Measurement of bioavailability

Measurement of bioavailability involves plasma drug concentration, urinary excretion data, and pharmacokinetic parameters.

• Bioavailability is measured using pharmacokinetic and pharmacodynamic methods to assess the extent and rate of drug absorption.

1. Pharmacokinetic Measurement

   • Pharmacokinetic methods analyze drug concentration in the body over time to determine how much reaches systemic circulation.
   • The two main approaches are:

     1. Plasma Level-Time Study

        • This method measures drug concentration in blood plasma at different time points after administration.
        • Process:
          1. Drug Administration – Given via a non-IV (e.g., oral) or IV route (reference).
          2. Blood Sampling – Collected at fixed intervals to track absorption, distribution, and elimination.
          3. Drug Concentration Analysis – Measured using HPLC, mass spectrometry, or immunoassays.
          4. Data Analysis – Generates a plasma concentration-time curve, determining:
             • AUC (Area Under the Curve) – Total drug exposure.
             • Cmax (Peak Plasma Concentration) – Maximum drug level in plasma.
             • Tmax (Time to Cmax) – Time taken to reach peak concentration.

     2. Urinary Excretion Study

        • Used for drugs primarily excreted unchanged in urine.
        • Process:
          • Drug Administration – Given via non-IV or IV routes.
          • Urine Collection – Samples taken at specific time intervals.
          • Drug Concentration Analysis – Using HPLC, mass spectrometry, or immunoassays.
          • Data Analysis – Calculates:
            • Drug excretion per interval.
            • Cumulative drug excretion over time.
            • Percentage of dose excreted to assess bioavailability.

2. Pharmacodynamic Measurement

   • Pharmacodynamic methods evaluate the relationship between drug concentration and its effects, especially when drug levels in blood do not directly correlate with its action.

     1. Acute Pharmacological Response

        • Measures short-term drug effects after administration.
        • Methods:
          • Physiological Changes – E.g., changes in blood pressure, heart rate, pupil dilation.
          • Biochemical Markers – E.g., reduction in blood glucose after an antidiabetic drug.
          • Specific Drug Effects – E.g., receptor occupancy, enzyme inhibition.

     2. Therapeutic Response

        • Evaluates long-term clinical effectiveness of a drug.
        • Methods:
          • Symptom Relief – E.g., pain reduction in chronic pain patients.
          • Disease Progression – E.g., reduced relapses in multiple sclerosis or tumor stabilization in cancer.
          • Survival Rates – E.g., overall survival, progression-free survival in life-threatening conditions.

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