Maximal Tubular Reabsorption Of Phosphate Research Articles

Burosumab Efficacy and Safety in Patients with X-Linked Hypophosphatemia: Systematic Review and Meta-analysis of Real-World Data.

To assess the efficacy and safety of burosumab in children and adults with X-linked hypophosphatemia based on real-world evidence. MEDLINE (via PubMed) and Cochrane Library were searched until 18 October 2023 for single-arm (before-after) studies. Registries including Clinicaltrials.gov, EU Clinical Trials, WHO International Clinical Trials Registry Platform, and conference abstracts. The outcomes were a change in serum phosphorus concentrations and change in RSS, a change in serum ALP, bone-specific ALP, a change in the ratio of Tubular maximum reabsorption of Phosphate to Glomerular Filtrate rate, a change in serum 1,25(OH)2D and 25(OH)2D concentrations, change in height Z-score, McMaster Universities Osteoarthritis Index (WOMAC) and safety outcomes. An inverse variance random-effects meta-analysis was applied for data synthesis. Fifteen studies (289 participants) were included. Burosumab treatment improved serum phosphate concentrations [mean difference 0.88mg/dl, 95% confidence interval 0.70 to 1.07, I2 = 92%), Rickets Severity score (mean difference -1.86, 95% confidence interval -2.5 to -1.21, I2 = 71%), serum alkaline phosphate concentrations (mean difference -1.86, 95% confidence interval -2.5 to -1.21, I2 = 71%), serum 1,25(OH)2D concentrations (mean difference 18.91pg/ml, 95% confidence interval 6.39 to 31.43, I2 = 96%) and renal phosphate reabsorption (mean difference 1.22 mg/dl, 95% confidence interval 0.70 to 1.74, I2 93%). Burosumab treatment improved overall clinical and laboratory findings in patients with X-linked hypophosphatemia.

#2015 Burosumab therapy in adult patients with X-linked hypophosphatemia: a three-year follow-up study

Abstract Background and Aims X-linked hypophosphatemic rickets (XLH) arises from mutations in the PHEX gene, resulting in elevated FGF23 levels, renal phosphate loss, hypophosphatemia, low vitamin D, heightened alkaline phosphatase (AP), and associated bone disorders. In challenging the conventional therapy of vitamin D and phosphate supplements, Burosumab, a monoclonal antibody against FGF23, has shown promise in normalizing phosphate and vitamin D levels in pediatric patients. However, its effectiveness in adult patients with established bone lesions remains an area of investigation. This study aims to present the three-year outcomes of Burosumab therapy on both biochemical and clinical aspects in adult XLH patients. Method The study assessed the efficacy of Burosumab in adult XLH patients (age >18, drug-naïve) before and continuously over three years post-therapy. Monthly assessments of biochemical parameters (phosphatemia, alkaline phosphatase, Tubular Maximum Reabsorption of Phosphate per Glomerular Filtration Rate, TMP/GFR) were conducted before the monthly administration of Burosumab, while clinical improvements were monitored every six months using established tests, including the six-minute walk test, UP and go timed test, and WOMAC Index. Burosumab was administered monthly at a consistent dosage of 1 mg/kg throughout the entire follow-up period. Results In a cohort of five genetically diagnosed XLH patients (average age 45 ± 10 years, average weight 60 ± 10 kg), the average monthly Burosumab dose was 60 ± 10 mg. Baseline values and subsequent changes in Ps, TMP/GFR, WOMAC Index, six-minute walk, and UP and go test are presented in Table 1 and Fig. 1. Importantly, no significant adverse events were reported during the initial year of treatment. Conclusion Burosumab therapy demonstrated excellent tolerability and yielded substantial improvements in biochemical parameters (Ps and TMP/GFR), maintaining stability over the entire three-year follow-up. Alkaline phosphatase levels, initially elevated, exhibited a progressive reduction in the second and third years, eventually reaching values within the normal range. Functional indices, reflective of enhanced strength (improved walk test and UP and go timed test) and diminished pain (optimal WOMAC test score), showcased notable improvements in the first year, persisting with stability during the follow-up period, with the exception of the walk test, which demonstrated a gradual and sustained enhancement. The findings suggest that Burosumab is a well-tolerated and effective therapeutic option for adult XLH patients. The observed reduction in alkaline phosphatase after a year of treatment implies a normalization of bone turnover, while the progressive improvement in the walk test indicates a sustained clinical enhancement likely associated with the chronic positive effects of maintaining biochemical stability.

