' future in biology and medicine will depend on acquiring a comprehensive understanding of their activity and delivery potential, and developing cost-effective and simple methods for their application.

Read Shana Kelley et al's perspective 'Cell-Penetrating Peptides as Delivery Vehicles for Biology and Medicine' in a forthcoming issue of Organic and Biomolecular Chemistry.