Metabolism Phase Drug Interactions

Drug Interactions

Drug interactions occur when one drug’s pharmacokinetic, pharmacodynamic or toxicological characteristics are influenced by a second substance. Usually, the second substance is another drug—termed a “drug-drug interaction”—but it can also be a component of the diet (such as proteins, fats, minerals, vitamins). In describing these interactions we use the terms “victim” and “perpetrator”. The victim is the drug whose PK, PD or tox characteristics are changed whereas the perpetrator is the drug (or sometimes a nutrient) who causes the change. In many instances, both drugs are affected and the victim/perpetrator model breaks down.

Metabolism Phase Drug Interactions Overview

Metabolism phase drug interactions occur when one drug alters the metabolic clearance of another drug, typically through enzyme inhibition or induction. These interactions commonly involve cytochrome P450 enzymes and less-commonly Phase II conjugating enzymes. They are among the most clinically significant and probably the most common type of drug interactions. Because there are so many victim—perpetrator combinations in metabolism phase interactions, it is more efficient to predict them based on the underlying mechanism rather than rote learning. As always, understanding these mechanisms will helps prevent and manage unwanted changes in drug pharmacokinetics that can lead to toxicity or therapeutic failure.

The magnitude of a metabolic interaction depends on several factors: the fraction of the victim drug metabolised by the affected pathway (f_m) discussed in Metabolism, the potency of the perpetrator as an inhibitor or inducer, the therapeutic index of the victim drug, and the availability of alternative metabolising pathways. Drugs that are the most susceptible to a clinically significant interaction are predominantly metabolised by a >single enzyme pathway and often have a narrow therapeutic window.

Strength of Inhibition and Induction

CYP450 inhibitors and inducers are often described as being strong, moderate or weak depending on their effect on a victim drug. Since inhibitors/inducers change CL of the victim drug, victim drug AUC is expected to also change. We therefore classify inhibitors/inducers based on changes in AUC or CL.

Classification of inhibitors/inducers based on changes in victim drug AUC.

	Inhibitor	Inducer
strong	>5x increase	<20% decrease
moderate	2–5x increase	20–50% decrease
weak	1.25–2x increase	<20% increase

Classification of inhibitors/inducers based on changes in victim drug CL.

	Inhibitor	Inducer
strong	>80% decrease	>5x increase
moderate	50–80% decrease	2–5x increase
weak	20–50% decrease	1.25–2x increase

Enzyme Inhibition

Enzyme inhibition occurs when a perpetrator drug reduces the activity of a metabolic enzyme. This has the effect of decreasing the clearance of victim drugs metabolised by that enzyme and can lead to accumulation, increased plasma concentrations, prolonged duration of action, enhanced pharmacological effects, or toxicity. Enzyme inhibition typically develops rapidly (within hours to days) as the inhibitor accumulates to steady-state concentrations.

Mechanisms of Inhibition

Drug metabolising enzymes have an active site where substrate drug binds and undergoes a chemical transformation. In reversible competitive inhibition, the perpetrator competes with the victim drug for the enzyme’s active site. Both victim and perpetrator drugs want to bind to the same site, but only one can occupy it at a time. The degree of inhibition depends on the relative concentrations and binding affinities (K_i) of both drugs. When the inhibitor has been cleared, enzyme activity returns to normal. Most interactions that involve CYP450 inhibition are competitive in nature. Examples include ketoconazole inhibiting CYP3A4, fluoxetine inhibiting CYP2D6, and fluvoxamine inhibiting CYP1A2. In theory, any substrate for a CYP450 enzyme can act as a competitive inhibitor. Ketoconazole, ritonavir, ciclosporin, clarithromycin, verapamil and many more are all substrates of CYP3A4. Any combination of two of these drugs represents a potential drug interaction!

Enzymes are complicated entities some drugs are able to bind to a part of the enzyme that is not the active site and alter the overall conformation of the enzyme. This reduces the catalytic activity of the enzyme without obscuring the active site. As such, drug substrates can still bind to the active site so this type of inhibition is considered non-competitive. This is less common for drug interactions but can occur with certain CYP450 inhibitors.