Dynamics of Urine Electrolytes in Term Neonates during the 1st Week of Life

Background: Urine electrolyte assessment is vital for diagnosing and managing neonatal conditions. However, the challenge of urine collection in neonates has resulted in a lack of standardized urine electrolyte reference values. Aim: This study seeks to explore the reference levels and potential trends in serum and urine electrolytes to better understand how the kidneys handle these substances. Methods: Healthy neonates were prospectively enrolled following normal births. Using biochemical methods, blood and urine samples were collected and analyzed on the 1st and 5th postnatal days. Statistical analysis was performed using descriptive statistics and the Wilcoxon matched-pairs signed-rank test. Results: This prospective study enrolled 55 healthy neonates. Significant changes in serum electrolyte concentrations were observed between the 1st and 5th days after birth. Notably, sodium, creatinine, urea nitrogen, and uric acid levels decreased, whereas potassium, calcium, and phosphate levels increased. Urine analysis revealed significant increases in the tubular maximum phosphate reabsorption per glomerular filtration rate and decreases in the fractional excretion of potassium and uric acid by Day 5. Conclusion: This study challenges prevailing assumptions about the stability of neonatal urine electrolytes and highlights dynamic changes in the first postnatal week. These insights lay the groundwork for further research into electrolyte disorders in newborns and have potential implications for improving neonatal care practices.

Multiparametric renal function assessment in cirrhotic patients shows high prevalence of medically actionable changes in multiple modules.

Renal dysfunction is a common complication of cirrhosis, occurring either as part of multiorgan involvement in acute illness or secondary to advanced liver disease. To date, no study has comprehensively assessed multiple renal function parameters in hospitalized patients with cirrhosis through a multiparametric analysis of renal biochemistry markers. We conducted a retrospective, observational study including all consecutive patients hospitalized with cirrhosis who underwent a 43-multiparametric renal function assessment between January 1, 2021, and June 30, 2023. All patients showed at least one of the following renal abnormalities: Kidney Disease: Improving Global Outcomes stage G2 or higher, sodium and/or chloride excretion fraction <1%, electrolyte-free water clearance <0.4mL/min, or tubular maximum phosphate reabsorption capacity <0.8mmol/L. The estimated glomerular filtration rate equations significantly overestimated the measured creatinine clearance with median differences of +14mL/min/1.73m2 (95% CI 6-29) and +9mL/min/1.73m2 (95% CI 2-15) for European Kidney Function Consortium equations, respectively. Notably, 54% and 39% of patients demonstrated estimated glomerular filtration rates exceeding 30% of the measured creatinine clearance when the Chronic Kidney Disease - Epidemiology Collaboration and European Kidney Function Consortium formulas were employed, respectively. Substantial discrepancies in Kidney Disease: Improving Global Outcomes stage assignments were observed between the estimated glomerular filtration rate- and measured creatinine clearance-based assessments. This study underscores the value of a multiparametric renal function assessment as a routine tool for evaluating renal function in patients with cirrhosis. A high prevalence of medically actionable renal abnormalities spanning multiple renal function modules, including alterations in glomerular function, salt and solute-free water excretion, and proximal tubule phosphate reabsorption, has been demonstrated in hospitalized patients with cirrhosis.

Experience of X-linked hypophosphatemic rickets in the Gulf Cooperation Council countries: case series.

X-linked hypophosphatemic rickets (XLH), the most prevalent form of inherited hypophosphatemic rickets, is caused by loss-of-function mutations in the gene encoding phosphate-regulating endopeptidase homolog, X-linked (PHEX). This case series presents 14 cases of XLH from Gulf Cooperation Council (GCC) countries. The patients' medical history, biochemical and radiological investigative findings, as well as treatment responses and side effects from both conventional and burosumab therapy, are described. Cases were aged 2-40 years at diagnosis. There were two male cases and 12 female cases. All cases were treated with conventional therapy which resulted in a lack of improvement in or worsening of the clinical signs and symptoms of rickets or biochemical parameters. Side effects of conventional therapy included nausea, diarrhea, abdominal pain, nephrocalcinosis, and hyperparathyroidism, which affected the patients' quality of life and adherence to treatment. In the 10 patients treated with burosumab, there was a marked improvement in the biochemical markers of rickets, with a mean increase in serum phosphate of +0.56 mmol/L and tubular maximum phosphate reabsorption (TmP) to glomerular filtration rate (GFR) ratio (TmP/GFR) of +0.39 mmol/L at 12 months compared to baseline. Furthermore, a mean decrease in serum alkaline phosphatase (ALP) of -80.80 IU/L and parathyroid hormone (PTH) of -63.61 pmol/L at 12 months compared to baseline was observed in these patients. Additionally, patients treated with burosumab reported reduced pain, muscle weakness, and fatigue as well as the ability to lead more physically active lives with no significant side effects of treatment. Conventional therapy resulted in a suboptimal response, with a lack of improvement of clinical signs and symptoms. Side effects of conventional therapy included nausea, diarrhea, abdominal pain, nephrocalcinosis, and hyperparathyroidism, which affected the patients' quality of life and adherence to treatment. Burosumab demonstrated marked improvements in the biochemical markers of rickets, in addition to reducing pain, muscle weakness, and fatigue. There were no significant side effects associated with burosumab therapy.

