Schemi e spiegazioni di Molecular pharmacology

Decimo Ilaria

Molecular

Pharmacology

Molecular and Medical Biotechnology (LM9)

ANNA BACCI

2023/2024 Bacci Anna – University of Verona

LESSON 1 – 03/10/2023

1 P HARMACOLOGY

It studies drugs and their interactions with living organisms.

A drug is any substance or molecule that induces functional and specific changes in an organism through a

chemical or physical action, regardless of whether the resulting effect is beneficial or detrimental to the

health of the receiving organism. A drug is a substance used in the diagnosis, treatment, or prevention of

diseases or as a component of a medication.

At first drugs were medicinal herbs or minerals or animal substances with therapeutic activity. Before the

1970s, all marketed drugs contained molecules either obtained by chemical synthesis (still representing

most of the pharmacological compounds) or extracted from organs, tissues, and biological fluids of animals

(such as insulin, extracted from pigs) or humans (such as the growth hormone, obtained from the

hypophysis of dead individuals, or gonadotropins, purified from the urine of pregnant or postmenopausal

women). So, from then we are in biotechnological era that is also called biosimilar medicine: it is a medicine

containing macromolecules (proteins, glycoproteins, polysaccharides) made by or derived from living

organisms. But we can also have gene therapy or nanoparticles.

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Drug Name Drugs may be indicated by:

- the chemical name, which identifies the chemical and molecular composition.

- the generic or official name, meaning the name under which the drug is licensed by the

manufacturer and internationally identified, with the initial in lower case.

- the trade name or proprietary/brand name (patented) indicating the medicine containing the drug,

with the initial in large upper case.

The active ingredient is generally associated with inert components called excipient. The excipient should

not alter the pharmacological properties of the active ingredient but may modify its pharmacokinetics.

Receptor is the molecule whose function is modulated by interaction with the drug, the ligand. The ligand

can bind its receptor at orthosteric and/or allosteric sites. Occupation of orthosteric sites modifies the

receptor function, usually activating it; ligand occupation of allosteric sites does not activate/inhibit

receptor function, but modifies the receptor response to its physiological ligand.

Competition is the condition occurring when two drugs recognize the same binding site. Reciprocal

hindrance between two drugs reduces their apparent affinity for the binding site. If one of two drugs binds

irreversibly to the binding site, the interaction is called noncompetitive.

Agonist is a drug that increases the probability of a biological response by activating or stabilizing the

receptor active state. Antagonist is a drug that binds to a receptor and does not activate it but prevents the

agonist from interacting with the binding site. The inverse agonist reduces the probability of the receptor

(that is active also in absence of a ligand) to be in its active state.

Effectiveness is the magnitude of the effect generated.

Potency is the dose (or concentration) required to produce an effect of a given entity.

1 Bacci Anna – University of Verona

Selectivity At molecular level, it is the ability of a drug to interact with a limited number (preferably only

one) of macromolecules.

Desensitization is a reduction in the capacity of a receptor system to generate a response or to transduce a

signal upon prolonged exposure to an agonist.

Downregulation is a decrease in receptor number generally occurring upon prolonged activation by an

agonist.

Tolerance is the reduced ability of an organism to respond to repeated administrations of a drug. Tolerance

can be consequence of receptor desensitization and/or downregulation. It should not be confused with

“addiction.” A tolerance that develops rapidly is called tachyphylaxis. A drug can induce tolerance and

tachyphylaxis to another drug.

Sensibilization is a gradual increase in response following repeated drug administrations.

Addiction is the compulsive use of a psychoactive drug as a result of its repeated and continuous use.

Side Effects are effects of a drug different from those required for therapeutic purpose.

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Risk/Benefit Ratio It is the ratio between patient’s health risk (due to side or toxic effects) and beneficial

effects produced by a drug. It is a relative evaluation, often subjective, that must consider severity of the

treated disease, benefits expected, and quality of life as perceived by the treated patient.

