Schemi e spiegazioni di Molecular pharmacology

Main Features of Cellular Barriers

1. Endothelium is a labile barrier for molecules at low molecular weight (60 kDa is the molecular cut-off and corresponds to the albumin molecular weight and diameter <10 nm). The small drug can go in and out by transcellular diffusion (liposoluble) and/or by using the pores.

2. Only the free molecules (not bound to any other protein) can go in and out from the capillaries.

3. Morphological and functional variations of the vessels (i.e. phlogosis) may affect the diffusion velocity from blood to the tissue.

4. The diffusion may be limited by the extension of the irroration: 50 capillaries/mm in the skin, 2000 in the myocardium tissue). Some changes in the morpho-functional organization of the capillary endothelium (e.g., inflammatory processes) may influence the velocity of drug diffusion from vascular bed to interstitial fluids at specific sites.

2.5.1 Capillary Endothelium

The endothelial cells are like the fibroblasts. The fibroblast can migrate because...

they need to assemble the vessels. We have a different way of assemblage. They could be continuous, fenestrated or sinusoid endothelium. - permeability + When we have sinusoid type, like in the liver or in the spleen we know that in these organs we have high permeability of the drug, so it flows faster inside the tissue, for the lack of a basal lamina and the endothelium is discontinuous. The plasma proteins can easily diffuse to hepatocytes. ➔ They have different permeabilities of capillarity beds, depending on the different tissues because of differences in endothelial cell thickness and pore number. The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. In brain, spinal cord and blood brain barrier (BBB) we have a different cell type. The endothelium of cerebral

capillaries is particularly impermeable and sheathed by astrocytic projections to constitute the so‐called BBB. The BBB limits the penetration of hydrophilic drug into the CNS. Drugs can reach CNS cells through two paths: from the blood or from the cerebrospinal fluid (CSF). Encephalic capillaries display very limited exo‐/endocytic and transcytotic activity, lack the large pores that characterize the normal capillary endothelium, and are almost entirely sheathed by glial cells.

In order to cross the BBB, a drug needs to be either actively transported (it should have some transporter) or lipophilic. Drugs with a high OWPC enter the CNS very rapidly, due to the high perfusion index of the encephalon. Lipophilic drugs tend to accumulate in the nervous tissue, especially in the white matter. On the other hand, hydrophilic drugs do not enter the CNS to a significant extent and may lack central effects unless they are directly administered in the CSF.

Main features of cellular barriers:

1. The BBB limits the penetration of hydrophilic drugs into the CNS.
2. Encephalic capillaries lack large pores and are sheathed by glial cells.
3. Drugs need to be actively transported or lipophilic to cross the BBB.
4. Drugs with a high OWPC enter the CNS rapidly.
5. Lipophilic drugs tend to accumulate in the white matter.
6. Hydrophilic drugs do not enter the CNS to a significant extent unless administered in the CSF.

1- The permeability properties of the capillary endothelium are different in different tissues, such as the liver and brain.

2- The blood-brain barrier (BBB) limits the access of hydrophilic drugs to the central nervous system (CNS), despite the high level of perfusion (0.5 ml/min/gr tissue).

3- The permeability properties of the capillary endothelium are modulated by specific physio-pathological conditions and drugs. For example, histamine can increase capillary permeability, and meningitis can also increase permeability. However, the choroid plexus and pituitary gland allow drugs to enter the CNS, even if they usually don't pass through the BBB (e.g., penicillin in meningitis).

4- The placental barrier is less impermeable compared to the BBB, and its permeability varies during gestation. In early pregnancy, it is about 25 um, while at the end of pregnancy, it is 2 um.

The BBB and placental barrier differ in several respects. First, the placental barrier is less tight than the BBB, and hydrophilic drugs can cross it, mainly limited by their molecular size. Second, maternal blood flows very

slowly in the placenta, allowing drugs more time to move into fetal circulation. Thus, placental transfer is negligible for those drugs that are hydrophilic and large in size and are rapidly cleared from the blood of the mother. ¹⁰ Around capillaries we have special cells called pericytes. Pericytes affect capillaries permeability. This did not allow any passage of drugs or molecules. They are important in the maintenance of the permeability of capillaries. Article "Blood-spinal cord barrier pericyte reductions contribute to increased capillary permeability" by Ethan A Winkler [source]. Not having the pericytes cause membrane permeability. In pathological conditions, the pericytes are detached from the vessels in some diseases like tumors. In this way, the capillaries increase the permeability. BBB: Blood Brain Barrier. In the schematic representation of BBB, there is the nervous tissue, in whichthere are different cell types(neurons, microglia, astrocytes,…). All of them contribute to the formation of BBB, that is the contact between the nerve tissue and the endothelium. The BBB is almost absent at the level of the choroid plexus and other CNS regions where CSF is usually formed. At these sites, hydrophilic drugs can interact with the exposed brain tissue. If we can inject directly a molecule in CSF (cerebrum-spinal fluid) we can have some permeability of the drug inside the brain parenchyma. This is called intrathecal route of administration (injection directly in CSF) through the spinal cord, in the subdural space. The dura mater, leptomeninges and the pia are the three layers that cover the brain. Inside the brain you have these ventricles, in which there’s ONLY the CSF. We can have an intraventricular injection. Inside the ventricles we released the drug. At this point we are in CSF. Why BBB is particularly impermeable? a. Endothelial cells have tight junctions b. They are

unsheathing by glial cells.

