Clinical Kinetics Flashcards
What is the most common route where a drug can act
- oral administraiton is the most commonly used route
When is the gut the site of action
- Erythromycin as a prokinetic agent
- vancomycin for clostridium difficile infection
- neomycin for gut decontamination
What are the enteral realted routes
- oral route
- rectal route
what is the downside of the rectal route
- rectal mucosa is vascular, absoprtion is inconsistent and may by pass the portal circulation
What routes allow the drug to be absrobed directly intot he systemic circulation
- buccal
- sublingual
- intranasal routes
What is a buccal route
- drug is held next to the cheek
what is an example of a drug that uses the buccal route
- midazolam for sedation in children
What is an example of a drug that uses a sublingual route
- sublingual glyceryl trinitrates to relieve anginal pain
What drugs use the inhalational route
- used for anaesthetic gases
- pulmonary vasodilators (nitric oxide)
- bronchodilators (salbutamol)
What does parenteral mean
- bypasses the gut
describe when you can use a drug by parenteral administration
- your patient is unable to take drugs orally as they are vomiting
- rapid onset of action is required (anaesthetic induction)
- the drug is not available as an oral formulation (adrenaline, insulin)
What are the two ways to deliver a drug Intravenously
- bolus - single dose
- infusion - over a defined peroid of time
What is the dowside of giving a drug via the intramuscular route
- has to be absorbed from the muscle into the systemic circulation and this can be slow and inconsistent
Describe giving drugs through subcutaneous administration
- involves direct injection of a drug into the fatty tissue beneath the skin
- allows a drug reseroir effect with slow release
- absorption can be inconsistent
What is topical application useful for
- Skin conditions such as antifungal or corticosteroid cream
- local anaesthetic e.g. lidocaine wiht prilocaine as an EMLA cream
- systemic absorption of drugs to act as a slow release prearation (e.g. nicotine patch for smoking cessation)
What is bioavailability
- this is the extent to which a drug reaches the systemic circulation where it is available to act at the effector site
what way to administer the drug results in 100% bioavailability
- if the drug was given intravenously
What factors that effect bioavailability
- route of administration
- properties of the drug
- plasma protien binding
- metabolism
- elimination
Describe the factors that effect bioavailability of drugs
Route of administration
- a drug given orally has to be asborbed in the gut before reaching the systemic circulation
Properties of the drug
- a highly lipid soluble drug will be absorbed better
Plasma protien binding
- highly protein bound drugs have less free drug to act at the receptor site
Metabolism
- drugs absorbed by the gut pass through the liver via the portal circulation where they are metabolised
Elimination
- a drug eliminated through the kidneys will accumulate in renal dysfunction
What happens when you give a drug as an IV bolus
- its plasma concentration rises to a peak almost at once before falling
- fall is due to the uptake of the drug into tissues, metabolism and elimination from the body
What ahppens when a drug is given enterally
- its plasma concentration will still rise but more slowly as absorption from the gut takes time
What are the key pharmacokinetic parameters that describe bioavailability
- Area under the curve (AUC)
- Peak plasma concentration (Cmax)
- time to peak plasam concentration (Tmax)
What is a steady state concentration
- When the inflow rate is less than rate of outflow
- this causes the plasma concentration to begin to fall
What is the half life
- the time for plasma concentration to fall by half is called the half life (T1/2) of the drug
- for most drugs the fall is related to elimination from the body and so it is called the elimination half life
What is the primary location for drug absorption
- Small intestine which is the primary location for drug absorption
Where are acidic and basic drugs better absorbed in
- acidic drugs - better absorbed in the stomach
- basic drugs - better absorbed in the small intestine
Give examples of concomitant drug administration
- erythromycin inhibits warfarin metabolism
- phenytoin increases metabolism of other antiepileptics
- rifampicin increases the metabolism of oestrogen in the oral contraceptive pill
What do formulation coatings do
- these are designed to protect the drug from degradation along the GI tract or utilise these changes to control the rate of delivery and absorption of drug into the systemic circulation