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Pharmacology > A Clinicians Guide to Pharmacokinetics > Flashcards

A Clinicians Guide to Pharmacokinetics Flashcards

1
Q

What is C”max”?

A

maximum plasma concentration

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2
Q

What is AUC?

A

area under plasma concentration vs time curve from time of inj to the limit of quantification of the assay

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3
Q

What is t”max”?

A

time point @ which C”max” is reached

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4
Q

What is T”1/2”?

A

plasma half-life of drug

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5
Q

What is t”+0.2”?

A

time plasma concentrations remain above 0.2 micrograms/mL

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6
Q

how does clinical pharmacokinetics work?

A
  • drug companies work out dosage regime (costs millions of dollars)
  • research paper will suggest modifications
  • individualizing treatment for specific patient requires that you have an understanding of the basics
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7
Q

What do you know about pharmacodynamics so far?

A

Dose is not proportional to effect!

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8
Q

What is the therapeutic window?

A

range btwn minimum EFFECTIVE & minimum TOXIC plasma concentrations in an INDIVIDUAL
(you will not know what these numbers will be)

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9
Q

What is the therapeutic range?

A

plasma concentration range in POPULATION likely to produce a desired effect

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10
Q

What is pharmacokinetics the study of ?

A

how drugs get into & out of the body & where they go when they are in the body

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11
Q

why do we need mathematical models?

A

to develop dosing regimes

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12
Q

What are 2 problems associated with pharmacokinetics?

A
  1. math involves logs
  2. studies are mainly done on blood (plasma) - & most drugs dont work in the blood
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13
Q

what are the clinical limitations of pharmacokinetic data?

A
  1. PK data is based on plasma drug concentrations, which may not be as relevant as tissue drug concentrations (ex: UTI)
  2. PK/PD models
    - PK values may be model dependent
    - often highly variable btwn studies
    - numbers are NOT absolute
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14
Q

Where else can you use drugs that is not in the bloodstream?

A

Not all drug use is systemic
1. skin
2. ears
3. inhaled
4. intra-mammary
5. intra uterine
6. wounds

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15
Q

what is the problem of extrapolating plasma kinetics?

A
  1. most studies are done on sm grps of similar animals & dont account for effects of: age, gender, sex, body type, breed, pregnancy, lactation, &/or genetics
    also 2. plasma vs tissue drug concentrations may vary or some drugs get sequestered in fat
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16
Q

How does first order drug elimination work?

A
  • rate of elimination is proportional to amt of drug remaining
  • constant PROPORTION of drug is eliminated per unit time
  • this results in curved line when plotted on LINEAR/linear graph
  • (straight line when plotted on LOG/linear graph)
  • most drugs follow 1st order kinetics (@ label doses)!
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17
Q

why do pharmacokineticists tend to stop at 7 half lives?

A

only 0.8 % of drug remaining

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18
Q

How do you know if graph is linear/linear or log/linear?

A

look at y axis scale!

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19
Q

what is K”el”?

A

elimination rate constant; slope of line of LOG/linear graph

20
Q

what happens if you infuse a drug at a constant rate?

A
  • concentration will rise logarithmically to reach a steady state
  • final concentration is related to rate of infusion
21
Q

How long does it take to reach steady state?

A

7 half-lives

22
Q

How do you get a drug concentration to steady state faster?

A

w/ a loading dose (rapid IV bolus followed by a sustained infusion)

23
Q

How to calculate a loading dose?

A

Vol of distribution - total dose of drug/ C”max” = L/Kg (how many L of water per kg of body weight is drug dissolved in?)

24
Q

What does calculating the volume of distribution give you an idea of ?

A

how drug distributes in body
- blood only 0.08
- body water 0.6
- highly fat soluble drugs 200!

24
Q

What does volume of distribution greater than 100 mean?

A

drug has gone out of blood

25
Q

How to roughly calculate a loading dose?

A

loading dose = desired concentration X volume of distribution

26
Q

How else can drugs be given other than IV?

A

IM, SC, oral, rectally, nasally, trans-dermal, vaginally

27
Q

If you are giving a drug any way other than IV, what has to happen?

