Session 7: Pharmoacokinetics Flashcards
What are the main 4 factors affecting rates of drug in-drug out?
Absorption
Distribution
Metabolism
Elimination
What are the main routes of drug administration?
Oral Transdermal Topic Subcutaneous Intravenous Intrathecal Intramuscular Rectal Inhalation Sublingual
Relating to drug administration, what is the difference between enteral and parenteral?
Enteral is delivered into the internal environment of the body
Parenteral is delivery to all the other routes that are not the GI
If a drug is given in tablet or capsule form, the stomach is largely where what occurs?
Disintegration and disaggregation of the tablet
Does much absorption take place in the stomach for most drugs? Why is this?
No, little absorption takes place here because of the thick mucous layer that protects it from self digestion limits the absorption
How do the majority of enteral drugs enter the circulation?
They mix with gastric fluid and chyme and enter the weakly acidic environment of the small intestine, where the majority of drug absorption takes place
What are the plicae circulares?
The small circular folds of the small intestine
What is the role of the plicae circulares?
To increase the overall surface area of the small intestine
Apart from the plicae circulares, what other features of the small intestine help in increase the surface area?
Villi
Microvilli
What is the typical transit time through the small intestine?
3-5 hours
Apart from the physical dimensions of the small intestine, what other feature is a key factor in its role in drug absorption? Why is this important?
pH: weakly acidic (between 6-7)
It is important in determining the ratio of drug molecule that exists in ionised and unionised state
What are the 4 major mechanisms of drug absorption?
Passive diffusion
Facilitated diffusion
Active transport
Pinocytosis
What is the most common way that drugs can pass across lipid membranes?
By being of small molecular weight and not strongly ionic (lipophillic)
How do lipophilic molecules pass through the lipid membrane?
Passive diffusion
What kind of molecules are lipophilic?
Steroids
How is the concentration gradient of the drug maintained when the drug diffuses across the lipid membrane?
The drug is constantly carried away from the gut by the capillary supply of the small intestine
At physiological pH, if a drug is acidic, what does this mean?
It will release a proton (H+) to go from HA to H+ and A-?
At physiological pH, if a drug is basic, what does this mean?
It will gain a proton (H+) to go from B to BH+
Does the ionised form of a drug diffuse readily across the lipid bilayer?
No, it cannot diffuse directly through
How does the ionised form of a drug pass through the lipid bilayer?
Facillitated diffusion
The rate of uptake of weak acids or bases depends on what?
pKa or pKb
What is the pKa or pKb?
The pH value at which the weak acid or base exists in the 50% ionised and 50% unionised form
What determines the rate of diffusion across from the gut lumen into the epithelial cells?
The proportion of the drug that exists in the unionised/ionised state
What equation do we use to work out how well a drug will diffuse passively out of the lumen of the small intestine?
Henderson-Hasselbach
pKa= pH + log10 [AH]/[A-]
If we know that the proportion of [HA]/[A-] is 0.1, what does this tell us about the drugs ability to pass through the lipid membrane?
Irrespective of drug concentration, 10% is unionised at that point and therefore 10% of the drug will be able to diffuse across the membrane
When the drug moves out of the gut lumen as an AH (unionised) species, how is the equilibrium, as defined by Henderson-Hassebach equation, kept?
More A- ions will combine with free protons to become AH and move across the membrane by passive diffusion
Give examples of molecules that have low lipid solubility with a nett residual ionic charge
Glucose
Amino acids
Neurotransmitters
How do molecules such as glucose, amino acids and neurotransmitters pass through the lipid membrane, despite their low lipid solubility and ionic charge?
Facilitated diffusion
Is facilitated diffusion an active or a passive process?
Passive
How does facilitated diffusion work?
Using the electrochemical concentration gradient across the cell membrane
What are OCTs?
Organic Cation Transporters
What are OATs?
Organic Anion Transporters
What are the solute carrier proteins?
OCTs and OATs
Where are OCTs and OATs expressed in the body?
Throughout all body tissues
What are the two types of active transport and how do they differ?
Primary- The direct use of ATP to transport molecules across a lipid membrane
Secondary- The indirect use of ATP to transport molecules across a lipid membrane (to set up the electrochemical gradient)
OCTs and OATs can use which type of active transport to move ionised molecules across the lipid bilayer?
Secondary active transport
What process is important for the transport of large molecules across the blood brain barrier?
Endocytosis
When would exocytosis be used during drug absorption?
Delivery of hormones more locally for neurotransmitters into the synaptic cleft
What anatomical/physiological factors of the gut could affect drug absorption?
The structure of the stomach and gut lumen:
e.g. total surface area available, gut lumen pH, density of active and passive transport mechanisms, blood flow
Drug metabolism in the gut:
e.g. First pass metabolism
What is a xenobiotic?
A compound that is foreign to the body that is not normally produced or found in the body
Virtually all molecules that are absorbed by the gut will travel via what to where?
The hepatic portal vein
Liver
What happens to the drug during passage through the liver?
It will enter liver cells and be metabolised by Phase I and phase II enzymes
What is bioavailability?
The relative amount of a drug that reaches the greater systemic circulation (once the drug has gone through their first passage of hepatic circulation)
What does the fraction of bioavailability tell us about the drug?
It reflects all the physiochemical and biological barriers to its absorption and post hepatic systemic distribution
How do you work out the bioavailability of a drug?
Amount of drug reaching systemic circulation / total amount of drug administered
Which route of administration will usually involve more barriers to systemic uptake than intramuscular or subcutaneous routes?
Oral (Enteral)
What is the bioavailability of drugs administered by the IV route?
1
How do we work out oral bioavailability (F)?
Amount of drug getting in by oral route/ amount of drug administered from the IV route
What can oral bioavailability fraction tell us about a drug?
Tells us the optimal route a drug needs to be given to reach its therapeutic concentration and therefore determines the amount of drug the oral formulation needs to be given
What is AUC?
What does this tell us?
Area under curve
It is the total drug exposure overtime
What happens to AUC as you decrease oral bioavailability?
AUC decreases
What is the drug distribution defined as?
How the drug molecule reaches and interacts with its cellular targets
Once a drug gets into the cardiovascular system proper, how it is rapidly distributed around the body over large distances?
Bulk flow through the arterial system
Following bulk flow via arteries, drugs will transverse over much shorter distances via what method of transport?
From where to where?
Diffusion
Capillaries to surrounding interstitial fluid and tissues
The overall rate of delivery to a given tissue bed is dependent on what?
The density of the capillary supply
The ‘micro-leakiness’ of the capillaries
The drug will reach well __________ organs such as ______, ________, _________ more rapidly than skin, bone or adipose tissue
vascularised
kidneys
heart
lungs
True or false: There are varying degrees of “leakiness” depending on the tissues supplied by capillaries
True
Which tissues have more “leaky” capillary beds?
How is this achieved?
Intestinal Endocrine Pancreatic Kidney Achieved through cells being fenestrated by pores
In tissues of the liver, bone marrow, lymph and spleen, endothelial cells are separated by what?
What does this allow?
Slit junctions or large intercellular gaps
Allows large movements of molecular material
How do lipophilic drugs enter the brain?
Diffusion