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Pharmaceutics Year 2 > Section 3 - Solubility > Flashcards

Section 3 - Solubility Flashcards

1
Q

What makes preparation of a solution dose form problematic?

A

Drugs with limited water solubility

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2
Q

What causes many drugs to have limited water solubility?

A

Molecules interact with repulsive forces, which prevent molecular interpenetration, and attractive forces, which cause molecules to cohere

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3
Q

What happens when attractive and repulsive forces are in eq’m?

A

The potential energy between the 2 molecules is minimal and the system is most stable

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4
Q

What is the solubilization theory?

A

In the process of dissolution, both the solute-solute and solvent-solvent bonds must be broken to allow the formation of solute-solvent bonds

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5
Q

What happens in bond breaking with respect to energy?

A
  • Energy is required

- Breaking of bonds will absorb heat

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6
Q

What happens in bond formation with respect to energy?

A

Evolution of heat

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7
Q

Most dissolution processes show net effect of _____ heat

A

Absorbing

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8
Q

How can you increase solubility?

A

Increase temp.

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9
Q

What is an example of a drug that shows a decrease in solubility with an increase in temp?

A

Calcium citrate

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10
Q

The solubility of any substance at a stated temperature is _____

A

Constant

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11
Q

A solubility less than __% is considered insoluble

A

1%

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12
Q

What does solubility depend on for molecules with ionizable functional groups?

A

pH of the solvent

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13
Q

What are barbiturates?

A
  • Weak acids with low solubility

- Form water soluble salts at alkaline pH

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14
Q

What situation causes a barbiturate to come out of solution?

A

If the pH of a solution of one of the water soluble salts is lowered by the addition of acid

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15
Q

The sodium salt of phenobarbital has a solubility of about 1 g in __ mL

A

1

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16
Q

The free acid phenobarbital has a solubility of about 1 g in ____ mL

A

1000

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17
Q

How can you predict solubility in a general way?

A
  • Molecular structure
  • Functional groups
  • Physicochemical properties of the solute and solvent
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18
Q

What does “like dissolves like” mean?

A

A solvent w/ a chemical structure or functional groups similar to the solute will most likely dissolve it

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19
Q

Organic molecules are generally more soluble in ____ solvents than water

A

Organic

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20
Q

What will addition of a polar functional group (such as -OH) do to the water solubility of an organic molecule?

A

It will increase water solubility of an organic molecule

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21
Q

What is the rule for water solubility of an inorganic molecule if both the cation and anion are monovalent and why?

A
  • The compound is soluble

- B/c solute-solute forces are easily overcome

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22
Q

What is the rule for water solubility of an inorganic molecule if one of the ions in a salt is monovalent?

A

The compound is soluble

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23
Q

What is the rule for water solubility of an inorganic molecule if both the anion and cation are multivalent and why? What are the exceptions?

A
  • The compound will have limited water solubility b/c the solute-solute forces are too great to be easily overcome by solute-solvent interactions
  • Exceptions - ZnSO4 and FeSO4
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24
Q

What is the rule for water solubility of an inorganic molecule if it is a salt of alkali metals and ammonia and what is the exception?

A
  • Water soluble

- Exception - Li2CO3

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25
Q

Are nitrates, sulfates, and halides water soluble? What are the exceptions?

A
  • Yes

- Exceptions - AgCl2, BaSO4

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26
Q

Are hydroxides and oxides water soluble? What are the exceptions?

A
  • No

- Exceptions - alkali metals and ammonium salts

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27
Q

Are phosphates, carbonates, silicates, and borates water soluble? What are the exceptions?

A
  • No

- Exceptions - salts of alkali metals and ammonium

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28
Q

What is the rule for water solubility of an organic molecule if the molecule has one polar functional group?

A

Usually soluble up to a chain length of 5 carbons

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29
Q

What is the rule for water solubility of an organic molecule if the molecule has a branched chain?

A

More soluble than the corresponding straight chain compound

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30
Q

For organic solubility, what is the relationship between water solubility and molecular weight?

A

Inverse (solubility decreases with increasing molecular weight)

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31
Q

Generally, a drug must be _____ before it is able to be absorbed into the bloodstream

A

In solution

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32
Q

What determines how much of a drug is available for absorption?

A
  • Dose
  • Solubility
  • Dissolution rate
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33
Q

For a liquid dose form, is water solubility desirable?

A

Yes

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34
Q

For a suspension, is water solubility desirable?

