Applying the Retro-Enantio Approach To Obtain a Peptide Capable of Overcoming the Blood-Brain Barrier

From In Vitro Promise to In Vivo Reality: An Instructive Account of Infection Model Evaluation of Antimicrobial Peptides

Antimicrobial peptides (AMPs) are regarded as a promising alternative to traditional antibiotics in the face of ever-increasing resistance. However, many AMPs fail to progress into clinics due to unexpected difficulties found in preclinical in vivo phases. Our research has focused on crotalicidin (Ctn), an AMP from snake venom, and a fragment thereof, Ctn[15-34], with improved in vitro antimicrobial and anticancer activities and remarkable serum stability. As the retroenantio versions of both AMPs maintained favorable profiles, in this work, we evaluate the in vivo efficacy of both the native-sequence AMPs and their retroenantio counterparts in a murine infection model with Acinetobacter baumannii. A significant reduction in bacterial levels is found in the mice treated with Ctn[15-34]. However, contrary to expectations, the retroenantio analogs either exhibit toxicity or lack efficacy when administered to mice. Our findings underscore the critical importance of in vivo infection model evaluation to fully calibrate the therapeutic potential of AMPs.

Chikusetsu Saponin IVa liposomes modified with a retro-enantio peptide penetrating the blood-brain barrier to suppress pyroptosis in acute ischemic stroke rats

BACKGROUND

The therapeutic strategies for acute ischemic stroke were faced with substantial constraints, emphasizing the necessity to safeguard neuronal cells during cerebral ischemia to reduce neurological impairments and enhance recovery outcomes. Despite its potential as a neuroprotective agent in stroke treatment, Chikusetsu saponin IVa encounters numerous challenges in clinical application.

RESULT

Brain-targeted liposomes modified with THRre peptides showed substantial uptake by bEnd. 3 and PC-12 cells and demonstrated the ability to cross an in vitro blood-brain barrier model, subsequently accumulating in PC-12 cells. In vivo, they could significantly accumulate in rat brain. Treatment with C-IVa-LPs-THRre notably reduced the expression of proteins in the P2RX7/NLRP3/Caspase-1 pathway and inflammatory factors. This was evidenced by decreased cerebral infarct size and improved neurological function in MCAO rats.

CONCLUSION

The findings indicate that C-IVa-LPs-THRre could serve as a promising strategy for targeting cerebral ischemia. This approach enhances drug concentration in the brain, mitigates pyroptosis, and improves the neuroinflammatory response associated with stroke.

Overcoming Barriers in Glioblastoma-Advances in Drug Delivery Strategies

The world of cancer treatment is evolving rapidly and has improved the prospects of many cancer patients. Yet, there are still many cancers where treatment prospects have not (or hardly) improved. Glioblastoma is the most common malignant primary brain tumor, and even though it is sensitive to many chemotherapeutics when tested under laboratory conditions, its clinical prospects are still very poor. The blood-brain barrier (BBB) is considered at least partly responsible for the high failure rate of many promising treatment strategies. We describe the workings of the BBB during healthy conditions and within the glioblastoma environment. How the BBB acts as a barrier for therapeutic options is described as well as various approaches developed and tested for passing or opening the BBB, with the ultimate aim to allow access to brain tumors and improve patient perspectives.

Efficient systemic CNS delivery of a therapeutic antisense oligonucleotide with a blood-brain barrier-penetrating ApoE-derived peptide

Antisense oligonucleotide (ASO) has emerged as a promising therapeutic approach for treating central nervous system (CNS) disorders by modulating gene expression with high selectivity and specificity. However, the poor permeability of ASO across the blood-brain barrier (BBB) diminishes its therapeutic success. Here, we designed and synthesized a series of BBB-penetrating peptides (BPP) derived from either the receptor-binding domain of apolipoprotein E (ApoE) or a transferrin receptor-binding peptide (THR). The BPPs were conjugated to phosphorodiamidate morpholino oligomers (PMO) that are chemically analogous to the 2'-O-(2-methoxyethyl) (MOE)-modified ASO approved by the FDA for treating spinal muscular atrophy (SMA). The BPP-PMO conjugates significantly increased the level of full-length SMN2 in the patient-derived SMA fibroblasts in a concentration-dependent manner with minimal to no toxicity. Furthermore, the systemic administration of the most potent BPP-PMO conjugates significantly increased the expression of full-length SMN2 in the brain and spinal cord of SMN2 transgenic adult mice. Notably, BPP8-PMO conjugate showed a 1.25-fold increase in the expression of full-length functional SMN2 in the brain. Fluorescence imaging studies confirmed that 78% of the fluorescently (Cy7)-labelled BPP8-PMO reached brain parenchyma, with 11% uptake in neuronal cells. Additionally, the BPP-PMO conjugates containing retro-inverso (RI) D-BPPs were found to possess extended half-lives compared to their L-counterparts, indicating increased stability against protease degradation while preserving the bioactivity. This delivery platform based on BPP enhances the CNS bioavailability of PMO targeting the SMN2 gene, paving the way for the development of systemically administered neurotherapeutics for CNS disorders.

Prediction of blood-brain barrier penetrating peptides based on data augmentation with Augur

BACKGROUND

The blood-brain barrier serves as a critical interface between the bloodstream and brain tissue, mainly composed of pericytes, neurons, endothelial cells, and tightly connected basal membranes. It plays a pivotal role in safeguarding brain from harmful substances, thus protecting the integrity of the nervous system and preserving overall brain homeostasis. However, this remarkable selective transmission also poses a formidable challenge in the realm of central nervous system diseases treatment, hindering the delivery of large-molecule drugs into the brain. In response to this challenge, many researchers have devoted themselves to developing drug delivery systems capable of breaching the blood-brain barrier. Among these, blood-brain barrier penetrating peptides have emerged as promising candidates. These peptides had the advantages of high biosafety, ease of synthesis, and exceptional penetration efficiency, making them an effective drug delivery solution. While previous studies have developed a few prediction models for blood-brain barrier penetrating peptides, their performance has often been hampered by issue of limited positive data.

