Drug Stability & Stability Testing - Part 1

Question 1

What is quality measured by?

Answer 1

• the CONTENT of the ACTIVE SUBSTANCE in the product
• the PURITY of the active substance in the product (presence of impurities and decomposition products of the active)
• the PHYSICO-CHEMICAL PROPERTIES of the product
• the MICROBIOLOGICAL PROPERTIES of the product
• the ORGANOLEPTIC AND AESTHETIC PROPERTIES of the product

Question 2

Biological molecule can be..

Answer 2

activated or de-activated

Question 3

Which Modes of Drug/Product Degradation are frequently closely linked

Answer 3

• chemical
• biological
• physical
• microbiological

Question 4

Classes of degradation - chemical

Define hydrolysis

Answer 4

Hydrolysis is frequently catalysed by hydrogen ions or hydroxyl ions and by acidic or basic species (buffers).

Question 5

Can hydrolysis be removed?

Answer 5

Hydrolysis can only be reduced, cannot get rid of it completely

Question 6

What are the main 4 classes of drugs susceptible to hydrolysis - ‘labile carbonyls’

Answer 6

• esters - e.g. aspirin, cocaine, procaine, tetracaine
• amides - e.g. dibucaine, chloramphenicol
• lactones - e.g. pilocarpine, spironolactone
• lactams - e.g. penicillins, cephalosporins, nitrazepam

Question 7

Hydrolysis of tetracaine

Answer 7

draw

Question 8

How do we prevent/reduce hydrolysis?

Answer 8

• Minimise contact/removal of moisture:
  solid dosage forms; desiccant (silica gel at high humidity,
  molecular sieves (including bentonites, zeolites) at low humidity)
• Adjust the pH:
  reduce acid-base catalysed hydrolysis
• Reduce solubility/less soluble salts/solvents:
  if drug is not in solution it can’t be hydrolysed (eg. procaine, penicillin suspensions)
• Form complexes:
  eg. addition of caffeine to benzocaine, procaine and amethocaine

Question 9

What does oxidation cause?

Answer 9

Major cause of drug degradation:
morphine, dopamine, adrenaline, some steroids, some
antibiotics

Question 10

Functional groups subject to oxidation include:

Answer 10

phenols
ethers
thiols
carboxylic acids
catechols
aldehydes
thioethers
nitrites

Question 11

What is chemical oxidation and what are the equations for reversible loss of e-?

Answer 11

Removal of an electropositive atom, radical or electron, or addition of an electronegative atom or electron.

Fe2+ - e- ↔ Fe3+
Fe3+ + e- ↔Fe2+
Red ↔ Oxid + n e-

reversible loss of electrons without the addition of oxygen

Question 12

What are free radicals?

Answer 12

Chemicals containing at least one unpaired electron.

Highly unstable and highly reactive

Question 13

What is Auto-oxidation?

Answer 13

many pharmaceutical oxidations are irreversible chain reactions proceeding
slowly under the influence of atmospheric oxygen.
Reactions may occur over several years.
Involves formation of free radicals

Question 14

Auto-oxidation

What are the steps in the chain reaction

Answer 14

initiation → propagation → termination

Question 15

What happens at initiation?

Answer 15

free radicals formed by the action of light, heat or trace
metals (such as iron, from where?):
RH → R● + H●

Question 16

What happens at propagation?

Answer 16

formation of peroxy radical, ROO ●
removal of H from organic molecule to form hydroperoxide ROOH
and a new free radical

R● + O2→ROO●
ROO● + RH → ROOH + R●

Question 17

What happens at termination?

Answer 17

free radicals combine to form inactive (new) products

ROO● + R●→ ROOR
R● + R●→ RR

Question 18

How do we prevent / reduce oxidation?

Answer 18

Precautions during manufacturing
! avoid contact of drug with heavy metal ions
! replace oxygen in container with nitrogen (salads…)
! well-filled containers – less room for oxygen…
! store at reduced temperatures (i.e. rate of reaction / solubility of O2 in solvents)

Formulation approaches
! use antioxidants
! primary antioxidants – interfere with propagation step
! synergists – promote activity of primary antioxidants
! reducing agents – have a lower REDOX potential than the drug – more likely to be oxidised than the drug
use chelating agents
! e.g. ethylene diamine tetra-acetic acid (EDTA) ! adjust pH
! decreasing pH often increases REDOX potential (the tendency of a chemical to acquire electrons and thereby be reduced – higher REDOX potential = more likely to be reduced) and increases resistance to oxidation,
! e.g. adrenaline solution – reducing pH from 6 to 4 results in a 2-fold increase in chemical stability

Question 19

What do antioxidants do?

Answer 19

Interrupt propagation by interaction with free
radicals
– leads to formation of an antioxidant free radical which Is not sufficiently reactive to maintain the chain reaction
– commonly used antioxidants include: 
• gallic acid
• butylated hydroxyanisole 
• butylated hydroxytoluene 
• tocopherols

Question 20

How to prevent/stop initiation?

Answer 20

INHIBIT
! protect from light
! chelate metals
! reduce temperature ! change pH

Question 21

How to prevent/stop propagation?

Answer 21

INHIBIT
! antioxidants
! reduce/remove free oxygen

Question 22

Which pharmaceuticals are light sensitive?

Answer 22

• corticosteroids – hydrocortisone; prednisolone;
• phenothiazine tranquillisers – chlorpromazine;
• tetracyclines
• catecholamines – adrenaline
• sulphonamides
• benzodiazepines - diazepam

Question 23

What happens in Photochemical degradation (photolysis)?

