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PHARMACOKINETICS · DRUG INTERACTIONS

Enzyme inhibition

A process by which a drug reduces CYP450 or other drug-metabolising enzyme activity, causing substrate drug concentrations to rise — potentially to toxic levels. Onset is rapid (hours); severity depends on inhibitor potency (Ki) and whether inhibition is reversible or mechanism-based.

Onset
Hours (reversible) to days (MBI accumulation)
Offset (reversible)
Hours after inhibitor stopped (2–5 t½)
Offset (MBI)
5–14 days (new enzyme synthesis required)
Net effect on substrate
Increased AUC / Cmax → potential toxicity

Clinically important enzyme inhibitors

Inhibitor Enzyme(s) Type Key substrates at risk
Fluconazole CYP2C9, 3A4 Reversible Warfarin, phenytoin, sulfonylureas, tacrolimus
Clarithromycin CYP3A4 MBI (irreversible) Simvastatin/lovastatin, colchicine, QT drugs
Fluoxetine/paroxetine CYP2D6 MBI (irreversible) Codeine, tramadol, tamoxifen, TCAs
Amiodarone CYP2D6, 2C9 Reversible (long t½) Warfarin (INR doubles), digoxin
Ritonavir CYP3A4 (pan-inhibitor) Reversible + MBI Most CYP3A4 substrates; used as deliberate booster
Gemfibrozil CYP2C8 Reversible Repaglinide (AUC ×8), cerivastatin (withdrawn)
Grapefruit juice Intestinal CYP3A4 MBI (gut wall) Simvastatin, felodipine, colchicine, sildenafil

CYP Inhibitors and the Cytochrome P450 System

The cytochrome P450 (CYP450) system is a superfamily of haem-containing enzymes responsible for the oxidative metabolism of approximately 75% of all drugs in clinical use. CYP450 metabolism occurs primarily in the liver (hepatocytes) and, to a lesser extent, in the intestinal wall, kidney, and lung. The major drug-metabolising CYP isoforms — CYP3A4, CYP2D6, CYP2C9, CYP2C19, CYP1A2, and CYP2B6 — each have distinct substrate specificities, genetic polymorphisms, and inhibitor profiles.

CYP inhibitors reduce the metabolic activity of these enzymes, causing substrate drug concentrations to rise — potentially to toxic levels. Common CYP inhibitors encountered in clinical practice include the azole antifungals (ketoconazole, fluconazole, itraconazole, voriconazole), macrolide antibiotics (clarithromycin, erythromycin), HIV protease inhibitors (ritonavir), and even dietary components like grapefruit juice (which irreversibly inhibits intestinal CYP3A4 via furanocoumarins). The NCBI StatPearls chapter on cytochrome P450 provides a comprehensive overview of enzyme function and clinical significance.

Understanding the cytochrome P450 system is essential for predicting drug-drug interactions. The FDA Drug Development and Drug Interactions Table classifies inhibitors as strong (>=5-fold AUC increase), moderate (2-5-fold), or weak (1.25-2-fold) based on the magnitude of substrate AUC change. Clinicians should consult this resource and the EMA Drug Interaction Guideline when prescribing known CYP substrates alongside inhibitors.

