Receptor-mediated gene targeting to tissues in vivo following intravenous administration of pegylated immunoliposomes

The legacy of William M. Pardridge (1947-2024) on the science and fields concerned with the physiology of the blood-brain barrier and the transport of drugs to the brain

This article highlights the scientific achievements and professional career of William M. Pardridge, who passed away on 18th of June 2024. He was born in 1947 in Bethesda, Maryland, USA. William, known affectionately as Bill, was one of the most influential researchers in the blood-brain barrier (BBB) field. Bill's research contributions to the field spun over 6 decades at the University of California, Los Angeles, and it was focused on the molecular physiology of the BBB, including the carrier-mediated transport of nutrients and small molecule drugs, the receptor-mediated transport of peptides and molecular Trojan horses, gene therapy of the brain with Trojan horse liposomes, and BBB genomics and proteomics. He founded ArmaGen Inc. in 2004, a biotech company that provided the basis for the treatment of CNS disorders with the molecular Trojan horse technology. This technology continues to be widely used in academia and in the biotech industry. He has been a cherished friend and mentor to many within the BBB community.

Targeting strategies with lipid vectors for nucleic acid supplementation therapy in Fabry disease: a systematic review

Fabry disease (FD) results from a lack of activity of the lysosomal enzyme α-Galactosidase A (α-Gal A), leading to the accumulation of glycosphingolipids in several different cell types. Protein supplementation by pDNA or mRNA delivery presents a promising strategy to tackle the underlying genetic defect in FD. Protein-coding nucleic acids in FD can be either delivered to the most affected sites by the disease, including heart, kidney and brain, or to specialized organs that can act as a production factory of the enzyme, such as the liver. Lipid-based systems are currently at the top of the ranking of non-viral nucleic acid delivery systems, and their versatility allows the linking to the surface of a wide range of molecules to control their biodistribution after intravenous administration. This systematic review follows the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement guidelines and provides an overview and discussion of the targeting ligands that have been employed so far to actively vectorize intravenously administered non-viral vectors based on lipid carriers to clinically relevant organs in the treatment of FD, for protein-coding nucleic acid (pDNA and mRNA) supplementation. Among the thirty-two studies included, the majority focus on targeting the liver and brain. The targeting of the heart has been reported to a lesser degree, whereas no articles addressing kidney-targeting have been recorded. Although a great effort has been made to develop organ-specific nucleic acid delivery systems, the design of active-targeted carriers with high quality, good clinical translation, and large-scale manufacturing capacity is still challenging.

Treating late-onset Tay Sachs disease: Brain delivery with a dual trojan horse protein

Tay-Sachs (TS) disease is a neurodegenerative disease resulting from mutations in the gene encoding the α-subunit (HEXA) of lysosomal β-hexosaminidase A (HexA). We report that (1) recombinant HEXA alone increased HexA activity and decreased GM2 content in human TS glial cells and peripheral mononuclear blood cells; 2) a recombinant chimeric protein composed of HEXA linked to two blood-brain barrier (BBB) entry elements, a transferrin receptor binding sequence and granulocyte-colony stimulating factor, associates with HEXB in vitro; reaches human cultured TS cells lysosomes and mouse brain cells, especially neurons, in vivo; lowers GM2 in cultured human TS cells; lowers whole brain GM2 concentration by approximately 40% within 6 weeks, when injected intravenously (IV) to adult TS-mutant mice mimicking the slow course of late-onset TS; and increases forelimbs grip strength. Hence, a chimeric protein equipped with dual BBB entry elements can transport a large protein such as HEXA to the brain, decrease the accumulation of GM2, and improve muscle strength, thereby providing potential treatment for late-onset TS.

Magnetic resonance imaging with upconversion nanoprobes capable of crossing the blood-cerebrospinal fluid barrier

The central nervous system (CNS) maintains homeostasis with its surrounding environment by restricting the ingress of large hydrophilic molecules, immune cells, pathogens, and other external harmful substances to the brain. This function relies heavily on the blood-cerebrospinal fluid (B-CSF) and blood-brain barrier (BBB). Although considerable research has examined the structure and function of the BBB, the B-CSF barrier has received little attention. Therapies for disorders associated with the central nervous system have the potential to benefit from targeting the B-CSF barrier to enhance medication penetration into the brain. In this study, we synthesized a nanoprobe ANG-PEG-UCNP capable of crossing the B-CSF barrier with high targeting specificity using a hydrocephalus model for noninvasive magnetic resonance ventriculography to understand the mechanism by which the CSF barrier may be crossed and identify therapeutic targets of CNS diseases. This magnetic resonance nanoprobe ANG-PEG-UCNP holds promising potential as a safe and effective means for accurately defining the ventricular anatomy and correctly locating sites of CSF obstruction.

Brain gene therapy with Trojan horse lipid nanoparticles

The COVID-19 mRNA vaccine was developed by the scalable manufacture of lipid nanoparticles (LNPs) that encapsulate mRNA within the lipid. There are many potential applications for this large nucleic acid delivery technology, including the delivery of plasmid DNA for gene therapy. However, gene therapy for the brain requires LNP delivery across the blood-brain barrier (BBB). It is proposed that LNPs could be reformulated for brain delivery by conjugation of receptor-specific monoclonal antibodies (MAbs) to the LNP surface. The MAb acts as a molecular Trojan horse to trigger receptor-mediated transcytosis (RMT) of the LNP across the BBB and subsequent localization to the nucleus for transcription of the therapeutic gene. Trojan horse LNPs could enable new approaches to gene therapy of the brain.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Overcoming barriers in non-viral gene delivery for neurological applications

