Azathioprine ( AZA ), sold under the brand name Imuran among others, is an immunosuppressive medication.
Common side effect include bone marrow suppression and vomiting.
Azathioprine was first made in 1957.
1 Medical uses
1.2 Rheumatoid arthritis
1.3 Inflammatory bowel disease
2 Adverse effects
2.2.1 Skin cancers
5 Pregnancy and breastfeeding
6.2 Mechanism of action
10 External links
Azathioprine is used alone or in combination with other immunosuppressive therapy to prevent rejection following organ transplantation, and to treat an array of autoimmune diseases, including rheumatoid arthritis, pemphigus, systemic lupus erythematosus, Behçet’s disease, and other forms of vasculitis, autoimmune hepatitis, atopic dermatitis, myasthenia gravis, neuromyelitis optica (Devic’s disease), restrictive lung disease, and others.
In the United States, it is currently approved by the Food and Drug Administration (FDA) for use in kidney transplantation from human donors, and for rheumatoid arthritis.
Azathioprine is used to prevent rejections of kidney or liver allografts, usually in conjunction with other therapies including corticosteroids, other immunosuppressants, and local radiation therapy.
Being a disease-modifying antirheumatic drug (DMARD), azathioprine has been used for the management of the signs and symptoms of adult rheumatoid arthritis.
Inflammatory bowel disease
Azathioprine has been used in the management of moderate to severe chronically active Crohn’s disease,
Azathioprine treatment is associated with an increased risk of lymphoma, but it is unclear if this is due to the drug or a predisposition related to Crohn’s disease.
In Crohn’s disease, treatment with azathioprine shortly after diagnosis was no more likely to result in corticosteroid-free remission than standard care or placebo.
It may also be used to prevent flares in those with ulcerative colitis.
Azathioprine is sometimes used in systemic lupus erythematosus patients who require a maintenance dose of 15 mg or higher of prednisone and those who experience recurrent flares.
It is used as an add on therapy when steroid therapy is given by mouth for pemphigus and myasthenia gravis, as a “steroid-sparing” agent.
It was shown to be very effective in eczema and atopic dermatitis in researches, even though it is not commonly used.
It was widely used for the treatment of multiple sclerosis until the first half of the 1990s. Concerns about increased risk of malignancy has led to a decreased use, yet it is still used in maintenance treatment for patients who frequently relapse.
A widely used therapy for idiopathic pulmonary fibrosis was azathioprine in combination with prednisone and N – acetylcysteine. A 2012 study showed that there were worse outcomes with this combination than with placebo.
Two generic azathioprine oral tablets, 50 mg each
Nausea and vomiting are common adverse effects, especially at the beginning of a treatment. Such cases are met with taking azathioprine after meals or transient intravenous administration. Side effects that are probably hypersensitivity reactions include dizziness, diarrhea, fatigue, and skin rashes. Hair loss is often seen in transplant patients receiving the drug, but rarely occurs under other indications. Because azathioprine suppresses the bone marrow, patients can develop anaemia and will be more susceptible to infection ; regular monitoring of the blood count is recommended during treatment.
Under FDA rules, like many other immunosuppressants, the use of this drug excludes eligibility for blood donation.
It is listed by the International Agency for Research on Cancer as a group 1 carcinogen (carcinogenic to humans).
The enzyme thiopurine S-methyltransferase (TPMT) is responsible for various activation and deactivation steps in azathioprine’s mechanism of action. The first metabolic step that azathioprine undergoes in the body is the conversion to 6-mercaptopurine (6-MP; see Pharmacokinetics ), which is itself an immunosuppressant prodrug.
Azathioprine is listed as a human carcinogen in the 12th Report on Carcinogens by the National Toxicology Program of U.S. Department of Health and Human Services, asserting that it is “known to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in humans.”
The risks involved seem to be related both to the duration and to the dosage used. People who have previously been treated with an alkylating agent may have an excessive risk of cancers if treated with azathioprine. Epidemiological studies by International Agency for Research on Cancer (IARC) have provided “sufficient” evidence of azathioprine carcinogenicity in humans ( Group 1 ),
The various diseases requiring transplantation may in themselves increase the risks of non-Hodgkin’s lymphoma, squamous cell carcinomas of the skin, hepatobiliary carcinomas, and mesenchymal tumours to which azathioprine may add additional risks. Those receiving azathioprine for rheumatoid arthritis may have a lower risk than those undergoing transplantation.
Cases of hepatosplenic T-cell lymphoma – a rare type of lymphoma – have been reported in patients treated with azathioprine. The majority occurred in patients with inflammatory bowel disease. Adolescents and young adult males were the majority of cases.
In transplant patients, skin cancer is 50 to 250 times more common than in the general population, and between 60% and 90% of patients are affected 20 years after transplantation. The use of immunosuppressive medication including azathioprine in organ transplantation has been linked to increased rates of developing skin cancer.
Large single doses are generally well tolerated; a patient who took 7.5 g azathioprine (150 tablets) at once showed no relevant symptoms apart from vomiting, slightly decreased white blood cell count and marginal changes in liver function parameters. Main symptoms of long-term overdosing are infections of unclear origin, mouth ulcers and spontaneous bleeding, all of which are consequences of the bone marrow suppression.
