Brain Physiology & CNS 🧠

Question 1

CNNS

Answer 1

Brain (cranium)
Spinal cord (vertebral column)

3 protective layers of connective tissue cover brain & spinal cord – meninges:

Dura mater
Outer layer – tough, durable

Arachnoid mater
Middle layer

Pia mater
Inner layer next to brain

Question 2

Meningitis

Answer 2

Inflammation of the meninges

Bacterial meningitis – caused by meningococcal infection

When using a glass test, a non-blanchable rash may indicate sepsis associated with meningitis

Treatment:

Benzylpenicillin
–Allergy – Cefotaxime

Immediate hospitalisation

Question 3

Epidural pain relief

Answer 3

Administered into the epidural space of the spine e.g. labour.
–Bupivacaine HCl, Ropivacaine HCl.

Spinal cord extends 2/3 length of spinal column, with individual neurons in lower 1/3

Epidural pain relief & lumbar puncture (CSF) carried out in this lower region to avoid potential damage to spinal cord

Question 4

Brain Vasculature

Answer 4

Adequate blood supply is essential to maintain brain function

Disruption (mins) (eg clot/ small barrier) leads to tissue damage (stroke)

Brain has a vast capillary network (~400 miles, 20 m2 )

Brain capillaries form the blood-brain barrier which separates the CNS from the periphery

Capillaries contain specialised endothelial cells

Question 5

BBB vs capillary in general

Answer 5

Endothelial cells locked in by tight junctions
High levels of mitochondria as require high levels of mitochondria and ATP

Question 6

BBB

Answer 6

Intact BBB crucial:

•Maintains CNS homeostasis by restricting movement of water, ions, hormones etc from blood to brain

•Ensures composition of brain interstitial fluid is optimal for neuronal function e.g. [Na+], [K+], [Ca2+]

Permeability hard as:
BBB has high expression of efflux pumps and tight junctions between cells (paracellular transport hard).
Can only get the amount that made it to the blood

….

• BBB limits brain exposure to noxious circulating compounds e.g. drugs, and dietary and environmental xenobiotics and carcinogens.

Question 7

Transport across BBB

Answer 7

• Only some small hydrophilic ions/molecules can pass through tight junctions into brain (paracellular route)

• Other molecules must cross the BBB cells to enter brain (transcellular route)

Passive diffusion
•Some small lipophilic molecules may cross BBB by passive diffusion

•E.g. the general anaesthetic Propofol
–Propofol (IV) - onset of action 15-30 sec

In general, drug diffusion across the BBB is “aided” by:
•Low molecular weight (<400-500 Da)
•Optimal lipophilicity - ideal logP is 1.5-2.5
•Low polar surface area (60-90 Å2)
•Up to 5 H-bond donors and 10 H-bond acceptors

Question 8

Why is dopamine not administered?

Answer 8

Can’t pass BBB
2 hydroxyl groups (very polar) & an amine group ( hydrogen bond doner)

Question 9

EBL:
The main function of the blood brain barrier is to protect the brain from potentially harmful substances while allowing essential nutrients to pass through. As a result, drug delivery to the brain remains a complex and challenging task, and the process of developing CNS-active drugs is significantly longer and more expensive than development of peripherally-active drugs.

A. What features of the blood brain barrier could potentially impact delivery of therapeutic drugs to the CNS?

B. Identify some characteristics that a new drug in development should aim to possess to
potentially increase its chances of diffusing across the BBB.

Answer 9

A) features of the blood-brain barrier (BBB) could impact the delivery of therapeutic drugs to the CNS…
The BBB is a highly selective barrier that controls what gets into the brain, thus limiting drug penetration and hence making drug delivery challenging. Key features that impact drug penetration include:

Tight Junctions – Endothelial cells in CNS capillaries are joined by tight junctionswhich reduces paracellular transport of hydrophilic / large molecules. This means drugs must pass through cells (transcellularly) rather than around them and,prevents most large or water-soluble molecules from passing between i

No Fenestrations – Unlike other peripheral capillaries brain capillaries don’t have small pores (aka fenestrations), so drugs can’t pass through gaps in the vessel walls.

Efflux Transporters – The BBB expresses ATP-binding cassette (ABC) transporters, such as P-glycoproteinp, and multidrug resistance-associated proteins (MRPs), which actively pump many drugs out of the brain, reducing their CNS bioavailability..