RESULTS

In this study, we present Augur, a novel prediction model using borderline-SMOTE-based data augmentation and machine learning. we extract highly interpretable physicochemical properties of blood-brain barrier penetrating peptides while solving the issues of small sample size and imbalance of positive and negative samples. Experimental results demonstrate the superior prediction performance of Augur with an AUC value of 0.932 on the training set and 0.931 on the independent test set.

CONCLUSIONS

This newly developed Augur model demonstrates superior performance in predicting blood-brain barrier penetrating peptides, offering valuable insights for drug development targeting neurological disorders. This breakthrough may enhance the efficiency of peptide-based drug discovery and pave the way for innovative treatment strategies for central nervous system diseases.

A transferrin receptor targeting dual-modal MR/NIR fluorescent imaging probe for glioblastoma diagnosis

The prognosis of glioblastoma (GBM) remains challenging, primarily due to the lack of a precise, effective imaging technique for comprehensively characterization. Addressing GBM diagnostic challenges, our study introduces an innovative dual-modal imaging that merges near-infrared (NIR) fluorescent imaging with magnetic resonance imaging (MRI). This method employs superparamagnetic iron oxide nanoparticles coated with NIR fluorescent dyes, specifically Cyanine 7, and targeted peptides. This synthetic probe facilitates MRI functionality through superparamagnetic iron oxide nanoparticles, provides NIR imaging capability via Cyanine 7 and enhances tumor targeting trough peptide interactions, offering a comprehensive diagnostic tool for GBM. Notably, the probe traverses the blood-brain barrier, targeting GBM in vivo via peptides, producing clear and discernible images in both modalities. Cytotoxicity and histopathology assessments confirm the probe's favorable safety profile. These findings suggest that the dual-modal MR\NIR fluorescent imaging probe could revolutionize GBM prognosis and survival rates, which can also be extended to other tumors type.

BrainBike peptidomimetic enables efficient transport of proteins across brain endothelium

Protein therapeutics cannot reach the brain in sufficient amounts because of their low permeability across the blood-brain barrier. Here we report a new family of bicyclic peptide shuttles, BrainBikes, capable of increasing transport of proteins, including antibody derivatives, in a human cell-based model of the blood-brain barrier.

Topoisomeric Membrane-Active Peptides: A Review of the Last Two Decades

In recent decades, bioactive peptides have been gaining recognition in various biomedical areas, such as intracellular drug delivery (cell-penetrating peptides, CPPs) or anti-infective action (antimicrobial peptides, AMPs), closely associated to their distinct mode of interaction with biological membranes. Exploiting the interaction of membrane-active peptides with diverse targets (healthy, tumoral, bacterial or parasitic cell membranes) is opening encouraging prospects for peptides in therapeutics. However, ordinary peptides formed by L-amino acids are easily decomposed by proteases in biological fluids. One way to sidestep this limitation is to use topoisomers, namely versions of the peptide made up of D-amino acids in either canonic (enantio) or inverted (retroenantio) sequence. Rearranging peptide sequences in this fashion provides a certain degree of native structure mimicry that, in appropriate contexts, may deliver desirable biological activity while avoiding protease degradation. In this review, we will focus on recent accounts of membrane-active topoisomeric peptides with therapeutic applications as CPP drug delivery vectors, or as antimicrobial and anticancer candidates. We will also discuss the most common modes of interaction of these peptides with their membrane targets.

Receptor-Targeted Carbon Nanodot Delivery through Polymer Caging and Click Chemistry-Supported LRP1 Ligand Attachment

Carbon nanodots present resistance to photobleaching, bright photoluminescence, and superior biocompatibility, making them highly promising for bioimaging applications. Herein, nanoprobes were caged with four-armed oligomers and subsequently modified with a novel DBCO-PEG-modified retro-enantio peptide ligand reL57, enhancing cellular uptake into U87MG glioma cells highly expressing low-density lipoprotein receptor-related protein 1 (LRP1). A key point in the development of the oligomers was the incorporation of ε-amino-linked lysines instead of standard α-amino-linked lysines, which considerably extended the contour length per monomer. The four-armed oligomer 1696 was identified as the best performer, spanning a contour length of ~8.42 nm for each arm, and was based on an altering motive of two cationic ε-amidated lysine tripeptides and two tyrosine tripeptides for electrostatic and aromatic stabilization of the resulting formulations, cysteines for disulfide-based caging, and N-terminal azidolysines for click-modification. This work highlights that well-designed four-armed oligomers can be used for noncovalent coating and covalent caging of nanoprobes, and click modification using a novel LRP1-directed peptide ligand facilitates delivery into receptor-expressing target cells.

Cell-Penetrating and Targeted Peptides Delivery Systems as Potential Pharmaceutical Carriers for Enhanced Delivery across the Blood-Brain Barrier (BBB)

Among the challenges to the 21st-century health care industry, one that demands special mention is the transport of drugs/active pharmaceutical agents across the blood-brain barrier (BBB). The epithelial-like tight junctions within the brain capillary endothelium hinder the uptake of most pharmaceutical agents. With an aim to understand more deeply the intricacies of cell-penetrating and targeted peptides as a powerful tool for desirable biological activity, we provide a critical review of both CPP and homing/targeted peptides as intracellular drug delivery agents, especially across the blood-brain barrier (BBB). Two main peptides have been discussed to understand intracellular drug delivery; first is the cell-penetrating peptides (CPPs) for the targeted delivery of compounds of interest (primarily peptides and nucleic acids) and second is the family of homing peptides, which specifically targets cells/tissues based on their overexpression of tumour-specific markers and are thus at the heart of cancer research. These small, amphipathic molecules demonstrate specific physical and chemical modifications aimed at increased ease of cellular internalisation. Because only a limited number of drug molecules can bypass the blood-brain barrier by free diffusion, it is essential to explore all aspects of CPPs that can be exploited for crossing this barrier. Considering siRNAs that can be designed against any target RNA, marking such molecules with high therapeutic potential, we present a synopsis of the studies on synthetic siRNA-based therapeutics using CPPs and homing peptides drugs that can emerge as potential drug-delivery systems as an upcoming requirement in the world of pharma- and nutraceuticals.

