Assessment of tubular reabsorption of phosphate as a surrogate marker for phosphate regulation in chronic kidney disease

Calcification Propensity (T50) Predicts a Rapid Decline of Renal Function in Kidney Transplant Recipients

BACKGROUND

Serum creatinine level, proteinuria, and interstitial fibrosis are predictive of renal prognosis. Fractional excretion of phosphate (FEP)/FGF23 ratio, tubular reabsorption of phosphate (TRP), serum calcification propensity (T50), and Klotho's serum level are emerging as determinants of poor kidney outcomes in CKD patients. We aimed at analysing the use of FGF23, FEP/FGF23, TRP, T50, and Klotho in predicting the rapid decline of renal function in kidney allograft recipients.

METHODS

We included 103 kidney allograft recipients in a retrospective study with a prospective follow-up of 4 years. We analysed the predictive values of FGF23, FEP/FGF23, TRP, T50, and Klotho for a rapid decline of renal function defined as a drop of eGFR > 30%.

RESULTS

During a follow-up of 4 years, 23 patients displayed a rapid decline of renal function. Tertile of FGF23 (p value = 0.17), FEP/FGF23 (p value = 0.78), TRP (p value = 0.62) and Klotho (p value = 0.31) were not associated with an increased risk of rapid decline of renal function in kidney transplant recipients. The lower tertile of T50 was significantly associated with eGFR decline >30% with a hazard ratio of 3.86 (p = 0.048) and remained significant in multivariable analysis.

CONCLUSION

T50 showed a strong association with a rapid decline of renal function in kidney allograft patients. This study underlines its role as an independent biomarker of loss of kidney function. We found no association between other phosphocalcic markers, such as FGF23, FEP/FGF23, TRP and Klotho, with a rapid decline of renal function in kidney allograft recipients.

Endogenous markers of kidney function and renal drug clearance processes of filtration, secretion, and reabsorption

The kidneys are responsible for maintaining physiologic homeostasis. The kidneys clear a variety of drugs and other substances through passive (filtration) and active processes that utilize transport proteins. Renal clearance is comprised of the processes of glomerular filtration, tubular secretion, and tubular reabsorption. Endogenous biomarkers, such as creatinine and cystatin C, are routinely used to estimate renal clearance. Understanding the contributing components of renal function and clearance, through the use of biomarkers, is necessary in elucidating the renal pharmacology of drugs and other substances. While exogenous markers of kidney function have been known for decades, several complexities have limited their usage. Several endogenous markers are being evaluated and hold promise to elucidate the individual components of kidney function that represent filtration, secretion, and reabsorption.

Association between Serum Inorganic Phosphorus Levels and Adverse Outcomes in Chronic Kidney Disease: The Fukushima CKD Cohort Study

Objective Although an association between serum inorganic phosphorus levels and a poor prognosis has been noted in dialysis patients, these associations have been insufficiently reported in non-dialysis-dependent chronic kidney disease (NDD-CKD) patients. This study attempted to determine the association between serum inorganic phosphorus levels and adverse outcomes in Japanese NDD-CKD patients. Methods We investigated the relationships between serum inorganic phosphorus levels and adverse outcomes, such as kidney events, cardiovascular events, and all-cause death, in Japanese NDD-CKD patients using longitudinal data from the Fukushima CKD Cohort Study with a median follow-up period of 2.8 years. The study evaluated 822 patients with NDD-CKD enrolled between June 2012 and July 2014. A kidney event was defined as a combination of doubling of the baseline serum creatinine or end-stage renal disease. Cox regression was performed to analyze the relationships of the quartile of the serum inorganic phosphorus with kidney events, cardiovascular events, and all-cause death. Results The frequency of kidney events per 1,000 person-years exhibited a U-shaped distribution based on serum inorganic phosphorus levels, with these levels not significantly associated with an increased risk of cardiovascular events and all-cause death. A multivariable Cox regression analysis showed an increased risk of kidney events for the highest quartile of the serum inorganic phosphorus levels (≥3.7 mg/dL) versus the second quartile (2.9-3.2 mg/dL, hazard ratio, 3.30; 95% confidence interval, 1.50-7.28; p=0.003). There were no significant associations between the serum calcium levels and adverse outcomes. Conclusion Serum inorganic phosphorus levels were associated with an increased risk of CKD progression in Japanese NDD-CKD patients.

Serum Phosphate, BMI, and Body Composition of Middle-Aged and Older Adults: A Cross-Sectional Association Analysis and Bidirectional Mendelian Randomization Study

BACKGROUND

Observational studies have reported associations between serum phosphate and BMI in specific clinical settings, but the nature of this relation in the general population is unclear.

