Pharmacokinetics Flashcards
pharmacokinetics
the discipline that describes the absorption, distribution, metabolism, and excretion of drugs
*how the body affects the drugs
criteria for getting a drug to work
- unbound and active form of drug has to bind to receptor
- usually, the drug gets to the receptor via the plasma
- usually, the concentration of the drug at the active site decreases over time
steady state serum level
serum concentration when the amount of drug going into the patient = amount going out of the patient
*must be continued dosing to acheive
loading dose
one-time dose to get to a therapeutic level quickly
*usually followed by smaller amounts of continued dose
volume of distribution - definition
THEORETICAL volume that the amount of drug administered would occupy (it it were uniformly distributed) to provide the same concentration as it currently is in the blood plasma
volume of distribution - equation
Vd = amount of drug in body / concentration
concentration =
amount / volume
what will INCREASE volume of distribution (Vd)
extensive binding and/or distribution to the TISSUES
what will DECREASE volume of distribution (Vd)
increased binding to SERUM proteins
clearance - definition
represents a volume of plasma from which the drug is eliminated per unit time (volume/time)
rate of elimination = ? (for a first order process)
rate of elim = clearance x concentration
clearance - equation (for a first order process)
clearance = Vd x Ke
*note - Ke is the elimination rate constant
zero-order elimination of a drug
a CONSTANT AMOUNT is eliminated over time, regardless of concentration
*ex = 1 mg is eliminated every hour
*linear plot
first-order elimination of a drug
a constant PROPORTION is eliminated over time
*amount eliminated decreases as concentration decreases
*ex = 20% is eliminated every hour
*logarithmic plot
*MOST drugs are first-order
half-life - definition
time required to decrease the amount of drug in the body by HALF
half-life - equation
half-life = (0.7 x Vd) / clearance
how many half lives does it take to achieve steady state
~5 half-lives for ANY DOSAGE CHANGE (starting or stopping drugs)
are loading doses affected by changes in elimination
NO (but maintenance doses do)
how can you decrease concentration variability when giving a drug
give the drug more frequently
how does albumin affect volume of distribution
albumin is a serum protein, and many medications bind to it, SO:
*LOW serum albumin means more drug going into the tissues (HIGHER Vd)
*HIGH serum albumin means more drug staying in the plasma (LOWER Vd)
routes of absorption (how drugs cross membranes)
- passive diffusion
- facilitated passive diffusion
- active transport
- pinocytosis (rarely)
benefits of absorption of drug in small intestine
*largest surface area for drug absorption
*membranes more permeable than those in stomach
bioavailability (f) - definition
fraction of unchanged drug reaching SYSTEMIC CIRCULATION following administration from any route
bioavailability (f) for IV administration
= 1 (100% bioavailability)
