Recent advances in understanding of blood–brain tumor barrier (BTB) permeability mechanisms that enable better detection and treatment of brain tumors

Introduction

Blood–Brain Barrier Permeability

The blood–brain barrier (BBB) is a cerebrovascular permeability barrier that strictly regulates the entry of variety of substances, including therapeutics and imaging agents into the brain. Unlike blood vessels that circulate blood to other areas of the body, the microvessels that perfuse the brain consist of special endothelial cells and pericytes that lack fenestrations and are sealed by endothelial tight junction proteins (TJp) (Ningaraj et al., 2003; Black and Ningaraj, 2004). These tight endothelium, pericytes, and astrocytic foot processes provide a physical barrier that, together with metabolic barriers, form the BBB (Fig. 17.1). At the blood–brain tumor barrier (BTB) the TJp and astrocytoic foot processes are compromised, leading to a “leaky” BBB. The BBB protects the brain against pathogens and other damaging agents in the circulatory system, including electrolytes that changes the composition of the systemic blood. At the same time the BBB permits entry of certain substances, such as small fat-soluble (lipophilic) molecules that can freely diffuse through the barrier. The BBB also permits entry of essential nutrients, such as glucose and amino acids through receptor-mediated endocytosis to maintain vital brain functions. These nutrients are generally water soluble (hydrophilic), and require more complex mechanisms to cross the BBB, such as carriermediated transport, receptor-mediated transcytosis and absorptive-mediated transcytosis .

While protective under normal circumstances, the BBB prevents the delivery of drugs, other therapeutic molecules and imaging agents to the brain. Furthermore, the BBB blocks delivery of more than 98% of central nervous system (CNS) drugs (Pardridge, 2002). Therefore, the challenge posed by the BBB is compelling, particularly as the population ages and the incidence of neurodegenerative diseases such as stroke, Alzheimer’s disease, and Parkinson’s disease increase in prevalence. The problem is particularly acute for patients with malignant brain tumors, who cannot benefit from anticancer drugs effective in treating tumors elsewhere in the body. A solution to drug delivery across the BBB would be to produce an exponential increase in the number of drugs available for the treatment and prevention of CNS disorders. Regulation of BBB/BTB permeability function may involve endogenous nitric oxide production and a cyclic GMP-dependent mechanism (Liu and Sundqvist, 1997)

Blood–Brain Tumor Barrier Permeability

An ideal strategy for delivery of therapeutic molecules to brain tumors would involve selective opening of only that portion of the BBB that serves the brain tumor. The portion of the BBB that surrounds a brain tumor is known as the BTB. Over the years, many methods have been used to enhance delivery of anticancer drugs and imaging agents to brain tumors (Misra et al., 2003). Some of these efforts have focused on changing the therapeutic molecule or encapusalting the drugs in nanospheres or nanoparticles (Amrawy et al., 2016), rather than altering the barrier. Well considered drug design or drug delivery techniques were aimed at “lipidizing” otherwise poorly-lipid soluble compounds, by either developing lipophilic analogs or packing hydrophilic drugs in liposomes. This approach has been limited by the relative instability of lipophilic analogs in the blood, and the rapid removal of these analogs from the blood as a direct result of their increased lipid solubility. Other strategies have focused on circumventing the barrier, for example by directly injecting drugs into the brain or through the use of implantable drug delivery devices, such as Gliadel. These strategies are highly

Biochemical Modulation of the Blood–Brain Tumor Barrier

Biochemical disruption is based on the finding that the permeability of tumor capillaries is enhanced relative to that of normal brain capillaries by administration of certain vasoactive molecules (Fig. 17.2). BTB function is generally impaired in brain tumors because the endothelium-dependent regulation of cerebral blood vessel function is abnormal (Morimoto et al., 2002), which might affect BTB permeability in response to vasomodulators (Cobbs et al., 1995). Accordingly, biochemical opening with vasoactive agents holds promise for increased anticancer drug delivery selectively to brain tumors across BTB, compared to the toxic Mannitol-induced osmotic disruption. This is due to its nonselectivity and unwanted distribution of anticancer drugs to normal brain. We and others have tested a variety of vasoactive compounds, such as leukotriene (LTC4 ), bradykinin (BK), cGMP, and certain potassium channel agonists to selectively disrupt the BTB without affecting the BBB, for enhanced anticancer drug delivery in experimental brain tumor models (Sugita and Black, 1998; Hashizume and Black, 2002).