Gene therapy for neurological disorders has attracted significant interest as a way to reverse or stop various disease pathologies. Typical gene therapies involving the central and peripheral nervous system make use of adeno-associated viral vectors whose questionable safety and limitations in manufacturing has given rise to extensive research into non-viral vectors. While early research studies have demonstrated limited efficacy with these non-viral vectors, investigation into various vector materials and functionalization methods has provided insight into ways to optimize these non-viral vectors to improve desired characteristics such as improved blood-brain barrier transcytosis, improved perfusion in brain region, enhanced cellular uptake and endosomal escape in neural cells, and nuclear transport of genetic material post- intracellular delivery. Using a combination of various strategies to enhance non-viral vectors, research groups have designed multi-functional vectors that have been successfully used in a variety of pre-clinical applications for the treatment of Parkinson's disease, brain cancers, and cellular reprogramming for neuron replacement. While more work is needed in the design of these multi-functional non-viral vectors for neural applications, much of the groundwork has been done and is reviewed here.

Aptamers: Cutting edge of cancer therapies

The development of an aptamer-based therapeutic has rapidly progressed following the first two reports in the 1990s, underscoring the advantages of aptamer drugs associated with their unique binding properties. In 2004, the US Food and Drug Administration (FDA) approved the first therapeutic aptamer for the treatment of neovascular age-related macular degeneration, Macugen developed by NeXstar. Since then, eleven aptamers have successfully entered clinical trials for various therapeutic indications. Despite some of the pre-clinical and clinical successes of aptamers as therapeutics, no aptamer has been approved by the FDA for the treatment of cancer. This review highlights the most recent and cutting-edge approaches in the development of aptamers for the treatment of cancer types most refractory to conventional therapies. Herein, we will review (1) the development of aptamers to enhance anti-cancer immunity and as delivery tools for inducing the expression of immunogenic neoantigens; (2) the development of the most promising therapeutic aptamers designed to target the hard-to-treat cancers such as brain tumors; and (3) the development of "carrier" aptamers able to target and penetrate tumors and metastasis, delivering RNA therapeutics to the cytosol and nucleus.

Crossing the blood-brain barrier: A review on drug delivery strategies using colloidal carrier systems

The blood-brain barrier represents the major challenge for delivering drugs to the central nervous system (CNS). It separates the blood circulation from the brain tissue, thereby protecting the CNS and maintaining its ion homeostasis. Unfortunately, most drugs are not able to cross this barrier in vivo despite promising in vitro results. One approach to solve this problem is the delivery of drugs via surface modified nanocarrier systems. This review will give an overview on currently tested systems, mainly liposomes and solid nanoparticles and inform about new developments.

Delivery of Therapeutic Agents to the Central Nervous System and the Promise of Extracellular Vesicles

The central nervous system (CNS) is surrounded by the blood–brain barrier (BBB), a semipermeable border of endothelial cells that prevents pathogens, solutes and most molecules from non-selectively crossing into the CNS. Thus, the BBB acts to protect the CNS from potentially deleterious insults. Unfortunately, the BBB also frequently presents a significant barrier to therapies, impeding passage of drugs and biologicals to target cells within the CNS. This review provides an overview of different approaches to deliver therapeutics across the BBB, with an emphasis in extracellular vesicles as delivery vehicles to the CNS.

Brain Delivery of Nanomedicines: Trojan Horse Liposomes for Plasmid DNA Gene Therapy of the Brain

Non-viral gene therapy of the brain is enabled by the development of plasmid DNA brain delivery technology, which requires the engineering and manufacturing of nanomedicines that cross the blood-brain barrier (BBB). The development of such nanomedicines is a multi-faceted problem that requires progress at multiple levels. First, the type of nanocontainer, e.g., nanoparticle or liposome, which encapsulates the plasmid DNA, must be developed. Second, the type of molecular Trojan horse, e.g., peptide or receptor-specific monoclonal antibody (MAb), must be selected for incorporation on the surface of the nanomedicine, as this Trojan horse engages specific receptors expressed on the BBB, and the brain cell membrane, to trigger transport of the nanomedicine from blood into brain cells beyond the BBB. Third, the plasmid DNA must be engineered without bacterial elements, such as antibiotic resistance genes, to enable administration to humans; the plasmid DNA must also be engineered with tissue-specific gene promoters upstream of the therapeutic gene, to insure gene expression in the target organ with minimal off-target expression. Fourth, upstream manufacturing of the nanomedicine must be developed and scalable so as to meet market demand for the target disease, e.g., annual long-term treatment of 1,000 patients with an orphan disease, short term treatment of 10,000 patients with malignant glioma, or 100,000 patients with new onset Parkinson's disease. Fifth, downstream manufacturing problems, such as nanomedicine lyophilization, must be solved to ensure the nanomedicine has a commercially viable shelf-life for treatment of CNS disease in humans.

In vivo Glioblastoma Therapy Using Targeted Liposomal Cisplatin

Background

Drug delivery systems have demonstrated promising results to cross blood–brain barrier (BBB) and deliver the loaded therapeutics to the brain tumor. This study aims to utilize the transferrin receptor (TR)-targeted liposomal cisplatin (Cispt) for transporting Cispt across the BBB and deliver Cispt to the brain tumor.

Methods

Targeted pegylated liposomal cisplatin (TPL-Cispt) was synthesized using reverse phase evaporation method and thiolated OX26 monoclonal antibody. The formulation was characterized in terms of size, size distribution, zeta potential, drug encapsulation and loading efficiencies, bioactivity, drug release profile, stability and cellular uptake using dynamic light scattering, flame atomic absorption spectroscopy (AAS), ELISA, dialysis membrane, and fluorescence assay. Next, the potency of the formulation to increase the therapeutic effects of Cispt and decrease its toxicity effects was evaluated in the brain tumor-bearing rats through measuring the mean survival time (MST), blood factors and histopathological studies.