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Therapeutic Index It is the ratio between the dose (or concentration) that can produce useful effects

(ED: Effective Dose) and the dose (or concentration) responsible for side/ toxic effects (e.g., toxic dose (TD)

or lethal dose (LD)).

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Meta‐analysis It is a statistical method to compare and combine results of different studies and

experiments with the aim of extrapolating statistically significant conclusions on a particular therapeutic

effect of a drug treatment.

Important data:

1958: Frederick Sanger and the nobel prize for the protein sequence of insulin.

2023: Katalin Karikò and Covid’s vaccine.

The therapeutic use of insulin is one of the most significant medical advances of the 20th century. The main

problems are the safety and standardization. The human insulin is the first biotech drug. The great

discovery is the transfer and cloning of insulin gene. It’s formed from two chain a (21 aa) and b (30 aa) held

together by two disulphide bridges. First, we have pre-pro-insulin (110 aa), then pre-insulin by cutting the

amino-terminal which enables the secretion of the protein. Then the pro-insulin by cutting the c-peptide

with a protease. So at the end we have the correct folding of the insulin that is active.

Synthesis of human recombinant insulin was produced by Genentech (deduced DNA from protein

sequence) and Biogen (cloned the human insulin sequence from a library). Of course, Genentech was the

first that produced it because is easier and faster. 2 Bacci Anna – University of Verona

2 P .

HARMACOKINETICS

It’s a field of pharmacology that deal with how the drug is transferred to the body: absorbtion, distribution

and elimination.

The drug needs to reach the target with the sufficient concentration to be effective. It’s important to

understand the quantity, the frequency and by which route.

The specificity is also important. Sometimes drug can be not so specific. Because if concentration is high, it

could have bad effect. →

The journey of the drug ADME:

❖ Absorption: a drug may occasionally be administered directly at the site of its action. However, in

general, the target organ is distant from the site of drug administration. To cover this long distance,

the drug must enter the systemic circulation. So the absorption is a process that allows the drug to

enter the bloodstream. If the drug is injected in an interstitial space (i.e., intramuscular or

subcutaneous administrations), the drug diffuses from the injection site and enters the capillary

vessels. For any other administration route (e.g., oral, rectal, sublingual, and transcutaneous), the

drug must also cross the epithelial barrier.

The rate of absorption depends on the route of administration.

❖ Distribution: once the drug has entered the organism and reached the bloodstream, it can diffuse

to the whole organism. But the distribution is not homogeneous, the drug initially reaches the

various body compartments with different velocities, mainly due to their different perfusion rates.

Later on, its local concentration in each tissue may significantly depart from plasma concentration

for drug different tropism.

❖ Metabolism: metabolic enzymatic processing is another route of drug elimination; it could happen

in the liver. These metabolic modifications change pharmacokinetic properties and/or efficacy of

most drugs.

❖ Excretion: about 4% of the whole plasma fluid is filtered every minute by renal glomeruli. All drug

molecules that are not reabsorbed along the nephron will be eliminated with the urine. So kidney is

the most important organ for drug elimination from the body. Some drugs are eliminated at the

level of the pulmonary alveoli or through other secretions such as sweat, milk, saliva, and bile.

2.1 F .

EATURES OF DRUG

2.1.1 Drug formulation

There are many different types of formulations or

medicinal preparation:

- pills, capsules, tablets

- liquid

- patch (cerotto)

- liposome/micelle.

The formulation influences the absorption and the effects of the drug. The best formulation depends on

the effects you want, such as higher or constant. 3 Bacci Anna – University of Verona

The concentration of the drug during the time can be modulated by the pharmaceutic formulations.

