Introduction: pharmaceutics

My name is Ian Larson, and I’m going to take you through the pharmaceutics part of this course. Pharmaceutics is about the science of different types of medicines and how medicines move into and around the body. In this course, you’re going to learn about both of these aspects. In this particular video, you’re going to start learning about how medicines move around the body in terms of the pharmacokinetics, that Dan mentioned earlier. Earlier, you heard from Dan about the relationship between drug concentration and the response that it causes. In most situations, there is a relationship between the concentration of drug at the site of action, for example, the receptor or enzyme, and the concentration of drug in blood.

For a small number of drugs, such as aspirin, the blood is the site of action, as they’re designed to affect blood itself. However, generally, we can use the drug concentration in blood as an indicator of the drug concentration at the site of action. This benefits us, as it is not easy to measure the drug concentration at the actual site of action. To this end, if we make a graph, we can plot on the y-axis the concentration of drug in blood and on the x-axis we can plot time. So, we have a graph that shows the concentration of drug in blood. From now on, we will refer to this as blood concentration as a function of time.

For a tablet, we can get a curve that looks like this. We see that the concentration initially increases due to the absorption of drug into our blood. For some medicines there is a distribution phase, where the drug distributes from the blood into tissue in the body. There is always an elimination phase, where the drug is eliminated from our body. Our body has recognised it as a foreign entity, and it is removed from our system. The minimum effective concentration is the minimum blood concentration required for the drug to be effective, that is to produce the desired response in a person. The shading indicates person-to-person variability. Everyone is different, and the minimum effective concentration is, therefore, an average value.

Different drugs behave differently in our bodies and can be eliminated at different rates. This is easily seen on graphs of blood concentration as a function of time during the elimination phase. To measure this difference in elimination, we mark the blood concentration at a particular time. We then mark the time at which the blood concentration drops by half. The time it takes the blood concentration to drop by half in the elimination phase is called the drug’s elimination half-life.

The duration of action is the period of time the blood concentration is above the minimum effective concentration and would, therefore, have a pharmacological response. A drug with a longer elimination half-life at a similar absorption phase, will have a longer duration of action.

A medicine needs to be retaken when the blood concentration drops below the minimum effective concentration and is, therefore, influenced by the drug’s elimination half-life. A longer elimination half-life will generally mean that the time between doses is longer. The minimum toxic concentration is the minimum blood concentration that produces toxic effects. As shown here, ideally we want the blood concentration of drug to always be below the minimum toxic concentration. Again, the shading indicates person-to-person variability and the minimum toxic concentration is an average value. If we include the minimum effective concentration and the minimum toxic concentration on the same graph, we can see that we have an ideal maximum and minimum drug concentration.

The term, therapeutic window, is often used to refer to the concentration span between the minimum effective concentration and the minimum toxic concentration. The larger the therapeutic window, the easier it is to use that drug without causing toxicity. So now you’ve seen that the drug concentration in blood is an indicator of how effective the medicine is, and you’ve also seen that how a drug behaves in the body influences how often medicines need to be retaken. In later weeks, you’ll also learn about how medicine design or formulation also influences how often medicines need to be retaken.