Introduction to PD - Concentration-response relationship
Hi everyone,
Let’s continue this discussion by considering the concentration-response relationship.
Because, the drug-receptor interaction is a very specific one, the shape of the concentration response relationship is quite unique. Instead of a straight line, it assumes a S shaped curve. We call this a Sigmoidal curve.
It is also concentration dependent, but the concentration axis is logarithmic or exponential.
The shape of the curve makes the concentrations dependence quite unique. In the upper and lower concentration ranges, the drug response is quite insensitive to big changes in concentrations. In the uppermost range, we see that the response cannot go higher because it is already at its maximum.
In the middle of the sigmoidal curve, however, you will see that the response is quite linear in response to exponential changes in concentrations. This part of the curve is very important as many drugs are developed to exploit this straight part of the curve. This is the reason why many drug formulations are in doses that are doubling, for example – 5mg, 10mg, 20 mg 40 mg etc.
Consider the analgesics. Morphine operates largely in the middle part of the concentration response curve, so you can vary the dose over a fairly wide range to adjust the level of analgesia required.
A drug like paracetamol, however, operates near the maximum, so there is very little analgesic advantage in increasing the dose of paracetamol.
The affinity of the drug molecule for the receptor can be measured by determining the EC50, or the effective dose producing half the maximum response. The maximum response is termed the efficacy of the drug.
Comparing the EC50 of various drugs allows us to determine the relative potencies of these. If the EC50 and the curve is shift to the left, the potency is higher. Conversely, if the EC50 and the curve is shifted to the right, the potency is lower.
If the maximum response for a particular drug is lower than other drugs acting at the same receptor, we call this drug a partial agonist. Such partial agonists can also function as partial antagonists. The analgesic Pentazocine is a partial agonist at the opioid receptor.
So that brings us to the end of the video , and the end of the entire series of videos introducing you to the fundamental principles in Pharmacology. If you want to know more, there are several other videos in this channel that cover the topics in a bit more detail. If there is anything you would like to ask me, just leave a message.
All the best! Hope to see you all again soon! This is Edmund Lee signing off!
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