Uric Acid and Urate in Urolithiasis: The Innocent Bystander, Instigator, and Perpetrator

Association of urinary excretion rates of uric acid with biomarkers of kidney injury in patients with advanced chronic kidney disease

BACKGROUND

The potential influence of hyperuricemia on the genesis and progression of chronic kidney disease (CKD) remains controversial. In general, the correlation between blood levels of uric acid (UA) and the rate of progression of CKD is considered to be modest, if any, and the results of relevant trials oriented to disclose the effect of urate-lowering therapies on this outcome have been disappointing. Urinary excretion rates of UA could reflect more accurately the potential consequences of urate-related kidney injury.

METHOD

Using a cross-sectional design, we investigated the correlation between different estimators of the rates of urinary excretion of UA (total 24-hour excretion, mean urinary concentration, renal clearance and fractional excretion)(main study variables), on one side, and urinary levels of selected biomarkers of kidney injury and CKD progression (DKK3, KIM1, NGAL, interleukin 1b and MCP)(main outcome variables), in 120 patients with advanced CKD (mean glomerular filtration rate 21.5 mL/minute). We took into consideration essential demographic, clinical and analytic variables with a potential confounding effect on the explored correlations (control variables). Spearman's rho correlation and nonlinear generalized additive regression models (GAM) with p-splines smoothers were used for statistical analysis.

MAIN RESULTS

Multivariate analysis disclosed independent correlations between urinary UA concentrations, clearances and fractional excretion rates (but not plasma UA or total 24-hour excretion rates of UA), on one side, and the scrutinized markers. These correlations were more consistent for DKK3 and NGAL than for the other biomarkers. Glomerular filtration rate, proteinuria and treatment with statins or RAA axis antagonists were other independent correlates of the main outcome variables.

CONCLUSIONS

Our results support the hypothesis that urinary excretion rates of UA may represent a more accurate marker of UA-related kidney injury than plasma levels of this metabolite, in patients with advanced stages of CKD. Further, longitudinal studies will be necessary, to disclose the clinical significance of these findings.

Overview of the pharmacokinetics and pharmacodynamics of URAT1 inhibitors for the treatment of hyperuricemia and gout

INTRODUCTION

Hyperuricemia is a common metabolic disease, which is a risk factor for gouty arthritis and ureteral stones and may also lead to cardiovascular and chronic kidney disease (CDK). Therefore, hyperuricemia should be treated early. Xanthine oxidase inhibitors (XOIs) and uricosuric agents (UAs), which target uric acid, are two types of medications that are used to treat gout and hyperuricemia. XOIs stop the body from producing excessive uric acid, while UAs eliminate it rapidly via the kidneys. Urate transporter 1 (URAT1) belongs to the organic anion transporter family (OAT) and is specifically localized to the apical membrane of the epithelial cells of proximal tubules. Unlike other organic anion transporter family members, URAT1 identifies and transports organic anions that are primarily responsible for urate transport.

AREAS COVERED

This article reviews the pharmacokinetics and pharmacodynamics of the existing URAT1 inhibitors to serve as a reference for subsequent drug studies.

EXPERT OPINION

The URAT1 inhibitors that are currently used as clinical drugs mainly include dotinurad, benzbromarone, and probenecid. Results indicate that RDEA3170 may be the most promising inhibitor, in addition to SHR4640, URC-102, and MBX-102, which are in the early stages of development.

Increased risk of chronic kidney disease in uric acid stone formers with high neutrophil-to-lymphocyte ratio

Urolithiasis is associated with an increased risk of chronic kidney disease (CKD), irrespective of stone compositions. Chronic inflammation is an important factor for CKD progression. Neutrophil-to-lymphocyte ratio (NLR) has been recognized as a reliable biomarker of inflammation, yet its use in predicting renal deterioration in patients with urolithiasis remains limited. We aimed to explore whether the combination of stone composition and NLR could be useful as a predictor for CKD risk. A total of 336 stone formers with at least one stone submission for analysis were enrolled in the retrospective study. Stones were classified into uric acid and calcium groups. Renal functions were assessed at least one month after stone treatment. Uric acid stone formers had significantly lower estimated glomerular filtration rate (eGFR) compared with calcium stone formers (p < 0.001). NLR was significantly higher in uric acid stone formers (p = 0.005), and a significantly negative correlation (p < 0.001) between NLR and eGFR had been observed only in uric acid stone group. Univariate and multivariate logistic regression analyses showed that higher proportion of uric acid stone composition and higher NLR were both significantly associated with CKD risks. A nomogram integrating independent predictors was generated for CKD prediction, yielding an AUC of 0.811 (0.764-0.858). In conclusion, our study demonstrated that stone formers with higher proportion of uric acid composition and higher NLR levels were associated with higher CKD risk.

