Pharmacokinetics and Pharmacodynamics in Older Adults

Study Notes

Responses to drugs can be additive (enhancing effects) or antagonistic (reducing effects).
Therapeutic uses include antihypertensives, cancer chemotherapy, and pain management.
Harmful effects may occur in patients taking multiple drugs with anticholinergic properties.

Gastrointestinal (GI) absorption can be inhibited by:
- Medications with large surface area, such as antacids.
- Binding resins like cholestyramine.
- Drugs affecting GI motility.
- Agents altering GI pH (e.g., antacids, iron, kaolin-pectin).

Drugs may compete for binding sites on proteins and tissues, impacting pharmacokinetics for several days.
Clinically significant displacement occurs when:
- The displaced drug has high protein binding.
- It has a limited volume of distribution (VoD).
- Elimination is slow.
- It possesses a low therapeutic index.
Example: Phenylbutazone can dramatically displace warfarin from protein binding sites, enhancing bleeding risk.

Drug metabolism can be altered by concurrent medication administration leading to varying effects:
- Enzyme inducers, like Barbiturates, Carbamazepine, and Phenytoin, may enhance metabolism (effects evident in 1-3 weeks).
- Enzyme inhibitors (primarily affecting P450 3A4) can lead to increased drug concentration and possible toxicity. Common inhibitors include:
  - Amiodarone, fluoxetine, cimetidine, erythromycin, and others.
Inhibition of drug metabolism typically begins rapidly and reaches maximum effect by the time steady-state concentrations are achieved.