Tumor-induced osteomalacia due to a phosphaturic mesenchymal maxillary tumor

ABSTRACT Tumor-induced osteomalacia is a rare paraneoplastic syndrome with hypophosphatemia due to renal phosphate wasting. The clinical presentation is bony pains and muscle wasting. A 23-year-old male presented to us in a wheelchair with joint pain and muscle weakness for 5 years. There was no history of a similar disease in the family. Treatment with non steroidal anti inflammatory drugs, steroids, and infliximab was unsuccessful. Positive examination findings revealed tenderness and swelling in both ankle and knee joints, tenderness in hip joints, proximal myopathy, and power grade 4/5 in the proximal leg muscles. On evaluation, he was found to have severe hypophosphatemia [serum phosphate 1.2 mg/dl], decreased renal phosphate absorption [tubular maximum for phosphate reabsorption adjusted for glomerular filtration rate (TMP/GFR) of 1.04], increased fibroblast growth factor 23 (FGF23), and decreased 1,25 dihydroxy vitamin D levels. Fluorodeoxyglucose-positron emission tomography (FDG PET-CT) revealed a left maxillary tumor. Histopathology revealed a mesenchymal tumor. Tumor excision resulted in cure of the patient’s symptoms and the normalization of serum phosphate levels and TMP/GFR.

LMS-Based Pediatric Reference Values for Parameters of Phosphate Homeostasis in the HARP Cohort.

The assessment of phosphate homeostasis in children is challenging due to the marked changes in laboratory parameters during growth and development, and the lack of adequate reference values. To develop Lambda-Mu-Sigma (LMS)-based continuous pediatric reference percentiles for 7 key laboratory parameters of phosphate homeostasis. This cross-sectional, single-center study, the HAnnover Reference values for Pediatrics (HARP) study, included 455 children aged 0.1-18 years (254 boys) from outpatient hospital clinics and a secondary school program. Main outcome measures were LMS-based continuous reference percentiles for serum phosphate, plasma intact fibroblast growth factor 23 (iFGF23), and its cofactor soluble Klotho (sKlotho), tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR), fractional tubular reabsorption of phosphate (TRP), and urinary calcium/creatinine (Ca/Crea) and phosphate/creatinine (Pi/Crea) ratios. LMS-based percentiles and z-scores were established for 7 key laboratory parameters of phosphate homeostasis, which were all found to be age-dependent. Serum phosphate, TmP/GFR, and sKlotho associated with sex. Serum phosphate, TmP/GFR, and urinary Ca/Crea and Pi/Crea levels were highest in infancy and declined until age 18 years, while phosphate and TmP/GFR values reached adult levels earlier in girls compared to boys. iFGF23 concentrations are highest in infancy and fall to a stable plateau by 4 years of age, while sKlotho peaks during adolescence. This is the first report of LMS-based continuous pediatric reference percentiles for key laboratory parameters of phosphate homeostasis that allow calculation of standardized patient z-scores to facilitate test result interpretation in children and adolescents.

Elderly-onset calcinosis of hyperphosphataemic familial tumoural calcinosis/hyperostosis-hyperphosphataemia syndrome: the role of comorbid scleroderma.

