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Module 5 Pharmacokinetcs Pt. 2 By Lauren McCormick (Bioavailability…
Module 5
Pharmacokinetcs Pt. 2
By Lauren McCormick
Bioavailability
amount of drug that actually reaches systemic circulation (protein bound or unbound)
compares amount of drug absorbed after administration by a route (often oral) to that absorbed after IV administration
Bioequivalency
bioequivalent drug products show no significant difference in rate and extend of absorption of therapeutic ingredient
Models of Drug Distribution
one compartment
considers body as a single well-mixed container w/ volume equal to volume of distribution for that drug
drug distributes instantaneously after IV administration
drug freely removed by metabolism or excretion
plotted: elimination from plasma demonstrates straight line on logarithmic scale y-axis
two compartment
drug accumulates in specific compartments prior to distribution throughout the entire body
too complex to represent every organ, so we look at two compartments
first compartment = liver, heart, lungs, and brain (all grouped together)
second compartment = entire body
plotted: biphasic curve
Orders
first order kinetics
most drugs disappear from plasma by concentration dependent processes
constant % of drug lost per unit time
elimination rate constant is present
half life
period of time required for concentration of drug to decrease by 1/2
dose does not affect half life
rate of elimination if proportional to plasma drug concentration
unique for each drug & environment
pharmacologic half life:
time taken for pharmacological effect of drug to decay by half
administration route does not affect half life
most drugs follow first order kinetics
zero order kinetics
drugs that saturate elimination routes disappear from plasma in non-concentration dependent process
constant amount of drug lost per unit time
ex. alcohol dehydrogenase reaches a maximum catalytic rate at low alcohol concentrations
Multiple Dosing
first order kinetics
additional dose added prior to first dose being eliminated = reaches higher peak concentration
until plateau/
maintenance state
is reached
Drug swing = interval between each dose
Dose administered = amount eliminated in dose interval
equation may be rearranged if either dose or dosing interval is requried
dosing rate may be calculated by steady infusion via IV if clearance is known
steady state for maintenance achieved after ~3-5 half lives
long half life? may take weeks to achieve steady maintenance state
Loading/Priming Dose
sometimes needed to reach maintenance rapidly
reached in second dose via IV or repeated dose administration
contraindicated for drugs w/ low therapeutic index
must be followed by repeated doses or graded continuous infusion
