F1. Pharmacokinetic parameters Flashcards
what is bioavailability?
-Important term related to absorption, but not ONLY to absorption
-The fraction of the dose which reaches the systemic circulation as intact drug
-In most cases bioavailability is going to be less than 100%
what will affect absorption?
-Movement across membranes
-Passive diffusion
-Lipophilicity (partition coefficient)
-Size
-Ionisation
Bioavailability in the case or oral administration?
-Causes of reduced oral bioavailability
-Loss in the faeces
-Decomposition in the lumen
-Destruction within the wall of the gastrointestinal tract (intestinal first-pass metabolism)
-Destruction within the liver (hepatic first-pass metabolism) – this aspect of reduced bioavailability happens AFTER absorption took place
ONE NOTE
Describe distribution
-Drug is carried by blood and reversibly transferred to tissue and back to blood
-An equilibrium is reached with time
-But…..drug also lost from body (hence not true equilibrium)
-Many factors affect rate and extent of distribution
Describe volume of distribution (Vd)?
-Volume of distribution is ONE of TWO major independent pharmacokinetic parameters
-It is not a ‘real’ volume (rarely reflects real volumes) – therefore is frequently called Apparent Volume of Distribution
-Describes the relationship between the amount of the drug in the body and the concentration in blood (plasma)
Volume of Distribution (Vd) equation?
Vd= total amount of drug in body (Ab) / Plasma drug concentration (C)
Describe the elimination of drugs
-Two main routes of drug elimination: excretion and metabolism
-Kidneys (mostly): excreted unchanged
-Liver (mostly): metabolised and then excreted
-Metabolism and excretion can occur all over the body
-Kidneys and the liver are among the most highly perfused organs in the body
Describe clearance
-Clearance is a SECOND of TWO (first was Vd) major independent pharmacokinetic parameters
-Describes efficiency of irreversible elimination of a drug molecule from the systemic circulation
-Clearance may be through excretion of the unchanged drug or metabolic conversion to a different molecule
-The metabolite may still be in the body – but the parent molecule has been cleared
-Uptake into tissues is not clearance if the unchanged drug eventually returns to the systemic circulation
ONE NOTE
Describe extraction ratio
ONE NOTE
Describe clearance
ONE NOTE
Describe rate of elimination
ONE NOTE
