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PHARMACOKINETICS · DRUG INTERACTIONS

Enzyme induction

A process by which a drug increases expression of drug-metabolising enzymes — most commonly CYP450 — leading to faster metabolism of substrate drugs, lower plasma concentrations, and potential loss of efficacy.

Mechanism
Nuclear receptor activation (PXR, CAR, AhR) → CYP gene transcription
Onset
5–14 days (new enzyme synthesis required)
Offset
1–3 weeks after stopping the inducer
Net effect
Reduced substrate AUC / plasma concentration → potential treatment failure

Clinically important enzyme inducers

Inducer Enzymes induced Key interactions Potency
Rifampicin CYP3A4, 2C9, 2C19, 2B6, P-gp Warfarin, OCPs, ARVs, tacrolimus, voriconazole Strong
Carbamazepine CYP3A4, 2C9, 2B6 (autoinduction) OCPs, warfarin, phenytoin, antipsychotics Strong
Phenytoin CYP3A4, 2C9, 2C19 OCPs, warfarin, statins, corticosteroids Strong
Phenobarbital CYP3A4, 2C9, 2C19, 1A2 OCPs, warfarin, many Strong
St John's wort CYP3A4, P-gp Ciclosporin, ARVs, OCPs, digoxin Moderate–strong
Efavirenz CYP3A4, 2B6 (autoinduction) Methadone, other ARVs, OCPs Moderate
Rifabutin CYP3A4, P-gp ARVs, OCPs (weaker than rifampicin) Moderate

CYP Enzyme Induction: Clinical Impact and Common Inducers

Enzyme induction is one of the most clinically significant pharmacokinetic drug interaction mechanisms. When a drug activates nuclear receptors (PXR, CAR) in the liver, the resulting upregulation of CYP induction increases the rate of metabolism for all substrate drugs processed by the induced enzyme — reducing their plasma concentrations and potentially causing therapeutic failure.

The three most important clinical CYP enzyme inducers are rifampicin (rifampin), phenytoin, and carbamazepine. Rifampicin is the most potent known clinical inducer, affecting CYP3A4, CYP2C9, CYP2C19, CYP2B6, and P-glycoprotein simultaneously. Its impact is profound: co-administration reduces the AUC of oral contraceptives by approximately 50%, warfarin by 50%, and immunosuppressants (tacrolimus, cyclosporine) by 85-95%. The FDA Drug Interactions Table classifies rifampicin as a strong inducer of multiple CYP pathways.

Phenytoin and carbamazepine are particularly challenging because they are both NTI drugs and potent inducers — they can reduce the efficacy of co-prescribed medications while themselves requiring careful TDM. St John's wort (Hypericum perforatum) is an OTC herbal product that induces CYP3A4 and P-gp, leading to clinically significant interactions with cyclosporine, oral contraceptives, and antiretrovirals. The EMA Drug Interaction Guideline specifically warns prescribers about herbal inducers.

Frequently asked questions

What is enzyme induction in pharmacology?
Enzyme induction occurs when a drug activates nuclear receptors — primarily PXR (pregnane X receptor) ↗, CAR (constitutive androstane receptor), or AhR (aryl hydrocarbon receptor) — triggering transcription of CYP450 genes. Result: increased enzyme protein → faster substrate metabolism → reduced substrate plasma levels. Unlike inhibition, induction takes 5–14 days to develop and reversal takes 1–3 weeks after the inducer is stopped. Key inducers: rifampicin (potent CYP3A4/2C9 inducer), carbamazepine, St John's wort. See FDA DDI table ↗.
Which drugs are the most clinically important enzyme inducers?
Clinically critical inducers: Rifampicin — most potent clinical CYP3A4/2C9/P-gp inducer; reduces oral contraceptive (OCP) efficacy by ~50%, warfarin by ~50%, HIV ARVs (protease inhibitors, integrase inhibitors) dramatically, tacrolimus/cyclosporin by 85–95%. Carbamazepine/phenytoin/phenobarbital — classic anticonvulsant inducers causing OCP, warfarin, and statin failures. St John's wort — OTC herbal CYP3A4 inducer; significant interaction with OCPs, antiretrovirals, ciclosporin (MHRA warning ↗). Efavirenz/nevirapine — autoinduction of own metabolism within 2–4 weeks. See drug interactions guide.
How does induction differ from inhibition pharmacokinetically?
Inhibition: onset within hours to days; proportional to inhibitor concentration; reversible on drug removal within 2–3 half-lives of inhibitor; mechanism = enzyme active-site blockade or MBI (mechanism-based inhibition). Induction: onset 5–14 days (new enzyme synthesis required); maximum effect after 7–14 days; offset 1–3 weeks post-inducer (enzyme degradation rate); mechanism = nuclear receptor (PXR, CAR) gene transcription. Clinical trap: when rifampicin is stopped, substrates accumulate — warfarin can double or triple if dose was increased during rifampicin therapy. Monitor closely and reduce dose. See inhibition for contrast and CYP450 guide.
What is autoinduction?
Autoinduction: a drug induces its own CYP-mediated metabolism. Classic example: Carbamazepine — initial t½ 25–65 h → falls to 12–17 h after 3–5 weeks of therapy due to CYP3A4/2C8 autoinduction. Serum levels drop progressively; dose increases are commonly required 2–4 weeks after initiation. Monitor levels (target 4–12 mg/L) and titrate. Carbamazepine DailyMed ↗ notes autoinduction in its PK section. Other examples: efavirenz (reduces its own levels by ~30% after 2 weeks), nevirapine. Related concepts: half-life, steady-state, TDM.
What drugs are CYP enzyme inducers?
Key CYP enzyme inducers: Rifampicin — strongest clinical inducer (CYP3A4/2C9/2C19/2B6/P-gp); reduces OCP efficacy by ~50%, warfarin by ~50%, tacrolimus by 85-95%. Carbamazepine — strong CYP3A4/2C9/2B6 with autoinduction (own t½ drops from 25-65h to 12-17h). Phenytoin — strong CYP3A4/2C9/2C19. Phenobarbital — broad inducer. St John's wort — OTC CYP3A4/P-gp inducer; FDA DDI table ↗. Efavirenz/nevirapine — moderate, with autoinduction. Rifabutin — moderate, weaker alternative to rifampicin. See CYP450 guide.
Does rifampicin induction affect oral contraceptives?
Rifampicin reduces ethinylestradiol AUC by ~50–60% via CYP3A4 induction + P-gp enhancement → contraceptive failure. Use additional barrier contraception during treatment and for ≥28 days after stopping rifampicin. Note: enterohepatic circulation of ethinylestradiol is also disrupted by gut flora changes. The FSRH drug interaction guidance ↗ classifies rifampicin as a "significant enzyme inducer". Rifabutin is a weaker inducer — check local guidance for OCP compatibility. IUDs (hormonal or copper) and depot medroxyprogesterone are unaffected.