Molecular architecture determines brain delivery of a transferrin receptor–targeted lysosomal enzyme

Delivery of biotherapeutics across the blood–brain barrier (BBB) is a challenge. Many approaches fuse biotherapeutics to platforms that bind the transferrin receptor (TfR), a brain endothelial cell target, to facilitate receptor-mediated transcytosis across the BBB. Here, we characterized the pharmacological behavior of two distinct TfR-targeted platforms fused to iduronate 2-sulfatase (IDS), a lysosomal enzyme deficient in mucopolysaccharidosis type II (MPS II), and compared the relative brain exposures and functional activities of both approaches in mouse models. IDS fused to a moderate-affinity, monovalent TfR-binding enzyme transport vehicle (ETV:IDS) resulted in widespread brain exposure, internalization by parenchymal cells, and significant substrate reduction in the CNS of an MPS II mouse model. In contrast, IDS fused to a standard high-affinity bivalent antibody (IgG:IDS) resulted in lower brain uptake, limited biodistribution beyond brain endothelial cells, and reduced brain substrate reduction. These results highlight important features likely to impact the clinical development of TfR-targeting platforms in MPS II and potentially other CNS diseases.     

Extent and direction of ghrelin transport across the blood-brain barrier is determined by its unique primary structure

The novel hormone ghrelin is a potent orexigen that may counterbalance leptin. Ghrelin is the only secreted molecule requiring post-translational acylation with octanoic acid to ensure bioactivity. Ghrelin, predominantly derived from the stomach, may target neuroendocrine networks within the central nervous system (CNS) to regulate energy homeostasis. This would require ghrelin to cross the blood-brain barrier (BBB). In mice, we examined whether ghrelin crosses the BBB and whether its lipophilic side chain is involved in this process. We found that saturable systems transported human ghrelin from brain-to-blood and from blood-to-brain. Mouse ghrelin, differing from human ghrelin by two amino acids, was a substrate for the brain-to-blood but not for the blood-to-brain transporter and so entered the brain to a far lesser degree. des-Octanoyl ghrelin entered the brain by nonsaturable transmembrane diffusion and was sequestered once within the CNS. In summary, we show that ghrelin transport across the BBB is a complex, highly regulated bidirectional process. The direction and extent of passage are determined by the primary structure of ghrelin, defining a new role for the unique post-translational octanoylation.     

Carrier-mediated transport of the quaternary ammonium neuronal nicotinic receptor antagonist n,n'-dodecylbispicolinium dibromide at the blood-brain barrier

The quaternary ammonium compound N,N'-dodecyl-bispicolinium dibromide (bPiDDB) potently and selectively inhibits nicotinic receptors (nAChRs) mediating nicotine-evoked [(3)H]dopamine release and decreases nicotine self-administration, suggesting that this polar, charged molecule penetrates the blood-brain barrier (BBB). This report focuses on 1) BBB penetration of bPiDDB; 2) the mechanism of permeation; and 3) comparison of bPiDDB to the cations choline and N-octylnicotinium iodide (NONI), both of which are polar, charged molecules that undergo facilitated BBB transport. The BBB permeation of [(3)H]choline, [(3)H]NONI, and [(14)C]bPiDDB was evaluated using in situ rat brain perfusion methods. Cerebrovascular permeability surface-area product (PS) values for [(3)H]choline, [(3)H]NONI, and [(14)C]bPiDDB were comparable (1.33 +/- 0.1, 1.64 +/- 0.15, and 1.3 +/- 0.3 ml/s/g, respectively). To ascertain whether penetration was saturable, unlabeled substrate was added to the perfusion fluid. Unlabeled choline (500 microM) reduced the PS of [(3)H]choline to 0.15 +/- 0.06 microl/s/g (p < 0.01). Likewise, unlabeled bPiDDB (500 microM) reduced the PS of [(14)C]bPiDDB to 0.046 +/- 0.005 microl/s/g (p < 0.01), whereas unlabeled NONI reduced the PS for [(3)H]NONI by approximately 50% to 0.73 +/- 0.31 microl/s/g. The PS of [(14)C]bPiDDB was reduced (p < 0.05) in the presence of 500 microM choline, indicating that the BBB choline transporter may be responsible for the transport of bPiDDB into brain. Saturable kinetic parameters for [(14)C]bPiDDB were similar to those for [(3)H]choline. The current results suggest that bPiDDB uses the BBB choline transporter for approximately 90% of its permeation into brain, and they demonstrate the carrier-mediated BBB penetration of a novel bisquaternary ammonium nAChR antagonist.     

