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.

A. 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.

B. 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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