Introduction: Mammalian target of rapamycin (mTOR) inhibitors are the major alternative immunosuppressive treatment to prevent the adverse effects of calcineurin inhibitor (CNI) based immunosuppressive regimen in kidney transplant (KT) recipients but the timing of conversion to mTOR inhibitors is still a controversial clinical decision. In our study, we aimed to compare the effects of early and late conversion to mTOR inhibitors in our KT recipient population to contribute the data of timing of mTOR inhibitor initiation in patients who were planned to use CNI-free immunosuppressive regimens.
Materials and Methods: 108 kidney recipients who were converted to mTOR inhibitor based immunosuppressive regimen from CNI-based immunosuppressive regimen were enrolled to our study according to their conversion time. KT recipients who were converted to mTOR inhibitor regimen in the first 12 months after KT were in the early conversion group (Group 1)  and KT recipients who were converted to mTOR inhibitor regimen after the first year following KT were in the late conversion group (Group 2). The demographic, clinical and laboratory values of the patients were recorded and patients in both groups were followed-up for 24 months after conversion. Group 1 and Group 2 were compared according to their basic laboratory values, clinical situation and also graft kidney function.
Results: 32 patients who were converted to mTOR inhibitor based therapy in the first year after KT (Group 1) were compared to 76 patients who were converted to mTOR inhibitor therapy after the first year (Group 2).  After 2 years of follow-up of all patients after conversion; serum creatinine levels and 24 hours proteinuria were significantly lower and creatinine clearance and serum high-density lipoprotein levels were significantly higher in the early conversion group compared to late conversion group. Other laboratory values showed no significant difference between two groups. No graft loss was obtained in the 2 years follow-up.
Conclusion: CNI-based immunosuppressive regimens after KT are still the first choice in the very early period but long-term results are controversial. Until recently, the major adverse effects of CNIs that result with conversion to mTOR inhibitors are malignancy, chronic allograft nephropathy, viral infections and metabolic disturbances. Current studies showed that, early conversion to mTOR inhibitor-based regimens are clearly associated with preserving short and long graft kidney function in the KT recipients. In our study, we determined the early conversion in the first year after KT showed better results compared to late conversion. We suggest that elective conversion to mTOR inhibitors in patients with stable kidney function rather than salvage conversion in patients with worsening kidney function would help maintaining graft kidney and patient survival.