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Immunosuppressants
I. OVERVIEW
The importance of the immune system in protecting the body against harmful foreign
molecules is well recognized. However, this protection can result in serious problems.
For example, rejection of the transplanted tissue. Transplantation of organs and tissues
(for example, kidney, heart, or bone marrow) has become routine due to improved
surgical techniques and better tissue typing. Also, drugs are now available that more -
electively s inhibit rejection of transplanted tissues while preventing the patient from
becoming immunologically compromised. Earlier drugs were nonselective, and patients
frequently succumbed to infection due to suppression of both the antibody-mediated
(humoral) and cell-mediated arms of the immune system. Today, the principal approach
to immunosuppressive therapy is to alter lymphocyte function using drugs or antibodies
against immune proteins. Because of their severe toxicities when used as monotherapy, a
combination of immunosuppressive agents, usually at lower doses, is generally
employed. [Note: Immunosuppressive therapy is also used in the treatment of auto-
mmune diseases. For example, cori ticosteroids can control acute glomerulonephritis.]
Immunosuppressive drug regimens usually consist of anywhere from two to four agents
with different mechanisms of action that disrupt various levels of T-cell activation.
Immunosuppressive drugs can be categorized according to their mechanisms of action: 1)
Some agents interfere with cytokine production or action; 2) others disrupt cell
metabolism, preventing lymphocyte proliferation; and 3) mono- and polyclonal
antibodies block T-cell surface molecules.
II.
SELECTIVE INHIBITORS OF CYTOKINE PRODUCTION AND FUNCTION
Cytokines are soluble, antigen-nonspecific, signaling proteins that bind to cell surface
receptors on a variety of cells. The term cytokine includes the molecules known as
interleukins (ILs), interferons (IFNs), tumor necrosis factors (TNFs), transforming
growth factors, and colony-stimulating factors. Of particular interest when discussing
immunosuppressive drugs is IL-2, a growth factor that stimulates the proliferation of
antigen-primed (helper) T cells, which subsequently produce more IL-2, IFN-γ, and TNF-
a (Figure 40.2). These cytokines collectively activate natural killer cells, macrophages,
and cytotoxic T lymphocytes. Clearly, drugs that interfere with the production or activity
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of IL-2, such as cyclosporine, will significantly dampen the immune response and,
thereby, decrease graft rejection.
A.
Cyclosporine
Cyclosporine is a lipophilic cyclic polypeptide. The drug is extracted from the soil fungus
Beauveria nivea. Cyclosporine is used to prevent rejection of kidney, l - iver, and cardiac
allogeneic transplants. Cyclosporine is most effective in preventing acute rejection of
transplanted organs when combined in a double-drug or triple-drug regimen with
corticosteroids and an antimetabolite such as mycophenolate mofetil. Cyclosporine is an
alternative to methotrexate for the treatment of severe, active rheumatoid arthritis. It can
also be used for patients with recalcitrant psoriasis that does not respond to other
therapies, and it is also used for - erophthalmia. x
1.
Mechanism of action: Cyclosporine preferentially suppresses cellmediated
immune reactions, whereas humoral immunity is affected to a far lesser extent. After
diffusing into the T cell, cyclosporine binds to a cyclophilin (more generally called an
immunophilin) to form a complex that binds to calcineurin. The latter is responsible for
dephosphorylating NFATc (cytosolic Nuclear Factor of Activated T cells). Because the
cyclosporine-calcineurin complex cannot perform this reaction, NFATc cannot enter the
nucleus to promote the reactions that are required for the synthesis of a number of
cytokines, including IL-2. The end result is a decrease in IL-2, which is the primary
chemical stimulus for increasing the number of T lymphocytes.
2.