Frequently asked questions

What is enzyme inhibition in drug interactions?
Enzyme inhibition reduces CYP450 activity, slowing metabolism of substrate drugs → higher substrate plasma concentrations → potential toxicity. Types: Reversible competitive: inhibitor competes at active site (e.g., fluconazole–CYP2C9); reverses within hours of drug removal. Mechanism-based (irreversible/time-dependent): inhibitor is activated by the enzyme into a reactive species that covalently binds CYP (e.g., clarithromycin, erythromycin — CYP3A4; fluoxetine — CYP2D6). New enzyme synthesis (5–14 days) required for recovery. Contrast with enzyme induction where substrate levels fall. See CYP450 guide.
Which drugs are the most clinically dangerous CYP inhibitors?
Highest-risk clinical inhibitors: Azole antifungals — fluconazole (CYP2C9/3A4), itraconazole/voriconazole/posaconazole (strong CYP3A4); cause dangerous accumulation of statins, warfarin, tacrolimus, cyclosporin, benzodiazepines. Clarithromycin/erythromycin — mechanism-based CYP3A4 inhibition; rhabdomyolysis risk with simvastatin/lovastatin. Gemfibrozil — CYP2C8 inhibitor; increases repaglinide AUC >8-fold (hypoglycaemia). Amiodarone — CYP2D6/2C9; t½ 40–55 days → inhibition persists months after stopping. Ritonavir — deliberate CYP3A4 inhibition as "booster". Grapefruit juice — furanocoumarins inhibit intestinal CYP3A4. See FDA DDI table ↗.
What is mechanism-based (irreversible) inhibition?
MBI: the drug is metabolised by CYP into a reactive species that covalently inactivates the enzyme. New enzyme synthesis (5–14 days) required for recovery. Clinical examples: Clarithromycin: CYP3A4 MBI → dangerous simvastatin accumulation (rhabdomyolysis); avoid combination. Fluoxetine/paroxetine: CYP2D6 MBI; interaction lasts 4–6 weeks after fluoxetine stopped (norfluoxetine t½ = 7–15 days) — critical before switching to codeine, tramadol, or MAOIs. Diltiazem/verapamil: moderate CYP3A4 MBI; statin dose reduction needed. Mibefradil: withdrawn 1998 — fatal MBI with CYP3A4 substrates. Unlike reversible inhibition, MBI severity increases with repeated dosing. See induction for comparison.
What are common CYP inhibitors?
Common CYP inhibitors: Ketoconazole — strong CYP3A4 (FDA index inhibitor for DDI studies). Fluconazole — CYP2C9/moderate CYP3A4. Clarithromycin/erythromycin — CYP3A4 MBI (rhabdomyolysis with simvastatin). Ritonavir — pan-CYP inhibitor used as PK booster in HIV regimens. Fluoxetine/paroxetine — CYP2D6 MBI (blocks codeine activation for weeks). Amiodarone — CYP2D6/2C9 (t½ 40-55 days: inhibition persists months). Gemfibrozil — CYP2C8 (repaglinide AUC x8). Grapefruit juice — intestinal CYP3A4 MBI. See FDA DDI table ↗.
What is the cytochrome P450 system?
The cytochrome P450 system is a superfamily of haem-containing monooxygenases responsible for ~75% of all drug metabolism. Major isoforms: CYP3A4 (~50% of drugs; expressed in liver and gut wall), CYP2D6 (~25%; highly polymorphic — PM, IM, EM, UM phenotypes), CYP2C9 (warfarin, phenytoin, NSAIDs), CYP2C19 (clopidogrel, PPIs), CYP1A2 (caffeine, theophylline, clozapine), CYP2B6 (efavirenz, cyclophosphamide). CYP enzymes are susceptible to inhibition (raises substrate levels) and induction (lowers substrate levels). See NCBI StatPearls — Cytochrome P450 ↗ and our CYP450 guide.
How does grapefruit juice cause drug interactions?
Grapefruit furanocoumarins (bergamottin, DHB) irreversibly inhibit intestinal CYP3A4 (not hepatic) → increased bioavailability of substrates without hepatic effect. One glass inhibits gut CYP3A4 for 24–72 hours. High-risk substrates: Simvastatin/lovastatin — AUC can increase 3–7-fold → rhabdomyolysis (FDA grapefruit warning ↗); use pravastatin/rosuvastatin instead. Felodipine and other dihydropyridines — hypotension. Colchicine — toxicity. Sildenafil — enhanced hypotension. Switch to non-CYP3A4 statin, or avoid grapefruit in patients on high-risk substrates. CYP450 guide.
How is CYP inhibition quantified?
Ki (inhibition constant) = [I] that halves metabolic rate. [I]/Ki ratio: FDA considers >0.1 a signal for potential clinical interaction (in vitro to in vivo extrapolation). For reversible inhibition, the interaction magnitude ≈ 1 + [I]/Ki. For MBI, a time-dependent factor applies. FDA in vitro DDI guidance ↗ classifies strong inhibitors as ≥5-fold AUC increase for substrates, moderate as 2–5-fold, and weak as 1.25–2-fold. See AUC definition and drug interactions guide.