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ICPP > Pharmacokinetics 2 > Flashcards

Pharmacokinetics 2 Flashcards

1
Q

What is elimination?

A

How drugs are excreted from the body.

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2
Q

How is elimination done?

A

By metabolism and excretion

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3
Q

Where does excretion take place?

A

Predominantly done in the kidney (renal glomerular filtration)

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4
Q

Where does metabolism take place?

A

Predominantly in the liver

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5
Q

Metabolism is done in different phases. What happens in phase 1?

A 
Oxidation
Reduction
Deacylation
Hydrolysis
This usually inactivates drugs. However it can also activate them.
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6
Q

What happens in phase 2?

A

Some drugs enter phase 2 directly. Groups of enzymes add a hydrophilic group to the drug in order for it to be able to be filtered out by the kidneys. They also sometimes get rid of the lipophilic groups.

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7
Q

Why is it important to add a hydrophilic group or increase the ionic charge as well as making the drug less lipophilic?

A

In order for it to more easily be excreted. Lipophilic drugs diffuse out of the renal tubules back in to the plasma again.

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8
Q

What enzymes are responsible for the processes in phase 1?

A

Cytochrome P450 enzymes.

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9
Q

Where in the cell can you find CYP450s?

A

The cytoplasmic face of the ER.

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10
Q

How do CYP450s work?

A

They catalyse redox, diacylation and hydroxylation reactions.

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11
Q

What are enzyme inducers?

A

Drugs that enhance the activity of an enzyme. In this case CYP450s.

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12
Q

What consequences would increasing the activity of CYP450s have?

A

It would increase the metabolism of a drug, meaning the drug won’t last as long in the body or might not have the desired effect.

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13
Q

What are enzyme inhibitors in CYP450s case?

A

Drugs that inhibit CYP450s. This will decrease the activity of CYP450s and metabolism.

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14
Q

What consequences would decreasing the activity of CYP450s have?

A

It would mean that drug metabolism would decrease meaning a drug might build up in the system to toxic levels.

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15
Q

Give an example of a drug that is affected by enzyme inducers and enzyme inhibitors.

A

Warfarin. Phenobarbitone induces metabolism meaning the warfarin’s effect goes down.

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16
Q

What are prodrugs?

A

Drugs that are activated by phase 1 metabolism.

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17
Q

Give an example of a prodrug activated by phase 1 metabolism.

A

Codeine metabolised into morphine. Morphine then has a higher affinity for u-opioid receptors.

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18
Q

Where can you find phase 2 enzymes?

A

In the liver. Cellularly they are mainly cytosolic.

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19
Q

How do phase 1 CYP450s and phase 2 enzymes differ?

A

Phase 2 enzymes have a broader specificity and more rapid kinetics than CYP450s.

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20
Q

What is the purpose of phase 2 enzymes?

A

To reduce lipophilicity.
To add ionic charges
To increase hydrophilicity
This is in order to enhance renal elimination and therefore excrete the drugs.

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21
Q

There are two orders of kinetics. Which?

A

1st order and zero order.

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22
Q

What are first order kinetics?

A

Where the rate of elimination is proportional to the drug level.
Constant fraction of drug eliminated in unit time.
Half-life t1/2 can be defined.

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23
Q

How do first order kinetics look on a linear scale vs. a logarithmic scale.

A

On a linear scale they look like a reverse exponential curve. Going down that is.
On a logarithmic scale they look like a linear curve. As time passes the plasma concentration of a drug goes down.

24
Q

How do zero order kinetics look on a linear scale?

A

They have a linear curve on a linear scale. No logarithmic scale is used.

25
Q

How do first order and zero order differ?

A

1s order kinetics are predictable. The effect vs dose is linear. The therapeutic response from dose increases and it is easy to predict the outcome.
Zero order kinetics are not predictable. The effect vs. dose is exponential so with an increased dose it can quickly get out of hand. The therapeutic window might be breached and unwanted adverse effects might come.

26
Q

Why are drug interactions in metabolism important?

