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Physiology 2 > pharmacokinetics > Flashcards

pharmacokinetics Flashcards

1
Q

what is pharmacokinetics?

A

what the body does to the drug

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2
Q

what is pharmacodynamics?

A

what the drug does to the body

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3
Q

what are the stages of pharmacokinetics?

A

absorption
distribution
metabolism
excretion

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4
Q

what is absorption?

A

the passage of a drug from its site of administration into the plasma

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5
Q

when must absorption be considered?

A

for all routes of admin except IV where the drug is administered directly into the plasma

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6
Q

how do drug molecules move round the body?

A

bulk flow - blood, lymph or CSF

diffusion - short distances only

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7
Q

how are drugs absorbed?

A

with the exception of IV drugs, a drug molecule must cross at least one cell membrane in order to move from its site of administration into the general circulation

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8
Q

what are the main mechanisms of movement of small molecules?

A

passive diffusion through lipid
diffusion through aqueous pores
carrier-mediated transport
pinocytosis

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9
Q

what increase diffusion through a lipid?

A

high concentration gradient
low molecular weight
high lipid solubility
low degree of ionisation

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10
Q

where does simple diffusion occur?

A 
absorption through GI tract or skin
intracellular site of action
BBB
placental transfer
renal tubule reabsorption
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11
Q

ionisation in a base

A

at a high pH = low ionisation

at a low pH = high ionisation

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12
Q

Ionisation in an acid

A

at a high pH = high ionisation

at a low pH = low ionisation

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13
Q

why is ionisation important?

A

for absorption, unionised molecules cross cell membranes easily but ionised will not so readily cross

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14
Q

what can change in environmental pH do?

A

influence drug effectiveness

affect drug secretion

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15
Q

major routes of administration

A 
oral
sublingual
rectal
topical application
inhalation
transdermal - skin
injection
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16
Q

types of injection

A 
subcutaneous 
intramuscular
intravenous
intrathecal
intravitral
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17
Q

factors affecting absorption from GI tract?

A 
particle size and formulation
physiochemical factors
gut content 
GI motility
splanchnic blood flow
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18
Q

IV administration

A

directly into plasma
rapid onset and full absorption
used when rapid effect needed or oral absorption likely to be poor
when drug is rapidly metabolised a loading dose may need to be given followed by infusion to maintain the concentraion

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19
Q

loading dose

A

bolus

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20
Q

bioavailability

A

the fraction of administered dose that reaches the systemic circulation as the parent drug

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21
Q

how to calculate bioavailability

A

F (bioavailability) = AUC O/ AUC IV

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22
Q

what is bioavailability of and IV drug?

A

1, 100% of drug enters system

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23
Q

what results from oral administration

A

incomplete bioavailability
incomplete absorption and loss in faeces - too polar or not all released from tablet
first pass metabolism in gut lumen, during passage across gut wall or by liver before drug reaches systemic circulation

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24
Q

why is bioavailability important?

A

major factor that determines drug dosage for different routes of admin

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25
Q

what is distribution

A

the process by which the drug is transferred reversibly between the plasma and tissues
usually by passive diffusion

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26
Q

distribution of drugs

A

is uneven between tissues

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27
Q

what is distribution dependent on?

A

blood flow for speed of delivery

lipid solubility for tissue accumulation

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28
Q

lipid solubility

A

if a drug has a high lipid solubility it will accumulate in the tissues and so there will be a lower concentration in the blood than tissue and so the same blood concentration may represent very different tissue stores

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29
Q

importance of lipid solubility

A

tissue stores of lipid soluble drugs will be higher resulting in slower elimination from the body
which can result in increased duration of therapeutic or side effects
can be used to create a depot effect

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30
Q

tissue diffusion

A

time to reach equilibrium differs between different tissue groups depending on perfusion. equilibrium is reached must faster in well perfused organs

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31
Q

which organs are well perfused?

A 
brain
liver
lungs
kidneys
Gi tract
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32
Q

Which organs are poorly perfused?

A

skin
skeletal muscle
bone
fat

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33
Q

perfusion of different administrations

A

acute IV dose may not equilibrate across all tissues but repeated administration of an oral dose would be expected to equilibrate across all tissues
this is because orally allows even filling of all compartments as it is slow and IV is rapid so the drug is taken up quickly by vessel rich organs and then redistributed to other tissues

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34
Q

what happens at equilibrium

A

administration rate equals elimination rate

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35
Q

what drug factors affect distribution

A 
lipid solubility
molecular size
degree of ionisation
cellular binding 
duration of action
therapeutic effects
toxic effects
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36
Q

what body factors affect distribution?

A 
vascularity
transport mechanisms
blood/ placental barriers
free and bound forms of drugs
drug interaction
drug reservoirs 
plasma binding proteins
37
Q

what is protein binding?

A

many drugs bind to plasma proteins or intracellular proteins
usually reversible

38
Q

what does protein binding do?