OBJECTIVES

The aim of this study was twofold: to investigate the association between serum phosphate and BMI and body composition, as well as to explore evidence of causality through a bidirectional one-sample Mendelian randomization (MR) in the population-based Rotterdam Study (RS).

METHODS

Observational associations between phosphate (mg/dL) and BMI, lean mass, and fat percentage (fat%), estimated by DXA, were analyzed using multivariable regression models in 9202 participants aged 45-100 y from 3 RS cohorts. The role of serum leptin was examined in a subgroup of 1089 participants. For MR analyses, allele scores with 6 single-nucleotide polymorphisms (SNPs) for phosphate and 905 SNPs for BMI were constructed in 7983 participants.

RESULTS

Phosphate was inversely associated with BMI in the total population (β: -0.89; 95% CI: -1.17, -0.62), and stronger in women (β: -1.92; 95% CI: -2.20, -1.65) than in men (β: -0.37; 95% CI: -0.68, -0.06) (P-interaction < 0.05). Adjustment for leptin did not change results in men. In women, adjustment for leptin attenuated the association, but it was not abolished (β: -0.94; 95% CI: -1.45, -0.42). Phosphate was inversely associated with fat%, but not with lean mass, in both sexes. MR analyses suggested a causal effect of BMI on serum phosphate (β: -0.01; 95% CI: -0.02, 0.00) but not vice versa.

CONCLUSIONS

Serum phosphate was inversely associated with BMI and fat% in a population-based study of middle-aged and older adults, with a stronger effect in women than in men. Adjusting for leptin attenuated this relation in women only. MR results suggest a causal effect of BMI on phosphate but not vice versa. An underlying sex dimorphism in phosphate homeostasis should be further explored.

Alterations in the Mineral Bone Metabolism of Living Kidney Donors After Uni-Nephrectomy: Prospective Observational Study

We investigated the dynamic change of mineral bone metabolism and explored factors associated with the alteration of mineral bone metabolism in the living kidney donors (LKDs) after uni-nephrectomy. One-hundred forty-four prospective LKDs who underwent kidney donation between May 2016 and September 2018 were enrolled. Laboratory evaluation regarding mineral bone metabolism including intact parathyroid hormone (iPTH), renal fractional excretion of phosphate (FEPi), and technetium-99m diethylenetriaminepentaacetate (^99mTc-DTPA) scan was performed predonation and 6 months after donation. We divided donors into two groups, the low ΔFEPi and high ΔFEPi groups, according to the change of FEPi after donation, and investigated significant risk factors associated with high ΔFEPi. At 6 months after uni-nephrectomy, estimated glomerular filtration rate (eGFR) significantly declined by 30.95 ml/min/1.73 m² (p < 0.001), but the measured GFR (mGFR) of the remaining kidney by ^99mTc-DTPA scan showed significant increase. Serum phosphorus decreased (p < 0.001), whereas FEPi (13.34–20.23%, p < 0.001) and serum iPTH (38.70–52.20 pg/ml, p < 0.001) showed significant increase. In the high ΔFEPi group, the proportion of preexisting hypertension (HTN) was higher, the baseline FEPi was lower, and the percent decline in eGFR was greater. Moreover, all of these factors were independently associated with high ΔFEPi upon multivariable logistic regression analysis. LKDs showed a significant change in mineral bone metabolism after uni-nephrectomy, especially when the donors had preexisting HTN, lower baseline FEPi, and showed greater loss of kidney function. Hence, strict monitoring of the mineral bone metabolism parameters and bone health may be required for these donors.

Usefulness of 68Ga-DOTATOC PET/CT to localize the culprit tumor inducing osteomalacia

Tumor-induced osteomalacia (TIO) is an uncommon paraneoplastic syndrome presenting with sustained hypophosphatemia. Treatment of choice is removal of the tumor causing the TIO, but identification of the culprit tumor by routine imaging is challenging. This study aimed to assess the usefulness of somatostatin receptor imaging, called ⁶⁸Ga-DOTATOC PET/CT, in the management of patients with TIO. Twelve patients who were suspected of having TIO underwent ⁶⁸Ga-DOTATOC PET/CT. Lesion detectability and maximum standardized uptake value (SUV_max) were determined and retrospectively compared with the clinical/imaging surveillance and histopathologic diagnosis. The median duration of suspected TIO with hypophosphatemia was 7.8 years (range 2.1–21.0). Conventional radiologic and/or nuclear medicine images failed to identify the culprit tumors. However, ⁶⁸Ga-DOTATOC PET/CT scans showed that 8 of the 12 patients had positive lesions, suggesting the presence of focal culprit tumors. The SUV_max of positive tumors was 1.9–45.7 (median: 11.5). Six skeletal lesions and two extra-skeletal lesions were identified. Seven of the lesions were pathologically confirmed as potential culprits of TIO. Hypophosphatemia was resolved in five patients who underwent lesion excision. The ⁶⁸Ga-DOTATOC PET/CT is a useful whole-body imaging modality for the detection of causative tumors in patients with suspected TIO.

