Drug delivery to brain Flashcards
What is a brain tumour?
Any abnormal tissue growth inside the brain; whether malignant (cancerous) or benign
How do brain tumours cause problems?
by the pressure they exert on the normal brain
What is a glioblastoma?
An aggressive (stage IV), cancerous brain tumour that progresses rapidly and is difficult to cure
What does treatment of glioblastoma involve?
removal via surgery followed by chemotherapy (Temozolomide) and radiation therapy
How much distance of original resection is required for glioblastoma tumours to recur?
within 3cm of original resection
What is the average survival rate with a glioblastoma?
12 to 15 month survival rate
What is the survival rate after 5 years of a glioblastoma?
<3% :(
What is the limiting step in the development of new treatments for diseases of the CNS?
drug delivery to the brain: the presence of the BBB which restricts drug delivery to CNS
What does the BBB do?
maintains brain function:
- Allows selective access to essential nutrients and signalling mols from vascular compartment
- Restricts entry of foreign bodies i.e. drugs
What are examples of disorders that require drug transport across the BBB?
depression, severe pain, epilepsy, GBM
Efficient delivery of imaging agents across the BBB is necessary for what purposes? 4
- Accurate diagnosis of neuropathology
- Monitoring disease progression
- Localization for surgical intervention,
- Introduction of therapeutic agents
why can you not just treat glioblastoma by pulling it out?
not just a solid tumour, also has fingers going into important functional brain tissue
What structures form the blood brain barrier?
- capillaries: tight junctions between endothelial cells
- fenestrations: openings to allow drugs in and out
- tight junctions: physical barrier to drugs
in brain capillary, fenestrations are much tighter, why?
Blood comes in, pumped around, dynamic. Then want it to pass through and get to brain tissue (which will be all around this vessel).
Why does the CNS need a BBB?
- CNS must maintain an extremely stable internal fluid environment surrounding the neurons
- maintains reliable synaptic communication between nerve cells
- protective barrier shielding CNS from neurotoxic substances in blood
What macromolecule does the BBB also prevent from entering the brain?
proteins: content of CSF when compared to plasma is very low and markedly different from that of plasma
BBB is very selective
What molecules can cross the BBB passively?
- low MW
- neutral
- hydrophobic
(wide range of lipid soluble mols too)
What characteristics of a drug RESTRICT its entry into the CNS?
high PSA >80 A2
>6 HBDs
a number of rotatable bonds in molecule
MW >450 Da
high affinity to plasma proteins w low off-rate
Why does having >6 HBDs restrict entry into CNS?
increases the free energy requirements of moving from aqueous phase -> lipid of cell membrane
Which better penetrate the BBB: bases or acids? Why?
- bases
- positive charge means they can interact w negatively charged phospholipid head groups of the cell membrane
What are the 4 possible approaches for enhancing drug influx across the BBB?
Modification of the drug’s chemical structure
Disruption of the BBB
Drug solubilisation/encapsulation in nano or microparticles
Bypass the BBB
What are 2 strategies that bypass the BBB?
Convection Enhanced Delivery
Implantable drug delivery devices
What 3 techniques comprise the drug modification approach to overcome the BBB?
- lipophilic drug modification
- prodrugs
- vector-mediated drug delivery
What does lipophilicity correlate with?
CNS permeability
What is lipophilic drug modification?
modify drug w lipophilic moiety
What is the best application for lipophilic drug modification?
- proteins modified w fatty acid residues
- point modification w 1-2 fatty acid residues per protein molecule
- remains water soluble but also acquires lipophilic anchors that can target cell surfaces
What is: Pharmacologically active compounds that are chemically modified to be inactive until they are activated inside the target tissue by a single activating step?
prodrugs
What is: Pharmacologically active compounds that are chemically modified to be inactive until they are activated inside the target tissue by a single activating step?
prodrugs
What 3 deficient properties of a CNS drug could prodrugs intend to improve?
Membrane permeability
Stability
Water solubility
How do prodrugs for overcoming the BBB work?
- Water-soluble drugs attached to lipid-soluble carriers by cleavable bonds carried across BBB, released into brain
- released + processed prodrug then ‘trapped’ as it cannot exist BBB
How do we chemically modify drugs to make a CNS prodrug?
Esterification/ amidation of hydroxy-, amino-, and carboxylic acid containing drugs enhances lipid solubility thus transport to brain
simple example:
How was morphine formulated into a prodrug to improve its brain uptake?
- OH replaced w methyl increasing lipid solubility
- add another acidic group to molecule to increase brain uptake
- heroin (morphine prodrug) rapidly metabolised to 6-acetyl-morphine then to morphine, which interacts with opioid receptor
- morphine due to being more polar is ‘trapped’ as it cannot diffuse back across BBB
What is vector-mediated drug delivery?
