Understanding AUC in Biopharmaceutics

Apr 23, 2025

Biopharmaceutics and Pharmacokinetics: Area Under the Curve (AUC)

Introduction

  • AUC Definition: Represents the total integrated area under the plasma level-time profile indicating the total amount of active drug reaching systemic circulation.
  • AUC Formula: AUC = Dose / Clearance
  • Units: Expressed in microgram per ml hours.
  • Independence: Independent of the route of drug administration and elimination rate, provided these rates do not change.
  • Proportionality: AUC is directly proportional to the dose of a drug.

Importance of AUC

  • Toxicology: Measures drug exposure, derived from drug concentration and time, indicating how long and how much drug stays in the body.
  • Efficacy: Some antibiotics’ effectiveness is related to AUC or MIC (Minimum Inhibitory Concentration).
  • Bioavailability: Crucial for evaluating the bioavailability of a drug from different dosage forms, representing the extent of absorption.
  • Pharmacokinetics: Used to determine pharmacokinetic parameters such as clearance or bioavailability.
  • Chronic Conditions: Important for drugs administered repetitively for chronic conditions like asthma or epilepsy.

Methods to Determine AUC

  • Available Methods:
    • Plenty meter method
    • Cut and weight method
    • Trapezoidal rule (focus of this lecture)
    • Counting square method

Trapezoidal Rule

  • Purpose: Simplest and most common method to determine AUC.
  • Steps:
    1. Divide AUC into Trapezoids: Segment the whole AUC into small trapezoids.
    2. Calculate Area of Each Segment: Use the formula:
      • AUC(1-2) = (Cp2 + Cp1) / 2 * (T2 - T1)
    3. Sum All Segment Areas: Add up all segment areas.
    4. Consider First and Last Segments:
      • First Segment: Calculate after determining zero plasma concentration (Cp0) by extrapolation.
      • Last Segment: Utilize the specific formula to calculate.

Conclusion

  • Utility: AUC calculated using the trapezoidal rule helps in determining the relative efficiency of drug products, total body clearance, and the apparent volume of distribution.
  • Application: Direct calculation from Cp versus time data.

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