Strategies for the Activation and Release of the Membranolytic Peptide Melittin from Liposomes Using Endosomal pH as a Trigger

Receptor-Targeted Dual pH-Triggered Intracellular Protein Transfer

Protein therapeutics are of widespread interest due to their successful performance in the current pharmaceutical and medical fields, even though their broad applications have been hindered by the lack of an efficient intracellular delivery approach. Herein, we fabricated an active-targeted dual pH-responsive delivery system with favorable tumor cell entry augmented by extracellular pH-triggered charge reversal and tumor receptor targeting and pH-controlled endosomal release in a traceless fashion. As a traceable model protein, the enhanced green fluorescent protein (eGFP) bearing a nuclear localization signal was covalently coupled with a pH-labile traceless azidomethyl-methylmaleic anhydride (AzMMMan) linker followed by functionalization with different molar equivalents of two dibenzocyclooctyne-octa-arginine-cysteine (DBCO-R8C)-modified moieties: polyethylene glycol (PEG)-GE11 peptide for epidermal growth factor receptor-mediated targeting and melittin for endosomal escape. The cationic melittin domain was masked with tetrahydrophthalic anhydride revertible at mild acidic pH 6.5. At the optimally balanced ratio of functional units, the on-demand charge conversion at tumoral extracellular pH 6.5 in combination with GE11-mediated targeting triggered enhanced electrostatic cellular attraction by the R8C cell-penetrating peptides and melittin, as demonstrated by strongly enhanced cellular uptake. Successful endosomal release followed by nuclear localization of the eGFP cargo was obtained by taking advantage of melittin-mediated endosomal escape and rapid traceless release from the AzMMMan linker. The effectiveness of this multifunctional bioresponsive system suggests a promising strategy for delivery of protein drugs toward intracellular targets. A possible therapeutic relevance was indicated by an example of cytosolic delivery of cytochrome c initiating the apoptosis pathway to kill cancer cells.

Recent Advances of Cell-Penetrating Peptides and Their Application as Vectors for Delivery of Peptide and Protein-Based Cargo Molecules

Peptides and proteins, two important classes of biomacromolecules, play important roles in the biopharmaceuticals field. As compared with traditional drugs based on small molecules, peptide- and protein-based drugs offer several advantages, although most cannot traverse the cell membrane, a natural barrier that prevents biomacromolecules from directly entering cells. However, drug delivery via cell-penetrating peptides (CPPs) is increasingly replacing traditional approaches that mediate biomacromolecular cellular uptake, due to CPPs' superior safety and efficiency as drug delivery vehicles. In this review, we describe the discovery of CPPs, recent developments in CPP design, and recent advances in CPP applications for enhanced cellular delivery of peptide- and protein-based drugs. First, we discuss the discovery of natural CPPs in snake, bee, and spider venom. Second, we describe several synthetic types of CPPs, such as cyclic CPPs, glycosylated CPPs, and D-form CPPs. Finally, we summarize and discuss cell membrane permeability characteristics and therapeutic applications of different CPPs when used as vehicles to deliver peptides and proteins to cells, as assessed using various preclinical disease models. Ultimately, this review provides an overview of recent advances in CPP development with relevance to applications related to the therapeutic delivery of biomacromolecular drugs to alleviate diverse diseases.

Endosomal escape for cell-targeted proteins. Going out after going in

Protein-based nanocarriers are versatile and biocompatible drug delivery systems. They are of particular interest in nanomedicine as they can recruit multiple functions in a single modular polypeptide. Many cell-targeting peptides or protein domains can promote cell uptake when included in these nanoparticles through receptor-mediated endocytosis. In that way, targeting drugs to specific cell receptors allows a selective intracellular delivery process, avoiding potential side effects of the payload. However, once internalized, the endo-lysosomal route taken by the engulfed material usually results in full degradation, preventing their adequate subcellular localization, bioavailability and subsequent therapeutic effect. Thus, entrapment into endo-lysosomes is a main bottleneck in the efficacy of protein-drug nanomedicines. Promoting endosomal escape and preventing lysosomal degradation would make this therapeutic approach clinically plausible. In this review, we discuss the mechanisms intended to evade lysosomal degradation of proteins, with the most relevant examples and associated strategies, and the methods available to measure that effect. In addition, based on the increasing catalogue of peptide domains tailored to face this challenge as components of protein nanocarriers, we emphasize how their particular mechanisms of action can potentially alter the functionality of accompanying protein materials, especially in terms of targeting and specificity in the delivery process.

