Pharmaceutical solutions Flashcards
Solution?
a system in which molecules of a solute are dissolved in a solvent vehicle.
Solute?
the substance dissolved within a solution
Solvent?
a substance (generally liquid) capable of dissolving other materials (solutes)
Concentration?
the strength of a solution; the number of molecules of solute dissolved in a solvent
Solubility?
the quantity of a given substance that can dissolve in a given solvent under certain conditions
Dissolution?
the process in which a material enters into a solution to become a solute
Dissolution rate?
the rate at which dissolution proceeds
Saturated solution?
a solution in which the solute is dissolved in the solvent at the limit of its solubility (under certain conditions)
Examples of solution formulations
- Parenteral liquids
- Eye drops
- Mouthwashes/gargles
- Ear drops
- Topical lotions
Parenteral liquids need to be?
- Must be sterile
- Small or large volume injections or infusions
- Large volumes need to be isotonic and pH controlled
Eye drops must be?
• Small volume formulations applied onto eyeball for local
effect
• Need to be sterile, isotonic and pH controlled
Mouthwashes must be?
- non-sterile
* wide pH range acceptable
Potential issues with solution formulations?
- Solubility of the drug
- Vehicle acceptability
- pH
- Sterility (and anti- microbial preservatives)
- Chemical stability (and stability enhancers)
- Tonicity
- Viscosity and density
- Aesthetic considerations
- Reproducibility of dosing
- Patient acceptability
- Ease of use
- Ease of manufacture
- Cost
Importance of solubility and
dissolution to pharmacy?
- Drugs must be in solution to be absorbed into the blood
* Drugs can be given as solutions (IV injections and oral solutions
what is vapour pressure?
It is the pressure of this vapour in an closed system in equilibrium whereby some of the more energetic molecules have enough energy to overcome the intermolecular attractions and escape from the surface to form a vapour
Raoult’s Law for vapour pressure?
= mole fraction x vapour pressure of that content
What is Complete miscibility?
– liquids that can be mixed together in all proportions to give homogenous mixtures
– e.g. polar and semi-polar liquids
– ethanol-water, water- acetone, carbon tetrachloride-benzene
What is Partial miscibility?
– When two liquids with very different attractive and repulsive forces are mixed (water-chloroform) two layers are formed
– The molecules cannot mix well (although very small amounts of each liquid will dissolve in the other)
– The two phases represent • Water saturated with
chloroform
• Chloroformsaturatedwith water
how logP works?
- Add a small amount of a drug into an octanol-water system
- Shake well to mix the two liquids and leave to separate (equilibrate)
- Sample water and octanol layers to determine how much is in each layer
what are the importance of logP
Formulation
• Simple assessment of hydrophilic/lipophilic character
• Formulate as solution or oily emulsion?
• Predict movement of preservative from water to
oil phase in emulsions?
Pharmacokinetics
• Will drug stay in blood or accumulate in fat tissue?
why is Drug solubility important?
- Ionisable drugs show different solubility limits at different pH values
- Formulation scientists adjust pH to increase ionisation of drug = increased solubility
What Chemical properties of drug affect the absorption across the gut?
• Drug needs to be hydrophilic to dissolve in GI fluids
• Drug needs to be lipophilic to be absorbed across
the gut mucosae into the blood
• Partition coefficient (Log P, octanol-water) is a measure of lipophilicity and is measured early in the drug development process
• Drug ionisation is a key factor
why is Aqueous(water)solubility important for drug absorption?
generallydetermines the extent of drug dissolution in the GI fluids
why is dissolution rate important for drug absorption?
determines how quickly the blood is absorbed in the bloodstream (for most drugs)
whys consider the effect of buffered pH on
- stability of drug in formulation
- other formulation components
- taste of the formulation
How to increase drug solubility?
• Often the most efficient way to increase drug solubility is to make a salt form