Insulin in the brain: There and back again

Insulin performs unique functions within the CNS. Produced nearly exclusively by the pancreas, insulin crosses the blood-brain barrier (BBB) using a saturable transporter, affecting feeding and cognition through CNS mechanisms largely independent of glucose utilization. Whereas peripheral insulin acts primarily as a metabolic regulatory hormone, CNS insulin has an array of effects on brain that may more closely resemble the actions of the ancestral insulin molecule. Brain endothelial cells (BECs), the cells that form the vascular BBB and contain the transporter that translocates insulin from blood to brain, are themselves regulated by insulin. The insulin transporter is altered by physiological and pathological factors including hyperglycemia and the diabetic state. The latter can lead to BBB disruption. Pericytes, pluripotent cells in intimate contact with the BECs, protect the integrity of the BBB and its ability to transport insulin. Most of insulin's known actions within the CNS are mediated through two canonical pathways, the phosphoinositide-3 kinase (PI3)/Akt and Ras/mitogen activated kinase (MAPK) cascades. Resistance to insulin action within the CNS, sometimes referred to as diabetes mellitus type III, is associated with peripheral insulin resistance, but it is possible that variable hormonal resistance syndromes exist so that resistance at one tissue bed may be independent of that at others. CNS insulin resistance is associated with Alzheimer's disease, depression, and impaired baroreceptor gain in pregnancy. These aspects of CNS insulin action and the control of its entry by the BBB are likely only a small part of the story of insulin within the brain.     

Investigation of efflux transport of dehydroepiandrosterone sulfate and mitoxantrone at the mouse blood-brain barrier: a minor role of breast cancer resistance protein

Breast cancer resistance protein (Bcrp/Abcg2) is a new efflux transporter found at the blood-brain barrier (BBB) of humans and pigs. Since it has been hypothesized that Bcrp may act as a new type of efflux transporter at the BBB, we investigated the involvement of Bcrp in the efflux transport of typical substrates, dehydroepiandrosterone sulfate (DHEAS) and mitoxantrone, across the mouse BBB. The expression of Bcrp in mouse brain capillaries was confirmed by quantitative polymerase chain reaction, Western blot, and immunohistochemical analysis. The role of Bcrp as an efflux transporter was evaluated using the in situ brain perfusion method in wild-type and P-glycoprotein (P-gp) knockout mice with or without treatment with GF120918 (Elacridar), an inhibitor of both Bcrp and P-gp. The increased brain uptake of [(3)H]DHEAS and [(3)H]mitoxantrone by GF120918 in wild-type and P-gp knockout mice suggested the existence of a GF120918-sensitive and P-gp-independent efflux transporter for DHEAS and mitoxantrone across the BBB. However, the brain uptake of [(3)H]DHEAS in Bcrp knockout mice was comparable with that in wild-type mice, and the effect of GF120918 was still observed in Bcrp knockout mice. In addition, the brain uptake of [(3)H]mitoxantrone was also similar in wild-type and Bcrp knockout mice. These results suggest that although BCRP is expressed at the BBB it plays a minor role in active efflux transport of DHEAS and mitoxantrone out of brain and that one or more GF120918-sensitive efflux transporters distinct from BCRP or P-gp contributes to the brain efflux of DHEAS and mitoxantrone.     

Organic anion-transporting polypeptides mediate transport of opioid peptides across blood-brain barrier

