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CMOD Week 6 > Pharmacokinetics > Flashcards

Pharmacokinetics Flashcards

1
Q

Clinical Pharmacokinetics is concerned with what four quantitative parameters?

A
  1. Clearance - body’s efficiency of drug removal
  2. Volume of distribution - apparent space the drug resides in
  3. Elimination half-life - rate of drug removal
  4. Bioavailability- fraction of drug absorbed
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2
Q

What is the most important concept to be considered when a rational regimen for long-term drug administration is to be designed?`

A

Clearance (CL)

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3
Q

What is Clearance?

A

the theoretical volume of fluid (i.e. blood, plasma) from which a drug is removed per unit time.

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4
Q

The clinician usually wants to maintain steady-state concentrations of a drug within the therapeutic window. How can this be done?

A

Administering the drug at the same rate it is eliminated

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5
Q

What is the equation for dosing rate?

A

F*Dosing rate(dose/T)= CL x CSS

where CL is clearance from the systemic circulation and Css is the steady-state concentration of the drug. F= Bioavailability

When administered at this rate the drug will eventually reach a plateau

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6
Q

What is the equation for clearance for the organ?

A

CL = Q[(CA – CV)/ CA] = Q x E where E is referred to as the extraction ratio.

CL = rate of elimination/ concentration= dose/AUC

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7
Q

What is the limiting variable in clearance?

A

blood flow (i.e. presentation of the drug) to the organ is the limiting variable.

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8
Q

For elimination, most drugs follow _ order kinetics.

A

First order kinetics (most drugs obey first order kinetics)

Others follow Zero order kinetics (also known as saturation kinetics).

Only a few, but some very important drugs obey zero order kinetics.

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9
Q

A straight line is obtained plotting blood concentration with time when the y axis is log while x axis is linear. The slope of this straight line is the _______.

A

elimination rate constant (Ke) for that particular drug.

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10
Q

What is Ke? Equation?

A

elimination rate constant (Ke) for that particular drug. This means that a constant fraction of drug is eliminated per unit time. The absolute amount of drug removed from per unit time will be concentration-dependent

Ke= 0.693/(t1/2)= CL/Vd

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11
Q

What is Half-life for a drug?

A

(t1/2) is the time it takes for the plasma concentration or the amount of drug in the body to be reduced by 50%

t1/2= 0.7Vd/CL

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12
Q

What is the equation for volume of distribution?

A

Vd = Amount of drug in the body divided by C (Blood concentration)

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13
Q

What is the One compartment open model (single IV dose)?

A

Assumes entire human body is one compartment. This works for drugs that are distributed fairly uniformly throughout the body. Assumes an open system (excretion). Is not adequate for all situations.

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14
Q

What is the Two compartment open model?

A

Assumes that much of a drug is in a particular compartment and that an equilibrium exists between the blood and other areas. (single IV dose)

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15
Q

It takes about ___ half-lives to achieve steady state when multiple dose IV is given

A
  1. With time, a steady state maximum (Css,max) and a steady state minimum (Css,min) are achieved. Time to steady state is independent of dose or dose interval but strictly dependent on t 1/2 (plateau principle)
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16
Q

There are situations where one cannot wait for 5 half-lives to achieve a therapeutic range (heart attacks, serious heart failure, overwhelming bacterial infections, etc). In these situations, the use of a loading dose of a drug is warranted. What is a loading dose? Equation?

A

the desired steady state of a drug times the volume of distribution adjusted for bioavailability

Loading dose (LD) = (Css X Vd) / F

You can initially give a dose thats twice the normal dose and then give the normal dosage, and that will get you to standard state concentration much faster, such as in the event of an emergency

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17
Q

Loading doses can be particulary dangerous due to the high concentrations that are achieved. Following the loading dose patients are given maintenance doses in order to keep the Css within the desired therapeutic window such that:

A

Dosing rate = target Css x CL / F

18
Q

If the enzymes that metabolize a drug are rate limiting i.e. the enzyme is saturated at usual levels of drug in the body then the same amount of drug (ug/hr) is metabolized regardless of the level of drug. What is this called?

A

Zero-order kinetics.

19
Q

Which drugs exhibit zero order kinetics? Name 6

A

Ethanol Aspirin Heparin Amobarbital Phenytoin Tetracycline

PA HEAT

20
Q

For zero order elimination kinetics the following equations are employed

A 
LD = (Vd x Css)/F- same as first order 
Css = (Km x DR)/(Vm-DR)
DR = (Css x (Vm – DR))/ Km

Km is the dose that produces 50% of the maximal elimination rate Vm is the maximum rate of the process

21
Q

A higher dose with lower bioavailability would give the same curve as a lower dose with 100% bioavailability

A

A higher dose with lower bioavailability would give the same curve as a lower dose with 100% bioavailability

22
Q

T or F. When you reach steady state is independent of dosage interval

A

T. It only depends on half-life

23
Q

Saturation kinetics is equal to __ order kinetics

A

Zero.