Association of serum bicarbonate with the development of kidney stones in patients with chronic kidney disease: a retrospective cohort study

BACKGROUND

Epidemiological studies demonstrate an association between kidney stones and risk of chronic kidney disease (CKD) and CKD progression. Metabolic acidosis, as a consequence of CKD, results in a reduced urine pH which promotes the formation of some types of kidney stones and inhibits the formation of others. While metabolic acidosis is a risk factor for CKD progression, the association of serum bicarbonate with risk of incident kidney stones is not well understood.

METHODS

We used an Integrated Claims-Clinical dataset of US patients to generate a cohort of patients with non-dialysis-dependent CKD with two serum bicarbonate values of 12 to <22 mmol/L (metabolic acidosis) or 22 to <30 mmol/L (normal serum bicarbonate). Primary exposure variables were baseline serum bicarbonate and change in serum bicarbonate over time. Cox proportional hazards models evaluated time to first occurrence of kidney stones during a median 3.2-year follow-up.

RESULTS

A total of 142 884 patients qualified for the study cohort. Patients with metabolic acidosis experienced post-index date kidney stones at greater frequency than patients with normal serum bicarbonate at the index date (12.0% vs 9.5%, P < .0001). Both lower baseline serum bicarbonate [hazard ratio (HR) 1.047; 95% confidence interval (CI) 1.036-1.057] and decreasing serum bicarbonate over time (HR 1.034; 95% CI 1.026-1.043) were associated with increased risk of kidney stone development.

CONCLUSIONS

Metabolic acidosis was associated with a higher incidence of kidney stones and shorter time to incident stone formation in patients with CKD. Future studies may investigate the role of correcting metabolic acidosis to prevent stone formation.

Type 4 renal tubular acidosis and uric acid nephrolithiasis: two faces of the same coin?

PURPOSE OF REVIEW

The present review summarizes findings of recent studies examining the epidemiology, pathophysiology, and treatment of type 4 renal tubular acidosis (RTA) and uric acid nephrolithiasis, two conditions characterized by an abnormally acidic urine.

RECENT FINDINGS

Both type 4 RTA and uric acid nephrolithiasis disproportionately occur in patients with type 2 diabetes and/or chronic kidney disease. Biochemically, both conditions are associated with reduced renal ammonium excretion resulting in impaired urinary buffering and low urine pH. Reduced ammoniagenesis is postulated to result from hyperkalemia in type 4 RTA and from insulin resistance and fat accumulation in the renal proximal tubule in uric acid nephrolithiasis. The typical biochemical findings of hyperkalemia and systemic acidosis of type 4 RTA are rarely reported in uric acid stone formers. Additional clinical differences between the two conditions include findings of higher urinary uric acid excretion and consequent urinary uric acid supersaturation in uric acid stone formers but not in type 4 RTA.

SUMMARY

Type 4 RTA and uric acid nephrolithiasis share several epidemiological, clinical, and biochemical features. Although both conditions may be manifestations of diabetes mellitus and thus have a large at-risk population, the means to the shared biochemical finding of overly acidic urine are different. This difference in pathophysiology may explain the dissimilarity in the prevalence of kidney stone formation.

Epidemiology of Kidney Stones

In the past two decades, major breakthroughs that improve our understanding of the pathophysiology and therapy of kidney stones (KS) have been lacking. The disease continues to be challenging for patients, physicians, and healthcare systems alike. In this context, epidemiological studies are striving to elucidate the worldwide changes in the patterns and the burden of the disease and identify modifiable risk factors that contribute to the development of kidney stones. Our expanding knowledge of the epidemiology of kidney stones is of paramount importance and largely upgrades the modern management of the disease. In this paper, we review the variables affecting prevalence and incidence, including age, gender, race, ethnicity, occupation, climate, geography, systemic diseases, diabetes, vascular disease, chronic kidney disease, and dietary risk factors relevant to kidney stones.

