drug disposition Flashcards
what are the fates of drugs in the body ?
absorption
distribution
metabolism
excretion
absorption
a process by which a drug enters the body from its site of administration
distribution
process by which drug leaves circulation and enters the tissues perfused by blood
metabolism
process by which tissue enzymes catalyse chemical conversion of a drug to a more polar form that is more readily excreted from the body
excretion
process that remove the drug from the body
what factors control drug absorption ?
solubility
chemical stability
lipid to water partition coefficient
degree of ionisation
how does solubility control drug absorption ?
must dissolve
how does chemical stability control drug absorption ?
some drugs may be destroyed by acid
how does lipid to water partition coefficient control drug absorption ?
rate of diffusion of drug increases with lipid solubility
how does degree of ionisation control drug absorption ?
only unionised form readily diffuse across the lipid bilayer
what does degree of ionisation depend on ?
depends upon pKa of the drug and local pH
pKa
pH at which 50% of drug is ionised and 50% unionised
what is henderson-hasslbalch ?
an equation that determines the pH of a buffer
pH - pKa = log(A-/AH) = acid
what is oral availability ?
fraction of drug that reaches the systematic circulation after oral ingestion
what is systemic availability ?
fraction of drug that reaches the systemic circulation after absorption
what are routes of administration for drugs ?
enteral
parenteral
what are examples of enteral routes ?
oral
sublingual
rectal
what are examples of parental routes ?
intravenous
intramuscular and subcutaneous
inhalation
topical
what does systemic circulation allow ?
allows drug to be distributed to one or more body fluid compartments
are drugs evenly distributed in the body ?
no
what is the volume of distribution (Vd) ?
apparent volume is which drug is dissolved
how can you figure out the dose/plasma concentration ?
IV - Vd = dose/plasma concentration