Organic anion-transporting polypeptides (Oatps) are a rapidly growing gene family of polyspecific membrane transporters. In rat brain, Oatp1 (gene symbol Slc21a1) and Oatp2 (Slc21a5) are localized at the apical and basolateral domains, respectively, of the choroid plexus epithelium. Furthermore, Oatp2 is strongly expressed at the rat blood-brain barrier (BBB). This study localizes the human OATP (now called OATP-A; SLC21A3) at the BBB in humans. Furthermore, with the Xenopus laevis oocyte system the delta-opioid receptor agonists [D-penicillamine(2,5)]enkephalin (DPDPE) and deltorphin II were identified as new transport substrates of OATP-A. This OATP-A-mediated DPDPE and deltorphin II transport exhibited apparent K(m) values of approximately 202 and 330 microM, respectively, and OATP-A-mediated deltorphin II transport was inhibited by the mu-opioid receptor agonist Tyr-D-Ala-Gly-N-methyl-Phe-glycinol, the endogenous peptide Leu-enkephalin, and the opiate antagonists naloxone and naltrindole. DPDPE also was transported by rat Oatp1 (K(m) approximately 48 microM) and Oatp2 (K(m) approximately 19 microM), whereas deltorphin II was only transported by Oatp1 (K(m) approximately 137 microM). These results demonstrate that OATP-A can mediate transport of the analgesic opioid peptides DPDPE and deltorphin II across the human BBB. Furthermore, because rat Oatp1 and Oatp2 exhibit similar but not identical transport activities as OATP-A, the results generally indicate that members of the Oatp/OATP gene family of membrane transporters play an important role in carrier-mediated transport of opioid peptides across the BBB and blood-cerebrospinal fluid barrier of the mammalian brain.     

Blood-brain barrier permeability of novel [D-arg2]dermorphin (1-4) analogs: transport property is related to the slow onset of antinociceptive activity in the central nervous system

To clarify the pharmacological characteristics of Nalpha-amidino-Tyr-D-Arg-Phe-betaAla-OH (ADAB) and Nalpha-amidino-Tyr-D-Arg-Phe-MebetaAla-OH (ADAMB), mu1-opioid receptor-selective [D-Arg2]dermorphin tetrapeptide analogs, the plasma pharmacokinetics, and the in vivo blood-brain barrier (BBB) transport of these peptides were quantitatively evaluated. The mechanism responsible for the BBB transport of these peptides was also examined. The in vivo BBB permeation influx rates of 125I-ADAB and 125I-ADAMB after an i.v. bolus injection into mice were determined to be 0.0515 +/- 0.0284 microl/(min.g of brain) and 0.0290 +/- 0.0059 microl/(min.g of brain), respectively, both rates being slower than that of 125I-Tyr-D-Arg-Phe-betaAla-OH (125I-TAPA), a [D-Arg2]dermorphin tetrapeptide analog. To elucidate the BBB transport mechanism of ADAB and ADAMB, a conditionally immortalized mouse brain capillary endothelial cell line (TM-BBB4) was used as an in vitro model of the BBB. The internalization of both 125I-ADAB and 125I-ADAMB into cells was concentration-dependent with half-saturation constant (Kd) values of 3.76 +/- 0.83 and 5.68 +/- 1.75 microM, respectively. The acid-resistant binding of both ADAB and ADAMB was significantly inhibited by dansylcadaverine (an endocytosis inhibitor) and poly-l-lysine and protamine (polycations), but it was not inhibited by 2,4-dinitrophenol, or at 4 degrees C. These results suggest that ADAB and ADAMB are transported through the BBB with slower permeation rates than that of TAPA, and this is likely to be a factor in the slow onset of their antinociceptive activity in the central nervous system. The mechanism of the BBB transport of these drugs is considered to be adsorptive-mediated endocytosis.     

Charting the course across the blood-brain barrier

The blood-brain barrier (BBB) presents a significant obstacle to delivery of targeted therapies to brain tumors. In this issue of the JCI, Staquicini and colleagues apply an in vivo phage-displayed library of random peptides to identify differentially expressed peptides that can be used to transport targeted agents across the intact BBB. The authors uncover a non-canonical, peptide-mediated iron-mimicry mechanism to induce transport of the transferrin/transferrin receptor complex across the BBB. They then demonstrate the ability of phage-targeting approaches to deliver therapeutic cargo and molecular imaging reporters across the BBB in an intracranial glioblastoma mouse model.     

Intravenous human interleukin-1alpha impairs memory processing in mice: dependence on blood-brain barrier transport into posterior division of the septum

