2.6 Drug Interactions
Drug interactions occur when one drug alters the pharmacological effect of another. They can be beneficial (enhanced therapeutic efficacy) or harmful (increased toxicity, therapeutic failure). With polypharmacy affecting over 40% of elderly patients, understanding interaction mechanisms is a core clinical competency. This chapter covers pharmacokinetic interactions (ADME), pharmacodynamic interactions, CYP450 enzymology, and quantitative prediction models.
1. Classification of Drug Interactions
Drug interactions are classified into three major categories based on mechanism:
2. Pharmacokinetic Interactions (ADME)
Pharmacokinetic interactions alter the concentration-time profile of a drug by modifying its absorption, distribution, metabolism, or excretion. The net effect is a change in AUC, Cmax, or half-life.
2.1 Absorption Interactions
Absorption interactions alter the rate or extent of drug reaching the systemic circulation. Key mechanisms:
Chelation & Complexation
Metal cations (Ca²⁺, Mg²⁺, Fe²⁺, Al³⁺) form insoluble chelates with drugs, preventing absorption.
- • Tetracyclines + antacids: 50–90% reduction in AUC
- • Fluoroquinolones + iron: 30–50% reduction
- • Levothyroxine + calcium: clinically significant
Management: separate administration by 2–4 hours
Altered GI Motility
Drugs that change gastric emptying or intestinal transit alter absorption kinetics:
- • Metoclopramide (prokinetic): faster emptying → faster peak
- • Opioids / anticholinergics: slow transit → delayed/reduced absorption
- • Laxatives: reduced contact time → lower bioavailability
Altered Gastric pH
PPIs and H2-blockers raise gastric pH, affecting pH-dependent solubility:
- • Ketoconazole/itraconazole: require acidic pH for dissolution — AUC reduced 60–80% with omeprazole
- • Atazanavir: reduced absorption with PPIs (contraindicated)
- • Enteric-coated formulations: may dissolve prematurely in alkaline stomach
P-glycoprotein (P-gp) Interactions
P-gp is an efflux transporter in the intestinal epithelium that pumps drugs back into the lumen:
- • P-gp inhibitors (verapamil, cyclosporine, quinidine): increase oral bioavailability of P-gp substrates
- • P-gp inducers (rifampin, St John's wort): decrease bioavailability
- • Digoxin + quinidine: digoxin levels double (P-gp inhibition)
2.2 Distribution Interactions
Protein binding displacement was historically overemphasized. The key equation:
\( f_u = \frac{C_{\text{free}}}{C_{\text{total}}} \qquad C_{\text{free}} = f_u \cdot C_{\text{total}} \)
When a displacer drug increases \( f_u \), the free concentration transiently rises, but hepatic clearance of the free drug also increases, establishing a new steady state where:
\( \text{At new SS:} \quad C_{\text{total}}^{\text{new}} = \frac{\text{Dose rate}}{CL_u} \cdot f_u^{\text{new}} \)
Total concentration falls, but free concentration returns to baseline — displacement alone is rarely significant
When displacement IS clinically significant
- • Narrow therapeutic index + high protein binding: warfarin (99% bound), phenytoin (90% bound)
- • Simultaneous inhibition of metabolism: the real danger — displacement + CYP inhibition = sustained free drug elevation
- • Example: valproate displaces phenytoin from albumin AND inhibits CYP2C9 → phenytoin toxicity
2.3 Metabolism Interactions (CYP450)
Cytochrome P450 enzymes mediate ~75% of all drug metabolism. Inhibition or induction of these enzymes is the most clinically important interaction mechanism.