24
Q

What are some principles of zero order kinetics (aka saturation kinetics)? 4 things

A

A constant amount (not fraction) of the drug is eliminated per unit time. (First order= constant fraction)

The biological half-life is dependent on the dose administered

A linear slope is obtained when plasma concentration vs time is plotted on a linear-linear scale

No plateau principle (i.e. following multiple dosing, you will not reach steady-state plasma concentration in 4-5 half-lives)

25
Q

For first order kinetics, half of the drug is eliminated for each half life. For zero order kinetics, it is the AMOUNT, not fraction, that is eliminated that determined dosing.

A

For first order kinetics, half of the drug is eliminated for each half life. For zero order kinetics, it is the AMOUNT, not fraction, that is eliminated that determined dosing.

26
Q

What is normal renal clearance rate?

A

120ml/min (GFR)

27
Q

What might lead to a renal clearance of 70 ml/min?

A

extensive protein binding

28
Q

The bioavailability of Drug A is 20% while Drug B is 80%. Assuming they have similar molecular weights, effective concentrations, and elimination kinetics, if Drug A is given at 400 mg doses every 6 hours, how much of Drug should you give?

A

100 mg every 6 hrs

DRF= DRF
(400/6)(0.2)= (x/6)(0.8)
x=100mg every 6 hrs

want to give lower dose in same time interval to avoid fluctuations

29
Q

Drug A has a half-life of 24 hours. If administered every 12 hours, when will it reach steady state?

A

5 half lives- 120hrs

this is independent of dosage interval

30
Q

Drug Y causes a reduction in the plasma concentration of drug X regardless of route of administration. This is most likely because drug Y effectively reduces which pharmacokinetic parameters of X?

a) absorption
b) bioavailability
c) Half-life
d) Metabolism

A

Half-life

Because route of admin doesn’t matter, absorption and bioavailability aren’t an issue (think IV)

31
Q

Drug X has the following parameters: CL=1.6L/hr, Vd=80L; t1/2= 35 hr; F=0.5. IF the target plasma concentration is 5 mg/L, what is the appropriate loading dose for the drug?

A

800mg

Solution:
loading dose= (80L)*(5mg/L)/0.5
LD=800 mg

32
Q

To calculate the loading dose of a drug, one must know the target plasma concentration and?

A

volume of distribution (Vd) (and bioavailability)

LD= CssVd/F

33
Q

The VD and CL of Drug X are 40L/kg and 0.5L/hr/kg, respectively. If your patient is a 60k female, what is the expected t1/2?

A

56hrs

independent of her weight

Solution:
t1/2= 0.7Vd/CL
t1/2= 0.7(40L/kg)/(0.5L/hr/kg)= 56hrs

34
Q

A 3rd yr med student is trying to determine the CSs of an experimental drug. Treatment was initiated 6 hrs earlier at a rate of 3mg/min. t1/2=3hrs, Vd=120L, and CL=0.6L/min. At this infusion rate, what will the Css plasma drug level be?

A

5mg/L

Solution:
Dosing rate = target Css x CL / F
(3mg/min)/(0.6L/min)=Css
Css=5mg/L

35
Q

T or F. highly charged drugs in urine would increase excretion.

A

T.

36
Q

Drug X exhibits an AUC of 10mg/L/hr when 350 mg is administered as an IV bolus and distributes to 100L, What is the t1/2 of the drug?

A

2hrs

CL=dose/AUC=350/10mg/L/hr=35

t1/2=0.7Vd/CL=0.7(100)/35=2 hrs

37
Q

A continuous infusion of lidocaine is given to a 120kg male. Cl=9ml/min/kg, Vd=70L, t1/2=2hr. How long will it take after initiation of the infusion for the drug to reach 87.5% steady-state level?

A

3 half-lifes=6 hrs

38
Q

If a drug obey first-order kinetics, this implies:

a) There is only one mechanism for its removal
b) The half-life is the same regardless of concentration
c) The drug is largely metabolized in the liver
d) the drug is orally administered
e) The drug is retained within the vascular system

A

b

39
Q

Your patient exhibits changes in his average plasma concentration over the period of several days with a chronic dosing regimen as depicted (it is exponential). Which of the following is unlikely to explain this behavior?

a) liver toxicity
b) Impaired renal blood flow
c) Competition for protein sites from another drug
d) The drug obeys zero-order kinetics
e) The patient took more the prescribed amount

A

c

you can get accumulation with zero-order kinetics

40
Q

Equation for Dose

A

Conc= (dose/Vd)*Bioavailability

CMOD Week 6 (15 decks)

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