Clinical Pharmacokinetics II

Clinical pharmacokinetics II explores advanced dosing strategies across diverse patient populations including pediatric, geriatric, and obese patients. This comprehensive course examines how age, body composition, and organ dysfunction affect drug absorption, distribution, metabolism, and elimination. Students learn to calculate dose adjustments for renal and hepatic impairment while understanding extracorporeal drug removal methods. JoVE Coach provides essential preparation for pharmacy and medical students tackling complex pharmacokinetic applications in real US clinical settings.

• Understand physiological changes affecting drug pharmacokinetics in pediatric, geriatric, and obese patients
• Learn dose calculation methods for different patient populations using body weight, surface area, and age-based formulas
• Identify how renal impairment affects drug clearance and apply dose adjustment strategies using creatinine clearance
• Explore hepatic disease effects on drug metabolism and determine appropriate dosing modifications
• Analyze pharmacokinetic differences in special populations and their clinical implications
• Apply advanced dosing principles for patients requiring hemodialysis, peritoneal dialysis, or continuous renal replacement therapy
• Understand population pharmacokinetics and sources of pharmacokinetic variability in clinical practice
• Evaluate drug removal mechanisms during extracorporeal therapies and adjust dosing accordingly

1. Pediatric Pharmacokinetics and Dosing Strategies Pediatric patients exhibit unique pharmacokinetic profiles requiring specialized dosing approaches. Neonates have higher total body water content (80% vs 60% in adults) and immature organ systems affecting drug disposition. For example, gentamicin dosing in newborns requires 24-48 hour intervals compared to 8-12 hours in adults due to reduced renal function. Children aged 3-10 years often demonstrate enhanced hepatic metabolism, requiring higher per-kilogram doses of drugs like carbamazepine. Body surface area calculations using the Mosteller formula provide more accurate dosing than weight-based methods for many medications in pediatric populations.

2. Geriatric Pharmacokinetics and Age-Related Changes Aging significantly impacts all aspects of pharmacokinetics, necessitating careful dose modifications in patients over 65 years. Reduced albumin levels increase free drug concentrations for highly protein-bound medications like warfarin, requiring lower doses to prevent toxicity. Decreased lean body mass and total body water reduce distribution volumes for hydrophilic drugs such as digoxin, while increased body fat enhances distribution of lipophilic drugs like benzodiazepines. The Cockcroft-Gault equation estimates creatinine clearance in elderly patients: CrCl = [(140-age) × weight] / (72 × serum creatinine), with results multiplied by 0.85 for females.

3. Obesity Effects on Drug Pharmacokinetics Obesity, defined as BMI ≥30 kg/m² or weight exceeding ideal body weight by 20%, significantly alters drug distribution and clearance. Hydrophilic drugs like gentamicin should be dosed using ideal body weight to prevent overdosing, while lipophilic drugs such as propofol require total body weight dosing due to extensive adipose tissue distribution. Obese patients often exhibit increased hepatic blood flow and enhanced renal clearance for certain medications. Bariatric surgery patients may show altered drug absorption, with metformin bioavailability increasing by 50% post-gastric bypass surgery compared to non-surgical patients.

4. Renal Impairment and Dose Adjustment Methods Kidney dysfunction requires systematic dose adjustments to prevent drug accumulation and toxicity. Creatinine clearance estimation using the Cockcroft-Gault equation or eGFR calculation with CKD-EPI equations guides dosing decisions. For drugs primarily eliminated by kidneys, the dose adjustment factor equals patient CrCl divided by normal CrCl (120 mL/min). Aminoglycoside antibiotics exemplify renally eliminated drugs requiring significant dose reductions or interval extensions in renal impairment. The fraction excreted unchanged (fe) determines the magnitude of dose adjustment needed, with drugs having fe >0.7 requiring substantial modifications.

5. Hepatic Impairment and Drug Metabolism Changes Liver disease affects drug metabolism through reduced hepatic blood flow, decreased enzyme activity, and altered protein synthesis. Child-Pugh classification (Classes A, B, C) helps assess hepatic function severity, though it doesn't directly correlate with cytochrome P450 enzyme activity. High-extraction drugs like propranolol are more affected by hepatic blood flow changes, while low-extraction drugs such as phenytoin are influenced by intrinsic clearance alterations. Drug metabolites may be more or less active than parent compounds, requiring monitoring of both species in hepatic impairment patients.

6. Extracorporeal Drug Removal and Dialysis Hemodialysis, peritoneal dialysis, and continuous renal replacement therapy (CRRT) remove drugs through diffusion and filtration mechanisms. Drug removal depends on molecular weight, protein binding, and distribution volume. Water-soluble, low molecular weight drugs with minimal protein binding are effectively removed by dialysis. For example, lithium requires post-dialysis dosing due to significant removal, while highly protein-bound drugs like warfarin are poorly dialyzed. CRRT provides continuous drug removal with sieving coefficients (S) indicating clearance efficiency - when S=1, complete drug passage occurs through the membrane.