Link a non-transportable drug with a vector to the BBB which act as molecular Trojan horses
What types of molecules is vector-mediated drug delivery used for?
proteins, DNA, small molecules, micro- and nanocarriers
Will stay in and slowly release drug over time
–The choice of a vector moiety and type of linker is crucial for success
What natural molecules can effectively pass the BBB and are hence used in vector-mediated drug delivery?
natural peptides:
- insulin
- transferrin
What are examples of insulin being used in vector-mediated drug delivery? What is its limitations?
- Conjugation of insulin with the anticancer drug, methotrexate, resulted in receptor mediated endocytosis of the conjugate to brain tumour cells
- insulin fragments also used for delivery of peptides but short half-life + hypoglycemic effect limits its suitability
no major sucess yet
What is are examples of transferrin being used in vector-mediated drug delivery? What is its limitation?
- conjugation to mutated diphtheria toxin resulted in increase in brain tumour response by reduction in tumour volume
- however limited use as receptors are saturated under physiologic conditions
What strategy is designed to overcome transferrin’s limitation? Give an example.
- antibody for transferrin’s receptors
- mab OX26 binds to receptor’s extracellular domain shown to pass BBB
- single IV injection conjugated to fibroblast GF produced 80% reduction in stroke volume in brain of rats with a significant improvement of neurological deficit
What is the most invasive approach to overcoming the BBB?
disruption of BBB
What is disruption of the BBB used in conjunction w?
systemic administration, there it’s an adjunct and efficacious as one
What is the nature of the side-effect profile of disrupting the BBB? What is the consequence of its use?
- Associated with a high risk of adverse effects
- Only used in extreme cases of rapidly growing high grade gliomas
What are the 3 techniques that comprise the disruption of the BBB approach?
- Osmotic disruption
- Biochemical disruption
- Ultrasound (main)
How does osmotic disruption work?
- Open BBB tight junctions using hypertonic solutions of mannitol, arabinose, lactamide, saline, urea etc
- Initiates endothelial cell shrinkage and opening of BBB tight junctions for a period of a few hours allowing drug entry
How is osmotic disruption done in clinical trials?
intracarotid injection of mannitol (into carotid artery)
By how much did the median survival of patients with primary CNS lymphoma improve w the use of osmotic disruption?
increased from 17.8 to 44.5 months
What is the additional benefit that was observed w osmotic disruption?
- 25% increase in the permeability of the tumour microvasculature
- Compared to a 10-fold increase in the permeability of normal brain endothelium
What observation is biochemical disruption based on?
some substances can selectively open only brain tumour capillaries leaving normal brain capillaries unaffected
What substances are used in biochemical disruption? 3
Vasoactive leukotrienes
Vasoactive amines
Cereport
What is Cereport?
a nonpeptide known as RMP-7, bradykinin B2 agonist
What was the effect of leukotrienes in biochemical disruption shown to be related to?
abundance of g-glutamyl transpeptidase (g-GTP) in normal capillaries and its decreased amount in tumours
What is the result of biochemical disruption?
-> reduction of enzymatic barrier in tumour endothelial cells, and in elevated effect of leukotrienes
What is the effect of Cereport mediated specifically through?
bradykinin B2 receptors (as it’s an agonist)
How fast is biochemical disruption and how quickly does it reverse?
- drug effects occur rapidly within mins of infusion initiation
- barrier restores 2-5 upon termination
How does using ultrasound to disrupt the BBB work?
Sonication of brain, applied in presence of ultrasound contrast agent injected IV
increases num of:
- vesicles,
- vacuoles fenestration,
- channel formation,
- reversible openings of the BBB tight junctions
Which area of the tumour is important to deliver drug to and why?
area of the brain immediately around the tumour boundary = where tumour cells infiltrate, often escaping detection, surgery, and therapeutic levels of antineoplastic agents
Have to debulk first to remove pressure
What are the risk factors associated w disruption of the BBB?
- Passage of plasma proteins into the brain
- Altered glucose uptake
- The expression of heat shock proteins
- Microembolism, and abnormal neuronal function
Disruption of the BBB often used as an adjunct therapy to ?
pharmacological treatments of CNS disorders is efficacious
Lecture 2…….
Strategies to overcome the BBB include?
Drug solubilisation/encapsulation in nano or microparticles
why do CNS drugs need a protective durg carrier?
due to their low hydrolytic activity (hide in carrier)
Subject to degradation by blood proteins or enzymes encountered in the BBB
Drug carriers can be targeted using brain-specific vector moieties, what type of transport will take place?
receptor-mediated
single unit of given drug carrier can incorporate many drug molecules, resulting in high payloads per single targeting moiety
2 benefits of this?
o Improved efficacy
o Low number of receptors required
what 2 carriers can you encapsulate drugs into, to cross the BBB?
liposomes
nanoparticles (similar)
why are drugs encapsulated into liposomes? 3
- Prolongs time of drug circulation in bloodstream
- Reduces adverse effects
- Enhances therapeutic effects of CNS agents
why are conventional liposomes not good for drug delivery to brain?
theyre cleared rapidly form circulation by reticuloendothelial system
what type of liposomes remain in circ with half life as long as 50h in humans and are therefore suitable for drug delivery to brain 😊?
