Exam 1 Protein Binding and Drug Distribution Flashcards
What is plasma?
- the liquid component of blood in which the blood cells are suspended (yellow colored)
- plasma makes up about 50% of total blood volume
- plasma contains many vital proteins including fibrinogen, globulins, and albumin
- PK measures drug concentration in plasma
How does serum form?
when blood is allowed to coagulate and clotting occurs so it settles at the bottom of the tube → the supernatant fluid
What does serum contain/consist of?
serum is devoid of blood cells, fibrin, and clotting factors → but contains plasma proteins like albumin
What is the major drug binding plasma protein?
albumin
What is the relative size between drugs and proteins?
most drugs have a diameter of about 1 nm while RBCs are about 5 micrometers (so drugs are much smaller than RBCs)
Where are most drug molecules found?
they are mostly found in plasma (protein bound or free) and drug concentrations are typically measured in plasma
What are the two exceptions of drugs that are not found in the plasma?
tacrolimus and cyclosporine bind to RBCs significantly → blood samples instead of plasma samples are used for drug concentration measurement
For drugs that do not bind to blood cells (distributed in the plasma), what is the Cb relative to the Cp?
the Cb is half (smaller) than Cp
For drugs that are highly bound to blood cells (like tacrolimus) what is the Cb relative to the Cp?
the Cb is greater than Cp
What are the three major drug binding plasma proteins?
- albumin (65,000 kDa, 35-50 g/L)
- alpha1-acid glycoprotein (44,000 kDa, 0.4-1 g/L)
- lipoproteins (200,000-3,400,000 kDa, variable concentration)
What are some important things to know about albumin?
- the most abundant plasma protein (50% of human plasma proteins)
- produced in the liver
- binds free fatty acids, hormones, and weakly acidic (anionic) drugs
- main function is to maintain the oncotic pressure of blood
- serum levels are decreased in liver and kidney diseases → only small things can be filtered through but with kidney disease, large molecules can pass through so serum levels are decreased
What are some important things to know about alpha1-acid glycoprotein (AAG)?
- also called orosomucoid
- produced in the liver (like albumin)
- binds primarily weakly basic (cationic) drugs such as tertiary and quaternary amines → since it has an acidic moiety
- serum levels increase during acute phase reaction like inflammation and burns
- serum levels are decreased in liver and kidney diseases (like albumin)
What are some important things to know about lipoproteins?
- is a lipophilic protein and/or complex of proteins and lipids
- includes HDL and LDL
- binds hydrophobic drugs (aka lipophilic drugs)
- altered in some disease states such as heart disease
What is unbound fraction (fu)?
the ratio of free drug concentration/total drug concentration in which free drug concentration is Cu (measured by measuring the drug concentration after sample processing to exclude protein bound drug molecules) and total drug concentration is C (measured by measuring the drug concentration after sample processing to extract both protein bound and free drug molecules)
What are the two main drug transport processes?
- passive diffusion
2. carrier mediated
What is passive diffusion?
- down the concentration gradient
- not saturated
- most common
What are the two types of carrier mediated transport?
- facilitated diffusion
2. active transport
What is facilitated diffusion?
- down the concentration gradient
- via carriers
- saturable
- example includes organic anion transporting polypeptides (OATP)
What is active transport?
- against the concentration gradient
- via transporter
- uses energy via ATP
- saturable
- example is p-glycoprotein (pgp)
What are the effects of protein binding on drug transport?
- only free (unbound) drug exits capillaries to reach extravascular sites of action
- only free (unbound) drug crosses cell membrane via diffusion
- only free (unbound) drug binds to transporters
- unionized drug can go through!!
What is the nature of protein binding?
plasma protein binding of drugs is reversible!
What is the free drug concentration on either side of the membrane?
free (unbound) drug concentrations are the same on both sides of the membrane
What is the process of sample preparation?
obtain plasma from a blood sample → add organic solvent (like ethyl acetate) for extraction → dry out organic phase and reconstitute → quantitative analysis using HPLC
True/false: In the example of the sample preparation, the measured drug concentration reflects free drug concentration (Cu)?
false → the sample preparation is everything in the plasma (aka free drug and bound drug)
What are the three different types of capillaries?
- continuous
- fenestrated
- sinusoidal
What are continuous capillaries?
- most common
2. the endothelial junction is 10-15 nm → big enough for 1 RBC to go through
What are fenestrated capillaries?
- the size of fenestra is 60-80 nm
2. intestine, renal glomeruli
What are sinusoidal capillaries?
- size of gap: 100-1000 nm
2. liver
What are intercellular junctions?
a junction between endothelial cells → but not a tight junction so there is a hole
What is the theme going from continuous capillary, fenestrated capillary, and then to sinusoidal capillary?
more holes → so sinusoidal capillaries have the most holes
Where would antibody drugs distribute?
plasma and interstitial fluid
What type of compounds experience perfusion rate limitation?
lipophilic compounds → passes through cell membranes easily
What type of compounds experience permeability rate limitation?
hydrophilic compounds → have issues going through the cell membrane
The more lipophilic a drug is, what type of rate limiting factor in tissue distribution would it have?
perfusion rate limited
The more hydrophilic a drug is, what type of rate limiting factor in tissue distribution would it have?
permeability rate limited
What is the order of drugs from more lipophilic to less lipophilic (more hydrophilic)?
thiopental > phenobarbital > barbital > salicylic acid
What is propofol?
- general anesthesia inducing agent
2. nonpolar lipophilic compound → distribution is perfusion rate limited
Which of the following two tissues would show faster propofol distribution? (perfusion rate of brain is 0.5 and perfusion rate of muscle is 0.025)
brain
What is the equation for tissue binding?
V = Vp + VTW*(fu/fuT) (in which Vp is plasma volume and VTW is volume outside of plasma)
What is the relationship between volume of distribution (Vd) and fraction of drug unbound?
linear/direct relationship! (V = Vp + VTW*(fu/fuT))
- volume of distribution increases when fu is increased
- volume of distribution decreases when fuT is increased
What is the tissue binding and Vd of digoxin?
- digoxin plasma protein binding is 25%
- digoxin binds to cardiac Na+/K ATPase
- it has a large Vd (7 L/kg)
How does the blood brain barrier (BBB) have special tissue distribution?
- is a unique property of CNS capillaries
- highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from non-selectively crossing into the extracellular fluid of CNS
What is the difference between brain capillaries and most capillaries in the body?
- brain capillaries: use carrier mediated transport, have efflux transporters, and have tight junctions that have no pores → lipid soluble substances can pass through
- most capillaries: have a pore passage, have water lined pores → lipid soluble substances can pass through
What is the relationship between BBB permeability and drug lipophilicity?
generally, BBB permeability increases progressively with increasing lipophilicity
True/false: Lipophilic compounds tend to show greater membrane permeability.
true
True/false: Lipophilic compounds tend to show permeability rate limited distribution.
false → lipophilic compounds show perfusion rate limited distribution
True/false: Decreased protein binding of high protein bound drugs will likely increase Vd.
true
True/false: Vd is expected to be small for a drug that exhibits strong binding to tissue components.
false → increased tissue binding means decreased fuT (since free unbound drug in the tissue would decrease) which would lead to increase in Vd (from V = Vp + VTW*(fu/fuT))
True/false: For drugs that are highly bound to blood cells, drug concentrations in blood is higher than those in plasma.
true
