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Pharmacy > Science of Medicines Week 6 > Flashcards

Science of Medicines Week 6 Flashcards

(49 cards)

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1
Q

Why do we need to study the solubility of drugs?

A

there are changes in pH within the GI tract, so changes in ionisation and solubility and drugs could precipitate

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2
Q

Henderson-Hasselbalch equation for a weak acid

A

log [A-] / [HA] = pH - pKa

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3
Q

When is [A-] greater than [HA]?

A

when pH is greater than pKa

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4
Q

When is [HA] greater than [A-]?

A

when pH is less than pKa

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5
Q

Henderson-Hasselbalch equation for a weak base

A

log [B] / [BH+] = pH - pKa

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6
Q

When is [B] greater than [BH+]?

A

when pH is greater than pKa

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7
Q

When is [BH+] greater than [B]?

A

when pH is less than pKa

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8
Q

What is the solubility of a weak acid like in low pH?

A

you decrease pH by adding H+, so there will be an increased proportion of HA, which is unionised, so low solubility

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9
Q

What is the solubility like of a weak acid at high pH?

A

you increase pH by removing H+, so higher proportion of ionised form, which is highly soluble

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10
Q

Weak acid as salt equilibrium equation

A

Na+ A- + H2O <=> NaOH + HA

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11
Q

What is S0?

A

the saturation solubility of the undissociated species HA

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12
Q

What is Cs?

A

the ‘total saturation solubility’ of the weak acidic drug –> interpreted as the initial concentration of salt added to solution

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13
Q

What does Cs equal?

A

Cs = S0 + [A-]

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14
Q

What information do you need to predict the solubility of a weakly acidic drug?

A
  1. pH of the solution
  2. the pKa
  3. the solubility of the free (unionised) form of the drug (S0)
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15
Q

What is the solubility equation for a weakly acidic drug?

A

log (Cs-S0) / S0 = pH - pKa

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16
Q

When pH = pKa, what is the Cs of a weakly acidic drug?

A

Cs = 2 x S0

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17
Q

The solubility of a weakly acidic drug increase by about …. for each pH unit above the pKa

A

10x

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18
Q

The solubility of a weakly acidic drug approaches S0 as….

A

pH decreases below the pKa

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19
Q

The solubility of a weakly acidic drug equal 2 x S0 when…

A

the pH equals the pKa

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20
Q

When is a weakly basic drug more soluble?

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A

when pH is decreased by adding H+, so there is an increased proportion of BH+, so higher solubility

21
Q

When is a weakly basic drug less soluble?

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A

when pH is increased by removing H+, so there is a smaller proportion of BH+ and higher of B, so lower solubility

22
Q

Equation for weak bases forming salts

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A

BH+ Cl- + H2O <=> B + H+ Cl- + H2O

23
Q

What is Cs equal to for a weakly basic drug?

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A

Cs = S0 + [BH+]

24
Q

When pH = pKa, what is the Cs of a weakly basic drug?

25
The solubility of a weakly basic drug increases by about ... for each unit of pH below the pKa
10x
26
The solubility of a weakly basic drug approaches S0 as...
the pH increases above the pKa
27
The solubility of a weakly basic drug equals 2 x S0 when...
pH = pKa
28
What happens to the pH of a solution when the salt of a weak acid is used instead of the free form?
the pH increases
29
What happens to the solubility to the salt of a weak acid instead of the free form?
solubility increases
30
When may the precipitation of the salt of a weak acid occur?
when pH is the solution is lowered
31
What happens to the pH of the solution of the salt of a weak base instead of the free form?
the pH of the solution decreases
32
What happens to the solubility of the salt of a weak base instead of the free form?
solubility increase
33
When may precipitation occur with the salt of a weakly basic drug?
if pH is increased
34
BCS class 1
high solubility and permeability, use simple solid oral dosage form
35
BCS class 2
low solubility, high permeability, need techniques to increase surface area OR use solvents or surfactants
36
BCS class 3
high solubility, low permeability, need to include permeability enhances
37
BCS class 4
low solubility and permeability, combine 2 and 3
38
What happens to a weak base in the GI tract?
it will have a high dissolution rate in the stomach, but this falls as the pH of the GI tract rises
39
What happens to a weak acid in the GI tract?
it will have minimal dissolution in the stomach as pH is so low, but it increases down the gut with pH
40
How does the salt of a weak acid increase the pH of the diffusion layer?
1. Na+ + Cl- + H2O  Na+ + OH- + HA 2. Cl- reacts with water, forming -OH, increasing pH
41
How does the salt of a weak base decrease the pH of the diffusion layer?
1. BH+ + Cl- + H2O = H2O + B + H+ + Cl- 2. BH+ can donate a proton to water through H30+, which forms H2O and H+, decreasing pH
42
What is the major advantage of using the salt form of a drug?
results in a controlled pH of the diffusion layer no matter where in the GI tract it is
43
When does precipitation of a drug occur?
when the pH of solution is adjusted to a value that produces mostly unionised molecules exceeding solubility
44
How can the solubility of non-polar compounds in water be improved?
adding water-miscible solvents in which the compound IS soluble
45
define co-solvent
vehicles used with water to increase the solubility of a drug
46
What is the purpose of cosolvents? (3)
1. get aqueous systems where the drug is more soluble 2. formulate higher concentrations of the drug 3. improve stability of the formulation
47
What are the 3 factors a cosolvent needs?
1. to be an organic compound 2. miscible with water 3. better solvent than water for the drug
48
How do cosolvents works?
1. they decrease the hydrogen bond density of the aqueous system 2. reduce the cohesive interactions of water 3. reduce the polarity of the solution
49
How does solubility increase with increasing cosolvent concentration?
exponentially

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