Mechanism-based inhibition (aka “irreversible” or sometimes “suicide” inhibition) occurs when the enzyme metabolises the inhibitor into a reactive intermediate that rapidly forms a covalent bond with an amino acid in the active site. This permanently inactivates the enzyme. Enzyme activity can only return when new enzyme is synthesised, which can take days to weeks. This causes particularly prolonged and potent inhibition. Examples include clarithromycin and erythromycin inhibiting CYP3A4, paroxetine inhibiting CYP2D6, and ritonavir inhibiting CYP3A4.

CYP3A4 Inhibition

CYP3A4 inhibition is the most clinically relevant class of enzyme inhibition because CYP3A4 metabolises ~50% of drugs, making interactions extremely common. There are many CYP3A4 inhibitors exhibiting very strong inhibition—some by >90%.

Strong CYP3A4 inhibitors include most of the azole antifungals (ketoconazole, itraconazole, posaconazole, voriconazole), HIV protease inhibitors (ritonavir, lopinavir, indinavir), clarithromycin, telithromycin and grapefruit juice (in large quantities). Moderate CYP3A4 inhibitors include erythromycin, non-dihydropyridine calcium channel blockers (diltiazem and verapamil), fluconazole, aprepitant, ciclosporin and anticancer kinase inhibitors (imatinib, encorafenib, idelalisib etc). Weak CYP3A4 inhibitors include cimetidine, ranitidine, fluvoxamine and grapefruit juice.

Important CYP3A4 substrates at high risk for interactions include some statins (simvastatin, atorvastatin and lovastatin but not pravastatin or rosuvastatin), calcium channel blockers (amlodipine, felodipine, nifedipine), immunosuppressants (ciclosporin, tacrolimus, sirolimus), benzodiazepines (midazolam, triazolam, alprazolam—but not lorazepam, oxazepam, or temazepam), ergot alkaloids (ergotamine, bromocriptine), pimozide, quetiapine and many others.

For example, simvastatin taken together with clarithromycin greatly increases risk of side effects including life-threatening rhabdomyolysis. Simvastatin is extensively metabolised by CYP3A4 and potent inhibition by clarithromycin increases exposure up to 10x, dramatically increasing risk of muscle toxicity. This combination should be avoided. Since the statin is usually a long-term medication and the clarithromycin is short-term, management usually involves selecting an alternative antibiotic. Azithromycin has a similar spectrum of activity and is only a weak CYP3A4 inhibitor; tetracyclines also have similar spectrum. Other feasible strategies could be to withold the simvastatin for the duration of antibiotic treatment or select an alternative statin such as pravastatin or rosuvastatin.

Tacrolimus with itraconazole antifungal dramatically increases levels of the immunosuppressant up to 6x, causing nephrotoxicity and other serious adverse effects. Due to the nature of the diseases they treat, this and similar combinations are sometimes unavoidable in transplant patients who have a fungal infection. In such cases careful dose reduction and therapeutic drug monitoring is indicated.

Grapefruit juice deserves special mention as a dietary constituent that can strongly inhibit CYP3A4. Grapefruit contains furanocoumarins such as 6′,7′-dihydroxybergamottin that are mechanism-based inhibitors of CYP3A4 in the intestine wall. Even as little as one glass can reduce intestinal CYP3A4 by 50%. The effect persists for 24-72 hours. This increases oral bioavailability of CYP3A4 substrates. Patients taking CYP3A4 substrates should generally avoid grapefruit and grapefruit juice. Other citrus fruits generally do not contain furanocoumarins and don’t inhibit CYP3A4 (an exception is Seville orange which does have moderate furanocoumarin content). Tragically, the variety of grapefruit often said to be the favourite among lovers of grapefruit, Duncan variety, is also the highest in furanocoumarin content.