Non-Viral Carriers for Nucleic Acids Delivery: Fundamentals and Current Applications

Over the past decades, non-viral DNA and RNA delivery systems have been intensively studied as an alternative to viral vectors. Despite the most significant advantage over viruses, such as the lack of immunogenicity and cytotoxicity, the widespread use of non-viral carriers in clinical practice is still limited due to the insufficient efficacy associated with the difficulties of overcoming extracellular and intracellular barriers. Overcoming barriers by non-viral carriers is facilitated by their chemical structure, surface charge, as well as developed modifications. Currently, there are many different forms of non-viral carriers for various applications. This review aimed to summarize recent developments based on the essential requirements for non-viral carriers for gene therapy.

Chemically Enhanced Peptide and Protein Therapeutics

Proteins and peptides are on the rise as therapeutic agents and represent a higher percentage of approved drugs each year: 24% in 2021 vs [...]

Polymersome-based protein drug delivery - quo vadis?

Protein-based therapeutics are an attractive alternative to established therapeutic approaches and represent one of the fastest growing families of drugs. While many of these proteins can be delivered using established formulations, the intrinsic sensitivity of proteins to denaturation sometimes calls for a protective carrier to allow administration. Historically, lipid-based self-assembled structures, notably liposomes, have performed this function. After the discovery of polymersome-based targeted drug-delivery systems, which offer manifold advantages over lipid-based structures, the scientific community expected that such systems would take the therapeutic world by storm. However, no polymersome formulations have been commercialised. In this review article, we discuss key obstacles for the sluggish translation of polymersome-based protein nanocarriers into approved pharmaceuticals, which include limitations imparted by the use of non-degradable polymers, the intricacies of polymersome production methods, and the complexity of the in vivo journey of polymersomes across various biological barriers. Considering this complex subject from a polymer chemist's point of view, we highlight key areas that are worthy to explore in order to advance polymersomes to a level at which clinical trials become worthwhile and translation into pharmaceutical and nanomedical applications is realistic.

Short Peptoid Evolved from the Key Hydrophobic Stretch of Amyloid-β42 Peptide Serves as a Potent Therapeutic Lead of Alzheimer's Disease

Amyloid-β 42(Aβ42), an enzymatically cleaved (1-42 amino acid long) toxic peptide remnant, has long been reported to play the key role in Alzheimer's disease (AD). Aβ42 also plays the key role in the onset of other AD-related factors including hyperphosphorylation of tau protein that forms intracellular neurofibrillary tangles, imbalances in the function of the neurotransmitter acetylcholine, and even generation of reactive oxygen species (ROS), disrupting the cytoskeleton and homeostasis of the cell. To address these issues, researchers have tried to construct several strategies to target multiple aspects of the disease but failed to produce any clinically successful therapeutic molecules. In this article, we report a new peptoid called RA-1 that was designed and constructed from the hydrophobic stretch of the Aβ42 peptide, ¹⁶KLVFFA²¹. This hydrophobic stretch is primarily responsible for the Aβ42 peptide aggregation. Experimental study showed that the RA-1 peptoid is stable under proteolytic conditions, can stabilize the microtubule, and can inhibit the formation of toxic Aβ42 aggregates by attenuating hydrophobic interactions between Aβ42 monomers. Furthermore, results from various intracellular assays showed that RA-1 inhibits Aβ42 fibril formation caused by the imbalance in AchE activity, reduces the production of cytotoxic reactive oxygen species (ROS), and promotes neurite outgrowth even in the toxic environment. Remarkably, we have also demonstrated that our peptoid has significant ability to improve the cognitive ability and memory impairment in in vivo rats exposed to AlCl₃ and d-galactose (d-gal) dementia model. These findings are also validated with histological studies. Overall, our newly developed peptoid emerges as a multimodal potent therapeutic lead molecule against AD.

Directing the Way-Receptor and Chemical Targeting Strategies for Nucleic Acid Delivery

Nucleic acid therapeutics have shown great potential for the treatment of numerous diseases, such as genetic disorders, cancer and infections. Moreover, they have been successfully used as vaccines during the COVID-19 pandemic. In order to unfold full therapeutical potential, these nano agents have to overcome several barriers. Therefore, directed transport to specific tissues and cell types remains a central challenge to receive carrier systems with enhanced efficiency and desired biodistribution profiles. Active targeting strategies include receptor-targeting, mediating cellular uptake based on ligand-receptor interactions, and chemical targeting, enabling cell-specific delivery as a consequence of chemically and structurally modified carriers. With a focus on synthetic delivery systems including polyplexes, lipid-based systems such as lipoplexes and lipid nanoparticles, and direct conjugates optimized for various types of nucleic acids (DNA, mRNA, siRNA, miRNA, oligonucleotides), we highlight recent achievements, exemplified by several nucleic acid drugs on the market, and discuss challenges for targeted delivery to different organs such as brain, eye, liver, lung, spleen and muscle in vivo.

Recent advances in drug delivery and targeting to the brain

Our body keeps separating the toxic chemicals in the blood from the brain. A significant number of drugs do not enter the central nervous system (CNS) due to the blood-brain barrier (BBB). Certain diseases, such as tumor growth and stroke, are known to increase the permeability of the BBB. However, the heterogeneity of this permeation makes it difficult and unpredictable to transport drugs to the brain. In recent years, research has been directed toward increasing drug penetration inside the brain, and nanomedicine has emerged as a promising approach. Active targeting requires one or more specific ligands on the surface of nanoparticles (NPs), which brain endothelial cells (ECs) recognize, allowing controlled drug delivery compared to conventional targeting strategies. This review highlights the mechanistic insights about different cell types contributing to the development and maintenance of the BBB and summarizes the recent advancement in brain-specific NPs for different pathological conditions. Furthermore, fundamental properties of brain-targeted NPs will be discussed, and the standard lesion features classified by neurological pathology are summarized.

