PHARMACODYNAMICS

Pharmacodynamics is the study of the physiological effects of drugs on the body or on microorganisms or parasites within or on the body and the mechanisms of drug action and the relationship between drug concentration and effect.

Effects on the body

The majority of drugs either

 (a) mimic or inhibit normal physiological/biochemical processes or inhibit pathological processes in animals    (b) inhibit vital processes of endo- or ectoparasites and microbial organisms.

There are 5 main drug actions:

depressing

stimulating

destroying cells (cytotoxicity)

irritation

replacing substancesDesired activity

The desired activity of a drug :

Cellular membrane disruption

Chemical reaction

Interaction with enzyme proteins

Interaction with structural proteins

Interaction with carrier proteins

Interaction with ion channels

Ligand binding to receptors:

Hormone receptors

Neuromodulator receptors

Neurotransmitter receptors

General anesthetics were once thought to work by disordering the neural membranes, thereby altering the Na+ influx. Antacids and chelating agents combine chemically in the body. Enzyme-substrate binding is a way to alter the production or metabolism of key endogenous chemicals, for example aspirin irreversibly inhibits the enzyme prostaglandin synthetase (cyclooxygenase) thereby preventing inflammatory response. Colchicine, a drug for gout, interferes with the function of the structural protein tubulin, while Digitalis, a drug still used in heart failure, inhibits the activity of the carrier molecule, Na-K-ATPase pump. The widest class of drugs act as ligands which bind to receptors which determine cellular effects. Upon drug binding, receptors can elicit their normal action (agonist), blocked action (antagonist), or even action opposite to normal (inverse agonist).In principle, a pharmacologist would aim for a target plasma concentration of the drug for a desired level of response. In reality, there are many factors affecting this goal. Pharmacokinetic factors determine peak concentrations, and concentrations cannot be maintained with absolute consistency because of metabolic breakdown and excretory clearance. Genetic factors may exist which would alter metabolism or drug action itself, and a patient's immediate status may also affect indicated dosage.

Undesirable effects

Undesirable effects of a drug

Increased probability of cell mutation (carcinogenic activity)

A multitude of simultaneous assorted actions which may be deleterious

Interaction (additive, multiplicative, or metabolic)

Induced physiological damage, or abnormal chronic conditions