14.3 High-Risk Medications & Therapeutic Drug Monitoring

High-Risk Medications & Therapeutic Drug Monitoring

Therapeutic Drug Monitoring (TDM) is the clinical practice of measuring specific drug concentrations in blood to individualize dosage regimens, ensuring optimal therapeutic efficacy while avoiding concentration-dependent toxicity. TDM is reserved for drugs that possess a narrow therapeutic index (NTI), marked inter-individual pharmacokinetic variability, poor correlation between dose and clinical response, and established target plasma ranges.

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Principles of Therapeutic Drug Monitoring

The Pharmacokinetic Foundation of TDM

• Steady State ($C_{ss}$): Occurs when the rate of drug administration equals the rate of elimination. Steady-state plasma concentration is achieved after approximately 4 to 5 elimination half-lives ($t_{1/2}$) following initiation or dose adjustment. Sampling before steady state leads to premature, erroneous dose changes.
• Trough Concentration ($C_{min}$): The lowest plasma drug concentration during a dosing interval, sampled immediately prior to (within 30 minutes of) the next scheduled dose. Trough levels reflect drug clearance, drug accumulation, and risk of toxicity.
• Peak Concentration ($C_{max}$): The highest drug concentration achieved, sampled shortly after drug administration and distribution. Peak levels reflect bactericidal efficacy for concentration-dependent antimicrobials.

Summary of Core TDM Parameters (BNF & NICE Guidelines)

The British National Formulary (BNF) and NICE prescribing guidelines mandate specific sampling times, target therapeutic concentrations, and toxicological thresholds for narrow therapeutic index agents:

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Antimicrobial TDM: Aminoglycosides & Glycopeptides

1. Gentamicin (Aminoglycoside)

Gentamicin exerts bactericidal activity against aerobic Gram-negative bacilli (e.g., Pseudomonas aeruginosa, Escherichia coli, Klebsiella) by irreversibly binding the bacterial 30S ribosomal subunit to arrest protein synthesis. It displays concentration-dependent killing and a prolonged post-antibiotic effect (PAE).

• Extended-Interval Once-Daily Dosing (5–7 mg/kg IV): In modern UK hospital practice, once-daily high-dose administration is preferred over multiple-daily dosing because it maximizes peak concentration ($C_{max}$/MIC ratio ≥8–10) while allowing plasma concentrations to drop to near-zero for part of the 24-hour cycle, mitigating accumulation.
• Sampling Schedule & Target Thresholds:
  • Trough Concentration: Sampled immediately prior to (within 30 minutes of) the next scheduled dose (typically before the 2nd dose in once-daily regimens, or using the 6–14 hour single-sample Hartford Nomogram).
  • Target Trough Level: Must be <1 mg/L (<2 mg/L in multi-dose divided regimens) to prevent drug accumulation.
• Toxicity Profile:
  • Ototoxicity: Vestibular damage (vertigo, ataxia, oscillopsia) and cochlear damage (tinnitus, high-frequency sensorineural hearing loss). Damage results from progressive accumulation in endolymph and perilymph, causing apoptosis of sensory hair cells. Crucial point: Aminoglycoside ototoxicity is frequently permanent and irreversible.
  • Nephrotoxicity: Accumulation in proximal renal tubular epithelial cells triggers acute tubular necrosis (ATN), characterized by non-oliguric renal impairment, rising serum creatinine, and proteinuria. Nephrotoxicity is typically reversible upon drug cessation.

2. Vancomycin (Glycopeptide)

Vancomycin is a large glycopeptide antimicrobial active exclusively against Gram-positive bacteria, including Methicillin-Resistant Staphylococcus aureus (MRSA), Enterococcus faecalis, and coagulase-negative staphylococci. It inhibits peptidoglycan synthesis by binding the D-alanyl-D-alanine terminus of cell wall precursors, displaying time-dependent killing (optimised by an $\text{AUC}_{24}/\text{MIC} \ge 400$).

• Sampling Schedule & Target Trough Levels:
  • Administered as intermittent IV infusions; levels are measured immediately prior to the 4th dose (at steady state).
  • Standard Target Trough Level: 10 – 15 mg/L for mild-to-moderate Gram-positive infections.
  • High Target Trough Level (15 – 20 mg/L): Required for deep-seated or severe infections: MRSA bacteraemia, infective endocarditis, osteomyelitis, hospital-acquired pneumonia, and septic arthritis.
• Adverse Effects & Toxicity:
  • Nephrotoxicity: Toxic acute tubular necrosis and interstitial nephritis, exacerbated by concomitant loop diuretics, aminoglycosides, or piperacillin-tazobactam. Monitor creatinine.
  • Ototoxicity: Auditory nerve damage, especially at high sustained trough levels (>20 mg/L).
  • Red Man Syndrome (Vancomycin Infusion Reaction): A non-immunological anaphylactoid reaction triggered by rapid intravenous infusion. Rapid administration directly stimulates mast cells and basophils to degranulate, releasing large quantities of histamine without IgE involvement. Features: Intense pruritus, erythema, and erythematous flushing over the face, neck, and upper trunk ('mantle' distribution), occasionally accompanied by hypotension and bronchospasm. Prevention & Management: Slow the IV infusion rate to a maximum of 10 mg/min (or infuse each 1 g dose over at least 100 minutes); pre-treatment with oral antihistamines (cetirizine or chlorphenamine).