Application of Peptides in Construction of Nonviral Vectors for Gene Delivery

Gene therapy, which aims to cure diseases by knocking out, editing, correcting or compensating abnormal genes, provides new strategies for the treatment of tumors, genetic diseases and other diseases that are closely related to human gene abnormalities. In order to deliver genes efficiently to abnormal sites in vivo to achieve therapeutic effects, a variety of gene vectors have been designed. Among them, peptide-based vectors show superior advantages because of their ease of design, perfect biocompatibility and safety. Rationally designed peptides can carry nucleic acids into cells to perform therapeutic effects by overcoming a series of biological barriers including cellular uptake, endosomal escape, nuclear entrance and so on. Moreover, peptides can also be incorporated into other delivery systems as functional segments. In this review, we referred to the biological barriers for gene delivery in vivo and discussed several kinds of peptide-based nonviral gene vectors developed for overcoming these barriers. These vectors can deliver different types of genetic materials into targeted cells/tissues individually or in combination by having specific structure-function relationships. Based on the general review of peptide-based gene delivery systems, the current challenges and future perspectives in development of peptidic nonviral vectors for clinical applications were also put forward, with the aim of providing guidance towards the rational design and development of such systems.

Bio-nano scale modifications of melittin for improving therapeutic efficacy

INTRODUCTION

Melittin (MLT), a natural membrane-active component, is the most prominent cytolytic peptide from bee venom. Remarkable biological properties of MLT, including anti-inflammatory, antimicrobial, anticancer, anti-protozoan, and antiarthritic activities, make it an up-and-coming therapeutic candidate for a wide variety of human diseases. Therapeutic applications of MLT may be hindered due to low stability, high toxicity, and weak tissue penetration. Different bio-nano scale modifications hold promise for improving its functionality and therapeutic efficacy.

AREAS COVERED

In the current review, we aimed to provide a comprehensive insight into strategies used for MLT conjugations and modifications, cellular delivery of modified forms, and their clinical perspectives by reviewing the published literature on PubMed, Scopus, and Google Scholar databases. We also emphasized the MLT structure modifications, mechanism of action, and cellular toxicity.

EXPERT OPINION

Developing new analogs and conjugates of MLT as a natural drug with improved functions and fewer side effects is crucial for the clinical translation of this approach worldwide, especially where the chemicals and synthetic drugs are more expensive or unavailable in the healthcare system. MLT-nanoconjugation may be one of the best-optimized strategies for improving peptide delivery, increasing its therapeutic efficacy, and providing minimal nonspecific cellular lytic activity. [Figure: see text].

Preparation and application of pH-responsive drug delivery systems

Microenvironment-responsive drug delivery systems (DDSs) can achieve targeted drug delivery, reduce drug side effects and improve drug efficacies. Among them, pH-responsive DDSs have gained popularity since the pH in the diseased tissues such as cancer, bacterial infection and inflammation differs from a physiological pH of 7.4 and this difference could be harnessed for DDSs to release encapsulated drugs specifically to these diseased tissues. A variety of synthetic approaches have been developed to prepare pH-sensitive DDSs, including introduction of a variety of pH-sensitive chemical bonds or protonated/deprotonated chemical groups. A myriad of nano DDSs have been explored to be pH-responsive, including liposomes, micelles, hydrogels, dendritic macromolecules and organic-inorganic hybrid nanoparticles, and micron level microspheres. The prodrugs from drug-loaded pH-sensitive nano DDSs have been applied in research on anticancer therapy and diagnosis of cancer, inflammation, antibacterial infection, and neurological diseases. We have systematically summarized synthesis strategies of pH-stimulating DDSs, illustrated commonly used and recently developed nanocarriers for these DDSs and covered their potential in different biomedical applications, which may spark new ideas for the development and application of pH-sensitive nano DDSs.

