By Samrah Siddiqi
Often naively thought of as the wall which protects our brains, the blood-brain barrier (BBB) is a complex physiological barrier comprising a multitude of endothelial cells lining blood vessels. These vessels separate the blood from the brain to maintain brain homeostasis (Zuhorn, 2016). The homeostatic purpose of the BBB is to provide the brain with necessary nutrients (such as oxygen and glucose) whilst also helping to remove unwanted waste products (Tocris, n.d.). Its integral role in human health and disease was highlighted by Peter Searson of Johns Hopkins University who said that ‘almost every disease of the CNS is associated with disruption or dysfunction of the BBB’ (Keener, 2017). Numerous debilitating neurological conditions such as Alzheimer’s disease (AD) and Parkinson’s disease (PD) have been linked to disturbance of the BBB (Tocris, n.d.). To date, many therapeutic avenues have been explored in order to halt disease progression via BBB dysfunction. However, the selective nature of the BBB presents many challenges when developing drugs which can pass through it to treat brain disorders. Therefore, researchers have tried to exploit the unique features of the BBB in order to bypass it for delivery of therapeutics (Keener, 2017).
Firstly, it is important to understand the anatomy and physiology of the BBB in order for researchers to be able to design novel methods of targeted, non-invasive drug delivery to the brain. The BBB is a vascularised system which regulates the movement of molecules, ions and cells between the blood and the brain (Daneman & Prat, 2015). It has various components which are essential for the formation and maintenance of the BBB: pericytes; astrocytes; microglia; and a basement membrane comprised of structural proteins such as collagen and laminin (Tocris, n.d.). Each component is responsible for different properties of the BBB, collectively enabling it to protect the brain from harmful molecules which can result in brain damage. Water molecules are able to traverse the BBB due to aquaporin (AQP1 and AQP4) channel expression in the central nervous system (Bonomini & Rezzani, 2010). Excess water transport into the brain via aquaporins has been linked to brain oedema (Papadopoulos & Verkman, 2013). Another key feature of the BBB are the tight junctions which hold the endothelial cells together, preventing entry of not only bacteria but also antibodies and antibiotics into the brain (Daneman & Prat, 2015). This explains why it is difficult to treat infections of the brain (Tocris, n.d.). Additionally, the BBB has multidrug transporters, notably P-glycoprotein, which export therapeutic drugs out of cells before they are able to migrate through endothelial cells into the CNS. This area of reversing multidrug resistance has received a lot of interest from researchers, as the aim is to try to treat cancers of the brain with chemotherapeutics which are currently not able to penetrate the BBB due to multidrug transporters (Tocris, n.d.).
Designing in vitro models of the BBB has helped scientists to better understand the BBB and the way it functions. Searson’s team have spent decades trying to create a three-dimensional replica of the human BBB, progressively increasing the complexity of their model to mirror the complexity of the BBB. Initially a collection of human induced pluripotent stem cells, the team then progressed to culturing a 3-D gel matrix of human astrocytes which avoided triggering a cellular stress response – crucial for avoiding changes in gene expression in astrocytes as well as other BBB cells (Keener, 2017).
The challenge for scientists is to attempt to make the BBB temporarily permeable enough to allow small-molecule drugs to enter the neural tissue without compromising the integrity of the barrier. Since the 90s, invasive procedures have been favoured, with doctors directly injecting a drug into neural tissue or into cerebral-spinal fluid (CSF) (Keener, 2017). However, in 2015, a new non-invasive therapeutic technique using microbubbles and focused ultrasound waves emerged as a potentially effective method of drug delivery to the brain (Keener, 2017). Microbubbles are intravenously administered to the patient, after which they travel to the capillaries of the brain. Subsequent application of ultrasound waves on the microbubbles results in blood vessel expansion and contraction, forming temporary gaps for the drugs to pass through into the targeted neural areas (Keener, 2017). Additionally, this technique has shown to reduce the number of tight junctions and increases the leakiness in brain blood vessels in rats (Sheikov et al, 2008). Focused ultrasound has progressed to use in Human Phase 1 brain cancer trials with AD patients among others. This focused ultrasound technique has proved successful in breaching the BBB, but researchers are still refining this procedure and searching for improved methods which are less invasive and more precise.
The BBB’s role is not necessarily to prevent entry of molecules into the brain, as essential transport of nutrients such as sugars, amino acids and electrolytes are all encouraged (Keener, 2017). Researchers have tried to exploit this mechanism as an approach to disguise drugs so they can ‘piggyback’ into the central nervous system. Endothelial cells in the BBB have a variety of channels, transporters and receptors which can all be used to attach to therapeutic compounds so that they can be transported across the cell using a process known as transcytosis (Keener, 2017). An example of this approach being used is by the Netherlandic company 2-BBB Medicines which has developed a new technique involving coating the brain cancer drug ‘pegylated liposomal doxorubicin’ with the antioxidant glutathione, aiding enhanced drug delivery across the BBB through glutathione transporters (Gaillard P.J. et al., 2014). Using this technique, approximately five times more of the chemotherapeutic drug was able to cross the BBB and enter the brains of the treated rats, proving its efficacy (Keener, 2017). 2-BBB Medicines is now looking at using their technique to deliver other lipophilic molecules (e.g. steroids) across the BBB in order to treat other neurological conditions (Keener, 2017).
The future of this field is very exciting. Researchers are trying to localise drug delivery to specific brain regions in order to target diseased areas and avoid unwanted brain damage. Viruses, immune cells and antibodies are all being explored as potential drug-delivery vehicles for crossing the BBB. However, safety issues in human clinical trials have to be addressed and the low quantity of drug each technique can deliver into the CNS still poses a big challenge.
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