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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


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


What are the ways in which a drug can be cleared

Major: kidney, liver

Minor: lungs, other (milk, sweat, saliva, etc.)

Cl=Cl (kidneys) + Cl (liver) + etc.


What is the steady state or plateau principle?

As you give a person more of a drug, the concentration in the blood of the drug increases rapidly at first, then it slows down and reaches a maximum. This is because as the plasma concentration increases, the rate of elimination increases until the input rate and elimination rate are equal. This is the steady state.


How do you calculate steady state concentration?

Css=(F x dosing rate) / Clearance

Dosing rate= dose / time interval

Css= F x (dose/time interval) / (ke x Vd)


How long does it take for a drug to reach steady state?

5 half-lives


What is the effect of dosing intervals and what things must be kept in mind?

The dosing interval won't effect the time it takes to reach steady state; it will always be 5 halflives

shorter dosing intervals without changing the dose will lead to a higher steady state conc.

Longer dosing intervals will lead to more fluctuation, so care must be taken that the drug isn't completely eliminated and that the fluctuations don't reach toxic levels or non therapeutic levels.

Also, patient compliance should be considered.


What is the purpose of a loading dose? How are the loading dose and maintenance dose calculated?

If a drug has a long half-life, it may take a long time to get to steady state. To overcome this, a large loading dose will be given initially to get to therapeutic levels followed by maintenance doses to keep it there.

Dl=(Vd x Css) / F

Dm=(Css x Cl x time interval) / F