Results

The results showed that TPL-Cispt with a size of 157±8 nm and drug encapsulation efficiency of 24%±1.22 was synthesized, that was biologically active and released Cispt in a slow-controlled manner. The formulation compared to Cispt-loaded PEGylated liposome nanoparticles (PL-Cispt) caused an increase in the cellular uptake by 1.43-fold, as well as an increase in the MST of the brain tumor-bearing rats by 1.7-fold compared to the PL-Cispt (P<0.001). TPL-Cispt was potent enough to cause a significant decrease in Cispt toxicity effects (P<0.001).

Conclusion

Overall, the results suggest that targeting the Cispt-loaded PEGylated liposome is a promising approach to develop formulation with enhanced efficacy and reduced toxicity for the treatment of brain tumor.

Breaking Barriers: Bioinspired Strategies for Targeted Neuronal Delivery to the Central Nervous System

Central nervous system (CNS) disorders encompass a vast spectrum of pathological conditions and represent a growing concern worldwide. Despite the high social and clinical interest in trying to solve these pathologies, there are many challenges to bridge in order to achieve an effective therapy. One of the main obstacles to advancements in this field that has hampered many of the therapeutic strategies proposed to date is the presence of the CNS barriers that restrict the access to the brain. However, adequate brain biodistribution and neuronal cells specific accumulation in the targeted site also represent major hurdles to the attainment of a successful CNS treatment. Over the last few years, nanotechnology has taken a step forward towards the development of therapeutics in neurologic diseases and different approaches have been developed to surpass these obstacles. The versatility of the designed nanocarriers in terms of physical and chemical properties, and the possibility to functionalize them with specific moieties, have resulted in improved neurotargeted delivery profiles. With the concomitant progress in biology research, many of these strategies have been inspired by nature and have taken advantage of physiological processes to achieve brain delivery. Here, the different nanosystems and targeting moieties used to achieve a neuronal delivery reported in the open literature are comprehensively reviewed and critically discussed, with emphasis on the most recent bioinspired advances in the field. Finally, we express our view on the paramount challenges in targeted neuronal delivery that need to be overcome for these promising therapeutics to move from the bench to the bedside.

A window into the brain: Tools to assess pre-clinical efficacy of biomaterials-based therapies on central nervous system disorders

Therapeutic conveyance into the brain is a cardinal requirement for treatment of diverse central nervous system (CNS) disorders and associated pathophysiology. Effectual shielding of the brain by the blood-brain barrier (BBB) sieves out major proportion of therapeutics with the exception of small lipophilic molecules. Various nano-delivery systems (NDS) provide an effective solution around this obstacle owing to their small size and targeting properties. To date, these systems have been used for several pre-clinical disease models including glioma, neurodegenerative diseases and psychotic disorders. An efficacy screen for these systems involves a test battery designed to probe into the multiple facets of therapeutic delivery. Despite their wide application in redressing various disease targets, the efficacy evaluation strategies for all can be broadly grouped into four modalities, namely: histological, bio-imaging, molecular and behavioural. This review presents a comprehensive insight into all of these modalities along with their strengths and weaknesses as well as perspectives on an ideal design for a panel of tests to screen brain nano-delivery systems.

In Vivo Use of a Multi-DNA Aptamer-Based Payload/Targeting System To Study Dopamine Dysregulation in the Central Nervous System

The delivery of therapeutics across the blood-brain barrier remains a considerable challenge in investigating central nervous system related processes. In this work, a liposome vehicle was surface-modified with an aptamer that binds to the transferrin receptor and was loaded with two different dopamine-binding aptamer payloads. This system was effectively used to promote the delivery of the aptamer cargo from the peripheral injection site into the brain. The effect of these delivered aptamers on behavior was investigated in vivo in a locomotor task. The first dopamine binding aptamer assessed was a DNA aptamer, the binding of which had been previously validated through the aptamer-based biosensor development reported by several independent research groups. The second aptamer investigated was the result of a novel in vitro selection experiment described herein. Our data suggest that systemic administration of the modified liposomes led to delivery of the dopamine aptamers into the brain. Fluorescence microscopy revealed differential distribution of fluorescence based on the presence or absence of the transferrin receptor aptamer on the surface of fluorescently modified liposomes. In a behavioral experiment using cocaine administration to induce elevated concentrations of neural dopamine, systemic pretreatment with the dopamine aptamer-loaded liposomes reduced cocaine-induced hyperlocomotion. Multiple controls including a transferrin-negative liposome control and transferrin-positive liposomes loaded with either a nonbinding, base-substituted dopamine aptamer or a random oligonucleotide were investigated. None of these controls altered cocaine-induced hyperlocomotion. Chronic systemic administration of the modified liposomes produced no deleterious neurobehavioral or neural degenerative effects. Importantly, this work is one example of an application for this versatile multiaptamer payload/targeting system. Its general application is limited only by the availability of aptamers for specific neural targets.