Metabolic Enzymes – Enzymes like monoamine oxidase (MAO), cytochrome P450 (CYP450), and carboxylesterases are present in the endothelial cells of the BBB and can metabolise or degrade drugs before they reach their targets.

Selective Transport Systems – The brain has transporters that allow in essential nutrients (like glucose and amino acids), through carrier-mediated transporters, many therapeutic drugs do not have the necessary structure to utilise these pathways.

B) characteristics a drug can have to improve its ability to cross the BBB…
To increase CNS penetration, a drug should ideally have:

Small Molecular Weight (<400–500 Da) – Smaller molecules are more likely to cross the BBB via passive diffusion.

Moderate Lipophilicity (LogP 1.5–2.5) – The drug should be lipophilic enough to pass through the lipid bilayer of endothelial cells but not so lipophilic that it becomes overly retained in membranes or rapidly metabolised.

Low Polar Surface Area (PSA 60–90 Ų) – Molecules with a lower PSA have better permeability, as excessive polarity reduces membrane diffusion.

Limited Hydrogen Bonding (≤5 H-bond donors, ≤10 H-bond acceptors) – Too many hydrogen bonds increase water solubility, making it harder for the drug to cross the lipid-rich BBB.

Efflux Avoidance – Drugs should be designed to avoid recognition by P-gp, BCRP, and MRPs, as these transporters actively remove many compounds from the brain.

Metabolic Stability – The drug should be resistant to enzymatic degradation by CYP450, MAO, or esterases, which can break down molecules before they reach their site of action.

Use of Active Transport Mechanisms – If a drug cannot passively diffuse, it may be modified to resemble endogenous substrates that can enter the brain via carrier-mediated transport (CMT) or receptor-mediated transcytosis (RMT).

Nanomicro formulation

SUMMARY
Features of the BBB that Impact Drug Delivery

Tight Junctions:
Endothelial cells joined tightly; prevents paracellular diffusion.
Hydrophilic or large drugs must use transcellular (through-cell) routes.

No Fenestrations:
Brain capillaries lack fenestrae (pores), unlike systemic capillaries.
No passive diffusion through capillary gaps.

Efflux Transporters:
P-glycoprotein (P-gp), MRPs, and BCRP actively pump drugs out of CNS.
Reduces drug accumulation and therapeutic efficacy.

Metabolic Enzymes:
MAO, CYP450, carboxylesterases present in BBB endothelial cells.
Metabolize drugs before they reach brain tissue.

Selective Transport Systems:
Allows essential nutrients (e.g., glucose, amino acids) via carrier-mediated transport (CMT).
Most drugs lack suitable structure to use these pathways.

Drug Characteristics That Improve BBB Penetration

Small Molecular Weight:
<400–500 Da preferred for passive diffusion.

Moderate Lipophilicity:
Ideal LogP between 1.5–2.5 to cross lipid bilayer efficiently.

Low Polar Surface Area (PSA):
PSA between 60–90 Ų optimal for BBB permeability.

Limited Hydrogen Bonding:
≤5 H-bond donors, ≤10 H-bond acceptors.
Reduces water solubility, enhances lipid membrane diffusion.

Efflux Avoidance:
Structure should avoid P-gp/BCRP/MRP recognition to prevent CNS exclusion.

Metabolic Stability:
Should resist enzymatic degradation (e.g., by CYP450, MAO).

Use of Active Transport Mechanisms:
Modify drug to mimic endogenous ligands for CMT or receptor-mediated transcytosis (RMT).

Nanoparticle/Microcarrier Formulation:
Can enhance delivery via:
Receptor targeting
Surface PEGylation
Endocytosis-based uptake

Question 10

EBL: A. What is the difference between the incidence rate of a disease and its prevalence rate?
B. What are the incidence and prevalence rates of schizophrenia per 10,000 in the year 2021?
Continued

Question 11

Function of the Blood-Brain Barrier

Answer 11

Ensures homeostasis for neuronal activity.

Prevents toxic, inflammatory, or pathogenic substances from reaching neurons.

Allows precise selectivity of molecules (permits essential nutrients, blocks xenobiotics).

Protects CNS by controlling what enters and leaves the brain.

