Lecture 20: Kinetics III Flashcards
What do environmental elements do?
They provide the energy for decomposition reactions to occur
What are the decomposition processes we want to prevent?
Hydrolysis
Oxidation
Isomerisation (including racemisation and epimerisation)
Photolysis
What are some examples of common environmental elements?
Heat
Moisture content
Oxygen
Uv light
What factor does temperature play on degradation?
The arrhenius expression gave us k, the rate constant as a function of activation energy and temperature.
As T increases, the degradation rate constant also increases
How can decomposition be prevented, specifically targeting temperature?
We can store temperature sensitive drug products in the cold
What is hydrolysis?
Involves the reaction of drugs with water.
It is the major cause of drug deterioration, especially in aqueous conditions
What are the three scenarios that may cause a drug to decompose via hydrolysis?
Catalytic: catalysis of drug by hydroxyl or hydronium ions (due to specific acid base catalysis)
Catalytic: the alteration of the buffer species (due to general acid base catalysis)
Uncatalytic: decomposition with water
What is the extent of hydrolysis dependent on?
The amount of moisture and pH of the drug solution
How is moisture introduced to solid dosage forms?
Through excipients such as starch.
What does the moisture content affecting stability depend on?
How strongly moisture is bound to the excipient, whether moisture can get in contact with the drug
Moisture can also facilitate microbial growth and cause more decomposition of the drug
How can hydrolysis be prevented.
Using buffers in formulation to prevent pH based hydrolysis
Adding dessicants to the package
Protecting the drug by formulation means e.g. Micelles, encapsulation etc.
What is oxidation?
Involves the reactions with oxygen
What is auto oxidation?
This is the reaction with atmospheric oxygen and causes the compound to go into a free radical phase with unpaired electrons
What is a free radical.
Highly unstable and reactive species, catalyses oxidative degradation reactions
They are like robbers deficient in energy and snatch energy from other cells
What factors affect the rate of auto oxidation?
Degree of unsaturation of organic compound, the higher the degree, the more prone they are to oxidation
The presence of free fatty acids, the free carboxylic group enhances the reactivity
Temperature: increased oxidation occurs with increased temperature
Physical state of compound: solids are oxidised at a lesser rate than liquids. As in the presence of water, acid or bases are catalytic to oxidation reactions
How can oxidation be preventd
Exclusion of oxygen with product, via air tight packaging, but this is hard to do especially after opening
Adding antioxidants which more readily oxidises than the drug
Adding a reducing agent e,g, sodium metabisulfite
Diluting with an inert solvent e.g. Carbon tetrachloride, ethyl ether
What are some examples of antioxidants that may be added to prevent oxidation?
Ascorbic acid vitamin C
Tocopherols vitamin E
Butylated hydroxytoluene BHT
Butylated hydroxyanisole BHA
What increases the oxidation rate of vitamin C?
Even traces of copper increases the rate by a factor of 10,000
Why is vitamin C frequently added to food and pharmaceutical productS.
It is a cheap antioxidant
Why are storage conditions for vitamin C a non metal, absent from light and in a cool temperature container?
Because ascorbic acid is easily oxidised to form dehydroascorbate. The conditions of that container prevent extensive oxidation
What is isomerisation?
The transformation of a molecule into another which has exactly the same structural formula but different stereochemical configurations
What is racemisation?
The conversion of chiral molecules from the levorotory form to the dextrorotory form
Levo is pharmacologically more active than dextro
What order of reaction does racemisation usually follow?
First order kinetics,
What does racemisation depend on?
Temperature
Solvent
Catalyst
Light