Cortisol and Phosphate Homeostasis: Cushing's Syndrome Is Associated With Reversible Hypophosphatemia

OBJECTIVES

The influence of hypercortisolism on phosphate homeostasis is relatively unknown. A few previous studies have reported on patients with Cushing's syndrome (CS) with hypophosphatemia in whom serum phosphate normalized after initiation of treatment for CS. We aimed to investigate the prevalence of hypophosphatemia in CS, the association between the degree of hypercortisolism and serum phosphate and the change in serum phosphate after remission of CS. We compared the prevalence of hypophosphatemia in CS with the prevalence in the population-based Rotterdam Study (RS).

METHODS

Patients diagnosed with CS and treated at the Department of Endocrinology of Erasmus MC in the period of 2002-2020 were included and data was collected on age at diagnosis, sex, serum phosphate, calcium and potassium levels, kidney function and BMI. Using multivariate linear regression, we analyzed the association between 24h urinary free cortisol excretion (UFC) and serum phosphate. Changes in serum phosphate and covariates were tested with a repeated measurement ANOVA, using mean levels of laboratory values for the periods before remission, and 0-14 days and 15-180 days after remission.

RESULTS

Hypophosphatemia before treatment was present in 16% of the 99 CS patients with data on serum phosphate, 24h UFC and covariates. In comparison, the prevalence of hypophosphatemia in RS was 2.0-4.2%. Linear regression showed a negative association between the level of UFC and serum phosphate at diagnosis, which remained significant after adjusting for covariates [β -0.002 (95%CI -0.004; -0.0004), p=0.021]. A subset of 24 patients had additional phosphate measurements at 0-14 days and 15-180 days after remission. In this subgroup, serum phosphate significantly increased from 1.03 ± 0.17 mmol/L prior to remission to 1.22 ± 0.25 mmol/L 15-180 days after remission (p = 0.008). BMI decreased after remission [-1.1 kg/m², (95%CI -2.09 to -0.07), p=0.037]. Other covariates did not show an equivalent change over time.

CONCLUSION

In this retrospective study, we found that 16% of patients with CS had hypophosphatemia. Moreover, serum phosphate was related to the level of cortisoluria and increased after remission of CS. Potential underlying mechanisms related to urinary phosphate excretion and possibly involving FGF23, BMI and parathyroid hormone levels should be further explored.

New Potential Biomarkers for Chronic Kidney Disease Management-A Review of the Literature

The prevalence of chronic kidney disease (CKD) is increasing worldwide, and the mortality rate continues to be unacceptably high. The biomarkers currently used in clinical practice are considered relevant when there is already significant renal impairment compromising the early use of potentially successful therapeutic interventions. More sensitive and specific biomarkers to detect CKD earlier on and improve patients' prognoses are an important unmet medical need. The aim of this review is to summarize the recent literature on new promising early CKD biomarkers of renal function, tubular lesions, endothelial dysfunction and inflammation, and on the auspicious findings from metabolomic studies in this field. Most of the studied biomarkers require further validation in large studies and in a broad range of populations in order to be implemented into routine CKD management. A panel of biomarkers, including earlier biomarkers of renal damage, seems to be a reasonable approach to be applied in clinical practice to allow earlier diagnosis and better disease characterization based on the underlying etiologic process.