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Cardiovascular TDM: Digoxin

Digoxin is a cardiac glycoside extracted from Digitalis purpurea, indicated for rate control in permanent atrial fibrillation and as an adjunct in symptomatic heart failure with reduced ejection fraction (HFrEF).

• Mechanism of Action: Selectively inhibits the myocardial cell membrane Na⁺/K⁺-ATPase pump. Intracellular sodium rises, which blunts the activity of the sodium-calcium (Na⁺/Ca²⁺) exchanger. Consequently, intracellular calcium stores in the sarcoplasmic reticulum increase, producing positive inotropy (increased myocardial contractility). Concurrently, digoxin stimulates central vagal nuclei, slowing AV node conduction and prolonging the refractory period (negative chronotropy).
• Target Therapeutic Plasma Range: 0.5 – 2.0 mcg/L (In chronic heart failure, the optimal target is narrower: 0.5 – 0.9 mcg/L; levels >1.2 mcg/L increase mortality without conferring added functional benefit).
• Sampling Timing Rule: Blood sampling must occur at least 6 hours post-dose (or immediately prior to the next scheduled dose). Digoxin undergoes a prolonged tissue distribution phase; sampling earlier than 6 hours yields falsely elevated serum concentrations that do not reflect myocardial tissue saturation.

Precipitants of Digoxin Toxicity

1. Hypokalaemia (#1 Precipitant): Potassium and digoxin compete for the same binding site on the extracellular alpha-subunit of the myocardial $\text{Na}^+/\text{K}^+$-ATPase pump. Under hypokalaemic conditions (e.g. secondary to furosemide or bendroflumethiazide), decreased potassium competition allows excessive digoxin binding and severe pump inhibition even at low or normal serum digoxin concentrations.
2. Hypomagnesaemia & Hypercalcaemia: Low magnesium potentiates myocardial irritability; hypercalcaemia compounds intracellular calcium overload, precipitating malignant arrhythmias.
3. Renal Impairment: Digoxin is 70–80% eliminated unchanged by the kidneys; acute or chronic renal decline causes rapid drug accumulation.
4. Pharmacological Drug Interactions: Drugs that inhibit P-glycoprotein or displace digoxin from tissue binding sites (e.g. amiodarone, verapamil, diltiazem, clarithromycin, quinidine, spironolactone) double plasma digoxin levels unless the maintenance dose is halved.

Clinical Manifestations of Digoxin Toxicity

• Gastrointestinal (Earliest Symptoms): Anorexia, nausea, vomiting, abdominal pain, diarrhea.
• Neurological & Visual: Confusion, delirium, headache, fatigue, and pathognomonic visual disturbances: blurred vision, photophobia, and xanthopsia (yellow-green visual haloes or altered colour perception).
• Cardiovascular Arrhythmias: Can induce virtually any cardiac arrhythmia:
  • Ventricular bigeminy and ventricular ectopics (commonest overall arrhythmia).
  • Paroxysmal atrial tachycardia with variable (2:1) AV block (highly characteristic).
  • Bidirectional ventricular tachycardia (pathognomonic of severe digoxin cardiotoxicity).
  • Severe sinus bradycardia, sinoatrial arrest, and complete heart block.

Management of Digoxin Toxicity

1. Stop Digoxin: Cease drug administration immediately; monitor continuous cardiac rhythm.
2. Correct Electrolytes: Correct hypokalaemia urgently (target serum potassium 4.0–5.0 mmol/L using IV potassium chloride infusions); correct hypomagnesaemia.
3. Manage Bradyarrhythmias: Administer IV atropine (500 micrograms) for symptomatic bradycardia or high-grade AV block.
4. Definitive Antidote (DigiFab / Digibind): Administer Digoxin-specific antibody fragments (DigiFab) IV. Indications for DigiFab:
   • Life-threatening ventricular arrhythmias or haemodynamic instability.
   • Severe, refractory hyperkalaemia (serum potassium >5.5 mmol/L in acute overdose; elevated potassium reflects massive poisoned systemic $\text{Na}^+/\text{K}^+$ pump failure and predicts near-100% mortality without antibody rescue).
   • Acute ingestion >10 mg in adults or serum digoxin concentration >10 mcg/L.

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Neuropsychiatric TDM: Lithium & Phenytoin

1. Lithium Carbonate

Lithium is the gold-standard mood stabilizer indicated for the acute treatment of mania and long-term prophylaxis of bipolar affective disorder. It possesses a very narrow therapeutic index and is eliminated exclusively by renal excretion via glomerular filtration.