Pharmacokinetics: Cyclosporine may be given either orally or by intravenous (IV)
infusion. Oral absorption is variable. Interpatient variability may be due to metabolism by
a cytochrome P450 (CYP3A4) in the gastrointestinal (GI) tract, where the drug is
metabolized. Cyclosporine is also a substrate for P-glycoprotein (P-gp), a drug efflux
pump, which limits cyclosporine absorption by transporting the drug back into the gut
lumen. About 50 percent of the drug is associated with the blood fraction. Half of this is
in the erythrocytes, and less than one tenth is bound to the lymphocytes. Excretion of the
metabolites is through the biliary route, with only a small fraction of the parent drug
appearing in the urine.
3.
Adverse effects: Many of the adverse effects caused by cyclosporine are dose
dependent. Therefore, it is important to monitor blood l -evels of the drug. Nephrotoxicity
is the most common and important adverse effect of cyclosporine, and it is critical to
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monitor kidney function. Reduction of the cyclosporine dosage can result in reversal of
nephrotoxicity in most cases, although nephrotoxicity may be irreversible in 15 percent
of patients. Hepatotoxicity can also occur, liver function should be periodically assessed.
Infections in patients taking cyclosporine are common and may be life-threatening. Viral
infections due to the herpes group and cytomegalovirus (CMV) are prevalent. Lymphoma
may occur in all transplanted patients due to the net level of immunosuppression and has
not been linked to any one particular agent. Anaphylactic reactions can occur on
parenteral administration. Other toxicities include hypertension, hyperlipidemia,
hyperkalemia (it is important not to use K+-sparing diuretics in these patients), tremor,
hirsutism, glucose intolerance, and gum hyperplasia.
B.
Tacrolimus
Tacrolimus (originally called FK506) is a macrolide that is isolated from the soil fungus
Streptomyces -tsukubaensis. Tacrolimus is approved for the prevention of rejection of
liver and kidney transplants and is given with a corticosteroid and/or an antimetabolite.
This drug has found favor over cyclosporine, not only because of its potency and
decreased episodes of rejection, but also because lower doses of corticosteroids can be
used, thus reducing the likelihood of steroid-associated adverse effects. An ointment
preparation has been approved for moderate to severe atopic dermatitis that does not
respond to conventional therapies.
1.
Mechanism of action: Tacrolimus exerts its immunosuppressive effect in the same
manner as cyclosporine, except that it binds to a different immunophilin, FKBP-12 (FK-
binding protein).
2.
Pharmacokinetics: Tacrolimus may be administered orally or IV. Tacrolimus is
subject to gut metabolism by CYP3A4/5 isoenzymes and is a substrate for P-gp.
Together, both of these mechanisms limit the oral bioavailability of tacrolimus.
Absorption is decreased if the drug is taken with high-fat or high-carbohydrate meals.
Tacrolimus is from 10- to 100-fold more potent than cyclosporine. It is highly bound to
serum proteins and is also concentrated in erythrocytes. Like cyclosporine, tacrolimus
undergoes hepatic metabolism. At least one metabolite of tacrolimus has been shown to
have immunosuppressive activity. Renal excretion is very low, and most of the drug and
its metabolites are found in the feces.
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3.
Adverse effects: Nephrotoxicity and neurotoxicity (tremor, seizures, and
hallucinations) tend to be more severe in patients who are treated with tacrolimus than in
patients treated with cyclosporine. Development of posttransplant, insulin-dependent
diabetes mellitus is a problem. Other toxicities are the same as those for cyclosporine,
except that tacrolimus does not cause hirsutism or gingival hyperplasia. Compared with
cyclosporine, t - acrolimus has also been found to have a lower incidence of
cardiovascular toxicities, such as hypertension and hyperlipidemia. Anaphylactoid
reactions to the injection vehicle have been reported.
C.
Sirolimus
Sirolimus is a macrolide obtained from fermentations of the soil mold Streptomyces
hygroscopicus. The earlier name is rapamycin. Sirolimus is approved for use in renal
transplant- tion, to be used together with cyclosporine and a corticosteroids, allowing
lower doses of those medications to be used, thereby lowering their toxic potential. The
combination of sirolimus and cyclosporine is apparently synergistic because sirolimus
works later in the immune activation cascade. The antiproliferative action of sirolimus
has found use in cardiology. Sirolimus-coated stents inserted into the cardiac vasculature
inhibit restenosis of the blood vessels by reducing proliferation of the endothelial cells.