Peptides as Pharmacological Carriers to the Brain: Promises, Shortcomings and Challenges

Central nervous system (CNS) diseases are among the most difficult to treat, mainly because the vast majority of the drugs fail to cross the blood-brain barrier (BBB) or to reach the brain at concentrations adequate to exert a pharmacological activity. The obstacle posed by the BBB has led to the in-depth study of strategies allowing the brain delivery of CNS-active drugs. Among the most promising strategies is the use of peptides addressed to the BBB. Peptides are versatile molecules that can be used to decorate nanoparticles or can be conjugated to drugs, with either a stable link or as pro-drugs. They have been used to deliver to the brain both small molecules and proteins, with applications in diverse therapeutic areas such as brain cancers, neurodegenerative diseases and imaging. Peptides can be generally classified as receptor-targeted, recognizing membrane proteins expressed by the BBB microvessels (e.g., Angiopep2, CDX, and iRGD), "cell-penetrating peptides" (CPPs; e.g. TAT_47-57, SynB1/3, and Penetratin), undergoing transcytosis through unspecific mechanisms, or those exploiting a mixed approach. The advantages of peptides have been extensively pointed out, but so far few studies have focused on the potential negative aspects. Indeed, despite having a generally good safety profile, some peptide conjugates may display toxicological characteristics distinct from those of the peptide itself, causing for instance antigenicity, cardiovascular alterations or hemolysis. Other shortcomings are the often brief lifetime in vivo, caused by the presence of peptidases, the vulnerability to endosomal/lysosomal degradation, and the frequently still insufficient attainable increase of brain drug levels, which remain below the therapeutically useful concentrations. The aim of this review is to analyze not only the successful and promising aspects of the use of peptides in brain targeting but also the problems posed by this strategy for drug delivery.

Peptide Shuttles for Blood–Brain Barrier Drug Delivery

The blood–brain barrier (BBB) limits the delivery of therapeutics to the brain but also represents the main gate for nutrient entrance. Targeting the natural transport mechanisms of the BBB offers an attractive route for brain drug delivery. Peptide shuttles are able to use these mechanisms to increase the transport of compounds that cannot cross the BBB unaided. As peptides are a group of biomolecules with unique physicochemical and structural properties, the field of peptide shuttles has substantially evolved in the last few years. In this review, we analyze the main classifications of BBB–peptide shuttles and the leading sources used to discover them.

Examining Topoisomers of a Snake-Venom-Derived Peptide for Improved Antimicrobial and Antitumoral Properties

Ctn[15-34], the C-terminal section of crotalicidin (Ctn), a cathelicidin from a South American pit viper, is an antimicrobial and antitumoral peptide with remarkably longer stability in human serum than the parent Ctn. In this work, a set of topoisomers of both Ctn and Ctn[15-34], including the retro, enantio, and retroenantio versions, were synthesized and tested to investigate the structural requirements for activity. All topoisomers were as active as the cognate sequences against Gram-negative bacteria and tumor cells while slightly more toxic towards normal cells. More importantly, the enhanced serum stability of the D-amino-acid-containing versions suggests that such topoisomers must be preferentially considered as future antimicrobial and anticancer peptide leads.

Peptide-decorated nanocarriers penetrating the blood-brain barrier for imaging and therapy of brain diseases

Blood-Brain Barrier (BBB) is one of the most important physiological barriers strictly restricting the substance exchange between blood and brain tissues. While the BBB protects the brain from infections and toxins and maintains brain homeostasis, it is also recognized as the main obstacle to the penetration of therapeutics and imaging agents into the brain. Due to high specificity and affinity, peptides are frequently exploited to decorate nanocarriers across the BBB for diagnosis and/or therapy purposes. However, there are still some challenges that restrict their clinical application, such as stability, safety and immunocompatibility. In this review, we summarize the biological and pathophysiological characteristics of the BBB, strategies across the BBB, and recent progress on peptide decorated nanocarriers for brain diseases diagnosis and therapy. The challenges and opportunities for their translation are also discussed.

Targeting nucleic acid-based therapeutics to tumors: Challenges and strategies for polyplexes

The current medical reality of cancer gene therapy is reflected by more than ten approved products on the global market, including oncolytic and other viral vectors and CAR T-cells as ex vivo gene-modified cell therapeutics. The development of synthetic antitumoral nucleic acid therapeutics has been proceeding at a lower but steady pace, fueled by a plethora of alternative nucleic acid platforms (from various antisense oligonucleotides, siRNA, microRNA, lncRNA, sgRNA, to larger mRNA and DNA) and several classes of physical and chemical delivery technologies. This review summarizes the challenges and strategies for tumor-targeted nucleic acid delivery. Focusing primarily on polyplexes (polycation complexes) as nanocarriers, delivery options across multiple barriers into tumor cells are illustrated.

Enhancing autophagy in Alzheimer's disease through drug repositioning

Alzheimer's disease (AD) is one of the biggest human health threats due to increases in aging of the global population. Unfortunately, drugs for treating AD have been largely ineffective. Interestingly, downregulation of macroautophagy (autophagy) plays an essential role in AD pathogenesis. Therefore, targeting autophagy has drawn considerable attention as a therapeutic approach for the treatment of AD. However, developing new therapeutics is time-consuming and requires huge investments. One of the strategies currently under consideration for many diseases is "drug repositioning" or "drug repurposing". In this comprehensive review, we have provided an overview of the impact of autophagy on AD pathophysiology, reviewed the therapeutics that upregulate autophagy and are currently used in the treatment of other diseases, including cancers, and evaluated their repurposing as a possible treatment option for AD. In addition, we discussed the potential of applying nano-drug delivery to neurodegenerative diseases, such as AD, to overcome the challenge of crossing the blood brain barrier and specifically target molecules/pathways of interest with minimal side effects.