| CYP Enzyme | % Drug Metabolism | Key Substrates | Inhibitors | Inducers |
|---|---|---|---|---|
| CYP3A4 | ~50% | Midazolam, cyclosporine, statins, fentanyl, apixaban | Ketoconazole, itraconazole, ritonavir, grapefruit, clarithromycin | Rifampin, carbamazepine, phenytoin, St John's wort |
| CYP2D6 | ~25% | Codeine, metoprolol, tamoxifen, fluoxetine, dextromethorphan | Paroxetine, fluoxetine, bupropion, quinidine | Not readily inducible |
| CYP2C9 | ~15% | Warfarin (S-), phenytoin, losartan, NSAIDs | Fluconazole, amiodarone, metronidazole | Rifampin |
| CYP2C19 | ~10% | Omeprazole, clopidogrel, diazepam, phenytoin | Omeprazole (self-inhibition), fluoxetine, fluvoxamine | Rifampin |
| CYP1A2 | ~5% | Theophylline, caffeine, clozapine, olanzapine | Ciprofloxacin, fluvoxamine | Smoking, chargrilled food, omeprazole |
CYP Inhibition: Quantitative Prediction
Competitive inhibition follows the relationship:
\( \text{AUC ratio} = \frac{AUC_{\text{with inhibitor}}}{AUC_{\text{alone}}} = 1 + \frac{[I]}{K_i} \)
where \( [I] \) is the inhibitor concentration at the enzyme site and \( K_i \) is the inhibition constant. For mechanism-based (irreversible) inhibitors (e.g., erythromycin, ritonavir), the onset is slower but the effect persists until new enzyme is synthesized (recovery \( t_{1/2} \approx 1{-}3 \) days for hepatic CYPs).
Ketoconazole + midazolam
AUC ratio ~15× (CYP3A4 inhibition). Midazolam dose must be reduced 75%+.
Ritonavir boosting
Intentional CYP3A4 inhibition to boost HIV protease inhibitor levels 10–100×.
CYP Induction: PXR/CAR Pathway
Induction occurs via nuclear receptor activation (PXR, CAR, AhR), upregulating enzyme transcription. Onset is gradual (days to weeks), and offset follows enzyme degradation kinetics:
\( E_{\text{induced}}(t) = E_0 \cdot \left(1 + E_{\max} \cdot \frac{[I]^n}{EC_{50}^n + [I]^n}\right) \cdot \left(1 - e^{-k_{\text{deg}} t}\right) \)
- • Rifampin: the most potent inducer. Reduces oral contraceptive efficacy (breakthrough pregnancy), reduces warfarin effect (need 2–3× dose), reduces cyclosporine levels (transplant rejection risk).
- • St John's wort: OTC herbal, induces CYP3A4/P-gp. Caused transplant rejections, HIV breakthrough, serotonin syndrome.
- • Smoking (CYP1A2): clozapine and theophylline doses must be increased 50–100% in smokers. On cessation, dose reduction is critical.
2.4 Excretion Interactions
Renal Tubular Secretion
- • Probenecid + penicillin: blocks OAT-mediated secretion → higher penicillin levels (historically beneficial)
- • Methotrexate + NSAIDs: NSAIDs block MTX secretion → severe bone marrow toxicity
- • Metformin + cimetidine: OCT2 inhibition → metformin accumulation
Altered Renal Blood Flow & GFR
- • NSAIDs + lithium: NSAIDs reduce renal PG → decreased GFR → lithium toxicity
- • ACE inhibitors + NSAIDs: double hit on renal perfusion
- • Urinary pH manipulation: alkalinization increases salicylate excretion (ion trapping)
3. Pharmacodynamic Interactions
PD interactions occur when drugs modify each other's effects at the target or downstream signaling, without altering plasma concentrations. Quantified using isobolograms and combination indices.
\( CI = \frac{D_A}{ED_{50,A}} + \frac{D_B}{ED_{50,B}} \)
Combination Index: \( CI < 1 \) synergistic, \( CI = 1 \) additive, \( CI > 1 \) antagonistic
Additive (1 + 1 = 2)
Same mechanism or downstream target:
- • Alcohol + benzodiazepines (both enhance GABAA)
- • Two NSAIDs (both inhibit COX)
- • Thiazide + loop diuretic (sequential nephron blockade)
Synergistic (1 + 1 > 2)
Different points in same pathway:
- • TMP-SMX: sequential folate synthesis block
- • β-lactam + aminoglycoside (cell wall + ribosome)
- • ACE inhibitor + Ca-channel blocker (different BP mechanisms)
Antagonistic (1 + 1 < 2)
Opposing pharmacological actions:
- • Naloxone reverses opioid effects (competitive antagonist at μ)
- • Flumazenil reverses benzodiazepines (GABAA antagonist)
- • Bacteriostatic + bactericidal antibiotics (controversial)
Potentiation (0 + 1 > 1)
Drug with no intrinsic effect enhances another:
- • Clavulanic acid + amoxicillin (β-lactamase inhibitor has no antibacterial activity alone)
- • Carbidopa + levodopa (peripheral DOPA decarboxylase inhibitor increases CNS delivery)
Clinically Dangerous PD Interactions
Serotonin Syndrome
SSRI + MAO inhibitor, tramadol + SSRI, linezolid + SSRI. Triad: mental status changes, autonomic instability, neuromuscular hyperactivity. Can be fatal.
