General Drug Interaction Flashcards
How does absorption occur?
The drug needs to pass across a membrane (mostly by diffusion), typically containing protein molecules.
What is absorption?
Process by which drug moves from site of administration to bloodstream to get to the target site.
Name three variations that affect absorption.
- absorptive surface
- blood flow (circulation)
- solubility of the drug (the easier for it to be absorbed)
- ionisation
- formulation (eg. Enteric coating)
- route of administration (effects rate of onset and scope of therapeutic response
What is bioavailability?
Determined by the hepatic first pass effect, and usually expressed as a percentage. A drug with low bioavailability usually administered by route other than oral.
What is distribution?
Once a drug enters the bloodstream it can be distributed to various sites around the body.
Once a drug enters the bloodstream, what happens next?
Some free drug molecules bind to proteins forming drug protein complexes. In this bound state, the drug does not exert an effect, and binding is reversible.
Give an example of a lipid-soluble drug and how does it relate to adipose tissue.
Heparin or general anaesthetics.
Sometimes adipose tissue acts as a reservoir for the drug, releasing it slowly back into the bloodstream.
Explain blood-brain barrier and placental barrier.
Blood-brain barrier only allows lipid-soluble drugs, unless damaged, whilst the placental barrier is a non-selective passage of drug.
What is biotransformation and where is the principle site for metabolism?
Biotransformation (metabolism) is the process of chemical alteration of the drug to more water-soluble metabolite which has less action than parent drug and can then be excreted. The exception to this is prodrugs which are inactive until converted in the liver.
The principle site for metabolism is the liver.
What are the two phases of metabolism?
Phase 1:
- chemical reactions cause more water-soluble metabolites by specific drug metabolising enzymes.
Phase 2:
- metabolite from phase one joins with a polar substance, increasing water-solubility and enhancing excretion.
Name three drug influences on the metabolism.
- genetic factors
- environmental factors
- disease states
- prematurity and neonates
- other drug interactions
- percentage of protein binding of the drug
- ageing process
Explain the process of the Hepatic First Pass Effect.
Drugs travel fast through the portal system and the liver before entering the systemic circulation. Depending on whether the drug will be removed there or not, some of the drug will be removed there, never entering the systemic circulation.
Name two routes of drug excretion.
- kidneys
- biliary excretion
- respiratory expiration
- sweat and saliva
- breast milk
What is the half life?
The time taken for the plasma concentration of a drug to fall by 50%. Factors influencing half life are volume of distribution and clearance rate.
Explain loading dose.
Usually twice the normal dose, and is given to achieve therapeutic level faster, especially if long half lives are expected.