CSB601 Chap.2 Pharmacokinetics and Pharmacodynamics
Pharmacokinetics and Pharmacodynamics
From dose to concentration
Pharmacokinetics tracks absorption, distribution, metabolism and excretion, and four parameters summarise that journey: clearance, bioavailability, volume of distribution and half-life.
Clearance fixes the steady-state level and therefore the maintenance dose, volume of distribution governs the peak after a single dose, and half-life links the two while setting how often doses are given and how long levels take to settle.
From concentration to effect
Pharmacodynamics explains what a drug does once it reaches its target.
Agonists activate receptors, antagonists block them, and partial agonists stop short of a full response. On a log concentration curve, potency is read from the EC50 and efficacy from the ceiling, a distinction the unit flags as high-yield. Patient factors such as kidney function, age and other medicines shift these relationships and justify dose individualisation.
What this chapter covers
- 01
ADME and the PK-PD Chain
- 02
Clearance and Maintenance Dose
- 03
Bioavailability and Volume of Distribution
- 04
Half-Life and Steady State
- 05
Agonists, Antagonists, Partial Agonists
- 06
Potency Versus Efficacy
Worked example · free
Timing the plateau when clearance halves
- 2Steady state takes about four to five half-lives, so the first patient settles after roughly 32 to 40 hours of regular dosing.
- 2Half-life reflects both clearance and volume; halving clearance with volume unchanged doubles it to 16 hours, so the second patient needs about 64 to 80 hours.
- 2Clearance determines the steady-state concentration, so the second plateau sits about twice as high; with a narrow therapeutic range, check the monograph for a reduced maintenance dose.
Key terms
- Bioavailability
- The fraction of an administered dose, between 0 and 1, that reaches the systemic circulation, with an intravenous dose defined as 1.
- Volume of Distribution
- An apparent volume describing how widely a drug spreads into body tissues, which governs the peak concentration after a dose.
- Clearance
- The volume of blood cleared of drug per unit of time, which determines the steady-state concentration and the maintenance dose.
- Steady State
- The plateau reached during regular dosing when the amount removed in each interval equals the amount given, after about four to five half-lives.
- Partial Agonist
- A drug that activates its receptor but produces a lower maximal response than a full agonist.
Pharmacokinetics and Pharmacodynamics FAQ
Do I have to calculate doses from half-life in this unit?
The pharmacokinetics lecture states that regimen calculations were not worked through and points students to drug monographs and dosing nomograms instead. Prepare to reason about direction and timing: which parameter changed, whether levels rise or fall, and roughly how many half-lives the change takes to settle. Simple multiplication of a half-life by four or five is the most arithmetic you should expect to use.
Why does the steady-state rule say four to five half-lives rather than one exact number?
Levels approach the plateau gradually, closing half the remaining gap with every half-life, so they never land on it at one precise moment. After four half-lives a regimen sits at roughly 94 per cent of its plateau and after five at about 97 per cent. Lecture slides show both a figure marked at about four and text stating four to five, so quote the range.
Is a more potent drug always the better choice?
No. Potency describes how little drug is needed to reach half of its own maximum effect, while efficacy describes the size of that maximum. A highly potent drug can still have a low ceiling, and a less potent drug may achieve a larger effect at a higher dose. Choice depends on the response needed, adverse effects and patient factors, not potency alone.
What does a very large volume of distribution suggest about a drug?
A value above about 100 litres suggests the drug is lipid soluble or binds strongly to particular tissues, so much of it sits outside the bloodstream. Because the figure is apparent rather than a real anatomical space, treat it as a clue about distribution. A larger volume also lowers the peak plasma concentration after a dose and tends to lengthen half-life.
Exam move
Start by drawing the dose to plasma concentration to site of action to effect chain from memory and labelling which half belongs to pharmacokinetics and which to pharmacodynamics. Next, build a four-row table for clearance, bioavailability, volume of distribution and half-life, writing for each what it controls and one patient factor that moves it.
Practise one-line cause and effect statements such as reduced clearance raising the plateau and lengthening the time to reach it, because closed-book multiple choice questions reward fast, confident direction calls. Sketch a log concentration response curve for two full agonists and one partial agonist, then explain potency and efficacy aloud using only the sketch.
Finish by looking up the pharmacokinetic properties of one familiar medicine in a product information document and classifying each statement under absorption, distribution, metabolism or excretion, which mirrors the workshop task and makes the vocabulary stick.
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