Performance of nanoparticles for biomedical applications: The in vitro/in vivo discrepancy

Nanomedicine has a great potential to revolutionize the therapeutic landscape. However, up-to-date results obtained from in vitro experiments predict the in vivo performance of nanoparticles weakly or not at all. There is a need for in vitro experiments that better resemble the in vivo reality. As a result, animal experiments can be reduced, and potent in vivo candidates will not be missed. It is important to gain a deeper knowledge about nanoparticle characteristics in physiological environment. In this context, the protein corona plays a crucial role. Its formation process including driving forces, kinetics, and influencing factors has to be explored in more detail. There exist different methods for the investigation of the protein corona and its impact on physico-chemical and biological properties of nanoparticles, which are compiled and critically reflected in this review article. The obtained information about the protein corona can be exploited to optimize nanoparticles for in vivo application. Still the translation from in vitro to in vivo remains challenging. Functional in vitro screening under physiological conditions such as in full serum, in 3D multicellular spheroids/organoids, or under flow conditions is recommended. Innovative in vivo screening using barcoded nanoparticles can simultaneously test more than hundred samples regarding biodistribution and functional delivery within a single mouse.

Transferrin Receptor-Targeted Nanocarriers: Overcoming Barriers to Treat Glioblastoma

Glioblastoma multiforme (GBM) is the most common and lethal type of brain tumor, and the clinically available approaches for its treatment are not curative. Despite the intensive research, biological barriers such as the blood–brain barrier (BBB) and tumor cell membranes are major obstacles to developing novel effective therapies. Nanoparticles (NPs) have been explored as drug delivery systems (DDS) to improve GBM therapeutic strategies. NPs can circumvent many of the biological barriers posed by this devastating disease, enhancing drug accumulation in the target site. This can be achieved by employing strategies to target the transferrin receptor (TfR), which is heavily distributed in BBB and GBM cells. These targeting strategies comprise the modification of NPs’ surface with various molecules, such as transferrin (Tf), antibodies, and targeting peptides. This review provides an overview and discussion on the recent advances concerning the strategies to target the TfR in the treatment of GBM, as their benefits and limitations.

Nanotechnology-based drug delivery for the treatment of CNS disorders

Approximately 6.8 million people die annually because of problems related to the central nervous system (CNS), and out of them, approximately 1 million people are affected by neurodegenerative diseases that include Alzheimer's disease, multiple sclerosis, epilepsy, and Parkinson's disease. CNS problems are a primary concern because of the complexity of the brain. There are various drugs available to treat CNS disorders and overcome problems with toxicity, specificity, and delivery. Barriers like the blood-brain barrier (BBB) are a challenge, as they do not allow therapeutic drugs to cross and reach their target. Researchers have been searching for ways to allow drugs to pass through the BBB and reach the target sites. These problems highlight the need of nanotechnology to alter or manipulate various processes at the cellular level to achieve the desired attributes. Due to their nanosize, nanoparticles are able to pass through the BBB and are an effective alternative to drug administration and other approaches. Nanotechnology has the potential to improve treatment and diagnostic techniques for CNS disorders and facilitate effective drug transfer. With the aid of nanoengineering, drugs could be modified to perform functions like transference across the BBB, altering signaling pathways, targeting specific cells, effective gene transfer, and promoting regeneration and preservation of nerve cells. The involvement of a nanocarrier framework inside the delivery of several neurotherapeutic agents used in the treatment of neurological diseases is reviewed in this study.

Protease-Resistant Peptides for Targeting and Intracellular Delivery of Therapeutics

Peptides show high promise in the targeting and intracellular delivery of next-generation bio- and nano-therapeutics. However, the proteolytic susceptibility of peptides is one of the major limitations of their activity in biological environments. Numerous strategies have been devised to chemically enhance the resistance of peptides to proteolysis, ranging from N- and C-termini protection to cyclization, and including backbone modification, incorporation of amino acids with non-canonical side chains and conjugation. Since conjugation of nanocarriers or other cargoes to peptides for targeting and cell penetration may already provide some degree of shielding, the question arises about the relevance of using protease-resistant sequences for these applications. Aiming to answer this question, here we provide a critical review on protease-resistant targeting peptides and cell-penetrating peptides (CPPs). Two main approaches have been used on these classes of peptides: enantio/retro-enantio isomerization and cyclization. On one hand, enantio/retro-enantio isomerization has been shown to provide a clear enhancement in peptide efficiency with respect to parent L-amino acid peptides, especially when applied to peptides for drug delivery to the brain. On the other hand, cyclization also clearly increases peptide transport capacity, although contribution from enhanced protease resistance or affinity is often not dissected. Overall, we conclude that although conjugation often offers some degree of protection to proteolysis in targeting peptides and CPPs, modification of peptide sequences to further enhance protease resistance can greatly increase homing and transport efficiency.

Transferrin Receptor Targeted Polyplexes Completely Comprised of Sequence-Defined Components

Human transferrin protein (Tf) modified polyplexes have already displayed encouraging potential for receptor-mediated nucleic acid delivery into tumors. The use of a blood-derived targeting protein and polydisperse macromolecular cationic subunits however presents a practical challenge for pharmaceutical grade production. Here, Tf receptor (TfR) targeted small interfering RNA (siRNA) polyplexes are designed that are completely composed of synthetic, monodisperse, and sequence-defined subunits generated by solid-phase supported synthesis. An optimized cationizable lipo-oligoaminoamide (lipo-OAA) is used for siRNA core polyplex formation, and a retro-enantio peptide (reTfR) attached via a monodisperse polyethylene glycol (PEG) spacer via click chemistry is applied for targeting. Improved gene silencing is demonstrated in TfR-expressing KB and DU145 cells. Analogous plasmid DNA (pDNA) polyplexes are successfully used for receptor-mediated gene delivery in TfR-rich K562 cells and Neuro2a cells. Six lipo-OAAs differing in their lipidic domain and redox-sensitive attachment of lipid residues are tested in order to evaluate the impact of core polyplex stability on receptor-dependent gene transfer.

