topic 4 Flashcards
what is pharmacokinetics?
The mathematical way to describe absorption, distribution, biotransformation, and elimination of a drug.
What are some important parameters for pharmacokinetics?
Bioavailability-The fraction of drug absorbed into circulation
Volume of distribution-the apparent volume the drug occupies in the body
Half-life
Clearance-body’s ability to eliminate a drug
What is a first order process dependent on and what first order processes occur in the body?
It is dependent upon concentration at a given time.
If the log of the concentration is plotted on a graph vs. time, it is a straight line with a slope of -K.
Loss of drug from the blood is first order. Absorption, distribution, and elimination are first order.
How is rate of elimination calculated?
Since it basically a first order process, we usually use the half-life of the drug?
How is half life defined and calculated?
It is the time taken for a drug to be reduced by half.
t1/2=.693/k (remember slope is -k)
what are some factors that affect bioavailabiliity
1st pass metabolism
Low absorption in GI tract (oral forms are poorly water soluble, physiochemical properties of drug)
Hydrolysis by gastic acid or digestive enzymes
nature of drug formulation (incomplete release from tablet)
How to calculate rate of absorption
usually a rate isn’t calculated, rather the time it takes to get to a max is calculated b/c distribution is faster than absorption and doesn’t affec things too much.
After absorption occurs (shortly after max value), the curve is a first order elimination
What is bioavailability and how is it calculated?
It is the extent of absorption
F=area under curve oral/area under curve IV
or if different doses are given
F=area under curve oral x dose of IV/ area under curve IV x dose of oral.
what is the therapeutic significance of bioavailability
Differences in plasma concentrations can cause a very different effect
If the therapeutic index is small, you need to be careful, especially if the different between therapeutic and toxic is small.
What is the important of bioequivalence?
Drugs with the same bioavailability may not have the same bioequivalence which would result in a different therapeutic effect.
What are the two types of time of distribution of drugs?
Instantaneous-fast, described by linear curve (1 order elimination constant
Finite-slow, complicated curve, not described by single exponential constant.
How is volume of distribtion calculated?
For instantaneous,
Vd=total amount of drug in body (taken from dose in systemic circulation which depends on bioavailability)/plasma concentration at a point in time (Cp).
For finite
Vd=total amount of drug in body (dose)/Cp at time 0 (found by extrapolating elimination rate line back to time 0…shows amt. of drug after absorption).
Can be corrected by dividing by the weight of the individual.
What is capacity-limited elimination and what are its effects?
It is when the carriers are saturated or there is so much plasma concentration that tubular secretion is overwhelmed so the elimination is described as zero order meaning it is constant, not depedent on conc. and a half-life can’t be calculated.
What is mixed elimination?
When a drug exhibits zero order kinetics initially, then 1st order. This is due to the fact that it is saturated at first then when the concentration gets lower and its not saturated, it becomes 1st order. It looks linear then exponential on a group.
What is clearance and how is it calculated?
Clearance is the rate of loss of a drug from the body.
Cl=Fracture of drug cleared per unit time X Plasma volume
Cl (clearance)=ke (elimination rate constant) x Vd (volume of distribution)
ke=.693 / t1/2
