CYP1A2

What it metabolises human data

CYP1A2 prefers planar aromatic amines and heterocyclics, which is why its substrate list looks chemically coherent in a way that the CYP3A4 list does not. Caffeine is its probe and, for most people, its highest-volume substrate; the same enzyme handles theophylline, clozapine, olanzapine, several antidepressants, tizanidine, melatonin and ramelteon, and the R-enantiomer of warfarin. Two of those substrates — theophylline and clozapine — have a narrow margin and severe toxicity, and both are the classic casualties of the induction and de-induction cycle described below. Tizanidine deserves a specific mention because it is the extreme case: adding fluvoxamine has been shown to raise tizanidine exposure by a very large multiple, with profound hypotension and sedation, and ciprofloxacin produces a similar order of effect. CYP1A2 also bioactivates rather than detoxifies in one important class — the heterocyclic aromatic amines formed when meat is cooked at high temperature — which is why the enzyme appears in the cancer-epidemiology literature as well as the drug-interaction one.

Sources: Flockhart DA 2021 · U.S. Food 2023 · Culm-Merdek KE 2005 · Faber MS 2004 · Gunes A 2008 · Guengerich FP 2008

Drug inhibitors and inducers, with FDA potency classes human data

Same FDA definitions as elsewhere on this shelf: strong raises a sensitive substrate AUC five-fold or more, moderate at least two-fold and under five-fold, weak at least 1.25-fold and under two-fold. Fluvoxamine is the strongest common inhibitor and the reference case — it reduced single-dose caffeine clearance roughly fivefold in healthy volunteers, without changing what caffeine did at a given concentration, which is a neat demonstration that a pharmacokinetic interaction can be large while the pharmacodynamics are untouched. The fluoroquinolones are the interaction most likely to be encountered by accident, because a one-week antibiotic course for a urinary infection is not something anyone thinks of as a psychotropic event. Oral contraceptives are a genuinely under-appreciated moderate inhibitor of this enzyme.

Sources: U.S. Food 2023 · U.S. Food 2020* · Flockhart DA 2021 · Culm-Merdek KE 2005 · Faber MS 2004 · Anderson GD 2016

Smoke, quitting, and cruciferous vegetables: when a behaviour is the interaction contested human data

CYP1A2 induction by smoke is the clearest case in all of pharmacology that an interaction need not involve a second drug. Polycyclic aromatic hydrocarbons produced by combustion activate the aryl hydrocarbon receptor, which upregulates CYP1A1 and CYP1A2 transcription; a heavy smoker therefore clears caffeine, theophylline, clozapine and olanzapine substantially faster than a non-smoker and has been titrated to a dose that assumes that clearance. The interaction is the cessation. Measured CYP1A2 activity falls by roughly a third within about a week of stopping heavy smoking, which means clozapine or theophylline concentrations rise over days in a patient who has changed nothing about their prescription. This is a documented cause of toxicity in exactly the settings where smoking stops abruptly and not by choice — a psychiatric admission to a smoke-free ward, an intensive care unit, a surgical stay. Nicotine replacement does not hold the induction, because the inducer was never the nicotine. Cruciferous vegetables (broccoli, Brussels sprouts, cabbage, kale, watercress) induce CYP1A2 too, via glucosinolate breakdown products including indole-3-carbinol which is also an AhR ligand, and charbroiled meat induces it through its polycyclic aromatic hydrocarbon content; both effects are real and both are smaller and far more variable than the smoking effect. The honest statement is that a large sustained change in cruciferous intake can move a CYP1A2 substrate measurably, and that a single serving of broccoli will not.

• The inducer is the smoke. Vaping, patches, gum and lozenges do not maintain the induction.
• CYP1A2 activity falls by about 36 percent within roughly a week of stopping heavy smoking.
• Highest-risk substrates on cessation: clozapine, olanzapine, theophylline. Clozapine levels can roughly double.
• Smoked cannabis induces CYP1A2 too, by the same combustion route — a fact that complicates every attempt to attribute an interaction in a cannabis user to the cannabinoids.
• Cruciferous vegetables and charbroiled meat: real but smaller and more variable induction.
• Starting to smoke is the mirror event: levels fall and a stable patient relapses.

