3. Disposition of Drugs Flashcards Preview

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Flashcards in 3. Disposition of Drugs Deck (35):
1


What is Drug Disposition? 

the study of the movement of drugs in the body across biological membranes from the
time of absorption until elimination

2

What are the stages of drug disposition?

Absorption
Distribution
Biotransformation
Excretion

3

Describe what the biological cell membrane is composed of

Composed of a lipid bilayer
Membranes also contain membrane proteins and carbohydrates
There are aqueous pores or junctions between cells

4

List the ways drugs can be transported across the cell membrane

Passive diffusion
Facilitated diffusion
Active transport
Pinocytosis

5


What movements are included in passive diffusion?


Transmembrane and paracellular movement
Simple diffusion

6

What is Transmembrane Movement?

movement of water is either
by passive diffusion through aqueous protein
channels or by flow resulting from osmotic or hydrostatic difference across the membrane

7

What is paracellular transport?

transport of water is through intercellular aqueous pores

8

Tissues that have a barrier mechanism have?

 “tight” intercellular junctions

9


What can bulk flow of water carry with it?

small water-soluble substances

10

What are factors that affect simple diffusion?

Concentration gradient
Lipid solubility
Degree of ionization

11

Describe Concentration Gradient

Drugs move across membranes from high concentration to low concentration

12

Describe Lipid Solubility


Is measured by lipid partition coefficient (the ratio between solubility of drug in
lipid to its solubility in water).
The higher the lipid solubility of the drug the faster the drug crosses cell
membranes

13

Describe Degree of ionization

Drugs cross biological membranes in the non
ionized form (lipid soluble and do not
carry electric charges).
The rate of diffusion depends on the ratio between the non ionized form of the drug
to the ionized form (N/I)

14

Ionization of the drug depends on:

The pH of the drug
The pka of the drug
The pH of the medium (environment)

15

What is the pka of a drug?

the pH at which a weak electrolyte is 50% ionized and 50%
nonionized

16

Most drugs are?

weak electrolytes (weak acids or weak bases)

17


Acidic drugs ionize in?

alkaline medium

18

Basic drugs ionize in?

acidic medium

19

Henderson and Hasselbalch equation


pka of an acid = pH of the medium + Log N/I

pka of a base = pH of the medium + Log I/N

N = Non ionized
I = Ionized

20

What happens to the N/I value when the Pka of a weak acid is higher?

its higher

21

What happens to the N/I value when the Pka of a weak base is lower?

Its higher

22

Quaternary ammonium compounds such as d-tubocurarine are?

mostly ionized

23

Phenylbutazone, a weak acidic drug (pka 4.4) is?

mostly non ionized in the stomach
(pH 1.4)

Then it is absorbed from the stomach because it is lipid soluble.
When it reaches the plasma (pH 7.4) it is mostly ionized and is trapped in plasma

24

What is facilitated diffusion?


A carrier-mediated transport that does not need energy.
A drug moves across the membrane from high concentration to low concentration at a faster rate (downhill movement)

25

Transporters can move drugs from?

 from outside the cell into the cell or from inside the cell
outside the cell

26

Transporters may act as a what?

a barrier system to protect cell from toxic substances by moving them from inside the cell outside the cell

27

What is an example of facilitated diffusion?

Movement of glucose across a muscle cell membrane by insulin-sensitive glucose transporter

28

What is active transport?

A drug moves across the membrane from low concentration to high concentration (uphill movement).
A carrier-mediated transport that requires energy

29

Primary active transport uses energy that comes directly from what?

ATP such as Na+-K+ ATPase

30

Secondary active transport uses stored energy in the?

in the Na+ electrochemical gradient that
was created using ATP in primary active transport

31


Secondary active transporters are classified into what?

symporters (cotransporters) such as
the Na+ -depended glucose transporters in the GI epithelium and renal tubules or antiporters (exchangers) such as Na+-Ca+2 exchanger protein

32

Active transport is characterized by?

saturablility
selectivity
competitive inhibition by cotransported compounds

33

What is Pinocytosis?

Is a specific type of endocytosis where the cell engulfs the drug molecules dissolved in water

34

An example of pinocytosis

Sequestration of aminoglycoside antibiotics by renal tubular cells

35

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