Protein Binding & Distribution

Question 1

What is distribution?

Answer 1

Reversible movement of drug between the systemic circulation and the tissues of the body (ex. organs, fat, muscle)

Question 2

For drugs that extensively distribute into the tissues, do they have high plasma concentrations?

Answer 2

No, for drugs that are extensively distributed into the tissues, most of the drug has left the systemic circulation (low Cp)

Question 3

For drugs that experience limited distribution into the tissues, are they in relative high concentration in the systemic circulation?

Answer 3

Yes, most of the drug is in the blood (high Cp)

Question 4

What is protein binding?

Answer 4

It is the non-covalent, reversible interaction of a drug with a protein to form a drug-protein complex

Question 5

What are some examples of drug-protein interactions important to PK and PD?

Answer 5

1. Drug-transporter complex (drug is transported across membrane)
2. Drug-enzyme complex (drug is metabolized to metabolite)
3. Drug-plasma protein complex (drug is bound to plasma protein)
4. Drug-receptor complex (drug produces therapeutic effect)

Question 6

What is the basic tenet of pharmacology?

Answer 6

Pharmacological response is related to the unbound drug concentration at the receptor site

Question 7

Review slide 6 for movement of unbound drug in the body

Question 8

Review slide 8 for the mathematical relationship describing drug-protein interactions

Question 9

What are the variables that describe affinity for binding in drug-protein complexes?

Answer 9

Kd, Km, EC50

These variables represent concentration required to reach half-maximal capacity, therefore higher the variable value, lower affinity

Question 10

What variables describe capacity for binding in drug-protein complexes?

Answer 10

nP, Tmax, Vmax, Emax

At saturation, as drug concentration increases, fraction of unbound increases because no more vacant receptors are left

Question 11

What are some factors that influence drug-protein binding?

Answer 11

1. Drug properties
2. Protein properties
3. Affinity
4. Drug interactions (competitive binding)
5. Pathophysiological condition

Question 12

Review slide 11 for the equation that relates concentration of bound drug to concentration of unbound drug, binding capacity, dissociation constant

Question 13

What is fu(b) (fraction unbound)?

Answer 13

This variable indicates the relationship between unbound drug plasma concentration with total plasma drug concentration

remains constant until saturation

Question 14

What are the three types of plasma binding proteins?

Answer 14

1. Albumin
2. Alpha-Acid Glycoprotein (AAG)
3. Cortisol Binding Globulin Lipoproteins

Question 15

What are some characteristics of albumin in relation to protein binding?

Answer 15

Principal binding plasma protein

Acidic lipophilic drugs bind more readily and extensively than basic lipophilic drugs

Small, neutral, water-soluble drugs are negligibly bound

Question 16

What are some characteristics of Alpha-Acid Glycoproteins (AAG)?

Answer 16

Lower concentration than albumin, but still important

Concentrations of AAG rapidly increase with inflammation (reducing unbound drug)

Binds to basic drugs

Question 17

Can protein-bound drug exert a pharmacological effect?

Answer 17

No, only unbound drug exerts pharmacological effect

Question 18

Can protein-bound drug be eliminated?

Answer 18

No, most elimination mechanisms require free drug

Question 19

What types of drugs are we most concerned about the effects of protein binding?

Answer 19

Drugs that are extensively bound to plasma protein. These drugs have a unbound fraction below 0.2 (80% of drug in the plasma is bound to plasma proteins)

Question 20

What factors determine the rate and extent of drug distribution?

Answer 20

1. Tissue perfusion
2. Drug permeability characteristics
3. Protein binding and partitioning (Kp)

Question 21

What conditions slow down approach to tissue equilibrium?

Answer 21

Poor perfusion

Higher tissue partitioning

Question 22

What are the rate-limiting steps of distribution?

Answer 22

1. Perfusion rate-limited
2. Permeability rate-limited

Question 23

What are the characteristics of a perfusion rate-limited drug distribution?

Answer 23

Tissue membrane presents no barrier to drug distribution (rapid equilibrium)

Small, lipophillic drugs are at risk of experiencing perfusion rate-limited distribution

Affects loosely knit membranes or porous capillaries

Question 24

What are the characteristics of a permeability rate-limited drug distribution?

