BBB 2 and 3

3 over arching solutions to bypass BBB

• Exploit transport mechanisms

• Drug modifications and delivery systems

• Delivery methods to bypass or disrupt BBB

What are the two exchange mechanisms

Lipid mediated diffusion (lipophilic, <500Da)

Catalyzed transport (active efflux, facilitated, receptor-mediated)

What is permeability proportional to

Lipid solubility

Oil-water partition coefficient

What is the partition coefficient 

Ratio of concentration of a compound in two phases of a mixture of immiscible solvents

What are the exceptions to the partition rule

• Really high- bind to albumin less permeable

• Some hydrophilic like D-glucose are easily taken up by selective endothelial transport

What can enter through transcytosis

Large molecular weight solutes, proteins, peptides 

Molecules in receptor-mediated transcytosis

Transferrin, IgG, insulin, leptin, TNF-a, EGF

Molecules in adsorptive-mediated transcytosis

Cationised albumin

Specialized feature of the BBB

The ability of the primary sorting end-some to route away from lysosomes

5 major transport systems

1. Facilitative transporters

2. Active Na+-dependent transporters

3. Na+/K+ ATPase keeps gradient

4. Facilitative transporter

5. ATP-binding cassete (ABC)

Facilitative transporters

• Both membranes

• Move molecules along concentration gradient

• GLUT-1, L system

Active Na-dependent transporters

• Abluminal membrane

• Against gradient

• Glutamate, glycine

Facilitative transporters 2

• Luminal membrane

• Extrude amino acids accumulated

• Xg, N, Glu, Gln

ABC transporters

• Efflux metabolites, drugs, xenobiotics out of the brain in the blood

• P-glycoprotein, MRP

P-gp

• Determinant of effect of drug resistance

• Also known as MDR1 and ACBB1

• Two homologs halves- 6 trans

• Hydrolyzes ATP 

• Expressed in many tissues

Substrates of P-gp

Large flexible drug-binding pocket with low specificity

Amino acid important in stabilizing P-gp binding to substrate

Cyan

Where is P-gp expression high?

Enterocytes, kidney cells, hepatocytes, placenta, BBB

Examples of limited absorption, limited distribution and active elimination

• Limited absorption: enterocytes

• Active elimination: proximal tubule cells

• Limited distribution: endothelial cells

Maternal P-gp cells in placenta

Syncytiotrophoblast

Role of MDR polymorphism in CNS disease

• Pesticides-induces Parkinson is associated with MDR-1 polymorphisms

• Drug-resistant epilepsy is associated with a specific MDR-1 polymorphism 

Drug notifications and delivery systems to allow passage through the BBB

• Inhibition of P-glycoprotein

• Production of lipophilic drugs

• Pre-existing BBB (Trojan horse)

• nanoparticles

Delivery to bypass or disrupt BBB

• Intrathecal and intracerebroventricular 

• Focused ultrasound 

• Intranasal

Three ways to leverage P-gp

• Modified small molecule 

• Coadministration with competitor P-gp substrate

• Coadministration with noncompetitive inhibitor

H-bonds

Polar surface area- PSA or N or H atoms counts

Number of H-bonds other than forms with water

Each pair of H bonds permeability decreases by 10-fold

Two small molecule modifications to help with delivery

• Modifications of groups that form H-bonds

• Lipidation of molecules (Could increase uptake into other organs)

Molecular Trojan Horses

• Endogenous peptides, modified proteins, monoclonal antibodies

• Target specific receptor/transport systems (receptor-mediated transcytosis through the BBB)

• Molecules attached to the molecular Trojan Horses are co-transported into the brainGeneral structure o

General structure of IgG antibodies

• Fab region, antigen binding

• Fc, heavy chain

• VL and VH variable domain (light and heavy)

BBB transport vehicle

Fc antibody fragment engineered to bind with LOW AFFINITY 

Molecular shuttle system

In trial for Hunter syndrome

EPiC technology 

• Engineered Peptide- compound technology 

• Synthetic peptic angiopep2 that can bind to LRP-1

• LRP-1 is one of the most abundant in the BBB and has a very fast recycling rate

• Ideal and hard to saturate

ANG1005

• Novel taxane derivative

• One small peptide to 3 paclitaxol

Liposomes

One or more lipid bilayers of amphiphilic lipids delimiting an internal aqueous compartment loaded with drugs

Cationic liposomes

Positively charged lipids and undergo adsorptive-mediated endocytosis and internalization in endosomes

Polymeric nanoparticles

Solid colloidal particle composed of biodegradable polymers

Polylactic acid acid, polyglycolides

Liposomes and nanoparticles?

Liposomes and nanoparticles bypass the P-glycoprotein

Advantages of nanoparticles vs liposomes

• A smaller number of excipients are used for production

• Simpler manufacturing processes

• Higher physical stability and longer plasma half-life 

• More efficient drug release

• Can be multi functionalized more easily to improve brain delivery and target engagement 

Intrathecal drug delivery

Catheter into subarachnoid space in spinal cord

their targets are mainly in the dorsal horn of the spinal cord 

What do intrathecal drugs usually treat?

Pain, spasticity, dystonia

Intraventricular drug delivery

• Ommaya reservoir (for repeat)

• Mic with CSF- still has to diffuse into parenchyma

• Approved for infusion of recombinant lysosomal enzymes 

Intranasal drug delivery

• Olfactory and trigeminal pathway

• Nasal mucosa to perineurial olfactory and trigemini space to olfactory bulb to pons

• 5% of the drug is transported through the perivascular space

• Can exploit rhythmic contraction of adjacent blood vessels

Advantages of intranasal

• Direct route to brain (no BBB)

• Bypasses GI and hepatic first-pass metabolism

• Decreases systemic exposure

Nasal bioavailability

• Poor for hydrophilic, large, biologics

• Rapid clearance and tight junctions

• Drug diffusion is limited to the ependymal surface of the brain

Current developments for nasal absorption

• Permeability enhancers

• Enzyme inhibitors

• Mucoadhesive substances

• Goal: increase residence time and improve passage

Focused ultrasound for drug delivery

• Microbubbles contain an inert gas in their core (perfluoropropane) and resonate at ultrasound frequency

• Change in volume in response ultrasound- disrupt barrier

• Can be co-administered with drug or drug carried on bubble 

What do we used FUS for?

• Experimental

• Localized brain tumour

• Biologics, neurodegenerative conditions

• Long term disruption may cause issues