Nano carriers for drug transport across the blood–brain barrier

Effective therapy lies in achieving a therapeutic amount of drug to the proper site in the body and then maintaining the desired drug concentration for a sufficient time interval to be clinically effective for treatment. The blood–brain barrier (BBB) hinders most drugs from entering the central nervous system (CNS) from the blood stream, leading to the difficulty of delivering drugs to the brain via the circulatory system for the treatment, diagnosis and prevention of brain diseases. Several brain drug delivery approaches have been developed, such as intracerebral and intracerebroventricular administration, intranasal delivery and blood-to-brain delivery, as a result of transient BBB disruption induced by biological, chemical or physical stimuli such as zonula occludens toxin, mannitol, magnetic heating and ultrasound, but these approaches showed disadvantages of being dangerous, high cost and unsuitability for most brain diseases and drugs. The strategy of vector-mediated blood-to-brain delivery, which involves improving BBB permeability of the drug–carrier conjugate, can minimize side effects, such as being submicrometre objects that behave as a whole unit in terms of their transport and properties, nanomaterials, are promising carrier vehicles for direct drug transport across the intact BBB as a result of their potential to enter the brain capillary endothelial cells by means of normal endocytosis and transcytosis due to their small size, as well as their possibility of being functionalized with multiple copies of the drug molecule of interest. This review provids a concise discussion of nano carriers for drug transport across the intact BBB, various forms of nanomaterials including inorganic/solid lipid/polymeric nanoparticles, nanoemulsions, quantum dots, nanogels, liposomes, micelles, dendrimers, polymersomes and exosomes are critically evaluated, their mechanisms for drug transport across the BBB are reviewed, and the future directions of this area are fully discussed.     

P-glycoprotein mediates brain-to-blood efflux transport of buprenorphine across the blood–brain barrier

The involvement of P-glycoprotein (P-gp) in buprenorphine (BNP) transport at the blood–brain barrier (BBB) in rats was investigated in vivo by means of both the brain uptake index technique and the brain efflux index technique. P-gp inhibitors, such as cyclosporin A, quinidine and verapamil, enhanced the apparent brain uptake of [³H]BNP by 1.5-fold. The increment of the BNP uptake by the brain suggests the involvement of a P-gp efflux mechanism of BNP transport at the BBB. [³H]BNP was eliminated with an apparent elimination half-life of 27.5 min after microinjection into the parietal cortex area 2 regions of the rat brain. The apparent efflux clearance of [³H]BNP across the BBB was 0.154 ml/min/g brain, which was calculated from the elimination rate constant (2.52 × 10^{? 2} min^{? 1}) and the distribution volume in the brain (6.11 ml/g brain). The efflux transport of [³H]BNP was inhibited by range from 32 to 64% in the presence of P-gp inhibitors. The present results suggest that BNP is transported from the brain across the BBB via a P-gp-mediated efflux transport system, at least in part.     

Targeted delivery across the blood–brain barrier

The safest and most effective way of targeting drugs to the entire brain is via delivery systems directed at endogenous receptor-mediated uptake mechanisms present at the cerebral capillaries. Such systems have been shown to be effective in animal models including primates, but no clinical trials have been performed so far. This review focuses on the well-characterised transferrin and insulin receptor-targeted systems, as well as on the more recently described systems that use the low-density lipoprotein-related protein 1 receptor, the low-density lipoprotein-related protein 2 receptor (also known as megalin and glycoprotein 330) or the diphtheria toxin receptor (which is the membrane-bound precursor of heparin-binding epidermal growth factor-like growth factor). The possibilities and limitations of these systems are compared and their future for human application is discussed.     

Nano-enabled delivery systems across the blood–brain barrier

The development of drugs to treat disorders of the central nervous system (CNS) faces difficulties in achieving penetration of a drug through the blood–brain barrier (BBB) and allowing the drug to reach its intended target in the brain. There have been strategies to improve drug delivery to the brain through endogenous transport pathways such as passive diffusion, endocytosis, and active transport. Among various strategies, nano-enabled delivery systems offer a promising solution to improve the uptake and targeted delivery of drugs into the brain. Various nanocarriers including liposomes, bolaamphiphiles and nanoparticles can be used as a means to encapsulate drugs, either alone or in combination with targeting ligands. Moreover, most of materials used in nanocarrier fabrication are both biodegradable and biocompatible, thereby increasing the clinical utility of them. Here, we review the possibility to employ nano-enabled materials for delivery of drug across the BBB and the recent advances in nanotechnologies for therapy of the CNS diseases.     

