Principles of drug action
The entire course of a drug’s action, from dose to effect, can be understood in three phases of action:
- Drug administration phase
- Pharmacokinetic phase
- Pharmacodynamic phase
The drug administration phase
The drug administration phase describes the method by which a drug dose is made available to the body. Its
- Drug dosage forms
- Routes of administration
Active and inactive components
All drugs contain both active and inactive components.
Active (or active principle) is the drug component which exerts the therapeutic action.
Inactive components are preservatives, propellants, stabilizers, pH modifiers, etc. Some side effects are
related to the inactive components, ex. bronchoconstriction might be seen with the use of MDIs containing propellants.
Drug dosage forms
Dosage form is the physical state of the drug in association with non-drug components such as the vehicle.
Examples are: Tablets, capsules, injectable solutions, aerosols, ointments, patches, etc.
The form in which a drug is available must be compatible with the route of administration.
Routes of administration
Route of administration is the portal of entry for the drug into the body. They have been divided into five
-Applicable to administration of drugs intended for absorption anywhere along the gastrointestinal tract.
-The most common is by mouth (oral), because it is convenient, painless and offers flexibility.
-The oral route requires the patient to be able to swallow and airway protective reflexes should be intact.
-It is commonly taken to mean injection of a drug.
-It includes three different options: intravenous (IV), intramuscular (IM) and subcutaneous (SC).
-It can supply long-term continuous delivery to the systemic circulation.
-Drugs can be given by inhalation for either a systemic effect (ex. anesthesia) or
a local effect in the lung (ex. aerosolized agents used in asthma).
-Drugs can be applied directly to the skin or the mucous membranes to produce a local effect (ex. nasal drops
containing an α agonist to relieve congestion).
The pharmacokinetic phase
The pharmacokinetic phase of drug action describes the time, course and disposition of a drug in the body,
based on its absorption, distribution, metabolism and elimination.
Process by which a drug moves across different barriers to become available for distribution.
Biovailability: Proportion of a drug that reaches the systemic circulation.
Distribution, metabolism and elimination
-To be effective at its desired site of action, a drug must have a certain concentration, ex. MIC for antibiotics.
-It is a group of processes by which drug molecules are biotransformed to inactive metabolites. The liver
is the principal organ for drug metabolism and its major enzyme system is the cytochrome P450 oxidase system.
-The primary site for drug excretion in the body is the kidney, either for drug metabolites produced by the
liver or for drugs which are not metabolized and are eliminated from the circulation enterely by the kidney.
Metabolism and elimination are important factors when considering alternative therapies, because liver or
kidney diseases may affect the clearance of a drug by these organs.
The pharmacodynamic phase
The pharmacodynamic phase describes the mechanisms of drug action by which a drug molecule exerts its effects
in the body.
Drugs having the greatest relevance for RT act through receptor proteins, although enzymes are important targets
for antibiotics and other drugs.
The matching of a drug molecule with a receptor or enzyme in the body is based on a structural similarity
between the drug and its binding site. This relationship is named Structure-activity relation (SAR). Similar drugs which structures
differ, bind to different receptors and exhibit critical differences in action, side effects, etc.
The Isoproterenol vs. Albuterol example
Response to a drug is proportional to the drug concentration. As drug concentration increases, the number
of occupied receptors increases and the drug effect also increases up to a maximal point: once all the receptors are
occupied, no further increase occurs.
Chemical antagonism: A direct, inactivating interaction between a drug and a biologic mediator.
Functional antagonism: Each one of two drugs produce an effect, and the two effects cancel out each other.
Competitive antagonism: A drug has affinity for a receptor, but no efficacy, and at the same time blocks the
active agonist from binding to the receptor.
Synergism: Two drugs act on a target organ by different mechanisms of action, and the effect of the drug pair
is greater than the sum of the separate effects.
Additivity: Two drugs act on the same receptors and the combined effect is the simple linear sum of both effects.
Potentiation: A special case of synergism in which one drug has no effect but can increase the activity of
the other drug.
Terms for drug responsiveness
Drug effect that is opposite to, unusual or no effect, compared with the usual predicted effect in an individual.
An allergic or immune-mediated reaction to a drug, which can be serious.
It describes a decreasing intensity of response to a drug over time.
It describes a rapid decrease in responsiveness to a drug.