1.
Mechanism of action: Sirolimus and tacrolimus bind to the same cytoplasmic FK-
binding protein, but instead of forming a complex with calcineurin, sirolimus binds to
mTOR. Binding of sirolimus to mTOR blocks the progression of activated T cells from
the G1 to the S phase of the cell cycle and, consequently, the proliferation of these cells.
Unlike cyclosporine and - acrolimus, sirolimus does not owe its effect to lowering IL-2
prot duction but, rather, to inhibiting the cellular responses to IL-2.
2.
Pharmacokinetics: The drug is available only as oral preparations. Although it is
readily absorbed, high-fat meals can decrease the drug’s absorption. Sirolimus has a long
half-life (57 to 62 hours) compared to those of cyclosporine and tacrolimus, and a loading
dose is recommended at the time of initiation of therapy, but only requires once daily
dosing. Sirolimus also increases the drug concentrations of cyclosporine, and careful
blood level monitoring of both agents must be done to avoid harmful drug toxicities. The
parent drug and its metabolites are predominantly eliminated in feces.
3.
Adverse effects: A common side effect of sirolimus is hyperlipidemia. The
combination of cyclosporine and sirolimus is more nephrotoxic than cyclosporine alone.
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Although the administration of sirolimus and tacrolimus appears to be less nephrotoxic,
sirolimus can still potentiate the nephrotoxicity of tacrolimus, and drug levels of both
must be monitored closely. Other untoward problems are headache, nausea and diarrhea,
leukopenia, and thrombocytopenia. Impaired wound healing has been noted with
sirolimus in obese patients and those with diabetes.
D.
Everolimus
Everolimus (another mTOR inhibitor) was recently approved by the U.S. Food and Drug
Administration for use in renal transplantation in combination with low-dose
cyclosporine and corticosteroids. It was originally approved in 2009 for second-line
treatment in patients with advanced renal cell carcinoma.
1. Mechanism of action: Everolimus has the same mechanism of action as sirolimus. It
inhibits activation of T cells by forming a complex with FKBP-12 and subsequently
blocking mTOR.
2. Pharmacokinetics: Everolimus differs from sirolimus in its pharmacokinetic profile.
Everolimus is rapidly absorbed, attaining maximal concentrations in 1 to 2 hours post
dose, but absorption is decreased with high-fat meals. Everolimus is a substrate of
CYP3A4 and P-gp and, thus, is subject to the same drug interactions as previously
mentioned immunosuppressants. Everolimus avidly binds erythrocytes, and monitoring
of whole blood trough concentrations is recommended. It has a much shorter half-life
than does sirolimus at 30 ± 11 hours and requires twice-daily dosing. Everolimus
increases drug concentrations of cyclosporine, thereby enhancing the nephrotoxic effects
of cyclosporine, and is, therefore, recommended to be used with reduced doses of
cyclosporine.
3. Adverse effects: Everolimus has similar side effects to sirolimus, including
hyperlipidemia, impaired or delayed wound healing following transplantation, and
enhanced nephrotoxicity in combination with higher doses of cyclosporine. An additional
adverse effect noted with everolimus is angioedema, which may increase with
concomitant use of angiotensin-converting enzyme inhibitors. There is also an increased
risk of kidney arterial and venous thrombosis, resulting in graft loss, usually in the first
30 days posttransplantation.
III. IMMUNOSUPPRESSIVE ANTIMETABOLITES
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Immunosuppressive antimetabolite agents are generally used in combination with
corticosteroids and the calcineurin inhibitors, cyclosporine and tacrolimus.
A.