A 73-year-old woman with type 2 diabetes mellitus was referred to our department for glycaemic control. Physical examination revealed two subcutaneous hard masses around the left shoulder and the right hip joint. The patient could not fully extend her fingers because of skin sclerosis in both hands. Laboratory studies showed hyperphosphataemia and a high ratio of renal tubular maximum reabsorption of phosphate to glomerular filtration rate. There were no abnormalities in serum calcium, creatinine, alkaline phosphatase, and intact parathyroid hormone levels, whereas serum fibroblast growth factor 23 was low. Hyperphosphataemic familial tumoural calcinosis/hyperostosis-hyperphosphataemia syndrome (HFTC/HHS) was diagnosed using whole genome sequencing that revealed a novel frameshift beyond the 584th threonine located in the lectin domain of UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 associated with a duplication of the 1748th thymine in the coding region of the corresponding gene. Furthermore, anti-nuclear, anti-centromere, and anti-cardiolipin antibodies were positive, implying that comorbid limited type scleroderma might play a role in tumoural calcinosis (TC) development. A low phosphate diet was prescribed with phosphate-lowering medications, including aluminium hydroxide, acetazolamide, and sevelamer hydrochloride. The patient displayed a decrease in serum phosphate levels from 6.5 to 5.5 mg/dL 10 months after the initiation of treatment, but her TC had not improved during treatment for more than 1 year. This case was interesting because the patient with HFTC/HHS exhibited TC despite being over her 60s, and subsequent scleroderma might contribute to the specific clinical course. When HFTC/HHS presents with elderly-onset TC, the involvement of comorbidities in exacerbating TC should be considered. HFTC/HHS occurs on an autosomal recessive basis, but its clinical course and manifestations differ significantly throughout the cases. HFTC/HHS may be undiagnosed until later in life because of its rarity, unfamiliarity, and phenotype diversity; therefore, HFTC/HHS should be included in the differential diagnosis of elderly patients with unexplained hyperphosphataemia or ectopic calcinosis. Comorbidities, including rheumatologic disorders, may contribute to developing HFTC/HHS-associated calcinosis.

Uncertainty in measurement and the renal tubular reabsorption of phosphate.

The ratio of tubular maximum reabsorption of phosphate to glomerular filtration rate (TmP/GFR) is used toevaluate renal phosphate transport. TmP/GFR is most probably calculated using the formula described by Kenny and Glen or obtained from the nomogram described by Walton and Bijvoet. Even though the calculation itself is well described, no attention has been given to its measurement uncertainty (MU). The aim of this study is to provide a procedure for evaluating the MU of the Kenny and Glen formula; a procedure which is based on the Evaluation of measurement data- Guide to the expression of uncertainty in measurement (GUM). TmP/GFR is a quantity value calculated from the input of measured values for serum (plasma) phosphate and creatinine, plus measured values of urine phosphate and creatinine. Given the measurement uncertainty associated with these input quantities, the GUM describes the mathematical procedures required to determine the uncertainty ofthe calculated TmP/GFR. From a medical laboratory perspective, these input uncertainties are the standard deviations of the respective internal quality control estimates for serum and urine phosphate, plus serum and urine creatinine. Based on representative measurements for the input quantities and their associated standard uncertainties, the expanded relative uncertainty for a calculated TmP/GFR is approximately 3.0-4.5 %. With the continued relevance of the TmP/GFR procedure and the use of creatinine clearance as an estimate of GFR, the addition of an uncertainty estimate is important as an adjunct to this diagnostic procedure.

Commentary on Severe Hypophosphatemia: The Hidden Truth.

Abnormal phosphate results are common in daily practice, but often neglected by laboratorians and clinicians. This case of severe hypophosphatemia highlights the critical implications that a delayed diagnostic workup of the underlying cause can have. Early diagnosis and treatment could have prevented years of continuous bone loss and immobility. Therefore, severe hypophosphatemia should always trigger additional investigations to find the underlying cause. Determination of the tubular maximum reabsorption of phosphate for glomerular filtration rate was pivotal for establishing renal phosphate wasting and for guiding further diagnostic measures. This rarely used parameter can easily be determined in most laboratories, but pathologists are often unaware of its diagnostic value. Consequently, many laboratories limit themselves to the measurement of timed urinary phosphate excretion, despite the fact that a wide reference interval limits its diagnostic utility. In adult patients with renal phosphate wasting, measurement of fibroblast growth factor-23 (FGF-23) helps to further narrow the spectrum of potential causes. Elevated FGF-23 concentrations are typical in tumor-induced osteomalacia, but normal values can occur when the underlying tumor secretes other phosphatonins or in the presence of interfering heterophile antibodies or human antimouse antibodies that disturb the assay (1).

Severe Hypophosphatemia: The Hidden Truth.