Question 12

BBB comprises the …

Answer 12

Special brain capillary construction

Direct drainage to bloodstream to/from the CSF

Active effluent transport in brain capillary and support cells

Destructive enzymes in the brain supports cells

Question 13

Key component of BBB: special capillary construction

Answer 13

1) Special capillary construction:
the brain has a complex vasculature
main arteries connect with carotid and vertebral veins return via the jugular
large vessels and sinuses occupy the sub-arachnoid
space
surrounding brain surface
from these the smaller capillaries supply the brain interior
capillaries leave main vessels in sub-arachnoid branch out into brain tissue
are extremely close to each other (40um)
bring nutrients & remove metabolites to brain tissue
How do capillaries interact with other brain cells?
an almost one-to-one relationship with neurones specialised glial cells (astrocytes)
plug into both neurones and capillaries to ensure
repal.
physical support
enrymas action that can destroy drug molecules
Efflux pumps to remove drug
Tight junctions between endothelial cells (no paracellular transport).
No fenestrations unlike systemic capillaries
the capillaries are unusual in construction closely tesellated endothelial cells
tight junctions in brain capillaries make very much smaller pores than in ordinary capillaries

The BBB threshold molecular weight is 500Da or below and must be lipophilic

The BBB consists if extra thickness in the endothelial cells provided by glial cells.

Ionised = polar = cannot transverse the endothelial membrane in brain capillaries

Question 14

Key component of BBB: Direct drainage to / from CSF

Question 15

Key component of BBB: Direct drainage to / from CSF

Question 16

Key component of BBB: active efflux transport ( AET) molecular pumps

Answer 16

present in:
these are membrane molecular pumps present in
the capillary luminal (inner) surface
the astrocyte foot processes
the CSF-carrying sub-arachnoid membranes

all have a functional protein in the membrane
Eg glycoprotein mechanism
Expels foreign molecules taken up by cells: returns drug to the capillaries, the venous sinuses

Question 17

⚠️ BBB Disruption in Disease

Answer 17

BBB hyperpearmability caused

Stroke (ischemic/hemorrhagic) → ↓ expression of tight junction proteins → BBB becomes leaky

Lower expression / localisation of proteins that form tight junctions

Alters rate & extent endothelial cells paracellular transport (through junctions)

Leads to:
↑ uptake of hydrophilic drugs
Potential CNS toxicity if unintended drugs reach brain

Clinical Implication: Stroke patients may require dose adjustments for BBB-impermeable drugs. May require decreased doses of hydrophilic drugs as these at now reach the brain and cause brain toxicity.

Question 18

BBB points

Answer 18

BBB maintain CNS homeostasis by controlling permeability

BBB comprises special brain capillary construction, CSF drainage stem, AET & metabolic enzymes; all acting as barriers against drug delivery

Lipophilic small drugs (<500Da) CAN normally reach brain cells via transcellular transport

Hydrophilic and/or large drugs (200-10,000Da) CANNOT travel paracellularly because of the tighter junctions

Little drug transport between CSF & brain, but more drug loss to the large venous sinuses

AET pumps drugs to capillaries & venous sinuses via active transport through transmembrane glycoprotein channels

Stroke can cause hyperpermeability! Effect on drug delivery & dose

Question 19

BBB: CSF dynamics & implications

Answer 19

CSF flows through:
Subarachnoid space
Ventricles
Hypothalamus

Drains to blood via arachnoid granulations

Drug exchange between CSF and:
Blood (more common)
Brain (only in specific areas: choroid plexus, 3rd/4th ventricles, hypothalamus)

CSF can lose drug to venous sinus → loss of delivered drug

Question 20

3 strategises for CNS drug entry

Answer 20

1. Improve Diffusion Across BBB
   Aim: Enhance passive diffusion.
   Key Properties (not hard rules):
   LogP: ~2–4 (too low: poor membrane crossing; too high: insoluble)
   LogD: 2-3
   Molecular weight: < 450Da
   pKa: 6-10.5
   Hydrogen bond donors: ideally 0 (≤3)
   Polar Surface Area (PSA): < 90 Ų recommended
   Note: These properties only increase probability of BBB penetration.