Urinary Risk Profile, Impact of Diet, and Risk of Calcium Oxalate Urolithiasis in Idiopathic Uric Acid Stone Disease

The role of diet in the pathogenesis of uric acid (UA) nephrolithiasis is incompletely understood. This study investigated the effect of dietary intervention on the risk of UA stone formation under standardized conditions. Twenty patients with idiopathic UA stone disease were included in the study. Dietary intake and 24 h urinary parameters were collected on the usual diet of the patients and a standardized balanced mixed diet. Although urinary UA excretion did not change, the relative supersaturation of UA decreased significantly by 47% under the balanced diet primarily due to the significant increase in urine volume and pH. Urinary pH was below 5.8 in 85% of patients under the usual diet, and in 60% of patients under the balanced diet. The supersaturation of calcium oxalate declined significantly under the balanced diet due to the significant decrease in urinary calcium and oxalate excretion and the increase in urine volume. Dietary intervention is a key component in the management of UA nephrolithiasis. Urinary calcium and oxalate excretion should also be monitored in patients with pure UA calculi to reduce the risk of mixed stone formation with calcium oxalate. Lower urinary pH in UA stone patients can only be partially explained by diet.

Uric Acid in Parkinson's Disease: What Is the Connection?

Numerous studies have linked Parkinson's disease (PD) with low levels of uric acid (UA). Low UA has been associated with the risk of developing PD, and its progression and severity. The biological mechanisms underlying these relationships have never been firmly established. The most frequently proposed mechanism is that UA is an antioxidant. Low UA is thought to predispose to oxidative stress, which contributes to dopamine neuron degeneration, and leads to initial appearance of symptoms of PD and its worsening over time. Several recent studies have questioned this explanation. In this review, we describe the biology of UA, its many links with PD, evidence regarding UA as an antioxidant, and we question whether UA causes PD or contributes to its progression. We also address the possibility that something about PD causes low UA (reverse causation) or that low UA is a biomarker of some other more relevant mechanism in PD. We hope the evidence provided here will stimulate additional studies to better understand the links between UA and PD. Elucidating these mechanisms remains important, because they may provide new insights into the pathogenesis of PD or novel approaches to treatments. © 2022 International Parkinson and Movement Disorder Society.

Correlation of Glucose and Lipid Metabolism Levels and Serum Uric Acid Levels with Diabetic Retinopathy in Type 2 Diabetic Mellitus Patients

Objective. The aim of this study was to observe the association between the development of diabetic retinopathy (DR) in type 2 diabetes mellitus (T2DM) and the levels of glucose and lipid metabolism and serum uric acid (SUA) levels. Methods. A retrospective analysis was performed on 97 patients with T2DM who were admitted to our endocrinology department from June 2019 to April 2021 with complete data; the patients were divided into DR and no DR groups (NDR) according to the presence or absence of DR. Their clinical history and biochemical test indexes were collected, and the fundus was examined by fundus photography and the fundoscopic examination method, and the vascular diameter was measured by using a computer software. All clinical data, medical history, and biochemical test indexes were compared between the two groups, and logistic regression was used to analyze the risk factors of DR. Results. The duration of DM disease, fasting blood glucose (FBG), glycosylated hemoglobin, type A1C (HbA1c) levels, cholesterol (TC), triacylglycerol (TG), low-density lipoprotein cholesterol (LDL-C), and SUA levels were higher in the DR group than those in the NDR group, and the differences were significant ( $P<0.05$ ). The difference between the NDR group and the DR group in terms of gender, age, BMI, DBP, SBP, family history of DM, FINS, and HDL-C levels was not significant ( $P>0.05$ ). The results of multifactorial analysis showed that the four variables of DM duration, HbA1c, TG, and SUA were still risk factors for the development of DR ( $P<0.05$ ). Further receiver operating characteristic (ROC) analysis showed that the areas under the curves (AUCs) for the duration of DM disease, HbA1c, TG, and SUA to predict the occurrence of DR were 0.740 (95% CI 0.639–0.841), 0.767 (95% 0.672–0.862), 0.721 (95% CI 0.617–0.826), and 0.693 (95% CI 0.588∼0.797), respectively. Conclusion. The lesions of DR in T2DM patients have a close relationship with the course of DM, HbA1c, TG, and SUA, and the course of DM, HbA1c, TG, and SUA has a good predictive value for the occurrence of DR.

