Pharmacology of the blood-brain barrier Flashcards
What is the blood-brain barrier?
Paul Ehrlich: important contributions to pharmacology, e.g.:
* receptor theory: postulated that action of drugs are mediated via binding to the organism / cellular structures / receptors
* discovery of the BBB by trypan blue injection into different compartments (1885)
* interpreted as BBB by his student Edwin Goldmann
The Blood Brain Barrier: Bottleneck in Brain Drug Development
- 98 % of small molecule drugs do not cross the BBB
- 100 % of large molecule drugs do not cross the BBB
- 1 % of drug companies have a BBB drug targeting program
- 1 % of academic neuroscience programs emphasize BBB transport biology
What is the blood-brain barrier?
a selective barrier between blood and CNS compartments
* to pathogens
* to small hydrophilic molecules
* to proteins
* to leukocytes
a gateway between blood and CNS compartments
* for nutrient and oxygen supply of neurons
* for the regulation of blood pressure
* forming an interface for immune- and nervous system crosstalk
Structure - different barriers
- neurovascular unit (blood brain barrier)
- Chloroid plexus (blood CSF barrier)
- Meninges (arachnoid barrier)
- Neuroependyma (fetal CSF brain barrier)
- adult ependyma (free exchange)
Structure - circumventricular organs
- The circumventricular organs are characterized by extensive vasculature and fenestrated capillaries which lead to a “leaky” BBB.
- sensory organs: area postrema (AP), subfornical organ (SFO) and vascular organ of lamina terminalis
- secretory organs: posterior pituitary, pineal gland, choroid plexus and median eminence
- choroid plexus: CSF production and filtration
- pineal gland: Melatonin release
- median eminence: release of CRH, TRH, GnRH
- area postrema: trigger of vomiting
- subfornical organ: fluid balance
- vascular organ of lamina terminalis: fever regulation
- posterioir pituitary: Oxytocin and ADH release
Structure - brain vasculature
- 600 km vessels in human brain
- each neuron has its own capillary (by number)
- by far the largest barrier interface in the brain
- vessel composition differs between arterioles, capillaries and venues
1. Penetrating artery: Basement membrane, Astrocytic end foot, VSMC, Endothelial cell, Pia, Neuronal projection, Virchow-Robin space
2. vascular tree: penetrating artery, astrocyte, neuron, VSMC, arteriole, pre-capillary arteriole, pericyte, capillary, pericyte
3. Arteriole: endothelial cell, VSMC
4. Capillary: Pericyte, endothelial cell
Structure - brain vasculatur 2
- peripheral capillaries are fenestrated while cerebral capillaries are not
- lower transcytotic rate than peripheral endothelial cells
- different barrier properties: ~30 Ω/cm2, brain capillaries ~1500-2000 Ω/cm2
- increased number of mitochondria in BBB endothelial cells
- coverage with astrocytic endfeet
- highest coverage of pericytes compared to peripheral vessels
peripheral capillary: fenestration, pinocytotic vesicle, intracellular cleft, nucleus, pericyte, endothelial cell, mitochondriium, capillary lumen
cerebral capillary: nucleus, pericyte, endothelial cell, mitochondria, capillary lumen, astrocyte, neuron, tight junction
Structure - neurovascular unit
Central players of the neurovascular unit:
* endothelial cells
* pericytes / smooth muscle cells
* astrocytes
* neurons
Extended parts:
* microglia
* blood cells
Structure - pericytes
- surround the endothelial cell layer
- embedded in between the endothelial and the parenchymal basement membrane
- important for function and development of the BBB
- influence several mechanisms in the brain (immune cell infiltration, blood flow regulation)
- heterogenous cell type (dependent on vessel type)
Structure - astrocytes
- direct contact to vessels, neurons, synapses, other glial cells