Receptor Targeted Polymeric Nanostructures Capable of Navigating across the Blood-Brain Barrier for Effective Delivery of Neural Therapeutics

The window of neurological maladies encompasses 600 known neurological disorders. In the past few years, an inordinate upsurge in the incidences of neuronal ailments with increased mortality rate has been witnessed globally. Despite noteworthy research in the discovery and development of neural therapeutics, brain drug delivery still encounters limited success due to meager perviousness of most of the drug molecules through the blood-brain barrier (BBB), a tight layer of endothelial cells that selectively impedes routing of the molecules across itself. In this Review, we have tried to present a comprehensive idea on the recent developments in nanoparticle based BBB delivery systems, with a focus on the advancements in receptor targeted polymeric nanoparticles pertaining to BBB delivery. We have also attempted to bridge the gap between conventional brain delivery strategies and nanoparticle based BBB delivery for in-depth understanding. Various strategies are being explored for simplifying delivery of molecules across the BBB; however, they have their own limitations such as invasiveness and need for hospitalization and surgery. Introduction of nanotechnology can impressively benefit brain drug delivery. Though many nanoparticles are being explored, there are still several issues that need to be analyzed scrupulously before a real and efficient BBB traversing nanoformulation is realized.

Glaucoma: Biological Trabecular and Neuroretinal Pathology with Perspectives of Therapy Innovation and Preventive Diagnosis

Glaucoma is a common degenerative disease affecting retinal ganglion cells (RGC) and optic nerve axons, with progressive and chronic course. It is one of the most important reasons of social blindness in industrialized countries. Glaucoma can lead to the development of irreversible visual field loss, if not treated. Diagnosis may be difficult due to lack of symptoms in early stages of disease. In many cases, when patients arrive at clinical evaluation, a severe neuronal damage may have already occurred. In recent years, newer perspective in glaucoma treatment have emerged. The current research is focusing on finding newer drugs and associations or better delivery systems in order to improve the pharmacological treatment and patient compliance. Moreover, the application of various stem cell types with restorative and neuroprotective intent may be found appealing (intravitreal autologous cellular therapy). Advances are made also in terms of parasurgical treatment, characterized by various laser types and techniques. Moreover, recent research has led to the development of central and peripheral retinal rehabilitation (featuring residing cells reactivation and replacement of defective elements), as well as innovations in diagnosis through more specific and refined methods and inexpensive tests.

Targeting transferrin receptors at the blood-brain barrier improves the uptake of immunoliposomes and subsequent cargo transport into the brain parenchyma

Drug delivery to the brain is hampered by the presence of the blood-brain barrier, which excludes most molecules from freely diffusing into the brain, and tightly regulates the active transport mechanisms that ensure sufficient delivery of nutrients to the brain parenchyma. Harnessing the possibility of delivering neuroactive drugs by way of receptors already present on the brain endothelium has been of interest for many years. The transferrin receptor is of special interest since its expression is limited to the endothelium of the brain as opposed to peripheral endothelium. Here, we investigate the possibility of delivering immunoliposomes and their encapsulated cargo to the brain via targeting of the transferrin receptor. We find that transferrin receptor-targeting increases the association between the immunoliposomes and primary endothelial cells in vitro, but that this does not correlate with increased cargo transcytosis. Furthermore, we show that the transferrin receptor-targeted immunoliposomes accumulate along the microvessels of the brains of rats, but find no evidence for transcytosis of the immunoliposome. Conversely, the increased accumulation correlated both with increased cargo uptake in the brain endothelium and subsequent cargo transport into the brain. These findings suggest that transferrin receptor-targeting is a relevant strategy of increasing drug exposure to the brain.

Lipopolyplex for Therapeutic Gene Delivery and Its Application for the Treatment of Parkinson's Disease

Lipopolyplex is a core-shell structure composed of nucleic acid, polycation and lipid. As a non-viral gene delivery vector, lipopolyplex combining the advantages of polyplex and lipoplex has shown superior colloidal stability, reduced cytotoxicity, extremely high gene transfection efficiency. Following intravenous administration, there are many strategies based on lipopolyplex to overcome the complex biological barriers in systemic gene delivery including condensation of nucleic acids into nanoparticles, long circulation, cell targeting, endosomal escape, release to cytoplasm and entry into cell nucleus. Parkinson's disease (PD) is the second most common neurodegenerative disorder and severely influences the patients' life quality. Current gene therapy clinical trials for PD employing viral vectors didn't achieve satisfactory efficacy. However, lipopolyplex may become a promising alternative approach owing to its stability in blood, ability to cross the blood-brain barrier (BBB) and specific targeting to diseased brain cells.

Transcytosis in the blood-cerebrospinal fluid barrier of the mouse brain with an engineered receptor/ligand system

Crossing the blood-brain and the blood-cerebrospinal fluid barriers (BCSFB) is one of the fundamental challenges in the development of new therapeutic molecules for brain disorders because these barriers prevent entry of most drugs from the blood into the brain. However, some large molecules, like the protein transferrin, cross these barriers using a specific receptor that transports them into the brain. Based on this mechanism, we engineered a receptor/ligand system to overcome the brain barriers by combining the human transferrin receptor with the cohesin domain from Clostridium thermocellum, and we tested the hybrid receptor in the choroid plexus of the mouse brain with a dockerin ligand. By expressing our receptor in choroidal ependymocytes, which are part of the BCSFB, we found that our systemically administrated ligand was able to bind to the receptor and accumulate in ependymocytes, where some of the ligand was transported from the blood side to the brain side.

