FK506 (Tacrolimus)

FK506 (Tacrolimus) is a potent immunosuppressive drug for primary and rescue therapy of solid organ transplants. FK506 is a 100-fold more potent inhibitor of T lymphocyte activation and IL-2 expression than cyclosporine. Large multi-center trials in both liver and kidney transplantation have shown that FK506 immunosuppression is associated with a lower incidence of acute rejection than standard cyclosporine therapy. In renal transplantation, FK506 is commonly used as rescue therapy for refractory acute rejection in patients treated with cyclosporine. The reported rescue rate was 75% over a 5-year follow-up period.

FK506 has a narrow therapeutic window. The most frequent side effect is nephrotoxicity, followed by hyperkalemia, hyperglycemia, GI upset, and neurotoxicity. Trough blood levels are useful in guiding dosage adjustments to achieve optimal immunosuppression while minimizing toxicity. Immediately following transplant, an optimal response is obtained when the trough blood level is in the range of 15 ng/mL. Metabolism increases as renal and hepatic functions normalize and a steady state is reached two weeks or more after transplant. At that time, the ideal trough blood level is 3 to 8 ng/mL.

Specimen requirement is one lavender top (EDTA) tube of blood.

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