Pharmacokinetics: Absorption and Distribution Flashcards
What is pharmacokinetics?
- what the body does to a drug that is administered
What is pharmacoDynamics?
- what the Drug does to the body once administered
Pharmacokinetics is the branch of pharmacology that is concerned with the movement of drugs within the body. What are the 4 things that can affect the course of the drug once administered?
- use A.D.M.E
- A = absorption
- D = distribution
- M = metabolism
- E = excretion
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In order for any drug to be effective, what are the 2 main aspects of the A.D.M.E mnemonic that the drug must overcome?
1 - absorption = overcome physiological barriers
2 - digestion = directed to target tissue in appropriate concentration and overcoming internal physiological barriers)
Even if a drug can be absorbed and distributed around the body, what are 2 limitations that are part of the mnemonic A.D.M.E that the body imposes on any drug?
1 - metabolism = all or very little of the drug may be metabolised
2 - excretion = may be excreted slowly or quickly affecting therapeutic window or toxicity
If the body cannot absorb a drug, what problem can this cause for its clinical use?
- difficult to get its concentration up to a therapeutic level
If the body absorbs a drug, but it cannot be distributed, what problem can this cause?
- drug cannot reach target tissue
- may have off target effects
If the body absorbs a drug, but cannot metabolise the drug, what problem can this cause?
- if drug is not metabolised well, too much of it may circulate in the body
If the body absorbs, distributes and metabolises a drug, but cannot excrete the drug, what problem can this cause?
- drug level may become toxic to the body
What is the most common route of drug administration and why?
- oral
- cheap, convenient, acceptable
- no special skills needed
Injection is one form of drug administration, and there are 3 main types, what are they?
1 - intravenous (quickest)
2 - intramuscular
3 - subcutaneous
Inhalation is one form of drug administration, what is a key example where this is useful?
- asthma
- less adverse effects
- direct effect on tissue
Sublingual is one form of drug administration, what is a key example where this is useful?
- glyceryl trinitrate (GTN) for angina
- absorbed quickly under the tongue
Rectal suppository is one form of drug administration, what is this?
- drugs are place in the anus
When we talk about pharmaceutical properties of a drug, what are we referring to?
- syrups, pills etc…
- what form they take
When we talk about Physicochemical properties of a drug, what are we referring to?
- solubility of drugs
- pH of drug (acidic drugs better absorbed in the stomach; alkaline drugs in the intestine)
- molecular weight (affects diffusion)
When we talk about Physiological properties of a drug, what are we referring to?
- surface area =GIT has a huge surface area
- contact time = digestion is a long process, increaisng absorption
- concentration of the drug on the absorption site
- absorption site (blood flow, increases with food pH)
- interactions with enzyes and bacteria
- transport systems
The oral route of drug administration is by far the most common. What is the main aspect of A.D.M.E that oral drug administration must overcome?
- absorption
The digestive tract is one of the biggest barriers to taking drugs orally. What are a few of the things in the digestive tract that may affect drug digestion?
- pH
- enzymes
- bacteria
- motility
- food
- drugs
What is first pass metabolism?
- a phenomenon of drug metabolism whereby the concentration of a drug, specifically when administered orally, is greatly reduced before it reaches the systemic circulation
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Why are the physicochemical properties inside the GIT important to allow drugs to be administered orally, digested and then absorbed?
- crucial to ensure crossing of the GIT membrane
- drugs must be soluble to be absorbed
- drug must reach GIT where pH is optimally for absorption
- drugs molecular weight must be suitable for absorption
What does biotransformation systems relate to in drugs in the GIT?
- the ability of drugs to resist enzymatic action
- if enzymes break drug down in GIT, it may not be effective or absorbed
What is an example of where the physicochemical properties of a drug are dependent on pH?
- antacids are active once they reach a low pH
- they can raise the pH in the stomach reducing reflux
- patients must have a gap between taking them, otherwise pH will not be low enough for the next dose
What is an example of where the physicochemical properties of a drug are dependent on food being present in the GIT?
- formation of complexes / chelation
- tetracyclines taken with milk is bad
- Ca2+ binds and inactivates tetracyclines
What does bioavailability mean?
- the fraction of a drug that reaches the systemic circulation and the target tissue as an intact drug
What is the formula for calculating bioavailability?
- calculate how much drug reaches target via oral administration
- calculate how much drug reaches target via intravenous (IV) administration
- oral area under the curve (AUC) / IV AUC
- accounts for digestion and metabolism