Tacrolimus: a new agent for the prevention of graft-versus-host disease in hematopoietic stem cell transplantation.

Tacrolimus (FK506) is a macrolide lactone with potent immunosuppressive activity 100 times that of cyclosporine by weight. The molecular mechanism of action is mediated via an inhibition of the phosphorylase activity of calcineurin by drug-immunophilin complex, resulting in the inhibition of IL-2 gene expression. There are emerging studies now showing significant efficacy of tacrolimus in GVHD prevention in both related and unrelated donor transplantation. Three multicenter randomized studies comparing tacrolimus to cyclosporine have been completed, one each in related and unrelated donor transplantation; the remaining study involved both related and unrelated donor transplantation. All three studies showed a significantly lower incidence of grade II-IV acute GVHD in patients who received tacrolimus. One study in sibling donor transplantation showed that patients with advanced disease who received tacrolimus had a poorer survival than patients who received cyclosporine, but the survival was similar in patients with non-advanced disease. The remaining two studies, one in unrelated donors and the other combining both related and unrelated donors did not show any survival difference between the tacrolimus and cyclosporine groups. In addition, this review also highlights some of the critical questions regarding the role of this agent in allogeneic stem cell transplantation: (1) the contribution of methotrexate in combination with tacrolimus; (2) the starting i.v. dose of tacrolimus; (3) the suggested whole blood level of tacrolimus and its effect on nephrotoxicity; and (4) whether tacrolimus should be used in patients with advanced malignancy. Future studies using tacrolimus in combination with other immunosuppressants, and its use in patients with advanced malignancy will be warranted.