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MECHANISM OF DRUG ACTION

Mechanism of drug action simply means how the drug produces its effect on site of action.

In pharmacology, mechanism of action implies the certain biochemical reaction by which the drug produces certain effect on site of action.

The initial requirement for drug action is drug delivery to the site of action, as soon as the drug reaches the site of action it produces its action by certain mechanism that we will study further.

Generally drug is delivered to the site of action with the help of blood flow.

What you get below?

  1. Introduction to Mechanism of drug action
  2. Types of Drug action
  3. Drug Dosage
  4. Conclusion  
  5. Frequently Asked Questions (FAQs)

Introduction to Mechanism of drug action

Earlier, mechanism of drug action was given in physical and chemical manner but nowadays the drugs are acting on the site of action in various complex manners like enzymatic and receptor mediated, etc.

Only handful of drugs act through the simple physical and chemical properties, majority of drugs show their action by many complex manners.

Ultimately mechanism of drug action simply means how the drugs work? Or what is the process of acting of drug to the site of action?

Types of Drug action

Mechanism of action in pharmacodynamics

Broadly mechanism of drug action can be divided as Physical action, Chemical action, Through enzymes, Through receptors.

1) Physical action:

Mechanism of drug action is determined by its physical property.

Mass of the drugBulk laxative (Bran) and Ispaghula
Radio opacityBarium sulphate
Osmotic activityMannitol and magnesium sulphate
RadioactivityRadioactive atom of iodine (I-131)
Adsorptive propertyKaolin and charcoal
Absorption of  UV raysPara-amino benzoic acid

Mechanism of drug action of Bulk Laxatives:

They are polysaccharides forms a hardy mass in the intestine by absorbing water around it and blocks the lumen, this creates pressure and further peristalsis occur smoothly.

2) Chemical action:

Mechanism of drug action is as the drugs that react extracellular by chemical reaction.

Examples:

Neutralization of gastric HCl Antacids like Aluiminium hydroxide.
Alteration of pH of urineAcidifying agent like Ammonium chloride and alkalizing agent like Sodium biocarbonate.
Complexing agentsEDTA, BAL
Oxidizing agentPotassium permanganate, Iodine.
sequestration of bile acidsCholestyramine

3) Through Enzymes:

Enzyme substrate binding

Enzymes are the proteins which do not take part in chemical reaction but increase the rate of reaction, enzymes can be recovered after the reaction is completed.

Generally all the reactions in any living system are carried out with the help is enzyme mediated response; therefore enzymes play a very important role in drug action. Drugs can increase or decrease the rate of action.

The action is as, when the drug comes near the enzyme, it gets attached to the active site present on enzyme further the enzyme increases the rate and acts as biological catalyst, both drug and enzyme forms a complex, this complex is called as drug-enzyme complex.

Further after the action (product) the enzyme lives the drug and again enzyme recovers.

There are two sub-types of this mechanism:

1. Enzyme stimulation:

Enzyme stimulation by drugs is often less seen, but this is a physiological system of enzyme.

Drug are truly foreign substance which reacts with the enzyme. Many endogenous mediators and modulators get influence by the enzyme stimulation.

Like Pyridoxine acts by increasing decarboxylase activity and adenylyl cyclase is stimulated by adrenaline.

Stimulation of enzyme increases the affinity of substrate to get attached on the enzyme (Km decreases).

Apparent increase in the enzyme activity can also be seen during enzyme induction means synthesis of more enzyme protein (Km constant), this cannot be said enzyme stimulation.

2. Enzyme inhibition:

Types of enzymatic receptor: through enzymes

Inhibition of enzyme is more common mechanism of drug action as compared to enzyme stimulation.

1. Non-Specific Inhibition:

As the name indicates it is non-specific type of enzyme action, where the chemicals are capable of denaturating the proteins, they are not specific about proteins they just denaturate all the proteins coming in their way.

For example, alcohols denaturate all the proteins. The drugs alter the tertiary structure of the enzyme and also alter the function of enzyme and thus inhibit it.

Examples are heavy metals, phenols, acids and alkalis inhibit the enzyme nonspecifically.

2. Specific Inhibition:

As the name indicates it is specific type of enzyme action where the chemicals are capable of affecting the specific enzyme of which it has affinity to. They do not affect the other enzyme which it has no affinity.

Specific inhibition are basically divided into two sub-types and they are Competitive type and Non-competitive type.

(1) Competitive type:

Competitive inhibition is a drug competes with the normal substrate and forms a bond, gets attach to the affinity site.

For example, Physostigmine and Neostigmine competes with Ach to cholinesterase. Carbidopa and methydopa compete with each other for L-dopa. Organophosphate reacts covalently with the esteratic site of enzyme cholinesterase.

Here Km increases and Vmax either remains unchanged or decreases.

(2) Non-competitive type:

The inhibitor matter reacts with the enzyme on other site but not on the actual site or catalytic site and inhibits the structure of enzyme so that the substrate cannot attach on the catalytic site. This site is often called as allosteric site.

Here Km remains unchanged and Vmax decreases.

