Expansion
Agonists activate, antagonists block, partial agonists do both depending on context
Expansion
- Full agonist: binds and produces the maximal response. High affinity and full efficacy
- Partial agonist: binds but produces a submaximal response even at full occupancy. Acts as an agonist when no full agonist is present, and as an antagonist when one is. Examples: buprenorphine, aripiprazole, salmeterol
- Antagonist: binds with no efficacy, blocking the agonist
- Competitive: surmountable by more agonist; shifts the dose-response curve right without lowering the maximum
- Non-competitive or irreversible: not surmountable; lowers the maximum response
- Inverse agonist: produces the opposite effect to the agonist by reducing constitutive receptor activity
Physiological antagonism is different again: two drugs with opposing effects acting through different receptors, as with adrenaline opposing histamine in anaphylaxis.
The partial agonist concept is clinically important. Buprenorphine in an opioid-naive patient provides analgesia, but given to someone on a full agonist it can precipitate withdrawal by displacing it. The same logic explains why partial agonists are used in smoking cessation and in opioid substitution.