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From SNPedia

Tacrolimus, also known as FK-506 or Fujimycin, is the generic name for a calcineurin inhibitor drug sold under the brand names Prograf, Advagraf, and Protopic. Tacrolimus is an immunosuppressive agent for treating autoimmune disease, including myasthenia gravis and rheumatoid arthritis, as well as for preventing allograft rejection in organ transplantation. Wikipedia

Due to its narrow therapeutic window following organ transplantation, tacrolimus requires therapeutic drug monitoring (usually by assays using monoclonal antibody raised against it). If the concentration is lower than the optimal range, the transplanted organ will be rejected due to the insufficient immunosuppression by tacrolimus. If the concentration is higher than the range, adverse eŠects will be observed due to the excess concentration of tacrolimus.

Tacrolimus is primarily metabolized by CYP3A5, and CYP3A5 SNPs may therefore influence treatment with this drug. The most common CYP3A5 variant found to have an effect is CYP3A5*3, as determined by rs776746. CYP3A5*3 is a nonfunctional variant, so heterozygous and especially homozygous carriers of this allele tend not to break down tacrolimus as much as carriers of the common CYP3A5*1 allele, leading to higher blood concentrations of tacrolimus in these (CYP3A5*3) individuals.