Section 4.2: Clinical Pharmacokinetics & Therapeutic Drug Monitoring (TDM)

Clinical Pharmacokinetics & Therapeutic Drug Monitoring (TDM)

Principles of Therapeutic Drug Monitoring (TDM)

Therapeutic Drug Monitoring (TDM) is the clinical practice of measuring drug concentrations in blood, plasma, or serum to guide individual dosing regimens. TDM is indicated for drugs that meet specific criteria:

• Narrow Therapeutic Index (NTI): Small differences between therapeutic and toxic concentrations.
• Significant Pharmacokinetic Variability: Wide inter-individual variation in absorption, distribution, metabolism, or excretion.
• Established Relationship: Clear correlation between serum concentration and efficacy or toxicity.
• Lack of Easily Measurable Clinical Markers: Efficacy cannot be readily assessed by bedside monitoring (unlike blood pressure for antihypertensives or blood glucose for insulin).

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Key Drug Classes Monitored by TDM

1. Aminoglycosides (Gentamicin, Tobramycin, Amikacin)

Aminoglycosides exhibit concentration-dependent killing and a significant post-antibiotic effect (PAE).

• Traditional Dosing: Administered 2-3 times daily. Peak concentrations reflect efficacy, while trough levels correlate with toxicity. For Gentamicin and Tobramycin, traditional peaks are 5–10 mcg/mL, and troughs must be < 2 mcg/mL (ideally < 1 mcg/mL) to avoid nephrotoxicity and ototoxicity. For Amikacin, peaks are 20–30 mcg/mL, and troughs < 8 mcg/mL.
• Extended-Interval Dosing (EID): Administered once daily (typically 5–7 mg/kg for gentamicin/tobramycin). Takes advantage of concentration-dependent killing (high peak ~16–24 mcg/mL) and long PAE while allowing the trough to drop to undetectable levels, reducing drug accumulation in renal cortical cells. The Hartford nomogram utilizes a single serum concentration drawn 6–14 hours after the start of the infusion to determine the appropriate dosing interval (24, 36, or 48 hours).
• Exclusion Criteria for EID: Pregnancy, severe renal impairment (CrCl < 30 mL/min), extensive burns, ascites, cystic fibrosis, or endocarditis.

2. Vancomycin

Vancomycin is a glycopeptide antibiotic with time-dependent killing.

• Target Parameter: Efficacy and safety are best predicted by the ratio of the Area Under the Curve to the Minimum Inhibitory Concentration ($\text{AUC}/\text{MIC}_{BMD}$). The target $ ext{AUC}/ ext{MIC}$ ratio is 400–600 (assuming a MIC of 1 mcg/mL).
• Monitoring Strategy: While trough-guided monitoring (targeting 15–20 mcg/mL for serious infections like MRSA bacteremia, osteomyelitis, meningitis, and pneumonia) was historically common, recent consensus guidelines mandate AUC/MIC monitoring using Bayesian software or two-point pharmacokinetic equations to reduce the incidence of acute kidney injury (AKI). Nephrotoxicity risk increases significantly with sustained troughs > 20 mcg/mL or AUC values > 650 mg·h/L.

3. Phenytoin

Phenytoin is an anticonvulsant with unique, capacity-limited (non-linear, Michaelis-Menten) pharmacokinetics.

