Pharmacokinetics- Drug transfer and absorption Flashcards
What is F
Bioavailability
What is V
Volume of distribution
What is CL
Drug clearance
What does a change in absorption lead to
Change in area under curve
Tmax
F (bioavailability)
How will change In distribution affect drug
Many drugs bind to plasma proteins. If change in plasma conc of albumin, can change pharmacokinetics. Liver disease means less albumin. Will decrease binding of albumin with drug and affect elimination of drug
How will change in elimination of drug affect elimination of drug
AUC
CL
T1/2
if patient is renally impaired, decrease clearance of drug and increase the half life and so increase pharmacokinetic impact of the drug (shown by area under curve)
How do most drugs move across membranes
Passive diffusion
What is pKa and what does It mean
Ionisation constant
-pH ar which drug is charged:uncharged is 50:50
What does it mean if pH is at pKa value
Then charged:uncharged is 50:50
What happens if a weak acid goes to a decreasing pH environment
It turns to uncharged drug so promotes the drug going across the lipid bilayer
What happens if a weak base goes to a decreasing pH environment
Shift the equilibrium which favours the charged species which cannot be absorbed
Where are weak acids mainly absorbed
Stomach and intestines
Where are weak bases mainly absorbed
Intestine
negligible absorption in stomach
which drugs are likely/unlikelyto enter the brain easily
Weak acids unlikely so use these if you don’t want the drug to have an effect on the brain
Weak bases likely to enter the brain
What happens to acids and bases in the stomach
Weak acids- predominantly unionised and absorbed
Weak bases- predominantly ionised and absorbed
What is absorption like in acid stable drugs vs acid labile drugs
Stable- Slowed absorption
Labile- Stay linger and so more readily broken down
How are PPIs given
They are given in gastroresistant tablets because they are acid labile
How to PPIs act on stomach
DOn’t get broken down in stomach. Enter duodenum, and capsule degrades. Enters blood stream. Acts systemically to inhibit PPs in parietal cells in stomach
What is first pass metabolism and how does this affect bioavailability
Drug absorbed from small intestines go to liver via hepatic portal vein. Some drugs are metabolised by liver before reaching systemic circulation
-If drug has extensive First pass metabolism, bioavailability is decreased
What is buccal absorption
Under tongue for rapid absorption and also avoids first pass metabolism (e.g. GTN used for angina pain)
What is percutaneous absorption
Patches on skin and drug is absorbed because dermis is permeable