Two more special mentions are cobistat and ritonavir, so-called “therapeutic” inhibitors of CYP3A4. They are used in combination with other drugs to improve their pharmacokinetic properties. For example, ritonavir is used with the COVID-19 antiviral nirmetralvir. Nirmetralvir on its own has poor absopriotn and short t_½ due to extensive hepatic metabolism by CYP3A4. Hence, ritonavir is used to inhibit first pass metabolism (increasing bioavailability) and prolong t_½. Cobistat is used in a similar manner with the HIV integrase inhibitor elvitegravir.

CYP2D6 Inhibition

~25% of drugs are metabolised by CYP2D6 making it very important for drug interactions. CYP2D6 is not inducible like other CYP450 isoforms so inhibition is the only mechanism of drug interaction involving this enzyme.

Potent CYP2D6 inhibitors include many antidepressants (fluoxetine, paroxetine, bupropion), quinidine, terbinafine and ritonavir. Many of these are mechanism-based inhibitors causing prolonged inhibition.

Important CYP2D6 substrates include many opioids (codeine, tramadol, oxycodone), some beta blockers (metoprolol, propranolol, carvedilol), antiarrhythmics (flecainide, propafenone), many antidepressants (tricyclic antidepressants, venlafaxine, duloxetine, atomoxetine), antipsychotics (risperidone, haloperidol, aripiprazole) and tamoxifen.

Some important drugs to highlight are those that are prodrugs activated by CYP2D6. The opioid painkillers codeine and tramadol are O-demethylated to active metabolites. In both cases, the parent drug has very little activity at the µ-opioid receptor (although tramadol has weak activity at other targets that give it minor painkilling effects). Similarly, the anti-breast cancer drug tamoxifen is also inactive as parent drug but undergoes activation to afimoxifen (a hydroxylated metabolite) and endoxifen (a hydroxylated, N-demethylated metabolite). These active metabolites modulate the estrogen receptor in breast cancer with 25-50x greater affinity than the parent tamoxifen.

For example, if codeine is taken with fluoxetine, codeine’s analgesic efficacy will be decreased. Codeine is a prodrug requiring CYP2D6 metabolism to morphine for analgesic effect. When CYP2D6 is inhibited by paroxetine, codeine remains in the inactive parent drug, resulting in inadequate analgesia. To manage this interaction, patients may instead benefit from an alternative opioid painkiller such as morphine—equi-analgesic doses are easy to calculate and titration to effect can be rapid. Comparatively, switching to an alternative antidepressant can be difficult and generally takes a long time.

Tamoxifen with paroxetine is a well-documented interaction that reduces tamoxifen efficacy in breast cancer. When CYP2D6 is inhibited by paroxetine, tamoxifen cannot be metabolised to the active endoxifen and the patient may be at risk of breast cancer progression or recurrence. This interaction is particularly pernicious because tamoxifen is essentially a preventative medication; there is no overt signs or symptoms to alert the patient and their team that tamoxifen metabolic activation is inadequate. Women taking tamoxifen should avoid potent CYP2D6 inhibitors. Alternative antidepressants (venlafaxine, citalopram, escitalopram, sertraline, mirtazapine, fluvoxamine) may be preferred.

CYP2C9 Inhibition

CYP2C9 inhibition is most important for causing interactions with the narrow therapeutic range anticoagulant warfarin. Sudden increases in warfarin concentrations can cause serious bleeding.

CYP2C9 inhibitors include amiodarone, fluconazole, metronidazole, sulfamethoxazole, fluvastatin and fluoxetine.

Important CYP2C9 substrates include warfarin, phenytoin, tolbutamide, glipizide, NSAIDs (ibuprofen, diclofenac, celecoxib), losartan and irbesartan.

Warfarin with amiodarone increases INR and bleeding risk. Amiodarone’s long half-life (~60 days) and warfarin’s slow onset of action makes this interaction particularly challenging as it can develop even if warfarin is taken weeks after amiodarone has been ceased. Alternatives should be considered but this combination is sometimes used. Warfarin dose reduction is recommended when starting amiodarone.