Melittin-Based Nano-Delivery Systems for Cancer Therapy

Melittin (MEL) is a 26-amino acid polypeptide with a variety of pharmacological and toxicological effects, which include strong surface activity on cell lipid membranes, hemolytic activity, and potential anti-tumor properties. However, the clinical application of melittin is restricted due to its severe hemolytic activity. Different nanocarrier systems have been developed to achieve stable loading, side effects shielding, and tumor-targeted delivery, such as liposomes, cationic polymers, lipodisks, etc. In addition, MEL can be modified on nano drugs as a non-selective cytolytic peptide to enhance cellular uptake and endosomal/lysosomal escape. In this review, we discuss recent advances in MEL’s nano-delivery systems and MEL-modified nano drug carriers for cancer therapy.

In-depth drug delivery to tumoral soft tissues via pH responsive hydrogel

A pH responsive nanoparticle–hydrogel hybrid drug delivery system was investigated for in-depth anticancer drug delivery to solid tumours. It consists of acid susceptible polymer nanoparticles loaded in a chitosan hydrogel. The hybrid formulation was characterized by UV-visible spectroscopy, FTIR, SEM, TEM, particle size analysis, zeta potential measurement and viscosity measurement. Drug encapsulation and nanoparticle loading efficiencies were found to be 48% and 72% respectively which describes the efficient interaction of the chemical entities in this hybrid drug delivery system. The hydrogel exhibited pH responsive behaviour: minimal drug and nanoparticle release at physiological pH but an increase in viscosity under acidic conditions and fast nanoparticle and drug release. The cytotoxicity of the drug loaded hydrogel was investigated against the MCF-7 breast cancer cell line along with the drug and nanoparticles without hydrogel. The drug loaded hydrogel showed a better cytotoxic effect on MCF-7 cancer cells. Thus, drug loaded nanoparticles containing hydrogel could be a better option for maximum drug distribution in tumours.

A pH responsive nanoparticle–hydrogel hybrid drug delivery system was investigated for in-depth anticancer drug delivery to solid tumours.

Formulation and delivery solutions for the next generation biotherapeutics

In 2018 I was appointed full professor of Pharmaceutical Biotechnology & Delivery at the Pharmaceutics division of the department of Pharmaceutical Sciences at Utrecht University, The Netherlands. In this contribution to the Orations - New Horizons of the Journal of Controlled Release I will introduce my research group (see also www.uu.nl/pharmaceutics) and will highlight my current and future research projects. In coming years the focus of my research will be on the administration of biotherapeutics, aiming to control their fate from the site of injection to the site of action. I will discuss issues related to formulation of biotherapeutics into nanomedicines (NMs), intracellular delivery of nucleic acids as well as protein therapeutics, and targeted delivery of biotherapeutics beyond the liver. In addition, I will provide a forward view on how current developments in the drug delivery and gene therapy field may result in sustainable and cost-effective dosing regimens for biotherapeutics.

Co-delivery of Bee Venom Melittin and a Photosensitizer with an Organic-Inorganic Hybrid Nanocarrier for Photodynamic Therapy and Immunotherapy

