Pharmacokinetic drug interactions occur when the absorption, distribution, metabolism, or excretion of one drug affects the pharmacokinetic properties of another drug. These interactions can lead to altered drug concentrations in the body, potentially enhancing or diminishing therapeutic effects or increasing the risk of toxicity. Here are some primary classifications, along with examples:
1. Absorption Interactions
- Example: Antacids (like aluminum hydroxide) can decrease the absorption of certain antibiotics (like tetracycline) by binding to the drug, leading to reduced efficacy.
2. Distribution Interactions
- Example: Warfarin, an anticoagulant, is highly protein-bound. If a patient takes another highly protein-bound drug (like phenytoin), it can displace warfarin from its binding sites, increasing its free concentration and risk of bleeding.
3. Metabolism Interactions
- Example: The antibiotic rifampicin induces the cytochrome P450 enzyme CYP3A4, which can increase the metabolism of other drugs such as oral contraceptives, potentially leading to decreased contraceptive effectiveness.
4. Excretion Interactions
- Example: Probenecid can inhibit the renal tubular secretion of penicillin, leading to higher concentrations of penicillin in the blood and an increased duration of action.
Summary:
- Absorption: Antacids and tetracycline
- Distribution: Warfarin and phenytoin
- Metabolism: Rifampicin and oral contraceptives
- Excretion: Probenecid and penicillin
These interactions can have significant clinical implications and require careful management and monitoring when prescribing medications.