CYP2C19 Inhibition

CYP2C19 inhibition is clinically important for proton pump inhibitors and clopidogrel.

CYP2C19 inhibitors include fluconazole, fluvoxamine, omeprazole, esomeprazole, ticlopidine and some HIV protease inhibitors.

Important CYP2C19 substrates include clopidogrel, proton pump inhibitors, diazepam, citalopram, escitalopram and carisoprodol.

Clopidogrel is a prodrug requiring CYP2C19 activation into a more portent antiplatelet drug. Some proton pump inhibitor (particularly omeprazole and esomeprazole) inhibit CYP2C19, potentially reducing clopidogrel’s activation and therefore antiplatelet effect. Clinical studies show reductions in antiplatelet effect but there is uncertainty in whether this affects cardiovascular endpoints such as heart attack. An alternative proton pump inhibitor such as pantoprazole (a weak CYP2C19 inhibitor) may be suggested.

Phase II Enzyme Inhibition

Phase II enzyme inhibition is uncommon but can be very impactful for a small number of drug combinations.

For example, lamotrigine is glucuronidated by UGT1A4. Valproic acid inhibits UGT enzymes. Lamotrigine must be started at lower doses and titrated more slowly when combined with valproic acid to avoid severe rash and Stevens-Johnson syndrome due to decreased lamotrigine clearance.

Enzyme Induction

Enzyme induction occurs when a perpetrator drug increases the expression (synthesis) of metabolic enzymes, accelerating the clearance of victim drugs metabolised by those enzymes. This causes reduced victim drug concentrations and potentially therapeutic failure. Unlike inhibition, which occurs quickly (within hours), induction requires changes in protein synthesis and so develops slowly (over days to weeks).

Mechanism of Induction

Most CYP450 induction occurs through activation of nuclear receptors that regulate gene transcription. The inducer drug enters the hepatocyte and binds to a nuclear receptor in the cytoplasm forming a complex, the complex translocates into the nucleus and binds to a specific DNA response element in the promoter region of a CYP450 gene. This leads to increased transcription of the CYP450 gene and ultimately increased CYP450 enzyme levels in the hepatocyte. Examples of nuclear receptors involved in CYP450 induction include the pegnane X receptor (PXR), constitutive androstane receptor (CAR) and aryl hydrocarbon receptor (AhR), each targetting different CYP genes:

• Pregnane X receptor (PXR)—CYP3A4
• Constitutive androstane receptor (CAR)—CYP2B6 and various CYP2C members
• Aryl hydrocarbon receptor (AhR)—CYP1A members
• Peroxisome proliferator-activated receptor alpha (PPARα)—CYP4A members
• Liver X Receptor (LXR)—CYP7A1

When the inducer is discontinued, drug clearance returns to baseline levels only after the existing enzyme is degraded by normal cellular processes. This can take days to weeks.

Time Course of Induction

Depending on the specific inducer, maximal induction typically occurs after 1-3 weeks of treatment with the inducing agent. The antibiotic rifampicin is a notoriously rapid inducer, reaching maximal effect in ~1 week. St John’s wort is another strong inducer of CYP3A4 but require 2-3 weeks for maximal induction.

After discontinuing the inducer, enzyme levels gradually return to baseline over a few weeks as enzyme is degraded. During this de-induction period, victim drug concentrations gradually increase. This can create a “discovered” toxicity wherein drug dose is raised to counteract the increased CL due to CYP450 induction but the dose is not decreased after the inducer is removed.

The victim drug’s concentration changes are also gradual. When induction increases clearance, the victim drug is eliminated more rapidly until a new (lower) steady-state is reached (4–5 half-lives of victim drug). For victim drugs with short half-lives, this occurs quickly. For victim drugs with long half-lives (eg warfarin, t_½ ~40 hours), concentration decreases over days to weeks.

CYP3A4 Induction

CYP3A4 is highly inducible. Potent inducers can increase CYP3A4 expression 10–20x, dramatically increasing clearance of CYP3A4 substrates.

