A case of zoledronate-induced tubulointerstitial nephritis with Fanconi syndrome.

A 54-year-old female underwent mastectomy in 1992 for breast cancer, but later developed liver metastasis in 2004, which was treated with docetaxel for a short period, and then discontinued due to nausea. Bone metastasis diagnosed in 2005 was treated with the combination of trastuzumab and zoledronate (4 mg/month) as well as radiotherapy. Progressive hypokalemia and renal dysfunction were observed since 2006 and the patient was admitted to our department in 2009 for further management of hypokalemia (serum potassium 2.2 mEq/L) and renal dysfunction (serum creatinine: 2.0 mg/dL). Accelerated potassium excretion and metabolic acidosis of the normal anion gap were observed and the patient was diagnosed with hypokalemia associated with renal tubular acidosis. Moreover, development of glucosuria, hypophosphatemia, hyperphosphaturia, hyponatremia, hypouricemia, and high β2-microglobulin/NAG, together with histopathological findings compatible with renal tubular injury/interstitial nephritis on renal biopsy, allowed the diagnosis of Fanconi syndrome. Because the syndrome was considered to be induced by zoledronate based on the onset and clinical course, we stopped zoledronate. However, she continued the ambulatory chemotherapy with trastuzumab. This resulted in immediate and sustained improvement in renal function. To our knowledge, this is the first report of zoledronate-induced interstitial nephritis-Fanconi syndrome and indicates the close monitoring of renal function to avoid potential nephrotoxicity of zoledronate.