An expert update on novel therapeutic targets for hyperphosphatemia in chronic kidney disease: preclinical and clinical innovations

Hyperphosphatemia in Chronic Kidney Disease: The Search for New Treatment Paradigms and the Role of Tenapanor

Hyperphosphataemia represents a significant challenge in the management of chronic kidney disease, exerting a pronounced influence on the pathogenesis of cardiovascular complications and mineral bone disorders. Traditional approaches to address hyperphosphataemia involve implementing dietary phosphate restrictions, administering phosphate binders, and, in cases of end-stage renal disease, resorting to dialysis. Unfortunately, these interventions frequently prove inadequate in maintaining phosphate levels within recommended ranges. Additionally, commonly employed pharmacological agents are not immune to eliciting adverse events, thereby limiting their prescription and therapeutic adherence. There is a growing focus on exploring novel therapeutic strategies in this context. The current discussion centres on tenapanor, a pharmacological agent predominantly acting as a selective inhibitor of sodium/hydrogen exchanger isoform 3 (NHE3). Its mechanism of action involves modulating tight junctions, resulting in reduced sodium absorption and intestinal paracellular permeability to phosphate. Furthermore, tenapanor downregulates sodium-dependent phosphate 2b transport protein (NaPi2b) expression, thereby impeding active transcellular phosphate transport. Clinical trials have elucidated the efficacy and safety profile of tenapanor. This evidence hints at a potential paradigm shift in the management of hyperphosphataemia. However, the burgeoning optimism surrounding tenapanor warrants tempered enthusiasm, as further research remains indispensable. The imperative lies in meticulously delineating its efficacy and safety contours within the crucible of clinical practice. In this review, we synthesize the intricate interplay between hyperphosphataemia and Chronic Kidney Disease-Mineral Bone Disorder, and we discuss the existing pharmacological interventions for hyperphosphataemia and explore emerging treatment paradigms that offer novel perspectives in managing elevated phosphate levels in CKD patients.

The basics of phosphate metabolism

Phosphorus is an essential mineral that is, in the form of inorganic phosphate (Pi), required for building cell membranes, DNA and RNA molecules, energy metabolism, signal transduction and pH buffering. In bone, Pi is essential for bone stability in the form of apatite. Intestinal absorption of dietary Pi depends on its bioavailability and has two distinct modes of active transcellular and passive paracellular absorption. Active transport is transporter mediated and partly regulated, while passive absorption depends mostly on bioavailability. Renal excretion controls systemic Pi levels, depends on transporters in the proximal tubule and is highly regulated. Deposition and release of Pi into and from soft tissues and bone has to be tightly controlled. The endocrine network coordinating intestinal absorption, renal excretion and bone turnover integrates dietary intake and metabolic requirements with renal excretion and is critical for bone stability and cardiovascular health during states of hypophosphataemia or hyperphosphataemia as evident from inborn or acquired diseases. This review provides an integrated overview of the biology of phosphate and Pi in mammals.

Phosphate intake, hyperphosphatemia, and kidney function

Phosphate is essential in living organisms and its blood levels are regulated by a complex network involving the kidneys, intestine, parathyroid glands, and the skeleton. The crosstalk between these organs is executed primarily by three hormones, calcitriol, parathyroid hormone, and fibroblast growth factor 23. Largely due to a higher intake of ultraprocessed foods, dietary phosphate intake has increased in the last decades. The average intake is now about twice the recommended dietary allowance. Studies investigating the side effect of chronic high dietary phosphate intake suffer from incomplete dietary phosphate assessment and, therefore, often make data interpretation difficult. Renal excretion is quickly adapted to acute and chronic phosphate intake. However, at the high ends of dietary intake, renal adaptation, even in pre-existing normal kidney function, apparently is not perfect. Experimental intervention studies suggest that chronic excess of dietary phosphate can result in sustained higher blood phosphate leading to hyperphosphatemia. Evidence exists that the price of the homeostatic response (phosphaturia in response to phosphate loading/hyperphosphatemia) is an increased risk for declining kidney function, partly due by intraluminal/tubular calcium phosphate particles that provoke renal inflammation. High dietary phosphate intake and hyperphosphatemia are progression factors for declining kidney function and are associated with higher cardiovascular disease and mortality risk. This is best established for pre-existing chronic kidney disease, but epidemiological and experimental data strongly suggest that this holds true for subjects with normal renal function as well. Here, we review the latest advances in phosphate intake and kidney function decline.