Example: second generation anti-histamines staying out the CNS to avoid causing drowsiness. Eg fexofenadine.
It is a hydrogen bond acceptor, polarity, polar surface area. Has carbpxylic acid. Too big (MW)
First-gen (e.g. diphenhydramine) enters brain → drowsiness.
Second-gen (e.g. fexofenadine) modified to increase polarity, size, H-bond donors (e.g., –OH, COOH) → excluded from brain.

2) Use of active transport
Transporters at CNS ensures brain gets chemical resources from circulation eg glucose, amino acids
Designing drugs that resemble;es these can be them transported into CNS
Example: L-DOPA/Levodopa for Parkinson’s
Depleted dopamine levels in this disease
Has 3 hydrogen bond doners
Does not diffuse easily but carried by amino acid transporter proteins. It is decarboxylated to give dopamine by aromatic-L-amino-acid decarboxylase Structurally mimics tyrosine (amino acid) → actively transported into brain.
Decarboxylated inside CNS → dopamine.

3) Metabolic Lock-In Strategy
If molecule is converted in the bran from a form that can diffuse, to one that can’t, it will be retained in the brain.
Example: Temozolomide (for glioblastoma):
Weak BBB diffusion.
Once inside, undergoes hydrolysis → highly polar metabolite → trapped in brain. LogP decreased further and HBD increased
Poor molecule but only available option, shows limited efficacy.

Question 21

Measurement challenges in CNS drug discovery

Answer 21

Brain Concentration Measurement
Difficulties:
High vascularisation → contamination by blood during tissue sampling.
Blood drug levels usually much higher than brain.
Binding to brain proteins eg complicates free drug estimation.
Standard Method:
Sacrifice animal → homogenise brain → buffer extraction → centrifugation → assay.

So homogenise brainthen add homogenate to buffer then agitate and centrifuge the analyse concentration in the supernatant.

One significant challenge is that the brain is a very highly perfused organ so there can be blood retained in the sample so the concentration may not be inside the brain matter itself
Measuring the fraction unbound in brain is a further challenge
Few reports with lots of measured values of concentrations in brain because research is ethically & financially challenging

Question 22

🔑 Exam Takeaways – Answering Strategy Questions

Answer 22

• Key medicinal chemistry strategies to get molecules into the CNS include:

Improving diffusion by changing the physical properties that influence it make sure you have a sense of what these are)

Designing molecules that hijack the active transport system

Designing molecules that get locked in by transformation from a relatively well-diffusing form to a relatively poorly-diffusing form

Analysing the concentration of drugs in animal brains is very challenging and ethical considerations must be prominent

Don’t just list properties (e.g. “increase logP”).

State the strategy goal first, then explain how:

Improve passive diffusion → change physicochemical properties.

Use active transport → mimic endogenous substrates.

Retention via metabolism → convert into non-diffusible form / slower diffusion out in CNS.

Avoid CNS entry → increase PSA, H-bond donors, MW, polarity.

Question 23

General drug discovery conclusions

Answer 23

• SSRIs like sertraline and fluoxetine block uptake of serotonin more than their effects on uptake of norepinephrine or dopamine

• Drug discovery efforts started from the structures of molecules known to reduce nervous activity

• Animal tests used during the development of the SSRIs have either no link or a weak link to any human condition

• Establishing efficacy in animal tests helps to ensure that toxicity is well understood which can allow investigators to proceed to the clinic more confidently

• Vigilance will always be required to ensure that highest standards of research good practice are enforced

Question 24

Hydrogen acceptors/ doners!!! Exam??

Answer 24

The number of nitrogen’s and oxygens is… and thats the number of

The number of OHs r NHs is the hydrogen bond doners. Also carboxylic acids (COOH)
An atom that has a hydrogen attached to it and can donate it in a hydrogen bond.
Look for:
–OH (hydroxyl groups) → 1 HBD each
–NH / –NH₂ (amine groups) → 1 HBD per hydrogen

🔷 What Counts as a Hydrogen Bond Donor (HBD)?
An atom that has a hydrogen attached to it and can donate it in a hydrogen bond.
Look for:
–OH (hydroxyl groups) → 1 HBD each
–NH / –NH₂ (amine groups) → 1 HBD per hydrogen
In Fexofenadine:

Left aromatic ring: has a –OH = 1 HBD
Right side: another –OH = 1 HBD
The carboxylic acid (–COOH): contributes 1 HBD via its OH group
🟰 Total HBDs = 3
Diphenhydramine has no –OH or –NH →
🟰 HBDs = 0

