drug absorption Flashcards
what is the pharmacokinetic process
getting the drug to the site of action
what is the pharmacodynamic process
producing the correct pharmacological effect
what are the 4 basic factors that determine drug pharmacokinetics
(ADME)
Absorption
Distribution
Metabolism
Elimination
define pharmacokinetics
the science of the rate of movement of drugs within a biological system
what does pharmacokinetics allow the understanding of
dosage, drug administration, drug handling, patient variability, potential for harm
what do most drugs require to have biological action
enter the blood stream and be distributed to a site of action
methods of administration of drugs
oral, IV (not absorbed), subcutaneous, intramuscular, other GI (sublingual, rectal), inhalation, nasal, transdermal
define absorption
the process of movement of unchanged drug from the site of administration to the systemic circulation
regarding absorption, what 2 factors is there always a correlation between
plasma concentration of the drug and the therapeutic response
Tmax
time to peak concentration of the drug, depends on how acute the symptoms are to be treated
Cmax
peak concentration of the drug
AUC
area under the drug concentration-time curve, represents the amount of drug which reaches the systemic circulation and is used to work out bioavailability
during which point do we want Cmax to occur within
the therapeutic window
what would cause an earlier Tmax
a more rapid rate of absorption
what impact does dose have on Tmax and Cmax
doesnt effect Tmax but does increase Cmax
Define therapeutic range
the range of concentrations over which a drug is active
below this = insufficient or no pharmacological action
above this = toxicity
what is the therapeutic index
a measure of the range at which a drug is safe and active
what is the bioavailability of a drug given IV
100%
what factors affect bioavailability
formultion, ability of the drug to pass physiological barriers, GI effects, route of administration, first pass metabolism
how does formulation affect bioavailability
some medications are prepared in slow release preparations, capsules break down faster than tablets
what can alter the ability of drugs to cross physiological barriers
particle size, lipid solubility (blood-brain/ovary barrier, placenta etc), pH, ionisation
why can lipid soluble drugs have significant side effects
they can bypass lipid membranes e.g. cannabis
how do GI effects alter bioavailability
motility: speed of gastric absorption affects the speed at which the drug reaches the side of aborption (most drugs are absorbed in small intestine), can be affected by other drugs, food/drink and illnesses
food: can enhance or impair absorption
illness: malabsorption can increase/decrease rate of absorption, migraine reduces rate of stomach emptying and therefore rate of absorption of analgesic drugs
how does route of administration affect bioavailability
by mouth will get into the blood faster than rectally
first pass metabolism and its effects on bioavailability
- Varies markedly between individuals and increases during pregnancy
- Metabolism of drug prior to reaching systemic circulation, can be a limit on oral route for some drugs (oral route not useable - insulin), gut lumen (acid, enzymes), gut wall (metabolic enzymes), liver (hepatic enzymes). These factors can be changed by drugs and disease
subcutaneous/intramuscular drug administration
can change the rate of absorption from these sites with different physical properties of formulation, depends on blood flow to site, needs small volume, avoids first pass metabolism, some drugs not well absorbed from this route (water soluble better)
sublingual/buccal drug administration
sublingual absorption from the buccal mucosa bypasses first pass metabolism which will inactivate the drug, drugs given this way enter the circulation directly e.g. GTN for angina
rectal drug administration
bypasses first pass metabolism, absorption tends to be slow, often used for drugs that cause stomach irritation or local conditions
inhalation/nasal administration
absorption depends on the type of delivery system, particle size and patient technique. Better for volatile agents, can be metabolism in the lungs. Relatively rapid action, about 5-10% absorbed. Most commonly used for asthma, allows dosage to be reduces. Usually used for topical effect or to avoid problems of oral absorption e.g. nausea
transdermal drug administration
avoids first pass metabolism, can provide controlled released, few substances well absorbed, need to be non-irritant
what factors need to be taken into consideration when determining the mode of administration for a drug
- purpose and site of drug action (local, first pass metabolism)
- disease effects
- patient’s ability to take the medicine
- speed of action
- reliability of absorption
