Pharmacokinetics

Definition of Pharmacokinetics:

The study of the kinetics of absorption, distribution, metabolism, and excretion (ADME) of drugs, their pharmacologic, therapeutic, or toxic effects on humans and animals.

Why Study Pharmacokinetics?

Applications:

1. Dosage calculations (general, pediatric, geriatric).

2. IV infusion medications.

3. Hospital pharmacy and industrial pharmaceutical research

Pharmacokinetic Data Representation:

• Plotted as concentration vs. time, typically shown with an IV bolus dose where no absorption phase exists.

• Key Parameters:

  • C(0): Initial concentration.

  • t1/2: Elimination half-life.

  • k: Elimination rate constant (k = 0.693/t½).

Volume of Distribution (V or Vd)

• Describes theoretical homogeneous volume for drug distribution.

• Equation:

  • V=D/C (0) OR V = Cl/ k

  • D= dose

  • C (0) = Initial concentration

• Helps determine how much of a drug is in the blood vs. peripheral tissues.

Clearance of a Drug (Cl)

• Volume of plasma cleared of a drug per unit time.

• Equation:

  • Cl = k × V OR. Cl = Dose/AUC

Area Under the Curve (AUC)

• Measures total drug exposure over time, representing bioavailability.

• Equation for IV bolus dose:

  • AUC = C(0) / k

  • Mostly use trapezium rule (a+b/2) × h

Compartment Models

• One Compartment Model: Drug is absorbed and eliminated uniformly.

• Two Compartment Model: Drug spreads rapidly to highly perfused organs (e.g., brain, liver) and then to areas with lower blood flow (e.g., muscle, fat).

Bioavailability (F)

• Fraction of administered drug that reaches systemic circulation.

• Equation:

  • F = AUC⁰ / AUCIV

Elimination Rate

• Describes drug concentration change over time:

  • 𝑑𝐶/𝑑𝑡 = −𝑘𝐶

  • Integration yields: C(t) = C(0) × e−kt

Elemenation Rate Constant

• K = ln 2 / t½ = 0.693/t½