Endocytosis of Nanomedicines: The Case of Glycopeptide Engineered PLGA Nanoparticles

The success of nanomedicine as a new strategy for drug delivery and targeting prompted the interest in developing approaches toward basic and clinical neuroscience. Despite enormous advances on brain research, central nervous system (CNS) disorders remain the world's leading cause of disability, in part due to the inability of the majority of drugs to reach the brain parenchyma. Many attempts to use nanomedicines as CNS drug delivery systems (DDS) were made; among the various non-invasive approaches, nanoparticulate carriers and, particularly, polymeric nanoparticles (NPs) seem to be the most interesting strategies. In particular, the ability of poly-lactide-co-glycolide NPs (PLGA-NPs) specifically engineered with a glycopeptide (g7), conferring to NPs' ability to cross the blood brain barrier (BBB) in rodents at a concentration of up to 10% of the injected dose, was demonstrated in previous studies using different routes of administrations. Most of the evidence on NP uptake mechanisms reported in the literature about intracellular pathways and processes of cell entry is based on in vitro studies. Therefore, beside the particular attention devoted to increasing the knowledge of the rate of in vivo BBB crossing of nanocarriers, the subsequent exocytosis in the brain compartments, their fate and trafficking in the brain surely represent major topics in this field.

OX26/CTX-conjugated PEGylated liposome as a dual-targeting gene delivery system for brain glioma

Background

The successful gene delivery into the brain is a major challenge due to the presence of the blood–brain barrier (BBB). In order to transport plasmid DNA across the BBB and target the brain glioma, the PEGylated liposomes (PLs) modified with OX26 and chlorotoxin (CTX) were developed as a dual-targeting gene delivery system, and the therapeutic efficacy of OX26/CTX-PL/pC27 against glioma was evaluated using in vitro and in vivo experimental models.

Methods

The PEGylated liposome complexes were prepared by the reverse phase evaporation method, and their physicochemical properties were examined. The transfection efficiency, intracellular distribution, in vitro effects of OX26/CTX-PL/pC27 were determined on C6, F98 and HEK293T cell lines. The dual-targeting therapeutic efficacy of OX26/CTX-PL/pC27 against glioma were assessed using the BMVECs/C6 cells co-culture model and the rat orthotopic glioma model.

Results

The OX26/CTX-PL/pDNA complexes exhibited a subglobose shape, and possessed notably low toxicities to HEK293T and C6 cells post 4 h incubation. In the in vitro transfection experiment, gene expressions of hTERTC27 from C6 and F98 cells were significantly improved by OX26 and CTX modification. Our in vitro results also showed that OX26 endowed the PLs with the transport ability across the BBB. Using the BMVECs/C6 cells co-culture model, the viability of C6 cells was decreased to 46.0% after OX26/CTX-PL/pC27 transfection. The OX26/CTX-PL/pC27 complexes exhibited enhanced therapeutic effects on C6 cells. Moreover, the dual-targeting therapeutic effects were further conformed with diminished tumor volumes (18.81 ± 6.15 mm³) and extended median survival time (46 days) in C6 glioma-bearing rats. Immunohistochemical analysis revealed the therapeutic effects derived from enhanced hTERTC27 expression in the tumor site.

Conclusions

The PEGylated liposomes modified with OX26 and CTX are able to significantly promote cell transfection, increase the transport of plasmid DNA across the BBB and afterwards target the brain glioma cells in vitro and in vivo, exhibit the most significant therapeutic efficacy. The ligand OX26 plays a critical role in transporting the lipoplexes across the BBB, and CTX acts as a major role in targeting brain glioma cells. The results would encourage further developments for non-invasive targeting therapy of brain gliomas by intravenous injection.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-191) contains supplementary material, which is available to authorized users.

Transferrin-bearing polypropylenimine dendrimer for targeted gene delivery to the brain

The possibility of using genes as medicines to treat brain diseases is currently limited by the lack of safe and efficacious delivery systems able to cross the blood-brain barrier, thus resulting in a failure to reach the brain after intravenous administration. On the basis that iron can effectively reach the brain by using transferrin receptors for crossing the blood-brain barrier, we propose to investigate if a transferrin-bearing generation 3-polypropylenimine dendrimer would allow the transport of plasmid DNA to the brain after intravenous administration. In vitro, the conjugation of transferrin to the polypropylenimine dendrimer increased the DNA uptake by bEnd.3 murine brain endothelioma cells overexpressing transferrin receptors, by about 1.4-fold and 2.3-fold compared to that observed with the non-targeted dendriplex and naked DNA. This DNA uptake appeared to be optimal following 2h incubation with the treatment. In vivo, the intravenous injection of transferrin-bearing dendriplex more than doubled the gene expression in the brain compared to the unmodified dendriplex, while decreasing the non-specific gene expression in the lung. Gene expression was at least 3-fold higher in the brain than in any tested peripheral organs and was at its highest 24h following the injection of the treatments. These results suggest that transferrin-bearing polypropylenimine dendrimer is a highly promising gene delivery system to the brain.

Targeting nanoparticles across the blood-brain barrier with monoclonal antibodies

Development of therapeutics for brain disorders is one of the more difficult challenges to be overcome by the scientific community due to the inability of most molecules to cross the blood-brain barrier (BBB). Antibody-conjugated nanoparticles are drug carriers that can be used to target encapsulated drugs to the brain endothelial cells and have proven to be very promising. They significantly improve the accumulation of the drug in pathological sites and decrease the undesirable side effect of drugs in healthy tissues. We review the systems that have demonstrated promising results in crossing the BBB through receptor-mediated endocytic mechanisms for the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