Answer 23

Molecules exposed to electromagnetic radiation → absorb light at CHARACTERISTIC wavelengths (note: UV spectra; IR spectra)
Frequently results in dark-coloured products or fading of colouring agents

! This causes an increase in the energy state of the compound that can lead to
! decomposition
! conversion to heat
! emission of light at a new wavelength (fluorescence)

Question 24

Photo-degradation depends on..

Answer 24

! intensity and wavelength

! shorter the wavelength – higher the energy

Question 25

What is photo degradation mediated by

Answer 25

free radicals (in similar way to oxidation)

Question 26

How do we prevent/reduce photochemical degradation?

Answer 26

Storage conditions ! amber bottles ! darkness ! cool temperature UVabsorbers ! absorb harmful light radiation and dissipate the energy as heat of fuorescence or in a reversible (e.g. keto → enol) chemical reaction ! e.g. benzophenones

Question 27

Photo degradation of chlorpromazine

Answer 27

draw

Question 28

What is Isomerisation?

Answer 28

Conversion of a drug to its optical or geometric isomer

Question 29

Isomers are usually of different...

Answer 29

Isomers are usually of a different activity – e.g. low activity of adrenaline solutions at low pH – e.g. tetracycline – e.g. salbutamol – Formulated as a racemic mixture of the R- and S-isomers. – The R-isomer has ca. 150 times greater affinity for the β2-receptor than the S-isomer (and the S-isomer has been associated with toxicity). – Levalbuterol, the single R-isomer of salbutamol (cost issues).

Question 30

What is polymerisation?

Answer 30

The process by which two or more identical molecules form a complex – e.g. ampicillin (highly antigenic)

Question 31

Concomitant self-aminolysis (dimerisation) & hydrolysis of ampicillin

Answer 31

draw - drug reacting with itself - the higher the conc of the drug the more likely the reaction is to happen

Question 32

What are the 3 other chemical decomposition pathways

Answer 32

Dehydration ! breaking of covalent bond to eliminate water molecule from the structure, e.g. prostaglandin E2; tetracycline Decarboxylation ! elimination of carbon dioxide from a compound, e.g. when parenteral solutions of sodium bicarbonate are autoclaved Chemical incompatibilities

Question 33

Physical degradation

Answer 33

- Physical properties can change too but are form dependent

Question 34

Physical degradation | Examples for liquid formulations:

Answer 34

– appearance, colour, odour, pH, clarity (solutions) and freedom from visible particulate contamination, size range of particulate contamination (large volume parenterals), particle size distribution (suspensions), micelle size distribution (micellar solutions), resuspendability (suspensions), viscosity, moisture content (powders for reconstitution), phase separation (emulsions)

Question 35

Physical degradation | • Examples for tablets and capsules

Answer 35

dissolution rate (or dissolution profiles for sustained-release products), appearance, odour, hardness, friability, moisture content, brittleness

Question 36

Physical degradation | Examples for ointments and creams

Answer 36

appearance, odour, viscosity, softening range, loss of water, physical and chemical homogeneity, particle size distribution, particle formation, pH

Question 37

Physical degradation | Examples for transdermal patches

Answer 37

appearance, in vitro release rate, adhesive strength

Question 38

What are the forms of physical instability

Answer 38

– Those apparent to the user – Those causing inaccurate dosage – More subtle changes affecting product performance, i.e. during processing; loss or gain or water? (i.e. lidocaine / Karl Fisher titration) – Loss of water – Loss of volatiles, e.g sertraline base, glyceryl trinitrate – Water absorption, e.g effervescent tablets – Drug sorption onto / into packaging , e.g. diazepam / PVC bags; peptides, verapamil on / into glass – Excipient sorption, e.g. preservatives – Change in product colour – Change in odour / flavour – Crystal growth (i.e. suspensions – “ripening”) – Altered particle size distribution – Polymorphic changes – Caking of suspensions – Cracking of emulsions / syneresis – Ageing of solid dosage forms – Softening of tablets – Cracking / splitting of tablet coats – Release of materials from containers – Packaging changes

Question 39

How can physical instability be prevented?

Answer 39

– Formulation – Manufacture – Packaging – Storage

Question 40

Microbiological deterioration | What are microbial flora introduced via and what is the ability of microbes?

Answer 40

``` Microbial flora introduced via: • raw materials • water • equipment • operators • environment ``` Ability of microbes to: • survive manufacturing process • ingress during storage and use • adapt to low nutrient requirements

Question 41

Microbiological deterioration What are the specific microbial purity and preservation requirements

Answer 41

* product category, e.g orals; topicals; parenterals * formulation design * method of manufacture * route of administration – i.e. parenterals / aseptic

Question 42

What are the effects of Microbiological deterioration?

Answer 42

``` VISUAL/ APPARENT ! Breakdown of emulsion ! Formation of slime, pellicle or sediment ! Surface growth on solids ! Fermentation – formation of gas ! Pigmentation / change in colour ! Cloudiness ! Change in viscosity ! Separation ``` NON VISUAL ! Growth in fluids (to millions per ml) without opacity ! Growth in creams (to millions per ml) without visible changes ! Formation of endotoxins (≡ pyrogens) ! Formation of foul odours

Question 43

Microbiological deterioration | Microbiological Quality Assurance

Answer 43

* high levels of GLP and GMP * special licensed premises * use of authorised SOPs, raw material and release specifications * low bioburden prior to sterilisation * validation of sterility assurance * monitoring air and water quality * QP * stability testing for retention of microbial purity and preservation over shelf life

Question 44

Some applications – typical changes • Physical changes.. • Chemical changes.. • Microbial changes..

Answer 44

``` Physical changes • appearance • meltingpoint • clarityandcolourofsolution • water • crystal modification (polymorphism) • particle size ``` Chemical changes • increase in degradation products • decrease of assay Microbial changes • growth of microorganism