Measurement of blood-brain barrier permeability

Brain endothelial permeability, transport, and flow assessed over 10 orders of magnitude using the in situ brain perfusion technique

BACKGROUND

Cerebral blood flow normally places a limit on the magnitude of brain vascular permeability (P) that can be measured in vivo. At normal cerebral blood flow, this limit falls at the lower end of lipophilicity for most FDA-approved CNS drugs. In this study, we report on two methods that can be used to overcome this limitation and measure brain vascular permeability values that are up to ~1000 times higher using the in situ brain perfusion technique.

METHODS

Rat brain was perfused with physiological saline at increased flow rate and in the presence of various concentrations of plasma protein, serum albumin or alpha-acid glycoprotein. Plasma protein was added to the saline perfusion fluid to lower extraction into the measurable range using the Crone Renkin "diffusion-flow" equation to calculate brain PoS.

RESULTS

Cerebrovascular Po was determined for 125 solutes, of which 78 showed little or no evidence of active efflux transport. Fifty of the solutes were in the lipophilicity zone (Log Poct 1-5) of most FDA-approved CNS drugs. Care was taken to ensure the integrity of the brain vasculature during perfusion and to measure flow accurately using markers that had been verified for the flow rates. The results showed a linear relationship between Log Po and Log Poct over ~10 orders of magnitude with values for diazepam, estradiol, testosterone, and other agents that exceed prior published values by fivefold to 200-fold.

CONCLUSIONS

The results show that brain vascular permeability can be measured directly in vivo for highly lipophilic solutes and the PS values obtained match reasonably with that predicted by the Crone-Renkin flow diffusion equation with care taken to validate the accuracy for the component measurements and with no need to invoke "enhanced" or "induced" dissociation.

A Review of Mathematics Determining Solute Uptake at the Blood-Brain Barrier in Normal and Pathological Conditions

The blood-brain barrier (BBB) limits movement of solutes from the lumen of the brain microvascular capillary system into the parenchyma. The unidirectional transfer constant, K_in, is the rate at which transport across the BBB occurs for individual molecules. Single and multiple uptake experiments are available for the determination of K_in for new drug candidates using both intravenous and in situ protocols. Additionally, the single uptake method can be used to determine K_in in heterogeneous pathophysiological conditions such as stroke, brain cancers, and Alzheimer's disease. In this review, we briefly cover the anatomy and physiology of the BBB, discuss the impact of efflux transporters on solute uptake, and provide an overview of the single-timepoint method for determination of K_in values. Lastly, we compare preclinical K_in experimental results with human parallels.

Regulation of permeability across the blood-brain barrier

The blood-brain barrier refers to the very low permeability across microvessels in the Central Nervous System (CNS), created by the interaction between vascular endothelial cells and surrounding cells of the neurovascular unit. Permeability can be modulated (increased and decreased) by a variety of factors including inflammatory mediators, inflammatory cells such as neutrophils and through alterations in the phenotype of blood vessels during angiogenesis and apoptosis. In this chapter, some of these factors are discussed as well as the challenge of treating harmful increases in permeability that result in brain swelling (vasogenic cerebral edema).

Selectivity requirements for diagnostic imaging of neurofibrillary lesions in Alzheimer's disease: a simulation study

Whole-brain imaging is a promising strategy for premortem detection of tau-bearing neurofibrillary lesions that accumulate in Alzheimer's disease. However, the approach is complicated by the high concentrations of potentially confounding binding sites presented by beta-amyloid plaques. To predict the contributions of relative binding affinity and binding site density to the imaging-dynamics and selectivity of a hypothetical tau-directed radiotracer, a nonlinear, four-tissue compartment pharmacokinetic model of diffusion-mediated radiotracer uptake and distribution was developed. Initial estimates of nonspecific binding and brain uptake parameters were made by fitting data from a previously published kinetic study of Pittsburgh Compound B, an established amyloid-directed radiotracer. The resulting estimates were then used to guide simulations of tau binding selectivity while assuming early-stage accumulation of disease pathology. The simulations suggest that for tau aggregates to represent at least 80% of specific binding signal, binding affinity or density selectivities for tau over beta-amyloid should be at least 20- or 50-fold, respectively. The simulations also suggest, however, that overcoming nonspecific binding will be an additional challenge for tau-directed radiotracers owing to low concentrations of available binding sites. Overall, nonlinear modeling can provide insight into the performance characteristics needed for tau-directed radiotracers in vivo.

Cilostazol, a phosphodiesterase 3 inhibitor, protects mice against acute and late ischemic brain injuries

Cilostazol, a selective inhibitor of phosphodiesterase 3, exerts neuroprotective effects on acute brain injury after cerebral ischemia in rats. However, it is unknown whether cilostazol affects the subacute or chronic ischemic injury. In the present study, we evaluated the dose- and time-dependent effects of cilostazol on acute ischemic brain injury and the long-lasting effect on the late (subacute/chronic) injury in mice with focal cerebral ischemia induced by transient middle cerebral artery occlusion. We found that pre-treatment of cilostazol (injected i.p. at 30 min before ischemia) significantly ameliorated the acute injury 24 h after ischemia, and the effective doses were 3-10 mg/kg. The post-treatment of cilostazol (10 mg/kg) was effective on the acute injury when it was injected 1 and 2 h after ischemia. In addition, for the late injury, post-treatment of cilostazol (10 mg/kg, i.p., for 7 consecutive days after ischemia) attenuated neurological dysfunctions, brain atrophy and infarct volume. It also inhibited astrocyte proliferation/glial scar formation and accelerated the angiogenesis in the ischemic boundary zone 7 and 28 days after ischemia. Thus, we conclude that cilostazol protects against not only the acute injury, but also the late injury in mice with focal cerebral ischemia; especially it can modify brain remodeling, astrogliosis and angiogenesis.