A 52-year-old woman presented with a 14-year history of progressive proximal muscle weakness and myalgia. She had been wheelchair-bound for the previous 10 years, with a significant loss of height since the onset of symptoms. She was previously healthy, with no family history of metabolic bone disease. Clinical examination revealed a proximal myopathy, short stature with marked kyphoscoliosis and pectus carinatum. Her admission laboratory investigations showed severe hypophosphatemia, with a serum phosphate of 0.87 mg/dL (0.28 mmol/L [reference interval 2.5–4.5 mg/dL; 0.8–1.4 mmol/L]). 25-Hydroxy vitamin D (25-OHD) was reduced with a normal 1,25-dihydroxy vitamin D (1,25-OHD). Laboratory results are summarized in Table 1. Of note, the tubular maximum reabsorption of phosphate for glomerular filtration rate (TmP/GFR) was 0.8 mg/dL (0.22 mmol/L [reference interval 2.0–3.4 mg/dL; 0.80–1.35 mmol/L]), suggestive of renal phosphate wasting. Fibroblast growth factor 23 (FGF-23) was elevated at 9207 pg/mL (reference interval 12–49 pg/mL). Despite phosphate and vitamin D replacement, the hypophosphatemia persisted.

Commentary on Severe Hypophosphatemia: The Hidden Truth.

Mohamed & Raal highlight the importance of investigating hypophosphatemia biochemically to allow a diagnosis of the underlying aetiology. Both parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) facilitate phosphate excretion in the urine. The tubular maximum reabsorption of phosphate for glomerular filtration rate (TmP/GFR) is a measure of renal tubular capacity to reabsorb phosphate. TmP/GFR is therefore reduced in a calcipenic state (vitamin D and/or calcium deficiency) due to secondary hyperparathyroidism and in the phosphopenic state, either due to a primary renal tubular defect or excess circulating FGF23 (genetic defects in FGF23 degradation or increased FGF23 production such as tumor-induced osteomalacia [TIO]). PTH increases 1,25-(OH)2Vitamin D (calcitriol) production while FGF23 reduces it. In this case of severe symptomatic hypophosphatemia, the low TmP/GFR, inappropriately normal calcitriol in the context of hypophosphatemia, and elevated FGF23 confirmed FGF23-induced renal phosphate wasting. A nuclear medicine scan established the diagnosis of TIO and assisted in the localization of the lesion.

Presentation and Diagnosis of Pediatric X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is a rare type of hereditary hypophosphatemic rickets. Patients with XLH have various symptoms that lower their QOL as defined by HAQ, RAPID3, SF36-PCS, and SF36-MCS in adult patients and SF-10 and PDCOI in pediatric patients. Early diagnosis and treatment are needed to reduce the burden, but the condition is often diagnosed late in childhood. The present review aims to summarize the symptoms, radiological and biological characteristics, and long-term prognosis of pediatric XLH. Typical symptoms of XLH are lower leg deformities (age six months or later), growth impairment (first year of life or later), and delayed gross motor development with progressive lower limb deformities (second year of life or later). Other symptoms include dental abscess, bone pain, hearing impairment, and Chiari type 1 malformation. Critical, radiological findings of rickets are metaphyseal widening, cupping, and fraying, which tend to occur in the load-bearing bones. The Rickets Severity Score, validated for XLH, is useful for assessing the severity of rickets. The biochemical features of XLH include elevated FGF23, hypophosphatemia, low 1,25(OH)2D, and elevated urine phosphate. Renal phosphate wasting can be assessed using the tubular maximum reabsorption of phosphate per glomerular filtration rate (TmP/GFR), which yields low values in patients with XLH. XLH should be diagnosed early because the multisystem symptoms often worsen over time. The present review aims to help physicians diagnose XLH at an early stage.

Burosumab for X-linked hypophosphatemia in children and adolescents: Opinion based on early experience in seven European countries.

Given the relatively recent introduction of burosumab in the management of X-linked hypophosphatemia (XLH), there is limited real-world data to guide its use in clinical practice. As a group of European physicians experienced with burosumab treatment in clinical practice, we convened with the objective of sharing these practice-based insights on the use of burosumab in children and adolescents with XLH. We attended two virtual meetings, then discussed key questions via Within3, a virtual online platform. Points of discussion related to patient selection criteria, burosumab starting dose, dose titration and treatment monitoring. Our discussions revealed that criteria for selecting children with XLH varied across Europe from all children above 1 year to only children with overt rickets despite conventional treatment being eligible. We initiated burosumab dosing according to guidance in the Summary of Product Characteristics, an international consensus statement from 2019 and local country guidelines. Dose titration was primarily guided by serum phosphate levels, with some centers also using the ratio of tubular maximum reabsorption of phosphate to glomerular filtration rate (TmP/GFR). We monitored response to burosumab treatment clinically (growth, deformities, bone pain and physical functioning), radiologically (rickets and deformities) and biochemically (serum phosphate, alkaline phosphatase, 1,25-dihydroxyvitamin D, 25-hydroxyvitamin D, urine calcium-creatinine ratio and TmP/GFR). Key suggestions made by our group were initiation of burosumab treatment in children as early as possible, from the age of 1 year, particularly in those with profound rickets, and a need for clinical studies on continuation of burosumab throughout adolescence and into adulthood.