Photodynamic therapy (PDT) is a clinical cancer treatment modality based on the induction of therapeutic reactive oxygen species (ROS), which can trigger immunogenic cell death (ICD). With the aim of simultaneously improving both PDT-mediated intracellular ROS production and ICD levels, we designed a serum albumin (SA)-coated boehmite ("B"; aluminum hydroxide oxide) organic-inorganic scaffold that could be loaded with chlorin e6 (Ce6), a photosensitizer, and a honey bee venom melittin (MLT) peptide, denoted Ce6/MLT@SAB. Ce6/MLT@SAB was anchored by a boehmite nanorod structure and exhibited particle size of approximately 180 nm. Ce6/MLT@SAB could significantly reduce hemolysis relative to that of free MLT, while providing MLT-enhanced PDT antitumor effects in vitro. Compared with Ce6@SAB, Ce6/MLT@SAB improved Ce6 penetration of cancer cells both in vitro and in vivo, thereby providing enhanced intracellular ROS generation with 660 nm light treatment. Following phototreatment, Ce6/MLT@SAB-treated cells displayed significantly improved levels of ICD and abilities to activate dendritic cells. In the absence of laser irradiation, multidose injection of Ce6/MLT@SAB could delay the growth of subcutaneous murine tumors by more than 60%, compared to controls. When combined with laser irradiation, a single injection and phototreatment with Ce6/MLT@SAB eradicated one-third of subcutaneous tumors in treated mice. The addition of an immune checkpoint blockade to Ce6/MLT@SAB phototreatment further augmented antitumor effects, generating increased numbers of CD4⁺ and CD8⁺ T cells in tumors with concomitant reduction of myeloid-derived suppressor cells.

Bioresponsive polyplexes - chemically programmed for nucleic acid delivery

INTRODUCTION

The whole delivery process of nucleic acids is very challenging. Appropriate carrier systems are needed, which show extracellular stability and intracellular disassembly. Viruses have developed various strategies to meet these requirements, as they are optimized by biological evolution to transfer genetic information into host cells. Taking viruses as models, smart synthetic carriers can be designed, mimicking the efficient delivery process of viral infection. These 'synthetic viruses' are pre-programmed and respond to little differences in their microenvironment, caused by either exogenous or endogenous stimuli.

AREAS COVERED

This review deals with polymer-based, bioresponsive nanosystems (polyplexes) for the delivery of nucleic acids. Strategies utilizing pH-responsiveness, redox-responsiveness as well as sensitivity towards enzymes will be described more in detail. Systems, which respond to other endogenous triggers (i.e. reactive oxygen species, adenosine triphosphate, hypoxia), will be briefly illustrated. Moreover, some examples for combined bioresponsiveness will be presented.

EXPERT OPINION

Bioresponsive polyplexes are a smart way to facilitate programmed, timely delivery of nucleic acids to desired, specific sites. Nevertheless, further optimization is necessary to improve the still moderate transfection efficiency and specificity - also in regard to medical translation. For this purpose, precise carrier structures are desirable and stability issues of bioresponsive systems must be considered.

Nanoscale Melittin@Zeolitic Imidazolate Frameworks for Enhanced Anticancer Activity and Mechanism Analysis

The cytolytic peptide melittin (MLT) is an important candidate of anticancer drug owing to its hemolytic properties. Nevertheless, its clinical applications are severely restricted as a result of its nonspecific toxicities like hemolysis. In this work, we reported MLT-loaded zeolitic imidazolate framework-8 (MLT@ZIF-8) nanoparticles (NPs). The formed MLT@ZIF-8 NPs not only possess excellent stability but also efficiently inhibit the hemolysis bioactivity of MLT. Confocal scanning imaging and cytotoxicity experiments revealed that as-synthesized MLT@ZIF-8 NPs exhibit enhanced cellular uptake and cytotoxicity toward cancer cells compared to MLT. The mechanism is well investigated by a series of transcriptome analysis, which indicates that MLT@ZIF-8 NPs can regulate the expression of 3383 genes, and the PI3K/Akt-regulated p53 pathway is involved in MLT@ZIF-8 NPs induced A549 cells apoptosis. Finally, MLT@ZIF-8 NPs exhibit enhanced antitumor activity than free MLT in vivo, while no obvious systemic toxicity has been found. This work emphasizes the great potential of utilizing MOF as a simple and efficient nanoplatform for deliverying cytolytic peptides in cancer treatment, and also the investigation on the antitumor mechanism could provide theoretical support for clinical usage of MLT.