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Pharm > Tutorial: Drug variability > Flashcards

Tutorial: Drug variability Flashcards

1
Q

What determines the amount of drug reaching the end organ

A

ADME

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2
Q

What might be reasons for variabulity

A

Absolute differences in dose administered

Relative overdose or underdose

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3
Q

Reasons for absolute difference in dose administered

A

This may be deliberate or accidental eg. due to:

a) error in prescription or dispensing
b) patient non-compliance
c) drug formulation

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4
Q

Reasons for relative underdose or overdose

A

Environmental exposure to chemicals, including other drugs

Food intake – drugs may interact chemically with components of food; this may alter their absorption

Fluid intake

Age

Disease

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5
Q

How can Environmental exposure to chemicals, including other drugs affect overdose or underdose

A

enzyme induction

enzyme inhibition

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6
Q

Why would a patient be told to take a drug with food

A

foods delay gastric emptying and alter gastric pH.

means that drug is absorbed in the stomach

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7
Q

In which case would a patient take drug with water

A

most drugs are better absorbed if taken with water eg may dissolve better

fluids may stimulate gastric emptying. (so taken for drugs absorbed in intestine)

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8
Q

How might age affect relative underdose or overdose

A

NEWBORNS:

  1. more body water than adults
  2. poorer renal function, with immature tubular secretion
  3. an immature blood brain barrier
  4. lower capacity for drug metabolism

ELDERLY
deterioration n physiological function

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9
Q

How might absorption in elderly influence relative underdosing or overdosing

A
  1. ABSORPTION
    - decreased absorptive surface of small intestine
    - altered gastric and gut motility
    - increased rate of gastric emptying
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10
Q

How might distributionin elderly influence relative underdosing or overdosing

A
  1. DISTRIBUTION: reduced lean body mass and body water, relative increase in fat
  • lipid soluble drugs have increased Vd and decreased blood levels
  • water soluble drugs have decreased Vd and increased blood levels
  • reduced plasma albumin, so fewer plasma protein binding sites so increased amount of drug
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11
Q

How might metabolism in elderly influence relative underdosing or overdosing

A
  1. METABOLISM:
    - splanchnic and hepatic blood flow decrease by 0.3 – 1.5%/year
  • liver size and hepatocyte number decrease
  • hepatic enzyme activity and induction capacity decrease
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12
Q

How might excretion in elderly influence relative underdosing or overdosing

A

reduced renal mass

reduced renal perfusion

reduced glomerular filtration rate
reduced tubular excretion

These changes are normal – the situation may be compounded if the patient has renal disease

REDUCED EXCRETION OF DRUG AND ACCUMULATION

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13
Q

What is the most important affecting drug handling in elderly

A

changes in renal function are probably the most important factors affecting drug handling in the elderly

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14
Q

How might organ sensitivity in elderly influence relative underdosing or overdosing

A

the elderly tend to be more sensitive to CNS active drugs

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15
Q

How might general nutritional status affect overdosing or underdosing

A

unbalanced diets may lead to deficiency states and enzyme abnormalities

starvation – decreased plasma protein binding and metabolism

obesity – increased lipid fraction

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16
Q

How might GI status affect overdosing or underdosing

A

altered drug absorption

eg achlorhydria (lack of HCl in stomach… affects gastric pH and absorpton) , coeliac and Crohn’s

17
Q

How might congestive heart failure affect overdosing or underdosing

A

reduced splanchnic blood flow

intestinal mucosal oedema

reduced hepatic clearance

18
Q

How might kidney failure affect oversdosing or underdosing

A

decreased drug excretion leading to toxicity

water overload leading to changes in drug concentrations in different body fluid compartments

19
Q

How might liver failure affect oversdosing or underdosing

A

reduced metabolism

reduced first pass metabolism (hence increased bioavailability)

decreased biliary secretion and hence decreased removal

decreased albumin synthesis and hence reduced plasma protein binding

20
Q

Why would it be bad for patients to take bisphosphonates with milk

A

They are used osteoporosis.

Patients might think good to take with milk

But ions in milk chelate the bisphosphonates and affect the absorption

21
Q

What type of antibiotic is clarithryomycin

A

Macrolide

22
Q

What does the warfarin dose depend on

A

ADME…. it is standarised for each patient in relation to this

23
Q

How does clarithromycin affect warfarin

A

Warfarin metabolised by CYTP450

Clarithromycin inhibits CYTP450 which inhibits the warfarin metabolism and incrases plasma levels of it

24
Q

Which other drugs might reduce warfarin metabolism

A

Other amcrolide ABs

CYTP450 inhibitors:

ABs of other classes e.g. quinolones

Some systemic antifungals

PPIs

Some anti-HIV drugs

25
Q

What is INR

A

International normalised ration

INR=(pro-thrombin time (TEST)/ pro-thrombine time (CONTROLLED SAMPLE))^ISI

26
Q

What is ISI

A

International sensitivity index

(the tissue factor used in the clotting assay can be different and this also affects prothrombin time)…

between 1-1.4

it is standardised

27
Q

What is the normal INR

A

0.9-1.2

28
Q

What does reduced INR mean

increased INR?

A

Reduced INR= Increased propensity for clotting

Increased INR= increased propensity to bleed

29
Q

What might INR be in clotting and bleeding

A

Clotting=0.5

Bleedng=4

30
Q

How might St John’s wort affect warfarin

A

Upregulates CYTP450 to increase metabolism of the warfarin (hypericin)

more metabolism of warfarin so less of it so increased clotting

31
Q

What else might upregulate the warfarin metabolisn

A

CYTP450 inducers

Rifampicin (TB)

Griseofulvin (a systemic antifungal drug)

Anti-epilepsy drugs

32
Q

Which molecule in st johns wort induces CYTP450

A

Hypericin

33
Q

What would you want to do to INR if they have AF

A

You want to increase the INR (but be careful to not increase bleeding)

34
Q

Action of digoxin

A

Increases FoC and slows heart

By reducing the Na+/K+ ATPase

35
Q

What is the effect of hyperkalaemia and hypokalaemia for digoxin

A

Hyperkalaemia (renal problem?, so digoxin not cleared well, so more digoxin than you would expect

Hypokalamia- well digoxin competes with K+ for the Na/+K+ATPase, so digoxin will have a hige effect here and slow the heart

36
Q

Would you change the digoxin dose for increased ventricular rate

A

No, unless normokalaemic

If hypo or hyperkalaemic then no

37
Q

Why might effect of benzodiazapine for depression be greater on Mr Jones than his daughter

A
  1. ADME differences meanin worse metabolism and excretion so increased bioavailability
  2. OLDER PEOPLE HAVE INCREASED SENSITIVITY TO CNS DRUGS
38
Q

If Mr Jones had become anxious and lost lots of weight why might INR change

A

It may increase (increased bleeding)

Because malnutrition reduces CYTP450 activity so increased bioavailability of warfarin and increased propensity to bleed

Pharm (33 decks)

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