Azathioprine
Azathioprine was the first agent to achieve widespread use in organ transplantation. It is a
prodrug that is converted first to 6-mercaptopurine (6-MP) and then to the corresponding
nucleotide, thioinosinic acid. The immunosuppressive effects of azathioprine are due to
this nucleotide analog. The drug has little effect on suppressing a chronic immune
response. Its major toxicity is bone marrow suppression. Concomitant use with
angiotensin-converting enzyme inhibitors or cotrimoxazole in renal transplant patients
can lead to an exaggerated leukopenic response. Allopurinol, an agent used to treat gout,
significantly inhibits the metabolism of azathioprine. Therefore, the dose of azathioprine
must be reduced by 60 to 75 percent. Nausea and vomiting are also encountered.
B.
Mycophenolate mofetil
Mycophenolate mofetil has, for the most part, replaced azathioprine because of its safety
and efficacy in prolonging graft survival. It has been successfully used in heart, kidney,
and liver transplants. As an ester, it is rapidly hydrolyzed in the GI tract to mycophenolic
acid. This is a potent, reversible, uncompetitive inhibitor of inosine monophosphate
dehydrogenase, which blocks the de novo formation of guanosine phosphate. Thus, like
6-MP, it deprives the rapidly proliferating T and B cells of a key component of nucleic
acids. Mycophenolic acid is quickly and almost completely absorbed after oral
administration. Both mycophenolic acid and its glucuronidated metabolite are highly
bound (greater than 90 percent) to plasma albumin. The glucuronide metabolite is
excreted predominantly in urine. The most common adverse effects include diarrhea,
nausea, vomiting, abdominal pain, leukopenia, and anemia. Higher doses of
mycophenolate mofetil (3 g/day) were associated with a higher risk of CMV infection.
[Note: mycophenolic acid is less mutagenic or carcinogenic than azathioprine.]
Concomitant administration with antacids containing magnesium or aluminum, or with
cholestyramine, can decrease absorption of the drug.
C. Enteric-coated mycophenolate sodium
In an effort to minimize the GI effects associated with mycophenolate mofetil, enteric-
coated mycophenolate sodium was developed. The active drug (mycophenolic acid) is
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contained within a delayed-release formulation designed to release in the neutral pH of
the small intestine. The new formulation was found to be equivalent to mycophenolate
mofetil in the prevention of acute rejection episodes in kidney transplant recipients.
However, the rate of GI adverse events was similar to that with mycophenolate mofetil.
IV. ANTIBODIES
The use of antibodies plays a central role in prolonging allograft survival. The names of
monoclonal antibodies conventionally contain “muro” if they are from a murine (mouse)
source and “xi” or “zu” if they are chimerized or humanized, respectively. The suffix
“mab” (monoclonal antibody) identifies the category of drug. The polyclonal antibodies,
although relatively inexpensive to produce, are variable and less specific, which is in
contrast to monoclonal antibodies, which are homogeneous and specific.
A. Antithymocyte globulins
They are primarily used, together with other immunosuppressive agents, at the time of
transplantation to prevent early allograft rejection, or they may be used to treat severe
rejection episodes or corticosteroid-resistant acute rejection.The antibody-bound cells are
phagocytosed in the liver and spleen, resulting in lymphopenia and impaired T-cell
responses. The antibodies are slowly infused intravenously, and their half-life extends
from 3 to 9 days. Because the humoral antibody mechanism remains active, antibodies
can be formed against these foreign proteins. [Note: This is less of a problem with the
humanized antibodies.] Other adverse effects include chills and fever, leukopenia and
thrombocytopenia, infections due to CMV or other viruses, and skin rashes.
B. Muromonab-CD3 (OKT3)
Muromonab-CD3 is a murine monoclonal antibody that is synthesized by hybridoma
technology and directed against the glycoprotein CD3 antigen of human T cells.
MuromonabCD3 is used for treatment of acute rejection of renal allografts as well as for
corticosteroid-resistant acute allograft rejection in cardiac and hepatic transplant patients.
It is also used to deplete T cells from donor bone marrow prior to transplantation.
