Evaluation of exercise-induced acute renal failure in renal hypouricemia using Tc-99m DTPA renography

Targeted design of a recombinant tracer for SPECT renal imaging

Rationale: A robust radiopharmaceutical has high uptake in the target and low retention in non-target tissues. However, traditional tracers for renal imaging that chemically chelate ^99mTc are excreted through the renal route with transient resident time in the kidney. Following a rational design approach, we constructed a protein-based radiotracer, designated PBT-Fc, to sequentially bind tubular neonatal Fc-receptor and subsequently proximal tubular basement membrane for its targeted sequestration in kidney parenchyma. In this process, the tracer participates in physiologic glomerular filtration and tubular reabsorption while escaping lysosomal catabolism and urinary clearance. Methods: To specifically target renal receptors in navigating the urinary passage in the kidney, we produced a recombinant fusion protein with two separate functional parts: a polybasic PBT segment derived from human Vascular Endothelial Growth Factor and Fc segment of IgG1. The chimeric fusion of PBT-Fc was labeled with radionuclide ^99mTc and tested in rodent models of kidney diseases. Planar scintigraphy and single-photon emission computerized tomography (SPECT) were performed to evaluate renal-specificity of the tracer. Results: When injected in mouse and rat, following a brief 10 - 15 min dynamic redistribution phase in circulation, ~ 95% of the [^99mTc]-PBT-Fc signal was concentrated in the kidney and lasted for hours without urinary loss or surrounding tissue activities. Long-lasting tracer signals in the kidney cortex in conjunction with SPECT greatly augmented the image quality in detecting pathological lesions in a variety of disease models, including ischemic acute kidney injury, drug-induced renal toxicity, and chronic kidney disease from renin-angiotensin system (RAS) overactivation. Conclusion: Exclusive renal retention of the recombinant radiotracer greatly facilitated static-phase signal acquisition by SPECT and achieved submillimeter spatial resolution of kidney alternations in glomerular and tubular disease models.

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.

A novel homozygous GLUT9 mutation cause recurrent exercise-induced acute renal failure and posterior reversible encephalopathy syndrome

Renal hypouricemia (RHU) is an autosomal recessive hereditary disease characterized by impaired renal urate reabsorption and subsequent profound hypouricemia. There are two types of RHU, type 1 and type 2, caused by the loss-of-function mutation of SLC22A12 and SLC2A9 genes, respectively. RHU predisposes affected people to exercise-induced acute renal failure (EIARF), posterior reversible encephalopathy syndrome (PRES) and nephrolithiasis. A Chinese patient had experienced three episodes of EIARF and one episode of PRES. The investigations showed profound hypouricemia and significantly increased renal excretion of UA. Cranial magnetic resonance imaging showed communicating hydrocephalus. Renal biopsy displayed interlobular artery intimal thickening with reduction of lumen and acute tubulointerstitial injury. The mutational analysis revealed a homozygous splice-site mutation in the SLC2A9 gene encoding glucose transporter 9. The patient was diagnosed as RHU type 2 caused by a loss-of-function mutation of the SLC2A9 gene. Consequently, he was strictly prohibited from strenuous exercise. During the 5-year follow-up, EIARF and PRES never recurred. Strenuous exercise may induce systemic (including renal and cerebrovascular) vasoconstriction eventually resulting in EIARF and PRES in patients with RHU. To our knowledge, this is the first report of a homozygous splice-site mutation in the SLC2A9 gene, renal arteriolar chronic lesion, concurrence of RHU and communicating hydrocephalus.

Clinical characteristics of acute renal failure with severe loin pain and patchy renal vasoconstriction

BACKGROUND

Acute renal failure (ARF) with severe loin pain and patchy renal vasoconstriction (PRV) is a syndrome presenting with sudden loin pain after anaerobic exercise. We aimed to investigate the clinical characteristics and the efficacy of diagnostic imaging studies of patients with this syndrome.

METHODS

We retrospectively selected 17 patients with ARF accompanied by loin or abdominal pain who showed multiple patchy wedge-shaped delayed contrast enhancements on a computerized tomography scan. Information about the clinical characteristics, including the nature of pain and combined symptoms, suspected causes, such as exercise, drug or alcohol intake, and renal hypouricemia, and the results of laboratory and imaging tests were gathered.

RESULTS

The mean age of patients with episodes of ARF accompanied by loin pain was 23.0±6.5 (range 16-35) years old. Pain was mainly located in the loin (70.6%) or abdominal area (76.5%) and continued for approximately 3.5±4.0 days. Exercise was suspected as a primary cause of disease in 12 (70.6%) patients. Maximal serum creatinine was 5.42±3.16 (1.4-12.1) mg/dL 3.1±1.8 (1-7) days after the onset of pain. The peak level of serum uric acid was 9.41±2.91 (6.0-15.8) mg/dL. All of the patients recovered to near-normal renal function, and one patient showed hypouricemia after recovery.

CONCLUSION

ARF with severe loin pain and PRV can present with loin or abdominal pain, even without a history of anaerobic exercise. Careful history taking and appropriate imaging studies are critical in the diagnosis and management of this syndrome.

[Hereditary renal hypouricemia in a Caucasian patient: a case report and review of the literature]

Hereditary renal hypouricemia is characterized by a decreased serum uric acid, a uric acid fractional excretion above normal and the absence of another cause of hyperuricosuric hypouricemia. This pathology, generally caused by a mutation of urate renal transporter URAT1, is relatively common in Asia, but occurs very infrequently in Caucasian populations. The disease's association with exercise-induced acute renal failure is well known. This article reports the case of a 47-year-old man of Italian origin who was diagnosed with hereditary renal hypouricemia after an episode of exercise-induced acute renal failure. Molecular analysis of SLC22A12 encoding URAT1 for renal hypouricemia using peripheral blood genomic DNA of the patient was performed. Single-strand conformation polymorphism screening, amplification, and direct sequencing of SLC22A12 revealed no mutation in this patient. This suggests that another gene can be involved in this disease.