Rationally Designed Protein-Based Inhibitor of α-Synuclein Fibrillization in Cells

Misfolding of the neuronal protein α-synuclein (αSyn) into amyloid fibrils is involved in the development of Parkinson's disease (PD), and inhibition of this process is considered to be a promising therapeutic approach. In this work, we engineered protein inhibitors that bind to fibrils with higher affinity than the monomeric αSyn. They were developed based on the recent structural data of the αSyn fibrils and were shown to prevent fibril elongation upon binding to fibril ends. These inhibitors are highly selective to the misfolded αSyn, nontoxic, and active in cytosol in small concentrations. The best-performing inhibitor shows IC₅₀ ∼10 nM in a cell-based assay, which corresponds to the ∼1:60 molar ratio to αSyn. It can suppress the formation of αSyn aggregates in cells that can be potentially used to slow down the spreading of the pathological aggregates from cell to cell during the course of the PD.

Orally Active Peptide Vector Allows Using Cannabis to Fight Pain While Avoiding Side Effects

The activation of cannabinoid CB₁ receptors (CB₁R) by Δ⁹-tetrahydrocannabinol (THC), the main component of Cannabis sativa, induces analgesia. CB₁R activation, however, also causes cognitive impairment via the serotonin 5HT_2A receptor (5HT_2AR), a component of a CB₁R–5HT_2AR heteromer, posing a serious drawback for cannabinoid therapeutic use. We have shown that peptides reproducing CB₁R transmembrane (TM) helices 5 and 6, fused to a cell-penetrating sequence (CPP), can alter the structure of the CB₁R–5HT_2AR heteromer and avert THC cognitive impairment while preserving analgesia. Here, we report the optimization of these prototypes into drug-like leads by (i) shortening the TM5, TM6, and CPP sequences, without losing the ability to disturb the CB₁R–5HT_2AR heteromer, and (ii) extensive sequence remodeling to achieve protease resistance and blood–brain barrier penetration. Our efforts have culminated in the identification of an ideal candidate for cannabis-based pain management, an orally active 16-residue peptide preserving THC-induced analgesia.

Manickam D. Targeting the blood-brain barrier for the delivery of stroke therapies

A variety of neuroprotectants have shown promise in treating ischemic stroke, yet their delivery to the brain remains a challenge. The endothelial cells lining the blood-brain barrier (BBB) are emerging as a dynamic factor in the response to neurological injury and disease, and the endothelial-neuronal matrix coupling is fundamentally neuroprotective. In this review, we discuss approaches that target the endothelium for drug delivery both across the BBB and to the BBB as a viable strategy to facilitate neuroprotective effects, using the example of brain-derived neurotrophic factor (BDNF). We highlight the advances in cell-derived extracellular vesicles (EVs) used for CNS targeting and drug delivery. We also discuss the potential of engineered EVs as a potent strategy to deliver BDNF or other drug candidates to the ischemic brain, particularly when coupled with internal components like mitochondria that may increase cellular energetics in injured endothelial cells.

Harnessing molecular recognition for localized drug delivery

A grand challenge in drug delivery is providing the right dose, at the right anatomic location, for the right duration of time to maximize therapeutic efficacy while minimizing off-target toxicity and other deleterious side-effects. Two general modalities are receiving broad attention for localized drug delivery. In the first, referred to as "targeted accumulation", drugs or drug carriers are engineered to have targeting moieties that promote their accumulation at a specific tissue site from circulation. In the second, referred to as "local anchoring", drugs or drug carriers are inserted directly into the tissue site of interest where they persist for a specified duration of time. This review surveys recent advances in harnessing molecular recognition between proteins, peptides, nucleic acids, lipids, and carbohydrates to mediate targeted accumulation and local anchoring of drugs and drug carriers.

Lipid-Based Nanocarriers for The Treatment of Glioblastoma

Glioblastoma multiforme (GBM) is the most common and malignant neoplasia having origin in the brain. The current treatments involve surgery, radiotherapy, and chemotherapy, being complete surgical resection the best option for the patient survival chances. However, in those cases where a complete removal is not possible, radiation and chemotherapy are applied. Herein, the main challenges of chemotherapy, and how they can be overcome with the help of nanomedicine, are approached. Natural pathways to cross the blood-brain barrier (BBB) are detailed, and different in vivo studies where these pathways are mimicked functionalizing the nanomaterial surface are shown. Later, lipid-based nanocarriers, such as liposomes, solid lipid nanoparticles, and nanostructured lipid carriers, are presented. To finish, recent studies that have used lipid-based nanosystems carrying not only therapeutic agents, yet also magnetic nanoparticles, are described. Although the advantages of using these types of nanosystems are explained, including their biocompatibility, the possibility of modifying their surface to enhance the cell targeting, and their intrinsic ability of BBB crossing, it is important to mention that research in this field is still at its early stage, and extensive preclinical and clinical investigations are mandatory in the close future.

Brain penetrating peptides and peptide-drug conjugates to overcome the blood-brain barrier and target CNS diseases

Nearly one in six people worldwide suffer from disorders of the central nervous system (CNS). There is an urgent need for effective strategies to improve the success rates in CNS drug discovery and development. The lack of effective technologies for delivering drugs and genes to the brain due to the blood-brain barrier (BBB), a structural barrier that effectively blocks most neurotherapeutic agents from reaching the brain, has posed a formidable hurdle for CNS drug development. Brain-homing and brain-penetrating molecular transport vectors, such as brain permeable peptides or BBB shuttle peptides, have shown promise in overcoming the BBB and ferrying the drug molecules to the brain. The BBB shuttle peptides are discovered by phage display technology or derived from natural neurotropic proteins or certain viruses and harness the receptor-mediated transcytosis molecular machinery for crossing the BBB. Brain permeable peptide-drug conjugates (PDCs), composed of BBB shuttle peptides, linkers, and drug molecules, have emerged as a promising CNS drug delivery system by taking advantage of the endogenous transcytosis mechanism and tricking the brain into allowing these bioactive molecules to pass the BBB. Here, we examine the latest development of brain-penetrating peptide shuttles and brain-permeable PDCs as molecular vectors to deliver small molecule drug payloads across the BBB to reach brain parenchyma. Emerging knowledge of the contribution of the peptides and their specific receptors expressed on the brain endothelial cells, choice of drug payloads, the design of PDCs, brain entry mechanisms, and delivery efficiency to the brain are highlighted. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease.

