01.01 Introduction to Pharmacology

Pharmacology is the study of all aspects relating to drugs or medicines. Some of us, use the term ‘drugs’ to cover medicines (prescription and non-prescription) and drugs of abuse (legal and illicit), and this is how I am going to use ‘drugs’ in this Unit. The popular press often uses the term ‘drugs’ to mean drugs of abuse, and this can be confusing. Some medicines can also be drugs of abuse, which further complicates the issue. For example, morphine is both a potent pain reliever and a drug of abuse.

There are a huge number of drugs available in Australia. Currently there are 27,000 drugs on the Australian Register of Therapeutic Goods (ARTG) and, of these, 3,500 are registered as prescription only.

Pharmacology can be divided into two parts; pharmacokinetics and pharmacodynamics.

Pharmacokinetics is ‘What the body does to the drug’. After administration, the drugs usually has to be absorbed into the body, may be distributed around the body, and/or metabolised, and will eventually be excreted/eliminated. Absorption (A), distribution (D), metabolism (M) and excretion/elimination (E), sometimes referred to as ADME, are all pharmacokinetic processes and they may be altered in disease states. This is important, as we most commonly use drugs in disease states.

Pharmacodynamics is ‘What the drug does to the body’. When we administer a drug to a person, we hope that the drug will have a beneficial effect. In order to do so, the drug will have to cause an alteration in a physiological, biochemical or pathological process. This alteration is called the mechanism of action or pharmacodynamics of the drug. It is the molecular mechanism of action of the drug, and the actions of the drug in the human body that follow from this mechanism of action.

Pharmacokinetics and pharmacodynamics are related, as shown in Figure 1.1.

Figure 1.1 Relationship between pharmacokinetics and pharmacodynamics (Copyright QUT, Sheila Doggrell)

Pharmacokinetics includes ADME; Absorption, Distribution, Metabolism and Elimination. After administration, drugs undergo absorption into the systemic circulation, this is the circulation that takes blood to most parts of the body, and will take the absorbed drugs with it. Drugs can be further distributed into tissues or fat, and may also be released back into the circulation from the tissues and fat. Drugs are often targeted to have actions at specific sites, and when the drug reaches this site, the process of pharmacodynamics starts. Pharmacodynamics is a change in the body system – hopefully for a beneficial effect. After having an effect, the drug often returns to the circulation, where it will be taken in the circulation to its site of metabolism, which is commonly the liver. After metabolism drugs are removed from the body by excretion, commonly from the kidney. Some drugs are not metabolised but are directly eliminated via the kidneys.

In addition to pharmacokinetics and pharmacodynamics, there are some other pharmac…..terms used in pharmacology. The first is pharmaceutics, which has some overlap with pharmacokinetics. Pharmaceutics is designing formulations of drugs suitable for administration to people. Pharmacogenetics is the genetic basis for variation in drug response. This variation can have a basis in pharmacokinetics or pharmacodynamics, and we will discuss examples of these later. Pharmacovigilance is the detection, assessment, understanding, and prevention of adverse drug effects and adverse drug interactions. Some of these adverse effects and adverse drug interactions are at the level of pharmacokinetics and some at the level of pharmacodynamics. Examples of pharmacovigilance will be discussed later.