The primary outcome is change in urinary NGAL expression with conversion from IR tacrolimus to tacrolimus XR. In aim 1, changes in urinary biomarkers of tubular injury at 4 weeks after conversion to tacrolimus XR with stable trough levels will be assessed. These changes will be assessed by CYP3A5 expressor category that will be determined by genotyping of a single gene for variants. The changes in GFR using the creatinine-cystatin C CKD-EPI equation will be assessed. In aim 2, the rate of rejection as defined as treated rejection within the last 30 days for any grade \> 1R or AMR or acute graft dysfunction (LV ejection fraction drop \> 10%) will be looked. The cardiac allograft vasculopathy based on coronary angiography +/- IVUS and right heart catheterization hemodynamics at baseline and 1 year post conversion will also be evaluated. In addition to changes in cardiac function, the changes in blood pressure, serum glucose, and cholesterol in the first year after conversion will be assessed.
