Therapeutic drug monitoring (TDM) of immunosuppressive drugs is used to improve the immunosuppressive effect while minimizing the toxicity related to exposition to high serum levels. Although TDM is widely used in clinical practice, a significant number of kidney transplant recipients have acute allograft rejection in the first year after transplantation. To improve the use of immunosuppressive drugs, new approaches of TDM have been developed. Monitoring drug concentrations at lymphocytes of peripheral blood is considering promising because it indicates the availability of the drug directly in the target sites of immunosuppression. The present study intends to establish the concentration profile of tacrolimus in the peripheral blood in parallel with the concentration profile inside T and B lymphocytes of peripheral blood of kidney transplant recipients, and correlates them with the expected pharmacological effects. The pharmacological effects of tacrolimus in calcineurin dependent and calcineurin independent (mitogen-activated protein kinase (MAPK) dependent) activation pathways will be assessed by measuring activated nuclear factor of activated T cells (NFAT) and p38, respectively, by flow cytometry. The expression of interleukin (IL) - 2 and IL-10 by T and B lymphocytes, respectively, will be also used to monitoring the pharmacodynamic effects of tacrolimus.

Inclusion Criteria:

• Kidney transplant recipients with a well functioning graft (serum creatinine ≤ 2,0 mg/dL at inclusion)
• Must be on tacrolimus therapy
• Must be on short term follow up time (1 to 5 months) after surgery

Exclusion Criteria:

• A concomitant second solid organ transplant
• Immunosuppression not containing tacrolimus

The study will include kidney transplant recipients followed at Hospital das Clínicas da Faculdade de Medicina da USP.