Modulation of Brain Tumor Capillaries for Enhanced Drug Delivery Selectively to Brain Tumor

Background: The blood-brain tumor barrier (BTB) significantly impedes delivery of most hydrophilic molecules to brain tumors. Several promising strategies, however, have been developed to overcome this problem.

Methods: We discuss several drug delivery methods to brain tumor, including intracerebroventricular, convectionenhanced delivery, BBB/BTB disruption, and BTB permeability modulation, which was developed in our laboratory.

Results: Using immunolocalization, immunoblotting, and potentiometric studies, we found that brain tumor capillary endothelial cells overexpress certain unique protein markers that are absent or barely detectable in normal capillary endothelial cells. We biochemically modulated these markers to sustain and enhance drug delivery, including molecules of varying sizes, selectively to tumors in rat syngeneic and xenograft brain tumor models. We also demonstrated that the cellular mechanism for vasomodulator-mediated BTB permeability increase is due to accelerated formation of pinocytotic vesicles that transport therapeutic molecules across the BTB

Conclusions: Other methods to deliver drugs across the BTB are effective but have severe drawbacks. Our strategy targets BTB-specific proteins to increase antineoplastic drug delivery selectively to brain tumors with few or no side effects, thus increasing the possibility of improving brain tumor treatment.

Introduction

The cerebral microvessels and capillaries that form the blood-brain barrier (BBB) protect the brain, but they also pose an obstacle to the delivery to the brain of small and large therapeutic molecules. In fact, Pardridge1 reported that the BBB blocks delivery of more than 98% of central nervous system (CNS) drugs. As large water-soluble molecules (eg, therapeutic humanized monoclonal antibodies) are developed for the treatment of neurological diseases, the challenge to deliver them across the BBB has assumed critical importance. This article focuses on the biochemical modulation of the blood-brain tumor barrier (BTB), which surrounds brain tumors and has key characteristics that differentiate it from the BBB, as a strategy to enhance anticancer drug delivery to brain tumor. Our research has also shed light on the properties of the abnormal BBB, which results from damage caused by cerebral ischemia and other neurological disorders, yet is still somewhat different from the BTB

Every year in the United States, approximately 25,000 new primary brain tumor and more than 200,000 secondary (metastatic) brain tumor cases are reported. For the most part, even after surgery to remove a malignant brain tumor, brain cancer recurs, severely shortening life expectancy. Conventional treatment using radiation and intravenous (IV) chemotherapy are often unsuccessful primarily because the anticancer drugs fail to cross the BTB in sufficient quantities.2 Therefore,understanding the BTB and the biochemical regulation of the BBB in its normal and abnormal states will be increasingly important as efforts continue to deliver therapeutic compounds to CNS targets. In particular, successful treatment of brain tumors involves efficient anticancer drug delivery to brain tumors across the BTB. Although the BTB is leaky in the tumor center, the established microvessels feeding the proliferating edge of the tumor and the brain adjacent to the tumor are nearly as impermeable as the BBB.2 Therefore,the BTB still poses a major hurdle to anticancer drug delivery to tumors. Over the past 10 years, several promising strategies have been developed to open the BTB to increase anticancer drug delivery to brain tumors. In this article, we review the advantages and disadvantages of the major drug delivery methods

Invasive Drug Delivery

Invasive drug delivery strategies circumvent the BTB but require either a craniotomy or insertion of catheters into the carotid artery. This strategy includes either intracerebroventricular (ICV) infusion of drugs, which is similar to a slow IV infusion,3 or intracerebral implantation of controlled-release anticancer drugs encased in biodegradable polymers.4 While effective in delivering anticancer drugs to tumor, the drugs in this delivery strategy are not targeted specifically at brain tumor cells, so potentially noxious anticancer agents are also delivered to normal, healthy brain cells that can result in undesirable side effects.

In relation to the obstacle of developing a method to deliver drugs across the BTB,the drugs infused via the ICV strategy may not necessarily cross the BBB or the BTB. Such a strategy may deliver drugs to cerebrospinal fluid (CSF) via a circumventricular brain region (CVR) that surrounds the ventricular system but is not protected by the BBB or the BTB. Furthermore, the BBB and the blood-CSF barrier are anatomically and functionally distinct. Therefore, entry of a drug into CSF via CVR does not necessarily mean that the drug has crossed the BBB but is only a measure of blood-CSF barrier permeability.1 A recent study concluded that temozolomide (Temodar) crossed the intact BBB by showing the presence of temozolomide in CSF after systemic administration.5 However, their observation may not mean that the drug crossed the BTB or BBB because CVR lacks a BBB or BTB, and temozolomide or its metabolite methyl-triazenyl imidazole carboxamide (MTIC) level was not quantified in brain tumor following a systemic administration. In contrast, we found that [14C]-labeled temozolomide hardly crossed the BTB and, in fact, only a small amount was taken up by the tumor in a rat brain glioma model (unpublished data). Therefore, drug entry into CSF is not an index of BTB permeability unless drug levels in the tumor are quantified by quantitative autoradiography6 or by detection and identification of a drug or its metabolites in brain tumor tissue by a quantitative assay such as the high-pressure liquid chromatography-mass spectrum-mass spectrum method.7 For example, a circulating drug such as azidothymidine (AZT) gains access to CSF from the blood following a systemic administration. The AZT molecules are rapidly exported back to blood by the CSF, possibly due to an active efflux mechanism,8 but they do not cross the BBB or BTB and would not reach a brain tumor. This complicates the ability of a drug to effectively reach tumor across the BBB or BTB. The goal of crossing the BTB instead of going around it via the CVR is crucial in developing a delivery method for most brain tumors. The CVR is surrounded by more permeable capillaries than BBB capillaries, which may offer an opportunity to deliver drugs to some brain tumors. However, it is a limited opportunity since the surface area of the CVR is small compared with the surface area of the BBB and BTB. Therefore, while the CVR is an effective portal for anticancer drug delivery to tumors in close proximity to the CVR, it is not an effective route to most brain tumors.

Intracerebral Infusion

Intracerebral implants have a limited, narrow effect that is effective against small brain tumors. However, they are clinically ineffective against larger (greater than 500 µm) and highly diffused tumors such as gliomas because they do not allow anticancer agents to diffuse. This is a particularly critical factor in glioblastoma multiforme because it is essential that anticancer drugs be able to reach small pockets of tumor cells well away from the tumor core. For instance, the diffusion of 1,3-bis (2-cholorethyl) 1-nitrosourea (BCNU), a commonly used anticancer biodegradable implant, is limited to a tumor radius of 500 µm from the implanted site