A

lowers free concentration of drug available
only free drug can cause a pharmacological effect and be excreted
drug-protein complex can act as a reservoir, releasing bound drug when free drug is distributed to other compartments or eliminated

39
Q

volume of distribution

A

theoretical volume that would be necessary to contain the total amount of an administered drug at the same concentration that is observed in blood plasma

40
Q

calculating volume of distribution

A 
Vd = Q/Cp 
Vd = volume of distribution
Q = total amount of administered drug
Cp = concentration in blood plasma
41
Q

low Vd drugs

A

large, water soluble

stay within plasma and well perfused organs

42
Q

High Vd drugs

A

small, lipid soluble

distribute into all compartments

43
Q

elimination

A

larger molecules excreted in bile

smaller molecules excreted renally, if lipid-soluble need to be metabolised by liver into ionised form

44
Q

elimination via kidney

A

drugs need to be water soluble

45
Q

why is metabolism necessary?

A

for elimination of lipid soluble drugs

46
Q

what happens in metabolism?

A

a lipid soluble molecule is turned into a water soluble that can be excreted in urine

47
Q

metabolites

A

may show different pharmacological properties from the parent molecule

48
Q

phase 1 of metabolism

A

oxidation
reduction
hydrolysis
forms primary product

49
Q

when can metabolism happen?

A

before and during absorption, which can limit the amount of drug that reaches general circulation

50
Q

what affects metabolism?

A 
large surface area
high levels of enzyme activity
enzyme induction or inhibition 
blood flow to liver 
genetic variation in enzymes
51
Q

where is the main site of drug metabolims

A

liver

52
Q

what happens when there is impaired metabolism of a drug?

A

increased activity

53
Q

what happens when there is enhanced metabolism of a drug?

A

reduced activity

54
Q

what is a pro-drug?

A

a drug that is metabolised after administration to a pharmacologically active metabolite. the pro-drug may be inactive until metabolised

55
Q

what are the potential routes of excretion?

A

fluids - majority
solids
gases
bile

56
Q

what is maximum renal clearance?

A

700ml/min - renal blood flow

57
Q

what happens to molecules released into bile?

A

absorbed again in small intestine and returned to liver in enterohepatic circulation

58
Q

what is enterohepatic circulation?

A

liver > bile > small intestine > liver

59
Q

what does the rate of elimination of a drug determine?

A

duration of response to drug
time interval between doses
time to reach equilibrium during repeated dosing

60
Q

what are the main excretory organs?

A

kidneys

liver

61
Q

what are the orders of elimination

A

first

zero

62
Q

first order elimination kinetics

A

elimination of a constant fraction per time unit of the drug quantity present in the organism
proportional to drug concentration

63
Q

zero-order elimination kinetics

A

elimination of a constant quantity per time unit of the drug quantity present in the organsims

64
Q

which drugs follow zero order kinetics?

A

few
overdoses take longer to clear
e.g. alcohol

65
Q

measuring elimination

A

half life
elimination rate constant
clearance

66
Q

half life

A

the time it takes for the plasma concentration or amount of drug in body to be reduced by 50%
independent from concentration

67
Q

elimination rate constant

A

(k)

rate of drug removal from the body and can be used to calculate half life. Slope of graph = -k

68
Q

clearance

A

volume of plasma in the vascular compartment cleared of drug per unit time by the processes of metabolism and excretion

69
Q

calculating clearance

A

CL = rate of elimination from body/drug concentration in plasma
clearances are additive with respect to kidney and liver
CLtotal = CLrenal + CLnonrenal

70
Q

another way of calculating clearance

A

CL total = k x Vd

71
Q

repeated administration

A

administration of a drug of a fixed dose at a regular time interval through a given route
used to maintain constant concentration of drug in blood and site of action to maintain therapeutic effect

72
Q

calculating average steady state concentration

A

= bioavailability x dose/clearance x dosing interval (hrs)

73
Q

loading dose

A

avoids delay between starting treatment and reaching steady state concentration
a single dose needed to produce a desired steady state concentration

74
Q

how to calculate loading dose

A

steady state concentration x Vd

75
Q

compromised states

A

low cardiac output - distribution impaired

hepatic/ renal impairment - slower elimination

76
Q

adjustments for low cardiac output

A

reduce dose and give slowly

77
Q

adjustments for hepatic/ renal impairment

A

increase dosage intervals/ reduce dose - monitoring if possible

78
Q

what are drug-drug interactions?

A

occur when 1 drug affects the activity of another when the 2 are given in combination

79
Q

what can drug interactions cause?

A

increase or decrease in drug effect

synergistic effect

80
Q

how do drug interactions work?

A

can be pharmacokinetic or pharmacodynamic mechanisms

81
Q

therapeutic index

A

indicates safety margin of a drug

82
Q

how to calculate therapeutic index?

A

dose resulting in toxicity/ dose giving therapeutic response
or
50% lethal dose/ 50% effective dose

83
Q

high therapeutic index

A

low risk of toxicity

84
Q

low or narrow therapeutic index

A

high risk of toxicity

85
Q

when do pharmacokinetic drug interactions occur?

A

during absorption, distribution and elimination

86
Q

when are harmful drug interactions likely to occur

A

narrow therapeutic index drugs, as a small increase in plasma concentration can cause toxic effects

87
Q

pharmacokinetic drug interactions

A

physico-chemical reactions (outside body, in GI tract)
protein-binding displacement - increases free drug
enzyme inhibition or induction

88
Q

enzyme induction and inhibition

A

some drugs can inhibit or induce the enzymes that are involved in elimination of other drugs, causing enhanced or decreased activity

89
Q

how many hospital admissions are drug related

A

adverse drug reactions - 5-8% of that 20% are due to drug interactions

Physiology 2 (20 decks)

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