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Membranes and Receptors > Lecture 9 - Pharmacokinetics > Flashcards

Lecture 9 - Pharmacokinetics Flashcards

1
Q

define pharmacokinetics

A

what the body does to a drug

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2
Q

how can the ways of getting a drug to a patient be changed?

A

formulation

site of administration

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3
Q

how does formulation change how the drug gets to the patient?

A

solid (rate of action depends on dissolution)
liquid
patient compliance is important (less frequent is easier)

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4
Q

how does site of administration change how the drug gets to the patient?

A

local

systemic - enteral or parenteral

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5
Q

define oral bioavailability

A

proportion of the dose given orally that reaches the systemic circulation in an unchanged form

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6
Q

how can bioavailability be expressed?

A

amount or rate

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7
Q

how is bioavailability expressed as amount?

A

measured by area under curve of blood drug level vs time

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8
Q

how is bioavailability expressed as rate?

A

measured by peak height and rate of rise of drug level in blood

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9
Q

what is the therapeutic ratio?

A

maximum tolerated dose/minimum effective dose OR ld50 (lethal dose to 50% of people)/ed50 (effective dose in 50% of people)

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10
Q

what is the first pass effect?

A

substances absorbed from the lumen of the ileum enter venous blood, the hepatic portal vein and pass through the liver where it may be extensively metabolised

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11
Q

how much of an oral dose of paracetamol is usually metabolised by the first pass effect?

A

90%

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12
Q

define drug distribution

A

the theoretical volume into which a drug has distributed assuming that this is occurring instantaenously

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13
Q

how is drug distribution calculated?

A

amount given/plasma concentration at time 0

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14
Q

what level of the drug exerts an effect?

A

free level NOT the total

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15
Q

when is the free level of the drug important?

A

if the drug is highly bound to albumin

if the drug has a small volume of distributionif the drug has a low therapeutic index

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16
Q

what is an object drug?

A 
class 1 drug
used at a dose much lower than the number of albumin binding sites
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17
Q

what is a precipitant drug?

A 
class 2 drug
used at a dose greater than the available albumin binding sites
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18
Q

what happens when class 1 and class 2 drugs are administered simultaneously?

A 
class 1 are displaced by class 2, raising free levels of the object drug
higher risk of toxicity
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19
Q

what are the precipitant drugs for the object drug warfarin?

A

sulphonamides
aspirin
phenytoin

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20
Q

what are the precipitant drugs for the object drug tolbutamide?

A

sulphonamides

aspirin

21
Q

what is the precipitant drug for the object drug phenytoin?

A

valproate

22
Q

how is rate of drug metabolism calculated?

A

vmax[C]/km+[C]

23
Q

when is a drug metabolised by first order kinetics?

A

when a drug is used at a concentration lower than km

24
Q

what is the rate of metabolism with first order kinetics?

A

vmax[C]/km

25
Q

what does first order kinetics mean?

A

metabolism is proportional to drug concentration

26
Q

what is the graph of first order kinetics?

A

straight line when a log scale on the y axis vs time

27
Q

when is a drug metabolised by zero order kinetics?

A

when a drug is used at a concentration much higher than km

28
Q

what is the rate of metabolism with zero order kinetics?

A

vmax[C]/[C]

29
Q

what does zero order kinetics mean?

A

the enzyme is saturated so the rate of decline of plasma drug level is constant, regardless of concentration

30
Q

what is the graph of zero order kinetics?

A

straight line when normal (not log) plasma concentration is plotted against time

31
Q

when will a steady state be reached with zero order kinetics?

A

within 5 half lives of the drug

32
Q

if an immediate effect is needed with zero order kinetics, what is done?

A

loading dose

33
Q

what does first order kinetics give?

A

predictable therapeutic response from dose increases

34
Q

what does zero order kinetics give?

A

therapeutic response that can suddenly escalate as elimination mechanisms saturate

35
Q

what is phase 1 of drug metabolism?

A

a reactive group is exposed on the parent molecule or added to the molecule
generates a reactive intermediate that can be conjugated with a water soluble molecule to form a water soluble complex

36
Q

what are the most common chemical reactions in phase 1?

A

oxidation
reduction
hydrolysis

37
Q

what does phase 1 require?

A

cytochrome p450 and nadphenzymes are inducible and inhibitable

38
Q

what drugs does phenobarbitone induce?

A

warfarin

phenytoin

39
Q

what drug does rifampicin induce?

A

oral contraceptive

40
Q

what drug do cigarettes induce?

A

theophylline

41
Q

what drugs does cimetidine inhibit?

A

warfarin

diazepam

42
Q

how can drugs bypass phase 1?

A

already have a reactive group on their molecule

43
Q

what is phase 2 of drug metabolism?

A

reactive intermediate from phase 1 is conjugated with a polar molecule to form a water soluble complex

44
Q

what is the most common conjugate?

A

glucoronic acid

45
Q

what is glucoronic acid?

A

available by product of cell metabolism

46
Q

what can drugs be conjugated with?

A

glucoronic acid
sulphate ions
glutathione

47
Q

what does phase 2 require?

A

specific enzymes

uridine diphosphate glucoronic acid (udpga) - high energy cofactor

48
Q

how does the kidney control drug excretion?

A

only free unbound drug is filtered through glomerulus
drugs can be actively secreted by the tubule
urine pH can determine how much of the drug is excreted

49
Q

how will urine pH affect drug excretion?

A

for weak acids - alkaline urine will make the drug ionised so there will be less tubular absorption - increase excretion
for weak bases - acidic urine will increase excretion as the base will be ionised so the drug stays in the lumen

Membranes and Receptors (10 decks)

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