Answer 24

Membrane is a barrier to drug distribution (slow equilibrium)

Large or polar drugs are susceptible to permeability rate-limited drug distribution

Affects tight membrane barriers (BBB, blood-testes barrier, blood-milk barrier)

Question 25

How can cardiovascular function affect drug distribution?

Answer 25

Patients with reduced CO or tissue perfusion, will result in lower drug concentration in the tissue (also equilibrium will take longer)

Question 26

What factors determine extent of distribution?

Answer 26

1. Tissue Blood Flow 2. Tissue Blood Volume 3. Partitioning (Ctissue/Cblood) 4. Protein binding 5. Apparent volume of distribution

Question 27

What is apparent volume of distribution?

Answer 27

A measure of the extent of drug distribution (an indirect measure of tissue binding) It is a proportionality constant relating drug concentration in blood to amount in the body (The volume of fluid that the drug appears to distribute into)

Question 28

What does a large Vd (more than 0.6L/kg) reflect?

Answer 28

A large Vd value means more drug is in the tissue

Question 29

What are the two main compartments of the body?

Answer 29

1. Water compartment (blood, plasma, extracellular, and intracellular) 2. Tissue and Fat Compartment (Nonadipose and adipose)

Question 30

What are the specific factors that affect volume of distribution (Vd)?

Answer 30

1. Variation in binding parameter (capacity and affinity) 2. Competitive Binding Displacements (dependent on the combination of affinity and concentration) 3. Relative pH of tissue and fluid compartments (unionized vs ionized) 4. Blood flow (cardiac output) 5. Barrier integrity 6. Body Weight and Composition

Question 31

How does variation in binding parameters (capacity and affinity) affect volume of distribution (Vd)?

Answer 31

There are individual differences in the number of binding proteins (capacity) and how strongly these proteins bind to drugs (affinity) This affects unbound fraction in the blood. Any increases or decreases in fub, causes changes in Vd

Question 32

How does competitive binding displacement affect volume of distribution?

Answer 32

Drug-Drug interaction: Competition between two highly bound drugs for the same target (differences in affinity and concentration cause one of the drug to be less bound to plasma proteins, increasing the released drug's unbound fraction, therefore increasing Vd) Drug-endogenous molecule interaction: Compounds like bilirubin have high affinity to albumin, displacing other drugs that are also highly bound. This increases the unbound fraction of the drug, increasing volume of distribution (ex. patients with Jaundice need to take different doses of drugs that are highly bound)

Question 33

What are some characteristics of a displacer?

Answer 33

For significant displacements to occur, the displacer must have the following: 1. High affinity for the protein 2. Bind at the same site or affect the binding site of the displaced drug 3. Be in high concentration to saturate binding capacity (limiting factor)

Question 34

How does relative pH of tissue and fluid compartments affect volume of distribution (Vd)?

Answer 34

pH has an affect on the ratio of unionized and unbound drugs ex. Ion trapping (breast milk is acidic, but basic drugs in the blood are not able to leave the milk, causing basic drugs to be found in the milk more often than acidic drugs)

Question 35

How does blood flow affect volume of distribution (Vd)?

Answer 35

Cardiac output is a good measure for perfusion Increase in Vd: Decreases in CO can cause Vd to decrease for extensively distributed drugs because more is in the blood Decrease in Vd: Decreases in CO can increase Vd in drugs distributed in total body water (heart failure causes ascites, more water is in the tissues, therefore more of drug is in the tissues vs the plasma)

Question 36

How does barrier integrity affect volume of distribution?

Answer 36

Disease states: Loss of tissue barrier integrity allows increased penetration of drug into tissues (less drug in the fluid, therefore increased Vd) Transporter Expression: This can affect how much drug is pumped into cell and how much is pumped back out (this can affect apparent volume of distribution)

Question 37

How does body weight and composition affect volume of distribution (Vd)?

Answer 37

Drug does not partition into fat: Vd is proportional to lean body weight (water+bone+muscle) Drug does partition into fat: Vd is related to total body water