Tacrolimus

Background

Tacrolimus was discovered by Fujisawa pharmaceutical Co Ltd in 1984. It is a marcolide lactone isolated from Streptomyces tsukubaensis fungi. Tacrolimus is an immunosuppressant which is used to prevent transplant rejection in patients with allograft transplants especially in kidney and liver transplants.

Tacrolimus inhibits calcineurin which prevents T-cell activation. Tacrolimus is metabolised by the cytochrome P450 pathway, therefore drugs which affect the enzymes in this pathway may affect the tacrolimus concentration in the blood.

Tacrolimus has a narrow therapeutic range. Inadequate dosage may lead to graft rejection. Toxic levels can cause nephrotoxicity, neurotoxicity and susceptibility to infections. For these reasons tacrolimus levels need to be carefully controlled and regularly monitored.

The Dimension Tacrolimus (TACR) assay is an automated immunoassay technique in which free and tacrolimus-bound antibody-enzyme conjugates are separated using magnetic particles. The assay is performed using a specific flex reagent cartridge. The flex cartridge contains a pretreatment reagent, antibody – β-galactosidase conjugate, tacrolimus immobilised on chromium dioxide particles, chlorophenol red β-d-galactopyranoside (CPRG) substrate, and diluent to hydrate the tablets.

The dimension mixes and lyses the whole blood sample, the lysed sample is then mixed with the antibody enzyme conjugate. The tacrolimus present in the sample is bound by the tacrolimus antibody conjugate reagent. Magnetic particles coated with tacrolimus are added to bind free (unbound) antibody-enzyme conjugate. The reaction mixture is then separated magnetically.

Following separation, the supernatant containing the tacrolimus-antibody-enzyme complex is transferred to another cuvette and mixed with the substrate. β-galactosidase catalyses the hydrolysis of CPRG (chlorophenol red β-d-galactopyranoside) to produce CPR (chlorophenol red) that absorbs light maximally at 577 nm. The change in absorbance at 577 nm due to the formation of CPR is directly proportional to the amount of tacrolimus in the patient’s sample and is measured using a bichromatic (577, 700nm) rate technique

Reference ranges

Therapeutic ranges vary with type of transplant and length of time post transplant (A trough sample is desirable).

Specimen requirements

Sample type:

• Whole blood EDTA specimen
  • A trough sample is recommended for tacrolimus monitoring (usually 12 hours post dose.)

Sample identification:

• Three patient identifiers from
  • Name
  • D.O.B.
  • Address
  • N.H.S. number
  • Unit Number
  should match on the specimen and request form. This check may be performed prior to centrifugation by the Central Sort department.

Turnaround time

4 weeks