Pharmacokinetics 1 Flashcards
What is clearance?
body’s efficiency of drug removal. Theroretical volume of fluid from which a drug is removed per unit time.
CL = rate of elimination/concentration
What is volume of distribution?
apparent space the drug resides in. Fluid volume that would be required to contain all of the dose at the same concentration as exists in the blood or plasma.
Vd = amount of drug in body/ C
where C is blood concentration
What is elimination half-life?
rate of drug removal
What is bioavailability?
fraction of drug absorbed
What is the most important concept to be considered when a rational regimen for long-term drug administration is to be designed?
Clearance
What is the formula for dosing rate for long-term administered drugs?
Dosing rate = CL x Css (where CL is clearance and Css is steady-state concentration of drug
What is formula for rate of elimination of an organ?
Rate of elimination of organ = Q(Ca-Cv), where Q = blood flow to organ
What is the formula for clearance in terms of Ca and Cv?
CL = Q[(Ca-Cv)/Ca] = Q x E
Where E is the extraction ratio
What is the limiting variable for elimination from any organ?
blood flow, Q
What type of kinetic mode do most drugs eliminate by?
First order kinetics
What is formula for volume of distribution?
Vd = amount of drug in body/blood concentration
blood concentration = C
What is average total body water? Plasma volume? Extracellular fluid volume? Intracellular fluid?
Total = 40 L Plasma = 4 L Extracellular = 12 L Intracellular = 28L
What is Ke?
elimination constant = CL/Vd
What is the formula for half-life for zero order kinetic drug?
t1/2 = 0.7/Ke = 0.7xVd/CL
Why is half-life clinically important?
Helps determine the dosing interval (drugs are usually given at half-life intervals).
May determine route of administration (short half-lifes are usually given by IV or sustained release patches or tablets)
Provides good indication of time required to reach steady-state after a dosage regimen is initiated. Assumes one elimination rate (excretion)
What is the one-compartment model? What type of drugs is it acceptable for?
Assumes the entire human body is one compartment. This works for drugs that are distributed uniformly throughout the body
What is the two-compartment model?
Assumes that much of a drug is in a particular compartment and that an equilibrium exists between the blood and other areas. Have two different elimination rates (excretion and transfer rate between blood and compartment)
What type of model is most accurate for human body?
Multicompartment models. Requires use of computer to model, but is more widely used than one and two-compartment models.
CL = dose/AUC
AUC = area under curve
What is the equation to factor bioavailability into the dosing rate?
FxDosing rate = CL x Css
Taking bioavailability into account, what is formula to calculate steady state concentration?
Css = (FxDose)/(CLxT)
Where Dosing rate = Dose/T
T = dosing interval
Taking bioavailability into account, what is equation to calculate Dose?
Dose = (Css x CL x T)/F
About how long does it take to achieve steady state?
Usually takes about 5 half-lives to achieve steady state
What is time to steady-state strictly dependent on?
half life.
This is the plateau principle