A

For drugs with low therapeutic ratio
Drugs that are being used at minimum effective concentrations.
Drug metabolism that follows zero order kinetics

27
Q

Define drug clearance.

A

The rate of elimination of active drug from the body.

28
Q

What is the total drug clearance.

A

Clearance from both the hepatic system and the renal system, as well as other clearances.

29
Q

Give the formula for clearance.

A

Clearance=elimination rate/[Drug]plasma or (mass/time)/(mass/volume)

30
Q

Clearance and volume of distribution provides an estimate for something. What?

A

The drug half-life.

31
Q

Define drug half-life.

A

The period of time required for the concentration of a drug in the body to be reduced by one half.

32
Q

Why is drug half-life important?

A

In order to design dosing schedules
Defining therapeutic regimen levels
Minimising adverse drug reactions

33
Q

What is the rule of thumb in repeated dosing?

A

A steady state is reached in 5 half lives of the drug. This is regardless of the dose or the frequency of administration.

34
Q

Digoxin is a drug used in order to control heart rate quickly. Its half-life is 36 hours. How long will it take to reach a steady state? Is this favourable?

A

36 x 5 = 180 hours. It will take 180 hours to reach a steady state. No this takes way too long time.

35
Q

What is given instead?

A

A bolus loading dose is given instead. Usually intravenous.

36
Q

What is a loading dose?

A

A higher dose than usually given in maintenance dosage.

37
Q

Heparin is needed to quickly control anticoagulation. It has a half-life of 6 hours. How long will it take to reach a steady state? Is this quick enough?

A

6 x 5 = 30 hours. This is too long. A bolus loading dose is given instead.

38
Q

When would you used a loading dose?

A

When half-life is long and a rapid effect is desired.

39
Q

Look at page 24 for a good explanation.

A

Yup.

40
Q

Are both free drugs and protein bound drugs filtered in the kidneys?

A

Only free drugs. Some drug are actively secreted by the tubules.

41
Q

Where do free drugs enter the kidney?

A

In the glomerular filtrate.

42
Q

Where are drugs actively secreted into the kidney?

A

In the proximal tubule.

43
Q

What are they secreted by?

A

OATs and OCTs.

44
Q

Where does passive reabsorption happen? What drugs are being reabsorbed?

A

In the collecting duct. Lipophilic drugs and unionised drugs.

45
Q

What is passive reabsorption dependent on?

A

pH

46
Q

Why pH?

A

Since depending on the pK of the drug it may be protonated or deprotonated. This means they can be ionised or non-ionised. Usually both.

47
Q

What does acidic urine do to weak acids in the kidney?

A

Acid urine increases the absorption. This is because the more acidic the urine the more protonated the acid. This is because if the pH is less than the pK of the drug the predominant form will be protonated and therefore unionised.

48
Q

What does alkaline urine do to weak acids in the kidney?

A

Alkaline urine decreases the absorption. This is because a more alkaline urine means more deprotonated acids and therefore ionised. They can’t be reabsorbed then.

49
Q

What does acid urine do to weak base drugs?

A

It decreases absorption as the base will become protonated. A protonated base is an ionised base.

50
Q

What does alkaline urine do to weak base drugs?

A

It increases absorption as the base will become deprotonated. A deprotonated base is an unionised base.

51
Q

What charge does a protonated acid have?

A

No charge

52
Q

What charge does a protonated base have?

A

A positive charge

53
Q

What charge does a deprotonated acid have?

A

A negative charge

54
Q

What charge does a deprotonated base have?

A

No charge

55
Q

Is aspirin an acid or a base?

A

A weak acid.

56
Q

How would aspirin poisoning happen?

A

If the urine is acidic the aspirin will become protonated and therefore unionised. This means that more aspirin will be reabsorbed into the system.

57
Q

How would you treat aspirin poisoning?

A

By bicarbonate treatment in order to make the urine more alkaline and therefore make the aspirin deprotonated and ionised.

ICPP (20 decks)

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