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PHAR 423: Drugs in the Therapy
of Transplant Rejection
Dr. Thomas Abraham
Spring 2004
Transplantation of organs
Autograft: tissue from the
same individual.
Isograft: tissue from
genetically identical donor
e.g. monozygotic twins.
Allograft: tissue from
genetically distinct donor.
Xenograft: tissue from
donor of another species.
Transplantation of organs
• High demand for transplant tissue: about 64,000 recipients on waiting list in 1999
• Waiting period for kidney can be about 3 years!
• Generally blood and skin tissues are rejected most rapidly following transplantation.
•
Organ transplant is usually followed by:
1. Hyperacute rejection – due to ABO blood group incompatibility.
2. Delayed-type hypersensitivity reaction – cell mediated immunity.
Role of T Cells in Transplant Rejection
•
Cell-mediated immunity primarily carried out by CD4+ and CD8+ T cells.
•
Altered MHC molecules (type I and II) on graft tissue are recognized by T
cells to initiate the immune reaction.
•
Alternatively antigen-presenting cells (dendritic cells, macrophages,
passenger leukocytes) may process and present altered MHC molecules to T
cells.
•
Stages of Rejection
1. Sensitization – macrophages engulf and process graft MHC molecules,
proliferation of T cells after stimulation by APCs, release of cytokines (IL2, TNFb, IFNg).
2.
Effector stage – infiltration of macrophages/neutrophils; CD8+-mediated
killing of graft cells; IL-2 induced T cell proliferation & differentiation.
Role of T Cells in Transplant Rejection
•
CD4+ T cells bind altered MHC molecules and become activated: begin to
produce cytokines (mostly IL-2, TNFb, IFNg, etc.), cytokine receptors, and undergo
proliferation and differentiation (T helper cell).
•
CD8+ T cells become activated by IL-2 (from CD4+ cells), recognize graft tissue
and become cytotoxic T lymphocytes: bind MHC molecules of foreign cells and kill
them directly.
•
T helper cells stimulate the activation of monocytes, which migrate to graft site
and differentiate to macrophages: attack graft cells, cause inflammation, etc.
Role of T Cells in Transplant Rejection
Host Versus Graft Disease
Histological Stain of biopsies from a transplanted Kidney
2 Weeks after transplant
2 Years after transplant
Graft versus Host Disease
Immune response against the host following bone marrow transplant
Histological Stain of Liver tissue
Skin condition after transplant
Intracellular Signaling in T cells
Intracellular Signaling in T cells
•
Binding of altered MHC molecule to
TCR causes increase in intracellular
calcium.
Graft cell or APC
MHC molecule
•
Calcium ion combines with calmodulin
to bind and activate calcineurin (protein
phosphatase 2B).
T Cell Receptor
T cell
[Ca2+ ]i
Calmodulin
+
Calcineurin (PP2B)
P
NFAT c
NFAT c
Pi
Nucleus
NFAT c NFAT n
IL-2, IL-2 receptor
•
Calcineurin dephosphorylates
cytoplasmic NFAT which then migrates
into the nucleus of the T cell to increase
the transcription of specific genes
involved in T cell activation.
Agents that modify Intracellular Signaling in T cells
Cyclosporin A (Sandimmune®)
•
Isolated from fungus Tolypocladium
inflatum; consists of 11 amino acid residues
arranged in cyclic structure.
•
All amides are hydrogen-bonded or
methylated.
•
Very lipophilic agent – administered in ethanol/castor oil mixture for IV
solutions or as soft gelatin capsule or microemulsion for oral admin.
•
Cyclosporine binds to cyclophilin (intracellular receptor) to form a stable
complex that inhibits the actions of calcineurin.
Agents that modify Intracellular Signaling in T cells
Cyclosporin (cont.)
•
Inactive calcineurin prevents T cell
response (IL-2 production, differentiation,
etc.), which blunts overall T cell-mediated
immunity.
•
Cyclosporine also increases production of
TGFb by various cells that inhibits the
activation of T cells.
•
Oral bioavailability from soft gelatin capsule: slow and incomplete (20-50%);
newer microemulsion formulation (Sandimmune Neoral®) has better oral
bioavailability.
•
Peak plasma conc. occurs 1-4 hours after oral admin; delayed with fatty meals.
Large volume of distribution with 50-60% of blood content in RBCs. High cyclophilin
concentration allows heavy concentration in leukocytes.
Agents that modify Intracellular Signaling in T cells
Cyclosporin (cont.)
•
High hepatic metabolism to several products (mostly inactive); cyclic structure
resistant to metabolism but extensive side-chain modification by CYP450 system.
Predominant biliary excretion of metabolites (94%).
Drug Interactions
• Phenobarbital, phenytoin, rifampin etc. increase cyclosporine clearance;
amphotericin B, erythromycin, ketoconazole decrease clearance.
•
Adverse effects: renal toxicity (in most pts.), hypertension (up to 50% of pts.),
nervous system effects, hepatotoxicity, hirsutism, gingival hyperplasia and GI
toxicity.
•
Used in kidney, pancreas, liver and cardiac transplants; also in severe
rheumatoid arthritis and early type I diabetes.
Agents that modify Intracellular Signaling in T cells
Tacrolimus (Prograf®, FK506)
•