PEG coated long circulating liposomes
how can we further increase liposomes encapsulated drugs circulation time?
Want to avoid immune system taking it out or through general PK
Adding immunoreactive moieties to PEG-modified liposomes can target them to the BBB. Maybe hang around longer
Efficient delivery of PEG-liposomes conjugated with transferrin to the cerebral endothelium was achieved in rats
what molecules used for CNS DD usually have a cross linked core and do not disassemble upon dilution in bloodstream
NPs
For efficient transcytosis across BBB, nanoparticles must be …
less than 100-200nm in size.
But if go smaller, have to put more NPs in, risk of tox?
give an example of nanoparticles that can reach the CNS by passing the BBB? and how does it work?
Poly (butyl)cyanoacrylate (PBCA) nanoparticles coated with Tween 80 have successfully delivered a wide range of drugs to the CNS
–Tween 80 promotes binding of apolipoprotein E to the surface of the particle, which assists transportation across the BBB by endocytosis
how can nanoparticles be used to overcome the BBB? MoA
Bypassing of the P-glycoprotein efflux system is suggested as a mechanism for enhanced transport of CNS drugs incorporated into the nanoparticles
Nanoparticles - where do they accumulate to more, and where to less?
penetrate deeper into brain tissue
–Accumulate more + localised to brain tissue
–less in capillary endothelial cells of BBB
what increases nanoparticles transportation? and how? (surfactant)
coated with polysorbate 80 -> elevated transport across BBB due to
–Increased permeability induced by surfactant
–Possibility of toxic effect
what is convection enhanced drug (CED) delivery used for?
to bypass the BBB and increase transport of therapeutics in and around brain tumors
in CED, how is the drug delivered?
through one to several catheters placed stereotactically directly within the tumour mass / around tumour or the resection cavity
A pump provides positive pressure and constant flow of the drug
what type of drugs in CED used to administer?
Most anti-glioblastoma multiforme (GBM) drugs effective in vitro do not cross the BBB.
anti-GBM drugs directly to tumour thus circumvents problem with BBB penetration
CED uses fine intracranial catheters with flow rates of…
0.1-10ul/min
− Depends on development of pressure-gradient.
- Infusate distributes directly into the brain extracellular space
− Tendency to follow anatomical boundaries.
− Catheter tip position v important!
how does CED differ to injection in terms of time in therapeutic window??
CED in there longer
whats the difference between injection and CED?
CED: homogenous, distributes over large vols regardless of size of therapeutic, and possible to control tissue conc of therapeutic
Injection: heterogenous, over small vols, dependant on size and source conc of therapeutic agent
Drugs of various types can be delivered using CED, such as?
–Chemotherapeutics
–Recombinant proteins
Catheters with one opening at their tips have primarily been used with CED
problem with CED?
-Problems with homogenous and reproducible distribution of the drugs
–Prone to reflux of the infusates
what limits the distribution vol even if infuion vol increased, in CED?
Reflux
–>Potential for toxic SE due to possible ventricular/ subacrachnoid space leakage at rates above 3 μl/min
Reducing infusion rate may reduce chance of reflux
better to deliver drugs at varying flow rates: >/= 5 μl/min to maximize tissue vol for drug delivery
Reflux-Preventing Catheters currently being investigated
Infusates been formulated as monodispersed NPs in solutions of 3% sucrose or 3–6% polyethene glycol
Liposomes and their contents been successfully delivered through X
formulation of infusates is an important parameter to consider in X
for which technique?
CED
what is implantable drug delivery used for?
Provide localised delivery of a chemotherapeutic agent directly into the resection cavity and the site of the cancer
main problem with implantable DDD such as the Gliadel wafer for recurring GBM?
Associated with severe side effects
–Risk Vs Beneft?
Issues with wound healing
–Infection
Suitable for patients where a near gross total resection is possible
–Difficult to treat deep-seated tumours
what is the gliadel wafer (implantable DDD)?
200mg disc-shaped, biodegradable wafer containing 3.85% w/w of chemo agent Carmustine
API + polymer dissolved in dichloromethane
Spray dried into microspheres varying in size from 1- 20μm
microspheres then compressed into wafers.
how does implantable drug delivery work?
Following the surgical resection of a primary brain tumour, up to 8 wafers implanted in resection cavity
Carmustine is released form them over a five-day period
The polymer matrix degrades over a period of 6-8 weeks
what are the advantages of implantable drug delivery devices?
provide localised drug dleivery
name the implantable DDD:
Small biodegradable rods 2mm diameter, 6mm long
–Implanted into resection margin
–Provide site specific controlled release of
chemotherapeutic drugs
–Directly into the tumour over a sustained
period of time
–Reduced systemic toxicity
ChemoSeed
