Down-regulation of Na+ transporters and AQP2 is responsible for acyclovir-induced polyuria and hypophosphatemia

Acquired disorders of phosphaturia: Beyond tumor-induced osteomalacia

Phosphate is an integral part of human cellular structure and function. Though most recognised disorders of phosphaturia are genetic in origin, phosphate loss due to acquired conditions is commonly encountered in clinical practice. Acquired hypophosphatemia is most commonly due to renal phosphate wasting and can produce significant morbidity. It also heralds future kidney damage, and continued exposure can lead to progressive kidney injury and potentially renal failure. These conditions are a diverse group of disorders with common shared mechanisms causing loss of phosphate in the urine. Renal phosphate loss can occur as an isolated entity or as a part of generalised proximal tubular dysfunction, i.e., Fanconi's syndrome. An insight into the pathophysiological mechanisms of acquired phosphaturia can help clinicians monitor their patients better and avoid potential harms.

Frequent occurrence of hypophosphatemia among multiple myeloma patients treated with elotuzumab: a single clinic retrospective study

The purpose of this single-center retrospective study was to determine the incidence of decreased blood phosphorus levels and hypophosphatemia among multiple myeloma (MM) patients treated with elotuzumab. Hypophosphatemia, which is defined as a serum phosphorus concentration < 2.5 mg/dL, leads to complications ranging from muscle weakness and disorientation to seizures and heart failure. A total of 23 MM patients receiving care in a clinic specializing in treatment of MM from July 2018 to March 2020 and treated with an elotuzumab-containing therapy were evaluated, and 9 were investigated for this study. Elotuzumab was given at 10 mg/kg weekly for the first two treatment cycles (28 days/cycle), followed by 10 mg/kg every other week for all subsequent cycles. Four different elotuzumab combination therapies were administered: 1) elotuzumab and dexamethasone 2) elotuzumab, lenalidomide and dexamethasone 3) elotuzumab, pomalidomide and dexamethasone and 4) elotuzumab, carfilzomib, pomalidomide, and dexamethasone. Phosphorous levels were determined at a median of every 13 days at intervals ranging from once weekly to once monthly until a phosphate supplement was prescribed to the patient or when elotuzumab treatment was discontinued. We found that regardless of elotuzumab combination therapy, all patients treated showed decreased phosphorus levels after initiating elotuzumab treatment with reductions ranging from 12.5% to 44.1% below baseline. Six participants (67%) demonstrated an average serum phosphorus at or below 2.5 mg/dL after starting elotuzumab therapy. This retrospective study suggests that hypophosphatemia commonly occurs among MM patients receiving elotuzumab-containing therapies.

Effect of Npt2b deletion on intestinal and renal inorganic phosphate (Pi) handling

BACKGROUND

Hyperphosphatemia is common in chronic kidney disease and is associated with morbidity and mortality. The intestinal Na⁺-dependent phosphate transporter Npt2b is thought to be an important molecular target for the prevention of hyperphosphatemia. The role of Npt2b in the net absorption of inorganic phosphate (Pi), however, is controversial.

METHODS

In the present study, we made tamoxifen-inducible Npt2b conditional knockout (CKO) mice to analyze systemic Pi metabolism, including intestinal Pi absorption.

RESULTS

Although the Na⁺-dependent Pi transport in brush-border membrane vesicle uptake levels was significantly decreased in the distal intestine of Npt2b CKO mice compared with control mice, plasma Pi and fecal Pi excretion levels were not significantly different. Data obtained using the intestinal loop technique showed that Pi uptake in Npt2b CKO mice was not affected at a Pi concentration of 4 mM, which is considered the typical luminal Pi concentration after meals in mice. Claudin, which may be involved in paracellular pathways, as well as claudin-2, 12, and 15 protein levels were significantly decreased in the Npt2b CKO mice. Thus, Npt2b deficiency did not affect Pi absorption within the range of Pi concentrations that normally occurs after meals.

CONCLUSION

These findings indicate that abnormal Pi metabolism may also be involved in tight junction molecules such as Cldns that are affected by Npt2b deficiency.

Hypophosphatemia in Critically Ill Children: Risk Factors, Outcome and Mechanism

OBJECTIVES

To determine the prevalence of hypophosphatemia in critically ill children and its association with clinical outcomes; to determine risk factors and mechanism of hypophosphatemia.

