Marie Blanchette

Recent Advances in Blood-Brain Barrier Disruption as a CNS Delivery Strategy

The blood–brain barrier (BBB) is a complex functional barrier composed of endothelial cells, pericytes, astrocytic endfeets and neuronal cells. This highly organized complex express a selective permeability for molecules that bear, amongst other parameters, adequate molecular weight and sufficient liposolubility. Unfortunately, very few therapeutic agents currently available do cross the BBB and enters the CNS. As the BBB limitation is more and more acknowledged, many innovative surgical and pharmacological strategies have been developed to circumvent it. This review focuses particularly on the osmotic opening of the BBB, a well-documented approach intended to breach the BBB. Since its inception by Rapoport in 1972, pre-clinical studies have provided important information on the extent of BBB permeation. Thanks to Neuwelt and colleagues, the osmotic opening of the BBB made its way to the clinic. However, many questions remain as to the detailed physiology of the procedure, and its best application to the clinic. Using different tools, amongst which MRI as a real-time in vivo characterization of the BBB permeability and CNS delivery, we attempt to better define the osmotic BBB permeabilization physiology. These ongoing studies are described, and data related to spatial and temporal distribution of a molecule after osmotic BBB breaching, as well as the window of BBB permeabilization, are discussed. We also summarize recent clinical series highlighting promising results in the application of this procedure to maximize delivery of chemotherapy in the treatment of brain tumor patients.

Blood-brain barrier disruption in the treatment of brain tumors.

Standard chemotherapy administered systemically has a limited efficacy in the treatment of brain tumors. One of the major obstacles in the treatment of brain neoplasias is the impediment to delivery across the intact blood-brain barrier (BBB). Many innovative approaches have been developed to circumvent this obstacle. One such strategy is BBB disruption (BBBD), which successfully increases the delivery of antineoplastic agents to the central nervous system (CNS). This chapter describes the application of the BBBD technique in rats. Different methods to evaluate and measure BBB permeability following hyperosmolar mannitol infusion including Evans blue staining, albumin immunohistochemistry, and dynamic magnetic resonance imaging are also described.

Conversion of arterial input functions for dual pharmacokinetic modeling using Gd-DTPA/MRI and 18F-FDG/PET.

Reaching the full potential of magnetic resonance imaging (MRI)‐positron emission tomography (PET) dual modality systems requires new methodologies in quantitative image analyses. In this study, methods are proposed to convert an arterial input function (AIF) derived from gadolinium‐diethylenetriaminepentaacetic acid (Gd‐DTPA) in MRI, into a 18F‐fluorodeoxyglucose (18F‐FDG) AIF in PET, and vice versa. The AIFs from both modalities were obtained from manual blood sampling in a F98‐Fisher glioblastoma rat model. They were well fitted by a convolution of a rectangular function with a biexponential clearance function. The parameters of the biexponential AIF model were found statistically different between MRI and PET. Pharmacokinetic MRI parameters such as the volume transfer constant (Ktrans), the extravascular–extracellular volume fraction (νe), and the blood volume fraction (νp) calculated with the Gd‐DTPA AIF and the Gd‐DTPA AIF converted from 18F‐FDG AIF normalized with or without blood sample were not statistically different. Similarly, the tumor metabolic rates of glucose (TMRGlc) calculated with 18F‐FDG AIF and with 18F‐FDG AIF obtained from Gd‐DTPA AIF were also found not statistically different. In conclusion, only one accurate AIF would be needed for dual MRI‐PET pharmacokinetic modeling in small animal models. Magn Reson Med, 2013.

Real-time monitoring of gadolinium diethylenetriamine penta-acetic acid during osmotic blood-brain barrier disruption using magnetic resonance imaging in normal wistar rats.