Adverse effects: Anaphylactoid reactions may occur. Cytokinerelease syndrome may
follow the first dose. The symptoms can range from a mild, flu-like illness to a life-
threatening, shock-like reaction. High fever is common. Central nervous system effects,
such as seizures, encephalopathy, cerebral edema, aseptic meningitis, and headache, may
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occur. Infections can increase, including some due to CMV. Muromonab-CD3 is
contraindicated in patients with a history of seizures, in those with uncompensated heart
failure, in pregnant women, and in those who are breast-feeding. Because of these
adverse effects and the improved tolerability of rabbit antithymocyte globulin and the IL-
2 receptor antagonists, muromonab-CD3 is rarely used today.
C.
IL-2-receptor antagonists
Basiliximab is said to be “chimerized” because it consists of 25 percent murine and 75
percent human protein. Daclizumab is 90 percent human protein, and is designated
“humanized.” Both agents have been approved for prophylaxis of acute rejection in renal
transplantation in combination with cyclosporine and corticosteroids. They are not used
for the treatment of ongoing rejection. In late 2009, daclizumab was withdrawn from the
U.S. market by the manufacturer due to a diminished demand for the product.
1.
Mechanism of action: Both compounds are anti-CD25 antibodies and bind to the α
chain of the IL-2 receptor on activated T cells. They thus interfere with the proliferation
of these cells. Basiliximab is about 10-fold more potent than daclizumab as a blocker of
IL-2 stimulated T-cell replication. Blockade of this receptor foils the ability of any
antigenic stimulus to activate the T-cell response system.
2.
Pharmacokinetics: Both antibodies are given IV. The serum half-life of
daclizumab is about 20 days, and the blockade of the receptor is 120 days. Five doses of
daclizumab are usually administered, the first at 24 hours before transplantation, and the
next four doses at 14-day intervals. The serum half-life of basiliximab is about 7 days.
Usually, two doses of this drug are administered, the first at 2 hours prior to
transplantation, and the second at 4 days after the surgery.
3.
Adverse effects: Both daclizumab and basiliximab are well tolerated. Their major
toxicity is GI. No clinically relevant antibodies to the drugs have been detected, and
malignancy does not appear to be a problem.
D.
Alemtuzumab
Alemtuzumab, a humanized monoclonal antibody, exerts its effects by causing profound
depletion of T cells from the peripheral circulation. This effect may last for up to 1 year.
Alemtuzumab is currently approved for the treatment of refractory B-cell chronic
lymphocytic leukemia. Although it is not currently approved for use in organ
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transplantation, it is being used in combination with sirolimus and low-dose calcineurin
inhibitors in corticosteroid-avoidance protocols at many transplant centers. Preliminary
results are promising, with low rates of rejection with a prednisone-free regimen. Side
effects include first-dose cytokine-release syndrome, requiring premedication with
acetaminophen, diphenhydramine, and corticosteroids. Adverse effects include
neutropenia, anemia, and, rarely, pancytopenia. Intermediate term results have shown an
increase in B-cell mediated rejection and development of autoimmune disorders in a
small number of patients and, thus, this agent should be used with caution.
A summary of the major immunosuppressive drugs is presented in Figure 40.8.
V. CORTICOSTEROIDS
The corticosteroids were the first pharmacologic agents to be used as
immunosuppressives both in transplantation and in various autoimmune disorders. They
are still one of the mainstays for attenuating rejection episodes. For transplantation, the
most common agents are prednisone or methylprednisolone, whereas prednisone or
prednisolone are used for autoimmune conditions. The steroids are used to suppress acute
rejection of solid organ allografts and in chronic graft-versus-host disease. In addition,
they are effective against a wide variety of autoimmune conditions, including refractory
rheumatoid arthritis, systemic lupus erythematosus, temporal arthritis, and asthma. The
exact mechanism responsible for the immunosuppressive action of the corticosteroids is
unclear. The T lymphocytes are affected most. The steroids are able to rapidly reduce
lymphocyte populations. They bind to the glucocorticoid receptor. The complex passes
into the nucleus and regulates the translation of DNA. The use of these agents is
associated with numerous adverse effects. For example, they are diabetogenic and can
cause hypercholesterolemia, cataracts, osteoporosis, and hypertension with prolonged
use.