Tacrolimus

Tacrolimus is a macrolide immunosuppressant and calcineurin inhibitor used primarily to prevent and treat immune-mediated injury after solid-organ transplantation. It is central to maintenance immunosuppression in kidney, liver, and lung transplantation, where it is commonly combined with agents such as mycophenolate mofetil and glucocorticoids including prednisone. Tacrolimus may also be used in selected immune-mediated disorders and as a topical Therapy for inflammatory skin disease.

Tacrolimus exerts its principal effect by binding to the intracellular immunophilin FKBP12. The tacrolimus–FKBP12 complex inhibits calcineurin, a calcium- and calmodulin-dependent phosphatase required for activation of nuclear factor of Activated T Cells (NFAT). Calcineurin inhibition reduces NFAT-dependent transcription of interleukin-2 and other cytokines, thereby suppressing T-cell activation and proliferation. This mechanism distinguishes tacrolimus from cyclosporine while placing both drugs in the calcineurin-inhibitor class.

Systemic tacrolimus has a narrow therapeutic window and substantial pharmacokinetic variability. Dosing is therefore generally individualized using whole-blood trough-concentration monitoring, together with assessment of renal function, liver function, electrolytes, infection risk, and evidence of graft rejection or drug toxicity. Important toxicities include nephrotoxicity, neurotoxicity, hypertension, hyperglycemia, and increased susceptibility to infection and malignancy. Tacrolimus is metabolized predominantly through CYP3A enzymes and is transported by p-glycoprotein; consequently, its exposure can be altered by numerous medicines and dietary substances. Genetic variation affecting tacrolimus metabolism, particularly in CYP3A5, is being investigated as a basis for pre-emptive dose selection.