**6.2. Mechanism of action of CNI**

The target protein of both tacrolimus and cyclosporine is CNI which is a calcium-dependent phosphatase. This enzyme is ubiquitously expressed and associates with calmodulin to form an active enzyme complex that dephosphorylates and activates the transcription factor, nuclear factor of activated T cells (NFAT), after T-cell receptor signaling. Dephosphorylated NFAT can then translocate to the nucleus and initiate transcription of several key cytokine genes (e.g., IL-2, IL-4, TNF- and IFN-γ). Blockade of calcineurin leads to decreased NFAT activity and transcription of critical cytokines affecting T cell function, activation and prolif‐ eration. Both these drugs bind to cytoplasmic proteins to mediate their action. Cyclosporin binds to cyclophilin, while tacrolimus binds to FKBP-12.

#### **6.3. Clinical use**

Recommended starting dose for tacrolimus is 0.15-0.30 mg/kg, while that of cyclosporine is 6-10 mg/kg. For both drugs, total dose is administered in two divided doses. Intravenous dosing is 1/3rd of the total oral dose, administered as a continuous 24 hour infusion. Patient variability in drug kinetics can be attributed to the heterogeneity of metabolic activity of the enzyme responsible for calcineurin metabolism; the liver enzyme, CYP3A. In general, African Americans may require higher doses of tacrolimus, whereas patients with liver disease and elderly patients may need lower doses. Because of wide patient variability in metabolism, therapeutic drug monitoring is routinely performed with these agents. Most centers check a 12 hr trough level prior to the morning dose. More sophisticated monitoring with area under the curve (AUC) measurements is available but is not routinely performed because of technical and clinical difficulties. During the first 3 months post transplant, our center aims for a 12 hr tacrolimus trough in the range of 8-12 ng/dl, followed by a level of 6-10 ng/dl for months 4 to 12. After the first year, we reduce tacrolimus dosing aiming to achieve maintenance levels of of 4-6 ng/dl. For cyclosporine, a 12 hour trough of 250-350 mg/dl are maintained for the first few months and then target levels are gradually decreased. After the first year post transplan‐ tation the usual cyclosporine trough is between 100-200mg/dl. Targeted drug ranges vary across centers and are driven by center protocols that take into account patient risk, type of induction used and the strength of other agents used for maintenance.