Using immunoblot and immunolocalization studies, we established that BKCa channels were overexpressed in rat brain tumor capillary endothelium and tumor cells, and demonstrated the functional presence of channels in isolated rat brain tumor capillary endothelial and tumor cells (Ningaraj et al. 2002). The BKCa channel has been shown to be the convergence point of a BK signaling pathway involving nitric oxide, soluble guanylyl cyclase, and cGMP (Fig. 17.1). While BK has been shown to activate BKCa channels, other known activators of BKCa do not act as vasodilators; for example, 1,3-dihydro [2-hydroxy(trifluoromethyl)phenyl] (trifluoromethyl)-2H-benzimidazolone (NS- 1619) (Holland et al., 1996). Another major class of potassium channels, KATP channels, have also been shown as overexpressed on BTB and brain tumors (Ruoshlati, 2002; Ningaraj et al., 2003). We have developed methods of increasing delivery of anticancer agents to brain tumor using potassium channel activators, which include compounds that indirectly activate potassium channels, such as nitric oxide, nitric oxide donors, and other activators of soluble guanylyl cyclase. These potassium channel activators selectively increase the permeability of the BBB and, in particular the BTB, for small to large-sized molecules, drugs, and imaging agents (Fig. 17.2).

Functional Ion Channels on Blood–Brain Tumor Barrier

Several researchers, including us, have shown the importance of ion channels on BTB permeability regulation and their role in anticancer drug delivery (Ningaraj, 2006; Ningaraj et al., 2002, 2003; Black and Ningaraj, 2004). We recently reviewed the role of BTB-associated ion channels in increasing the BTB permeability for delivering therapeutic, prophylactic, and diagnostic agents to brain tumors (Ningaraj and Khaitan, 2015). We showed that the BTB can be modulated to increase delivery of combination of drugs: trastuzumab with temozolomide in glioma models (Ningaraj et al., 2009). The underlying mechanism is not completely understood, but it involves formation of brain vascular endothelial transcytotic vesicles to facilitate transport of the drugs, as demonstrated by us earlier (Ningaraj et al., 2002, 2003). Recently, we described the role of BKCa and KATP channels in brain tumor cell growth. We also showed that modulators of BKCa and KATP channels may be utilized to enhance the delivery of antineoplastic drugs and imaging agents to glioma cells in brain tumor models (Ningaraj and Khaitan, 2015)

Nanomedicines and Nanoimaging

Despite the challenges faced due to the presence of BBB/BTB, there has been some progress in developing new strategies to treat gliomas. Specifically, gliomas are diffusive with several leading edges, making it difficult to target them with anticancer drugs as well as imaging agents. A recent review article (Juratli et al., 2013) has described the nanomedicine approaches that have been developed and some of the technologies that are being translated to the clinic. It also discusses the hurdles of effective brain tumor treatment and how various nanomedicine techniques that are being explored to overcome BBB using liposomal and polymeric nanoparticles. It is well studied that the BBB becomes compromised both structurally and functionally to transform itself into the BTB, as the primary and or metastatic tumors grow more than 1–2 mm in diameter. Such tumors also have disorganized or distorted, tortuous blood vessels that are typically referred to as the BTB (Fig. 17.1) (Abbott and Friedman, 2012). It is important to note that BTB capillary ultrastructure is markedly different between a GBM and a brain metastasis (Van Tellingen et al., 2015; Hawkins and Davis, 2005). This leads to differential vascular permeability and often requires different strategies to deliver anticancer drugs and imaging agents across the BTB of primary and metastatic brain tumors. Accordingly, BTB permeability should be assessed with advanced methodologies such as dynamic contrastenhanced magnetic resonance imaging (DCE-MRI) to plan for targeted treatment of different types of brain tumors. In this aspect, we have validated a noninvasive method of BTB permeability measurement using DCE-MRI in brain tumor models (Ningaraj and Khaitan, 2013).