Antibody Pharmacokinetics

Question 1

Top 6 of the 10 protein drugs

Answer 1

• Humira
• Rituxan
• Herceptin
• Avastin
• Remicade
• Enbrel

Question 2

How many mono-clonal antibodies are currently approved for clinical use?

Answer 2

• 75

- several hundred in current clinical developement

Question 3

What is an important consideration between the different isotypes of antibodies?

Answer 3

• molecular weight

- serum half-life

Question 4

Characteristics of IgM

Answer 4

• pentamer/hexamer
• 950 kD, 1150 kD (largest one!)
• serum half-life of 5
• primary Ab responses: secretory immunoglobulin

Question 5

Characteristics of IgD

Answer 5

• monomer
• 175 kD
• Serum half-life of 3
• Unknown biological properties
• Useful as a B cell marker

Question 6

Characteristics of IgG

Answer 6

• Monomer
• 150 kD
• Serum half-life of 23 (longest one!)
• Hallmark of secondary immune responses

Question 7

Characteristics of IgA

Answer 7

• Monomer
• 160 kD, 400 kD
• Serum half-life of 6
• Main secretory immunoglobulin

Question 8

Characteristics of IgE

Answer 8

• Monomer
• 190 kD
• Serum half-life of 2.5
• Allergic and anti-parasite responses

Question 9

What drives the pharmacokinetics of antibody therapy?

Answer 9

-Compliment

Question 10

What is the most prevalent antibody isotype in man?

Answer 10

• IgG antibodies
• All that are approved at IgG
• Very large protein (150,000 Da)

Question 11

What is the FAB site?

Answer 11

Fragment of antigen binding

Question 12

What is the FC site

Answer 12

Fraction that crystalizes

-precipitates from solution

Question 13

What is a polyclonal IgG antibody?

Answer 13

• Rats are immunized and it leads to propagation of genetically distinct Ab-producing cells
• Many clones, each with a different protein that they are making
• Using animals is not always desirable and it is difficult

Question 14

What is a monoclonal antibody?

Answer 14

• Ab cells may be fused with myeloma (tumor cells) and cloned, allowing for production of large quantities of Ab from a monoclonal population of Ab producing cells
• when done in vitro, all will have the same genetics and same proteins without having to use an animal

Question 15

How do antibodies complete neutralization of a toxin or xenobiotic (Synthetic toxin)

Answer 15

-the antibody therapy can prevent the activity of the soluble substance
Examples:  
-anti-digoxin Fab therapy 
-adalimumab
-bevacizumab
-infliximab
-alircumab
MOA:   1: Bind, 2: neutralize,  3:  Affect PK

Question 16

How do antibodies eliminate cells?

Answer 16

-antibody mediated clearance
-Example:
Anti-CD4 IgG can attach to the CD4 on a T-cell to clear the cell.
-opsonizes the cell, marks it for destruction, depletes the cells for the target
(Used in cancer treatment)
Example therapies:
-basiliximab
-daclizumab
-muromonab-CD3
-rituximab

Question 17

How do antibodies cause alteration of cell function?

Answer 17

Example: Alteration of cell function/ cell signaling
-abciximab (anti-CD41) prevents platelet aggregation
-useful in treatment of acute MI and prophylaxis
-CD41 (GPIIb/IIIa fibrinogen receptor)
-Abciximab has a chimeric Fab against CD41 which binds to surface receptor and prevents signaling of receptor with out causing damage to the cell
-Additional examples:
basiliximab, daclizumab, efalizumab

Question 18

How to antibodies cause drug delivery?

Answer 18

Increasing the efficiency of drug delivery to desired sites
Example: Gemtuzumab ozogamicin
-The intracellular release of ozogamicin and induction of cell death
-Ozogamicin is the toxin (chemotherapeutic drug) that delivers the toxin to the tumor cell
-This decreases toxicity to healthy tissues A
Additional Examples:
-ibritumomab tiuxetan, ado-trastuzumab emtansine, brentuximab vedotin

Question 19

What are the general expectations of antibody pharmacokinetics

Answer 19

• Good absorption following SQ or IM dosing
• 50-100% oral bioavailability
• Bi-exponential dispostion
• long terminal half-life (20 days) and low rates of clearance (10ml/h)
• small volume of distribution (3-9L)
• some have long half-lives maybe 6 months

Question 20

What are the elimination mechanisms for peptides and proteins?

Answer 20

1. ) Fluid phase endocytosis (& catabolism)
2. ) Renal filtration (& catabolism and excretion)
3. ) Phagocytosis
4. ) “Component-specific” - receptor mediated endocytosis
5. ) “Component-specific” - receptor mediated protection
6. ) “Drug- specific” receptor mediated endocytosis

Question 21

Describe the elimination mechanism of Fluid-Phase Endocytosis and Proteolysis

Answer 21

-Considered to be “non-specific”
-Proteins and peptides broken down to component amino acids
-degradation products are not likely to be toxic
Mechanism:
1.) There is protein and fluid outside of the cell (interstitial fluid/plasma)
2.) Then endocytosis occurs when there is “cell drinking” and captures some extracellular fluid and some of the protein
3.) Before endocytosis occurs, the cell exchanges Na+ for H+ but after endocytosis occurs, H+ influxes into the cell
-it drops PH of the fluid to about 5 or 6 (starts to denature the protein)
-once it goes to the lysosome even more harsh conditions occur (enzymes that chop up the protein)
-Once in the lysosome it becomes catabolized into amino acids
the half-life for this type of AB process is 20-30 days

Question 22

Describe the elimination mechanism of renal filtration/ phagocytosis and proteolysis

Answer 22

• Size specific mechanism of elimination
• phagocytosis may be a primary mechanism of elimination for proteins and aggregates greater than 400 kDa
• Renal filtration & subsequent proteolysis is often a primary mechanism of elimination when MW is less that 50 kDa

Question 23

How much of IgG is eliminated unchanged in urine?

Answer 23

Essentially nothing comes out of the urine unchanged due to re-uptake in the kidney

Question 24

Describe the renal clearance of Mab and antibody fragments

Answer 24

• Renal CL of protein is underestimated by glomerular sieving coefficient
• current data suggests that renal CL accounts