METHODS

Levels of serum phosphate, phosphate intake, renal phosphate handling indices and blood gases were measured on days 1, 3, 7 and 10 of pediatric intensive care unit (PICU) stay. Hypophosphatemia was defined as any serum phosphorus <3.8 mg/dl for children younger than 2 y and <3.5 mg/dl for children 2 y or older. Renal phosphate loss was assessed using the ratio of tubular maximum reabsorption of phosphate (TmP) to glomerular filtration rate (GFR) [TmP/GFR].

RESULTS

Prevalence of hypophosphatemia was 71.6 % (95 % CI: 64.6-78.6). On adjusted analysis, hypophosphatemia was associated with prolonged PICU length of stay (PICU LOS > 6 d) (adjusted OR: 3.0 [95 % CI: 1.4-6.7; p = 0.005]) but not associated with increased mortality. Renal phosphate threshold was significantly lower on all the days in hypophosphatemic group compared to that of non-hypophosphatemic group. No statistically significant difference in the amount of phosphate intake was seen in both the groups.

CONCLUSIONS

Hypophosphatemia is highly prevalent in critically ill children and is associated with prolonged PICU LOS. Increased phosphate loss in urine is one of the mechanism responsible for hypophosphatemia in critically ill children.

[Tenofovir nephrotoxicity]

Tenofovir is currently the only commercially available nucleotidic reverse-transcriptase inhibitor of human immunodeficiency virus (HIV). It is overall very well tolerated and is prescribed to millions of patients-without any specific monitoring in developing countries. However a significant nephrotoxicity has been described. Acute nephrotoxicity is well characterized. Tenofovir is excreted in urine by proximal tubular epithelial cells. In case of cytoplasmic accumulation, tenofovir inhibits mitochondrial DNA polymerase γ, which causes a dysfunction of the respiratory chain, and in turn an alteration of the energy-deprived cells. Fanconi syndrome is the clinical expression of tenofovir acute toxicity, with sometimes an associated acute kidney failure. These abnormalities are usually reversible, at least partially, when tenofovir is discontinued. Tenofovir chronic toxicity has been debated but seems now well established by several cohort studies, even though it pathophysiology has yet to be understood. It manifests as an accelerated glomerular filtration rate decline in treated patients with no other renal abnormalities. The identification of this chronic toxicity was probably blurred by multiple cofactors, usually excluded from clinical trials. Simple measures such as dose adaptation to kidney function, identification of risk factors, and plasmatic tenofovir concentration monitoring can help decrease the risk of nephrotoxicity.

Hypomagnesemia is a risk factor for nonrecovery of renal function and mortality in AIDS patients with acute kidney injury

The objective of the present study was to determine the prevalence of electrolyte disturbances in AIDS patients developing acute kidney injury in the hospital setting, as well as to determine whether such disturbances constitute a risk factor for nephrotoxic and ischemic injury. A prospective, observational cohort study was carried out. Hospitalized AIDS patients were evaluated for age; gender; coinfection with hepatitis; diabetes mellitus; hypertension; time since HIV seroconversion; CD4 count; HIV viral load; proteinuria; serum levels of creatinine, urea, sodium, potassium and magnesium; antiretroviral use; nephrotoxic drug use; sepsis; intensive care unit (ICU) admission, and the need for dialysis. Each of these characteristics was correlated with the development of acute kidney injury, with recovery of renal function and with survival. Fifty-four patients developed acute kidney injury: 72% were males, 59% had been HIV-infected for >5 years, 72% had CD4 counts <200 cells/mm(3), 87% developed electrolyte disturbances, 33% recovered renal function, and 56% survived. ICU admission, dialysis, sepsis and hypomagnesemia were all significantly associated with nonrecovery of renal function and with mortality. Nonrecovery of renal function was significantly associated with hypomagnesemia, as was mortality in the multivariate analysis. The risks for nonrecovery of renal function and for death were 6.94 and 6.92 times greater, respectively, for patients with hypomagnesemia. In hospitalized AIDS patients, hypomagnesemia is a risk factor for nonrecovery of renal function and for in-hospital mortality. To determine whether hypomagnesemia is a determinant or simply a marker of critical illness, further studies involving magnesium supplementation in AIDS patients are warranted.