Solid lipid nanoparticles carrying chemotherapeutic drug across the blood–brain barrier through insulin receptor-mediated pathway

Abstract

Carmustine (BCNU)-loaded solid lipid nanoparticles (SLNs) were grafted with 83–14 monoclonal antibody (MAb) (83–14 MAb/BCNU-SLNs) and applied to the brain-targeting delivery. Human brain-microvascular endothelial cells (HBMECs) incubated with 83–14 MAb/BCNU-SLNs were stained to demonstrate the interaction between the nanocarriers and expressed insulin receptors (IRs). The results revealed that the particle size of 83–14 MAb/BCNU-SLNs decreased with an increasing weight percentage of Dynasan 114 (DYN). Storage at 4?°C for 6 weeks slightly deformed the colloidal morphology. In addition, poloxamer 407 on 83–14 MAb/BCNU-SLNs induced cytotoxicity to RAW264.7 cells and inhibited phagocytosis by RAW264.7 cells. An increase in the weight percentage of DYN from 0% to 67% slightly reduced the viability of RAW264.7 cells and promoted phagocytosis. Moreover, the transport ability of 83–14 MAb/BCNU-SLNs across the blood–brain barrier (BBB) in vitro enhanced with an increasing weight percentage of Tween 80. 83–14 MAb on MAb/BCNU-SLNs stimulated endocytosis by HBMECs via IRs and enhanced the permeability of BCNU across the BBB. 83–14 MAb/BCNU-SLNs can be a promising antitumor drug delivery system for transporting BCNU to the brain.     

Drug delivery across the blood–brain barrier using focused ultrasound

Introduction: The presence of the blood–brain barrier (BBB) is a significant impediment to the delivery of therapeutic agents to the brain for treatment of brain diseases. Focused ultrasound (FUS) has been developed as a noninvasive method for transiently increasing the permeability of the BBB to promote drug delivery to targeted regions of the brain.

Areas covered: The present review briefly compares the methods used to promote drug delivery to the brain and describes the benefits and limitations of FUS technology. We summarize the experimental data which shows that FUS, combined with intravascular microbubbles, increases therapeutic agent delivery into the brain leading to significant reductions in pathology in preclinical models of disease. The potential for translation of this technology to the clinic is also discussed.

Expert opinion: The introduction of magnetic resonance imaging guidance and intravascular administration of microbubbles to FUS treatments permits the consistent, transient and targeted opening of the BBB. The development of feedback systems and real-time monitoring techniques improve the safety of BBB opening. Successful clinical translation of FUS has the potential to revolutionize the treatment of brain disease resulting in effective, less-invasive treatments without the need for expensive drug development.     

Murine in vitro model of the blood–brain barrier for evaluating drug transport

In vitro blood–brain barrier (BBB) models help predict brain uptake of potential central nervous system drug candidates. Current in vitro models are composed of brain microvascular endothelial cells (BMEC) that are isolated from rat, bovine, or porcine. However, most in vivo studies on drug transport through the BBB are performed in small laboratory animals, specially mouse and thus murine in vitro BBB models serve as better surrogates to correlate with these studies. Here we describe the functional characterization of a reproducible in vitro model composed of murine BMEC co-cultured with rat primary astrocytes in the presence of biochemical inducing agents. The co-cultures presented high TEER and low sodium fluorescein permeability. Expression of specific BBB tight junction proteins (occludin, claudin-5, ZO-1) and the functionality of transporters (Pgp, GLUT1) were detected by immunocytochemistry and Western blotting. These results indicated a 2.5-fold increase in the expression levels of these proteins in the presence of astrocytes. In addition, a high correlation coefficient (0.98) was obtained between the permeability of a series of hydrophobic and hydrophilic drugs and their corresponding in vivo values. These results together establish the utility of this murine model for future drug transport, pathological, and pharmacological characterizations of the BBB.     

ABC transporters at the blood–brain barrier

Introduction: The blood–brain barrier (BBB) possesses an outstanding ability to protect the brain against xenobiotics and potentially poisonous metabolites. Owing to this, ATP binding cassette (ABC) export proteins have garnered significant interest in the research community. These transport proteins are predominantly localized to the luminal membrane of brain microvessels, where they recognize a wide range of different substrates and transport them back into the blood circulation.

