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Action of drugs

Created: 1/9/2005

Drugs act in four ways:

- Physico-chemical properties
- Activity at receptors
- Inhibition of enzyme systems
- Influence on synthesis of nucleic acid

Sodium citrate neutralises acid to aid prevention of aspiration pneumonitis.


Receptors bind with endogenous substances to transduce the chemical signal into initiation of a change in the cell.

: a compound which binds to a receptor and changes intracellular function
Partial agonist: Cmax is a lower % of the maximum effect
Maximum effect: (Cmax) is reached when the receptor become saturated
Efficacy: maximum effect is the same but occurs at a different dose
Antagonists: bind to the receptor but produce no effect. Can be reversible or irreversible.
In the reversible case, the dose-response curve is shifted to the right but Cmax remains the same (e.g. displacement of acetylcholine by non-depolarising muscle relaxants).
An irreversible, or non-competitive, antagonist shifts the dose-response curve to the right and also reduces Cmax (e.g. the alpha 1-blocker phenoxybenzamine, used in preoperative preparation of patients with phaeochromocytoma).

Drugs acting on enzymes

Either inhibit enzyme or compete for its substrate, e.g.:
- allopurinol (xanthine oxidase)
- aminophylline (phosphodiesterase)
- captopril (angiotensin-converting enzyme [ACE])
- omeprazole- irreversible (H+/K+ ATPase)

Drugs influencing nucleic acid synthesis

Affect synthesis of protein by production of messenger RNA via steroid receptors, e.g. corticosteroids.

Plasma proteins

- Only the unbound portion of a drug is active
- Albumin is important (binds acidic and neutral drugs)
- There is decreased albumin in the elderly, neonates, malnutrition, liver and in renal and cardiac failure

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