A

It is desirable to reduce solubility, provided bioavailability is not affected

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35
Q

What is the Biopharmaceutics Classification System (BCS)?

A

A classification for oral drug products into 4 groups based on solubility and ability to permeate membranes

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36
Q

What is needed for a drug to be considered highly soluble under the BCS?

A

The highest dose strength must be soluble in less than 250 mL water of a pH range of 1-7.5

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37
Q

What is needed for a drug to be considered highly permeable under the BCS?

A

When the extent of absorption is over 90% of an administered dose

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38
Q

What are class 1 drugs under BCS?

A

Highly soluble and highly permeable (present few problems)

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39
Q

What are class 2 drugs under BCS?

A

Low solubility and high permeability

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40
Q

What are class 3 drugs under BCS?

A

High solubility and low permeability

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41
Q

What are class 4 drugs under BCS?

A

Low solubility and low permeability (present problems)

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42
Q

What is the rate-limiting step for bioavailability for class 2 drugs under BCS?

A

Solubility/dissolution

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43
Q

What is the rate-limiting step for bioavailability for class 3 drugs under BCS?

A

Crossing biological membranes

44
Q

What is the required dose:solubility ratio (D:S) for a drug that will enter the stomach?

A

Under 250 mL b/c the volume of stomach fluid is about 250 mL, so the dose must be soluble in that volume

45
Q

What is the required dose:solubility ratio (D:S) for a drug that will enter the eye?

A

Under 0.02 mL b/c the lacrimal fluid volume is about 20 uL

46
Q

What is the required dose:solubility ratio (D:S) for a drug that will enter the nose?

A

Under 0.3 mL b/c the volume of nasal fluid is about 0.3 mL

47
Q

How can solubility be altered for a drug molecule that contains ionizable functional groups?

A

By altering the pH of the vehicle being used

48
Q

Most drug products are formulated between pH values of ___ and ___ for ____

A

4 and 8 for biocompatibility

49
Q

How is formulation pH determined?

A

From pH-solubility and pH-rate profiles of the drug

50
Q

Generally, when a drug is in ionized form it is ____ soluble

A

Water

51
Q

What determines the extent a molecule is ionized?

A
  • pH of the medium

- pKa of the drug

52
Q

What can the Henderson-Hasselbalch equation help you calculate with respect to solubility?

A
  • pH of maximum solubility

- pH at which precipitation occurs

53
Q

What is generally the target formulation pH?

A

+/- 1 pH unit from the pH of maximal solubility

54
Q

What must be considered when buffers are used and why?

A
  • Must consider the stability of the drug

- Some buffer species may accelerate degradation

55
Q

What is a salt?

A

The product of an ionizable drug with an ionizable counterion of opposite charge

56
Q

What can salt formation alter of a drug?

A
  • Physicochemical properties
  • Biopharmaceutical properties
  • Processing properties
57
Q

What will salt formation NOT alter?

A
  • Fundamental chemical structure

- Corresponding pharmacology

58
Q

Salt formation is an easy way to overcome ____

A

Undesirable properties of the parent drug

59
Q

Salts will differ from their parent drug and from each other in properties such as ____

A
  • Melting point
  • Solubility
  • Dissolution rate
  • Hydroscopicity
  • Stability
  • Crystal form
  • Solution pH
  • Organoleptic properties (like taste)
60
Q

What is the most common reason for using a salt?

A

To improve bioavailability

61
Q

When would reduced solubility be desired?

A
  • Some sustained release products

- Masking an unpleasant taste

62
Q

What should you consider when choosing a counter-ion for a salt?

A

The pKa of the conjugate base should be greater than the conjugate acid by at least 3 pKa units

63
Q

What is Ksp?

A

The concentration of the drug [A+] times the concentration of the counter-ion [B-]

64
Q

How can Ksp be kept constant if the counter-ion concentration is increasing?

A

Drug concentration in solution must decrease

65
Q

The advantage of increased solubility may be offset if the salt is _____

A

Hydroscopic

66
Q

What is polymorphism?

A

The property where a compound can exist in 2 or more crystalline forms

67
Q

What is pseudopolymorphism?

A

Where solvent molecules are incorporated into the lattice, usually when the drug is purified using recrystallization

68
Q

Is the dissolution rate faster in a salt or its parent molecule and why?

A

Salt b/c it can act as its own buffer in the diffusion layer

69
Q

Is the pharmacology of a parent drug usually changed when a salt is formed?