Other purine analogues such as allopurinol inhibit xanthine oxidase, the enzyme that breaks down azathioprine, thus increasing the toxicity of azathioprine.
Azathioprine decreases the effects of the anticoagulant warfarin and of non-depolarizing muscle relaxants, but increases the effect of depolarizing muscle relaxants.
Pregnancy and breastfeeding
Azathioprine can cause birth defects.
Traditionally, as for all cytotoxic drugs, the manufacturer advises not to breastfeed whilst taking azathioprine. However, the “Lactation Risk Category” reported by Thomas Hale in his book “Medications and Mothers’ Milk” lists azathioprine as “L3”, termed “moderately safe”.
Metabolic pathway for azathioprine (AZA). Active metabolites are highlighted. XO: xanthine oxidase 6-MP: 6- mercaptopurine TPMT: thiopurine methyltransferase 6-MMP: 6-methylmercaptopurine HPRT: hypoxanthine-guanine phosphoribosyltransferase TIMP: thioinosine monophosphate, thioinosinic acid MeTIMP: methyl-thioinosine monophosphate TGTP: thio guanosine triphosphate TdGTP: thio- deoxyguanosine triphosphate
Azathioprine is absorbed from the gut to about 88%. Bioavailability varies greatly between individual patients, between 30 and 90%, because the drug is partly inactivated in the liver. Highest blood plasma concentrations, counting not only the drug itself but also its metabolites, are reached after one to two hours; and the average plasma half-life is 26 to 80 minutes for azathioprine and three to five hours for drug plus metabolites. 20 to 30% are bound to plasma proteins while circulating in the bloodstream.
Azathioprine is a prodrug, a substance that is not an active drug itself but is activated in the body. This happens in several steps; at first it is slowly and almost completely converted to 6-mercaptopurine (6-MP) by reductive cleavage of the thioether (– S –). This is mediated by glutathione and similar compounds in the intestinal wall, the liver and on red blood cells, without the aid of enzymes. 6-MP is metabolized analogously to natural purines, giving thio guanosine triphosphate (TGTP) and thio- deoxyguanosine triphosphate (TdGTP) via thioinosine monophosphate (TIMP) and several further intermediates. On a second path, the sulfur atom of 6-MP and TIMP is methylated. The end products of azathioprine metabolism are thiouric acid (38%) and various methylated and hydroxylated purines, which are excreted via the urine.
Mechanism of action
Azathioprine inhibits purine synthesis. Purines are needed to produce DNA and RNA. By inhibiting purine synthesis, less DNA and RNA are produced for the synthesis of white blood cells, thus causing immunosuppression.
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Azathioprine is converted within tissues to 6-mercaptopurine (6-MP). Some 6-MP is converted in turn to 6-thioguanine by the addition of an amino group. Both 6-MP and 6-thioguanine are conjugated with ribose, and then phosphorylated to form the nucleotides thioinosinic acid and thioguanylic acid respectively. These nucleotides masquerade, respectively, as inosinic acid and guanylic acid : the former is the starting point for purine nucleotide biosynthesis, while the latter is one of the building blocks of DNA and RNA.
The nucleotides are incorporated into newly synthesized (but non-functional) DNA, halting replication.
The nucleotides act to inhibit glutamine- phosphoribosyl pyrophosphate amidotransferase (GPAT), one of the enzymes involved in purine biosynthesis, one of the earlier steps in the synthesis of DNA and RNA. They achieve GPAT inhibition through a form of negative feedback called product inhibition.
A portion of the nucleotides is additionally phosphorylated to the triphosphate forms. These bind to GTP -binding protein Rac1, blocking synthesis of the protein Bcl-xL and thus sending activated T cells and mononuclear cells into apoptosis (programmed cell death). Increased apoptosis of mononuclear cells is seen in inflammatory bowel disease patients treated with azathioprine.
Azathioprine is a thiopurine linked to a second heterocycle (an imidazole derivative) via a thioether. It is a pale yellow solid with a slightly bitter taste and a melting point of 238–245 °C. It is practically insoluble in water and only slightly soluble in lipophilic solvents such as chloroform, ethanol and diethylether. It dissolves in alkaline aqueous solutions, where it hydrolyzes to 6-mercaptopurine.
Azathioprine is synthesized from 5-chloro-1-methyl-4-nitro-1 H -imidazole and 6-mercaptopurine in dimethyl sulfoxide (DMSO). the nitro group is introduced with nitric and sulfuric acid.
Azathioprine was synthesized by George Herbert Hitchings and Gertrude Elion in 1957 (named BW 57-322 ) to produce 6-mercaptopurine (6-MP) in a metabolically active but masked form, and at first used as a chemotherapy drug.
Robert Schwartz investigated the effect of 6-MP on the immune response in 1958 and discovered that it profoundly suppresses the formation of antibodies when given to rabbits together with antigens. For many years, this kind of dual therapy with azathioprine and glucocorticoids was the standard antirejection regimen, until ciclosporin was introduced into clinical practice (by Calne as well) in 1978.
Ciclosporin has now replaced some of the azathioprine use due to a longer survival time, especially in heart-related transplantations.
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