BBB 4 Flashcards
what are biologics and what are there general advantages in therapeutics?
biologic - drug with biological origins
e.g. peptides, nucleic-acid based compounds, cytokines, enzymes, recombinant proteins, monoclonal antibodies
advantages:
1) highly specific targeting of a single aspect of a complex disease
e.g. reducing amyloid peptide production, inhibiting neuronal death by apoptosis or glutumate toxicity, delivering supporting growth factors
2) relatively few unwanted effects elsewhere in the body
compare the general characteristics of biologics vs small molecule drugs
size
- small molecules = low mw <1kDA
- biologics - large mw 1-200kDa
stability
- small molecules - stable
- biologics - unstable
structure
- small molecules - simple
- biologics - complex
specificity
- small molecules - non-specific
- biologics - specific
preferred route of administration
- small molecules - oral
- biologics - IV - invasive
permeability
- small mol - high
- biologics - low
major metabolising enzymes
- small mol - hepatic - oxidases, hydrolases, conjugating enzymes
- biologics - nucleases, peptidases
distribution
- small mol - via circulation - easily distributed
- biologics - via circulation + lymphatics - limited distribution
immunogenicity
- small mol - NO
- biologics - YES
disadvantages of biologics
very large molecules - roughly 150,000 Da
very small amounts get into brain <0.1%
why don’t most antibodies cross BBB - unsuccessful in treating CNS diseases?
large molecules 1-200 kDa
hydrophilic
no specific transporter
no mechanism to get them across BBB
- not right charge for AMT
- no receptors for RMT
- too big for other mechanisms
describe the use of trojan antibodies to help them cross BBB
modifying the antibody in a way which fools the BBB into taking it up by RMT
trojan antibodies were raised against a receptor for RMT - so it can recognise and stick to receptor - allowing BBB to take it in
- insulin receptor (INR)
- transferrin receptor (TFR)
- increases amount of antibody in brain
- trojan antibodies are recognised as natural ligands by RMT receptors
why is using trojan antibodies alone not a usable therapy
1) not all RMT transport systems can be used
2) insulin receptors are not specific to brain - also in pancreas - can disrupt glucose control and cause organ damage
3) problem of antibody sticking to BBB capillary
- choice of RMT target is crucial to prevent unwanted effects
how can you optimise trojan antibodies?
overcome the problem of antibodies sticking to BBB capillaries
using transferrin receptor not insulin for RMT
combining a therapeutic antibody to the to the trojan antibodies - as trojan is just used to cross BBB
describe the problem of antibody sticking to capillaries and solution to overcome this
problem:
- Antibodies are capable of very high affinity binding so bind to receptor very tightly
- although they target BBB receptor well - due to high affinity for RMT receptor
- when it is bound on tightly - instead of being released to brain side after being carried by vesicles
- they didnt release and stayed in endocytosis vesicles - still stuck to receptor
- Ab builds up in capillary endothelial cell
solution:
- reduce the affinity of trojan antibodies
- reduce it just enough where trojan Ab can be released from transcytosis vesicle after transport across BBB and get into brain
- but not too much or will lose its affinity and not bind to the RMT receptor in the first place
optimised affinity - allows Ab to enter brain while still targeting BBB RMT receptor
give 3 ways to combine a therapeutic antibody to trojan antibody
1) bi-specific antibody
2) the brain shuttle
3) antibody transport vehicle (ATV)
describe how the bi-specific antibody works?
start with 2 diff antibodies:
1) one is trojan antibody - anti Tfr - targeting transferrin receptor
2) other is therapeutic antibody- anti-BACE1 enzyme antibody inhibiting amyloid production in brain - prime target in alzheimer’s
take half of trojan and therapy Abs
- one light chain + one heavy chain of both and conjugate them
anti-BACE1/TfR bispecific antibody - leads to more in brain and less amyloid production compared to using anti-BACE1 alone
describe how the brain shuttle works?
- bispecific Ab as a compound wasnt too stable
- another modification of it is brain shuttle - more stable
instead of conjugating half of trojan and therapy antibody
- take fab - antigen binding fragment of anti-tfr Ab and combine it with anti-amyloid therapy Ab - to create
- sFab - 1 antigen site
- dFab - 2 antigen sites + anti-amyloid Ab
single was better as dFab Ab got stuck in endocytosis vesicles - due to higher affinity - didn’t get into brain as much as sFab
sFab currently being trialed as TRONTINEMAB
describe the antibody transport vehicle (ATV)?
instead of using an anti-tfr antibody
- the protein binding structure that targets the tfr was engineered artificially into the bottom part of the therapy AB (anti-BACE1)
- smaller than having a large FAB region of trojan anti-tfr antibody
- so you end up with 1 antibody of normal size and no bits hanging off
ATV consists of:
- therapy Ab FABS + trojan FC heavy chain with Tfr binding site
this is a very flexible approach as you can add diff biologics to the Tfr FC
summarise the ways to optimise Ab delivery to brain and target CNS diseases?
1 - trojan antibodies - raise antibody against receptor for RMT
- anti TfR
- anti InR
2 - reduce affinity of Ab - so more can be released from vesicles and get into brain but not too low to hinder BBB targeting
3 - combine therapeutic Abs to trojan Abs
- bispecific antibodies
- brain shuttle
- ATV
