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LCRS Pharma > Pharmokinetics > Flashcards

Pharmokinetics Flashcards

1
Q

what are main routes of drug administration?

A 
IV
IM
IP
dermal 
Subcutaneous 
inhalation
ingestion
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2
Q

what are the two ways drugs can be administered?

A

systemic- across the entire organism, into deep tissue like the brain

local- restricted to one area, not directly into the bloodstream

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3
Q

what are the sites used for administration in terms of GI?

A

enteral- GI administration

parenteral- outside the GIT

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4
Q

advantage and disadvantage of using IV administration?

A

rapid onset of the drug action

invasive and requires training

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5
Q

examples of drugs and their administration

A

Salbutamol- inhalation, system

Betnovate- dermal, local

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6
Q

what is the journey of the drug?

A 
ADME
A-dministration 
D-istribution 
M-etabolism 
E-xcretion
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7
Q

what are the two pathways that drugs use to move in a biological system?

A

1) bulk flow transfer- via the blood

2) diffusional transfer- short distances

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8
Q

what are the two areas a drug has to transverse?

A

compartments (aqueous)
- blood, lymph, ECF

barrier (lipid)
- cell membrane

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9
Q

which administration routes are fastest?

A

IV

then inhalation

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10
Q

what are the methods of crossing a lipid barrier?

A

diffusion
aqueous pores (polar only)
carrier molecules
pinocytosis

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11
Q

what type of drug can freely move into a lipid barrier?

A

non-polar substances, lipid soluble

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12
Q

what sort of chemicals are most drugs and how does that affect whether they cross lipid barriers or not?

A

most drugs are either weak acids or weak bases

the drugs therefore exists in an ionised and unionised form in equilibrium

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13
Q

what does the ratio of ionised and unionised states depend on?

A

the pH of the environment and the pKa

when the pKa= pH, there is a perfect dynamic equilibrium

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14
Q

examples of weak acid and weak base drugs?

A

weak acid- aspirin (proton donor)

weak base- morphine (proton acceptor)

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15
Q

what is the partition hypothesis?

A

the drug diffuses passively across a membrane when it is unionised. In the ionised state, a drug diffuses slowly and is absorbed slowly

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16
Q

how well is aspirin absorbed in different parts of the GIT?

A

in the stomach it diffuses inside well due to the low pH enable a dominant unionised state, whilst in the high pH of the late GIT the aspirin diffuses in slowly

when pKa is above the pH of the environment (acids), the unionised form is dominant and therefore diffusion occurs readily. This is the reverse for basic drugs

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17
Q

what is the Hasselbach equation for weak acids and bases? how can it be used to identify if a drug will be ionised or unionised predominantly?

A

acids: unionised over ionised [AH/A-]
bases: ionised over unionised [BH+/B]

rearrangement: 10 to the power pKa-pH
- -> a value >1 indicates the top state is more dominant (a ratio is provided)

pKa of a drug never changes, but the pH of a compartment can change

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18
Q

what is ion trapping?

A

when the pH of a compartment is very high above the pKa of the drug, the drug is unable to leave and stays ionised in the blood to be excreted.

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19
Q

what are the factors that affect distribution of the drug?

A
  • plasma protein binding
  • regional blood flow
  • capillary permeability
  • localisation in tissues
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20
Q

how does regional flow affect distribution of a drug?

A

when blood flow increases to an area i.e. a greater proportion of CO is delivered, the denser the capillary network, the more drug that is absorbed per unit time

e.g. in exercise

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21
Q

how does plasma protein binding affect distribution of a drug?

A

they provide a reservoir for drugs like warfarin and aspirin, so the drug cannot leave the drug. When bound tightly and then won’t access the tissue. This would require a higher dose.

aspirin 50-80% PP bound

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22
Q

how does capillary permeability affect distribution of a drug?

A

the more permeable, the freer the movement.
continuous- H20 filled gaps
BBB- very tight junctions

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23
Q

what are the ideal drugs to enter the BBB?

A

lipophilic as water soluble drugs can’t get in unless there is a mediator available

24
Q

how does localisation affect the distribution of a drug?

A

drugs persist in certain tissues

e.g. general anaesthetic which is very lipid soluble distributes itself in fat and leaks out very slowly.

25
Q

what are the two major routes of drug excretion?

A

1) liver- drugs are concentrated into the bile, usually involving larger MW drugs
2) kidneys- most drugs are removed this way. Low MW into urine

26
Q

kidney excretion of drugs

A

bioactive component is small enough to be filtered through. active secretion of the remain 80% in the blood acids and bases in the PCT, some reabsorption of lipid soluble drugs.

27
Q

liver excretion of drugs

A

drugs concentrated in bile at the bile canaliculi then excreted into the gut

28
Q

what is enterohepatic cycling of drugs and how does it lead to drug persistence?

