Adenosine 5 – triphosphate-sensitive Potassium Channel-mediated Blood-Brain Tumor Barrier Permeability Increase in a Rat Brain Tumor Model

INTRODUCTION

Understanding the biochemical regulation of the blood-brain barrier (BBB) and blood-brain tumor barrier (BTB) is critical to developing methods to deliver therapeutic compounds to central nervous system targets. Despite evidence of the antineoplastic effect of drugs such as Trastuzumab, a humanized anti-Her-2 monoclonal antibody (MAb; developed by Genentech, Inc., San Francisco, CA), alone or with chemotherapeutic agents such as docetaxel (1, 2), their clinical use for neuro-oncology remains limited because of the high doses necessary to achieve in vivo therapeutically effective concentrations in brain tumors (2). Brain tumor capillaries constitute the BTB, which has different structural and functional characteristics to that of normal brain capillaries that form the BBB. Among many distinct differences (3), we showed that BTB capillaries are responsive to vasoactive agents (4–14). This knowledge allowed us to develop a biochemical approach to increase BTB permeability and to enhance delivery of hydrophilic therapeutic drugs or small- to large-sized molecules, including contrast-enhancing agents, antitumor compounds, therapeutic proteins, and viral vectors (4–14) in vivo to brain tumors selectively with little or no drug delivery to normal brain. This drug delivery strategy exploits the responsiveness of brain tumor capillaries to intravascular infusion of low doses of vasomodulators, such as bradykinin (BK), causing BTB permeability increase via a mechanism involving calcium-activated potassium (KCa) channels (4, 5), BK type 2 receptors (6), nitric oxide (7), and cyclic GMP (11). Overall, our research has identified the molecular targets that selectively modulate BTB permeability and contributed to a better understanding of the biochemical changes that occur during permeability modulations. More recently, we observed that another major potassium channel subtype, the ATP-sensitive potassium (KATP) channel, is involved in brain tumor microvessel permeability regulation and may serve as another target for anticancer drug delivery. We tested whether minoxidil sulfate (MS)-induced activation of KATP channels increases BTB permeability and enhances carboplatin (CPN) delivery to tumor tissue, thereby increasing survival in rats with implanted brain tumor. We demonstrated that CPN could be delivered selectively to tumor tissue without increasing delivery to normal brain cells

KATP channels are heterodimers of sulfonylurea receptors and inwardly rectifying potassium channel subunits (Kir6.x) with a (SURKir6.x)4 stoichiometry. KATP channels are widely distributed, including in the vasculature of the brain. They couple intracellular metabolic changes to the electrical activity of the plasma membrane regulating cerebral vascular tone and mediate the relaxation of cerebral vessels to diverse stimuli, including vasomodulators, in normal (15) and disease states (16). The role of KATP channels in regulating the permeability of normal and brain tumor capillaries, however, has not been elucidated. Brain tumors thrive in a hypoxic environment; this fact could explain the up-regulation of KATP channel expression detected in and around brain tumors but also shown in hypoxic and ischemic conditions (3, 16). Furthermore, endothelium-dependent regulation of cerebral blood vessel functions is impaired in brain tumors (17, 18), which might affect tumor capillary permeability in response to vasomodulators.

Brain tumors, particularly gliomas, frequently exhibit up-regulated epidermal growth factor receptor genes, which is associated with tumor aggressiveness, poor prognosis, and shortened patient survival (19, 20). One study found that 17–20% of primary brain tumors were Her-2 positive (20), and Her-2 has been linked to tumor progression. Mutant Her-2 MAbs directed against Her-2 or Her-2 inhibitors have been shown to block kinase activation of epidermal growth factor receptor through the formation of nonfunctional heterodimeric receptor complexes (21), which prevent tumor growth. Trastuzumab was developed to treat Her-2-positive breast cancers and may, possibly, also be effective in treating gliomas. Although Trastuzumab alone or in combination with a chemotherapeutic drug (22) has the potential to treat breast and lung cancers metastasized to brain, as well as gliomas, their efficient delivery across the BTB to tumor is highly challenging. Our strategy involves selective modulation of BTB permeability via activation of KATP channels in tumor capillaries to enhance delivery of Neu and Her-2 MAb in syngeneic and xenograft rat tumor models, respectively.

This study sought to elucidate the role of KATP channels in BBB and BTB permeability modulation and the role of transendothelial vesicular transport in normal and tumor capillaries in rats harboring intracranial rat gliomas (RG2). We investigated whether MS causes BTB permeability increase in a human, brain tumor xenograft model. We also studied whether brain tumor cells induce overexpression of KATP channels in brain endothelial cells cocultured with tumor cells.

We further tested whether MS increases the transport of macromolecules, such as adenoviral vectors carrying green fluorescent protein (GFP) genes and Her-2 MAb, across the BTB in a human tumor xenograft model.