Tacrolimus

Absorption

Absorption of tacrolimus from the gastrointestinal tract after oral administration is incomplete and variable. The absolute bioavailability in adult kidney transplant patients is 17±10%; in adults liver transplant patients is 22±6%; in healthy subjects is 18±5%. The absolute bioavailability in pediatric liver transplant patients was 31±24%. Tacrolimus maximum blood concentrations (Cmax) and area under the curve (AUC) appeared to increase in a dose-proportional fashion in 18 fasted healthy volunteers receiving a single oral dose of 3, 7, and 10 mg. When given without food, the rate and extent of absorption were the greatest. The time of the meal also affected bioavailability. When given immediately after a meal, mean Cmax was reduced 71%, and mean AUC was reduced 39%, relative to the fasted condition. When administered 1.5 hours following the meal, mean Cmax was reduced 63%, and mean AUC was reduced 39%, relative to the fasted condition.

Toxicity

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. LD50=134-194 mg/kg (rat).

Description

Tacrolimus, isolated from the microorganism Streptomyces tsukubaensis, is a macrolide immunosuppressant developed by Fujisawa for organ transplantation. It displays similar but more potent immunosuppressive activity than cyclosporin. It inhibits both cell mediated and humoral immune responses. In animal models of organ transplantation, tacrolimus has been shown to prolong survival of hepatic, renal, cardiac, small intestine, pancreatic and skin allografts and to reverse cardiac and renal allograft rejection. It has been used effectively in humans as rescue or primary immunosuppressant therapy in liver or kidney transplantation. Compared to cyclosporin, tacrolimus causes reduced incidence of infectious complications and of hypertension and hypercholesterolemia for the allograft recipients. In common with cyclosporin, tacrolimus binds with high affinity to a family of cytoplasmic immunosuppressant binding proteins, the immunophilins. This tight complex is proposed as the biologically active moiety that interacts with intracellular molecules involved in signal transduction.It inhibits phosphatase activity of calcineurin, an action that may impair the generation and/or activation of nuclear transcription factors required for lymphokine (particularly interleukin-2) gene expression. Tacrolimus has also been reported to have potential in multiple sclerosis, psoriasis, rheumatoid arthritis and uveitis associated with Behcet‘s disease.

The Uses of Tacrolimus

FK-506 (Tacrolimus) is a macrolide immunosuppressive drug that is mainly used after allogeneic organ transplant to reduce the activity of the patient's immune system

Background

Tacrolimus (also FK-506 or Fujimycin) is an immunosuppressive drug whose main use is after 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, severe refractory uveitis after bone marrow transplants, and the skin condition vitiligo. It was discovered in 1984 from the fermentation broth of a Japanese soil sample that contained the bacteria Streptomyces tsukubaensis. 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 inhibits calcineurin which inhibits T-lymphocyte signal transduction and IL-2 transcription.

What are the applications of Application

FK-506 is an agent that binds to the immunophilin FKBP2 and inhibits calcineurin

Indications

Immediate-release formulations of tacrolimus are indicated for the prophylaxis of organ rejection in adult and pediatric patients receiving allogeneic liver, kidney, heart, or lung transplants, in combination with other immunosuppressants. Extended-release formulations of tacrolimus are indicated for the prophylaxis of organ rejection in adult and pediatric patients receiving kidney transplants, in combination with other immunosuppressants, and may be used in patients converted from immediate-release formulations.
Topical tacrolimus ointment is indicated as second-line therapy for short-term and non-continuous treatment of moderate-to-severe atopic dermatitis in non-immunocompromised adults and children who have failed to respond adequately to other topical treatments or for whom alternative treatments are not advisable. Both available strengths are indicated in adult patients, while only the lower strength (0.03%) formulation is indicated in pediatric patients between 2 and 15 years of age.

Pharmacokinetics

Tacrolimus acts by reducing peptidyl-prolyl isomerase activity by binding to the immunophilin FKBP-12 (FK506 binding protein) creating a new complex. This inhibits both T-lymphocyte signal transduction and IL-2 transcription. Tacrolimus has similar activity to cyclosporine but rates of rejection are lower with tacrolimus. Tacrolimus has also been shown to be effective in the topical treatment of eczema, particularly atopic eczema. It suppresses inflammation in a similar way to steroids, but is not as powerful. An important dermatological advantage of tacrolimus is that it can be used directly on the face; topical steroids cannot be used on the face, as they thin the skin dramatically there. On other parts of the body, topical steroid are generally a better treatment.

Metabolism

The metabolism of tacrolimus is predominantly mediated by CYP3A4 and secondarily by CYP3A5. Tacrolimus is metabolized into 8 metabolites: 13-demethyl tacrolimus, 31-demethyl tacrolimus, 15-demethyl tacrolimus, 12-hydroxy tacrolimus, 15,31-didemethyl tacrolimus, 13,31-didemethyl tacrolimus, 13,15-didemethyl tacrolimus, and a final metabolite involving O-demethylation and the formation of a fused ring. The major metabolite identified in incubations with human liver microsomes is 13-demethyl tacrolimus. In in vitro studies, a 31-demethyl metabolite has been reported to have the same activity as tacrolimus.