- are able to influence tightness and transport mechanisms across the BBB
- essential for water and ion homeostasis in the brain (special water and ion channels at their endfeet)
- are highly reactive after disturbance
Structure - endothelial cells
- binding partner and regulator of immune cell trafficking
- transport of essential molecules into the brain tissue (e.g. hormones, glucose, vitamins)
- build up the barrier itself by two main properties
- very tight intercellular junctions→no paracellular flux across the BBB
- low uptake frequency of luminal and parenchymal molecules→low transcytotic rate
Structure - tight junctions
Claudins: homophilic interaction leads to formation of tight junctions
* 24 different claudins in vertebrates, of which only Claudin-3, -5 and -12 are expressed in BBB
* Regulation of permeability for proteins of certain size (claudin-5 KO mice are neonatal lethal due to increased permeability in lower molecular weight range: <800 Da)
Occludin:
* N-Terminus important for tight junction formation
* serves as anchor
Adaptor proteins: Tethering of junctional transmembrane proteins to the cytoskeleton (actin filaments)
* Zonula occludens proteins (ZO-1, 2 und 3)
* Cingulin, MUPP1, MAGI
Junctional adhesion molecules (JAMs)
* JAM1 mainly expressed in endothelial and epithelial cells
Barrier dysfunction in diseases
- BBB breakdown, accumulation of blood-derived neurotoxic molecules
- Aberrant angiogenesis
- Disrupted phagocytosis, accumulation of neurotoxins
- CBF dysfunction and reductions
- Increased leukocyte trafficking and loss of immune privilege
- Compromised stem cell activity
Blood brain barrier leakage
- accumulation of neurotoxic serum proteins
- accumulation of iron→ROS production
- antibodies could lead to autoimmune diseases
Diseases linked to BBB dysfunction
- stroke
- epilepsy
- AD
- Familial ALS
- PD
- MS
- Natalizumab-PML with IRIS
- NMO
- primary CNS vasculitis
- secondary CNS vasculitis
- VZV vasculopathy
- cerebral malaria
- primary CNS lymphoma
- glioblastoma
- PRES
- TBI
- migraine
- diabetes
Strategies to overcome the barrier
- route of administration (transcranial, intracerebroventricular, intracerebral injection, transnasal)
- increasing lipid solubility of small molecules (drawback→increased uptake in other organs (e.g. liver) and decreased plasma levels)
- use of drug precursors or modified drugs that are substrates of transporters (e.g. L-Dopa)
- efflux-inhibition (inhibition of P-gp, etc.)
- temporary damage to BBB, e.g. osmotic opening, solvents, ultrasound (drawback: entry of plasma proteins!)
- molecular „trojan horses“ (Transferrin (Tf-R)-Ab, Insulin-receptor (IR)-Ab)
- viral vectors (rAAVs)
Strategies to overcome the barrier - lipophilicity
Which factors define the permeation of a substance?
* the distribution coefficient between octanol and water (Ko/w) is a measure of lipophilicity
* Ko/w can be predicted from structure
* substances with high Ko/w generally display high permeability into the CNS
there are exceptions from the rule:
* Paroxetin, a selective serotonin-reuptake inhibitor is used as an antidepressant
*despite being highly lipophilic, paroxetin displays only low cerebral uptake
Strategies to overcome the barrier - transporters
-> P-glycoprotein (P-gp)
- P-gp (multi drug resistance gene 1, MDR1) was discovered in tumor cells where it is mediating resistance against cytostatic drugs
- P-gp mediates the transport of pharmacons (e.g. paroxetin) from endothelial cells to the blood
- P-gp is a ABC-transporter: ATP-Binding Cassette transporter
- limited transport capacity, accumulation of high levels of pharmacons still possible
ABC-transporters such as P-gp mediate directed transport of various substances – resulting in net efflux of pharmacons or other substances from the brain, thereby affecting bioavailability of CNS-acting drugs.
