Is there a therapeutic role for osmotic breaching of the blood-brain barrier?

Current Therapeutic Options in Severe Guillain-Barre?? Syndrome

The classical form of Guillain-Barré syndrome (GBS) refers to an acute monophasic demyelinating motor and sensory polyneuropathy characterized by symmetric ascending flaccid weakness, along with sensory impairment and, less commonly, autonomic perturbations. Pure motor axonal forms, axonal motor, and sensory forms, as well as pure autonomic forms, have also been identified. A complex immune-mediated process leads to segmental demyelination accompanied with axonal involvement in protracted cases. Establishing strategies of immunomodulation may therefore halt and even reverse the harmful autoimmune insult to peripheral nerves. The present article reviews the current immunomodulatory options in severe GBS. A recent Cochrane meta-analysis of 6 randomized studies showed no significant improvement using corticosteroids, including either oral or intravenous methylprednisolone. Combined methylprednisolone and immunoglobulins shortened the time lapse to regain independent walking. Plasmapheresis (PE) was the first effectively proven method of immunomodulation, followed by intravenous immunoglobulins (IVIG). Both methods are comparable in their beneficial effect and were used either separately or in combination, but PE was more frequently associated with severe adverse effects requiring cessation of therapy, including a bleeding diathesis. In addition, PE is feasible only in major referral centers requiring the appropriate equipment and trained personnel. In addition, younger children may be at risk for bleeding after insertion of wide catheters. Therefore, in cases of severe GBS, IVIG is recommended as the first-line drug using a total empiric dose of 2 g/kg administered over 2 consecutive days, especially in children proven highly effective with negligible adverse effects. In protracted cases, the addition of intravenous corticosteroids to IVIG should be considered, which may shorten the duration to regain independent walking. If such therapy fails, PE should be applied using centrifugal blood separators with 5% albumin as the substitute solution.

Hypertonic saline for cerebral edema

Raised intracranial pressure (ICP) is a major contributor to the mortality of many conditions encountered in a neurologic intensive care unit. Achieving a sustained reduction in ICP in patients with intracranial hypertension remains a challenge. Treatment with hyperosmolar agents is one of the few options that are available, and mannitol is currently the most commonly used agent. However, hypertonic saline solutions have recently emerged as a potentially safer and more efficacious alternative to mannitol.

Quantification and pharmacokinetics of blood-brain barrier disruption in humans

Hyperosmolar blood-brain barrier disruption (HBBBD), produced by infusion of mannitol into the cerebral arteries, has been used in the treatment of brain tumors to increase drug delivery to tumor and adjacent brain. However, the efficacy of HBBBD in brain tumor therapy has been controversial. The goal of this study was to measure changes in vascular permeability after HBBBD in patients with malignant brain tumors. The permeability (K1) of tumor and normal brain blood vessels was measured using rubidium-82 and positron emission tomography before and repeatedly at 8- to 15-minute intervals after HBBBD. Eighteen studies were performed in 13 patients, eight with glioblastoma multiforme and five with anaplastic astrocytoma. The HBBBD increased K1 in all patients. Baseline K1 values were 2.1 +/- 1.4 and 34.1 +/- 22.1 microl/minute/ml (+/- standard deviation) for brain and tumor, respectively. The peak absolute increases in K1 following HBBBD were 20.8 +/- 11.7 and 19.7 +/- 10.7 microl/minute/ml for brain and tumor, corresponding to percentage increases of approximately 1000% in brain and approximately 60% in tumor. The halftimes for return of K1 to near baseline for brain and tumor were 8.1 +/- 3.8 and 4.2 +/- 1.2 minutes, respectively. Simulations of the effects of HBBBD made using a very simple model with intraarterial methotrexate, which is exemplary of drugs with low permeability, indicate that 1) total exposure of the brain and tumor to methotrexate, as measured by the methotrexate concentration-time integral (or area under the curve), would increase with decreasing infusion duration and would be enhanced by 130% to 200% and by 7% to 16%, respectively, compared to intraarterial infusion of methotrexate alone; and 2) exposure time at concentrations above 1 microM, the minimal concentration required for the effects of methotrexate, would not be enhanced in tumor and would be enhanced by only 10% in brain. Hyperosmolar blood-brain barrier disruption transiently increases delivery of water-soluble compounds to normal brain and brain tumors. Most of the enhancement of exposure results from trapping the drug within the blood-brain barrier, an effect of the very transient alteration of the blood-brain barrier by HBBBD. Delivery is most effective when a drug is administered within 5 to 10 minutes after disruption. However, the increased exposure and exposure time that occur with methotrexate, the permeability of which is among the lowest of the agents currently used clinically, are limited and the disproportionate increase in brain exposure, compared to tumor exposure, may alter the therapeutic index of many drugs.