• Target Therapeutic Range:
  • Maintenance Prophylaxis: 0.6 – 0.8 mmol/L.
  • Acute Mania / Relapse: 0.8 – 1.0 mmol/L.
• Sampling Protocol: Blood samples must be collected exactly 12 hours post-evening dose (typically 12 hours after the 20:00 or 22:00 dose). Levels are checked 1 week after starting and after each dose change, then weekly until stable; then every 3 months for the first year and every 6 months after that if stable (3-monthly for higher-risk groups; NICE CG185). Thyroid function tests (TFTs) and renal function tests (U&Es, eGFR) must be checked every 6 months (risk of nephrogenic diabetes insipidus, chronic tubulointerstitial nephritis, and hypothyroidism).
• Precipitants of Lithium Toxicity:
  • Anything causing intravascular volume depletion or decreased renal sodium delivery triggers compensatory increased sodium reabsorption in the proximal convoluted tubule. Because lithium is handled by the kidney identically to sodium, lithium is co-reabsorbed, leading to toxic retention.
  • Culprit Precipitants: Dehydration, febrile illness, salt-restricted diet, gastroenteritis, Thiazide diuretics (highest risk; deplete distal sodium), NSAIDs (inhibit renal prostaglandins, reducing GFR), and ACE inhibitors / ARBs.
• Clinical Manifestations of Toxicity:
  • Mild to Moderate Toxicity (1.5 – 2.0 mmol/L): Coarse tremor (distinguished from fine physiological tremor), anorexia, nausea, vomiting, diarrhea, lethargy, muscle weakness, hyperreflexia.
  • Severe Toxicity (>2.0 mmol/L): Gross cerebellar ataxia, dysarthria, nystagmus, myoclonus, fasciculations, acute confusion, seizures, renal failure, circulatory collapse, and coma.
• Management: Stop lithium; aggressive intravenous hydration with 0.9% sodium chloride (promotes natriuresis and renal lithium clearance); monitor electrolytes and lithium levels every 4–6 hours. Discuss with TOXBASE or the National Poisons Information Service: the EXTRIP workgroup recommends haemodialysis if lithium is above 4.0 mmol/L with impaired kidney function, or if there is decreased consciousness, seizures or life-threatening dysrhythmia whatever the level.

2. Phenytoin

Phenytoin is an antiepileptic drug indicated for tonic-clonic seizures and focal seizures, and as a second-line intravenous agent in status epilepticus. It blocks voltage-gated sodium channels in neuronal membranes.

• Non-Linear (Zero-Order) Saturation Kinetics: At low therapeutic doses, phenytoin clearance follows first-order kinetics (elimination proportional to concentration). However, within the therapeutic plasma range, the hepatic cytochrome P450 enzymes responsible for its metabolism (CYP2C9 and CYP2C19) become completely saturated (Michaelis-Menten kinetics). Once saturation occurs, elimination becomes constant regardless of concentration (zero-order). Consequently, a minor percentage increase in daily dose produces a massive, exponential, unpredictable surge in plasma concentration and acute toxicity.
• Target Therapeutic Range: 10 – 20 mg/L (Phenytoin is 90% bound to serum albumin; in patients with hypoalbuminaemia or renal disease, total plasma levels appear falsely low; calculate corrected phenytoin using the Sheiner-Tozer formula).
• Acute Concentration-Dependent Toxicity:
  • >20 mg/L: Nystagmus (horizontal gaze-evoked nystagmus is the earliest and most sensitive physical sign of toxicity).
  • >30 mg/L: Truncal and gait ataxia, diplopia, slurred speech (dysarthria), nausea, vomiting.
  • >40 mg/L: Lethargy, confusion, ophthalmoplegia, coma, and paradoxical seizure aggravation.
• Chronic Idiosyncratic / Cosmetic Adverse Effects: Gingival hyperplasia (gum overgrowth in up to 50%), coarsening of facial features, hirsutism, peripheral motor-sensory neuropathy, cerebellar atrophy, folate deficiency with megaloblastic anaemia, osteomalacia (accelerated vitamin D catabolism), and teratogenicity (Fetal Hydantoin Syndrome).

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Respiratory TDM: Theophylline / Aminophylline

Theophylline (and its water-soluble ethylenediamine salt aminophylline, ratio 1:0.8) is a methylxanthine phosphodiesterase (PDE) inhibitor that promotes intracellular cyclic AMP accumulation, inducing bronchodilation and mild diaphragm inotropic stimulation in acute severe asthma and COPD exacerbations.

• Target Therapeutic Range: 10 – 20 mg/L (or 55–110 micromol/L). Sampled 4–6 hours post-dose (oral) or 4–6 hours after initiating an IV infusion.
• Pharmacokinetics & CYP1A2 Interactions: Theophylline is heavily metabolized by hepatic CYP1A2. Enzyme inhibitors (ciprofloxacin, erythromycin, clarithromycin, cimetidine) cause rapid theophylline retention and severe toxicity.
• Toxicity Manifestations: Anorexia, persistent intractable vomiting, agitation, sinus tachycardia, atrial and ventricular tachyarrhythmias, severe hypokalaemia, and refractory, life-threatening epileptic seizures (often unresponsive to standard anticonvulsants).
• Management: Stop the drug; administer multiple doses of oral activated charcoal (enhances gastrointestinal clearance via gastrointestinal dialysis); manage tachyarrhythmias with beta-blockers; arrange emergency haemoperfusion or haemodialysis for severe poisoning.