Fibroblast growth factor 23-Klotho and mineral metabolism in the first year after pediatric kidney transplantation: A single-center prospective study.

The role of fibroblast growth factor 23 (FGF23) levels in mineral metabolism before and after kidney transplantation in pediatric patients is poorly understood. We prospectively evaluated 24 patients under 18 years of age (4.5 [3.3-9.8]years) who underwent living kidney transplantation between July 2016 and March 2018, and measured intact FGF23 and serum αKlotho levels, and other parameters of mineral metabolism before and after transplantation (Day 7, 1 and 4 months, and 1 year). Relationships between parameters were examined by linear analysis. FGF23 level was 440.8 [63.4-5916.3]pg/ml pre-transplant and decreased significantly to 37.1 [16.0-71.5]pg/ml at Day 7 post-transplant (-91.6%, p < .001). Thereafter, it remained at normal levels until 1 year. αKlotho level was 785 [568-1292]pg/ml pre-transplant and remained low at Day 7 and 1 month post-transplant, with an increasing trend at 4 months. Post-transplant phosphorus levels were significantly decreased compared with pre-transplant, with a lowest level of 1.7 [1.3-2.9]mg/dl, -5.7 [-6.8, -3.8] SD at Day 4, followed by gradual recovery. Phosphorus levels and the ratio of tubular maximum phosphate reabsorption were significantly and negatively associated with pre-transplant FGF23 until 4 months of post-transplant. Pre-transplant αKlotho was negatively associated with pre-transplant FGF23 but not FGF23 or other parameters after transplantation. FGF23 in pediatric kidney transplant patients decreased rapidly after transplantation and associated with post-transplant hypophosphatemia and increased phosphorus excretion. Post-transplant αKlotho was low early post-transplant but tended to increase subsequently. Post-transplant αKlotho was unaffected by pre-transplant FGF23 or other factors, suggesting pre-transplant chronic kidney disease status has no effect.

PSAT228 Hereditary Hypophosphatemic Rickets: Management in Adults

Abstract Introduction Hereditary Hypophosphatemic Rickets (HHR) is a group of rare inherited disorders consisting of X-linked hypophosphatemic rickets (XLHR, prevalence 1 in 20,000 births), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR). XLHR is caused by loss of function mutation of the PHEX gene, ARHR is caused by loss of function mutation of the DMP1/ENPP1 gene and ADHR is caused by activating point mutations in FGF23. These mutations result in renal phosphate wasting, hypophosphatemia, and impaired bone mineralization. It presents in childhood as skeletal deformities, abnormal gait, skeletal pain, dental abnormalities, and stunted growth. Adults have osteomalacia-related pain, fractures, and arthritis. The diagnosis is made by findings consistent with rickets and normal serum calcium, normal 25-hydroxy vitamin D, low-normal 1,25-dihydroxy vitamin D, elevated alkaline phosphatase activity, and low phosphate levels. Urine studies show low urine calcium and decreased tubular maximum reabsorption of phosphate, confirming renal wasting of phosphate. The goal of treatment in adults is to reduce pain, prevent fractures, enhance mobility, and decrease osteomalacia. Phosphate and vitamin D supplementations are used with close monitoring as it is associated with severe adverse events. It can cause hypercalcemia leading to nephrocalcinosis, nephrolithiasis, chronic kidney disease, and hyperparathyroidism. There is no evidence that the treatment affects long-term skeletal complications. Burosumab, a monoclonal antibody that inhibits FGF23, is approved for the treatment of XLHR. No studies are available about the use of Burosumab in ADHR or ARHR. Case Presentation A 18-year-old male with past medical history of hypophosphatemic rickets with multiple fractures, on oral phosphate tablets and calcitriol, presented to the Endocrinology clinic to establish care. He had been following Pediatric Endocrinology in the past. His blood work showed normal calcium, low phosphate of 1.6 mg/dl (normal: 2.5-4.5 mg/dl), vitamin D 25 hydroxy level of 26 ng/ml (normal: 20-100 ng/ml), vitamin D 1,25-dihydroxy level of 25 pg/ml (normal: 30-80 pg/ml), intact parathyroid hormone level of 549 pg/ml (normal: 15-65 pg/ml), and alkaline phosphatase of 438 U/l (normal: 40-150 U/l). Urine calcium was low and urine phosphorus was elevated with phosphate/creatinine ratio more than 2000 (normal: 60-962), consistent with renal phosphate wasting. Genetic testing in the past was negative for PHEX gene mutation, so he was not a candidate for Burosumab. Conclusion There is limited knowledge about Hereditary Hypophosphatemic Rickets, especially ADHR and ARHR due to limited cases (less than 100 cases reported so far). The current standard of treatment of HHR is sub-optimal and associated with adverse effects. Burosumab has not been tested in ADHR or ARHR, so it is unclear if it will be beneficial in this subgroup. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m., Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.

