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POS BVetMed 2 > 2.2 Pharmacokinetics > Flashcards

2.2 Pharmacokinetics Flashcards

1
Q

What is AD(M)E?

A

Summary of pharmacokinetics:

Absorption
Distribution
Metabolism (excretion)
Elimination

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2
Q

Water v lipid solubility

A

Water:
~ must be aqueous for absorption, distribution, interation with target, passagein circulation

Lipid:
~cross cell membranes by passive diffusion
~ distribute into fatty tissue
~ easy access to action sites in cells
~easily re-absorbed from kidney

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3
Q

What happens to drugs in plasma?

A

Drugs in plasma available for distribution to sites of action, passage to organs of excretion, metabolism

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4
Q

Passage of drugs into/from circulation

A

Cross the vascular endothelium via gaps between cells packed with matrix of proteins that act to retain molecules of high molecular weight

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5
Q

How do drugs pass across cell membranes?

A

Passive diffusion
Active/facilitated diffusion
~ pinocytosis
~ aqueous pores

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6
Q

Factors effecting drug movement

A

Molecular weight
Lipid solubility
Chemical nature

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7
Q

How does molecular weight alter absorption of a drug?

A

High molecular weight compounds will remain at the site of administration

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8
Q

How does lipid solubility alter absorption of a drug?

A

Lipid soluble drugs can readily cross cell membranes

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9
Q

How does chemical properties alter absorption of a drug?

A

Weak acid/base: partially ionised so unionised fraction readily cross cell membranes

Neutral drugs: readily cross cell membranes

Strong acid/base: 100% ionised - polar molecules do not readily cross cell membranes

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10
Q

What is pKA

A

pH + log (conc. protonated species / conc. non-protonated species)

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11
Q

What affects the rate of absorption?

A
  • drug properties
  • physiological properties
  • drug formulation (e.g. coating)
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12
Q

How does pH affect drug absorption?

A

pH varies at different parts of the body so drug pH effects how well it absorbs:

In plasma:
~ weak base (unionised) readily absorbed
~ weak acid (ionised) less readily absorbed

In acid conditions:
~ weak base (ionised) less readily absorbed
~ weak acid (unionised) readily absorbed

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13
Q

How does the area of absorbing surface affect absorption?

A

surface area
compromised surfaces

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14
Q

How does local blood flow effect absorption?

A

vasoconstriction reduces rate of diffusion of a drug from injection site

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15
Q

What is bioavailability?

A

The fraction of administered dose that reaches the circulation in active form
(expressed as a % of total amount)

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16
Q

How can bioavailability be determined?

A

By measuring the amount of drug in the plasma overtime.

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17
Q

What is first pass metabolism?

A

When the drug is extensively metabolised before it reaches the systemic circulation

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18
Q

Explain Cmax and Tmax

A

Cmax (maximal plasma conc. of a drug) at Tmax (time achieved at)

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19
Q

What is volume of distribution?

A

A measure of volume of fluid required to contain the total amount of drug at plasma conc.

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20
Q

How is volume of distribution calculated?

A

Vd = Q/Cp

Q = Dose (total amount of drug in the body) mg/Kg

Cp = Plasma conc ng/ml

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21
Q

Why is Vd important?

A

predicts if a drug is likely to reach target site at effective concs

Determines loadinh dose necessary to achieve target plasma conc

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22
Q

Why might drugs accumulate in target tissues?

A

High lipid solubility (accumulates in fate)

Ion trapping (pH difference across barriers)

Binding to unrelated site from target

23
Q

How are drugs eliminated?

A

Drug enters, reaches peak, plasma conc falls

24
Q

Which drugs are eliminated more quickly?

A

water soluble

lipid soluble (reabsorbed)

25
Q

How are drugs metabolised?

A

Foreign substance so removed

Converted to water soluble form

Metabolised so biologically inactive

26
Q

Main sites of drug metabolism

A

Liver, plasma, GIT wall, lung, kidney

27
Q

How are lipid soluble drugs metabolised?

