Pharmacology: Pharmacokinetics Flashcards
What does volume of distribution describe?
relationship between a drug dose and the drug’ plasma concentration
Vd assumes two things, what are they?
- the drug distributes instantaneously (full equilibration occurs at t=0)
- The drug is not subjected to biotransformation or elimination before it fully distributes.
Vd is a calculated value, it is not something we can directly measure, what is the formula?
Vd = amount of drug / desired plasma concentration
What is the total body water in liters of a 70kg patient?
42 L (60%)
If a drug’s Vd exceeds total body water it is known as?
in order to exceed total body water the Vd must be greater than what?
lipophilic
> 0.6 L/Kg or > 42 L
If a drug has a very large Vd then will the dose requirement to achieve a given plasma concentration be high or low?
higher dose to achieve a given plasm concentration
Having a high Vd means that the drug distributes into what kind of tissues?
distributes into the total body water as well as into the fat
What parameter would a drug meet to be considered hydrophilic?
a Vd that is less than total body water (less than 0.6L/Kg or < 42L). It distributes into some or all of the body water, but it does NOT distribute into fat.
If a drug has a small Vd and thus is considered hydrophilic, does it require a larger or lower dose to achieve a given plasma concentration?
lower dose is required
What are the two components of the ECF?
Plasma
interstitial fluid
What is the formula for loading dose?
Loading dose = (Vd x desired plasma concentration) / bioavailability
How do you calculate Vd for a loading dose question?
The Vd will be given to you, you then multiply it by the patients weight in Kg which will then give you the patient specific Vd in L and you multiply that by the desired Cp and then if it is an IV medication you divide by 1
The dose of a medication that achieves a given plasma concentration is dependent on?
a. timing
b. route of administration
c. gender
d. age
b. route of administration
What is clearance?
volume of plasma that is cleared of a drug per unit of time
Clearance is directly proportional to? (3)
a. half-life
b. drug dose
c. extraction ratio
d. drug concentration in the central compartment.
e. blood flow to clearing organ
blood flow to clearing organ
extraction ratio
drug dose
The most important clearing organs include?
liver
kidney
organ independent (Hofmann elimination and ester hydrolysis in the plasma)
rate of administration = rate of elimination describes?
steady state
when is steady state achieved?
when the amount of drug entering the body is equivalent to the amount of drug being eliminated from the body.
SS is achieved after five half-times
How many half-times must pass for SS to be achieved?
five
What is the central compartment?
The plasma
What is the peripheral compartments?
the tissues
In the two compartment model, what is the alpha phase, what is the beta phase?
alpha phase is the steepest portion of the curve which represents redistribution
beta phase is the less steep portion of the curve representing elimination from the plasma
In the two compartment model, what characteristic of a drug would create a steeper slope?
the more lipophilic the drug the the larger the Vd and the steeper the slope
True or false:
When a drug distributes from the central compartment to the peripheral compartment the plasma concentration continues to decline, as a result the concentration gradient reverses and the drug then redistributes from the peripheral compartment and re-enters the central compartment?
True
What does a rate constant tell you and what symbol is used to represent “rate constant”.
rate constant communicates the speed at which a reaction occurs.
k = rate constant
what does k12, k21, and ke mean?
k12 - rate constant for drug transfer from the central to peripheral compartment.
k21 - rate constant for drug transfer from the peripheral to central compartment.
ke - rate constant for drug elimination from the body
What is the difference between elimination half-life and elimination half-time?
half life is talking about removal of drug (50%) from the body after rapid IV injection.
half time is removal of drug (50%) from the plasma during the elimination phase
We say a drug has been cleared from the body when what % of the dose has been eliminated from the plasma?
96.9% or five half-times
Does half-time measure a constant fraction or constant amount?
half time measures a constant fraction NOT a constant amount.
What is context sensitive half time?
Time required for the plasma concentration to decline by 50% after the infusion has stopped.
It takes the duration of drug administration into account, “time”.
Explain context sensitive half time in relation to fentanyl and remifentanil?
The longer fentanyl was infused the longer it will take it to be eliminated by 50% from the plasma concentration, this is because it is highly lipid soluble and has gone out into the tissues and then when the infusion is stopped it goes back into the central compartment.
