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FUN D > D12 Physical stability > Flashcards

D12 Physical stability Flashcards

1
Q

why is stability in medicines important?

A
  • essential factor in safety, efficacy and quality of medicine
  • requires chemical and physical stability
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2
Q

what may physical instability cause?

A
  • undesired changes in performance of the medicine (eg. phase separation of suspensions / emulsions)
  • may cause over or under dosing
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3
Q

what factors of the environment can influence the stability of the medicine?

A
  • temperature
  • relative humidity
  • light
  • oxygen
  • packaging
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4
Q

state 3 states that drugs and excipients can be in

A

crystalline solid
liquid / solution
gas

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5
Q

explain the state of matter and mechanism in a metre dose inhaler

A
  • comprised of can containing a liquified propellant
  • gases liquified in a closed container in equilibria with gas phase (headspace of inhaler)
  • the change in high to low pressure from the can to the atmosphere causes the change in state of liquid to gas
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6
Q

what 3 actions can happen to water upon increases in temp?

A

melting
vaporisation (evaporation)
sublimation

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7
Q

what 3 actions can happen to water upon decreases in temp?

A

condensation
freezing
deposition

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8
Q

what is the triple point?

A

where solid, liquid and gas are in equilibrium at this specific temp and pressure

(the temp and pressure at which a substance can exist as solid, liquid or gas)

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9
Q

describe solid to liquid transitions (temperature-induced) in relation to drugs and medicines

A
  • atoms in a solid state vibrate in fixed positions and increases in temperature disrupt the ordered nature of the lattice
  • most drugs are crystalline solids with high MP
  • suppository bases are stable at room temp but will melt at body temp when inserted to release drug
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10
Q

describe liquid to solid transitions in relation to drugs and medicines

A
  • freezing water in a liquid medicine creates crystals of ice
  • these interfere with physical stability of suspensions and emulsions
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11
Q

describe solid to gas transitions in relation to drugs and medicines

A
  • this mechanism is used during freeze-drying (drug is frozen in water then dried by sublimation)
  • iodine can sublime directly to gas
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12
Q

describe liquid to gas transitions in relation to drugs and medicines

A
  • cytotoxics: high vapour pressure and can volatilise to a gas if split
  • hazard when working with cytotoxics as this can be toxic
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13
Q

describe evaporation

A
  • change of liquid into a vapour at a temperature below the boiling point (occurs at lower temps)
  • vapour pressure of solids is much lower than liquids due to stronger intermolecular forces in solids
  • aqueous phase may evaporate from liquid medicines if bottle lid is left off
  • non-volatile drugs / low vapour pressures don’t escape or form vapours (leads to increased concentration of drug as water can escape but drug cannot)
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14
Q

describe sublimation

A
  • change of solid into a vapour at a temperature below the triple point in the phase diagram
  • direct transition from solid to gas
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15
Q

what is vapour pressure?

A
  • condensed phase (liquid or solid) placed in a closed container is the vapour
  • pressure exerted by the vapour at equilibrium (at constant temp)
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16
Q

describe adsorption of water onto a solid

A
  • solid (eg. tablet) has an equilibrium formed in its surface at constant temp and pressure
  • monolayer of water is formed on the surface
  • means the drug is in solution so there is potential for chemical degradation / hydrolysis
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17
Q

what is adsorption dependent on? how do these factors affect it?

A
  • temperature
  • pressure
  • amount of water vapour in atmosphere (humidity)
  • adsorption decreases with increased temperature
  • adsorption increases with increased humidity
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18
Q

what is hygroscopicity dependent on?

A
  • surface area
  • surface energy of drug particle (more polar = more water absorbed)
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19
Q

how can a solution be described when no more solute dissolves?

A

saturated

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20
Q

what does it mean for something to be supersaturated?

A
  • when more solute is added to an already saturated solution
  • the solution becomes supersaturated and the drug can crystallise out
21
Q

what is equilibrium solubility?

A

the concentration of drug in a saturated solution when excess solid is present and solution and solid are at equilibrium

22
Q

how do solubility and concentration change with temperature?

A
  • solubility decreases with temperature
  • concentration remains unchanged when temperature decreases

medicines must be stored at their recommended conditions to ensure drugs do not ppt out!

23
Q

what is meant by crystalline?

A
  • a solid in which the constituent molecules are packed in a regularly ordered, repeating pattern extending in all 3 spatial dimensions
  • held together by non-covalent interactions (electrostatic interaction, hydrogen bonding)
24
Q

what is meant by amorphous?