Early switching of tenofovir disoproxil fumarate (TDF) in HIV-infected patients with TDF-induced nephrotoxicity: a prospective study

Background: Tenofovir disoproxil fumarate (TDF) can induce proximal renal tubulopathy (PRT) and necessitate changes in treatment regimen. This prospective study aimed to compare tubular function recovery following early switching versus late switching of TDF in human immunodeficiency virus (HIV)-infected patients with TDF-induced PRT. Methods: For this prospective study, conducted during 2017–2019, we enrolled HIV-1-infected, virologically suppressed adults undergoing TDF-containing combination antiretroviral therapy. Patients were separated into a late-switching group (LSG) and an early-switching group (ESG). The LSG included patients having an estimated glomerular filtration rate (eGFR) decrease ≥25% from the pretreatment level or Fanconi syndrome. The ESG included patients having ≥2 of the following indicators of PRT: fractional excretion of phosphate (FEUP) ≥10%, low tubular maximum reabsorption of phosphate (TmP)/GFR, or uricosuria; fractional uric acid excretion ≥10%; urine protein–creatinine index (UPCI) ≥500 mg/g creatinine, normoglycemic glycosuria, or decrease in eGFR of 15%–24%. Recovery of proximal tubular function at 6 and 12 months after TDF discontinuation was assessed. Complete recovery was defined as normalization of all abnormal tubular markers. Results: Thirty-three HIV-infected patients were enrolled (70% male). Except for tubular function markers, baseline characteristics were not significantly different between the two groups. The proportion of patients having complete recovery was significantly higher in the ESG (p = 0.007, log-rank test). FEUP improved significantly in the ESG after TDF discontinuation; improvements of eGFR and UPCI were greater in the LSG. An eGFR change of 10% from baseline was the only independent predictor of failure to achieve complete recovery after switching. After median follow-up of 2.25 years post-trial, sustained recovery of eGFR within 5% of pre-TDF eGFR was achieved only in the ESG. Conclusions: Early-switching of TDF in HIV patients with PRT may allow complete recovery of proximal renal tubular function.

Assessment of Proximal Tubular Function by Tubular Maximum Phosphate Reabsorption Capacity in Heart Failure.

The estimated glomerular filtration rate (eGFR) is a crucial parameter in heart failure. Much less is known about the importance of tubular function. We addressed the effect of tubular maximum phosphate reabsorption capacity (TmP/GFR), a parameter of proximal tubular function, in patients with heart failure. We established TmP/GFR (Bijvoet formula) in 2085 patients with heart failure and studied its association with deterioration of kidney function (>25% eGFR decrease from baseline) and plasma neutrophil gelatinase-associated lipocalin (NGAL) doubling (baseline to 9 months) using logistic regression analysis and clinical outcomes using Cox proportional hazards regression. Additionally, we evaluated the effect of sodium-glucose transport protein 2 (SGLT2) inhibition by empagliflozin on tubular maximum phosphate reabsorption capacity in 78 patients with acute heart failure using analysis of covariance. Low TmP/GFR (<0.80 mmol/L) was observed in 1392 (67%) and 21 (27%) patients. Patients with lower TmP/GFR had more advanced heart failure, lower eGFR, and higher levels of tubular damage markers. The main determinant of lower TmP/GFR was higher fractional excretion of urea (P<0.001). Lower TmP/GFR was independently associated with higher risk of plasma NGAL doubling (odds ratio, 2.20; 95% confidence interval, 1.05 to 4.66; P=0.04) but not with deterioration of kidney function. Lower TmP/GFR was associated with higher risk of all-cause mortality (hazard ratio, 2.80; 95% confidence interval, 1.37 to 5.73; P=0.005), heart failure hospitalization (hazard ratio, 2.29; 95% confidence interval, 1.08 to 4.88; P=0.03), and their combination (hazard ratio, 1.89; 95% confidence interval, 1.07 to 3.36; P=0.03) after multivariable adjustment. Empagliflozin significantly increased TmP/GFR compared with placebo after 1 day (P=0.004) but not after adjustment for eGFR change. TmP/GFR, a measure of proximal tubular function, is frequently reduced in heart failure, especially in patients with more advanced heart failure. Lower TmP/GFR is furthermore associated with future risk of plasma NGAL doubling and worse clinical outcomes, independent of glomerular function.