Isolated from streptomyces species; cyclic
lactone structure.
OH
CH3O
CH3
O
H3C
O
OH
CH2
O
N
O
OH
CH3
CH3
O
CH3
O
OCH3
OCH3
•
Binds to different intracellular receptor
immunophilin (FKBP-12) to form stable complex
with calcineurin. Inhibition of calcineurin prevents
T cell activation.
•
IV and oral preparations. Very high hepatic metabolism with predominant biliary
excretion (99%). Toxicities include nephrotoxicity, CNS toxicities, hypertension, and
metabolic toxicities.
•
Agents that interact with CYP3A4 e.g. metronidazole, diltiazem have resulted in
elevated tacrolimus levels.
Agents that modify Intracellular Signaling in T cells
Sirolimus (Rapamune®, Rapamycin)
•
HO
CH3 O
•
CH3
H
H3C
O
N
O
OH
H3C
O
HO
O
R
O
HO
H3C
O
H3C
CH3
•
Isolated from Streptomyces species.
Has comparative pharmacokinetics as
tacrolimus with about 20% oral bioavailability
and extensive hepatic metabolism and biliary
excretion.
•
Does not inhibit calcineurin activity to
prevent IL-2 gene expression.
CH3
Inhibits T cell proliferation and differentiation, probably by inhibition of cellular
protein kinases involved in regulating the cell division cycle (S6 kinase and cyclindependent kinases). May have actions to inhibit B cell function as well, at
therapeutic doses.
Agents that modify Intracellular Signaling in T cells
Sirolimus (contd.)
• Animal and human studies indicate sirolimus to be less nephrotoxic and
hypertensive than cyclosporine. May increased serum cholesterol and
triglycerides and potential to increase susceptibility to infections and
lymphomas.
•
Interactions: dose adjustment when coadministered with diltiazem, cyclosporin,
rifampin
Glucocorticoid Agents
Prednisone (Deltasone®), Prednisolone (Hydeltrasol®)
CH2 OH
O
C
CH3
OH
CH3
O
Prednisolone
OH
•
Immunosuppressive effects by inhibiting T cell
proliferation and prevention of cytokine gene
expression.
•
May also decrease rejection by blocking
inflammatory reactions (DTH) and infiltration of
macrophages, neutrophils to site of graft.
•
Adverse effects: adrenal insufficiency, GI ulcers, hyperglycemia, osteoporosis
and susceptibility to infection.
•
Used in acute transplant rejection, graft-vs-host disease, autoimmune disorders,
allergic reactions, cytokine storm due to OKT3
Cytotoxic Agents
Azathioprine (Imuran®)
•
N
N
O2N
S
N
N
CH3
H
N
N
Purine antimetabolite that is converted to 6mercaptopurine in the presence of nucleophiles such as
glutathione.
•
6-mercaptopurine blocks de novo purine synthesis in
lymphocytes to prevent T cell proliferation during antigen
challenge.
•
Admin. oral and IV; good oral bioavailability of azathioprine with peak plasma
levels at 1-2 h. Metabolites excreted in urine. Allopurinol prevents metabolism of
6-MP and leads to increased toxicity.
•
Adverse effects: affects mainly rapidly growing cells – bone marrow (leukopenia,
thrombocytopenia); GI tract (NVD); hepatotoxicity.
Cytotoxic Agents
Mycophenolate Mofetil (CellCept®)
O
OH
•
CH3
O
O
O
CH3
N
O
OCH3
2-morpholinoethyl ester of
mycophenolic acid. Hydrolyzed to
mycophenolic acid by hepatic/plasma
esterases. Mofetil group improves oral
bioavailability.
•
Mycophenolic acid potent noncompetitive, inhibitor of inosine monophosphate
dehydrogenase (rate-limiting enzyme of de novo pathway) which is involved in the
guanine nucleotide biosynthesis pathway. Leukocytes depend on this pathway for
production of purines and thus more selective immunosuppression is achieved
(little bone marrow toxicity).
Inosine
IMPD
monophosphate
•
Xanthine
monophosphate
GTP
RNA
dGTP
DNA
Guanosine
monophosphate
Decreased leukocyte proliferation and decreased antibody production by B cells
Cytotoxic Agents
Mycophenolate (contd.)
•
Good oral absorption of mycophenolate mofetil (~94% bioavailability); rapidly
converted to free acid which is further metabolized to glucuronide conjugates
(inactive) that are renally excreted. High plasma protein binding of MPA (95%).
•
Adverse effects: diarrhea, GI dysfunction, neutropenia, viral infections.
•
Used to prevent renal or cardiac transplant rejection; often in combination with
glucocorticoids and calcineurin inhibitors.
•
Other cytotoxic drugs used: cyclophosphamide, dactinomycin, mizoribine and
leflunomide.
Immunomodulating Antibodies
Antithymocyte globulin (ATG)
•
Polyclonal antibodies from sheep, horse, rabbit or goat that have been
immunized with thymic lymphocytes (immature).
•
The purified antibodies when administered IV bind to circulating T cells to cause
leukopenia and decreased immune responsiveness (plasma half-life 3-9 days).
•
Adverse effects: serum sickness, nephritis, chills, fever, leukopenia,
thrombocytopenia and skin rash.
Antilymphocyte globulin (ALG, Thymoglubulin®)
•
Animals (usually rabbits) immunized with human lymphocytes for polyclonal
antibodies or monoclonal antibodies are produced by recombinant technology.
•
Antibody binding to lymphocytes causes their opsonization and phagocytosis by
macrophages, neutrophils; induction of complement cascade and cytotoxic cell
killing.
Immunomodulating Antibodies
Muromonab-CD3 monoclonal antibody (OKT3, Orthoclone®)