Potent CYP3A4 inducers include rifampicin (rifampin), several anticonvulsants (carbamazepine, phenytoin, phenobarbital), androgen receptor blockers (enzalutamide, apalutamide), BRAF inhibitors (encorafenib, dabrafenib, vemurafenib) and an over-the-counter herbal supplement St John’s wort. Moderate CYP3A4 inducers include non-nucleoside reverse transcriptase inhibitors (efavirenz, nevirapine, etravirine), modafinil and bosentan.

Rifampicin is a potent inducer, affecting CYP3A4 and other pathways. For example, when taken with some oral contraceptives including levonorgestrel and ethinylestradiol, rifampicin can suppress contraceptive efficacy risking an unwanted pregnancy. If rifampicin must be used, patients should be advised to use alternative contraception (eg copper IUD, condoms) for the duration of treatment.

St John’s wort (Hypericum perforatum) is an over-the-counter herbal supplement sometimes used as an antidepressant. It is also a potent inducer of CYP3A4. It has caused numerous documented interactions including these two famous cases of heart transplant rejection due to reduced ciclosporin levels. Because it is readily available without a prescription in many countries, this interaction can go unnoticed without a thorough patient history. Patients taking a CYP3A4 substrate drug should be counselled about the risks of using over-the-counter products.

CYP2C9 and Other CYP Induction

Rifampicin, phenytoin, carbamazepine and phenobarbital also induce CYP2C9, CYP2C19 and CYP2B6. This affects metabolism of warfarin, phenytoin, proton pump inhibitors and many other drugs. The principles are similar to CYP3A4 induction—onset and offset are gradual, potential for therapeutic failure, and gradual offset requiring dose adjustments.

Auto-Induction

Some drugs induce their own metabolism—a phenomenon known as “auto-induction”. The classic example is the antiepilepsy medication carbamazepine. Carbamazepine induces CYP3A4, which in turn metabolises carbamazepine itself. This results in complex pharmacokinetics at the start of therapy. When carbamazepine therapy is initiated, concentrations initially rise normally, but over 2-4 weeks, induction develops and clearance increases, causing concentrations to decline despite consistent dosing. This necessitates several incremental dose increases and regular plasma concentration measurements during the first few months of therapy.

Phase II Enzyme Induction

Induction of Phase II enzymes is uncommon. Phenobarbital, rifampicin, phenytoin, polycyclic aromatic hydrocarbons and some natural antioxidants (polyphenols, flavonoids, isothiocyanates) have been reported to induce various glucuronidation, glutathione-S-transferase and sulfotransferase enzymes. These induction pathways are mediated through the nuclear factor erythroid 2-related factor 2 (Nrf2) system. This increases drug clearance by accelerating Phase II metabolism. A documented example is that of rifampicin with lamotrigine. Concurrent use may increase CL of lamotrigine an necessitate higher doses.

Predicting the Magnitude of Metabolic Interactions

The magnitude of a metabolic interaction can be estimated using the fraction metabolised (f_m) by the affected pathway and the fold-change in intrinsic clearance caused by the perpetrator.

For complete inhibition or complete loss of an enzyme pathway:

$$ AUC_{ratio} = \frac{1}{1 - f_m} $$

If a drug is 90% metabolised by CYP3A4 (f_m = 0.9) and CYP3A4 is completely inhibited, AUC increases 10-fold (1/(1−0.9) = 10). If only 50% is metabolised by CYP3A4 (f_m = 0.5), AUC only doubles with complete inhibition (1/(1−0.5) = 2).

This explains why drugs predominantly metabolised by a single pathway (high f_m) are more susceptible to large interactions. Simvastatin (f_m,CYP3A4 ≈ 0.95) can show 10-20 fold AUC increases with potent CYP3A4 inhibitors. Atorvastatin (f_m,CYP3A4 ≈ 0.6–0.7) shows more modest 2–4 fold increases with the same inhibitors because alternative pathways compensate.