A

No

70
Q

What are some examples of atoms that shouldn’t be used as counter-ions and why?

A
  • Lithium, aluminum, bromide, iodides, and salicylates

- They have pharmacologic activities, which may lead to the potential for toxicity

71
Q

What are examples of products that may be used as cosolvents for parenteral products with limited water solubility and why?

A
  • Glycerin
  • Alcohol
  • Propylene glycol
  • Polyethylene
  • May provide exponential increases in solubility and they allow reduction of water content for products susceptible to hydrolysis
72
Q

Which cosolvents are extremely toxic and shouldn’t be used in pharmaceutical products?

A
  • Ethylene glycol

- Methanol

73
Q

How is selection and quantity of cosolvent determined?

A
  • Route of administration
  • Rate of administration
  • Nature of the patient
74
Q

What happens to solubility when volume fraction of cosolvent is increased?

A

Solubility increases logarithmically

75
Q

Solubility is largely a function of ____

A

The polarity of the solvent

76
Q

What do polar solvents dissolve?

A

Ionic and other polar solutes

77
Q

What do semi-polar solvents do to non polar molecules?

A

May induce a degree of polarity and improve their water solubility

78
Q

What is a dielectric constant?

A
  • A measure of the effect of a solvent on the potential energy of interaction between 2 charges
  • Measure of how effective the material is as an electrical insulator (its ability to keep two charges separate)
79
Q

What is the unit for dielectric constants?

A

No units

80
Q

How can you determine the dielectric constant of a solvent mixture?

A

Multiply the volume fraction of each solvent by its respective dielectric constant and sum

81
Q

Why are there only a few cosolvents that are suitable for use?

A

Toxicity concerns

82
Q

What are common side effects of most cosolvents?

A
  • Local irritation
  • Hemolysis
  • Tissue damage
83
Q

What is important to note about benzyl alcohol as a cosolvent?

A

Toxic to neonates

84
Q

_____ products rarely contain cosolvents

A

Ophthalmic

85
Q

For parenterals, the quantity of ethanol rarely exceeds ___%

A

10%

86
Q

For parenterals, the quantity of propylene glycol rarely exceeds ___%

A

40%

87
Q

For parenterals, the quantity of PEG rarely exceeds ___%

A

50%

88
Q

What can result from dilution of a product prepared with cosolvents?

A

The drug may come out of solution

89
Q

How can cosolvents stabilize a drug subject to hydrolysis?

A

Reduce the amount of water present

90
Q

How do cosolvents affect reaction rates?

A

Shift the eq’m between reactants and the transition state

91
Q

What effect will increased polarity of the solvent have on a solution in which the transition state is more polar than the reactants?

A

Increase reaction rate

92
Q

What effect does increasing the ionic strength of a solution have on polarity?

A

Will generally increase polarity

93
Q

Which dose form are surfactants used to formulate?

A

Parenteral

94
Q

What are surfactants?

A

Molecules w/ distinct polar and non-polar regions

95
Q

What is normally the non-polar region of a surfactant?

A

A hydrocarbon segment

96
Q

What is normally the polar region of a surfactant?

A
  • Anionic (carboxylate or sulfonate)
  • Cationic (trialkylammonium or ammonium)
  • Non-ionic (PEG, glycerol, sugar)
97
Q

What happens as concentration of surfactant increases?

A

The surfactant will associate into soluble structures called micelles

98
Q

What is the critical micelle concentration?

A

The concentration at which micelles begin to form

99
Q

Are micelles soluble?

A

Yes

100
Q

What are drawbacks to surfactants?

A
  • Can cause extreme irritation and damage to tissues

- Can cause systemic toxicity in children and newborns

101
Q

What are the only 3 surfactants used in formulating parenterals?

A
  • Polysorbate-80 (Tween)

- Polyethoxylated fatty alcohol (Cremophor EL)

102
Q

What is a complexing agent used for?

A

Drugs are incorporated within a complexing agent so that the outer hydrophilic groups of the agent interact w/ water and make the whole complex soluble

103
Q

What are cyclodextrins?

A

Molecules composed of 6, 7, or 8 glucopyranoside units joined through 1-4 bonds

104
Q

What causes the exterior of a cyclodextrin to be hydrophilic?

A

Hydroxyl groups

105
Q

What can complex with the interior of a cyclodextrin?

A

A lipid-soluble drug molecule

Pharmaceutics Year 2 (17 decks)

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