Peripherally administered cytokines profoundly affect the central nervous system (CNS). One mechanism by which they could affect the CNS is by crossing the blood-brain barrier (BBB) to interact directly with brain receptors. Human and murine IL-1alpha (hIL-1alpha; mIL-1alpha) are transported across the murine BBB with a high rate of transport into the posterior division of the septum (PDS), but it is unknown whether BBB transport is relevant to their actions. Here, we injected species-specific blocking antibodies into the PDS to determine whether transport across the BBB is required for blood-borne hIL-1alpha to affect memory. Retention was impaired in a dose-dependent manner when hIL-1alpha was injected either by tail vein (i.v.) or into the PDS, with the PDS route being 1000 times more potent. About 70% of the memory impairment induced by i.v. hIL-1alpha was reversed by injecting a blocking antibody (Ab) specific for hIL-1alpha into the PDS. This shows that much of the memory impairment induced by hIL-1alpha depends on its ability to cross the BBB. Ab specific for mIL-1alpha was also effective in reversing memory impairment, showing that hIL-1alpha releases mIL-1alpha from endogenous stores. Whether the mIL-1alpha was released from peripheral stores, which would require it to cross the BBB, or from brain stores is unknown. In conclusion, these results show that exogenous, blood-borne hIL-1alpha affects memory by releasing mIL-1alpha from endogenous stores and by crossing the BBB to act at sites within the PDS.     

Effect of age on functional P-glycoprotein in the blood-brain barrier measured by use of (R)-[(11)C]verapamil and positron emission tomography

INTRODUCTION: P-glycoprotein (P-gp) is an efflux transporter responsible for the transport of various drugs across the blood-brain barrier (BBB). Loss of P-gp function with age may be one factor in the development and progression of neurodegenerative diseases. The aim of this study was to assess the effect of aging on BBB P-gp function. Furthermore, the relationship between BBB P-gp activity and peripheral P-gp activity in CD3-positive leukocytes was investigated. Finally, plasma pharmacokinetics of carbon 11-labeled (R)-verapamil was evaluated. METHODS: (R)-[(11)C]verapamil and positron emission tomography were used to assess gray matter P-gp function. Because (R)-[(11)C]verapamil is a substrate for P-gp, the volume of distribution of (R)-[(11)C]verapamil in the brain inversely reflects P-gp function in the BBB. RESULTS: Mean volume of distribution values for 5 young healthy volunteers (age range, 21-27 years) and 5 elderly healthy volunteers (age range, 59-68 years) were 0.62+/-0.10 and 0.73+/-0.07, respectively (P=.03). The activity index of P-gp activity in CD3-positive leukocytes was 2.88+/-0.77 in young volunteers and 1.76+/-0.58 in elderly volunteers (P=.02). CONCLUSION: This study showed decreased P-gp activity during aging. Consequently, the brain may be exposed to higher drug and toxin levels in elderly subjects.     

Medicinal Chemistry Based Approaches and Nanotechnology‐Based Systems to Improve CNS Drug Targeting and Delivery

The central nervous system (CNS) is protected by various barriers, which regulate nervous tissue homeostasis and control the selective and specific uptake, efflux, and metabolism of endogenous and exogenous molecules. Among these barriers is the blood–brain barrier (BBB), a physical and physiological barrier that filters very efficiently and selectively the entry of compounds from the blood to the brain and protects nervous tissue from harmful substances and infectious agents present in the bloodstream. The BBB also prevents the entry of potential drugs. As a result, various drug targeting and delivery strategies are currently being developed to enhance the transport of drugs from the blood to the brain. Following a general introduction, we briefly overview in this review article the fundamental physiological properties of the BBB. Then, we describe current strategies to bypass the BBB (i.e., invasive methods, alternative approaches, and temporary opening) and to cross it (i.e., noninvasive approaches). This section is followed by a chapter addressing the chemical and technological solutions developed to cross the BBB. A special emphasis is given to prodrug‐targeting approaches and targeted nanotechnology‐based systems, two promising strategies for BBB targeting and delivery of drugs to the brain.     

Transport of histone through the blood-brain barrier

The present studies were designed to determine if the endogenous cationic protein, e.g., histone, is capable of penetrating the blood-brain barrier (BBB) in vivo. Calf thymus histone was iodinated with [125I]iodine and was found to be taken up rapidly by isolated bovine brain capillaries used as an in vitro model system of the BBB via a time- and temperature-dependent mechanism. The binding was saturable and a Scatchard plot of the binding data was linear, yielding a KD = 15.2 +/- 2.8 microM and a maximal binding = 7.7 +/- 1.0 nmol/mg of protein. Other polycations such as protamine or polylysine markedly inhibited uptake of [125I] histone, but cationized albumin demonstrated minimal inhibition and cationized immunoglobulin caused no inhibition of bovine brain capillary uptake of [125I]histone. The in vivo brain VD of [125I] histone reached 159 +/- 70 microliters/g by 10 min of carotid arterial perfusion as compared to the 10-min VD for [3H]albumin, 17 +/- 7 microliter/g. Most of this uptake represented sequestration by the vasculature, but approximately 8% of the total histone taken up by brain was found to be transported unmetabolized (based on trichloroacetic acid precipitability of brain supernatant [( 125I]) into brain interstitium. These studies demonstrate that histone is transported through the BBB in vivo via absorptive-mediated transport. Thus, histone is an endogenous protein that is capable of transport through the BBB and may be a potential vector for pharmaceutical delivery through the BBB.     

