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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
what is distribution
the process by which the drug is transferred reversibly between the plasma and tissues usually by passive diffusion
26
distribution of drugs
is uneven between tissues
27
what is distribution dependent on?
blood flow for speed of delivery | lipid solubility for tissue accumulation
28
lipid solubility
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
29
importance of lipid solubility
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
30
tissue diffusion
time to reach equilibrium differs between different tissue groups depending on perfusion. equilibrium is reached must faster in well perfused organs
31
which organs are well perfused?
``` brain liver lungs kidneys Gi tract ```
32
Which organs are poorly perfused?
skin skeletal muscle bone fat
33
perfusion of different administrations
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
34
what happens at equilibrium
administration rate equals elimination rate
35
what drug factors affect distribution
``` lipid solubility molecular size degree of ionisation cellular binding duration of action therapeutic effects toxic effects ```
36
what body factors affect distribution?
``` vascularity transport mechanisms blood/ placental barriers free and bound forms of drugs drug interaction drug reservoirs plasma binding proteins ```
37
what is protein binding?
many drugs bind to plasma proteins or intracellular proteins usually reversible
38
what does protein binding do?
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
volume of distribution
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
calculating volume of distribution
``` Vd = Q/Cp Vd = volume of distribution Q = total amount of administered drug Cp = concentration in blood plasma ```
41
low Vd drugs
large, water soluble | stay within plasma and well perfused organs
42
High Vd drugs
small, lipid soluble | distribute into all compartments
43
elimination
larger molecules excreted in bile | smaller molecules excreted renally, if lipid-soluble need to be metabolised by liver into ionised form
44
elimination via kidney
drugs need to be water soluble
45
why is metabolism necessary?
for elimination of lipid soluble drugs
46
what happens in metabolism?
a lipid soluble molecule is turned into a water soluble that can be excreted in urine
47
metabolites
may show different pharmacological properties from the parent molecule
48
phase 1 of metabolism
oxidation reduction hydrolysis forms primary product
49
when can metabolism happen?
before and during absorption, which can limit the amount of drug that reaches general circulation
50
what affects metabolism?
``` large surface area high levels of enzyme activity enzyme induction or inhibition blood flow to liver genetic variation in enzymes ```
51
where is the main site of drug metabolims
liver
52
what happens when there is impaired metabolism of a drug?
increased activity
53
what happens when there is enhanced metabolism of a drug?
reduced activity
54
what is a pro-drug?
a drug that is metabolised after administration to a pharmacologically active metabolite. the pro-drug may be inactive until metabolised
55
what are the potential routes of excretion?
fluids - majority solids gases bile
56
what is maximum renal clearance?
700ml/min - renal blood flow
57
what happens to molecules released into bile?
absorbed again in small intestine and returned to liver in enterohepatic circulation
58
what is enterohepatic circulation?
liver > bile > small intestine > liver
59
what does the rate of elimination of a drug determine?
duration of response to drug time interval between doses time to reach equilibrium during repeated dosing
60
what are the main excretory organs?
kidneys | liver
61
what are the orders of elimination
first | zero
62
first order elimination kinetics
elimination of a constant fraction per time unit of the drug quantity present in the organism proportional to drug concentration
63
zero-order elimination kinetics
elimination of a constant quantity per time unit of the drug quantity present in the organsims
64
which drugs follow zero order kinetics?
few overdoses take longer to clear e.g. alcohol
65
measuring elimination
half life elimination rate constant clearance
66
half life
the time it takes for the plasma concentration or amount of drug in body to be reduced by 50% independent from concentration
67
elimination rate constant
(k) | rate of drug removal from the body and can be used to calculate half life. Slope of graph = -k
68
clearance
volume of plasma in the vascular compartment cleared of drug per unit time by the processes of metabolism and excretion
69
calculating clearance
CL = rate of elimination from body/drug concentration in plasma clearances are additive with respect to kidney and liver CLtotal = CLrenal + CLnonrenal
70
another way of calculating clearance
CL total = k x Vd
71
repeated administration
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
calculating average steady state concentration
= bioavailability x dose/clearance x dosing interval (hrs)
73
loading dose
avoids delay between starting treatment and reaching steady state concentration a single dose needed to produce a desired steady state concentration
74
how to calculate loading dose
steady state concentration x Vd
75
compromised states
low cardiac output - distribution impaired | hepatic/ renal impairment - slower elimination
76
adjustments for low cardiac output
reduce dose and give slowly
77
adjustments for hepatic/ renal impairment
increase dosage intervals/ reduce dose - monitoring if possible
78
what are drug-drug interactions?
occur when 1 drug affects the activity of another when the 2 are given in combination
79
what can drug interactions cause?
increase or decrease in drug effect | synergistic effect
80
how do drug interactions work?
can be pharmacokinetic or pharmacodynamic mechanisms
81
therapeutic index
indicates safety margin of a drug
82
how to calculate therapeutic index?
dose resulting in toxicity/ dose giving therapeutic response or 50% lethal dose/ 50% effective dose
83
high therapeutic index
low risk of toxicity
84
low or narrow therapeutic index
high risk of toxicity
85
when do pharmacokinetic drug interactions occur?
during absorption, distribution and elimination
86
when are harmful drug interactions likely to occur
narrow therapeutic index drugs, as a small increase in plasma concentration can cause toxic effects
87
pharmacokinetic drug interactions
physico-chemical reactions (outside body, in GI tract) protein-binding displacement - increases free drug enzyme inhibition or induction
88
enzyme induction and inhibition
some drugs can inhibit or induce the enzymes that are involved in elimination of other drugs, causing enhanced or decreased activity
89
how many hospital admissions are drug related
adverse drug reactions - 5-8% of that 20% are due to drug interactions

Physiology 2 (20 decks)

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