Nanoparticle size and chemical modification play a crucial role in the interaction of nano gold with the brain: extent of accumulation and toxicity

The blood brain barrier (BBB) is a very selective barrier that protects the brain and the central nervous system (CNS) from the entry of harmful substances and helps regulate the exchange of different molecules and nutrients from and into the brain and the CNS. This selectivity makes delivering therapeutic and diagnostic materials across the BBB very challenging. In this study, different shapes and sizes of gold nanoparticles (GNP) were synthesized and functionalized with five different thiolated ligands to obtain GNP with various surface chemistries. The potential of GNP of different properties to be accumulated into the brain through the BBB and into other organs was investigated in a mouse model using qualitative and quantitative approaches. Gold nanorods (GNR) functionalized with 4-mercaptophenol (Mph) showed the highest penetration ability across the BBB into the brain with no significant deposition in other organs. Interestingly, increasing the size of GNR retarded their delivery into the brain, while enhancing their accumulation in other organs. On the other hand, gold nanospheres (GNS) demonstrated high deposition percentages in the brain and other organs with possible toxic effects. The properties of GNP play a crucial role in their interaction with the BBB and accumulation in the brain and other organs. Thus, GNP can be considered a promising nano-platform for drug delivery into the brain and as a photothermal-inducing agent against brain cancer.

Exploring the Use of Helicogenic Amino Acids for Optimising Single Chain Relaxin-3 Peptide Agonists

Relaxin-3 is a highly conserved two-chain neuropeptide that acts through its endogenous receptor the Relaxin Family Peptide-3 (RXFP3) receptor. The ligand/receptor system is known to modulate several physiological processes, with changes in food intake and anxiety-levels the most well studied in rodent models. Agonist and antagonist analogues based on the native two-chain peptide are costly to synthesise and not ideal drug leads. Since RXFP3 interacting residues are found in the relaxin B-chain only, this has been the focus of analogue development. The B-chain is unstructured without the A-chain support, but in single-chain variants structure can be induced by dicarba-based helical stapling strategies. Here we investigated whether alternative helical inducing strategies also can enhance structure and activity at RXFP3. Combinations of the helix inducing α-aminoisobutyric acid (Aib) were incorporated into the sequence of the relaxin-3 B-chain. Aib residues at positions 13, 17 and 18 partially reintroduce helicity and activity of the relaxin-3 B-chain, but other positions are generally not suited for modifications. We identify Thr21 as a putative new receptor contact residue important for RXFP3 binding. Cysteine residues were also incorporated into the sequence and cross-linked with dichloroacetone or α, α'-dibromo-m-xylene. However, in contrast to previously reported dicarba variants, neither were found to promote structure and RXFP3 activity.

Importance of structure-based studies for the design of a novel HIV-1 inhibitor peptide

Based on the structure of an HIV-1 entry inhibitor peptide two stapled- and a retro-enantio peptides have been designed to provide novel prevention interventions against HIV transmission. The three peptides show greater inhibitory potencies in cellular and mucosal tissue pre-clinical models than the parent sequence and the retro-enantio shows a strengthened proteolytic stability. Since HIV-1 fusion inhibitor peptides need to be embedded in the membrane to properly interact with their viral target, the structural features were determined by NMR spectroscopy in micelles and solved by using restrained molecular dynamics calculations. Both parent and retro-enantio peptides demonstrate a topology compatible with a shared helix–turn–helix conformation and assemble similarly in the membrane maintaining the active conformation needed for its interaction with the viral target site. This study represents a straightforward approach to design new targeted peptides as HIV-1 fusion inhibitors and lead us to define a retro-enantio peptide as a good candidate for pre-exposure prophylaxis against HIV-1.

Human Serum Albumin-Inspired Glycopeptide-Based Multifunctional Inhibitor of Amyloid-β Toxicity

In Alzheimer's disease (AD), insoluble Aβ42 peptide fragments self-aggregate and form oligomers and fibrils in the brain, causing neurotoxicity. Further, the presence of redox-active metal ions such as Cu2+ enhances the aggregation process through chelation with these Aβ42 aggregates as well as generation of Aβ42-mediated reactive oxygen species (ROS). Herein, we have adopted a bioinspired strategy to design and develop a multifunctional glycopeptide hybrid molecule (Glupep), which can serve as a potential AD therapeutic. This molecule consists of a natural metal-chelating tetrapeptide motif of human serum albumin (HSA), a β-sheet breaker peptide, and a sugar moiety for better bioavailability. We performed different biophysical and docking experiments, which revealed that Glupep not only associates with Aβ42 but also prevents its self-aggregation to form toxic oligomers and fibrils. Moreover, Glupep was also shown to sequester out Cu2+ from the Aβ-Cu2+ complex, reducing the ROS formation and toxicity. Besides, this study also revealed that Glupep could protect PC12-derived neurons from Aβ-Cu2+-mediated toxicity by reducing intracellular ROS generation and stabilizing the mitochondrial membrane potential. All these exciting features show Glupep to be a potent inhibitor of Aβ42-mediated multifaceted toxicity and a prospective therapeutic lead for AD.