Medication-induced hypophosphatemia: a review

Hypophosphatemia (serum phosphorus concentration <2.5 mg/dl, 0.8 mmol/l), although rare in the general population, is commonly observed in hospitalized patients and may be associated with drug therapy. In fact, hypophosphatemia frequently develops in the course of treatment with drugs used in every-day clinical practice including diuretics and bisphosphonates. Proper diagnostic approach of patients with low serum phosphorus concentrations should involve a detailed medical history with special attention to the recent use of medications. The clinical manifestations of drug-induced hypophosphatemia are usually mild but might also be severe and potentially life-threatening. This review aims at a thorough understanding of the underlying pathophysiological mechanisms and risk factors of drug therapy-related hypophosphatemia thus allowing prevention and effective intervention strategies.

Pharmacogenetics of tenofovir treatment

Tenofovir disoproxil fumarate (TDF) is a nucleotide analog used as part of HIV therapy. Its favorable profile in terms of high efficacy, low toxicity and once-daily dosing makes TDF one of the most attractive antiretroviral agents, and therefore, it is widely used. However, cases of kidney tubular dysfunction have been reported and concern exists regarding the long term use of TDF. Owing to the high interindividual variability in the presentation of kidney function abnormalities, research has recently focused on host genetic factors predisposing to TDF-associated renal dysfunction. Transporter proteins involved in the renal elimination of TDF, such as organic anion transporter 1 or multidrug resistant protein 2 or 4, seem to be involved importantly and several genetic polymorphisms in these proteins have been associated with an increased risk of kidney tubulopathy in patients treated with TDF. In this review, all relevant pharmacogenetic factors that may play a role in the risk of renal toxicity associated with the use of tenofovir are summarized.

Rosiglitazone reverses tenofovir-induced nephrotoxicity

Tenofovir disoproxil fumarate (TDF) is a first-line drug used in patients with highly active retroviral disease; however, it can cause renal failure associated with many tubular anomalies that may be due to down regulation of a variety of ion transporters. Because rosiglitazone, a peroxisome proliferator-activated receptor-gamma agonist induces the expression of many of these same transporters, we tested if the nephrotoxicity can be ameliorated by its use. High doses of TDF caused severe renal failure in rats accompanied by a reduction in endothelial nitric-oxide synthase and intense renal vasoconstriction; all of which were significantly improved by rosiglitazone treatment. Low-dose TDF did not alter glomerular filtration rate but produced significant phosphaturia, proximal tubular acidosis, polyuria and a reduced urinary concentrating ability. These alterations were caused by specific downregulation of the sodium-phosphorus cotransporter, sodium/hydrogen exchanger 3 and aquaporin 2. A Fanconi's-like syndrome was ruled out as there was no proteinuria or glycosuria. Rosiglitazone reversed TDF-induced tubular nephrotoxicity, normalized urinary biochemical parameters and membrane transporter protein expression. These studies suggest that rosiglitazone treatment might be useful in patients presenting with TFV-induced nephrotoxicity especially in those with hypophosphatemia or reduced glomerular filtration rate.

A murine transgenic model for transcriptional regulation of the Na/Pi-IIa major renal phosphate cotransporter

Levels of the type IIa Na/P(i) (Na/Pi-IIa) cotransporter, which serves as the principal mediator of phosphate reabsorption in the kidney, can be modulated through posttranscriptional or posttranslational mechanisms by dietary, hormonal, and pharmacological influences. Previous studies have not demonstrated clear-cut evidence for modulation of Na/Pi-IIa cotransporter levels through transcriptional mechanisms. We have previously demonstrated that a 4.7-kb rat genomic fragment upstream of the rodent Npt2 gene encoding the Na/Pi-IIa cotransporter, is sufficient to mediate its transcriptional activity in vitro (Shachaf C, Skorecki KL, Tzukerman M. Am J Physiol Renal Physiol 278: F406-F416, 2000). Accordingly, we have established an in vivo experimental model in which this Npt2 genomic fragment fused upstream of a Lac Z reporter gene was expressed as a transgene in mice. The nine independent transgenic founder lines generated exhibited Lac Z reporter gene expression specifically in the renal cortex. This renal cortical-specific expression driven by the Npt2 promoter was confirmed at the mRNA and protein levels using RT-PCR, histochemistry, and Lac Z enzymatic activity. Furthermore, the expression of the transgene correlated with expression of the endogenous Npt2 gene during embryonic and early postnatal development. Thus we have generated a transgenic mouse model which will enable in vivo investigation of the contribution of transcriptional mechanisms to the overall regulation of Na/Pi-IIa expression under physiological and pathophysiological conditions.