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Pharmacy > Science of Medicines Week 6 > Flashcards

Science of Medicines Week 6 Flashcards

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?

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?

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

A

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

23
Q

What is Cs equal to for a weakly basic drug?

A

Cs = S0 + [BH+]

24
Q

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

A

2 x S0

25
Q

The solubility of a weakly basic drug increases by about … for each unit of pH below the pKa

A

10x

26
Q

The solubility of a weakly basic drug approaches S0 as…

A

the pH increases above the pKa

27
Q

The solubility of a weakly basic drug equals 2 x S0 when…

A

pH = pKa

28
Q

What happens to the pH of a solution when the salt of a weak acid is used instead of the free form?

A

the pH increases

29
Q

What happens to the solubility to the salt of a weak acid instead of the free form?

A

solubility increases

30
Q

When may the precipitation of the salt of a weak acid occur?

A

when pH is the solution is lowered

31
Q

What happens to the pH of the solution of the salt of a weak base instead of the free form?

A

the pH of the solution decreases

32
Q

What happens to the solubility of the salt of a weak base instead of the free form?

A

solubility increase

33
Q

When may precipitation occur with the salt of a weakly basic drug?

A

if pH is increased

34
Q

BCS class 1

A

high solubility and permeability, use simple solid oral dosage form

35
Q

BCS class 2

A

low solubility, high permeability, need techniques to increase surface area OR use solvents or surfactants

36
Q

BCS class 3

A

high solubility, low permeability, need to include permeability enhances

37
Q

BCS class 4

A

low solubility and permeability, combine 2 and 3

38
Q

What happens to a weak base in the GI tract?

A

it will have a high dissolution rate in the stomach, but this falls as the pH of the GI tract rises

39
Q

What happens to a weak acid in the GI tract?

A

it will have minimal dissolution in the stomach as pH is so low, but it increases down the gut with pH

40
Q

How does the salt of a weak acid increase the pH of the diffusion layer?

A
  1. Na+ + Cl- + H2O  Na+ + OH- + HA
  2. Cl- reacts with water, forming -OH, increasing pH
41
Q

How does the salt of a weak base decrease the pH of the diffusion layer?

A
  1. BH+ + Cl- + H2O = H2O + B + H+ + Cl-
  2. BH+ can donate a proton to water through H30+, which forms H2O and H+, increasing pH
42
Q

What is the major advantage of using the salt form of a drug?

A

results in a controlled pH of the diffusion layer no matter where in the GI tract it is

43
Q

When does precipitation of a drug occur?

A

when the pH of solution is adjusted to a value that produces mostly unionised molecules exceeding solubility

44
Q

How can the solubility of non-polar compounds in water be improved?

A

adding water-miscible solvents in which the compound IS soluble

45
Q

define co-solvent

A

vehicles used with water to increase the solubility of a drug

46
Q

What is the purpose of cosolvents? (3)

A
  1. get aqueous systems where the drug is more soluble
  2. formulate higher concentrations of the drug
  3. improve stability of the formulation
47
Q

What are the 3 factors a cosolvent needs?

A
  1. to be an organic compound
  2. miscible with water
  3. better solvent than water for the drug
48
Q

How do cosolvents works?

A
  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
Q

How does solubility increase with increasing cosolvent concentration?

A

exponentially

Pharmacy (16 decks)

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