Current Pharmaceutical Biotechnology - Volume 15, Issue 3, 2014

In recent years biomacromolecular drugs including proteins and nucleic acids have been emerging among the most promising therapeutics and globally recognized as one of the most cutting-edge areas of drug research. However, the nature of strong hydrophilicity and high molecular weight creates a formidable barrier for the biomacromolecules to diffuse through the cell membrane. Therefore, along with rapid development of biodrugs, cell-penetrating peptides (CPP) have also been intensively studied for the application in enhanced intracellular delivery of protein and nucleic acid drugs. CPP has also been found very effective in overcoming various bio-barriers at tissue levels, for instance, blood-brain barrier, [1] skin, [2, 3] nasal mucosa, [4] intestinal walls, [5] and intratumoral penetration barrier [6]. Hence, the research focus has gradually shifted from intracellular delivery to in vivo delivery. In this special issue, we include eight review papers that provide a glimpse into the CPP-based drug delivery. David and Choi provide a brief outline of the potential mechanisms for CPP-mediated cellular uptake including endocytosis and non-endocytosis pathways. Often, the translocation involves more than one pathway, as a result of multiple mechanisms varying with the diversity of CPPs and their cargoes. More recently, scientists have started to think more about the in vivo profiles of CPPs. The nature of nonselective penetration of CPP usually causes wide bio-distribution and unwanted drug exposure. Therefore, the pharmacokinetics information must be taken into account for rational design of CPP-based drug delivery, of which Mier and his colleagues provide a summary about the targeting and biodistribution issues. In particular, they summarized the strategy of using targeting motifs to modify the CPPs for altering their bio-fate. They also proposed that “smart delivery platforms” would be a potential solution for achieving targeted delivery. He et al. provide a detailed review on intelligent liposomal systems for targeted tumor delivery by employing the activatable CPP strategy. The activation is specifically triggered by the tumor-associated factors (e.g. low pH, proteases) or external stimuli (e.g. UV or exogenous agents). Of note, blood-brain barrier represents one of the most difficult challenges against effective drug delivery to the central nervous system, and Chen and his colleagues describe the promising CPP-based brain delivery techniques, in which CPPs play an important role in overcoming the formidable barrier. Topical application of CPP may benefit from its local action, because of limiting the unwanted wide drug distribution. Huang and Moon et al. give discussion on the transdermal and nasal delivery mediated by CPPs. Wang and He et al. outline the application of CPP-assisted oral insulin delivery. Due to charge interaction, conjugation of insulin with CPP still remains a problem, and they proposed a proper selection of linker would be helpful. Sun et al. pay attention to vaccine delivery. They mainly discuss the noninvasive vaccination based on CPP-mediated delivery including protein, peptide and DNA vaccines. Last but not least, Lee and his colleagues focus on an interesting topic of CPP-mediated delivery by non-covalent forms, which is featured by its ease to produce and use, and versatility to various types of cargos without the need of custom-tailored conjugation processes, compared to the chemical method. Growing understanding of the molecular mechanisms has promoted CPP’s application in pharmacotherapy against many diseases. Prospects for clinical translation of CPP-based therapy have emerged, and some clinical trials have been well underway [7]. Therefore, we believe such a featured topic of “Cell-penetrating Peptides and Drug Delivery” should be of great interest to the pharmaceutical community, as well as to colleagues in a broad biomedical field.

A major thrust in the biomedical and pharmaceutical industries is to develop diagnostic and therapeutic tools that have significantly improved selectivity and specificity compared to the current state-of-the-art. This has driven much of the effort to look at molecules and materials that are significantly larger than the traditional small molecule agents. Due to size restrictions, however, many of these materials are unable to penetrate the cell membrane and gain access to the intracellular components on which they exert their action. The relatively recent discovery of cell penetrating peptides (CPP) provides a powerful tool that has enabled the intracellular delivery of these materials. While a variety of proteins, DNA, polymers and even nanoparticles have been successfully delivered into cells, there still remains some debate as to the mechanism of entry utilized by the CPPs. In this review, we provide a brief outline of the different potential mechanisms for cellular uptake of CPPs.

Therapeutics are restricted from cellular internalization due to the biological barrier formed by the cell membrane. Especially for therapeutics with high molecular weight, strategies are required to enable delivery to intracellular targets. Cell-penetrating peptides (CPPs) represent a powerful tool to mediate the entry of large cargos such as proteins, siRNA and nanoparticles. The high diversity of CPPs is the prerequisite to use this class of carriers for various applications. However, therapies based on CPPs are hampered by their unfavorable pharmacokinetics, mainly dominated by their rapid renal clearance and their lack of specificity. Rational design is required to overcome these disadvantages and thereby exploits the actual potential of CPPs. We summarize and highlight the current state of knowledge with special emphasis on pharmacokinetics. The unclear internalization pathways of CPPs remain one of the main obstacles and therefore have been in the focus of research. In this review, several promising strategies such as the combination with targeting sequences, activatable CPPs and adjustment of the molecular weight are described. In addition, new absorption pathways such as nasal, pulmonary or transdermal uptake expand the applicability of CPPs and may be a promising prospect for clinical application.

Liposomes are widely used as drug delivery systems and several liposome-based nanomedicines have been approved for clinical use. Cell penetrating peptides (CPPs) have been decorated onto nanoparticulated vesicle such as liposomes to further improve the intracellular delivery efficiency. However, the poor selectivity of CPPs hindered their application, especially in the in vivo application. To resolve this issue, several strategies have been developed, including shielding and environment-triggered deshielding of CPPs as well as designing of environment-responsive CPPs and specific- targeting CPPs and last but not least, combination strategy. In this review, the abovementioned strategies were discussed.