Limited exo/endo-cytotic activity

The lipophilic drugs penetrate the brain easily and they may also accumulate (high content of lipid membranes and myelin).

Passage from the blood to the brain: we can have a simple diffusion (very small molecules and lipophilic substances), a special transport protein (membrane proteins that allow membrane's passage) or transcytosis (molecule is bound to a receptor that internalized the molecule).

What affect the drug passage through the BBB?

1. Binding to plasma proteins, only the unbound fraction can diffuse across the BBB or in the liquor CSF.
2. Ionization, as the ionized fraction is excluded from CNS
3. Partition coefficient (OWPC) for the non-ionized fractions

In pathological conditions the permeability of BBB could be alter and look different.

In Alzheimer disease the presence of β-amyloid proteins, that can accumulate in the brain but also in the BBB, leads to leakiness of

membrane and increase the permeability of this barrier. Also cerebral ischemia with its inflammatory reaction or the diabetic retinopathy cause a major permeability of membrane. The placental barrier. It’s less tight barrier compares to BBB, so the hydrophilic drug can cross it, being limited by their molecular size. Every drug can reach the fetus during the gestation. Maternal blood flows very slowly in the placenta allowing drugs more times to move into fetal circulation. Equilibrium kinetics are slow in the placenta also because of binding to fetal plasma proteins. In other compartments, interstitial fluids are essentially devoid of proteins capable of binding the drug; therefore, drug concentration rapidly equilibrates with the level of free drug in the plasma. In the placenta, fetal plasma proteins bind part of the drug, thus slowing down the rise of free drug level toward the equilibrium with the maternal blood. In other words, plasma proteins increase the apparent distribution.

volume, decrease the flux/volume ratio, and therefore slow down the kinetics.

Barbiturates or acidic drugs can accumulate and cause persistent sedation.

If the drug is rapidly eliminated by the mother, therapeutic doses can be administered with no significant effect on the fetus.

SUMMERY.

• The pharmacokinetic properties of the drug are defined by temporal changes in its local concentrations at different sites, including the therapeutic target
• The plasma concentration of a drug is the final result of its absorption, distribution, metabolism and excretion (ADME)
• Drug formulation play a role in adsorption
• During its journey through the body a drug crosses several cellular barriers, which may differ in permeability and in the variety of active transport
• After entering the bloodstream drugs diffuse to all organs and tissue and during their journey they are metabolized (liver) and/or eliminated (kidney)
• Capability to cross the membrane is regulated by OWPC

molecular size (physicochemical properties)• Drug absorption, distribution, and elimination mostly depend on passive diffusion processes: in general, the net molar fluxes between compartments are proportional to concentration differences (first‐order kinetics), and the rates of equilibration between plasma and tissues depend on the flux‐to‐volume ratio (perfusion index). 12 Bacci Anna – University of Verona

2.6 D .RUG ABSORPTION AND DISTRIBUTION

The term “systemic administration” indicates that the drug is administered in a way that allows it to enter the systemic circulation and diffuse to all organs and tissues including the targets. There is a specific time before the drug reaches its site of action.

The drug reaches the target through the blood circulation. An oral drug arrives at the bloodstream through intestine and pass through the liver before reaching their site of action. In the liver the drug is transformed and breakdown. Drug is excreted by the kidneys.

urine is processed by the liver, transported in bile to the intestine, and excreted in faeces.

The non-oral drug travel bloodstream to the site of action without passing through the liver first.

2.6.1 Bioavailability

It's the percentage of the administrated dose that reaches the systemic circulation.

The dosage that reaches the blood could be different. Besides that, we know there is an intrinsic difference of the concentration we found in the blood. It is related to the properties of the substances (partition coefficient and drug dispersion) and the adsorbing tissue (extension and permeability of the surface, vascularization). It could be dependent on the company that produces the drugs (different excipient may show different absorption rate and/or bioavailability and also the formulations).

The route of administration:

• Enteral (oral and sublingual)
• Parenteral (intravenous IV, intramuscular IM, subcutaneous SC)
• Other (inhalation, intrathecal, topical, transdermal)