Areas covered: This review summarizes recent findings on these transport proteins, including their expression in the endothelial cell membrane and their substrate recognition. Signaling cascades underlying the expression and function of these proteins will be discussed as well as their role in diseases such as Alzheimer’s disease, epilepsy, amyotrophic lateral sclerosis and brain tumors.

Expert opinion: ABC transporters represent an integral part of the human transportome and are of particular interest at the blood–brain barrier they as they significantly contribute to brain homeostasis. In addition, they appear to be involved in myriad CNS diseases. Therefore studying their mechanisms of action as well as their signaling cascades and responses to internal and external stimuli will help us understand the pathogenesis of these diseases.     

Lipid-based nanocarrier efficiently delivers highly water soluble drug across the blood–brain barrier into brain

Delivering highly water soluble drugs across blood–brain barrier (BBB) is a crucial challenge for the formulation scientists. A successful therapeutic intervention by developing a suitable drug delivery system may revolutionize treatment across BBB. Efforts were given here to unravel the capability of a newly developed fatty acid combination (stearic acid:oleic acid:palmitic acid?=?8.08:4.13:1) (ML) as fundamental component of nanocarrier to deliver highly water soluble zidovudine (AZT) as a model drug into brain across BBB. A comparison was made with an experimentally developed standard phospholipid-based nanocarrier containing AZT. Both the formulations had nanosize spherical unilamellar vesicular structure with highly negative zeta potential along with sustained drug release profiles. Gamma scintigraphic images showed both the radiolabeled formulations successfully crossed BBB, but longer retention in brain was observed for ML-based formulation (MGF) as compared to soya lecithin (SL)-based drug carrier (SYF). Plasma and brain pharmacokinetic data showed less clearance, prolonged residence time, more bioavailability and sustained release of AZT from MGF in rats compared to those data of the rats treated with SYF/AZT suspension. Thus, ML may be utilized to successfully develop drug nanocarrier to deliver drug into brain across BBB, in a sustained manner for a prolong period of time and may provide an effective therapeutic strategy for many diseases of brain. Further, many anti-HIV drugs cannot cross BBB sufficiently. Hence, the developed formulation may be a suitable option to carry those drugs into brain for better therapeutic management of HIV.     

Glutathione PEGylated liposomes: pharmacokinetics and delivery of cargo across the blood–brain barrier in rats

Abstract

Partly due to poor blood–brain barrier drug penetration the treatment options for many brain diseases are limited. To safely enhance drug delivery to the brain, glutathione PEGylated liposomes (G-Technology®) were developed. In this study, in rats, we compared the pharmacokinetics and organ distribution of GSH-PEG liposomes using an autoquenched fluorescent tracer after intraperitoneal administration and intravenous administration. Although the appearance of liposomes in the circulation was much slower after intraperitoneal administration, comparable maximum levels of long circulating liposomes were found between 4 and 24?h after injection. Furthermore, 24?h after injection a similar tissue distribution was found. To investigate the effect of GSH coating on brain delivery in vitro uptake studies in rat brain endothelial cells (RBE4) and an in vivo brain microdialysis study in rats were used. Significantly more fluorescent tracer was found in RBE4 cell homogenates incubated with GSH-PEG liposomes compared to non-targeted PEG liposomes (1.8-fold, p?<?0.001). In the microdialysis study 4-fold higher (p?<?0.001) brain levels of fluorescent tracer were found after intravenous injection of GSH-PEG liposomes compared with PEG control liposomes. The results support further investigation into the versatility of GSH-PEG liposomes for enhanced drug delivery to the brain within a tolerable therapeutic window.     

The influence of age on the distribution of morphine and morphine-3-glucuronide across the blood–brain barrier in sheep

Background and purpose:

The effect of age on the distribution of morphine and morphine-3-glucuronide (M3G) across the blood–brain barrier (BBB) was studied in a sheep model utilizing intracerebral microdialysis. The effect of neonatal asphyxia on brain drug distribution was also studied.

Experimental approach:

Microdialysis probes were inserted into the cortex, striatum and blood of 11 lambs (127 gestation days) and six ewes. Morphine, 1 mg·kg⁻¹, was intravenously administered as a 10 min constant infusion. Microdialysis and blood samples were collected for up to 360 min and analysed using liquid chromatography-tandem mass spectrometry. The half-life, clearance, volume of distribution, unbound drug brain : blood distribution ratio (K_p,uu) and unbound drug volume of distribution in brain (V_u,brain) were estimated.