Hyperosmolar blood-brain barrier disruption in baboons: an in vivo study using positron emission tomography and rubidium-82

Hyperosmolar blood-brain barrier (BBB) disruption remains controversial as an adjuvant therapy to increase delivery of water-soluble compounds to extracellular space in the brain in patients with malignant brain tumors. To understand the physiological effects of BBB disruption more clearly, the authors used positron emission tomography (PET) to study the time course of BBB permeability in response to the potassium analog rubidium-82 (82Rb, halflife 75 seconds) following BBB disruption in anesthetized adult baboons. Mannitol (25%) was injected into the carotid artery and PET scans were performed before and serially at 8-to 15-minute intervals after BBB disruption. The mean influx constant (K1), a measure of permeability-surface area product, in ipsilateral, mannitol-perfused mixed gray- and white-matter brain regions was 4.9 +/-2.4 microliter/min/ml (+/- standard deviation) at baseline and increased more than 100% (delta K1=9.4 +/-5.1 microliter/min/ml, 18 baboons) in brain perfused by mannitol. The effect of BBB disruption on K1 correlated directly with the total amount of mannitol administered (p< 0.005). Vascular permeability returned to baseline with a halftime of 24.0 +/- 14.3 minutes. The mean brain plasma volume rose by 0.57 +/- 0.34 ml/100 ml in ipsilateral perfused brain following BBB disruption. This work provides a basis for the in vivo study of permeability changes induced by BBB disruption in human brain and brain tumors.

Experimental models of altering the blood-brain barrier

The passage of a substance from blood to brain can be enhanced by altering the permeability of the substance itself or by altering the BBB characteristics. Example of the former is to increase lipid solubility, glycosylation and cationization, liposome entrapment and coupling to carriers. Unselective opening of the BBB can be obtained by a local or systemic increase of the intravascular pressure, by intracarotid injection of hyperosmolar solutions or by substances that alter the endothelial surface charge such as protamine sulphate. Substances that alter the fluidity of membranes can reduce or enhance the permeability of the BBB. In experimental models involving temporary opening of the BBB possible long-term hazardous effects must be considered.

Oleic acid reversibly opens the blood-brain barrier

This study examined the effect of intracarotid oleic acid infusion on blood-brain barrier permeability. Oleic acid was infused for 30 s at a rate of 6 ml/min into the right internal carotid artery at concentrations of 10(-6), 10(-5), 2 x 10(-5) and 5 10(-5) M. Extensive Evans blue-albumin extravasation was observed 15 min after the administration of 2 x 10(-5) M oleic acid. The permeability surface area product for alpha-aminoisobutyric acid (AIB), determined 1-11 min following the infusion of oleic acid was increased 10-fold following infusion of 10(-5) M oleic acid and 20-fold following the administration of 5 x 10(-5) M oleate. The blood-brain barrier opening to AIB proved to be reversible 80-90 min after the infusion of 2 x 10(-5) M oleic acid. The possible mechanisms of the oleic acid effect are discussed.

Model for drug uptake by brain tumors: effects of osmotic treatment and of diffusion in brain

A mathematical model describing drug uptake into brain tumors, directly from blood and indirectly from neighboring tissue, is presented. The model quantitatively describes uptake into tumor, brain surrounding tumor (BST), and normal brain and uptake following reversible osmotic blood-brain barrier (BBB) and blood-tumor barrier disruption. It employs published data on the time course for reclosure of the BBB following osmotic treatment and on the brain and tumor uptake of [14C]alpha-aminoisobutyric acid by Walker 256 carcinomas and C6 gliomas implanted into the rat brain. Constant infusion and bolus injection infusion schedules are considered. In untreated brain, the BST acts as a sink, reducing the integrated exposure of the adjacent tumor to the drug, whereas following osmotic treatment, tumor exposure to drug is enhanced, not only by increased delivery from blood but also by diffusion (and bulk flow) from neighboring brain. The model provides a quantitative framework for examining the efficacy of osmotic treatment to enhance chemotherapy of brain tumors.