Tubular phosphate handling: references from child to adulthood in the era of standardized serum creatinine.

The assessment of phosphate homeostasis in clinical practice relies not only on circulating phosphate levels but also on phosphate tubular reabsorption, ideally assessed using the tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR). TmP/GFR reference values were established before the onset of isotope-dilution mass spectrometry-standardized (IDMS) creatinine assays and thus need to be updated. Our objective is to provide reference values for TmP/GFR from childhood to adulthood, using the gold-standard of GFR assessment and IDMS-standardized creatinine values. We retrospectively analysed all the inulin and iohexol clearances [measured glomerular filtration rate (mGFR)] performed in children and in adults screened for a living-donation in our unit since the beginning of IDMS-creatinine assays. TmP/GFR was calculated on a fasting sample, using the conventional formula without correction for tubular reabsorption of phosphate (TRP) in subjects below 19 years of age. A total of 2051 subjects (1711 children, 340 adults), aged from 1.9 to 73.4 years with normal GFR, normal phosphate and normal calcium levels, were included for TmP/GFR analysis. As expected, there was a progressive decrease along puberty in both genders of plasma phosphate and TmP/GFR, the decrease occurring earlier in girls. After the age of 19 years, there was a stabilization of plasma phosphate and TmP/GFR levels until the age of 55 years, phosphate levels and TmP/GFR being slightly lower in men than in women. We present the largest cohort describing TmP/GFR reference values in the era of IDMS-standardized creatinine assays. We believe that these data will help physicians to better diagnose and manage patients with abnormal phosphate metabolism in daily clinical routine.

Recombinant growth hormone therapy for X-linked hypophosphatemia in children.

Conventional treatment of X-linked hypophosphatemia with oral phosphate and calcitriol can heal rickets, but it does not always raise serum phosphate concentrations significantly, nor does it always normalize linear growth. Some clinical trials suggest that combining recombinant human growth hormone therapy with conventional treatment improves growth velocity, phosphate retention, and bone mineral density, but some clinical trials suggest that it appears to aggravate the pre-existent disproportionate stature of such children. This is an updated version of a previously published review. To determine whether recombinant human growth hormone therapy for children with X-linked hypophosphatemia is associated with changes in longitudinal growth, mineral metabolism, endocrine function, renal function, bone mineral density, body proportions, and also with any adverse effects. We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register which comprises references identified from comprehensive electronic database searches and handsearches of relevant journals and abstract books of conference proceedings. In addition, we searched the Cochrane Central Register of Controlled Trials, Ovid MEDLINE and the reference lists of identified trials and other reviews. We also undertook some additional handsearching of relevant journals and conference proceedings. Date of the most recent search: 12 January 2021 SELECTION CRITERIA: All randomized controlled studies or quasi-randomized controlled studies comparing growth hormone (alone or combined with conventional treatment) with either placebo or conventional treatment alone in children with X-linked hypophosphatemia. Two authors independently assessed studies for risk of bias and extracted data from eligible studies. GRADE criteria were used to assess the certainty of the evidence for each outcome. We included two studies (20 participants) in the review. In one cross-over study, results showed that recombinant human growth hormone therapy may improve the height standard deviation (SDS) score (z score), but we are unsure whether the intervention was the reason behind a transient increase in serum phosphate and tubular maximum for phosphate reabsorption. In the second, parallel study, treatment may also have improved the height SDS from baseline in the rhGH group compared to the control group, although no significant difference was seen between groups after three years, MD 0.50 SDS (95 % CI -0.54 to 1.54) (low-certainty evidence). The treatment was possibly well-tolerated during both studies with only transient adverse effects seen in three participants (low-certainty evidence). We are uncertain whether growth hormone improves serum phosphate levels or change in TmP/GFR (very low-certainty evidence). The treatment may make little or no difference to alkaline phosphatase levels (low-certainty evidence). We do not have enough high-certainty evidence to recommend the use of recombinant human growth hormone therapy in children with X-linked hypophosphatemia.