Nanoscale drug delivery systems and the blood-brain barrier

The protective properties of the blood–brain barrier (BBB) are conferred by the intricate architecture of its endothelium coupled with multiple specific transport systems expressed on the surface of endothelial cells (ECs) in the brain’s vasculature. When the stringent control of the BBB is disrupted, such as following EC damage, substances that are safe for peripheral tissues but toxic to neurons have easier access to the central nervous system (CNS). As a consequence, CNS disorders, including degenerative diseases, can occur independently of an individual’s age. Although the BBB is crucial in regulating the biochemical environment that is essential for maintaining neuronal integrity, it limits drug delivery to the CNS. This makes it difficult to deliver beneficial drugs across the BBB while preventing the passage of potential neurotoxins. Available options include transport of drugs across the ECs through traversing occludins and claudins in the tight junctions or by attaching drugs to one of the existing transport systems. Either way, access must specifically allow only the passage of a particular drug. In general, the BBB allows small molecules to enter the CNS; however, most drugs with the potential to treat neurological disorders other than infections have large structures. Several mechanisms, such as modifications of the built-in pumping-out system of drugs and utilization of nanocarriers and liposomes, are among the drug-delivery systems that have been tested; however, each has its limitations and constraints. This review comprehensively discusses the functional morphology of the BBB and the challenges that must be overcome by drug-delivery systems and elaborates on the potential targets, mechanisms, and formulations to improve drug delivery to the CNS.

Comparative binding, endocytosis, and biodistribution of antibodies and antibody-coated carriers for targeted delivery of lysosomal enzymes to ICAM-1 versus transferrin receptor

Targeting lysosomal enzymes to receptors involved in transport into and across cells holds promise to enhance peripheral and brain delivery of enzyme replacement therapies (ERTs) for lysosomal storage disorders. Receptors being explored include those associated with clathrin-mediated pathways, yet other pathways seem also viable. Well characterized examples are that of transferrin receptor (TfR) and intercellular adhesion molecule 1 (ICAM-1), involved in iron transport and leukocyte extravasation, respectively. TfR and ICAM-1 support ERT delivery via clathrin- vs. cell adhesion molecule-mediated mechanisms, displaying different valency and size restrictions. To comparatively assess this, we used antibodies vs. larger multivalent antibody-coated carriers and evaluated TfR vs. ICAM-1 binding and endocytosis in endothelial cells, as well as in vivo biodistribution and delivery of a model lysosomal enzyme required in peripheral organs and brain: acid sphingomyelinase (ASM), deficient in types A-B Niemann Pick disease. We found similar binding of antibodies to both receptors under control conditions, with enhanced binding to activated endothelium for ICAM-1, yet only anti-TfR induced endocytosis efficiently. Contrarily, antibody-coated carriers showed enhanced binding, engulfment, and endocytosis for ICAM-1. In mice, anti-TfR enhanced brain targeting over anti-ICAM, with an opposite outcome in the lungs, while carriers enhanced ICAM-1 targeting over TfR in both organs. Both targeted carriers enhanced ASM delivery to the brain and lungs vs. free ASM, with greater enhancement for anti-ICAM carriers. Therefore, targeting TfR or ICAM-1 improves lysosomal enzyme delivery. Yet, TfR targeting may be more efficient for smaller conjugates or fusion proteins, while ICAM-1 targeting seems superior for multivalent carrier formulations.

Liposomes for brain delivery

INTRODUCTION

Development of drug delivery systems for brain delivery is one of the most challenging research topics in pharmaceutical areas, mainly due to the presence of the blood-brain barrier (BBB), which separates the blood from the cerebral parenchyma thus limiting the brain uptake of the majority of therapeutic agents. Among the several carriers, which have been studied to overcome this problem, liposomes have gained increasing attention as promising strategies for brain-targeted drug delivery. The most advantageous features of liposomes are their ability to incorporate and deliver large amounts of drug and the possibility to decorate their surface with different ligands.

AREAS COVERED

The purpose of this review is to explore the different approaches studied to transport and deliver therapeutics and imaging agents to the brain by using liposomes. In the first part of the review, particular attention is paid to describe the anatomy of the BBB and different physiological transport mechanisms available for drug permeation. In the second part, the different strategies for the delivery of a drug to the brain using liposomes are reviewed for each transport mechanism.

EXPERT OPINION

Over the last decade, there have been significant developments concerning liposomal brain delivery systems conjugated with selected ligands with high specificity and low immunogenicity. An universally useful liposomal formulation for brain targeting does not exist but liposome design must be modulated by the appropriate choice of the specific homing device and transport mechanism.

Cell penetrating peptide tethered bi-ligand liposomes for delivery to brain in vivo: Biodistribution and transfection

Targeted nano-particulate systems hold extraordinary potential for delivery of therapeutics across blood brain barrier (BBB). In this work, we investigated the potential of novel bi-ligand (transferrin-poly-l-arginine) liposomal vector for delivery of desired gene to brain, in vivo. The in vivo evaluation of the delivery vectors is essential for clinical translation. We followed an innovative approach of combining transferrin receptor targeting with enhanced cell penetration to design liposomal vectors for improving the transport of molecules into brain. The biodistribution profile of 1, 1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine iodide(DiR)-labeled liposomes was evaluated in adult rats after single intravenous injection at dose of 15.2μmoles of phospholipids/kg body weight. We demonstrated that bi-ligand liposomes accumulated in rat brain at significantly (p<0.05) higher concentrations as compared to the single-ligand (transferrin) or plain liposomes. In addition, the bi-ligand liposomes resulted in increased expression of β-galactosidase(β-gal) plasmid in rat brain tissue in comparison to the single-ligand liposomes. Histological examination of the transfected tissues did not show any signs of tissue necrosis or inflammation. Hemolysis assay further authenticated the biocompatibility of bi-ligand liposomes in blood up to 600 nmoles of phospholipids/1.4×10(7) erythrocytes. The findings of this study provide important and detailed information regarding the distribution of bi-ligand liposomes in vivo and accentuate their ability to demonstrate improved brain penetration and transfection potential over single-ligand liposomes.