Role of site-specific binding to plasma albumin in drug availability to brain

Many studies have reported greater drug uptake into brain than that predicted based upon existing models using the free fraction (f(u)) of drug in arterial serum. To explain this difference, circulating plasma proteins have been suggested to interact with capillary membrane in vivo to produce a conformational change that favors net drug dissociation and elevation of f(u). Albumin, the principal binding protein in plasma, has two main drug binding sites, Sudlow I and II. We tested this hypothesis using drugs that bind selectively to either site I (warfarin) or site II (ibuprofen), as well as mixed ligands that have affinity for both sites (tolbutamide and valproate). Brain uptake was determined in the presence and absence of albumin using the in situ rat brain perfusion technique. Unidirectional brain uptake transfer constants (K(in)) were measured and compared with those predicted using the modified Kety-Crone-Renkin model: K(in) = F(1-e(-f(u) x PS(u)/F)), where F is perfusion flow and PS(u) is the permeability-surface area product to free drug of brain capillaries. The results demonstrated good agreement between measured and predicted K(in) over a 100-fold range in perfusion fluid albumin concentration using albumin from three different species (i.e., human, bovine, and rat), as well as whole-rat serum. K(in) decreased in the presence of albumin in direct proportion to perfusion fluid f(u) with constant PS(u). The results show that brain uptake of selected Sudlow site I and II ligands matches that predicted by the modified Kety-Crone-Renkin model with no evidence for enhanced dissociation.

Blood-brain barrier integrity is unaltered in human brain cortex with diabetes mellitus

Diabetes-related cognitive dysfunction has been recognized for many years in humans, but the pathogenesis of this condition is poorly understood. Evidence from animal studies suggests that altered function of the blood-brain barrier (BBB) could be a potential cause contributing to this disease. This study aimed to investigate whether the permeability of the BBB is affected in the brains of persons with diabetes mellitus (DM). On postmortem prefrontal and temporal cortex of diabetic patients and controls, immunohistochemical stainings were carried out using specific antibodies against three proteins (PAL-E, IgG and albumin), which are considered as markers for the vascular permeability status of the BBB. Rare or no PAL-E staining was found in the capillaries of the prefrontal and temporal cortex parenchyma, in both DM and control materials. IgG and albumin were localized in and directly around blood vessel walls in the prefrontal and temporal cortex. No obvious differences in the staining pattern of IgG and albumin were observed between brain samples of persons with DM and controls. This study suggests that the BBB in diabetic patients is well maintained.

Changes in cerebral blood flow and distribution associated with acute increases in plasma sodium and osmolality of chronic hyponatremic rats

The cause of the osmotic demyelination syndrome that follows too rapid correction of chronic hyponatremia (CHN) is unknown. Recently, we reported in CHN rats an association between blood-brain barrier (BBB) disruption occurring as early as 3 h into correction and subsequent demyelination. Given the changes in brain water and blood volume which occur during correction of CHN, we hypothesized that the same correction protocol that causes demyelination might alter cerebral blood flow (CBF) during correction, thereby possibly contributing to BBB disruption and demyelination. Ten CHN rats were given hypertonic sodium intraperitoneally and its effect on CBF was continuously monitored for 3 h by magnetic resonance flow imaging. Over the subsequent 3 h, plasma sodium rose from 110.8 to 127.6 mEq/liter (P < 0.001) but neither mean arterial blood pressure nor arterial CO(2) tension changed significantly. By 30 min, CBF increased by 50% in cortical and subcortical areas (P < 0.001) and remained elevated for the next 60 min. After 2 h, cortical flow was no longer elevated significantly and by 3 h it had returned to control values. Subcortical flow, however, significantly exceeded control values throughout the 3 h so that after 2 h the ratio of cortical to subcortical blood flow had fallen from 1.17 to 0.91 (P < 0.05). Although the mechanism by which increased plasma sodium and osmolality alters CBF is uncertain, the results suggest that changes in CBF may be part of a cascade of cerebrovascular disturbances including endothelial or parenchymal damage, mechanical events, metabolic disturbances, or cytokine release which eventually lead to BBB disruption and subsequent demyelination.

Animal models for studying transport across the blood-brain barrier

There are many reasons for wishing to determine the rate of uptake of a drug from blood into brain parenchyma. However, when faced with doing so for the first time, choosing a method can be a formidable task. There are at least 7 methods from which to choose: indicator dilution, brain uptake index, microdialysis, external registration, PET scanning, in situ perfusion, and compartmental modeling. Each method has advantages and disadvantages. Some methods require very little equipment while others require equipment that can cost millions of dollars. Some methods require very little technical experience whereas others require complex surgical manipulation. The mathematics alone for the various methods range from simple algebra to complex integral calculus and differential equations. Like most things in science, as the complexity of the technique increases, so does the quantity of information it provides. This review is meant to serve as a starting point for the researcher who wishes to study transport and uptake across the blood-brain barrier in animal models. An overview of the mathematical theory, as well as an introduction to the techniques, is presented.