Crossing the Blood-Brain Barrier: Profiling Cognitive Safety in Clinical Development
Posted on 26 March 2015 in Clinical Trials / Research
Author: Kenton Zavitz, PhD, Director of Clinical Affairs
- The blood-brain barrier (BBB) functions to protect the brain from toxic molecules and infectious agents that can impact brain health and cognitive function
- An understanding of the structure and function of the BBB is essential for the development of new effective therapies to treat CNS disorders
- Common disease states, environmental toxins, certain medications and the aging process itself can compromise the integrity of the BBB
- Drug developers and regulatory agencies are increasingly interested in measuring and monitoring cognitive function as part of a drug’s safety profile and risk management strategy
The blood-brain barrier (BBB)
The blood-brain barrier (BBB) is a highly regulated and complex layer of cells that has evolved to protect the brain from toxic molecules and infectious agents. It serves as the interface between the brain and the other tissues of the body. Under normal conditions, the BBB permits the selective transport of molecules that are essential for brain function and metabolism while preventing the passage of most proteins and small molecules, including the majority of drugs in current use. Knowledge of the structure and function of the BBB is therefore essential for those in the pharmaceutical industry developing new small molecule drugs and biologics that are required to cross the BBB to reach their targets and treat disorders of the brain such as Alzheimer’s disease. This remains one of the most significant technical challenges in drug design and development.
There is, however, growing evidence that the integrity and permeability of the BBB can be compromised by many common medical conditions and comorbidities. Examples of these include systemic diseases (e.g. diabetes, hypertension), inflammatory conditions (Multiple sclerosis), neurodegenerative diseases (e.g. Alzheimer’s disease, Parkinson’s Disease), infections (HIV), stroke, traumatic brain injury and brain tumours [2,3,4]. In addition, the function of the BBB can be altered by certain medications, environmental toxins and the aging process itself. Therefore, there exists the potential for many new and commonly used drugs to gain access to the brain and have an unanticipated impact on cognitive function, when used by patients in both clinical trials and real-world settings.
Investigators involved in the development of new drugs for both CNS and non-CNS related medical conditions, regulatory agencies in Europe and the US, as well as practicing physicians, are increasingly interested in measuring and monitoring cognitive function as part of a drug’s safety profile and risk management strategy.
The BBB consists of a layer of highly specialized brain microvascular endothelial cells (BMECs) lining the inner surface of the capillaries that feed the brain. In the human brain, there are about 100 billion capillaries in total providing a combined length of brain capillary endothelium of approximately 650 km and a total surface area of about 20 m2 . To say the least, this is a vast system of plumbing to prevent from springing leaks! BCMEs are the primary site for the production and function of the structural proteins, enzymes and membrane transporters that tightly regulate access of privileged molecules to the brain parenchyma. The close physical association of BMECs is achieved by ‘tight junctions’ that severely limit the diffusion of molecules throughout the spaces between these cells. Tight junctions are composed of an elaborate assembly of structural proteins that provide strong physical connections by tying together the cytoskeletons of adjacent cells [2,3].
Entering the brain
The physical and biochemical properties of the healthy BBB mean that there are essentially two ways for small molecule drugs and larger biologic drugs in peripheral circulation to gain access to the brain: 1) lipid-mediated free diffusion through the BBB or 2) carrier- or receptor-mediated transport across the BBB. Most drugs for brain disorders currently in clinical use are small molecules (molecular weight <400 Da) that are also highly lipid soluble and can therefore diffuse through the BBB . However, of all drug molecules, very few fit these dual criteria and the design of new drug candidates that can utilize lipid-mediated diffusion to gain access to the brain remains incredibly challenging.
Carrier-mediated transport proteins are responsible for the selective movement of specific molecules across the membranes of the BCMEs from one side of the BBB to the other. For example, glucose transporters move glucose via a process of facilitated diffusion across the BBB to provide a constant supply of energy for neuronal activity. Other transporters mediate the entry of other sugars, amino acids, nucleosides, fatty acids and essential metabolites. Some drug molecules have been found to be transported, serendipitously, across the BBB by these transporters. For example, L-DOPA, used in the treatment of Parkinson’s disease and gabapentin, an anticonvulsant used to treat epilepsy, both cross the BBB via the large neutral amino acid carrier LAT1 because their structures mimic those of the endogenous substrates . Thus, impairments in these transporters could alter drug access to the brain resulting in sub-optimal treatment. Indeed, age-related defects in the transport of glucose, amino acids and hormones across the BBB have been observed and the resulting deficits in glucose and choline bioavailability may contribute to the vulnerability of older adults to hypoglycemia and cognitive decline [6,7].
Receptor-mediated transcytosis (RMT) is the transport pathway across the BBB for endogenous peptides such as insulin, insulin-like growth factor and transferrin. It is a highly specific process in which the bioactive peptide in the blood is taken up by its receptor on the luminal side of BCMEs and delivered to the brain, with the receptor recycled back to the luminal membrane . RMT is currently being exploited by the pharmaceutical industry for delivery to the brain of novel antibodies and protein therapeutics using an approach sometimes called a ‘molecular Trojan horse’ [1,2]. An innovative example of this approach is a ‘bispecific antibody’ currently in preclinical development for the treatment of Alzheimer’s disease . One end of the antibody binds with low affinity to the transferrin receptor of the BCME which then transports the antibody across the BBB. Once in the brain, the other end of the antibody binds with high affinity and inhibits an enzyme called b-secretase 1 (BACE1) that is necessary for the production of amyloid-b, the peptide responsible for the toxic amyloid plaque deposits in the Alzheimer’s brain. Using this antibody delivery system, researchers have demonstrated significantly reduced brain levels of amyloid-b in both mouse and primate models of Alzheimer’s disease .
An important active transporter in the BBB is the permeability glycoprotein (P-gp), a member of a family of ‘multidrug resistance’ (MDR) efflux pumps that play an essential role in the removal of metabolites, drugs and toxins from the brain to the blood. Recent studies using radiolabelled ligands for MDR pumps have shown that the activity of these pumps is impaired in healthy older subjects compared to younger controls . Several recent studies have suggested that impaired activity of MDR pumps in elderly subjects may lead to the accumulation of inflammatory mediators or opioid medication levels that increase the risk for delirium [10,11,12]. These are further examples of how the aging brain may become more vulnerable to adverse drug effects on cognitive function.
Impact on drug development
The BBB functions to protect the brain from toxic molecules and infectious agents that can impact brain health and cognitive function. An understanding of the structure and function of the BBB is essential for the development of new effective therapies to treat CNS disorders. However, common disease states, environmental toxins, certain medications and the aging process itself can compromise the integrity of the BBB. It is therefore of critical importance for drug makers to monitor cognitive function throughout the drug development process and to consider the impact of their target patient populations, comorbid medical conditions and concomitant drug treatments in assessing the safety and risk profile of their drugs.
Dr Zavitz will be presenting on ‘The Principles of cognitive safety assessment in clinical development’ at The World Drug Safety Congress, Chicago, 22-23 April.
For further information on assessing the cognitive safety profile of compounds in clinical development.
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