Fumaric acid ester-induced renal Fanconi syndrome: evidence of mitochondrial toxicity.

Background: Fumaric acid esters (FAEs) are used to treat chronic plaque psoriasis. Fumarate is a crucial component of the Krebs cycle and mitochondrial function. Proximal tubule cells have high energy demands and rely on aerobic respiration. Proximal tubular dysfunction can cause renal Fanconi syndrome and acute kidney injury. We sought to better understand the mechanism for this in the context of FAE therapy. Methods: We describe a case series of 10 patients with FAE-associated Fanconi syndrome. Patients were diagnosed and managed at a tertiary renal tubular disorder clinic, with examination of serum and urine biochemistry. Five patients had a renal biopsy with examination of the specimens by electron microscopy. Results: The median age was 36.5 years [interquartile range (IQR) 32.25-54.25]. The median dose of FAE was 720 mg/day (IQR 390-720). There was low molecular weight proteinuria: the median urinary retinol-binding protein (RBP) at presentation was 8385 μg/mL (IQR 2793-14 600) and the RBP:creatinine ratio was 710 (IQR 390-2415). All patients had hyperphosphaturia [median fractional excretion of phosphate 24.2% (IQR 20.8-26.9), normal range <20%] as well as relative hypophosphataemia, with a median serum phosphate concentration of 0.93 mmol/L (IQR 0.83-0.97). Renal histology showed proximal tubular damage and abnormal mitochondrial morphology. Two patients had a favourable biochemical response to treatment with probenecid. Conclusions: We document for the first time that FAE-associated renal Fanconi syndrome is associated with mitochondrial damage visible on electron microscopy. This effect may be ameliorated by antagonism of the organic anion transporter with probenecid.