Molecular therapy
Drug targeting and delivery 1
Characteristics of classical drugs
- Small organic molecules.
- Lipophilic enough to cross membranes (compatible with oral delivery).
- Distribute freely through the body.
- Act by inhibiting enzymes/receptors.
Limitations of classical drugs
- Limited chemical diversity due to size and lipophilicity requirements.
- Risk of side effects:
- Insufficient specificity (cross-reactivity).
- Inhibition of essential pathways in non-target organs (e.g. methotrexate
inhibits thymidine biosynthesis).
- Poor inhibitors of protein–protein interactions:
- Classical molecules fit into small, deep binding pockets.
- Protein–protein interfaces are large and shallow, making small molecule
disruption inefficient.
Oligonucleotides can directly block or alter protein expression with high specificity.
Drug targeting
Drug targeting: enhance the concentration of the drug at it’s site of action.
Advantages:
- Avoid side effects.
- Reduce total dose.
- Reduce cost (especially oligonucleotides).
- Higher concentration for a longer period of time, protection from degradation and
excretion.
- Form more established drug molecules (e.g. doxorubicine).
- For a molecule that otherwise could not be a drug (e.g. siRNA to downregulate
protein expression) = drug delivery.
Dual challenge:
1. Stealth effect: nanoparticles must evade immune recognition.
2. Targeting: must find and reach the correct site.
Nanomedicine: The application of molecular assemblies (1 – 100 nm) in diagnostics and
therapy. 1 nm is relevant in the context of inorganic nanomedicine; an antibody is already 10
nm.
,Main components of nanomedicine:
- Surface modifications: stealth effect to prevent rapid clearance.
- Targeting ligands (e.g. antibodies).
- The matrix: Encapsulates and controls release (e.g. liposomes, polymers). Must
balance stability and release. The drug/matrix ratio should be as high as possible.
Polylactide-co-glycolide (PLGA): polymer matrix.
Lactic acid and a glycolic acid bound by an ester bond.
Due to this bond, PLGA dissolves over time in the body,
and the natural metabolites LA and GA are released.
This is used in disappearing stitches.
PLGA advantages
- Can be formed in a variety of different formats.
- Adjustable degradation kinetics.
- Degradation into non-toxic metabolizable compounds.
PLGA disadvantages
- Concern of for denaturation protein drugs (hydrophobic environment, acid release).
- Burst release.
- Loading very much drug dependent.
Delivery strategy
- Minimal functional design (targeting/uptake/release). Production must be easy and
reproducible. Every added functionality makes it more complex and expensive.
- No toxicity.
- Elimination of the targeting vehicle (secretion versus break-down, PLGA is a
break-down example, while pills are covered in non-metabolizable plastic, which is
secreted).
- Compatibility with the drug to be targeted. Highly hydrophobic drugs can not be
carried in hydrophilic matrix.
- Stability (breakdown and shelf life).
The delivery strategy is different for every single drug, as the matrix interacts differently with
each molecule.
,Barriers in drug delivery
The outside of the body is separated by an
epithelial layer, e.g. the mucosa. From the
bloodstream into the surrounding tissue is an
endothelial layer. They are different barriers
that sometimes require different delivery
methods. The characteristics of each barrier
are organ-specific. Depending on the route
of application/localization, different barriers
have to be crossed. The more barriers that
can be crossed, the more comfortable the
route of administration (ingestion vs injection).
The blood-brain barrier (BBB) can be crossed via injection directly into the spinal fluid.
Administration routes are an important aspect to consider as intravenous and intrathecal
injection requires healthcare professionals. Therefore, an important aspect of drug delivery is
creating medicine that can be self-administered.
Endothelia
The type of capillary endothelia is tissue-dependent.
The different types of endothelia are:
- Continuous: Skeletal muscle (containing many transport vesicles) and the BBB
(containing few vesicles). The cells are tightly bound to each other forming a strong
layer.
- Fenestrated: Endocrine glands, intestines, pancreas, glomeruli. There are
fenestrations in the endothelium (60-80 nm). Small particles can pass.