The existence of blood-brain barrier (BBB) represents the most formidable challenge for drug delivery to the central nervous system (CNS). Modern breakthrough in biology offers multiple choices for overcoming this barrier but yields modest outcomes for clinical application due to various problems such as safety concerns, insufficient delivery efficiency and poor penetration. Cell penetrating peptides (CPPs) possessing powerful transmembrane capacity have been shown to be effective transport vectors for bioactive molecules and an attractive alternative to traditional active targeting approaches. However, the non-specificity of CPPs has hindered them from targeting a desired site of action. Promisingly, design of novel CPP-mediated nanoparticulate delivery systems with specific targeting property may extricate CPPs from the dilemma. In this review, both the traditional and novel applications of CPPs-based strategies for CNS drug delivery will be discussed.

Biomacromolecules play an important role in the treatment of many diseases, but their topical application has been limited by their poor cell membrane permeability. Cell-penetrating peptides (CPPs) have been investigated over the past few decades as a potential strategy to overcome this challenge. CPPs can effectively mediate the cellular uptake of poorly internalized biomacromolecules without unfavorable side effects and have therefore attracted considerable interest as a viable alternative to existing parenteral therapies involving biomacromolecules. In this review, we will highlight the application of CPPs to the non-invasive delivery of biomacromolecules, with a focus on percutaneous and nasal absorption of biomacromolecules administered using CPPs.

A variety of methods including penetrating enhancers, enzyme inhibitors, as well as cargo mediated drug delivery have been explored to improve the intolerance of parenteral administrated insulin, but little success has been achieved so far. Under this background, cell penetrating peptides (CPPs), with their ability to enhance transport efficiency of macromolecular drugs have been demonstrated to be able to increase insulin bioavailability (BA) in a number of studies, of which a BA up to 50.7% relative to subcutaneously administered insulin could be achieved by nasal route under optimal conditions. Furthermore, CPPs could be conveniently formulated with insulin, or be grafted onto drug-loaded cargoes to facilitate the cargo mediated insulin delivery. Here we reviewed the recent achievements on CPP-mediated insulin transport, and outlined various CPP-based delivery strategies which are expected to show potential in clinical translation in the future.

The poor permeability of cellular membrane is one of the principal hurdles of most drug administrations including vaccine system, consequently impairing drug efficiency. In order to address this natural barrier, cell-penetrating peptides (CPPs) have been developed to facilitate molecular cargos to enter cells. CPPs, generally short peptides with 5-30 amino acids, can be classified in to cationic, amphipathic and hydrophobic types according to physical-chemical properties. In this review, different processes of antigen uptake, subsequent processing and presentation of CPP-based vaccine are comprehensively summarized despite precise mechanisms are not still established. Some experimental factors such as cell types, concentration of CPPs and antigenic cargos have been reported to affect antigen uptake orientation. Moreover, this review discusses different antigenic cargos where CPPs can transport for vaccine delivery as well as the application of CPPs in vaccine system via different administration routes like transdermal, subcutaneous and mucosal immunization.

Cellular and nuclear delivery of biomolecules is limited by low membrane permeability. Cell-penetrating peptides (CPPs) can be covalently linked to cargos to improve cellular internalization. Our work indicates that arginine-rich CPPs are also able to interact with a variety of cargos, including DNA, RNA, proteins and nanomaterials, in a noncovalent manner and subsequently effect their delivery into cells. The advantages of noncovalent attachment in CPP-mediated transduction are multiple: ease of use, ease of production, and versatility with respect to both cargo composition and functional delivery (i.e., the cargo is not chemically modified). We have extended this approach to achieve simultaneous transduction of covalently and noncovalently associated complexes, opening a new method for delivering multiple types of cargos, including proteins, fluorescent nanomaterials, nucleic acid and others. These novel variations of CPP-mediated transport should be of broad utility in the transport of genes, small interfering RNAs, proteins and nanoparticles in biomedical research and therapeutic intervention.

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS). The exact pathomechanism is unknown, but an aberrant immune response against CNS antigens, leading to inflammation in brain and spinal cord followed by demyelination, axonal damage and scar formation, seems to play a major role. Later in the disease course, inflammation decreases, while neurodegeneration proceeds. Approximately 80% of the patients initially show a relapsing-remitting disease course (RRMS), but the majority of them later develops a secondary progressive MS (SPMS). A minority suffers from primary progressive MS (PPMS). Primary goals of long-term MS therapy are to prevent relapses and disease progression. Assuming that MS is an autoimmune disease, most therapeutics aim to modulate or suppress the immune system. Until now many drugs have proven efficacy in RRMS, but none in PPMS. Interferon-β (IFN-β) and glatiramer acetate are known in RRMS therapy for years. Based on preclinical research and clinical trials, new treatment strategies have emerged and have been transferred from bench to bedside. The α4β-integrin-antagonist natalizumab was approved in 2005. Fingolimod, dimethyl fumarate and teriflunomide were the first oral drugs introduced in MS therapy. Recently alemtuzuab, another monoclonal antibody, was approved in Europe. Promising future perspectives are alemtuzumab, daclizumab, and laquinimod. Here, we review drug mechanisms in the therapy of MS. The mechanisms of action and the effect of the drugs on the immune system are summarized. We report recent results of clinical trials, highlight special features of different treatment strategies, and discuss future perspectives and ongoing clinical trials.