Physiological regulation of phosphate homeostasis

Phosphate homeostasis is dependent on the interaction and coordination of four main organ systems: thyroid/parathyroids, gastrointestinal tract, bone and kidneys, and three key hormonal regulators, 1,25-hydroxyvitamin D3, parathyroid hormone and FGF23 with its co- factor klotho. Phosphorus is a critical nutritional element for normal cellular function, but in excess can be toxic to tissues, particularly the vasculature. As phosphate, it also has an important interaction and inter-dependence with calcium and calcium homeostasis sharing some of the same controlling hormones, although this is not covered in our article. We have chosen to provide a current overview of phosphate homeostasis only, focusing on the role of two major organ systems, the gastrointestinal tract and kidneys, and their contribution to the control of phosphate balance. We describe in some detail the mechanisms of intestinal and renal phosphate transport, and compare and contrast their regulation. We also consider a significant example of phosphate imbalance, with phosphate retention, which is chronic kidney disease; why consequent hyperphosphatemia is important, and some of the newer means of managing it.

Doctor-led intensive diet education on health-related quality of life in patients with chronic renal failure and hyperphosphatemia

BACKGROUND

Secondary hyperparathyroidism, renal osteodystrophy, and cardiovascular adverse events can occur if long-term hyperphosphatemia is not corrected, leading to the adverse prognosis of patients with chronic renal failure. Besides the use of phosphorus binders, clinical control measures for hyperphosphatemia in these patients should also incorporate diet control.

AIM

To observe doctor-led intensive diet education effects on health-related quality of life in patients with chronic renal failure and hyperphosphatemia.

METHODS

We assessed 120 patients with hyperphosphatemia and chronic renal failure on hemodialysis admitted to our hospital (July 2018 to March 2020). The control group (n = 60) was given routine nursing guidance, and the observation group (n = 60) was given doctor-led intensive diet education. The changes in EQ-5D-3L scores, disease-related knowledge, and compliance scores before intervention and 3 and 6 mo after intervention in the two groups were recorded. The levels of serum parathyroid hormone (iPTH), calcium (Ca), phosphorus (P), calcium-phosphorus product (Ca × P), serum creatinine (Scr), and blood urea nitrogen (BUN) before intervention and 3 and 6 mo after intervention in the two groups were assessed along with patient satisfaction.

RESULTS

There was no significant difference in blood iPTH, Ca, P, Ca × P, Scr, or BUN levels between the groups before intervention. After 3 and 6 mo of intervention, the blood iPTH, Ca, P, and Ca × P levels in the two groups decreased gradually (P < 0.05), but there were no significant differences in Scr or BUN. The blood iPTH, Ca, P, and Ca × P levels in the observation group were lower than those in the control group (P < 0.05). The satisfaction rate in the observation group after 3 mo was 93.33% and after 6, 90.00%, which was high compared with the 80.00% and 71.67%, respectively, in the control group (P < 0.05). There was no significant difference in EQ-5D-3L score between the two groups before intervention. After 3 and 6 mo of intervention, the visual analogue scale score of the two groups increased gradually (P < 0.05); and the scores of action ability, self-care, daily activities, pain and discomfort, and anxiety and depression decreased gradually (P < 0.05). The overall EQ-5D-3L score in the observation group was better than that in the control group (P < 0.05). There was no significant difference in disease-related knowledge or compliance scores between the groups before intervention. After 3 and 6 mo of intervention, the scores of disease, diet, and medication knowledge and compliance in the two groups increased gradually (P < 0.05). The scores of disease-related knowledge and compliance were higher in the observation group than in the control group (P < 0.05).

CONCLUSION

Doctor-led intensive diet education can improve patient satisfaction and the quality of life in patients with chronic renal failure and hyperphosphatemia and promote low-phosphorus diet behavior.