Mouse monoclonal antibody produced against human CD3 protein on T
cells that is required for recognition of altered MHC of the graft along with TCR.

Binding of muromonab-CD3 to CD3 results in lack of MHC binding to TCR
(blunts immune response) and causes loss of circulating T cells. When T cells
reappear in blood they are without TCR. Also causes release of cytokines by
activating T cells which decreases subsequent immune response.

Adverse effects: cytokine release syndrome, anaphylactoid syndrome, CNS
toxicity and propensity to infection and neoplasias.
•
Used to decrease acute
transplant rejection
Immunomodulating Antibodies
Daclizumab (Zenapax®), Basiliximab (Simulect®)

Newer human/mouse monoclonal (recombinant) antibodies that bind to asubunit of IL-2 receptor on activated T cells. Behave as IL-2 receptor antagonists
to minimize the stimulation, proliferation and differentiation of T cells.

Since the antibodies are mostly of human origin with just the antigen-binding
site from mouse, they are less immunogenic during repeated administration.
Immunomodulators in Immune disorders
Infliximab (Remicade®)
• Chimeric anti-TNFa antibody neutralizes TNFa to prevent its actions
•
Used in rheumatoid arthritis, Crohn’s disease
Etanercept (Embrel®)
• Composed of the ligand binding domain of the TNFa receptor fused to the FC
portion of IgG
•
Binds to and removes TNFa before it can bind to tissue receptors to produce
disease.