→P-gp inhibition could lead to increased availability in the CNS. - there are exceptions from the rule:
- the polar compound L-dopa can pass the BBB
Strategies to overcome the barrier - transporters
-> P-glycoprotein (P-gp) - (lower activity) polymorphisms affect effects of antidepressants
- P-gp substrates: Paroxetin, Citalopram, Venlafaxin, Amitriptylin →carriers of polymorphism show stronger effect of drug
- no P-gp substrate: Mirtazapin
→no difference between groups
Strategies to overcome the barrier - transporters
-> Strategy of L-DOPA
1961: decreased dopamine level in the striatum of PD (parkinson ́s disease) patients
→ dopamine couldn ́t be substituted → levels not high enough in the brain
→but L-Dopa was found to be effective (Arvid Carlsson, Nobel price in 2000)
* additional peripheral administration of decarboxylase- inhibitors increases the available amount of L-dopa in the brain
Example: LAT1 (large amino acid transporter), which transports L-Dopa but not dopamine
-> also limited capacity -> L-DOPA treatment not effective after protein-rich meal
L-DOPA and LAT1 exceptions from the rule
Which factors define the permeation of a substance?
*the polar compound L-dopa can pass the BBB→it is transported by solute carrier transporter (SLC, OATPs)
*SLC derive energy for transport from existing gradients, they act as:
* antiporter: simultaneous bidirectional transport
* symporter: simultaneous unidirectional transport
* uniporter: equilibrating transport
Strategies to overcome the barrier - transporters
Transporters not only mediate efflux but also influx into the tissue of molecules.
BSP.: GLUT1, Pgp, LRP and RAGE
Transporters as treatment options
Influencing efflux (e.g. P-gp) or influx mechanisms could be treatment options.
Influx blockade (antagonist effect) and efflux blockade (agonist effect)
Strategies to overcome the barrier - Trojan horse
- vesicle uptake and transcytosis could be used to transport drugs into the brain
- receptor mediated uptake of peptidomimetic, monoclonal antibodies (e.g. against insulin-receptor (IR) or transferrin receptor (Tf-R))
- antibodies coupled to peptides, recombinant peptides or siRNA (e.g. BDNF-coupled IR antibodies for stroke)
- antibodies coupled to PEGylated liposomes -> transport of enclosed substances across the BBB (e.g. Tf-R antibodies coupled to liposomes containing tyrosine hydroxylase for the treatment of Parkinson’s)
Strategies to overcome the barrier - rAAVs
Recombinant adeno-associated virus (rAAV) with modified capsid structure * changes in small peptide AA sequences results in altered tropism (1)
* uptake in endothelial cell (2), and either subsequent
* transcytosis and release of viral particles to the brain, or
* expression of therapeutic factors and secretion from endothelial cells (4)
BBB as target for specific diseases
Alzheimer’s disease
ABeta (enhanced)
LRP1 (inhibited)
→RAGE inhibition or LRP1 stimulation could be effective to clear Aß
Multiple sclerosis
-> endothelial cell proteins: selectins, ICAM/VCAM
-> Immune cell proteins: P selection ligand, alpha4 integrin, LFA1
-> Natalizumab inhibits immune cell infiltration into the CNS
COVID-19
up to 84 % of COVID 19 patients suffer from neurological symptoms
* imaging studies detected lesions comparable to microvascular pathology
* leakage of plasma proteins→parenchyma
1) immune cell infiltration
2) activated glial cells
3) plasma protein leakage
4) platelet aggregation
- Cytokine storm
- Immunoglobulins
- Complement
- SARS-CoV2
NEMO loss in brain endothelial cells
- induces cell death
- induces blood-brain barrier leakage
- induces neurological symptoms (e.g. seizures)
- RIPK inhibition rescues symptoms
Pro cleaves NEMO -> cell death -> RIPK inhibition?
Summary BBB
- functions and anatomy of the blood-brain barrier
- different cells of the neurovascular unit and their properties * diseased state of the BBB
- different ways to overcome the barrier
- the BBB as target for neurological diseases
- example: NEMO and COVID-19
„The BBB is not simply a barrier, but a complex, interactive, ever-adapting interface that serves the needs of the CNS… and provides an array of opportunities for drug development:“ (Banks, 2016)
