Decrease in serum tacrolimus level and rise in serum creatinine under late addition of cinacalcet in a renal...

Cinacalcet is a calcimimetic drug that has been approved for treatment of secondary and tertiary hyperparathyroidism in patients with renal failure requiring renal replacement therapy. A few cases of successful treatment in renal transplant patients immunosuppressed with cyclosporine have been reported. Herein we have reported the case of a 48-year-old renal transplant recipient presenting with secondary hypercalcemic hyperparathyroidism (parathyroid hormone [PTH] 896 pg/mL; total calcium, up to 3.3 mmol/L) under immunosuppressive therapy with tacrolimus. Owing to substantial comorbidity and a high operative risk, we decided to initiate a therapeutic trial with cinacalcet. Using a daily dose of 30 mg of Cinacalcet, normal calcium levels and a mild fall in PTH levels (decline of 62 pg/mL) were achieved within the first week of treatment. At this point, we also observed a marked decrease in tacrolimus levels (from 6.3 to 2.6 mg/dL) without any change in concomitant medications. Thus, we adapted the tacrolimus dosage. Concurrent with cinacalcet therapy, there was a rise in serum creatinine levels (from 3.9 to 4.9 mg/dL before discontinuation of cinacalcet), which was not reversible after termination of 3 weeks of treatment with cinacalcet, but continued. Cinacalcet and tacrolimus are both metabolized via cytochrome P 450. The documented decrease in tacrolimus serum levels, suggested a drug-drug interaction between tacrolimus and cinacalcet. The irreversible deterioration in renal function may be attributed to nephrotoxic properties of cinacalcet, but may also indicate an acceleration of the natural course of chronic allograft nephropathy.