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LF261 - Neuropharmacology > Pharmacokinetics -> lecture 2 > Flashcards

Pharmacokinetics -> lecture 2 Flashcards

1
Q

Why do we use models of bodies in drug production?

A

Allows predictions of drug activity on body e.g. how does drug act, what is the drugs active duration etc.

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2
Q

What is the single compartment model and what is it showing?

A
  • The body is one bucket with opening at top and outflow at bottom.
  • Full drug dose is added at once, enters bucket w/o absorption etc, is eliminated.
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3
Q

What is the apparent volume of distribution (Vd)?

A

Total amount of drug in body compared to plasma concentration.

Vd = amount of drug in body/plasma drug concentration.

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4
Q

What is the Vd an indicator of?

A

Indicates extravascular distribution.

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5
Q

Why is Vd called the apparent volume?

A

It may not be a real number, e.g. if Vd was 300l then that’s larger than avg vol of a human.

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6
Q

What would a high Vd indicate?

A

There is a large distribution of plasma into different sites i.e. more drug leaves the plasma and goes into other body compartments.

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7
Q

How to include elimination in simple model to calculate drug extraction ratio?

A
  • Blood will take drug into eliminator.
  • Blood will then flow away from eliminator.
  • Calculate the amount of drug extracted.
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8
Q

What is drug elimination also called?

A

Clearance

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9
Q

How to calculate clearance?

A

Clearance = rate of elimination/conc of input.

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10
Q

What are the units/inference of clearance?

A

How much of a set volume of plasma is cleared per unit time.

Vol/unit time.

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11
Q

How to measure clearance clinically?

A

Best way = creatinine clearance:
- Look at how much creatinine is cleared.
- Renal clearance correlated with clearance of creatinine.

Clinically done via:
- Single blood sample.
- Measuring urine collection over 24 hours.

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12
Q

What is creatinine and how to measure it’s clearance?

A
  • Product of the breakdown of creatine phosphate in muscle.

Measuring creatinine clearance for men:
Creatinine clearance = ((1.23(140-age))*weight)/serum creatinine.

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13
Q

How does clearance differ with age?

A

Decreases with age.

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14
Q

What precautions are needed due to age-related decreased clearance?

A
  • Pay attention to drug half-lives, don’t want drug accumulation.
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15
Q

What are the features of first order drug elimination?

A
  • Most common.
  • Exponential decay.
  • Rate of elimination is proportional to drug conc. (more drug = faster removal).
  • Constant half-lives.
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16
Q

What is the elimination rate constant ?

A

Kel => proportion of drug eliminated per unit time.

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17
Q

What is the association with elimination constant and Vd?

A

Kel = rate of elimination/amount of drug left. = clearance/Vd.

18
Q

On a natural log (concentration)-time graph what does the slope represent?

A
  • The negative elimination constant (-Kel)
19
Q

What is a drug half-life?

A
  • Time taken for [drug] to halve.
20
Q

How many half lives roughly for avg drug to decay?

A

~5 half-lives.

21
Q

How does half-life differ with the elimination constant?

A

Different Kel = different half-lives.

22
Q

What are the half-lives of drugs used for in clinical settings?

A
  • Used to set dosing intervals i.e. one hl then another dose etc.
23
Q

How do drug kinetics differ if distribution is added to the model?

A
  • More drug causes more rapid distribution.
  • Still a first order reaction.
  • Measure distribution via Kd.
24
Q

What happens if absorption is added to single compartment model?

A
  • Still a first order reaction.
  • Rate of absorption proportional to unabsorbed drug.
  • Measure absorbance via Ka
25
Q

How to calculate absorption from the area under curve?

A
  • AUC = dose/clearance
  • i.e. g/(l/time)
26
Q

How can bioavailability be measured using the area under curve?

A

Bioavailability = AUC (in dosage form)/AUC (via IV injection)

27
Q

What is a loading dose?

A
  • Top up drug dose to keep drug within therapeutic window.
28
Q

How are loading doses done?

A
  • Continuous IV injections.
29
Q

What are the advantages of loading doses?

A
  • Good for drugs with narrow therapeutic windows.
  • Stop toxicity due to too quick drug presentation |(immune, hypotensive, vein damage).
30
Q

What are the key features of a zero-order elimination?

A
  • Elimination rate isn’t proportional to drug conc. (the drug is removed at same rate no matter the conc).
  • Half-lives depend on [drug].
31
Q

Why do half-lives depend on [drug] for zero-orders reactions?

A

Using enzyme e.g. alcohol:
- Enzymes work at constant rate.
- HL depends on starting conc, higher conc = more saturated enzymes.

32
Q

Key differences between first and zero order elimination:

A

1st order:
- Amount of drug eliminated per unit time is proportional to [drug].
- A constant proportion of total drug is eliminated per unit time.

0 order:
- Constant amount of drug is eliminated per unit time.

33
Q

When would drug doses need to vary?

A

Mainly due to differing ages (neonates, elderly)

34
Q

Why are doses changed with neonates and elderly groups?

A

Neonates:
- Lower body vol.
- Higher water %

Elderly:
- Higher fat %
- Lower body weight.

35
Q

What are the effects of low and high age on drug dose?

A
  • Less plasma-drug binding = lower levels of albumin.
  • Less clearance = lower renal function.
  • Lower metabolism.
36
Q

What is therapeutic drug monitoring?

A
  • Monitor + measure conc of drug in body + determine effective dosage times etc.
37
Q

How to do therapeutic drug monitoring ?

A
  • Measure drug in blood, urine or saliva (if unbound).
  • Aim to avoid toxicity in patient.
38
Q

For different drugs when is monitoring important?

A
  • Antibiotics = measuring peak drug conc important to not be too high.
  • Anticonvulsants = measuring trough drug conc, don’t want them to get too low + cause seizures.
  • Want to measure time above threshold value measuring minimum inhibitory conc i.e. what’s the lowest conc of drug to produce max response.
  • Measure overall drug exposure in anticancer drugs etc.
39
Q

When is urine sampling used + why?

A
  • Instead of blood sampling.
  • Useful for drugs w partial/full elimination into urine.

As urine volume is variable it’s not using drug conc (which doesn’t mean anything) + can express drug quantity as absolute amount.

40
Q

How to urine sample?

A
  • Look at cumulative amount btwn two time points.
    OR
  • Look at amount excreted per interval btwn urine collections.

LF261 - Neuropharmacology (3 decks)

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