🔶 What Counts as a Hydrogen Bond Acceptor (HBA)?
Atoms like O or N that have available lone pairs to accept hydrogen bonds.
Includes:
Ethers (–O–)
Carbonyls (C=O)
Alcohols (–OH): the O is the acceptor
Amines (–N<): lone pair on N is an acceptor
In Diphenhydramine:

1 ether oxygen → 1 HBA
1 tertiary nitrogen → 1 HBA
🟰 Total HBAs = 2
In Fexofenadine:

2 hydroxyl oxygens = 2 HBAs
2 ether oxygens (in the middle chain) = 2 HBAs
1 carbonyl (C=O) in the carboxylic acid = 1 HBA
🟰 Total HBAs = 5
🔍 Which group is the COOH?
In Fexofenadine, the COOH (carboxylic acid) is the group on the far right, with:

A C=O double bond (HBA)
An –OH on the same carbon (HBD)
The other –OH groups are phenols (attached directly to aromatic rings).

Question 25

Transport through BBB: Transcytosis

Answer 25

Multiple specific transport systems placed on cytoplasmic membrane of endothelial cells permit selective molecules to actively cross BBB 2 types of transcytosis: receptor -mediated and adsorptive-mediated transcytosis 1) Receptor-mediated transcytosis • Macrmolecule binds & interacts with a specific receptor protein placed on the cellular membrane. • provokes endocytosis of the molecule receptor complex. The so-formed vesicle will cross the endothelial cell and then will be excreted at the opposite site with an exocytosis mechanism. • Many molecules use this to reach brain: transferrin, lactoferrin, insulin, glutathione, apolipoprotein E (apoE), & epidermal growth factor (EGF) Ligand-receptor interaction triggers vesicle transport (e.g., transferrin, insulin) 2) Adsorptive-mediated transcytosis Interaction of positive charges between macromolecule & endothelial membrane Negatively charged membrane interacts with positively charged macromolecules triggers transcytosis and subsequent exocytosis > Electrostatic interaction between cationic drugs and anionic membrane Examples: cat ionic proteins, heparin, cell-penetrating peptides, transportation, penetratin

Question 26

Transport through BBB: Active Efflux Transport (AET)

Answer 26

ATP-binding cassette (ABC) transporters Transmembrane channels spell the drug out the cells Brain protection: they prevent the assuage of endogenous compounds & xenobiotics which can be neurotoxic ABC sub-family G member 2 (ABCG2), multiple drug resistance protein 1 (MDR 1), multiple resistance protein 4 (MRP4), or ABC sub-family B member 1 (ABCB 1), better known as permeability glycoprotein (P-gp), are members of this family of proteins. Unfortunately, many drugs are substrates of these transporters. Active expulsion of drugs via ABC transporters (e.g., P-gp/MDR1)

Question 27

Drug Factors Affecting BBB penetration

Answer 27

Hydrogen bonding: Permeability decareses by one log of a magnitude for each pair of hydrogen bonds added to the molecule. The permeation of drugs through BBB more probable if less that 5 (3?) hydrogen bond donersexpresssed as the number of amine and hydroxyl groups. Molecular weight: Lipophilic drugs, molecular weight lower than 400-500 Da can cross the endothelial cells through passive diffusion.

Question 28

Strategies to deliver hydrophilic drugs: conventional

Answer 28

Reduce H-bonding groups (eg hydroxyl, amine) to improve lipophilicity Caution: May affect receptor binding & pharmacological activity. Conjugate with lipid carriers like acyl chains: atttach lipophilic groups to enhance permeability. Creating a lipid soluble prodrug (modifying drug action until metabolised to active) Risks: Increased systemic distribution & side effects as increased rapid permeation to other tissues from bloodstream (i.e. off-side side effect) May: Molecular weight ↑ → cellular uptake ↓. Therefore, they LOWER the likelihood of drug reaching brain