A

in the gut, the drug metabolite may be reabsorbed by the liver via the hepatic portal vein

after water soluble conjugate (glucuronide) is broken off by bacteria (post bilary excretion)

This leads to drug persistence in the system (toxicity)

29
Q

what are other methods that drugs can be excreted?

A

lungs, skin, GI secretions, saliva, sweat, milk and genetical secretions

30
Q

what is bioavailability?

A

key: in systemic circulation
GI is not systemic circulation

proportion of drug available within the body to exert a physiological effect

The amount of drug contained in a medicine that enters the systemic circulation in an unchanged form after administration of the product.

31
Q

apparent volume

A

indicator of apparent volume of distribution of the drug

32
Q

what is the biological half life?

A

the time taken for the concentration of a drug to halve

Indicates the rate of metabolism and indicates the dose required for chronic use.

33
Q

first order kinetics

A

rate of elimination of a drug

the amount of drug decreases proportional to the concentration of the drug remaining

i.e. slower removal of the drug at lower concentration

34
Q

zero order kinetics

A

linear elimination of a drug
implies an enzymatic removal of a drug

i.e. the same amount of drug is always removed regardless of concentration.

35
Q

what is entral administration?

A

involves the GI tract:

Oral, Rectal, Buccal (between gums and cheek), Sublingual (under tongue)

36
Q

what is parentral administration?

A

doesn’t involve the GI tract: Intravenous, Intramuscular, Intradermal, Subcutaneous, Inhalation

37
Q

where can lipid soluble drugs be reabsorbed in the kidney?

A

proximal and distal tubules

38
Q

what are some other routes of drug excretion?

A 
lungs
skin
GI secretions
saliva
sweat
milk
genital secretions
39
Q

what is pharmacokinetics?

A

the interaction of the body with the drug

40
Q

what is pharmacodynamics?

A

the interaction of the drug on the body

41
Q

advantages and disadvantages of oral administration

A

[+]

  • self-medication
  • limited sterile preparations
  • incidence of anaphylaxis is lower
  • capacity to prevent complete absorption

[-]

  • inappropriate for drugs that are labile in acid pH (stomach) i.e liable to alteration
  • undergo extensive first pass metabolism
  • requires patient compliance
42
Q

advantages and disadvantages of IV administration

A

[+]

  • rapid onset
  • avoids poor absorption/destruction in stomach

[-]

  • higher incidence of anaphylaxis
  • trained personnel required
  • complications possible
43
Q

advantages and disadvantages of inhalation administration

A

[+]

  • ideal for small molecules
  • large surface area so faster onset of action.

[-]
possible localised effect in lung

44
Q

advantages and disadvantages of IM administration

A

[+]

  • relatively high blood flow
  • depot therapy (prolonged absorption from pellet etc.)

[-]
possible infection and nerve damage.

45
Q

advantages and disadvantages of subcutaneous administration

A

[+]

  • local administration possible
  • enables depot therapy

[-]
pain
abscesses
tissue necrosis.

46
Q

what are the advantages and disadvantages of percutaneous administration

A 
[+]
- local application and action
- lipid soluble components diffuse rapidly.
[-]
- local irritation and skin reactions
- alteration of skin structure.
47
Q

why is bioavailability not an indicator of drug effectiveness?

A
  • drug may become ion trapped
  • drug may be a prodrug
  • cream for example don’t enter systemic circulation and don’t need to be bioavailable
48
Q

how is bioavailability measured?

A

‘plasma level vs. time curves

with comparison of IV administration
(100% bioavailable)

vs. intended route of administration.

49
Q

what is bioavailability in IV administration?

A

100%

50
Q

what factors influence bioavailability?

A
  • physiochemical properties of the drug i.e. pKA
  • GI pH
  • drug actively or passively diffused
  • size of drug particles
  • physiochemical interactions between drug and gut contents
  • illness e.g. GI disease, polymorphism
51
Q

what effect does 100% first pass metabolism of a drug have on

A

if the drug is completely metabolised at first pass then there is 0% bioavailable

useful for topical agents or prodrugs

52
Q

how is aspirin (pKa= 3.5) absorbed in the stomach?

A

pH of compartment < pKa of drug
i.e. pKa is higher than pH

therefore the drug will be in the unionised so the drug will be absorbed readily (lipophilic)

once inside the cells, pH is increased, the drug becomes ionised state and will not move out (ion trapped)

53
Q

hydrophilic drug

A

ionised

less well absorbed

54
Q

how would you ensure slow release of aspirin?

A

enteric coated aspirin

also used for pain relief of late GI tract not not everything is taken at the stomach

55
Q

what is bioequivalence?

A

a new generic drug behaves sufficiently similar to the existing one without causing clinical problems.

done when a patent expires, so any drug company can manufacture the drug

56
Q

what are the benefits of bioequivalence?

A

cheaper alternatives of various formulations

especially for drugs with a narrow therapeutic window

57
Q

what is a therapeutic window/index?

A

the difference in dose between a clinically useful dose and a toxic dose

LCRS Pharma (32 decks)

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