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Physiology > pharmacokinetics and clearance > Flashcards

pharmacokinetics and clearance Flashcards

1
Q

what is pharmacodynamics

A

what a drug does to our body

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2
Q

what is pharmacokinetics

A

what our body does to drugs

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3
Q

what is the changing concentration of drug in the body a result of

A

absorption, distribution, metabolism and excretion

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4
Q

what is an important consideration regarding drug concentrations

A

for a drug to have the clinically desired effect it usually needs to be maintained at a certain concentration, at its site of action for the required amount of time

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5
Q

what is the therapeutic window

A

at a dose between what is toxic and the minimal dose at which it has a therapeutic effect

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6
Q

what journey does the term absorption refer to

A

from where drug was administered to the blood plasma

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7
Q

where can drugs be adminstered

A

gut, skin, muscle, CSF or lungs

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8
Q

what drugs are not absorbed

A

intravenous drugs are introduced directly to the plasma

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9
Q

where do most drugs have to reach to have their physiological effect

A

blood plasma

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10
Q

what 2 forms do drugs usually exist

A

bound to proteins and unbound

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11
Q

what form of drug is clinically useful

A

unbound

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12
Q

what is bound drug referred to

A

drug reservoir as binding is reversible

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13
Q

describe drug distribution in well perfused tissues vs poorly perfused

A

drug circulation is achieved by circulation so..
well perfused eg heart liver kidneys - rapid
poorly perfused eg fat and skeletal muscle - slow

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14
Q

4 body fluid compartments

A

plasma water 5%
interstitial water 16%
intracellular water 35%
transcellular water 2%

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15
Q

where do water-soluble drugs stay largely

A

eg atenolol
largely in blood and interstitial fluid

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16
Q

where do lipid-soluble drugs largely stay

A

eg opioid fentanyl
concentrates in fat

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17
Q

where does the majority of drug metabolism occur

A

liver

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18
Q

what other organs and tissues contribute to drug metabolism

A

GI tract mucosa, kidneys and lungs

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19
Q

what does a drug have to do to be metabolised by the liver and why?

A

must enter a hepatocyte - crossing the plasma membrane - means hydrophobic drugs are more easily metabolised by the liver

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20
Q

how can hydrophilic drugs enter a hepatocyte

A

membrane transport proteins

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21
Q

what do liver enzymes do to drugs

A

generally modify them to make them inactive or more easily excreted

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22
Q

what are prodrugs

A

modified by the liver to form the pharmacologically active molecule
eg perindopril, ace inhibitor, metabolised to form perindoprilat, which has a therapeutic effects

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23
Q

what are the 2 types of metabolic reactions drugs can be modified by

A

phase 1 - catabolic
phase 2 - anabolic

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24
Q

describe phase 2 metabolic reactions

A

result in the attachment of a chemical group ie conjugation of the drug. most common groups incl sulfate, glucuronyl, added molecules which then form the point of attachment for the conjugation reaction

25
Q

describe where metabolising liver enzymes are embedded

A

in the SER

26
Q

what is the most important system of enzymes for drug metabolism in the liver

A

cytochrome P450 CYP system
these are haem proteins and add one atom of oxygen to the drug molecule to form a hydroxyl group

27
Q

what are other potential routes for drug excretion

A

biliary excretion, faeces, respiration

28
Q

what is the predominant route of drug excretion

A

renal

29
Q

why is it important to know a drugs clearance rate

A

determining dosage, higher clearance = higher dose
clearance low = low levels needed to maintain a therapeutic concnetration

30
Q

what does the rate of drug excretion depend on

A

on a balance between drug filtration, secretion and reaborption

31
Q

how does the drug get to the glomerulus

A

afferent arteriole

32
Q

what drug is filtered at the glomerulus

A

only the unbound

33
Q

what influences drug filtration

A

as well as protein binding, the size of the drug also
small drugs not protein-bound can be filtered but larger drugs eg Heparin, even when protein-bound cant be filtered and are poorly excreted by the kidneys

34
Q

3 factors affecting the amount of drug filtered into the Bownman’s capsule

A

renal blood flow
glomerular filtration rate
plasma protein binding

35
Q

what can drug molecules do for more rapid excretion

A

can be actively secreted into the renal tubule

36
Q

are secretory mechanisms specific?

A

not specific for the drug but take advantage of molecular similarities between the drug and naturally occuring substances

37
Q

how are organic bases transported

A

eg PAH
transported in cationic form, against their electrochemical gradient by OAT (organic anionic transporter) family proteins

38
Q

how are organic bases transported

A

eg morphine
in their cationic forms by OCT (organic cationic transporter) familt protein

39
Q

describe the effect of plasma binding on drug excretion

A

relatively small effect at the PCT since there are highly efficient transporters that mediate active secretion which removes unbound drug and decreases plasma protein binding

40
Q

what increases the conc of drug in tubular lumen

A

glomerular filtration and tubular secretion

41
Q

what decreases conc of drug in tubular lumen

A

reabsorption

42
Q

what factors influence amount of drug reabsorbed

A

urine flow rate, lipid solubility of the drug and whether or not it is ionised

43
Q

what happens if the tubule is freely permeable to a drug

A

as water is reabsorbed along the length of the tubule, the drug can be reabsorbed passively with its conc gradient
this means that the lipid-soluble drugs which can cross the tubular cell membrane are excreted poorly by the kidneys, whereas polar drugs which don’t cross the tubular membrane easily are excreted well

44
Q

what is degree of ionisation dependent on

A

on the pH of the fluid it is dissolved in

45
Q

does ionisation increase or decrease reabsorption

A

decreased reabsorption of drug

46
Q

describe drugs as acids or bases

A

can be weak acids or bases
weak acids = become anionic in an alkaline urine (give up H+) -> excretion increases
weak bases = become cationic in an acidic urine (bind to H+ ions) and excretion is increased

47
Q

what is ion trapping

A

when the charged form of the drug remains in the tubules and more of it is excreted

48
Q

how can we increase a drugs excretion

A

pH of urine can be changed to favour the charged form which cant be easily reabsorbed

49
Q

how can the effect of pH on drug excretion be predicted

A

Henderson-Hasselbalch relationship

50
Q

how would increasing urine flow rate affect drug reabsorption

A

decrease

51
Q

explain why polar drugs eliminated primarily by the kidneys which isn’t activated by metabolism must be used with special care in patients with renal disease

A

if not activated by metabolism, then renal excretion primarily determines duration of action, dose may need modified if GFR is reduced

52
Q

what pH would the renal tubular fluid normally be at the PCT and beyond, and as a result would the excretion of acids or bases usually be favoured

A

normally acidic pH favouring base excretion

53
Q

define reabsorption

A

the process by which water and solutes are removed by the nephron from the tubular fluid

54
Q

define secretion

A

the process by which substances are created and discharged by the gland and

55
Q

what is clearance

A

removal of substances from the plasma and their excretion in the urine

56
Q

what is the clearance of a solute

A

removal of substances from the plasma and their excretion in urine as a substance

57
Q

example of a substance which is completely cleared

A

PAH

58
Q

what is the GFR

A

vol of fluid filtered from the renal glomerular capillaries into the Bowman’s capsule per unit time

59
Q

criteria for a substance to be used to measure GFR

A

freely filterable by the glomerulus
neither reabsorbed nor secreted from the tubules
must be inert

Physiology (4 decks)

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