Discontinuation of study due to site staffing and resources available to conduct the study

Medication non-adherence is a major risk factor for graft dysfunction and graft loss among pediatric and adult transplant recipients. Rates of non-adherence in these populations are estimated between 30 and 70%, with the highest prevalence in adolescent and young adult (AYA) transplant recipients. Treatment-related factors known to impact rates of adherence include the number of medication doses per day and the number of tablets or capsules a patient takes per day, or "pill burden". One approach to minimizing dosing frequency and pill-burden includes transitioning patients to once-daily formulations. The current literature investigating utilization of once-daily immunosuppressive regimens in the AYA kidney transplant population is limited.

One approach to minimizing dosing frequency and pill-burden includes transitioning patients to once-daily formulations.3 The current literature investigating utilization of once-daily immunosuppressive regimens in the AYA kidney transplant population is limited. Two studies have demonstrated safe and effective conversion of twice-daily tacrolimus to the Astagraf® in stable pediatric solid organ transplant recipients.4,5 Patients maintained equivalent tacrolimus exposure and experienced similar rates of rejection and graft loss in the first year post-conversion.5 To date, experience with another once-daily extended release (XR) tacrolimus product, Envarsus XR®, has not been published in the AYA population. Additionally, adherence studies evaluating a once-daily immunosuppression regimen including extended-release tacrolimus and azathioprine (which is dosed once daily as opposed to the twice daily dosing required for azathioprine's alternative mycophenolate mofetil) have not been conducted.

Of note, and even though twice-daily mycophenolate has been shown to be superior to once-daily azathioprine early post-transplant, more long-term data suggest that this advantage may not persist.6 Furthermore, a recent Cochrane review addressed the question of mycophenolate versus azathioprine as primary anti-proliferative immunosuppression for kidney transplant recipients; it concluded that "balancing the benefits and harms of the two drugs remains a major task of the transplant physician to decide which agent" is appropriate for the individual patient. 7 Moreover, once-daily immunosuppression with tacrolimus extended-release and once-daily azathioprine has been used with excellent results at a British center that focusses on AYA kidney transplant recipients.

Inclusion Criteria:

1. Age 13-22 years
2. Tanner stage 4/5
3. "Stable" kidney transplant status, as determined by the primary transplant team
4. At minimum 1 year post-kidney transplant or patients meeting all of the above inclusion criteria except 4) who have instead been transitioned to azathioprine by the primary transplant team due to intolerability of mycophenolate mofetil secondary to adverse medication effects (i.e. neutropenia, gastrointestinal intolerance etc.)

Exclusion Criteria:

1. < Tanner stage 4
2. Kidney transplant performed at an institution other than Children's Hospital Colorado, Lurie Children's Hospital of Chicago, or Cincinnati Children's Hospital
3. Recipients of dual solid organ transplants (i.e. heart kidney, liver kidney).