Key results:

Morphine K_p,uu was 1.19 and 1.89 for the sheep and premature lambs, respectively, indicating that active influx into the brain decreases with age. Induced asphyxia did not affect transport of morphine or M3G across the BBB. Morphine V_u,brain measurements were higher in sheep than in premature lambs. The M3G K_p,uu values were 0.27 and 0.17 in sheep and premature lambs, indicating a net efflux from the brain in both groups.

Conclusions and implications:

The morphine K_p,uu was above unity, indicating active transport into the brain; influx was significantly higher in premature lambs than in adult sheep. These results in sheep differ from those in humans, rats, mice and pigs where a net efflux of morphine from the brain is observed.     

Ibogaine labeling with 99mTc-tricarbonyl: Synthesis and transport at the mouse blood–brain barrier

The ^99mTc-tricarbonyl core may be used as an ideal tool for gamma-labeling ligands in noninvasive SPECT imaging. However, most ^99mTc-tricarbonyl-labeled agents have difficulty crossing the blood–brain barrier (BBB). We radiolabeled the neuroactive indole ibogaine with ^99mTc-tricarbonyl and measured its transport into the mouse brain by in situ brain perfusion. We measured the interactions of [^99mTc(CO)₃-ibogaine]⁺ and ^99mTc-tricarbonyl with the main BBB efflux transporters P-gp and BCRP in vitro and in vivo. Ibogaine was radiolabeled (yield: over 95%). [^99mTc(CO)₃-ibogaine]⁺ entered the brain (K_in) poorly (0.18 µL/g/s), at about the same rate as ^99mTc-tricarbonyl (0.16 µL/g/s) and [^99mTc-sestamibi]⁺ (0.10 µL/g/s). The CNS tracer [^99mTc-HMPAO]⁰ entered the brain ～70-times higher than [^99mTc(CO)₃-ibogaine]⁺. In vitro studies revealed that neither [^99mTc(CO)₃-ibogaine]⁺ nor ^99mTc-tricarbonyl ion were substrates for P-gp or BCRP. But lowering the membrane dipole potential barrier with phloretin enhanced the brain transport of [^99mTc(OH₂)₃(CO)₃]⁺ ～3-fold. Thus, ibogaine directly labeled with ^99mTc-tricarbonyl is not suitable for CNS imaging because of its poor uptake. Brain transport is not restricted by efflux transporters but is reduced by its lipophilicity and interaction with the membrane-positive dipole potential.     

Localized delivery of therapeutic doxorubicin dose across the canine blood–brain barrier with hyperthermia and temperature sensitive liposomes

Most drugs cannot penetrate the blood–brain barrier (BBB), greatly limiting the use of anti-cancer agents for brain cancer therapy. Temperature sensitive liposomes (TSL) are nanoparticles that rapidly release the contained drug in response to hyperthermia (>40?°C). Since hyperthermia also transiently opens the BBB, we hypothesized that localized hyperthermia can achieve drug delivery across the BBB when combined with TSL. TSL-encapsulated doxorubicin (TSL-Dox) was infused intravenously over 30?min at a dose of 0.94?mg/kg in anesthetized beagles (age ～17?months). Following, a hyperthermia probe was placed 5–10?mm deep through one of four 3-mm skull burr holes. Hyperthermia was performed randomized for 15 or 30?min, at either 45 or 50?°C. Blood was drawn every 30?min to measure TSL-Dox pharmacokinetics. Nonsurvival studies were performed in four dogs, where brain tissue at the hyperthermia location was extracted following treatment to quantify doxorubicin uptake via high-performance liquid chromatography (HPLC) and to visualize cellular uptake via fluorescence microscopy. Survival studies for 6?weeks were performed in five dogs treated by a single hyperthermia application. Local doxorubicin delivery correlated with hyperthermia duration and ranged from 0.11 to 0.74?μg/g of brain tissue at the hyperthermia locations, with undetectable drug uptake in unheated tissue. Fluorescence microscopy demonstrated doxorubicin delivery across the BBB. Histopathology in Haematoxylin &; Eosin (H&;E) stained samples demonstrated localized damage near the probe. No animals in the survival group demonstrated significant neurological deficits. This study demonstrates that localized doxorubicin delivery to the brain can be facilitated by TSL-Dox with localized hyperthermia with no significant neurological deficits.