Remifentanil is an exception to this rule, it is highly lipophilic but it is quickly metabolized by plasma esterase’s and has a similar context -sensitive half time regardless of how long it is infused.
difference between a strong acid or base placed in water vs. a weak acid or base placed in water?
strong acids and bases in water will ionize completely
weak acids and bases in water will only partially ionize and the remaining fraction will be unionized
What does ionization mean?
ionization is the process where a molecule gains a positive or negative charge.
The amount of ionization is dependent on two things?
- The pH of the solution
2. The pKa of the drug (pKa = dissociation constant)
When pKa and pH are the same what % of the drug will be ionized and what % of the drug will be unionized?
50% ionized
50% unionized
True or false:
small changes in pH do not have much impact on the degree of ionization?
False,
small changes in pH can have profound impact on the degree of ionization.
When are the small changes in pH most profound in relation to ionization?
when the pKa and pH are very close to each other
less significant when the pKa is very far from the pH
are ionized particles (drugs) soluble in water or lipids?
ionized drugs are soluble in water
are unionized particles (drugs) soluble in water or lipids?
unionized drugs are soluble in lipids
pharmacologic effect of ionized drugs vs unionized drugs? (which one is active and which one is not active)
ionized = not active unionized = active
are ionized or unionized drugs more likely to undergo renal elimination?
ionized are more likely to undergo renal elimination
are ionized or unionized drugs more likely to undergo hepatic biotransformation?
unionized are more likely to undergo hepatic biotransformation
Is ionized or unionized drugs more likely to diffuse across a lipid bilayer?
unionized
acidic and basic drugs, tell me which one in which type of solution (acidic or basic solutions) will give you ionized or unionized drugs?
acid with a base = ionized (acid donates protons and base accepts protons)
acid with acid or base with base = unionized (if you place like with like then you will only be wanting to give away or only be wanting to accept the protons)
Weak acids are typically paired with a positive ion preparation, this would include which 3 elements?
sodium
calcium
magnesium
Weak bases are typically paired with a negative ion preparation, this would include what 2 elements?
chloride or sulfate
If you mix a weak base in what kind of solution will you have predominately unionized fraction of the drug?
weak base drug mixed with higher pH solution than the pKa of the drug
(base dissolved in a base)
if you mix a weak base in what kind of solution will you have a predominately ionized fraction of the drug?
weak base drug mixed with lower pH solution than the pKa of the drug
(base dissolved in an acid = ionized)
if you mix a weak acid in what kind of solution will you have a predominately unionized fraction of the drug?
weak acid drug mixed with lower pH solution than the pKa of the drug
(acid dissolved in an acid)
If you mix a weak acid in what kind of solution will you have a predominately ionized fraction of the drug?
weak acid drug mixed with higher pH solution than the pKa of the drug
(acid dissolved in an base = ionized)
are local anesthetics weak acids or bases?
LA = weak bases
what are the perfect conditions for fetal ion trapping?
basic drug
alkalotic mother
acidotic fetus
The basic drug is given IV to alkalotic mom thus the unionized fraction of the drug is increased and more basic drug is available to diffuse across the placenta and to the fetus. Once in the fetus where it is more acidic than mom or perhaps fetal acidosis is in place the drug becomes ionized (base with acid) and can not cross back over the placenta. This would cause more distress to the fetus and further increase fetal acidosis.
In this scenario usually a LA is being given because it is a weak base drug
in relation to ionized or unionized, which one is lipophilic and which one is hydrophilic?
unionized = lipophilic
ionized = hydrophilic
What is the pKa of lidocaine?
pKa = 7.9
is fetal pH usually a little lower or higher than maternal pH on average?
little lower, thus more acidic than maternal pH
Where do you find plasma proteins?
they are synthesized by the liver and remain in the circulation ( to big to pass through cell membranes )
intravascular drug storage compartment describes?
plasma proteins
Is the drug bound to a plasma protein active?
A drug bound to a plasma protein can not bind to a receptor, so any bound drug can be considered inert. It is only when the drug is released from the protein, is it able to affect the body.