Question 29

Strategies to deliver hydrophilic drugs: unconventional

Answer 29

Disrupt BBB transiently: drugs that opens tight junctions. Dangerous as brain exposed to toxins etc Examples: hyperosmolar mannitol (IV) , vasoactives ( eg bradykinin) selectively in the tumour Transient effect, variable, associated wit sever hypotension Blocking AET: Modify drug to avoid recognition by ABC transporters. Pump (AET) inhibitors or alternative blockade could be administered as either co-drugs or as prodrugs this proved useful for azidothymidine (AZT, zidovudine) delivery for management of CNS symptoms of HIV-AIDS AZT in the bloodstream can enter the CSF via the choroid plexus Remember this transit point in the brain ventricles However, the CSF rapidly loses AZT again by being flushed out via an efflux system (pump) into the peripheral blood in the venous sinus via the sub-arachnoio granulations Example: AZT–UDCA conjugate avoids efflux, converted intracellularly to active AZT. Blockade needed to extend activity of drug in brain: conjugate AZT to a second molecule to produce a prodrug eg by conjugation with ursodeoxyxholic acid (UDCA), conjugate not recognised by the AET pump, giving prodrug time to release AZT by metabolic enzymes and to act in situ to control HIV effectively. Invasive routes (ICV injection & IC implants): a. Intracerebroventricular (ICV) Injection: Drug injected into CSF via ventricles. Poor parenchymal diffusion, rapid CSF clearance due to catabolic enzymes & efflux pumps into blood vessels poor method for parenchymal delivery Synapses protected by lipid glial sheaths —extra barrier lining ventricle? Catabolic enzymes here Efflux pumps into blood vessels (AET) Diffusion itself is very,very slow, especially for large molecules CFS forms effective sink condition CSF bulk flow is MUCH faster so, CSF clears rapidly to bloodstream via venous sinuses delivering drug elsewhere before brain diffusion effective -so movement from CSF to blood is much greater than from CFS deep into tissue Used for diagnostic imaging, not drug delivery. b. Intracerebral Implants: Intracerebral (IC) implants into the parenchyma Used for: genetically altered cells reservoir devices matrix devices e.g. Gliadel (carmustine wafer): Implanted after brain tumour resection to prevent recurrence (localised delivery of carmustine) Implanted wafers (e.g., Gliadel®) release carmustine after tumour resection (glioblastoma). Local delivery targets residual tumour cells. Risks of implants (irritates tissue): Seizures, brain edema, CSF leakage, implant migration into ventricular system, impaired IC wound healing Nanocarriers:

Question 30

Most efficient strategy to deliver small hydrophilic drugs: exploiting specialised transport systems

Answer 30

Nutrients took into the brain Specialised transporter receptors for small hydrophilic Neutral amino acids taken into brain by LAT1 transporter This two-way route has receptors on brain and blood sides Other transporters. GLUT glucose transporter (two way) CAT1 the cationic amino acid stransporter (two way) MCT 1 lactate transporter (two way) CNT2 adenosine transporter (effectively one way) STRATEGIES: Synthesise drugs that have aspects of the nutrient a drug or prodrug (e.g. L-dopa, gabapentin, methy-Dopa through LAT1 etc…) Conjugate the nutrient to the drug Example Systems: L-Dopa: Prodrug, LAT1 substrate, converted to dopamine in brain. Immunoliposomes: Liposomes + OX26 mAb actively target glioma via TFR.