A2A adenosine receptor modulates drug efflux transporter P-glycoprotein at the blood-brain barrier

The blood-brain barrier (BBB) protects the brain from toxic substances within the peripheral circulation. It maintains brain homeostasis and is a hurdle for drug delivery to the CNS to treat neurodegenerative diseases, including Alzheimer’s disease and brain tumors. The drug efflux transporter P-glycoprotein (P-gp) is highly expressed on brain endothelial cells and blocks the entry of most drugs delivered to the brain. Here, we show that activation of the A2A adenosine receptor (AR) with an FDA-approved A2A AR agonist (Lexiscan) rapidly and potently decreased P-gp expression and function in a time-dependent and reversible manner. We demonstrate that downmodulation of P-gp expression and function coincided with chemotherapeutic drug accumulation in brains of WT mice and in primary mouse and human brain endothelial cells, which serve as in vitro BBB models. Lexiscan also potently downregulated the expression of BCRP1, an efflux transporter that is highly expressed in the CNS vasculature and other tissues. Finally, we determined that multiple pathways, including MMP9 cleavage and ubiquitinylation, mediated P-gp downmodulation. Based on these data, we propose that A2A AR activation on BBB endothelial cells offers a therapeutic window that can be fine-tuned for drug delivery to the brain and has potential as a CNS drug-delivery technology.     

Homogeneous antibody–angiopep 2 conjugates for effective brain targeting

Antibody-based therapy has shown great success in the treatment of many diseases, including cancers. While antibodies and antibody–drug conjugates (ADCs) have also been evaluated for central nervous system (CNS) disorders as well as brain tumors, their therapeutic efficacy can be substantially limited due to low permeability across the blood–brain barrier (BBB). Thus, improving BBB permeability of therapeutic antibodies is critical in establishing this drug class as a reliable clinical option for CNS diseases. Here, we report that, compared with a conventional heterogeneous conjugation, homogeneous conjugation of the synthetic BBB shuttle peptide angiopep-2 (Ang2) to a monoclonal antibody (mAb) provides improved binding affinity for brain microvascular endothelial cells in vitro and accumulation into normal brain tissues in vivo. In a mouse model, we also demonstrate that the homogeneous anti-EGFR mAb–Ang2 conjugate administered intravenously efficiently accumulates in intracranial tumors. These findings suggest that homogeneous conjugation of BBB shuttle peptides such as Ang2 is a promising approach to enhancing the therapeutic efficacy of antibody agents for CNS diseases.

Homogeneous conjugation of angiopep-2 to a monoclonal antibody improves binding affinity for brain microvascular endothelial cells and accumulation into brain tissues and tumors across the BBB.     

Hepatic disposition of the acyl glucuronide 1-O-gemfibrozil-beta-D-glucuronide: effects of clofibric acid, acetaminophen, and acetaminophen glucuronide

Glucuronidation of carboxylic acid compounds results in the formation of electrophilic acyl glucuronides. Because of their polarity, carrier-mediated hepatic transport systems play an important role in determining both intra- and extrahepatic exposure to these reactive conjugates. We have previously shown that the hepatic membrane transport of 1-O-gemfibrozil-beta-D-glucuronide (GG) is carrier-mediated and inhibited by the organic anion dibromosulfophthalein. In this study, we examined the influence of 200 microM acetaminophen, acetaminophen glucuronide, and clofibric acid on the disposition of GG (3 microM) in the recirculating isolated perfused rat liver preparation. GG was taken up by the liver, excreted into bile, and hydrolyzed within the liver to gemfibrozil, which appeared in perfusate but not in bile. Mean +/- S. D. hepatic clearance, apparent intrinsic clearance, hepatic extraction ratio, and biliary excretion half-life of GG were 10.4 +/- 1.4 ml/min, 94.1 +/- 17.9 ml/min, 0.346 +/- 0.046, and 30.9 +/- 4.9 min, respectively, and approximately 73% of GG was excreted into bile. At the termination of the experiment (t = 90 min), the ratio of GG concentrations in perfusate, liver, and bile was 1:35:3136. Acetaminophen and acetaminophen glucuronide had no effect on the hepatic disposition of GG, suggesting relatively low affinities of acetaminophen conjugates for hepatic transport systems or the involvement of multiple transport systems for glucuronide conjugates. In contrast, clofibric acid increased the hepatic clearance, extraction ratio, and apparent intrinsic clearance of GG (P <.05) while decreasing its biliary excretion half-life (P <.05), suggesting an interaction between GG and hepatically generated clofibric acid glucuronide at the level of hepatic transport. However, the transporter protein(s) involved remains to be identified.     