To improve the absorption is used SNAC (salcaprozate sodium), that is a synthetic N-acetylated aminoacid

derivate of salicylic acid. It was discovered as part of a screen to identify carrier-based permeation

enhancers that could “chaperone” poorly permeable payloads across the intestine. An absorption enhancer

helps facilitate semaglutide absorption in the stomach by increasing the local pH, which in turn leads to

imcreased drug solubility and protects against proteolytic degradation. The SNAC technology prevents

destruction of semaglutide in the stomach and facilitates transcellular absorption through the gastric

membrane enabling semaglutide to reach systemic circulation intact.

2.2 D :

RUG ADMINISTRATION ROUTE

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.

Routes of systemic administration are classified as enteral, if the site of absorption is in the digestive

apparatus, and parenteral, if anywhere else.

1- Oral

2- Mucous membranes

▪ rectal

▪ buccal or oral

3- Topical-transdermal

4- Inhaled

5- Injected

▪ Intravenous

▪ Intramuscolar

▪ Subcutaneous

▪ Intrathecal-spinal

▪ Intraperotoneal

When choosing the route of administration, we need to consider the physicochemical properties

(composition and hydrophobicity) of the drug and its therapeutic use. It’s necessary to consider the

concentration or volume that has to be administered and the type of organs target of the drug. But also

the time course of drug plasma concentration depends on the route of administration and the patient’s

compliance.

For example, highly hydrophilic drugs are usually poorly absorbed by the gastrointestinal apparatus and a

parenteral injection is preferred; by contrast, lipophilic drugs are very well absorbed even when applied

topically on the body surface (skin or mucosa). 4 Bacci Anna – University of Verona

The concentration of the drug in the blood is different for the type of administration route. By intravenous

I.V. injection may induce high plasma concentration and toxic effects, whereas the same dose administered

by intramuscular I.M. injection may result in lower peak values, no toxic effects, and longer action duration.

2.3 T HERAPEUTIC WINDOW

We need to know the concentration necessary to get the effects of the drug, that is called therapeutic

window. By using different administration route allows to avoid toxic plasma concentration od the drug

and to prolong therapeutic effects.

In addiction, if the absorption rate is too slow, drug plasma concentration may not reach the minimum level

within the therapeutic window.

The therapeutic window must be maintained by dosing.

We have a graph in which we consider time and drug concentration. During time this drug concentration

usually follows a decreasing curve.

Instead, in the blood the concentration of drug increases during the absorption phase, until the

achievement of the hematic pick. During the absorption the drug have to reach the target and it starts the

effects. Then the drug starts to be metabolized and excreted. This is the curve of different ADME phases.

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In clinical practice, it is often necessary to attain and maintain a given value of drug plasma concentration.

In this case, we need to set up a chronic therapy and predict the effect of each administration on the drug

plasma concentration produced by previous doses.

When the drug is in the blood needs to reach the target.

2.4 C ROSSING CELL MEMBRANES

In order to bind to its receptor and induce biological effects, a drug needs to diffuse from the site of

administration, enter general circulation, exit at capillary level, and, in some cases, enter the cells.

If we have an oral route, it’s passed through the mucosal epithelium in blood circulation and in capillaries

targeting the cell membranes. Also by direct injection of the drug in the blood pass through the capillaries

and arrives at the target cell membranes.

Three basic mechanisms sustain transcellular diffusion of a drug: passive diffusion, carrier‐mediated

transport (transmembrane channel), and endo‐/exocytotic events (receptor mediated endocytosis). In most

cases, passive diffusion governs the kinetics of drug transport across cell membranes. The membranes are

hydrophobic.

2.4.1 Passive diffusion across cell membranes

The capacity of a drug to cross cell membranes depends on its partition coefficient.

The matrix of cell membranes consists of lipids (the lipid tails of phospholipids), whereas cytoplasm and

extracellular spaces are aqueous solutions. In order to diffuse into a cell, a drug needs to be sufficiently

water soluble to stay in solution in the extra‐ and intracellular aqueous solutions but also liposoluble

enough to distribute in the lipidic environment of the membrane matrix.

