Drug dosing and elimination

Ammount of drug that reaches circulation

dependent upon routes of administration

Intravenous administration is 100% bioavailable

Can be used to compare drugs

Drugs with the same route of administration and
mechanism can compare efficiency by bioavailability

Bioequivilence

Two drugs with varying concentration of the same therapeutic ingredient and different routes of
administration, but same bioavailability

First order

Accounts for most drugs

Drug elimination dependent on concentration

Constant percentage of drug eliminated

Zero order

Elimination of drug is constant

irrespective of drug concentration

Some first order drugs at high concentrations
function as zero order drugs

Zero order drugs

Alcohol

salicylate

Phenytoin

Half life

Amount of time for half of initial
concentration to be cleared

Constant time irrespective of
initial concentration

Plasma half life does not always equal
pharmacological half life

Some things that can cause discrepency

Irreversible receptor binding

DNA damaging drugs

Does not apply to second order drugs

Rate constant can be calculated for a drug

A method of approximating drug distribution

One compartment model

Assumes drug is freely
distributed and eliminated

Used for:

Antibacterial drugs

Aminoglycosides

Multicompartment model

modeling every organ is unrerasonable

For many drugs 2 compartments is sufficient

Used to account for distribution phase

Can group together key organs

Second compartment is the rest of the body

adding additional dose before the origonal dose is excreted is cumulative and leads to a new peak of dose

Dosing interval

a consistent time period between drug doses

plateau state

After giving the same dose over the same interval for an amount of time, there will be a maximum dose that additional doses will not peak above

Drug swing

Peak dose minus trough dose when at plateau state

Maitinance dose

Can be solved for if you have:

Max concentration

Dosing interval

Practically, a steady state is
reached after 3-5 half lives

Loading dose

In instances where waiting for
steady state is not reasonable

Administering a larger initial dose
followed by smaller maintenance dose