The role of phosphate-containing medications and low dietary phosphorus-protein ratio in reducing intestinal phosphorus load in patients with chronic kidney disease

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is a common complication in patients experiencing end-stage renal disease (ESRD). It includes abnormalities in bone and mineral metabolism and vascular calcification. Hyperphosphatemia is a major risk factor leading to morbidity and mortality in patients with chronic kidney disease. Increased mortality has been observed in patients with ESRD, with serum phosphorus levels of >5.5 mg/dL. Therefore, control of hyperphosphatemia is a major therapeutic goal in the prevention and treatment of CKD-MBD. The treatment of hyperphosphatemia includes decreasing intestinal phosphorus load and increasing renal phosphorus removal. Decreasing the intestinal load of phosphorus plays a major role in the prevention and treatment of CKD-MBD. Among the dietary sources of phosphorus, some of the commonly prescribed medications have also been reported to contain phosphorus. However, drugs are often ignored even though they act as a potential source of phosphorus. Similarly, although proteins are the major source of dietary phosphorus, reducing protein intake can increase mortality in patients with CKD. Recently, the importance of phosphorus/protein ratio in food have been reported to be a sensitive marker for controlling dietary intake of phosphorus. This review summarizes the progress in the research on phosphate content in drugs as an excipient and the various aspects of dietary management of hyperphosphatemia in patients with CKD, with special emphasis on dietary restriction of phosphorus with low dietary phosphate/protein ratio.

Usefulness of urinary parameters in advanced chronic kidney disease

This review discusses the diagnostic value of urinary parameters in the setting of advanced chronic kidney disease and we present the key concepts that summarise the suggestions of the manuscript.

URINARY VOLUME

The amount of fluid intake may be a non-established risk factor for CKD. For these patients, a urinary output ≥2-3 l/day is a reasonable proposal. This recommendation is not applicable to patients with cardiorenal syndrome or fluid overload risk. NA: This determination is very useful to monitor salt intake. Reducing urinary Na<120 mEq/day (≅salt intake≤5-6g) is a reasonable objective.

URINARY UREA NITROGEN (UUN)

This parameter is useful to estimate protein intake (Maroni BJ equation). A protein intake between 48-72g (0.8-0.9g/kg/day according to weight) is equivalent to UUN 7-10g/day approximately.

ACID LOAD AND POTASSIUM

Acid load reduction may be an additional strategy in the nutritional management of this population. It may be estimated indirectly from a diet survey or by measuring the elimination of UUN and K_ur. The limits of this recommendation have not been established, but we propose a cautious and prudent diet of fruit and vegetables.

PHOSPHORUS

There is a significant positive correlation between phosphorus and protein, both in dietary records and urine elimination. Based on this information, we suggest a urinary P excretion<800mg/day or<600mg/day for patients with GFR<25ml/min or<15ml/min, respectively.

CONCLUSION

Urinary parameters provide sensitive and useful knowledge for clinical practice, provide information about the dietary habits of patients and the adherence to our recommendations.

Physiological regulation of phosphate by vitamin D, parathyroid hormone (PTH) and phosphate (Pi)

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4- and HPO42-). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

Urinary phosphorus excretion per creatinine clearance as a prognostic marker for progression of chronic kidney disease: a retrospective cohort study

Background

Whether phosphate itself has nephrotoxicity in patients with chronic kidney disease (CKD) is controversial, although phosphate excretion into urine may cause tubular damage in rat models. To evaluate actual phosphate load on each nephron, we examined the association between 24-h urinary phosphorus excretion per creatinine clearance (24-h U-P/C_Cr), a newly proposed index that is a surrogate for nephron load, and CKD progression in patients with CKD.

Methods

We conducted a single-center, retrospective cohort study. To avoid potential confounders for protein intake, only patients on our educational program for CKD with a fixed diet regimen and aged 20 years or older were included. The observation period was 3 years. Primary outcomes were CKD progression defined as a composite of end-stage kidney disease (ESKD) or 50 % reduction of estimated glomerular filtration rate. Patients were stratified by quartiles of 24-h U-P/C_Cr levels as Quartiles 1–4. The association was examined in three models: unadjusted (Model 1), adjusted for risk factors for CKD progression (Model 2), and factors that affect renal phosphate handling (Model 3).

Results

A total of 191 patients met the eligibility criteria. Patients with higher 24-h U-P/C_Cr showed a higher risk for the composite outcomes. The hazard ratios [95 % confidence interval] for 24-h U-P/C_Cr levels in Quartile 2, 3, and 4, respectively, versus Quartile 1 were 2.56 (1.15–6.24), 7.53 (3.63–17.62), and 12.17 (5.82–28.64) in Model 1; 1.66 (0.63–4.97), 3.57 (1.25–11.71), and 5.34 (1.41–22.32) in Model 2; and 3.07 (0.97–11.85), 7.52 (2.13–32.69), and 7.89 (1.74–44.33) in Model 3.

Conclusions

Our study showed that higher phosphorus excretion per creatinine clearance was associated with CKD progression.