Enhanced delivery of doxorubicin into the brain via a peptide-vector-mediated strategy: saturation kinetics and specificity

Doxorubicin delivery to the brain is often restricted because of the poor transport of this therapeutic molecule through the blood-brain barrier (BBB). To overcome this problem, we have recently developed a technology, Pep:trans, based on short natural-derived peptides that are able to cross efficiently the BBB without compromising its integrity. In this study, we have used the in situ mouse brain perfusion method to evaluate the brain uptake of free and vectorized doxorubicin. Doxorubicin was coupled covalently to small peptide vectors: L-SynB1 (18 amino acids), L-SynB3 (10 amino acids), and its enantio form D-SynB3. We first confirmed the very low brain uptake of free radiolabeled doxorubicin, which is most likely due to the efflux activity of the P-glycoprotein at the level of the BBB. Vectorization with either L-SynB1, L-SynB3, or D-SynB3 significantly increased the brain uptake of doxorubicin (about 30-fold). We also investigated the mechanism of transport of vectorized doxorubicin. We show that vectorized doxorubicin uses a saturable transport mechanism to cross the BBB. The effect of poly(L-lysine) and protamine, endocytosis inhibitors, on the transport across the brain was also investigated. Both inhibitors reduced the brain uptake of vectorized doxorubicin in a dose-dependent manner. These studies indicate that the transport of vectorized doxorubicin appears to occur via an adsorptive-mediated endocytosis.     

Clearance of Alzheimer's amyloid-ss(1-40) peptide from brain by LDL receptor-related protein-1 at the blood-brain barrier

Elimination of amyloid-ss peptide (Ass) from the brain is poorly understood. After intracerebral microinjections in young mice, (125)I-Ass(1-40) was rapidly removed from the brain (t(1/2) 相似文献   

The clinical toxicology of gamma-hydroxybutyrate,gamma-butyrolactone and 1,4-butanediol