Effect of hyperosmotic blood-brain barrier disruption on transcapillary transport in canine brain tumors

Whether hyperosmotic blood-brain barrier (BBB) disruption is a technique that can be used to increase permeability of brain-tumor capillaries and thereby transiently increase drug delivery to the brain tumor is controversial. Nine virally induced brain tumors were studied in seven dogs, before and after hyperosmotic BBB disruption with 1.4 osmolar mannitol. Each dog was studied with computerized tomography (CT) after administration of the water-soluble tracer meglumine iothalamate. Each study lasted 30 minutes. A baseline CT scan and 35 to 40 additional CT scans were obtained to provide a time-related measurement of the amount of meglumine iothalamate in tissue (Am(t], and 30 plasma samples were collected to provide the time-related measurement of meglumine iothalamate in plasma (Cp(t]. The data were analyzed by three different methods: 1) a two-compartment model and nonlinear curve fitting were used to calculate K1 (blood-to-tissue or influx constant), k2 (tissue-to-blood or efflux constant), and Vp (plasma vascular space); 2) K1 values were calculated with a two-compartment model, assuming no efflux, at the time point for each CT scan; and 3) a "tissue advantage ratio" was calculated that expressed the ratio of tissue uptake of meglumine iothalamate at each time point, comparing values before and after BBB disruption. Regardless of which method of data analysis was used, there was a marked and significant increase in transcapillary transport of meglumine iothalamate to tumor-free brain regions, while there was only a small, transient, and insignificant increase to the brain tumors. Although there were often marked increases in delivery to cortex in the same hemisphere as the tumors, there was no significant increase to brain immediately surrounding the tumors, perhaps due to altered circulatory dynamics in this region. These data raise serious questions as to the wisdom of using this technique to increase drug delivery to brain tumors in patients and strongly support the continued study of this technique in experimental brain tumors before it is used in patients.

The physiology of the blood-brain barrier

The BBB is a dynamic interface between blood and the central nervous system enabling the brain to keep an optimal internal environment. The endothelial cells of the brain capillaries are unique epithelial-like cells that are fused together by tight junctions and have a low pinocytotic activity. The entry of a specific substance will, therefore, mainly depend on its lipid solubility, and whether or not it has access to any of the carriers in the endothelial cells. Enzymatic degradation in the endothelium can prevent entry into the brain of substances that do enter the endothelial cells. Astrocytes may have an important role by inducing and upholding some barrier functions. An intact BBB is evidently important for optimal brain function. Manipulation of the BBB to allow entry of therapeutic agents may be justified under certain circumstances but should be done with caution until we know more about the long-term consequences of such manipulation.

A mouse model for the study of blood-brain barrier permeability

This article describes a C57BL/6 mouse model for the investigation of blood-brain barrier (BBB) alteration. Osmotic modification of BBB was achieved by infusion of 1.6 M arabinose solution into the internal carotid artery with or without occlusion of the external carotid artery. BBB alteration was measured by infusing 2% Evans blue dye. Only 1.6 M arabinose-treated animals but not 0.9% NaCl controls displayed prominent ipsilateral staining of frontal and temporal lobes. Light blue staining occurred in animals sacrificed within 10 min after injection. Prominent staining occurred in animals sacrificed 1-6 hours later. Identically treated animals were maintained for up to 6 months without signs of systemic or neurological dysfunction. This model may permit study of the effects of biological response modifiers (BRMs) upon the central nervous system (CNS) in healthy and diseased mice.

Sequelae of the osmotic blood-brain barrier opening in rats

Histopathological sequelae of the osmotic blood-brain barrier opening were studied in 69 adult Wistar rats sacrificed between 2 minutes and 6 days after infusion of 1.6 M mannitol into the unilateral internal carotid artery. The results were correlated with immunohistochemical localization of autologous albumin in the brain parenchyma on paraffin sections. Extravasation of serum albumin was evident in all rats, and the albumin immunoreactivity, commonly localized to the territories of the ipsilateral anterior, middle, and posterior cerebral arteries and contralateral anterior cerebral artery, showed maximum intensity in the rats sacrificed 30 minutes after infusion. The albumin immunoreactivity remained macroscopically visible in the brain parenchyma for 24 to 48 hours, and then gradually faded out. Serum extravasation was accompanied by widening of the perivascular space and focal edema, which largely subsided within 48 hours as the albumin immunoreactivity of the tissue diminished. Although no overt neurological sequelae were seen in the present experiment, minute but definite foci of infarction with focal accumulation of albumin were found in 23 (38%) of 61 rats surviving more than 30 minutes. In addition, ischemic neuronal death of delayed onset was encountered among neurons in the CA-1 region of the hippocampus, in the cerebellum, and in the thalamus in five (25%) of 20 rats sacrificed between Days 4 and 6. Thus, care should be exercised in the practice of this procedure.