Question 31

Strategies to deliver large hydrophilics: Receptor mediated transcytosis

Answer 31

Involves conjugating the hydrophilic (and possibly large) drug to a large carrier molecule that can target the receptor and fools the receptor into allowing these large constructs in; a 'Trojan Horse' strategy One commonly used approach is mainly related to the transferrin receptor (TfR): Highly expressed by brain capillary endothelial cells (that form the blood brain barrier (BBB)) & is therefore considered as a potential target for brain drug delivery. Monoclonal antibodies binding to the TfR, such as Ox26 have been shown to internalize into the endothelial cells in vivo. Therapeutic agents (radioactive, nonsense genetic material, cytotoxics ...etc.) can be piggybacked into the cells & from there into the brain tissue. Eg nanocarrier with drug conjugated with ligand (transferrin) that binds to TfR Enhancing cellular uptake Vital peptides trigger uptake in the brain by endothelial receptors (some one-way) e.g. peptides like transferrin (TfR), insulin (IR), growth factor (EGFR) -these are the Trojan horse structures Conjugated to the drug by a weak cleavable covalent bond forms 'chimera or fusion peptide' (the drug is the hidden army(1) Drug released by enzyme action in the brain parenchyma (2) to find its OWN specific receptor (3) Next strategy: for cancer Nanocarriers: Targeted delivery using nano immunoliposomes: passive targetting TR receptors can be fooled by OX26 monoclonal antibodies (Ox26 mab) Ox26 selectively bonds to transferrin receptors in cancer cells inside the brain, Ox26 decorates the liposome, hence called 'immunoliposome' the antibody is flexibly attached to a drug-carrying liposome the whole thing is internalised on receptor contact (receptor-medicated transcytosis) this is even more like a "trojan horse", as the drug is hidden in the liposome the liposome can carry cytotoxic and imaging markers, useful for targeted delivery to brain cancer (glioma) in this case the transferrin is replaced by the Ox26 mAb, which selectively binds to the transferrin receptor in endothelium liposomes have crossed the wall of the vessels and have entered glioma cells having found transferrin receptors Cancer vascularature: blood vessels ruptured, are very leaky, many gaps and not intact = passive targeting. Nanocarrier without any targeting ligand will still be taken up by cancer not healthy tissue. This is called enhanced permeation & retention (EPR) Enhanced permeation due to: angiogenesis, incomplete formation of vessel walls & basement membrane, gaps & defects in endothelium Enhanced retention due to: deficient lymphatic drainage Passive & Active Targeting in Cancer EPR Effect (Enhanced Permeation & Retention): Tumor vasculature is leaky → allows nanoparticle uptake. Poor lymphatic drainage → enhanced retention. Combination Example: Liposome (passive) + OX26 (active) → dual targeting of glioma. Strategies to target cancer: nano immunoliposomes (passive & also selective targeting with Ox26, which is a monoclonal antibody that selectively bind to transferrin receptors in giloma cells (brain cancer)

Question 32

Nose to brain route

Answer 32

———————————————————————— Rationale: Bypasses BBB using olfactory/trigeminal nerve pathways. Routes: Systemic: Absorbed by nasal blood vessels → re-encounters BBB. Pulmonary: Inhaled → lungs → systemic circulation → BBB. Neurological (desired): Direct transport via olfactory neurons or CSF route. Transcellular, paracellular, or RMT across olfactory epithelium. Key Design Features: Use mucoadhesive materials (e.g., chitosan) to prolong contact & reduce clearance. Include vasoconstrictors to minimize systemic absorption. Use nano-carriers: Biodegradable, biocompatible (e.g., PLGA, PEG, chitosan). Small size ↑ permeation. Surface ligands (e.g., lactoferrin) for active targeting.

Question 33

Summary of strategies

Answer 33

Chemical modification Improve lipophilicity Risk of off-target effects, receptor mismatch. Increases drug lipophilicity by reducing hydrogen bonding groups. Enhances passive diffusion across the BBB. Challenge: Risk of losing essential receptor binding; can affect pharmacological activity. Lipid conjugates/prodrugs, Increase BBB permeability, challenge: ↑ MW, systemic distribution. Attach lipophilic molecules to drug structure. Improves membrane permeability and can form prodrugs cleaved inside brain cells. Challenges: Increases molecular weight, reducing uptake. Promotes systemic distribution → potential off-target side effects. Efflux evasion Modify drugs to avoid recognition by efflux transporters (e.g. P-gp). Example: AZT–UDCA conjugate evades efflux, then releases active AZT intracellularly. Challenge: Few drugs successfully designed this way so far. Intracerebral implants Solid wafers (e.g., Gliadel®) implanted after tumor resection. Release chemotherapy (e.g., carmustine) locally into brain parenchyma. Challenges: Brain irritation (e.g., edema, seizures). Risk of implant migration, CSF leakage, and impaired healing. Receptor-mediated targeting (RMT) Use ligands or monoclonal antibodies to bind to BBB transporters (e.g., transferrin receptor). Example: OX26-conjugated immunoliposomes target glioma via transferrin receptor. Challenge: Requires specific receptor overexpression and ligand design. Passive targeting via EPR effect (tumors only) Tumor vasculature is leaky → allows entry of large nano-carriers. Retains carriers due to poor lymphatic drainage. Used for: Enhanced tumor drug delivery. Challenge: Does not apply to healthy brain vasculature. Nose-to-brain delivery Bypasses BBB using olfactory and trigeminal nerves. Delivers drug from nasal cavity directly to brain parenchyma. Formulation strategies: Use mucoadhesive polymers (e.g., chitosan). Add vasoconstrictors to limit systemic absorption. Apply biodegradable, non-toxic nano-carriers with or without targeting ligands. Challenge: Still experimental, with limited clinical evidence.