QT Prolongation & Torsades de Pointes
Two QT-prolonging drugs combined: erythromycin + cisapride, haloperidol + amiodarone, methadone + fluoroquinolone. Additive hERG channel block.
Hyperkalaemia
ACE inhibitor + K-sparing diuretic + NSAID (“triple whammy”). Each independently raises K⁺. Combined risk of fatal cardiac arrhythmia.
Bleeding Risk
Warfarin + aspirin + SSRI: anticoagulant + antiplatelet + impaired platelet serotonin uptake. GI bleed risk increases 5–10×.
4. High-Risk Drug Interaction Table
| Object Drug | NTI | Primary Danger | Key Precipitant Drugs | Mechanism |
|---|---|---|---|---|
| Warfarin | Yes | Bleeding / clotting | NSAIDs, fluconazole, amiodarone, rifampin, cranberry juice | CYP2C9 inhibition/induction + PD antiplatelet |
| Digoxin | Yes | Arrhythmia | Amiodarone, verapamil, quinidine, clarithromycin | P-gp inhibition + hypokalaemia (diuretics) |
| Lithium | Yes | Toxicity (tremor, seizures) | NSAIDs, ACE inhibitors, thiazides | Reduced renal clearance |
| Methotrexate | Yes | Bone marrow suppression | NSAIDs, penicillins, probenecid, TMP-SMX | Reduced renal secretion + folate antagonism |
| Phenytoin | Yes | Toxicity / seizure recurrence | Valproate, fluconazole, isoniazid, rifampin | CYP2C9/2C19 inhibition/induction + displacement |
| Theophylline | Yes | Seizures, arrhythmia | Ciprofloxacin, erythromycin, cimetidine | CYP1A2 inhibition |
| Cyclosporine | Yes | Rejection / nephrotoxicity | Ketoconazole, rifampin, St John's wort, grapefruit | CYP3A4 + P-gp inhibition/induction |
| Clopidogrel | No (prodrug) | Therapeutic failure | Omeprazole (CYP2C19 inhibitor) | Blocks activation of prodrug → stent thrombosis risk |
5. Clinical Management Framework
Risk Factors for Interactions
- • Narrow therapeutic index: warfarin, digoxin, lithium, phenytoin, theophylline, cyclosporine
- • Polypharmacy: risk increases exponentially — 5 drugs: 50% chance; 8+ drugs: 100%
- • Elderly patients: reduced hepatic/renal function, altered body composition
- • Genetic polymorphism: CYP2D6 poor metabolizers at 4× higher risk
- • Hepatic/renal impairment: reduced clearance magnifies interactions
Practical Management Strategies
- • Dose adjustment: reduce dose of object drug when adding inhibitor (e.g., halve warfarin with fluconazole)
- • Timing separation: chelation interactions (tetracycline + antacid: give 2h apart)
- • Alternative drug: choose non-interacting alternative (pantoprazole instead of omeprazole with clopidogrel)
- • TDM: measure drug levels for NTI drugs when adding/removing interacting drugs
- • Electronic alerts: use CPOE interaction checkers, but manage alert fatigue
Key Takeaways
- 1.
CYP450 metabolism interactions account for ~80% of clinically significant drug interactions. The AUC ratio for competitive inhibition is \( 1 + [I]/K_i \).
- 2.
CYP3A4 metabolises ~50% of all drugs. Key inhibitors: ketoconazole, ritonavir, grapefruit. Key inducer: rifampin.
- 3.
Pharmacodynamic interactions are quantified by the Combination Index: \( CI = D_A/ED_{50,A} + D_B/ED_{50,B} \). CI < 1 = synergy, CI > 1 = antagonism.
- 4.
Protein binding displacement alone is rarely clinically significant — the real danger is displacement combined with metabolic inhibition.
- 5.
High-risk scenarios: serotonin syndrome (SSRI + MAO-I), QT prolongation (additive hERG block), hyperkalaemia (ACEi + K-sparing + NSAID), and NTI drug polypharmacy in elderly.