- Sinusoid: Bone marrow, lymph nodes, liver, spleen (30-40 um). The cells do not
have proper cell-cell contact and there is an incomplete basement membrane. Large
particles can pass. There organs for example remove bacteria. This is important for
passive targeting.
, Transport across cellular barriers
- Transcellular: Nanoparticles pass through the cells (endocytosis/transcytosis).
- Paracellular: Nanoparticles move between neighboring cells.
There are two main strategies:
1. Using detergents, which disrupt and destroy cells, causing inflammation.
2. Activating cellular transport mechanisms enables effective delivery without damaging
the barrier. In nanomedicine, safe transport requires non-destructive methods.
The classical transcellular approach is to induce endocytosis by conjugating the drug to a
metabolic cargo. For instance, linking a drug to vitamin B12 can promote uptake in the small
intestine. This is a form of transcellular transport known as transcytosis.
Paracellular transport, involves movement of molecules between cells at the tight
junctions. To enhance this pathway, intracellular signaling can be activated to loosen
protein–protein interactions, creating small gaps. However, these gaps remain narrower than
those formed during transcytosis, making paracellular transport more limited. It is particularly
restrictive for very hydrophilic or larger molecules. Therefore, this method is not suitable
for nanomedicine.
The BBB is formed by endothelial cells that line the capillaries of the nervous system.
Transport across the BBB often relies on ligands that use transcytosis, and several
strategies exist to achieve this.
One challenge is the presence of efflux transporters, such as P-glycoprotein. These
multidrug-resistance (MDR) proteins actively expel substances that have crossed the lipid
bilayer, reducing drug accumulation in the brain. To overcome this, drug delivery systems
must avoid or bypass these transporters. A common strategy to cross the BBB is receptor
mediated transcytosis. Nanoparticles equipped with ligands bind endothelial receptors and
are internalized by endocytosis, then released into the brain. the drug can enter the tissue or
cell, but MDR proteins like P-glycoprotein actively pump it back out before it can accumulate
to an effective concentration.
Drug targeting and delivery 1
Characteristics of classical drugs
- Small organic molecules.
- Lipophilic enough to cross membranes (compatible with oral delivery).
- Distribute freely through the body.
- Act by inhibiting enzymes/receptors.
Limitations of classical drugs
- Limited chemical diversity due to size and lipophilicity requirements.
- Risk of side effects:
- Insufficient specificity (cross-reactivity).
- Inhibition of essential pathways in non-target organs (e.g. methotrexate
inhibits thymidine biosynthesis).
- Poor inhibitors of protein–protein interactions:
- Classical molecules fit into small, deep binding pockets.
- Protein–protein interfaces are large and shallow, making small molecule
disruption inefficient.
Oligonucleotides can directly block or alter protein expression with high specificity.
Drug targeting
Drug targeting: enhance the concentration of the drug at it’s site of action.
Advantages:
- Avoid side effects.
- Reduce total dose.
- Reduce cost (especially oligonucleotides).
- Higher concentration for a longer period of time, protection from degradation and
excretion.
- Form more established drug molecules (e.g. doxorubicine).
- For a molecule that otherwise could not be a drug (e.g. siRNA to downregulate
protein expression) = drug delivery.
Dual challenge:
1. Stealth effect: nanoparticles must evade immune recognition.
2. Targeting: must find and reach the correct site.
Nanomedicine: The application of molecular assemblies (1 – 100 nm) in diagnostics and
therapy. 1 nm is relevant in the context of inorganic nanomedicine; an antibody is already 10
nm.
,Main components of nanomedicine:
- Surface modifications: stealth effect to prevent rapid clearance.
- Targeting ligands (e.g. antibodies).
- The matrix: Encapsulates and controls release (e.g. liposomes, polymers). Must
balance stability and release. The drug/matrix ratio should be as high as possible.
Polylactide-co-glycolide (PLGA): polymer matrix.
Lactic acid and a glycolic acid bound by an ester bond.
Due to this bond, PLGA dissolves over time in the body,
and the natural metabolites LA and GA are released.