Contested — caveat. The smoking-cessation effect is well quantified. The cruciferous-vegetable effect rests on small dietary intervention studies with modest and variable effect sizes and depends heavily on the amount and preparation of the vegetables; it should not be treated as equivalent in magnitude to the smoking effect. The magnitude of CYP1A2 induction by smoked cannabis specifically is less well characterised than for tobacco.

Sources: Faber MS 2004 · Kall MA 1996 · Anderson GD 2016 · Flockhart DA 2021 · Gunes A 2008

Botanical and cannabinoid effects on this isoform contested in vitro

Cannabidiol and cannabinol are described in vitro as isoform-selective and relatively potent inhibitors of the CYP1 family, cannabidiol notably so at CYP1A1 and CYP1A2. That inhibition sits in direct opposition to the induction produced by smoking the plant, which is why the net CYP1A2 effect in a person who smokes cannabis is genuinely unpredictable: combustion products induce the enzyme, cannabinoids inhibit it, and the balance depends on route, frequency and dose. This is a case where the mechanism table gives two rules pointing opposite ways and the honest answer is that the net effect is unresolved. The operator corpus lists piperine as a CYP1A2 inhibitor alongside CYP3A4 and CYP2C9; the controlled in-vitro comparison found piperine relatively selective for CYP3A4, so carry that claim at lower confidence. Curcuminoids inhibit CYP1A2 in vitro. The in-vivo human probe study of four common botanicals found that kava, black cohosh and valerian did not significantly change CYP1A2 activity, and goldenseal did not either — useful negatives.

Contested — caveat. The cannabinoid CYP1 inhibition data are in-vitro IC50 determinations and have not been translated into a demonstrated human CYP1A2 interaction. The simultaneous presence of induction (combustion products) and inhibition (cannabinoids) in smoked cannabis means the net direction in a real user is not predictable from this table, and saying so is more useful than picking one.

Sources: Yamaori S 2011* · Stout SM 2014 · Nasrin S 2021 · Anderson GD 2016 · Faber MS 2004 · Kall MA 1996 · Volak LP 2008 · Bhardwaj RK 2002 · Gurley BJ 2005 · Bailey DG 2013 · Van Kush Family Research Institute (Temple Pharmacopoeia Project) 2026*

Clinical consequence, concretely human data

A patient stabilised on clozapine is admitted to a smoke-free psychiatric ward. Over the following week their clozapine concentration roughly doubles, and they become sedated, hypersalivating, and then have a seizure — with no change to the prescription and nothing in the chart that looks like a drug interaction. A patient on theophylline is given ciprofloxacin for a urinary infection and develops nausea, tachycardia, tremor and then a seizure. A patient on tizanidine is given fluvoxamine and collapses with hypotension. A patient whose caffeine habit was stable finds that the same three coffees now produce palpitations and insomnia after starting fluvoxamine or an oral contraceptive. Each of these is a CYP1A2 event and only the middle one looks like a drug interaction to an untrained eye; the first is normally recorded as a deterioration in mental state and the last as anxiety.

Sources: Faber MS 2004 · Culm-Merdek KE 2005 · Flockhart DA 2021 · U.S. Food 2023

Polymorphism contested human data

CYP1A2 variation between individuals is large — several-fold — but it is driven far more by environment than by coding sequence. The most studied variant, CYP1A2 star-1F (the minus-163 C to A change in intron 1), does not change baseline catalytic activity much; what it changes is INDUCIBILITY, so its effect appears mainly in smokers and in people with a high AhR-ligand intake. That makes it a gene-by-environment interaction rather than a straightforward pharmacogenetic marker, and it is the reason association studies of this variant with clozapine dose requirement or with caffeine consumption have been inconsistent. The practical consequence is that for CYP1A2, asking whether someone smokes is a better predictor than genotyping them.

Contested — caveat. CYP1A2 star-1F associations with clinical endpoints are inconsistent across studies, and the variant interacts with smoking status rather than acting independently. There is no established clinically actionable CYP1A2 pharmacogenetic guideline comparable to those for CYP2D6 or CYP2C19.

Sources: Gunes A 2008 · Zanger UM 2013 · Faber MS 2004