Selected bioavailability and pharmacokinetic calculations

1
Selected bioavailability and pharmacokinetic
calculations
Dr. Osama A. A. Ahmed
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Selected bioavailability and pharmacokinetic
calculations

• 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.
• Bioavailability the relative amount of drug
  from an administered dosage form that enters the
  systemic circulation.
• Pharmacokinetics the study and characterization
  of the time course of ADME of drugs.
• Plotting and interpreting drug dissolution data
  (page 322)
• Amount of dug bioavailable from a dosage form
• Drugs bioavailability factor (F) represents the
  decimal percentage of a drug substance available
• 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?
• 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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Selected bioavailability and pharmacokinetic
calculations

• 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
• 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?
• 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
• 0.0375 mg 1
  ml
• 0.15 mg
  X ml X 4 ml
• Plotting and interpreting a blood-drug
  concentration-time curve (Page 323)

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Selected bioavailability and pharmacokinetic
calculations

• The bioavailability of the drug may be determined
  by comparison of the AUC data for the particular
  dosage form against the intravenous form.
• 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 4.5 mg/ml/hr / 11.2 mg/ml/hr
  0.4 or 40

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Selected bioavailability and pharmacokinetic
calculations

• Different drugs administered in the same amount
  will show different volumes of distribution
  because of distribution characteristics
• Where, Vd is volume of distribution, D is the
  amount of drug in the body, and Cp is the drugs
  plasma concentration.
• 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
• 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