Kinetics of Drug Elimination

• Kinetics of drug elimination explains how drugs are removed from the body, following zero or first-order processes.
• Drug elimination kinetics describe how the concentration of a drug decreases over time.

Clearance and Half-Life

• Clearance (CL): The volume of plasma completely cleared of drug per unit time (mL/min or L/hr).
  • Total body clearance is the sum of individual clearances via each route (e.g., hepatic + renal + others).
• Half-life (t½): Depends on volume of distribution (Vd) and clearance (CL):

$t_{\frac{1}{2}} = \frac{0.693 \times V_d}{CL}$

• Knowing half-life helps determine dosing intervals and time to reach steady-state (usually 4–5 half-lives under first-order kinetics).

Types of Drug Elimination Kinetics

1. First-Order Kinetics (Exponential Elimination):

   • Most drugs follow this.
   • A constant fraction of drug is eliminated per unit time.
   • Rate of elimination is proportional to drug concentration.
   • Drug concentration decreases exponentially.
   • Half-life (t½):
     • Time taken for the plasma concentration to decrease by 50
     • Independent of dose in first-order kinetics.
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2. Zero-Order Kinetics (Linear Elimination):

   • A constant amount of drug is eliminated per unit time (not a fraction).
   • Occurs when elimination mechanisms are saturated.
   • Small dose increases can lead to toxicity.
   • Examples:
     • Ethanol
     • Phenytoin
     • High doses of aspirin
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