This is used in disappearing stitches.
PLGA advantages
- Can be formed in a variety of different formats.
- Adjustable degradation kinetics.
- Degradation into non-toxic metabolizable compounds.
PLGA disadvantages
- Concern of for denaturation protein drugs (hydrophobic environment, acid release).
- Burst release.
- Loading very much drug dependent.
Delivery strategy
- Minimal functional design (targeting/uptake/release). Production must be easy and
reproducible. Every added functionality makes it more complex and expensive.
- No toxicity.
- Elimination of the targeting vehicle (secretion versus break-down, PLGA is a
break-down example, while pills are covered in non-metabolizable plastic, which is
secreted).
- Compatibility with the drug to be targeted. Highly hydrophobic drugs can not be
carried in hydrophilic matrix.
- Stability (breakdown and shelf life).
The delivery strategy is different for every single drug, as the matrix interacts differently with
each molecule.
,Barriers in drug delivery
The outside of the body is separated by an
epithelial layer, e.g. the mucosa. From the
bloodstream into the surrounding tissue is an
endothelial layer. They are different barriers
that sometimes require different delivery
methods. The characteristics of each barrier
are organ-specific. Depending on the route
of application/localization, different barriers
have to be crossed. The more barriers that
can be crossed, the more comfortable the
route of administration (ingestion vs injection).
The blood-brain barrier (BBB) can be crossed via injection directly into the spinal fluid.
Administration routes are an important aspect to consider as intravenous and intrathecal
injection requires healthcare professionals. Therefore, an important aspect of drug delivery is
creating medicine that can be self-administered.
Endothelia
The type of capillary endothelia is tissue-dependent.
The different types of endothelia are:
- Continuous: Skeletal muscle (containing many transport vesicles) and the BBB
(containing few vesicles). The cells are tightly bound to each other forming a strong
layer.
- Fenestrated: Endocrine glands, intestines, pancreas, glomeruli. There are
fenestrations in the endothelium (60-80 nm). Small particles can pass.
- Sinusoid: Bone marrow, lymph nodes, liver, spleen (30-40 um). The cells do not
have proper cell-cell contact and there is an incomplete basement membrane. Large
particles can pass. There organs for example remove bacteria. This is important for
passive targeting.
, Transport across cellular barriers
- Transcellular: Nanoparticles pass through the cells (endocytosis/transcytosis).
- Paracellular: Nanoparticles move between neighboring cells.
There are two main strategies:
1. Using detergents, which disrupt and destroy cells, causing inflammation.
2. Activating cellular transport mechanisms enables effective delivery without damaging
the barrier. In nanomedicine, safe transport requires non-destructive methods.
The classical transcellular approach is to induce endocytosis by conjugating the drug to a
metabolic cargo. For instance, linking a drug to vitamin B12 can promote uptake in the small
intestine. This is a form of transcellular transport known as transcytosis.
Paracellular transport, involves movement of molecules between cells at the tight
junctions. To enhance this pathway, intracellular signaling can be activated to loosen
protein–protein interactions, creating small gaps. However, these gaps remain narrower than
those formed during transcytosis, making paracellular transport more limited. It is particularly
restrictive for very hydrophilic or larger molecules. Therefore, this method is not suitable
for nanomedicine.
The BBB is formed by endothelial cells that line the capillaries of the nervous system.
Transport across the BBB often relies on ligands that use transcytosis, and several
strategies exist to achieve this.
One challenge is the presence of efflux transporters, such as P-glycoprotein. These
multidrug-resistance (MDR) proteins actively expel substances that have crossed the lipid
bilayer, reducing drug accumulation in the brain. To overcome this, drug delivery systems
must avoid or bypass these transporters. A common strategy to cross the BBB is receptor
mediated transcytosis. Nanoparticles equipped with ligands bind endothelial receptors and
are internalized by endocytosis, then released into the brain. the drug can enter the tissue or
cell, but MDR proteins like P-glycoprotein actively pump it back out before it can accumulate
to an effective concentration.
