Lec.4 PHARMACODYNAMICS

Lec.4 PHARMACODYNAMICS
MACHANISM OF DRUG ACTION
Through Reception:
The drug interactions with receptors and alters the function of the cells by initiating the series of biochemical and physiological changes that are characteristic to the response to the drug.
Through simple chemical reaction:
The drug may act through chemical reactions such as the acidity of stomach is neutralized by taking a base without the help of any reception e.g. gastric antacids.
Through physical action:
Presence of a drug can produce response without the involvement of the drug and without chemical reaction e.g. mannitol as diuretic.
DRUG RECEPTORS
A drug receptor is a specialized target macromolecute, present on the cell surface or
intracellularly, that binds a drug and mediates its pharmacological actions. Drugs may interact with enzymes, nucleic acid or membrane receptors. Receptors are responsible for selectivity of drug actions of few drugs are not mediated by specific receptors, but depend on non- specific chemical or physical interactions e.g. anesthetic gases are thought to alter the structure of the membrane.
Affinity:
A drug which is able to fit into a particular receptor is said to have affinity for that receptor.
Intrinsic activity:
If a drug acts on a particular receptor and as a result elicits r response, it is said to have intrinsic activity.
AGONIST
• Drugs that bind to physiological receptors and mimic the effects of the endogenous regulatory compounds are termed agonists.
• The drug having affinity as well as intrinsic activity is called agonist(i.e. the combines with a particular receptor and produces response)
ANTAGONIST
• The drugs that bind to receptor and inhibit the action of agonist (e.g. by competition for agonist binding sites) are called antagonist.
• Antagonists do not directly alter the function of receptor; their effect is due to binding the agonist to the receptor and blocking its biological action.
• The drugs have affinity but no intrinsic activity.
Types of antagonist
Competitive antagonists
These agents interact with receptors at the same site as the agonist and thus compete for
binding of the agonist. In such condition concentration of an agonist required to produce a given effect is greater than the concentration required producing the same effect in the absence of the
antagonist i.e. it is a reversible condition.
Irreversible antagonists
The receptor antagonists who bind to the receptor in an irreversible fashion are called irreversible antagonists. The antagonist’s affinity for the receptor maybe so high that the, receptor is unavailable for binding of agonists, whatever the conc. Of agonist maybe.
Partial agonist
Partial agonists are those who block the agonist binding site but cause less response than a full agonist i.e. they have affinity for receptors but less intrinsic activity.
Other Mechanisms of Drug Antagonisms:
Not at all of the mechanisms of antagonism involve interaction of receptor with drug . Following are the example in which a drug acts as antagonism without the involvement of receptor.
Chemical antagonism
In chemical antagonism one during may antagonized the action of a second drug by binding to and intractivating the second drug e.g. Protamine counteract the effect of heparin.
Physiological antagonism
Actions produced by one drug can be antagonized by producing opposite actions by another drug on different receptor or without involvement of receptors is called physiological antagonism e.g.:
• Gulcocorticoids produce hyperglycemia, which is physiologically opposed by insulin.
• Effects of histamine are physiologically opposed by the adrenaline.
EFFICACY OF DRUG
Efficacy is the maximum effect of a drug. it depends on the number of drug-receptor complexes formed and the efficiency with which the activated receptors produces a cellular function.
POTENCY OF DRUG
The dose or concentration of a drug required to produce a given effect or the activity per unit weight of drug is called potency.
BIOLOGICAL VARIATION
The appearance of differences in the magnitude of response among individuals in the same
population given the same does of drug is called biological variation. Following are the example of the biological variations:
HYPERSENSITIVITY:
If a drug produces its usual effect at unexpectedly low dosage, the condition is called hypersensitivity.
IDIOSYNCRASY
The abnormal reactors to drug due to genetic abnormality are termed drug idiosyncrasy.
ALLERGY
It all can unusual response of a drug which is based on an immune mechanism, the symptoms are:
• Skin reactions e.g. urticaria, skin rashes angioneurotic edema and photosensitivity.
• Fever
• Asthmatic attack
• Anaphylactic shock
• Blood reaction (blood dyscrasias) e.g. thrombocytopenic purpura, aganulocytosis, a plastic enema and hemolytic reactions.
TOLERANCE
The failure of responsiveness to the usual does of a drug is called tolerance OR
If the effect of constant amount of drug decreases and larger does are required to elicit an equal response. This condition is called tolerance of drug e.g repeated use of morphine.
TAHCHYPHYLAXIS
Rapidly developed tolerance is called tachyphylaxis.
SOURCES OF DRUGS
Drugs are obtained from one of the following source:
1. Plant sources e.g. Quinine, atropine, and emetine.
2. Animal sources e.g. iodine, Mg, surface.
3. Microorganism: e.g. Penicillin.
4. Synthetic: e.g. Sulfonamides, aspirin, and barbiturates.
STUDY QUESTIONS
1. Following are the characteristic of a particular route of drug administration
a) L/V provides a rapid response
b) Oral route required that the patient be alert
c) I/M requires sterile technique
d) Subcutaneous route may cause local irritation
e) Inhalation provides slow access to the general circulation.
2. The first -pass effect occurs mostly after the following
a) Sublingual
b) Intravenous
c) Subcutaneous
d) Oral
e) Intramuscular
3. Bioavailability of a drug depend on its :
a) Its margin of safely
b) Half life
c) Clearance
d) First - pass manufacture
4. The Therapeutic Index of a drug indicates
a) Its margin of safely
b) Its potency
c) Its toxicity
d) The ration between the toxic and effective does
5. Phase II biotransformation or synthetic reaction include
a) Acetylation
b) Hydroxylation
c) Methylation
d) Oxidation
e) Glucuronide Conjugation
6. Factors affecting drug absorption are:
a) Solution
b) Degree of in ionization
c) Route of administration
d) Local blood flow
e) Surface area
7. Following are phase 1 or non- synthetic reaction of biotransformation.
a) Conjugation
b) Oxidation
c) Reduction
d) Hydrolysis
e) Methylation
8. Absorption of drug from GIT is influenced by :
a) Motility of gut
b) pH of stomach
c) presence of other substances in gut
d) Food
e) Stress
9. Drugs bound to plasma albumin are :
a) Biologically inactive
b) Can pass through the glomerulus’s
c) Usually can become unbound
d) Promptly metabolized
e) Can produce pharmacological actions
10. Combination of two or more drugs may result in:
a) Summation
b) Synergism
c) Protestation
d) Antagonism
e) Interference in absorption of other
ANSWER OF STUDY QUESTION
No. a b c d e
1 T T T T F
2 F F F T F
3 T F F T T
4 T F F F T
5 F F F F T
6 T T T T T
7 F T T T T
8 T T T T F
9 T F T F F
10 T T T T T