Crystals and crystalloids in cytopathology: Incidence and importance

Many crystals and crystal-like structures may be encountered in cytopathology practice and can represent both beautiful novelties and diagnostic aids. The authors present an organ-specific review of the published literature on crystals combined with personal experiences. The purpose is not only to serve as a reference guide by highlighting the clinical and morphologic features of crystals, crystalloids, and crystal-like structures but also to review their significance and to offer reporting strategies in cases that bear management implications.

Uric Acid and Oxidative Stress-Relationship with Cardiovascular, Metabolic, and Renal Impairment

Background: The connection between uric acid (UA) and renal impairment is well known due to the urate capacity to precipitate within the tubules or extra-renal system. Emerging studies allege a new hypothesis concerning UA and renal impairment involving a pro-inflammatory status, endothelial dysfunction, and excessive activation of renin–angiotensin–aldosterone system (RAAS). Additionally, hyperuricemia associated with oxidative stress is incriminated in DNA damage, oxidations, inflammatory cytokine production, and even cell apoptosis. There is also increasing evidence regarding the association of hyperuricemia with chronic kidney disease (CKD), cardiovascular disease, and metabolic syndrome or diabetes mellitus. Conclusions: Important aspects need to be clarified regarding hyperuricemia predisposition to oxidative stress and its effects in order to initiate the proper treatment to determine the optimal maintenance of UA level, improving patients’ long-term prognosis and their quality of life.

Serum and urine uric acid level may have different predictive value for urinary stone composition: a retrospective cohort study of 718 patients in Chinese population

PURPOSE

We launched a retrospective cohort study to explore the interactions among serum uric acid (UA), urine UA, and stone types.

METHODS

Clinical characteristics of urolithiasis patients in Beijing Tsinghua Changgung Hospital from October 2015 to August 2017 were retrospectively collected. Participants were categorized according to the quartiles of SUA and UUA respectively. Logistic regression model was built to identify the relationship between stone composition and UA level. Cubic spline was fitted to explore the correlation between 24-h urine UA and serum UA.

RESULTS

718 hospitalized patients (51.1 ± 14.3 years, male 63.4%) with urinary calculi were included. Higher serum UA is associated with male, alcohol use, multiple serum and urine electrolytes (e.g. potassium, chloride, calcium, phosphorus), and lower estimated glomerular filtration rate. The risk of UA stone and carbonate apatite stone was associated with serum UA while the risk of calcium oxalate (CaOx) stone and ammonium magnesium hexahydrate (AMH) was dependent on urine UA. In the unadjusted model (Model 1), higher risks of UA stones were observed in the third quartile (OR 3.26, 95% CIs 1.63-6.53, P = 0.001) and the fourth quartile (OR 3.55, 95% CIs 1.78-7.08, P < 0.001) of serum UA compared with the first quartile. The risks of carbonate apatite stone were lowered in the third (OR 0.48, 95% CIs 0.31-0.73, P = 0.001) and fourth quartile (OR 0.40, 95% CIs 0.42-0.98, P = 0.042) of serum UA. The risk of CaOx stone was increased in the fourth quartile (OR 2.14, 95% CIs 1.15-3.99, P = 0.017) while the risk of AMH stone was decreased in the third (OR 0.46, 95% CIs 0.22-0.94, P = 0.034) and fourth quartile (OR 0.35, 95% CIs 0.16-0.78, P = 0.009) of urine UA. The elevated risks of UA stones in high levels of serum UA were demonstrated in the adjusted model (Model 2). An M-shaped association was found between serum UA and urine UA in our population.

CONCLUSIONS

Serum UA and urine UA might cast different impact on urinary calculus composition. Proper control of the parameters should be considered based on different predisposing factors in individual patients.