Drug–Drug Interactions: CYP Inhibitors/Inducers

Clinical meaning

Drug–drug interactions (DDIs) occur when one drug alters the pharmacokinetics or pharmacodynamics of another, producing clinically significant changes in efficacy or toxicity. The NP must understand both pharmacokinetic interactions (affecting ADME: absorption, distribution, metabolism, elimination) and pharmacodynamic interactions (affecting drug action at the receptor level).

Cytochrome P450 (CYP450) system: The CYP450 enzyme superfamily in the liver (and intestinal wall) is responsible for Phase I metabolism (oxidation, reduction, hydrolysis) of approximately 75% of all medications. The most clinically important CYP enzymes are: - CYP3A4: metabolizes ~50% of all drugs (statins, calcium channel blockers, benzodiazepines, macrolide antibiotics, calcineurin inhibitors, most protease inhibitors) - CYP2D6: metabolizes ~25% of drugs (codeine → morphine, tamoxifen → endoxifen, many antidepressants, metoprolol, many antipsychotics) - CYP2C19: metabolizes PPIs, clopidogrel (→ active metabolite), some SSRIs, phenytoin - CYP2C9: metabolizes warfarin (S-warfarin), phenytoin, NSAIDs, sulfonylureas - CYP1A2: metabolizes theophylline, caffeine, clozapine, olanzapine

Enzyme INHIBITION: A CYP inhibitor DECREASES the metabolism of substrate drugs, causing INCREASED plasma levels → toxicity. Inhibition occurs rapidly (within hours to days of adding the inhibitor). Examples: fluconazole (CYP2C9/3A4 inhibitor) + warfarin → markedly elevated INR → bleeding risk; clarithromycin (strong CYP3A4 inhibitor) + simvastatin → massive statin level increase → rhabdomyolysis; fluoxetine (CYP2D6 inhibitor) + codeine → blocks conversion to morphine → loss of analgesic effect.

Enzyme INDUCTION: A CYP inducer INCREASES the production of CYP enzymes (via nuclear receptor activation → gene transcription), causing ACCELERATED metabolism of substrate drugs → decreased plasma levels → therapeutic failure. Induction develops gradually (takes 1-2 weeks to reach full effect and 1-2 weeks to resolve after discontinuation). Examples: rifampin (potent inducer of CYP3A4, 2C9, 2C19) + oral contraceptives → contraceptive failure; carbamazepine (CYP3A4 inducer) + warfarin → decreased INR → thrombotic risk; St. John's Wort (CYP3A4 inducer) + cyclosporine → transplant rejection.

Diagnosis & workup

Diagnostics & workup: - Medication reconciliation: complete list of all prescription medications, OTC drugs, herbal supplements, vitamins, and recreational substances — this is the FIRST and most critical step in identifying potential DDIs; discrepancies between what was prescribed and what is being taken are common - Drug interaction checking tools: use clinical decision support systems (CDSS) embedded in the EHR or standalone databases (Lexicomp, Micromedex, Epocrates) to screen all medication combinations; differentiate severity levels (major/contraindicated, moderate, minor) and focus clinical attention on major interactions - Therapeutic drug monitoring (TDM): measure serum drug levels for NTI medications when DDIs are suspected or when interacting drugs are added/removed — digoxin level (therapeutic: 0.5-2.0 ng/mL; toxic: >2.0), phenytoin level (therapeutic: 10-20 mcg/mL; adjust for hypoalbuminemia), lithium level (therapeutic: 0.6-1.2 mEq/L for maintenance), vancomycin AUC-based monitoring - INR monitoring: when adding or discontinuing a CYP2C9 or CYP3A4 inhibitor/inducer in a patient on warfarin, check INR within 3-5 days and then weekly for 2-4 weeks until stable — warfarin has a narrow therapeutic index and is metabolized by CYP2C9 (S-warfarin, more potent enantiomer) and CYP3A4 (R-warfarin) - ECG/QTc monitoring: when combining two or more QTc-prolonging drugs, obtain baseline ECG and repeat after initiation; QTc >500 ms or increase >60 ms from baseline → discontinue or substitute the offending agent; common QTc prolongers: fluoroquinolones, macrolides, antipsychotics, ondansetron, methadone, certain antiarrhythmics - Pharmacogenomic testing: consider CYP2D6 genotyping before prescribing codeine, tramadol, or tamoxifen; CYP2C19 genotyping before prescribing clopidogrel; HLA-B*5701 before prescribing abacavir; HLA-B*1502 before prescribing carbamazepine in patients of Southeast Asian descent - Renal and hepatic function assessment: obtain BMP (creatinine, BUN), calculate eGFR/CrCl, hepatic panel (AST, ALT, bilirubin, albumin) — organ function directly affects drug metabolism and elimination, modifying DDI risk

Risk factors: - Polypharmacy (≥5 medications): the risk of DDIs increases exponentially with the number of medications — a patient on 5 drugs has a 50% probability of a DDI; on 7+ drugs, the probability exceeds 80% - Narrow therapeutic index (NTI) medications: drugs where small changes in plasma concentration produce large changes in effect or toxicity — warfarin, digoxin, lithium, phenytoin, theophylline, cyclosporine, tacrolimus, aminoglycosides, methotrexate — these are the highest-risk drugs for clinically significant DDIs - Hepatic impairment: reduced CYP450 enzyme activity in liver disease decreases drug metabolism capacity, amplifying the effects of enzyme inhibitors and reducing the impact of enzyme inducers — Child-Pugh classification guides dose adjustment - Renal impairment: decreased elimination of renally cleared drugs or their active metabolites increases accumulation; combined with a CYP inhibitor, can produce dangerous supratherapeutic levels — always calculate CrCl or eGFR before prescribing interacting drugs - CYP2D6 and CYP2C19 genetic polymorphisms: poor metabolizers (PM) are at higher risk of toxicity from drugs metabolized by these enzymes; ultra-rapid metabolizers (UM) are at risk of prodrug toxicity or parent drug therapeutic failure — pharmacogenomic testing should be considered for high-risk drugs - Elderly patients (age ≥65): reduced hepatic blood flow (~40% decrease by age 65), decreased liver mass (~30% decrease), decreased Phase I (CYP) metabolism capacity, and increased sensitivity to pharmacodynamic interactions (especially CNS depression and QTc prolongation) — apply Beers Criteria and STOPP/START criteria - Herbal supplements and OTC medications: patients often do not report these to prescribers; St. John's Wort is a potent CYP3A4 inducer, grapefruit juice inhibits intestinal CYP3A4, and many OTC products contain ingredients that interact with prescription medications (pseudoephedrine + MAOIs → hypertensive crisis)

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