💡 AI Summary & Key Takeaways
– Prodrugs behave in the opposite way, because they need enzymes to become active.
– Learn a short list of common inducers, inhibitors and sensitive substrates.
Cytochrome P450 Interactions: Inducers and Inhibitors
CYP450 drug interactions are a core topic in pharmacology and pharmacy exams. The cytochrome P450 enzymes metabolize many drugs in the liver, and changing their activity changes drug levels. You do not need to memorize every enzyme, but a short list covers most questions.
The basic rules
- Inhibitors reduce metabolism, so levels of the substrate drug rise and toxicity may occur
- Inducers increase metabolism, so levels of the substrate fall and treatment may fail
- Prodrugs need activation by an enzyme, so inhibition reduces their effect and induction increases it
Inhibition acts quickly, while induction takes days to develop and fade, because new enzyme must be made.
Common inducers
Rifampicin, carbamazepine, phenytoin, phenobarbital, St John’s wort and chronic heavy alcohol use. A helpful memory aid is to remember the anticonvulsants, rifampicin and the herbal product. Smoking induces CYP1A2.
Common inhibitors
Macrolides such as clarithromycin and erythromycin, azole antifungals, protease inhibitors such as ritonavir, cimetidine, ciprofloxacin, amiodarone, metronidazole and grapefruit juice. Fluoxetine and paroxetine inhibit CYP2D6.
Sensitive substrates
- Warfarin: metabolized mainly by CYP2C9, so interactions change the INR
- Simvastatin and several calcium channel blockers: CYP3A4, so macrolides and azoles can cause toxicity
- Tacrolimus and cyclosporine: CYP3A4, with narrow therapeutic windows
- Theophylline: CYP1A2, affected by ciprofloxacin and smoking
- Oral contraceptives: levels fall with strong inducers
Prodrug examples
Codeine is converted to morphine by CYP2D6, so inhibitors can reduce pain relief and ultra-rapid metabolizers can have toxicity. Clopidogrel is activated by CYP2C19, so strong inhibitors can reduce its effect. Tamoxifen also depends on CYP2D6 for activation.
Pharmacogenetics
Genetic differences, such as poor or ultra-rapid metabolizer status, can change drug response. Exams may ask which patient group is at risk.
A classic exam pattern
A transplant recipient on tacrolimus starts a course of clarithromycin and develops tremor and kidney injury. The reason is CYP3A4 inhibition, which raises tacrolimus levels.
Study tips
- Memorize five inducers and five inhibitors
- Learn the four or five most sensitive substrates
- Review each wrong answer for the direction of change
This article is for exam revision and does not replace current clinical guidelines. Practice pharmacology in the Namazj free trial.