Introduction. Gamma-hydroxybutyrate (GHB) and its precursors, gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD), are drugs of abuse which act primarily as central nervous system (CNS) depressants. In recent years, the rising recreational use of these drugs has led to an increasing burden upon health care providers. Understanding their toxicity is therefore essential for the successful management of intoxicated patients. We review the epidemiology, mechanisms of toxicity, toxicokinetics, clinical features, diagnosis, and management of poisoning due to GHB and its analogs and discuss the features and management of GHB withdrawal. Methods. OVID MEDLINE and ISI Web of Science databases were searched using the terms “GHB,” “gamma-hydroxybutyrate,” “gamma-hydroxybutyric acid,” “4-hydroxybutanoic acid,” “sodium oxybate,” “gamma-butyrolactone,” “GBL,” “1,4-butanediol,” and “1,4-BD” alone and in combination with the keywords “pharmacokinetics,” “kinetics,” “poisoning,” “poison,” “toxicity,” “ingestion,” “adverse effects,” “overdose,” and “intoxication.” In addition, bibliographies of identified articles were screened for additional relevant studies including nonindexed reports. Non-peer-reviewed sources were also included: books, relevant newspaper reports, and applicable Internet resources. These searches produced 2059 nonduplicate citations of which 219 were considered relevant. Epidemiology. There is limited information regarding statistical trends on world-wide use of GHB and its analogs. European data suggests that the use of GHB is generally low; however, there is some evidence of higher use among some sub-populations, settings, and geographical areas. In the United States of America, poison control center data have shown that enquiries regarding GHB have decreased between 2002 and 2010 suggesting a decline in use over this timeframe. Mechanisms of action. GHB is an endogenous neurotransmitter synthesized from glutamate with a high affinity for GHB-receptors, present on both on pre- and postsynaptic neurons, thereby inhibiting GABA release. In overdose, GHB acts both directly as a partial GABA_b receptor agonist and indirectly through its metabolism to form GABA. Toxicokinetics. GHB is rapidly absorbed by the oral route with peak blood concentrations typically occurring within 1 hour. It has a relatively small volume of distribution and is rapidly distributed across the blood–brain barrier. GHB is metabolized primarily in the liver and is eliminated rapidly with a reported 20–60 minute half-life. The majority of a dose is eliminated completely within 4–8 hours. The related chemicals, 1,4-butanediol and gamma butyrolactone, are metabolized endogenously to GHB. Clinical features of poisoning. GHB produces CNS and respiratory depression of relatively short duration. Other commonly reported features include gastrointestinal upset, bradycardia, myoclonus, and hypothermia. Fatalities have been reported. Management of poisoning. Supportive care is the mainstay of management with primary emphasis on respiratory and cardiovascular support. Airway protection, intubation, and/or assisted ventilation may be indicated for severe respiratory depression. Gastrointestinal decontamination is unlikely to be beneficial. Pharmacological intervention is rarely required for bradycardia; however, atropine administration may occasionally be warranted. Withdrawal syndrome. Abstinence after chronic use may result in a withdrawal syndrome, which may persist for days in severe cases. Features include auditory and visual hallucinations, tremors, tachycardia, hypertension, sweating, anxiety, agitation, paranoia, insomnia, disorientation, confusion, and aggression/combativeness. Benzodiazepine administration appears to be the treatment of choice, with barbiturates, baclofen, or propofol as second line management options. Conclusions. GHB poisoning can cause potentially life-threatening CNS and respiratory depression, requiring appropriate, symptom-directed supportive care to ensure complete recovery. Withdrawal from GHB may continue for up to 21 days and can be life-threatening, though treatment with benzodiazepines is usually effective.     

MR-Guided Delivery of Hydrophilic Molecular Imaging Agents Across the Blood-Brain Barrier Through Focused Ultrasound

Purpose

A wide variety of hydrophilic imaging and therapeutic agents are unable to gain access to the central nervous system (CNS) due to the blood-brain barrier (BBB). In particular, unless a particular transporter exists that may transport the agent across the BBB, most agents that are larger than 500 Da or that are hydrophilic will be excluded by the BBB. Glutamate carboxypeptidase II (GCPII), also known as the prostate-specific membrane antigen (PSMA) in the periphery, has been implicated in various neuropsychiatric conditions. As all agents that target GCPII are hydrophilic and thereby excluded from the CNS, we used GCPII as a platform for demonstrating our MR-guided focused ultrasound (MRgFUS) technique for delivery of GCPII/PSMA-specific imaging agents to the brain.

Procedures

Female rats underwent MRgFUS-mediated opening of the BBB. After opening of the BBB, either a radio- or fluorescently labeled ureido-based ligand for GCPII/PSMA was administered intravenously. Brain uptake was assessed for 2-(3-{1-carboxy-5-[(6-[¹⁸F]fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid ([¹⁸F]DCFPyL) and YC-27, two compounds known to bind GCPII/PSMA with high affinity, using positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging, respectively. Specificity of ligand binding to GCPII/PSMA in the brain was determined with co-administration of a molar excess of ZJ-43, a compound of the same chemical class but different structure from either [¹⁸F]DCFPyL or YC-27, which competes for GCPII/PSMA binding.

Results

Dynamic PET imaging using [¹⁸F]DCFPyL demonstrated that target uptake reached a plateau by ～1 h after radiotracer administration, with target/background ratios continuing to increase throughout the course of imaging, from a ratio of ～4:1 at 45 min to ～7:1 by 80 min. NIRF imaging likewise demonstrated delivery of YC-27 to the brain, with clear visualization of tracer in the brain at 24 h. Tissue uptake of both ligands was greatly diminished by ZJ-43 co-administration, establishing specificity of binding of each to GCPII/PSMA. On gross and histological examination, animals showed no evidence for hemorrhage or other deleterious consequences of MRgFUS.

Conclusions

MRgFUS provided safe opening of the BBB to enable specific delivery of two hydrophilic agents to target tissues within the brain. This platform might facilitate imaging and therapy using a variety of agents that have heretofore been excluded from the CNS.