Pharmacodynamics:
The mechanisms of action of drugsA-Non receptor mechanism: Some of drugs act by non receptor mechanisms:
-Enzymes stimulation or inhibition ( MAO- inhibitors , ACE-inhibitors)
-Physical effect ( osmotic diuretic, bulk laxatives)
-Chemical effects ( Antacids)
-Local effects (counter irritants)
B- Receptors mechanism: Most of the drugs act by occupying specific receptors, which is a macromolecular component ( usually proteins) located on the l membrane, cytoplasm or nucleus of the cell.
Potency: It is the amount of drug required to produce a certain response
Efficacy: Maximal response that can be elicited by a drug
Agonist: An agent which activates a receptor to produce an effect similar to a that of the physiological signal molecule, e.g. Muscarine and Nicotine) Inverse agonist: an agent which activates receptors to produce an effect in the opposite direction to that of the agonist, e.g. DMCM Partial agonist: An agent which activates a receptor to produce submaximal effect but antagonizes the action of a full agonist, e.g. pentazocineAntagonist: an agent which prevents the action of an agonist on a receptor or the subsequent response, but does not have an effect of its own, e.g. atropine and muscarine
Antagonist could be:Physiologic (Functional) Antagonists Physiologic antagonists represent another type of antagonism in which the antagonist does not interact directly with the actions of the agonist at its molecular target. The agonist and antagonist each act on different molecular targets, but the responses elicited by these interactions are diametrically opposed and negate each other. Epinephrine and histamine are good examples of physiologic antagonists.Pharmacokinetic Antagonists One drug attenuates the action of another drug by decreasing its concentration at the site of action. This may occur through changes in absorption, distribution, metabolism, or excretion. An example is activated charcoal used in acute treatment of poisonings. Ingestion of activated charcoal binds drug in the intestine and reduces or prevents its absorption. Pharmacologic Antagonists The majority of antagonists used as drug therapy are pharmacologic antagonists that act by directly interfering with an agonist’s ability to activate its molecular target. The antagonist prevents agonist binding or agonist activation of the receptor and inhibits the biologic effects generated by the agonist.
The interaction between antagonist and agonist can take several forms, including competitive and noncompetitive antagonism.Competitive Antagonists: Compete with agonist for receptor binding => Agonist appears less potent, but can still achieve 100% effect (but at higher concentrations)Non-competitive Antagonists: Bind to receptor at different site and either prevent agonist binding or the agonist effect => maximal achievable response reduced
Dose Response relationshipThe exact relationship between the dose and the response depends on the biological object under observation and the drug employed.- If the concentration of the drug too low to produce the pharmacological effect, it means (no response) ie: sub-therapeutic dose.-The lowest concentration of a drug that elicits a response is minimal dose -The largest concentration after which further increase in concentration will not change the response is the maximal dose.
Major Receptor families: A) Ligand-gated ion channel B) G protein-coupled receptor C) Enzyme-linked receptors D) Intracellular receptorsA) Ligand-gated ion channel The most rapid cellular responses to receptor activation are mediated via ligand-gated ion channels. These kind of transmembrane receptors composed of multiple peptide subunits. The ligand binding causes conformational changes of receptor and opening of ion channel. Eg: nicotinic receptorsB) G protein-coupled receptor: It comprised of a single peptide that has seven membrane-spanning regions.- These receptors are linked to a G Protein (Gs and others) having three subunits, alpha (α) subunit (binds guanosine triphosphate GTP) and a beta-gamma (βY) subunit.- Binding of appropriate ligand to extracellular region of the receptor activates the G Protein, so that GTP replaces guanosine diphosphate GDP on the alpha subunit.-Dissociation of G Protein occurs, and both the alpha-GTP subunit and the βY subunit interact with other cellular effectors (an enzyme or an ion channel). - Effectors then change the concentration of the 2 nd messenger that are responsible for further actions within the cell.-Stimulation of these receptors results in responses last several seconds to minutes
C) Enzyme-linked receptors: These receptors have cytosolic enzyme activity as an integral component of their structure or function. - Binding of a ligand to an extracellular domain activates or inhibits this cytosolic enzyme activity. - Duration of responses to stimulation of these receptors is in order of "minutes to hours". Intracellular (cytoplasm or nucleus) receptors: Those receptors are not associated with cell membrane. Ligands are mostly lipid soluble and passively pass cell membrane. Ligand binding activates receptor the complex then translocates to nucleus and bind to specific DNA sequences mostly located in gene promoter region. This kind of signal tranduction is slow, but duration of response can last long. Eg: receptors for glucocorticoides and gonadal steroids.
There are two types of responses:1. Graded dose effect: As the dose administered to a single subject or tissue increases, the pharmacological response also increases in graded fashion up to ceiling effect. - It is used for characterization of the action of drugs. The concentration that is required to produce 50 % of the maximum effect is termed as EC50 or ED50. 2. Quantal dose effect ( All or none): It is all or none response, the sensitive objects give response to small doses of a drug while some will be resistant and need very large doses. The quantal doseeffect curve is often characterized by stating the median effective dose and the median lethal dose.
Median lethal dose or LD50: This is the dose (mg/kg), which would be expected to kill one half of a population of the same species and strain. Median effective dose or ED50: This is the dose (mg/kg), which produces a desiredresponse in 50 per cent of test population. Therapeutic index: It is an approximate assessment of the safety of the drug. It is the ratio of the median lethal dose and the median effective dose. Also called as therapeutic window or safety. LD50 Therapeutic index (T I) = ------- ED50 The larger the therapeutic index, the safer is the drug. Penicillin has a very high therapeutic index, while it is much smaller for the digitalis preparation.
Drug toleranceTolerance defined as a state of progressively decreased responsiveness to a drug as a result of which a larger dose of the drug is needed to achieve the effect originally obtained by a smaller doseThe sensitivity of the target cells is governed by genetic factors and adaptive changes by the body. Adaptive changes occur in response to the repeated exposure to a particular drug. The result is usually a loss of sensitivity to the drug. This decreased response is called tolerance.