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Flashcards in Drug Movement Deck (37)
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1
Q

What are the fates of drugs in the body?

A

Absorption
Distribution
Metabolism
Excretion

2
Q

What is absorption?

A

The process by which the drug enters the body from its site of administration and enters the general circulation

3
Q

What is distribution?

A

The transport of drugs by the general tissues. This usually involves the drug leaving the blood and entering tissues.

4
Q

What is metabolism?

A

The process by which tissue enzymes catalyze the chemical conversion of a lipid soluble drug into a less active and more polar form that can be readily excreted

5
Q

What is excretion?

A

The process that removes the drug or its metabolites from the body

6
Q

What factors affect drug absorption?

A

Solubility
Chemical stability
Lipid to water partition coefficient

7
Q

How does solubility affect drug absorption?

A

The drug must be able to dissolve in an aqueous solution in order to be absorbed

8
Q

How does chemical stability affect drug absorption?

A

Some drugs are destroyed by acid in the stomach or enzymes in the GI tract. In some cases, a compound is modified within the gut to release an active drug

9
Q

How does lipid to water partition coefficient affect drug absorption?

A

Absorption usually occurs via simple diffusion across membranes. For a given drug concentration gradeint across a membrane, the rate of diffusion increases with the lipid solubility of the drug.

10
Q

What is the partition coeffcient?

A

The ratio of the drug concentration in the membrane and concentration in water at equilibrium

11
Q

What does the degree of ionisation depend on?

A

The pKa of the drug and the local pH

12
Q

What is pKa?

A

The pH at which 50% of the drug is ionised and 50% of the drug is unionised

13
Q

How can the proportions of ionised and unionised drugs be calculated?

A

Henderson-Hasselbalch equations:
Acid: pKa - pH = log(AH/A-)
Base: pKa - pH = log(BH+/B)

14
Q

Where is the absorption of weak acids facilitated?

A

The pH of the stomach lumen.

15
Q

Where does the majority of absorption occur?

A

In the small intestine due to the large surface area, due to the presence of villi

16
Q

What do acidic drugs become in an acid environment?

A

Less ionised

17
Q

What do basic drugs do in a basic environment?

A

Less ionised

18
Q

What factors affect GI absorption?

A

GI motility - -rate at which stomach empties
pH at the absorption site
Blood flow to the stomach and intestines - increased by food
The way in which the tablet/capsule is manufactered
Physicochemical interactions
Transporters in the membranes of epithelial cells of the GI tract which facilitate drug absorption

19
Q

What is oral avaliability?

A

The fraction of drug that reaches the systemic circulation after oral ingestion

20
Q

What is systemic availability?

A

The fraction of drug that reaches the systemic circulation after absorption

21
Q

What way of administering drugs provides 100% systemic availability?

A

IV

22
Q

How can systemic availability be reduced by oral administration?

A

The drug can be inactivated by enzymes in the gut wall or liver before reaching the systemic circulation and body tissues in general

23
Q

What are common routes of administration?

A
Oral
Inhalation
Buccal/sublingual
Transdermal/subcutaneous
IV
Rectal
IM
24
Q

What routes of administration enter straight into the blood stream?

A
Buccal/sublingual
Subcutaneous
IV
Rectal
Intramuscular (IM)
25
Q

What routes of administration require absorption?

A

Oral

26
Q

What are some advantages to oral administration?

A

Simple, convenient, non-sterile, good absorption

27
Q

What are some disadvantages to oral administration?

A

Inactivation of drugs by acid/enzymes
Food binding
Swallowing may be difficult
Must pass the metabolism

28
Q

Where can drugs withing the systemic circulation move to?

A

Body fluid compartments

29
Q

What are the types of body fluid compartments?

A
Plasma water - 2.8L
Interstitial water - 11.2L
Intracellular water - 28L
Transcellular water - 1.4L
Fat
30
Q

What are the 2 states a drug molecule can exist in?

A

Bound or free

31
Q

What state of drug can move body compartments?

A

Free drugs. Ionisedand unionised drugs that are not bound to a protein can move freely via diffusion.

32
Q

What is the volume of distribution (Vd)

A

The apparent volume in which a drug is dissolved in the plasma membrane

33
Q

What is the Vd of a drug that is administered IV?

A

Vd = dose/plasma concentration

34
Q

What is more generally the equation to work out Vd?

A

Vd = amount of drug in the body/plasma concentration

35
Q

What does a Vd of less than 10L imply?

A

The drug is largely retained in the vascular compartment - this occurs for drugs that are extensively bound to membrane proteins such as aspirin or warfarin

36
Q

What does a Vd between 10 and 30L suggest?

A

The drug is largely restricted to extracellular water. This applies to drugs with a low lipid solubility e.g. gentamicin and amoxicillin

37
Q

What does a Vd of more than 30L suggest?

A

There is distribution throughout the total body water or accumulation in certain tissues