A

Liver - cross liver membrane to reach microsomal enzymes

Oxidation/reduction
Conjugation

28
Q

What is enterohepatic recycling?

A

Liver cells transfer drugs to bile, delivers to intestines, conjugated, drug reformed (free drug reabsorbed)

29
Q

What is half life?

A

Time taken for the conc in plasma to fall by 50%

30
Q

Explain elimination by first order kinetics

A

Expenontial decrease over time

rate of removal proportional to conc.

elimination pathways not saturated - fixed proportion removed over time

Half life independant on dose

31
Q

Explain elimination by zero order kinetics

A

Linear decrease in plasma drug conc. over time

Fixed amount of drug removed

Elimination pathways saturated

Half life increases with increase of dose

32
Q

What is the important of half life?

A

Indicates time to reach steady state of plasma

Time taken for drug to be removed from the body and duration of action

Estimation of appropriate dose intervals

33
Q

What are the 2 ‘groups’ of factors that affect of behaviour of a drug in the body?

A

Drug / product factors

Animal / human factors

34
Q

What does drug pharmacokinetics depend on? (6)

A

Lipid and water solubility

Stability

Dose

Plasma protein binding

Chirality

Effect of drugs on liver enzymes

35
Q

What will influence the pharmacokinetics of a drug?

A

Product formation

pH (renal excretion)

Other drugs being used

36
Q

Effect of drugs on liver enzymes

A

Enzyme induction caused by repeated administration of drugs

Liver enzyme inhibition

37
Q

Consequences of liver enzyme induction/inhibition (3)

A

Plasma conc. of the drug

Therapeutic efficacy

Time between doses need changing

38
Q

What does drug formulation effect?

A

Cmax

Tmax

Time drug remains at therapeutic levels

39
Q

How can food effect Cmax and Tmax?

A

↓ Cmax, ↑Tmax (Competition for absorption)

↓ Cmax, = Tmax (Reaction with food = excreted with food)

= Cmax, ↑Tmax (Delayed time for absorption)

↓ Cmax, ↑Tmax (Food required for absorption)

40
Q

Explain the effect of pH and renal excretion on the fate of drugs in the body

A

Acidic urine animals eliminate weakly basic drugs most effectively

Alkaline urine animals eliminate weakly acidic drugs mist effectively

41
Q

Effects of drug interaction (4)

A

Absorption
Plasma protein binding
Metabolism
Excretion

42
Q

What are the animal/human factors affecting the fate of drugs?

A

Disease
Age
Species/breed
Feeding schedule
Admin route
Time of day
Physiological state
Gender

43
Q

How does disease effect drug pharmacokinetics?

A

Changes in ggastric/intestinal motility

Damage to epithelial barrier

Reduced blood flow

Distribution

Inflammatory disease

Metabolism and excretion

44
Q

How does age effect pharmacokinetics?

A

In neonates:
~reduced renal excretion
~reduced hepatic metabolism
~reduced plasma protein binding
~increased GIT absorption
~increased volume of distribution

In aged animals:
~decline in liver and kidney elimination

45
Q

Define side effects

A

An unwanted action caused by a drug used at therapeutic dose

46
Q

Define adverse drug reaction

A

(ADR)
Any unwanted vent associated with the use of a drug at therapeutic doses

47
Q

What is type A ADR?

A

Augmented - expected but exaggerated response (predictable) dose dependant

48
Q

What is type B ADR?

A

Bizzare - unexpected/abnormal responses (unpredictable) not dose dependant

49
Q

What is type C ADR?

A

Chronic - only occurs after prolonged use

50
Q

What is type D ADR?

A

Delayed - occur at remote time from the treatment

51
Q

What is type E ADR?

A

End of treatment - occur when drug use is abruptly stopped

52
Q

What is type F ADR?

A

Failure - expected response is not acheived (rarely due to primary failure of a drug)

53
Q

What is the nocebo effect?

A

ADR reported isnt actually related to the drug

POS BVetMed 2 (43 decks)

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