OBJECTIVETreatment of malignant gliomas is hampered by several factors, one of which is the blood-brain barrier (BBB). Thus, innovative strategies to cross the BBB have been developed, such as the BBB disruption procedure. Although it has been studied extensively, details regarding the physiology of the procedure remain obscure. This study was undertaken to clarify these issues. METHODSForty Wistar rats were imaged with a 7T animal magnetic resonance imaging scanner in dynamic acquisitions during BBB disruption. Gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA) was injected to visualize and characterize the permeability of the BBB at different time points after disruption. The concentration of Gd-DTPA in the brain parenchyma was determined as a function of time after injection. RESULTSA typical pattern of signal change as a function of time was observed in the treated hemisphere of all animals. Initially, a slight signal decrease was observed in T1-weighted images followed by a strong increase corresponding to the injection of Gd-DTPA. Two different mechanisms seemed responsible for the distribution of Gd-DTPA within the parenchyma: 1) a direct diffuse increase in capillary permeability, and 2) a diffusion process in the interstitial compartment. Initial results showed that the barrier opens immediately after the procedure and for at least 30 minutes. CONCLUSIONThe methodology described in this article allows monitoring of the dynamics of the BBB disruption process and characterization of its physiology in vivo, and represents a marked advantage over postmortem static studies.

Impact of drug size on brain tumor and brain parenchyma delivery after a blood-brain barrier disruption.

Drug delivery to the brain is influenced by the blood–brain barrier (BBB) and blood–tumor barrier (BTB) to an extent that is still debated in neuro-oncology. In this paper, we studied the delivery across the BTB and the BBB of compounds with different molecular sizes in normal and glioma-bearing rats. Studies were performed at baseline as well as after an osmotic BBB disruption (BBBD) using dynamic contrast-enhanced magnetic resonance imaging and two T1, contrast agents (CAs), Magnevist (743 Da) and Gadomer (17,000 Da). More specifically, we determined the time window for the BBB permeability, the distribution and we calculated the brain exposure to the CAs. A different pattern of accumulation and distribution at baseline as well as after a BBBD procedure was observed for both agents, which is consistent with their different molecular size and weight. Baseline tumor exposure was threefold higher for Magnevist compared with Gadomer, whereas postBBBD tumor exposure was twofold higher for Magnevist. Our study clearly showed that the time window and the extent of delivery across the intact, as well as permeabilized BTB and BBB are influenced by drug size.

A new method of quantitatively assessing the opening of the blood-brain barrier in murine animal models.

The blood-brain barrier (BBB) restricts the delivery of drugs into the brain. Different strategies have been developed to circumvent this obstacle. One such approach, the osmotic BBB disruption (BBBD), has been under pre-clinical study since the 70s. Typically, qualitative ex vivo assessment of the extent of BBBD has been performed using Evans blue staining technique. In this study, we describe a simple quantitative technique based on albumin indirect immunohistochemistry to measure the extent of BBB breach. Thirty Fischer rats were assigned to one of 6 groups: a control group, and BBBD groups with escalation in IA mannitol infusion rate: 0.06, 0.08, 0.10, 0.12 and 0.15 cc/s. Fifteen minutes after the BBBD procedure, the animals were sacrificed, brain harvested and sections stained for albumin. Using an image analysis software, isolated albumin staining pixels were expressed as a fraction of the treated hemisphere. This ratio was used as a percentage value in the intensity of the BBB permeabilization. All sections studied harbored staining, averaging 0.37% for the controls (group 1), 5.69% for group 2 (0.06 cc/s), 10.44% for group 3 (0.08 cc/s), 6.99% for group 4 (0.1 cc/s), 18.50% for group 5 (0.12 cc/s) and reaching 61.70% for group 6 (0.15 cc/s). Important variations were observed between animals. A threshold effect was observed, and animals in group 6 presented a significant increase in BBB permeabilization compared to the other groups. We hereby detail a simple technique that can be applied to quantitatively measure the extent of the BBB breach notwithstanding the pathological process.

Optimization of the reference region method for dual pharmacokinetic modeling using Gd-DTPA/MRI and 18F-FDG/PET

The combination of MRI and positron emission tomography (PET) offers new possibilities for the development of novel methodologies. In pharmacokinetic image analysis, the blood concentration of the imaging compound as a function of time, [i.e., the arterial input function (AIF)] is required for MRI and PET. In this study, we tested whether an AIF extracted from a reference region (RR) in MRI can be used as a surrogate for the manually sampled 18F‐FDG AIF for pharmacokinetic modeling.