Pharmacodynamics
Introduced| English | Meaning |
|---|---|
| potency | The amount of drug needed for a given effect; a more potent drug needs less. |
| efficacy/ˈefɪkəsi/ | efficacy |
| partial agonist | partial agonist |
| competitive antagonist | competitive antagonist |
| therapeutic index | The ratio of toxic to effective dose; a small index means a narrow safety margin. |
The stronger drug that is not stronger
- One painkiller works at 1 mg; another needs 10 mg. The first is more potent, but if both reach the same maximum relief, neither is stronger.
- Dose–response curves separate how much drug you need from how much effect you can get.
Potency and efficacy
- On a log-dose curve, potency 效价 is position: the dose giving half the maximum effect (EC₅₀). Further left means more potent.
- Efficacy 效能 is the ceiling ($E_{max}$): the largest effect the drug can produce, however much you give.
Drug X has EC₅₀ 0.5 mg and drug Y has EC₅₀ 5 mg; both reach the same $E_{max}$. Which statement is right?
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Lower EC₅₀ means more potent; the same ceiling means the same efficacy.
Rank these drugs from most potent to least potent.
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The smaller the dose for half the effect, the more potent.
Full and partial agonists
- A full agonist reaches the system's full $E_{max}$. A partial agonist 部分激动剂 binds the same receptor but has a lower ceiling.
- Given with a full agonist, a partial agonist competes for receptors and can lower the combined effect.
A partial agonist has $E_{max}$ = 80% of the system's maximum. What effect does it give at its EC₅₀?
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EC₅₀ gives half the drug's own maximum, 80 ÷ 2 = 40%.
Adding a partial agonist to a full agonist always increases the total effect.
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It competes for receptors and can lower the combined effect toward its own ceiling.
Two kinds of antagonist
- A competitive antagonist 竞争性拮抗剂 shifts the curve right in parallel. More agonist overcomes it, so the ceiling is unchanged.
- A non-competitive antagonist lowers $E_{max}$. No amount of agonist restores the full effect.
Match each drug type to its effect on the agonist curve.
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Position and ceiling tell the drug types apart.
The therapeutic index
- Therapeutic index 治疗指数: $TI = TD_{50} / ED_{50}$, the dose toxic to half of people over the dose effective in half.
- A TI of 100 is forgiving; a TI near 2 needs care and often blood-level monitoring.
(Illustrative) $TD_{50}$ = 400 mg and $ED_{50}$ = 20 mg. What is the therapeutic index?
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400 ÷ 20 = 20.
Worked example: three curves
- (Fictional) Drug A: EC₅₀ 2 mg, $E_{max}$ 100%. Drug B: EC₅₀ 20 mg, $E_{max}$ 100%. B is 10 times less potent but just as efficacious.
- Drug C: EC₅₀ 2 mg, $E_{max}$ 60%: as potent as A, but a partial agonist with a lower ceiling.
Worked example: a therapeutic index
- (Illustrative) $TD_{50}$ = 100 mg, $ED_{50}$ = 10 mg: TI = 100 ÷ 10 = 10.
- If a second drug has $TD_{50}$ 30 mg and $ED_{50}$ 15 mg, TI = 2: the toxic and effective doses are close, so levels need watching.
Which drug most needs blood-level monitoring?
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The toxic and effective doses are closest together.
Three common slips
- Calling a potent drug strong. Potency is position; efficacy is the ceiling.
- Treating a partial agonist as a weak full agonist. With a full agonist it competes and can lower the effect.
- Reading TI from one curve. It needs two: effect and toxicity.
Recap
- Potency = EC₅₀ (position); efficacy = $E_{max}$ (ceiling).
- Partial agonist = lower ceiling; competitive antagonist = parallel right shift; non-competitive = lower ceiling.
- $TI = TD_{50}/ED_{50}$; a narrow TI means monitoring.