Bioavailability Flashcards
Describe three different formulations that may be used to deliver a drug via the oral route
lipid/syrup
capsule
tablet
Give three reasons why excipients are added to a drug formulation
improve taste
aid manufacture
improve biological stability
Suggest examples of agents that could be used as excipients
glucose, colouring, alcohol/water
What is bioavailability?
amount of a drug contained in a medicine that enters the systemic circulation in an unchanged form after administration of the products
Why is it important for a drug to be bioavailable?
reach target organs
ensure concentration at active site is sufficient
Does the measurement of bioavailability always reflect the effectiveness of the drug?
no eg. creams bypass the bloodstream and GI target organs so they act before they are absorbed into the blood
pro-drugs also do not reflect this relationship
How is the bioavailability of different formulations assessed?
comparing areas under plasma level-time curves after a) intravenous administration and b) administration of identical dose by intended route eg. oral
What are some key factors that influence bioavailability?
physicochemical drug characteristics eg. ionisation in gut
GI pH
passive vs active transport of drug
particle size
physicochemical interaction between drug and gut contents (eg. calcium + tetracyclines)
Aspirin has a pKa of 3.5. Where is it preferentially absorbed?
stomach –> unionised form easier to absorb because no need for transporters
Where do you find unionised aspirin and ionised aspirin?
unionised –> stomach
ionised –> gut
When would it be appropriate to take soluble aspirin and why would this formulation be advantageous over others?
trouble swallowing
makes overdosing more difficult due to water saturation
can be used in children
faster absorption as already dissolved
When would it be appropriate to take enteric aspirin and why would this formulation be advantageous over others?
when you want to slow absorption and release into the blood stream eg. chronic conditions such as ulcers + arthritis
What is bioequivalence?
evidence that a new ‘generic’ product behaves sufficiently similarly to the existing one it is being substituted for without causing clinical problems
Why is bioequivalence important when prescribing generic versions of a drug which have a narrow therapeutic index?
narrow therapeutic index –> small dose change can result in increased side effects/toxicity
Give examples of drugs to with narrow therapeutic indexes
digoxin, cyclophosphamide, warfarin, lithium
