Tacrolimus

Tacrolimus (also FK-506 or Fujimycin) is an immunosuppressive drug whose main use is after allogenic organ transplant to reduce the activity of the patient's immune system and so the risk of organ rejection. It is also used in a topical preparation in the treatment of severe atopic dermatitis ("eczema"), severe refractory uveitis after bone marrow transplants, and the skin condition vitiligo. It is a 23-membered macrolide lactone discovered in 1984 from the fermentation broth of a Japanese soil sample that contained the bacteria Streptomyces tsukubaensis.

History
Tacrolimus was discovered in 1987 by a Japanese team headed by T. Goto, T. Kino and H. Hatanaka; it was among the first macrolide immunosuppressants discovered, preceded by the discovery of rapamycin (sirolimus) on Rapa Nui (Easter Island) in 1975. Like ciclosporin, it was found in a soil fungus, although it is produced by a type of bacteria, Streptomyces tsukubaensis. The name tacrolimus is reportedly derived from 'Tsukuba macrolide immunosuppressant'.

The drug is owned by Astellas Pharma Inc., and is sold under the tradenames Prograf, Advagraf, and Protopic. It is sometimes referred to as FK-506, an early name relating to its action. It was first approved by the Food and Drug Administration (FDA) in 1994 for use in liver transplantation, this has been extended to include kidney, heart, small bowel, pancreas, lung, trachea, skin, cornea, bone marrow, and limb transplants.

Pharmacology
Tacrolimus is chemically known as a macrolide. It reduces peptidyl-prolyl isomerase activity by binding to the immunophilin FKBP-12 (FK506 binding protein) creating a new complex. This FKBP12-FK506 complex interacts with and inhibits calcineurin thus inhibiting both T-lymphocyte signal transduction and IL-2 transcription. Although this activity is similar to cyclosporin, studies have shown that the incidence of acute rejection is reduced by tacrolimus use over cyclosporin.

Immunosuppresion following transplantation
It has similar immunosuppressive properties to cyclosporin, but is much more potent in equal volumes. Also like cyclosporin it has a wide range of adverse interactions, including that with grapefruit which increases plasma-tacrolimus concentration. Several of the newer class of antifungals, especially of the azole class (fluconazole, posaconazole) also increase drug levels by competing for degradative enzymes. Immunosuppression with tacrolimus was associated with a significantly lower rate of acute rejection compared with cyclosporin-based immunosuppression (30.7% vs 46.4%) in one study.

Use in treating ulcerative colitis
In recent years, Tacrolimus has been used to suppress the inflammation associated with ulcerative colitis, a form of inflammatory bowel disease. Although almost exclusively used in trial cases only, Tacrolimus has shown to be significantly effective in the suppression of outbreaks of UC.

Dermatological use
See also: Immunomodulators in the treatment of eczema

As an ointment (Protopic), tacrolimus is a recent addition in the treatment of eczema, particularly atopic dermatitis. It suppresses inflammation in a similar way to steroids, and is equally as effective as a mid-potency steroid. An important advantage of tacrolimus is that unlike steroids, it does not cause skin thinning (atrophy), or other steroid related side-effects. It may therefore be used continuously on the body (clinical trials of up to one year in length have occurred), and applied to the thinner skin over the face and eyelids. Recently it has also been used to treat segmental vitiligo in children,especially on the face.

The most common adverse events associated with the use of Protopic included the sensation of skin burning, itching, flu-like symptoms, and headache. The use of Protopic should be avoided on known or suspected malignant lesions. The use of Protopic on patients with Netherton's syndrome or similar skin diseases is not recommended. Patients should minimize or avoid natural or artificial sunlight exposure. Skin infections should be cleared prior to application, and there may be an increased risk of certain skin infections. Protopic should not be used with occlusive dressings (http://www.protopic.com/).

Contraindications and Precautions

 * breast-feeding
 * hepatic disease
 * immunosuppression
 * infants
 * infection
 * intravenous administration
 * neoplastic disease
 * occlusive dressing
 * oliguria
 * pregnancy
 * QT prolongation
 * skin cancer
 * sunlight (UV) exposure
 * grapefruit juice

From oral and intravenous administration
Side effects can be severe and include blurred vision, liver and kidney problems (it is nephrotoxic), seizures, tremors, hypertension, hypomagnesemia, diabetes mellitus, hyperkalemia, itching, insomnia, confusion, loss of appetite, hyperglycemia, weakness, depression, cramps, and neuropathy, as well as potentially increasing the severity of existing fungal or infectious conditions such as herpes zoster or polyoma viral infections.

From topical use
A common side effect of tacrolimus ointment, if used over a wide area, is to cause a burning or itching sensation on the first one or two applications. Less common side effects include flu-like symptoms, headache, cough and burning eyes.

Cancer risks
Tacrolimus and a related drug for eczema (pimecrolimus) were suspected of carrying a cancer risk, though the matter is still a subject of controversy. The FDA issued a health warning in March 2005 for the drug, based on animal models and a small number of patients. Until further human studies yield more conclusive results, the FDA recommends that users be advised of the potential risks. Whereas current practice by UK dermatologists  is not to consider this a significant real concern and they are increasingly recommending the use of these new drugs.

Dermatologists agree that the drug should be used as a second-line remedy only after conventional methods of treatment have failed.