A
  • materials with no apparent crystalline form
  • no long range order
25
Q

describe the physical form of drugs

A
  • most drugs (organic molecules) are crystalline and designed to be inherently stable
  • except for surface amorphous content induced by processing on the surface of drugs (eg. milling / micronising)
  • most stable polymorph / hydrate is used
26
Q

why do drugs have high melting points?

A
  • so that they are stable (physically and chemically) at room temp and during processing
27
Q

which form is more stable, crystalline or amorphous?

A
  • crystalline
  • amorphous phases can crystallise with exposure to moisture
28
Q

define polymorphs

A
  • same molecule with different packing in the structure of the crystal
  • lots of one molecule arranged in a different packing
29
Q

what are polymorphs?

A
  • 2 crystals that have the same chemical composition but different molecular packing
  • have different physical and chemical properties
30
Q

state some properties that can vary between polymorphs

A

melting point
density
solubility
bioavailability
processing properties

31
Q

how are stability and solubility of polymorphs related?

A

the more stable a polymorph, the lower its solubility

32
Q

describe and explain what can happen to metastable forms of polymorphs and why this is bad

A
  • can convert to more stable form in both solid and liquids (tablets and suspensions) in storage
  • this means it changes to a form with lower solubility (less absorbed into GIT so lower drug-plasma conc and reduced effect)
  • if you start with the most stable polymorph, there are no changes in storage and high drug-plasma conc
33
Q

what are hydrates?

A
  • crystal structures with the presences of water molecules

influences:
- solubility
- stability
- dissolution rate
- bioavailability

34
Q

what material can molecules not pass through?

A

glass

35
Q

state 2 semi-permeable barriers to molecules. describe them further

A
  • rubber
  • plastics
  • can be permeable to oxygen, water vapour, carbon dioxide
  • components of medicines can also permeate through these packagings by diffusion
  • permeation depends on: area, thickness of the barrier, time, differences in partial vapour pressure across the barrier
36
Q

what is permeation described by?

A
  • Fick’s first law (rate of diffusion)
37
Q

describe how packaging problems arise and state some factors that can affect them

A

from:
- leachables
- discolouration
- absorption or adsorption (more common) from the medicine

  • all are temperature, humidity and storage time dependent
  • higher temp and humidity increase likelihood
38
Q

define leachables

A

‘organic and inorganic chemical species that can be released form the surfaces of the packaging used in storage of drug products (medicine) under conditions of normal use’

chemical components of the drug’s packaging migrate into the medicine itself

39
Q

what influences the accumulation of leachables in a medicine (solution)?

A
  • initial amount of leachable in container
  • solubility limit of the leachable in solution
  • partitioning of the leachable between the container and the solution
  • migration of the leachable from the container into solution
40
Q

describe how adsorption of drug or excipient can occur from solution to container surface, state factors that can affect the extent of adsorption

A
  • occurs when chemical groups are present that allow interaction

factors:
- solute concentration
- pH
- temperature
- surface area

41
Q

explain how pH can affect the extent of adsorption of drug or excipient molecules from a solution to container surface

A
  • affects extent of ionisation
  • unionised are more likely to be adsorbed
  • ionised will want to stay in solution
42
Q

explain how temperature can affect the extent of adsorption of drug or excipient molecules from a solution to container surface

A
  • exothermic process
  • lower temperature means more molecules adsorbed
43
Q

explain how surface area can affect the extent of adsorption of drug or excipient molecules from a solution to container surface

A
  • more surface area available for adsorption means more adsorption
44
Q

describe the types of glass (type I, II and III)

A

type I (borosilicate)
- chemically resistant and low leachables
- primarily used for parenteral medicines
- expensive, only used when necessary

type II and III (soda lime)
- type II suitable for buffered solutions
- type III used only for dry medicines (tablets / powders)

45
Q

what are the potential leachables from borosilicate glass?

A

alkaline earth & heavy metal oxides

46
Q

what are the potential leachables from soda lime glass?

A

alkaline earth & heavy metal oxides

47
Q

describe PVC in regards to being used for packaging

A
  • high potential for leachables, permeation and adsorption of insulin and other proteins
  • vitamin A and warfarin adsorbed (drugs present as additives)
48
Q

what are the potential leachables of natural, butyl and silicone rubber polymers?

A

natural:
heavy metal salts, lubricants, reducing agents

butyl:
heavy metal salts, lubricants, reducing agents

silicone:
minimal

FUN D (8 decks)

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