Examples:

                    Drugs                Enzymes
DisulfiramAldehyde dehydrogenase
AspirinCyclooxygenase
AcetazolamideCarbonic anhydrase
Digoxine or degitoxinSodium-potassium ion channel

4) Through Receptors:

We will study in detail about receptors in next blog but here we will study the basics and its terminologies along with its mechanism of action (drug action) in short.

Through receptors

Receptors are the macromolecules which provide a specific site for a drug to attach and carry out the function.

Receptors may be present on the cell surface or inside the cell. It provides the specific binding site for the drug.

In short, it provides the binding site for the substance or chemical molecule to carry out the further function.

There are certain terminologies that you must know before studying the receptors in detail:

Terminologies related to Receptors

1. Agonist: It attaches on the receptor site and produce action similar to that of signalling molecule.

2. Inverse agonist: It produces the effect exactly opposite of to that of agonist. It attaches on the receptor site and produce action opposite to that of signalling molecule.

3. Antagonist: Antagonist itself do not have any physiological action but acts as; it itself gets attached to the receptor and prevents the agonist to attach on the receptor site so the agonist cannot produce its effect.

4. Competitive antagonist: Antagonists which itself gets attached to the receptor competitively and prevents the agonist to attach on the receptor site so the agonist cannot produce its effect.

5. Partial agonist: It activates receptors to produce sub maximal effect but antagonise the effect of full agonist.

6. Ligand: Ligand is derived from a Latin word ligare meaning to bind. This is any molecule which attaches to receptor site. This do not specify the action whether it is agonist or antagonist.

7. Affinity: It is the ability and ease of molecule to gets attach with the receptors.

Drug Dosage

Dose is defined as the appropriate quantity of drug used to produce certain desired effect. Dose of drug is generally determined by quantifying the intensity of that is required.

For example, Dosage of aspirin varies according to the disease, for Anti-inflammatory dose of aspirin is 3-5mg and that of analgesic dose will be 0.3-0.6mg.

Dosage can be divided as Prophylactic dose, therapeutic dose and toxic dose on the basis of amount of drug or intensity of action produced.

Ultimately dose of drug is determined by the concentration of drug a physician desires to have on the target site to get desired effect.

If more concentration of drug is required at the site of action the dose should be more.

It also depends on pharmacokinetic property of the drug like its absorption, distribution and metabolism. If it metabolises very fast then the dosage should be increases accordingly.

Types of dose:

Standard dose: The dose is considered therapeutic for each individual meaning same dose will be applicable to most of the patient.

Regulated dose: Dose is determined on the basis of intensity of the particular disease.

Target level dose: Dose does not depend on the response but it depends on the concentration of drug at the site of action.

Titrated dose: The dose desired to produce the maximum therapeutic effect cannot be given because of intolerable adverse effects. Optimal dose is titrated with the acceptable adverse effect.

Conclusion

Here we have studied in detain about the mechanism of drug action, about how it acts? What are their consequences in its route? What are the dosage forms and many more.

The physical and chemical actions are bit old now, so the drugs which work on those mechanisms are less used now.

The enzyme mediated and receptor mediated drugs are preferred because they act better and more with more efficacy.

Majority of drugs act through the receptor mediated response. Receptor provides the site for natural material in the body and the drugs.

In detail we will study receptors in next blog as well. Agonists have good affinity and intrinsic activities, Antagonist have affinity but no intrinsic activity.

Frequently Asked Questions (FAQs)

1. What can affect drug action?

Drug action is affected by Age, sex, weight, Drug interaction, Pharmacodynamics, Pharmacokinetics, genetics, Drug tolerance, Dosage forms, etc

2. How to prolong the drug action?

Action of drug can be prolonged by several ways as if the drug gets metabolized earlier, then we can use its pro-drug.
If a drug disintegrates in the stomach by the affect of gastric juice, the enteric coated drugs can be used, enteric coated drugs are resistant to the acidic environment. There are many other ways by which action can be prolonged, as according to the challenges the action can be prolonged accordingly.

3. What is drug action as per enzyme?

Drug action as enzymatic, the enzyme provides the active site on which drug gets attached and it produces action.

4. Drug action vs. drug effect?

Drug action and drug effect are one and the same. But in detailed discussion the basic difference is as; Drug action is physicochemical action which is inside the body but Drug effect can be seen on the patient’s body externally.

5. Drug action vs. drug interaction?

Drug action is an action produced on the site of action but drug interaction can be broadly said that drug interacts with the biological substance or on other drug and produce undesired effect.

6. What is drug action of paracetamol?

Paracetamol acts on CNS as the analgesic but the primary action is inhibition of prostaglandin synthesis.

7. What are the factors affecting drug action in CNS?

The main factor which influences the drug action in CNS or in brain is its lipid solubility along with water solubility. Lipid solubility because it has to cross the blood brain barrier (BBB) and water solubility because it has to first get solublized in biological fluid.

Grateful to Share My World with You.

Gaurav Gururaj Shanbhag
Grateful to Share My World with You.
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