An incomplete summary of important CYP450 drug inhibitors, inducers and substrates.

CYP Enzyme	Potent Inhibitors	Potent Inducers	Important Substrates
CYP3A4	Azole antifungalsketoconazole, itra’, posa’, vori’, flu’ HIV antiviralsritonavir, darun’, lenacap’, lopin’ Macrolide antibioticsclarithromycin, ery’, teli’ Non-dihydropyridine calcium channel blockersdiltiazem, verapamil Several anticancer kinase inhibitorsimatinib, encorafe’, cobimeti’, idelalisib ciclosporin Grapefruit juice Therapeutic CYP3A4 inhibitors (cobicistat)	rifampicin carbamazepine phenytoin St John’s wort	Statinsatorvastatin, sim’, flu’ Benzodiazepinesmidazolam, alpr’, tri’, diazepam, nitr’, flunitr’ Immunosuppressantstacrolimus, si’, evo’, ciclosporin Some glucocorticoidsmethylprednisolone, triamcin’, hydrocortisone, dexametha’, flutica’, budesonide Dihydropyridine calcium channel blockersnifedipine, amlo’, lercani’, felo’, nimo’ ethinylestradiol and progestinslevonorgestrel, etono’, norethisterone (aka norethindrone), drospirenone Antipsychoticshaloperidol, chlorpromazine, thiorid’, caripr’, aripiprazole, brex’, lurasidone, respira’, ziprasi’, quetiapine, olanz’ cocaine Phosphodiesterase 5 inhibitorssildenafil, tadal’, avan’ Many anticancer kinase inhibitorsimatinib, acalabruti’, ascimi’, axiti’, brigati’, cabozanti’, ceriti’, cobimeti’, crizoti’, dabrafe’, dasati’, encorafe’, entrecti’, erloti’, gefitinib’, giltertinib’, ibruti’, lapati’, larotrecti’,lorlati’, niloti’, osimerti’, pazopa’, ponati’, regorafe’, ripreti’, ruxoliti’, selpercati’, selumeti’, sorafe’, suniti’, tofaciti’, upadaciti’, vandetan’, vemurafe’, zanubruti’, cannabidiol
CYP2D6	Some SSRIsfluoxetine, par’, duloxetine and bupropion quinidine terbinafine	Not inducible	Some opioidscodeine, tramadol, dextromethorphan, oxycodone tamoxifen metoprolol Tricyclic antidepressantsamitriptyline, nor’, imipramine, SSRIsfluoxetine, par’, fluvoxamine, SNRIsvenlafaxine, duloxetine, vortioxetine Antiemeticsmetoclopramide, ondansetron Antipsychoticshaloperidol, clozapine, olan’, aripiprazole, brex’, chlorpromazine, risperidone
CYP2C9	amiodarone Azole antifungalsfluconazole, vori’, mi’ metronidazole	rifampicin phenytoin carbamazepine aprepitant Androgen receptor blockersenzalutamide, ap’	warfarin phenytoin losartan amitriptyline Some NSAIDsibuprofen, diclofenac, celecoxib, meloxicam, naproxen Sulfonylureasglipizide, glyburide, glimepiride, glibenclamide, gliclazide tolbutamide cyclophosphamide
CYP2C19	Azole antifungalsfluconazole, vori’, mi’, keto’ fluvoxamine omeprazole and esomeprazole	rifampicin Androgen receptor blockersenzalutamide, ap’ St John’s wort	SSRIsescitalopram, citalopram, TCAsamitriptyline, imipramine Clopidogrel PPIsomeprazole, esome’, panto’, lanso’ cannabidiol diazepam, phenobarbital
CYP1A2	fluvoxamine ciprofloxacin cannabidiol verapamil	Smoking tobacco phenobarbital omeprazole	Atypical antipsychoticsclozapine, olan’ theophylline and caffeine paracetamol warfarin Random antidepressantsagomelatine, amitriptyline, duloxetine, imipramine

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