Question 34

Main CNS challenges

Answer 34

Getting the drug into the brain (BBB). Knowing if it's working as intended. Human CNS = uniquely complex and poorly understood. Psychiatric conditions (like depression) have: Multiple causes Variable presentation Poorly defined biological markers No reliable animal models for human psychiatric experiences (e.g., “depressed rats”). Core concepts • The drug discovery process is particularly challenging for drugs to arget the CNS; particular issues include the challenge of getting drugs across the blood-brain barrier and keeping them there and the need for animal models for efficacy that allow researchers to work out what the safety margins might be for their drugs before testing on humans in clinical trials. • There are lots of motivating factors for researchers in drug discovery that might cause them to make choices that can have negative consequences for patients - not only financial but also having invested a lot of time and energy in a particular concept. • Humans are complex creatures • Our CNS is poorly understood • There is a lot of scope for alternative interpretations • Pre-clinical safety and efficacy testing is required to obtain a licence for a clinical trial Animal testing is critical Animal models for many human conditions are challenging to create Conditions of the CNS are a particular problem Depression - is there a non-human equivalent?

Question 35

💊 Drug Discovery Process Overview

Answer 35

Target identification & molecule design Lead optimization (chemical tweaks to improve activity/selectivity) Preclinical testing (usually in animals) Clinical trials (Phase I-III in humans) Regulatory approval & marketing Key Point: After lead optimization, the same molecule goes through the entire pipeline. Medicinal chemists aim to streamline this to reduce cost and failure risk. Phase III trials = most expensive, but opportunity costs (delayed returns) mean earlier stages are costly too. Example: $1M spent early in the process = ~$24M in “opportunity cost” if delayed over ~20 years. 🌐 Alternative Drug Discovery Models Philanthropy and public/private initiatives for underfunded diseases: Bill & Melinda Gates Foundation: neglected tropical diseases. Medicines for Malaria Venture Cancer Research UK: large role in oncology trials.

Question 36

Helpful chemical structures

Answer 36

Aromatic rings Hydrocarbons Basic amines ( although polar) CHs

Question 37

EBL Find the structures and discuss common features of the following drugs and neurotransmitters: serotonin, fluoxetine, duloxetine, mirtazapine, haloperidol, clozapine, L-DOPA, dopamine and selegiline. Learning by doing to deduce common chemical strategies to achieve CNS penetration.

Answer 37

These structures share key features that influence their ability to cross the blood-brain barrier (BBB) and interact with CNS targets effectively. All structures include at least one aromatic ring, which plays a crucial role in receptor interactions and lipophilicity. Within fluoxetine, haloperidol, and clozapine, lipophilic halogens (F, Cl) (fluorine and chlorine) are present, increasing drug lipophilicity and enhancing CNS penetration. These aromatic rings and halogens facilitating passive diffusion into the BBB are beneficial when crossing this lipid rich environment, therefore promoting effective CNS activity. Dopamine has two hydroxyl (-OH) groups, making it too hydrophilic to cross the BBB. L-DOPA, which has an added carboxyl (-COOH) group, uses LAT1 transporters to reach the CNS. Drugs like L-DOPA, use active transport mechanisms instead of passive diffusion to enter the brain. Primary amines, though more hydrophilic, enable transporter-mediated uptake, ensuring efficient CNS delivery and receptor interactions (serotonin, dopamine, L-DOPA). Mirtazapine and clozapine contain bicyclic or tricyclic systems, enhancing receptor selectivity.The presence of fused rings improves binding at serotonin (5-HT), dopamine (D2), and adrenergic receptors. Low molecular weight Optimal Lipophilicity Low polar surface area Number of h bond donors and acceptors

Question 38

Ideal LogP

Answer 38

Ideal LogP for BBB: 1.5–2.5 The BBB is composed of lipophilic membranes, so drug molecules must be lipophilic to cross via passive diffusion. Too lipophilic (LogP >3–5): Drug may get trapped in the lipophilic membrane, reducing brain entry. Lipinski’s limit (LogP ≤5) is for oral drugs, not ideal for CNS delivery.