The voltage-gated potassium channel KV1.3 as a therapeutic target for venom-derived peptides

The voltage-gated potassium channel K_V1.3 is a well-established therapeutic target for a range of autoimmune diseases, in addition to being the site of action of many venom-derived peptides. Numerous studies have documented the efficacy of venom peptides that target K_V1.3, in particular from sea anemones and scorpions, in animal models of autoimmune diseases such as rheumatoid arthritis, psoriasis and multiple sclerosis. Moreover, an analogue of the sea anemone peptide ShK (known as dalazatide) has successfully completed Phase 1 clinical trials in mild-to-moderate plaque psoriasis. In this article we consider other potential therapeutic applications of inhibitors of K_V1.3, including in inflammatory bowel disease and neuroinflammatory conditions such as Alzheimer's and Parkinson's diseases, as well as fibrotic diseases. We also summarise strategies for facilitating the entry of peptides to the central nervous system, given that this will be a pre-requisite for the treatment of most neuroinflammatory diseases. Venom-derived peptides that have been reported recently to target K_V1.3 are also described. The increasing number of autoimmune and other conditions in which K_V1.3 is upregulated and is therefore a potential therapeutic target, combined with the fact that many venom-derived peptides are potent inhibitors of K_V1.3, suggests that venoms are likely to continue to serve as a rich source of new pharmacological tools and therapeutic leads targeting this channel.

Ultrasound-Activated Sensitizers and Applications

Modalities for photo-triggered anticancer therapy are usually limited by their low penetrative depth. Sonotheranostics especially sonodynamic therapy (SDT), which is different from photodynamic therapy (PDT) by the use of highly penetrating acoustic waves to activate a class of sound-responsive materials called sonosensitizers, has gained significant interest in recent years. The effect of SDT is closely related to the structural and physicochemical properties of the sonosensitizers, which has led to the development of new sound-activated materials as sonosensitizers for various biomedical applications. This Review provides a summary and discussion of the types of novel sonosensitizers developed in the last few years and outlines their specific designs and the potential challenges. The applications of sonosensitizers with various functions such as for imaging and drug delivery as well as in combination with other treatment modalities would provide new strategies for disease therapy.

Chemical biology tools for probing transcytosis at the blood-brain barrier

A BBB vehicle library is validated, establishing a SAR for uptake. Merging AMT/RMT motifs chemoselectively increased permeability.

Absorptive- and receptor-mediated transcytosis (AMT/RMT) are widely studied strategies to deliver therapeutics across the blood–brain barrier (BBB). However, an improved understanding of the mechanism surrounding trafficking is required that could promote delivery. Accordingly, we designed a flexible platform that merged AMT and RMT motifs on a single scaffold to probe various parameters (ligand, affinity, valency, position) in a screening campaign. During this process we adapted an in vitro BBB model to reliably rank transcytosis of the vehicle library. Our results demonstrate heightened uptake and trafficking for the shuttles, with a structure–activity relationship for transcytosis emerging. Notably, due to their small size, the majority of shuttles demonstrated increased permeation compared to transferrin, with the highest performing shuttle affording a 4.9-fold increase. Consequently, we have identified novel peptide conjugates that have the capacity to act as promising brain shuttles.

Microwave-assisted synthesis and in vitro stability of N-benzylamide non-steroidal anti-inflammatory drug conjugates for CNS delivery

More effective delivery of non-steroidal anti-inflammatory drugs (NSAIDs) to the brain could treat the underlying inflammatory pathology of a range of CNS diseases and conditions. Use of a blood-brain barrier shuttle such as the N-benzylamide moiety, which has been largely unexplored for this purpose, could improve the brain bioavailabilities of NSAIDs. A series of novel N-benzylamide NSAID conjugates was synthesized via a three-step process with a microwave-assisted bimolecular nucleophilic substitution as the final step. We explored conditions to promote substitution over a competing elimination reaction, which was successfully suppressed with isopropyl alcohol solvent. All molecules exhibit physicochemical properties consistent with those of brain-penetrant molecules. Furthermore, they exhibit long (>48 h) half-lives in phosphate-buffered saline (PBS; pH 7.4) and short to moderate half-lives in human plasma. N-Benzylamide NSAID conjugates represent promising CNS drug discovery leads.

Potential Neuroprotective Peptide Emerged from Dual Neurotherapeutic Targets: A Fusion Approach for the Development of Anti-Alzheimer's Lead

Amyloid-beta (Aβ) peptide misfolds into fibrillary aggregates (β-sheet) and is deposited as amyloid plaques in the cellular environment, which severely damages intraneuronal connections leading to Alzheimer's disease (AD) pathogenesis. Furthermore, neurons are rich in tubulin/microtubules, and the intracellular network of microtubules also gets disrupted by the accumulation of Aβ fiber in the brain. Hence, development of new potent molecules, which can simultaneously inhibit Aβ fibrillations and stabilize microtubules, is particularly needed for the efficient therapeutic application in AD. To address these issues, here we introduced an innovative fusion strategy to design and develop next generation anti-AD therapeutic leads. This unexplored fusion strategy entails design and development of a potent nonapeptide by taking into account both the hydrophobic core (17-21) of Aβ peptide and the taxol binding region of β-tubulin. In vitro results suggest that this newly designed peptide interacts at the taxol binding region of β-tubulin with a moderate binding affinity and promotes microtubule polymerization. It has the ability to bind at the hydrophobic core (17-21) of Aβ, responsible for its aggregation, and prevent amyloid fibril as well as plaque formation. In addition, it interacts at the CAS site (catalytic anionic site) of acetylcholinesterase (AChE) and significantly inhibits AChE induced Aβ fibrillation, stimulates neurite branching, and provides stability to intracellular microtubules and extensive protection of neurons against nerve growth factor (NGF) deprived neuron toxicity. Moreover, this newly designed peptide shows good stability in serum obtained from humans and efficiently permeates the blood-brain barrier (BBB) without showing any toxicity toward differentiated PC12 neurons as well as primary rat cortical neurons. This excellent feature of protecting the neurons by stabilizing the microtubules without showing any toxicity toward neurons will make this peptide a potent therapeutic agent of AD in the near future.