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Drug Transport Across the Blood Brain Barrier with Dr. Sadhana Jackson
NIH Clinical Center
Overview
This lecture explains the blood-brain barrier (BBB) as a highly selective interface that protects the central nervous system. It details the cellular components of the BBB, including endothelial cells, pericytes, astrocytes, and the basement membrane, collectively forming the neurovascular unit. The presentation then elaborates on the factors influencing drug transport across the BBB, such as lipid solubility, molecular size, charge, and the presence of efflux transporters. Various modes of transport, including paracellular, transcellular, carrier-mediated, receptor-mediated transcytosis, absorptive transcytosis, and cell-mediated transcytosis, are discussed. Finally, the lecture touches upon ongoing research aimed at manipulating BBB permeability to improve drug delivery for neurological disorders and mentions other biological barriers.
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Chapters
- The BBB acts as a highly selective barrier, controlling what enters the central nervous system (CNS).
- It can be conceptualized as a 'club' with strict entry requirements, protecting the brain's 'celebrities' (neurons).
- Understanding the BBB is crucial for developing effective drug therapies for brain conditions.
This chapter introduces the fundamental concept of the BBB and its importance in regulating brain access, setting the stage for understanding drug delivery challenges.
The analogy of the BBB as a selective club where only certain individuals (drugs) are allowed entry.
- The BBB is composed of a neurovascular unit (NVU) including endothelial cells, pericytes, astrocytes, neurons, and microglia.
- Endothelial cells form the primary barrier with exceptionally tight junctions, unlike those in peripheral organs.
- Pericytes support endothelial cells, astrocytes act as 'bartenders' facilitating communication, and the basement membrane provides structural support.
- Microglia and neurons play supporting roles, becoming more active during inflammation or injury.
Knowing the cellular components helps understand how the BBB maintains its integrity and how different cells contribute to its function and regulation.
Endothelial cells are described as 'bodyguards' due to their role in regulating what passes through the blood vessels in the brain.
- Drug transport is influenced by lipid solubility (higher is better), molecular size (smaller is better), and electrical charge (neutral is better).
- Efflux transporters actively pump drugs out of the brain, reducing their concentration.
- Protein binding can limit a drug's ability to cross the BBB, as only unbound drugs are available for transport.
- Regional blood flow also affects drug delivery; areas with poor blood flow receive less of the drug.
These factors determine a drug's potential to reach the brain, guiding the selection and design of therapeutic agents.
Lipid-soluble drugs are likened to 'life of the party' guests who easily gain entry, while highly charged drugs are like overly energetic individuals who are denied entry.
- Paracellular transport allows small, water-soluble molecules to pass through the tight junctions between endothelial cells.
- Transcellular transport (passive diffusion) enables lipid-soluble, non-polar molecules to cross directly through the endothelial cells.
- Carrier-mediated transport uses specific proteins to facilitate the passage of essential nutrients like glucose and amino acids.
- Efflux pumps, like P-glycoprotein, actively remove drugs from the brain.
- Transcytosis (receptor-mediated, absorptive, and cell-mediated) involves vesicular transport across endothelial cells for larger molecules or specific substances.
Understanding these diverse transport mechanisms is key to developing strategies for both delivering drugs into the brain and preventing unwanted substances from entering.
Receptor-mediated transcytosis is compared to a friend inside the club vouching for someone to get them past security.
- Research focuses on temporarily increasing BBB permeability to allow more drugs to enter the brain, especially for conditions like brain tumors.
- Methods include using endogenous mediators, nanoparticles, microspheres, or disrupting tight junctions with agents like mannitol.
- Circumvention strategies, such as biodegradable wafers impregnated with chemotherapy, bypass the BBB entirely.
- Other delivery methods being explored include convection-enhanced delivery and intranasal delivery.
These innovative approaches aim to overcome the BBB's protective nature, offering new hope for treating challenging neurological diseases.
A biodegradable wafer impregnated with chemotherapy placed directly at the tumor site during surgery to bypass the BBB.
- There is a need for agents that can temporarily 'close' the BBB to prevent leakage in conditions like Alzheimer's, head injury, or stroke.
- Currently, there are limited agents to attenuate BBB leakage, and supportive care is often the primary treatment.
- Ongoing research continues to explore various biological barriers beyond the BBB, each with unique transport characteristics.
- Visual aids and detailed diagrams are essential for understanding the complex transport mechanisms within the BBB.
This highlights the ongoing challenges and the continuous pursuit of knowledge to better manage brain health and disease through improved drug delivery and barrier regulation.
The analogy of needing to 'close the doors' of the BBB temporarily to prevent harmful substances from entering or beneficial ones from exiting during conditions like stroke or Alzheimer's.
Key takeaways
- The blood-brain barrier is a highly selective and dynamic interface crucial for protecting the central nervous system.
- The integrity and function of the BBB are maintained by a complex interplay of various cell types within the neurovascular unit.
- Drug properties like lipophilicity, size, and charge significantly influence their ability to cross the BBB.
- Multiple transport mechanisms exist across the BBB, ranging from passive diffusion to active efflux and specialized transcytosis pathways.
- Efflux transporters play a critical role in limiting drug entry into the brain by actively pumping substances out.
- Research is actively pursuing strategies to enhance drug delivery across the BBB for treating neurological diseases, often by manipulating its permeability or utilizing specific transport pathways.
- Understanding the BBB is essential for neuropharmacology, impacting the development of treatments for a wide range of CNS disorders.
Key terms
Blood-Brain Barrier (BBB)Neurovascular Unit (NVU)Endothelial CellsTight JunctionsPericytesAstrocytesLipid SolubilityEfflux TransportersP-glycoprotein (P-gp)TranscytosisReceptor-Mediated TranscytosisParacellular TransportTranscellular TransportCarrier-Mediated Transport
Test your understanding
- What are the primary cellular components of the blood-brain barrier and what is the main function of each?
- How do lipid solubility, molecular size, and charge affect a drug's ability to cross the blood-brain barrier?
- Describe at least three different modes of drug transport across the blood-brain barrier and provide an example for each.
- What role do efflux transporters play in drug delivery to the brain, and how can this be overcome?
- What are some of the current research strategies being employed to enhance drug delivery across the blood-brain barrier for treating neurological diseases?