• Michaelis-Menten Kinetics: At low doses, clearance is constant. As serum concentrations approach the therapeutic range, metabolizing enzymes (CYP2C9 and CYP2C19) become saturated. Consequently, small increases in dose lead to disproportionately large increases in serum concentrations and potential toxicity.
• Protein Binding & Hypoalbuminemia: Phenytoin is highly protein-bound (~90% to albumin). Only the unbound (free) fraction is active. In conditions with low albumin (e.g., liver disease, nephrotic syndrome, pregnancy, or in older adults) or reduced binding affinity (e.g., end-stage renal disease with CrCl < 10 mL/min), the measured total phenytoin level appears falsely low, while the active free level may be therapeutic or toxic.
• Sheiner-Tozer Equation: To evaluate phenytoin levels in patients with hypoalbuminemia (albumin < 3.5 g/dL) or renal impairment, the corrected phenytoin level must be calculated: $\text{Corrected Phenytoin} = \frac{\text{Measured Total Phenytoin}}{(0.2 \times \text{Albumin}) + 0.1}$ In patients with severe renal failure (CrCl < 10 mL/min), the binding affinity is further reduced, and the equation is modified: $\text{Corrected Phenytoin} = \frac{\text{Measured Total Phenytoin}}{(0.1 \times \text{Albumin}) + 0.1}$
• Therapeutic Ranges:
  • Total Phenytoin: 10–20 mcg/mL.
  • Free Phenytoin: 1–2 mcg/mL.

4. Digoxin

Digoxin inhibits the $\text{Na}^+/\text{K}^+$-ATPase pump, increasing intracellular calcium.

• Therapeutic Ranges:
  • Heart Failure: 0.5–0.9 ng/mL (higher levels are associated with increased mortality).
  • Atrial Fibrillation: 0.8–2.0 ng/mL.
• TDM Timing: Serum samples must be drawn at least 6–8 hours after the oral dose (preferably 12 hours) to allow complete tissue distribution.
• Toxicity and Electrolytes: Digoxin toxicity manifests as gastrointestinal symptoms (nausea, vomiting), neurological signs (confusion), and cardiac arrhythmias (bradycardia, heart block). Hypokalemia, hypomagnesemia, and hypercalcemia predispose patients to digoxin toxicity even at "therapeutic" serum levels because potassium competes with digoxin for binding on the $\text{Na}^+/\text{K}^+$-ATPase pump.

5. Lithium

Lithium is used for bipolar disorder and has a narrow window.

• Therapeutic Ranges:
  • Acute Mania: 0.8–1.2 mEq/L.
  • Maintenance: 0.6–1.0 mEq/L.
• Elimination & Drug Interactions: Lithium is 100% renally eliminated, handled by the proximal tubules in a manner identical to sodium. Any state that decreases sodium levels or renal blood flow increases lithium reabsorption, leading to toxicity. Key interactants include thiazide diuretics (deplete sodium, leading to compensatory proximal tubule lithium reabsorption), ACE inhibitors/ARBs (reduce aldosterone, affect GFR), and NSAIDs (constrict afferent arteriole, reducing GFR).

6. Calcineurin Inhibitors (Cyclosporine, Tacrolimus)

These immunosuppressants are utilized in solid organ transplantation to prevent rejection.

• Tacrolimus: Target trough range is typically 5–15 ng/mL (whole blood). Side effects include nephrotoxicity, neurotoxicity (tremor, headache), hyperglycemia, alopecia, and hyperkalemia.
• Cyclosporine: Target trough range is 100–400 ng/mL (whole blood). Alternatively, monitoring the 2-hour post-dose concentration ($C_2$, target 800–1200 ng/mL) provides a better correlation with systemic exposure. Side effects include nephrotoxicity, hypertension, hyperlipidemia, hirsutism, and gingival hyperplasia.

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Clinical Dosing Adjustments in Renal Impairment

For renally cleared drugs, dosing adjustments are guided by GFR or creatinine clearance (CrCl) calculated using the Cockcroft-Gault equation: $\text{CrCl} = \frac{(140 - \text{Age}) \times \text{Ideal Body Weight (kg)}}{72 \times \text{Serum Creatinine (mg/dL)}} \times [0.85 \text{ for females}]$

Adjustments can be made by either reducing the dose (maintaining therapeutic levels with smaller peaks, preferred for time-dependent antibiotics) or extending the dosing interval (allowing peaks to remain high but giving time for clearance, preferred for concentration-dependent antibiotics like aminoglycosides).