Tacrolimus Measurement: Building a Better Immunoassay

Tacrolimus (formerly known as FK506) is a macrolide immunosuppressant that is used for primary immuno-suppression after organ transplantation (1). The land-mark work of Starzl at the University of Pittsburgh paved the way for the clinical use of tacrolimus as a primary immunosuppressant, leading to a shift away from cyclosporine use (2–4). Today, tacrolimus is used in themajority of transplant patients (5). Tacroli-mus binds to several cytosolic immunophilins of im-portance; binding to the FK506 binding protein leads to the formation of a complex that interacts with calcium-dependent calcineurin calmodulin transloca-tion pathways that result in prevention of T-cell activa-tion (6). Monitoring blood tacrolimus concentrations is essential in assessing organ rejection and toxicity be