Download Selected bioavailability and pharmacokinetic calculations

Selected bioavailability and pharmacokinetic
calculations
Dr. Osama A. A. Ahmed
Selected bioavailability and pharmacokinetic calculations
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Biopharmaceutics: the area of study that deals with the properties of drug substances
and dosage forms that influence the release of the drug for biologic activity.
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Bioavailability: the relative amount of drug from an administered dosage form that
enters the systemic circulation.
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Pharmacokinetics: the study and characterization of the time course of ADME of
drugs.
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Plotting and interpreting drug dissolution data (page 322)
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Amount of dug bioavailable from a dosage form
Drug’s bioavailability factor (F) represents the decimal percentage of a drug substance
available
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Example: if the bioavailability factor (F) for a drug substance in a dosage form is 0.60,
how many milligrams of drug would be available for absorption from a 100-mg tablet
of the drug?
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F indicates that only 60% of the drug present in the dosage form is available for
absorption
100mg X 0.60 = 60 mg
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Dr. Osama A. A. Ahmed
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Selected bioavailability and pharmacokinetic calculations
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Bioequivalent amounts of bioinequivalent dosage forms:
The bioavailability of a given drug substance may vary when in different dosage forms
or in the same dosage form but from a different manufacturer. Thus, it may be desired
to calculate the equivalent doses for two bioinequivalent products
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Example: If the biovailability (F) of digoxin in a 0.25-mg tablet is 0.60 compared to
the bioavailability (F) of 0.75 in a digoxin elixir (0.05 mg/ml), calculate the dose of the
elixir equivalent to the tablet?
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Digoxin biovailable from tablet = 0.25 mg X 0.60 = 0.15 mg
Digoxin biovailable per milliliter of the elixir = 0.05 mg X 0.75 = 0.0375 mg
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0.0375 mg
0.15 mg
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Plotting and interpreting a blood-drug concentration-time curve (Page 323)
Dr. Osama A. A. Ahmed
1 ml
X ml
X = 4 ml
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Selected bioavailability and pharmacokinetic calculations
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The bioavailability of the drug may be determined by comparison of the AUC data for
the particular dosage form against the intravenous form.
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F
AUCdosage form
AUCint ravenous
Example: If the AUC for an oral dose of a drug administered by tablet is 4.5 mg/ml/hr,
and the intravenous dose is 11.2 mg/ml/hr, caclculate the bioavailability of the oral dose
of the drug?
F
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Dr. Osama A. A. Ahmed
AUCoral tablet
AUCIV
F = 4.5 mg/ml/hr / 11.2 mg/ml/hr
=
0.4 or 40%
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Selected bioavailability and pharmacokinetic calculations
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Different drugs administered in the same amount will show different volumes of
distribution because of distribution characteristics
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D
Vd 
Cp
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Where, Vd is volume of distribution, D is the amount of drug in the body, and Cp is the
drug’s plasma concentration.
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Example: A patient received a single intravenous dose of 300 mg of a drug substance
that produced an immediate blood concentration of 8.2 mg of drug per ml. Calculate the
apparent volume of distribution?
Vd = 300 mg / 8.2 mg/ml = 300 mg / 8.2 mg/L = 36.6 L
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Elimination half life and elimination rate constant
Elimination t1/2 is the time takes for the plasma drug conc. to fall by one half
Elimination rate constant (Kel) is the fractional rate of drug removal per unit time
e.g. Kel = 0.01 min-1, meaning 1 % per minute
Kel = 0.693/ t1/2
Example: Page 327
Dr. Osama A. A. Ahmed
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