Anti-HIV-1 nanotherapeutics: promises and challenges for the future

The advent of highly active antiretroviral therapy (HAART) has significantly improved the prognosis for human immunodeficiency virus (HIV)-infected patients, however the adverse side effects associated with prolonged HAART therapy use continue. Although systemic viral load can be undetectable, the virus remains sequestered in anatomically privileged sites within the body. Nanotechnology-based delivery systems are being developed to target the virus within different tissue compartments and are being evaluated for their safety and efficacy. The current review outlines the various nanomaterials that are becoming increasingly used in biomedical applications by virtue of their robustness, safety, multimodality, and multifunctionality. Nanotechnology can revolutionize the field of HIV medicine by not only improving diagnosis, but also by improving delivery of antiretrovirals to targeted regions in the body and by significantly enhancing the efficacy of the currently available antiretroviral medications.

Delivery of gold nanoparticles to the brain by conjugation with a peptide that recognizes the transferrin receptor

The treatment of Alzheimer's disease and many other brain-related disorders is limited because of the presence of the blood-brain barrier, which highly regulate the crossing of drugs. Metal nanoparticles have unique features that could contribute to the development of new therapies for these diseases. Nanoparticles have the capacity to carry several molecules of a drug; furthermore, their unique physico-chemical properties allow, for example, photothermal therapy to produce molecular surgery to destroy tumor cells and toxic structures. Recently, we demonstrated that gold nanoparticles conjugated to the peptide CLPFFD are useful to destroy the toxic aggregates of β-amyloid, similar to the ones found in the brains of patients with Alzheimer's disease. However, nanoparticles, like many other compounds, have null or very low capacity to cross the blood-brain barrier. In order to devise a strategy to improve drug delivery to the brain, here we introduced the peptide sequence THRPPMWSPVWP into the gold nanoparticle-CLPFFD conjugate. This peptide sequence interacts with the transferrin receptor present in the microvascular endothelial cells of the blood-brain barrier, thus causing an increase in the permeability of the conjugate in brain, as shown by experiments in vitro and in vivo. Our results are highly relevant for the therapeutic applications of gold nanoparticles for molecular surgery in the treatment of neurodegenerative diseases such as Alzheimer's disease.

Two-order targeted brain tumor imaging by using an optical/paramagnetic nanoprobe across the blood brain barrier

Surgical resection is a mainstay of brain tumor treatments. However, the completed excision of malignant brain tumor is challenged by its infiltrative nature. Contrast enhanced magnetic resonance imaging is widely used for defining brain tumor in clinic. However its ability in tumor visualization is hindered by the transient circulation lifetime, nontargeting specificity, and poor blood brain barrier (BBB) permeability of the commercially available MR contrast agents. In this work, we developed a two-order targeted nanoprobe in which MR/optical imaging reporters, tumor vasculature targeted cyclic [RGDyK] peptides, and BBB-permeable Angiopep-2 peptides are labeled on the PAMAM-G5 dendrimer. This nanoprobe is supposed to first target the α(V)β(3) integrin on tumor vasculatures. Increased local concentration of nanoprobe facilitates the association between BBB-permeable peptides and the low-density lipoprotein receptor-related protein (LRP) receptors on the vascular endothelial cells, which further accelerates BBB transverse of the nanoprobe via LRP receptor-mediated endocytosis. The nanoprobes that have penetrated the BBB secondly target the brain tumor because both α(V)β(3) integrin and LRP receptor are highly expressed on the tumor cells. In vivo imaging studies demonstrated that this nanoprobe not only efficiently crossed intact BBB in normal mice, but also precisely delineated the boundary of the orthotropic U87MG human glioblastoma xenograft with high target to background signal ratio. Overall, this two-order targeted nanoprobe holds the promise to noninvasively visualize brain tumors with uncompromised BBB and provides the possibility for real-time optical-image-guided brain tumor resection during surgery.

The Trojan Horse Liposome Technology for Nonviral Gene Transfer across the Blood-Brain Barrier

The application of blood-borne gene therapy protocols to the brain is limited by the presence of the blood-brain barrier (BBB). Viruses have been extensively used as gene delivery systems. However, their efficacy in brain is limited by the lack of transport across the BBB following intravenous (IV) administration. Recent progress in the “Trojan Horse Liposome” (THL) technology applied to transvascular non-viral gene therapy of the brain presents a promising solution to the trans-vascular brain gene delivery problem. THLs are comprised of immunoliposomes carrying nonviral gene expression plasmids. The tissue target specificity of the THL is provided by peptidomimetic monoclonal antibody (MAb) component of the THL, which binds to specific endogenous receptors located on both the BBB and on brain cellular membranes, for example, insulin receptor and transferrin receptor. These MAbs mediate (a) receptor-mediated transcytosis of the THL complex through the BBB, (b) endocytosis into brain cells and (c) transport to the brain cell nuclear compartment. The expression of the transgene in brain may be restricted using tissue/cell specific gene promoters. This manuscript presents an overview on the THL transport technology applied to brain disorders, including lysosomal storage disorders and Parkinson's disease.

Novel and emerging strategies in drug delivery for overcoming the blood-brain barrier

Two decades of molecular research have revealed the presence of transporters and receptors expressed in the brain vascular endothelium that provide potential novel targets for the rational design of blood-brain barrier-penetrating drugs. In this review, we briefly introduce the reader to the molecular characteristics of the blood-brain barrier that make this one of the most important obstacles towards the development of efficacious CNS drugs. We highlight recent attempts to rationally target influx and bidirectional transport systems expressed on the brain endothelial cell and avoid the important obstacle presented in the form of efflux transporters. Many of these approaches are highly innovative and show promise for future human application. Some of these approaches, however, have revealed significant limitations and are critiqued in this review. Nonetheless, these combined efforts have left the field of CNS drug delivery better positioned for developing novel approaches towards the rational design of CNS-penetrating drugs.