A

drug has to be absorbed into the plasma

28
Q

Why does route of administration matter?

A
  1. absorption phase
  2. bioavailability
  3. sustained release
29
Q

What is bioavailability?

A

how much drug gets into blood & system

30
Q

how to calculate bioavailability?

A

area under the curve oral / area under curve IV

31
Q

What is flip-flop kinetics?

A
  • describes situation where “depot” injection is slowly absorbed over time (bit like an infusion)
  • elimination rate is controlled by absorption rate (rate that it is coming in is controlling rate @ which is is released from blood)
32
Q

What kind of meds have flip-flop kinetics?

A

long acting meds

33
Q

What happens when dosage interval is shorter than the half-life?

A

concentration will slowly increase over time (drug accumulation - C”max” at steady state is much higher than C”max” after a single dose); there is minimal fluctuations btwn C”max” & C”min”; missing 1 dose has minimal impact on [plasma]; loading dose may be useful to reach desired [plasma] more quickly

34
Q

What happens when dosage interval is equal to the half-life?

A

will get a little accumulation but much more variation; as long as the drug stays above the therapeutic margin you are good

35
Q

What happens when dosage interval is longer than the half-life?

A

not much difference btwn C”max” @ steady state & C”max” of single dose (little drug accumulation); marked fluctuations in drug concentration btwn C”max” & C”min” (ex: b/c penicillin is super safe you can give a massive dose and let it drop & repeat); missing dose greatly affects concentration; minimal lag time to achieve desired concentration in body (no loading dose needed)

36
Q

What does little T (tau) mean?

A

dosing interval

37
Q

When can you make changes to a dosing schedule?

A

need to be v cautious; only if you have scientific paper to support you

38
Q

What can vet’s do with PK info?

A

adjust dose regimens

39
Q

How can vets adjust dose regimens?

A
  1. change dose
    - w/ diff route of administration (using relative bioavailability)
    - w/ likely changes in drug clearance
    > renal or hepatic disease, enzyme inhibition: decrease dose
    > increased clearance due to enzyme induction: increase dose
  2. change dose interval
    - based on therapeutic drug monitoring
    > calc T”1/2elim”
    > useful: phenobarb or KBr therapy
    > not so much: theophylline, aminoglycosides, cyclosporine, etc.
    - based on likely changes in drug clearance
    > ex: renal or hepatic failure
40
Q

What else can PK info be used for?

A

drug development:
- drug formulation optimized based on PK & PD data
- correlation btwn PK/PD & clinical efficacy, but only if PK & PD data are well known (even then there is wide PK variability btwn animals & PD outcomes are not uniform)

41
Q

What can you not used PK for?

A

Knowing which drug is better
- PK parameters do NOT indicate safety or efficacy
- PK-PD together improves predictions about clinical efficacy/safety, but correlation isnt necessarily that strong!

42
Q

What are the clinical limitations of PK data?

A
  1. vol of distribution is just a ratio!
  2. elimination half-life
  3. PK data is based on plasma drug concentrations
  4. PK/PD models
43
Q

explain how PK data is limited by volume of distribution:

A

higher volume of distribution means there is more drug in the tissue OR less drug in the plasma, but we dont know which tissues the drug might be in; higher V”D” does not mean the drug works better

44
Q

explain how PK data is limited by elimination half-life:

A
  • longer half-life doesnt mean better, it may influence the dosing interval but that is it!
  • the elimination half-life from tissues may not appear to match the elimination half-life from plasma -> means that withdrawal periods cannot be accurately estimated from plasma half-life
45
Q

explain how PK data is limited by PK data being based on plasma drug concentrations:

A

these may not be as relevant as tissue drug concentrations; ex: UTIs: are treated by therapeutic urine! drug has already left the body & kidneys are just used to distribute drug (could get same effect from using a catheter)

46
Q

explain how PK data is limited by using PK/PD models to predict efficacy:

A

PK models may be model-dependent; they are often highly variable btwn studies & #s are not absolute, PK/PD models are PREDICTIVE only and their predictions may not always correlate w/ clinical results

Pharmacology (14 decks)

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