A case of exercise-induced acute renal failure in a patient with idiopathic renal hypouricemia developed during antihypertensive therapy with losartan and trichlormethiazide

Hypouricemia: what the practicing rheumatologist should know about this condition

We presented an update in the field of hypouricemia, which is defined as a serum urate concentration of < 2 mg/dL (119 μmol/L), for the practicing rheumatologist, who usually is the consulting physician in cases of disorders of urate metabolism. We performed a narrative review through a literature search for original and review articles in the field of human hypouricemia published between January 1950 and July 2018. We divided the etiology of hypouricemia into two main categories: those associated with a decrease in urate production and those promoting the elimination of urate via the kidneys. The most common conditions associated with these categories are discussed. Furthermore, the etiology of hypouricemia may be associated with certain medications prescribed by the practicing rheumatologists, such as the following: urate-lowering drugs (allopurinol and febuxostat); recombinant uricase (pegloticase); uricosuric agents (probenecid, benzbromarone); urate transporter URAT1 inhibitor (lesinurad); angiotensin II receptor blocker (losartan); fenofibrate; high-dose trimethoprim-sulfamethoxazole; some NSAID; and high-dose salicylate therapy. The rheumatologist is considered an expert in the metabolism of urate and its associated pathological conditions. Therefore, specialists must recognize hypouricemia as a biomarker of various pathological and potentially harmful conditions, highlighting the importance of conducting a deeper clinical investigation to reach a more accurate diagnosis and treatment.

Lesinurad: what the nephrologist should know

Lesinurad is an oral inhibitor of the monocarboxylic/urate transporter URAT1 encoded by the SLC22A12 gene. Market authorization was granted in February 2016 in Europe and December 2015 in the USA. As a potentially nephrotoxic uricosuric drug acting on the kidney, nephrologists should become familiar with its indications and safety profile. The approved indication is treatment of gout in combination with a xanthine oxidase (XO) inhibitor in adult patients who have not achieved target serum uric acid levels with an XO inhibitor alone. It is not indicated for asymptomatic hyperuricaemia or for patients with estimated creatinine clearance <45 mL/min. The only authorized daily dose is 200 mg and cannot be exceeded because of the nephrotoxicity risk. Nephrotoxicity is thought to be related to uricosuria. At the 200 mg/day dose, serum creatinine more than doubled in 1.8% of lesinurad patients (versus 0% in placebo) and in 11% of these it was not reversible. In addition, it is subject to a risk management plan given the potential association with cardiovascular events. In randomized clinical trials, the association of lesinurad with either allopurinol or febuxostat achieved a greater reduction in serum uric acid (∼1 mg/dL lower) than the XO inhibitors alone, and this allowed the serum uric acid target to be met in a higher proportion of patients, which was the primary endpoint. However, no clinical differences were observed in gout flares or tophi, although these were not the primary endpoints.

A case of renal hypouricemia and a G774A gene mutation causing acute renal injury that was improved by hemodialysis

A 16-year-old man came to our hospital complaining of loin pain after a middle-distance race. Following admission, his renal dysfunction worsened rapidly, requiring several hemodialysis sessions. A renal biopsy showed no change in the glomeruli, although interstitial edema was observed. Following the recovery of renal function, we confirmed that his uric acid level was abnormally low and urate clearance was abnormality high. Gene analysis showed that he had a G774A mutation which dominated the SLC22A12 gene encoding the urate transporter 1.

Clinical and functional characterization of URAT1 variants

Idiopathic renal hypouricaemia is an inherited form of hypouricaemia, associated with abnormal renal handling of uric acid. There is excessive urinary wasting of uric acid resulting in hypouricaemia. Patients may be asymptomatic, but the persistent urinary abnormalities may manifest as renal stone disease, and hypouricaemia may manifest as exercise induced acute kidney injury. Here we have identified Macedonian and British patients with hypouricaemia, who presented with a variety of renal symptoms and signs including renal stone disease, hematuria, pyelonephritis and nephrocalcinosis. We have identified heterozygous missense mutations in SLC22A12 encoding the urate transporter protein URAT1 and correlate these genetic findings with functional characterization. Urate handling was determined using uptake experiments in HEK293 cells. This data highlights the importance of the URAT1 renal urate transporter in determining serum urate concentrations and the clinical phenotypes, including nephrolithiasis, that should prompt the clinician to suspect an inherited form of renal hypouricaemia.

Oxidative imbalance in idiopathic renal hypouricemia

An important complication of idiopathic renal hypouricemia is exercise-induced acute renal failure (ARF). The most plausible explanation for this complication is that decreased antioxidant potential leads to kidney injury by reactive oxygen species (ROS). We demonstrated this oxidative imbalance by a concomitant assessment of ROS production and antioxidant system capability in a 15- year-old girl with idiopathic renal hypouricemia caused by a mutation in the urate transporter (URAT1) gene. Her serum level of ROS increased with decreasing antioxidant potential capacity soon after the initiation of anaerobic stress due to treadmill exercise. Thereafter, serum levels of ROS and antioxidant potential showed a parallel course, returning to the baseline values at 240 min after exercise. Some patients with idiopathic renal hypouricemia demonstrate oxidative imbalance soon after exercise with a predisposition to exercise-induced acute renal failure. Antioxidant properties may alter this imbalance by augmenting the antioxidant activity.

Exercise acutely increases renal transit time of Tc-99m mercaptoacetyltriglycine (MAG3) in a post-liver transplant patient

This case demonstrates the effect of exercise on the clearance of Tc-99m MAG3 in a patient with renal insufficiency status post-liver transplant. Even after furosemide administration, the tracer was retained in the kidneys after exercise. This is in contrast to normal clearance demonstrated on a baseline study performed 3 days previously.

Serum uric acid-lowering therapies: Where are we heading in management of hyperuricemia and the potential role of uricase

Although allopurinol has been available for approximately 50 years, hyperuricemia and its sequelae are not only prevalent, but the incidence and costs associated with this disorder continue to increase. However, several new therapies have been developed. Recombinant urate oxidase has been useful in the treatment of tumor lysis hyperuricemia, and pegylated urate oxidase shows promise in patients with hyperuricemia and gout. Febuxostat and Y-700 are new oral xanthine oxidase inhibitors that are in human clinical trials. Tailoring of antilipid therapy in selected hyperuricemic and hyperlipidemic patients with fenofibrate may be of benefit in lowering blood cholesterol and uric acid levels. Similarly, treatment of selected hyperuricemic patients who also are hypertensive with losartan or amlodipine may be beneficial in lowering blood pressure and hyperuricemia. Despite these advances, new treatments for hyperuricemia are needed.