PEGylated Poly(amidoamine) dendrimer-based dual-targeting carrier for treating brain tumors

A dual-targeting drug carrier (PAMAM-PEG-WGA-Tf) based on the PEGylated fourth generation (G = 4.0) PAMAM dendrimer with transferrin (Tf) and wheat germ agglutinin (WGA) on the periphery and doxorubicin (DOX) loaded in the interior was synthesized and its BBB penetration and tumor targeting properties were explored. DLS and TEM measurements revealed the size of PAMAM-PEG-WGA-Tf was in the range of 14-20 nm. It reduced the cytotoxicity of DOX to the normal cells greatly, while efficiently inhibited the growth rate of the C6 glioma cells. The assay of transport across the BBB showed that PAMAM-PEG-WGA-Tf delivered 13.5% of DOX in a period of 2 h, demonstrating an enhanced transport ratio as compared to the ratio of 8% for PAMAM-PEG-WGA, 7% for PAMAM-PEG-Tf and 5% for free DOX in the same period of time. The accumulation of DOX in the tumor site was increased due to the targeting effects of both Tf and WGA, leading to the complete breakage of the avascular C6 glioma spheroids in vitro.

A comparative study of transfection efficiency between liposomes, immunoliposomes and brain-specific immunoliposomes

This study investigated the transfection ability and efficiency of liposomes and immunoliposomes for exogenous gene delivery into the brain via the venous system. Four groups of rats underwent tail vein injection with one of the following: liposomes encapsulating pCMV (human cytomegalovirus promoter)-LacZ plasmid 80 microg (low dose) or 300 microg (high dose); general immunoliposomes encapsulating 80 microg transferrin receptor antibodies (OX26)-pCMV-LacZ plasmid; or brain-specific immunoliposomes encapsulating 80 microg OX26-pGFAP (glial fibrillary acidic protein promoter)-LacZ plasmid. A control group received no injected agent. The LacZ mRNA levels (1 h post-injection) and beta-galactosidase activity (48 h post-injection) in the brain and peripheral organs were assayed using real-time reverse transcription-polymerase chain reaction and histochemical staining, respectively. Both immunoliposomes delivered exogenous DNA containing the LacZ gene into the brain after venous injection, resulting in extensive LacZ expression in the brain. Furthermore, the brain-specific OX26-pGFAP-LacZ immunoliposome decreased the non-specific expression of LacZ in peripheral organs without affecting transfection efficiency in the brain. Thus, brain-specific immunoliposomes are an efficient and brain-specific targeting vector.

Nanoparticle-mediated brain-specific drug delivery, imaging, and diagnosis

Central nervous system (CNS) diseases represent the largest and fastest-growing area of unmet medical need. Nanotechnology plays a unique instrumental role in the revolutionary development of brain-specific drug delivery, imaging, and diagnosis. With the aid of nanoparticles of high specificity and multifunctionality, such as dendrimers and quantum dots, therapeutics, imaging agents, and diagnostic molecules can be delivered to the brain across the blood-brain barrier (BBB), enabling considerable progress in the understanding, diagnosis, and treatment of CNS diseases. Nanoparticles used in the CNS for drug delivery, imaging, and diagnosis are reviewed, as well as their administration routes, toxicity, and routes to cross the BBB. Future directions and major challenges are outlined.

Brain-targeting mechanisms of lactoferrin-modified DNA-loaded nanoparticles

Ligand-mediated brain-targeting drug delivery is one of the focuses at present. Elucidation of exact targeting mechanisms serves to efficiently design these drug delivery systems. In our previous studies, lactoferrin (Lf) was successfully exploited as a brain-targeting ligand to modify cationic dendrimer-based nanoparticles (NPs). The mechanisms of Lf-modified NPs to the brain were systematically investigated in this study for the first time. The uptake of Lf-modified vectors and NPs by brain capillary endothelial cells (BCECs) was related to clathrin-dependent endocytosis, caveolae-mediated endocytosis, and macropinocytosis. The intracellular trafficking results showed that Lf-modified NPs could rapidly enter the acidic endolysosomal compartments within 5 mins and then partly escape within 30 mins. Both Lf-modified vectors and NPs showed higher blood-brain barrier-crossing efficiency than unmodified counterparts. All the results suggest that both receptor- and adsorptive-mediated mechanisms contribute to the cellular uptake of Lf-modified vectors and NPs. Enhanced brain-targeting delivery could be achieved through the synergistic effect of the macromolecular polymers and the ligand.

Receptor-mediated therapeutic transport across the blood–brain barrier

The blood–brain barrier (BBB) hinders drug delivery to the brain parenchyma. The ultimate goal of brain drug targeting technology is to deliver therapeutics across the BBB with a diverse collection of molecular transport systems. Receptor-mediated transcytosis (RMT) is one such class of transport system. Insulin and transferrin, as well as other endogenous peptides, employ the vesicular trafficking machinery of the endothelium to transport substances between the blood and the brain. In addition to vector development, strategies for coupling drugs to the vector that give high-efficiency coupling are the other important element for RMT. After the BBB-targeting vector–therapeutic conjugates have crossed the BBB, there may still be a need to target them to a specific population of cells in the brain. This review will focus on two major aspects of RMT brain drug delivery: new advances of existing RMT systems and development of new BBB transport vectors and specific RMT targets.