The degree of hydro‐/lipophilicity of a compound can be determined by measuring its distribution in a

volume containing water and oil: the ratio between its concentrations in the oily and aqueous phases is

called oil–water partition coefficient (OWPC). When the OWPC is greater than 1, the compound is mostly

lipophilic; when it approaches 0 (less then 1), the compound is highly hydrophilic. Since drugs must interact

with both water and lipid environments, their OWPCs usually have intermediates values.

2.4.1.1 OWPC properties

The OWPC of a substance depends on its chemical and physical properties.

Charged compounds are fully hydrophilic. The presence of groups capable of forming hydrogen bonds with

water (carboxyl, alcohol, amine, aldehyde, and ketone moieties) also confers hydrophilicity.

Molecules with a very low OWPC are excluded from the lipid phase and therefore have a negligible ability

to penetrate and cross membranes. A hydrophilic drug filtered in the preurine is poorly reabsorbed along

the nephron and will be disposed of in the urine.

By contrast, molecules with a high OWPC can freely cross cell barriers: they can be completely absorbed in

the intestine, diffuse across barriers such as the blood–brain barrier (BBB), or even be absorbed through

the skin (transcutaneous route).

Finally, drugs with a particularly high OWPC do not easily diffuse across membranes because they tend to

accumulate within the lipid matric.

In general, metabolism produces more hydrophilic compounds, thus favoring renal elimination by

preventing passive tubular reabsorption. 6 Bacci Anna – University of Verona

Many drugs contain acidic or alkaline residues. Depending on solution pH, such drugs may be electrically

neutral or charged. For these drugs, OWPC depends on the surrounding pH, because the OWPC of their

ionized fraction is virtually null and only the nonionized fraction can enter the membrane lipid phase. The

nonionized fraction depends on the absolute difference between drug acidity constant (pKA) and pH of the

solution.

If we imagine having an oral administration the pH of gastric is 2.0, this means that if drug is acid, it’s not

ionized and can cross the membrane; but if the drug is basic it’s ionized and can’t cross the membrane.

➔ Only the not ionized drug can enter the membranes.

The property of the drug is described by dissociation constant, also called pKa. Based on the pKa we can

understand if the drug will pass the membrane in an acid or basic environment.

[][+] [] (−)

→ 10

Ka = and the negative decimal logarithm of Ka is indicated by pKA = =

[] [] +

Examples of aspirin or the Warfarin, a weak acid (pKa=3) that in a low pH such as stomach (gastric pH 1) the

neutral species (not ionized) predominates to the ionized, so the drug can pass trough the membrane into

the blood vessels. In the plasma the pH is 7.4 and predominates the ionised species.

The absorption depends on:

➢ the pharmaceutics formulation, that is important because of the dosing.

➢ Route of administration

➢ Liposolubility

➢ pH and dissociation constant

In particular in the brain it’s very difficult to deliver the drug, so we need a liposoluble drug.

2.4.2 Diffusion

The drug pass through the blood to the tissue, from the tissue to the blood and from the blood to the

organs of excretion (kidney) always by passive transport.

The passive transport depends on the nature of the membrane or the nature of the molecules.

Factors affecting passive diffusion across cell membrane:

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If we have a impermeable barrier no drugs can pass it. If we have a semipermeable barrier some molecules

of drug can pass through the membrane. But by having a permeable membrane the drug pass until we have

the equilibrium flux =0.

The rate of the drug on passing the membrane depends on:

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1- The magnitude of the gradient if the concentration of the two compartment is highly different

will have a fast passage of drugs.

2- The maintenance of the gradient (maintenance of the gradient depends on the blood supply). If we

have a tissue that is highly irrorates by capillaries the drug concentration will be drain very fast.

3- Thickness of semi-permeable membrane (in vivo thinner is the barrier, faster is the passage).

4- Extension of semi-permeable membrane (in vivo the larger is the membrane the faster pass the

drug through it).

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