Phosphate and Cellular Senescence

Cellular senescence is one type of permeant arrest of cell growth and one of increasingly recognized contributor to aging and age-associated disease. High phosphate and low Klotho individually and synergistically lead to age-related degeneration in multiple organs. Substantial evidence supports the causality of high phosphate in cellular senescence, and potential contribution to human aging, cancer, cardiovascular, kidney, neurodegenerative, and musculoskeletal diseases. Phosphate can induce cellular senescence both by direct phosphotoxicity, and indirectly through downregulation of Klotho and upregulation of plasminogen activator inhibitor-1. Restriction of dietary phosphate intake and blockage of intestinal absorption of phosphate help suppress cellular senescence. Supplementation of Klotho protein, cellular senescence inhibitor, and removal of senescent cells with senolytic agents are potential novel strategies to attenuate phosphate-induced cellular senescence, retard aging, and ameliorate age-associated, and phosphate-induced disorders.

Nicotinamide and acute kidney injury

In a recent issue of ckj, Piedrafita et al. reported that urine tryptophan and kynurenine are reduced in cardiac bypass surgery patients that develop acute kidney injury (AKI), suggesting reduced activity of the kynurenine pathway of nicotinamide (NAM) adenine dinucleotide (NAD+) synthesis from tryptophan. However, NAM supplementation aiming at repleting NAD+ did not replete kidney NAD+ and did not improve glomerular filtration or reduce histological injury in ischaemic-reperfusion kidney injury in mice. The lack of improvement of kidney injury is partially at odds with prior reports that did not study kidney NAD+, glomerular filtration or histology in NAM-treated wild-type mice with AKI. We now present an overview of research on therapy with vitamin B3 vitamers and derivate molecules {niacin, Nicotinamide [NAM; niacinamide], NAM riboside [Nicotinamide riboside (NR)], Reduced nicotinamide riboside [NRH] and NAM mononucleotide} in kidney injury, including an overview of ongoing clinical trials, and discuss the potential explanations for diverging reports on the impact of these therapeutic approaches on pre-clinical acute and chronic kidney disease.

Multidisciplinary Perspectives of Current Approaches and Clinical Gaps in the Management of Hyperphosphatemia

Population-based studies have shown that most patients with advanced chronic kidney disease (CKD) do not have optimal phosphate levels. Meta-analyses suggest that there is a morbidity and mortality benefit associated with the lowering of serum phosphate levels. However, to date there is no conclusive evidence from randomized controlled trials (RCTs) that lowering serum phosphate levels reduces the risk of morbidity and mortality. However, hyperphosphatemia may pose a risk to patients and treatment should be considered. We therefore sought to conduct a multidisciplinary review to help guide clinical decision-making pending results of ongoing RCTs. Restricting dietary phosphate intake is frequently the first step in the management of hyperphosphatemia. Important considerations when proposing dietary restriction include the patient’s socioeconomic status, lifestyle, dietary preferences, comorbidities, and nutritional status. While dietary phosphate restriction may be a valid strategy in certain patients, serum phosphate reductions achieved solely by limiting dietary intake are modest and should be considered in conjunction with other interventions. Conventional dialysis is also typically insufficient; however phosphate removal may be augmented by increased frequency or duration of dialysis, or through enhanced methods such as hemodiafiltration. Phosphate binders have been shown to reduce absorption of dietary phosphate and lower serum phosphate levels. There are several phosphate binders available, and while they all lower phosphate levels to variable degrees, they differ with respect to their pill burden, potential to induce or exacerbate vascular calcification or ectopic calcification, tissue accumulation, safety, and tolerability. The widespread treatment of hyperphosphatemia requires convincing data from RCTs to ascertain whether lowering serum phosphate levels improves patient-important outcomes, as well as the optimal method and degree of phosphate control. In the interim, the decision and approach used to treat hyperphosphatemia should be based on the best available data, as well as patient needs and clinical judgment.

Inflammation: a putative link between phosphate metabolism and cardiovascular disease

Dietary habits in the western world lead to increasing phosphate intake. Under physiological conditions, extraosseous precipitation of phosphate with calcium is prevented by a mineral buffering system composed of calcification inhibitors and tight control of serum phosphate levels. The coordinated hormonal regulation of serum phosphate involves fibroblast growth factor 23 (FGF23), αKlotho, parathyroid hormone (PTH) and calcitriol. A severe derangement of phosphate homeostasis is observed in patients with chronic kidney disease (CKD), a patient collective with extremely high risk of cardiovascular morbidity and mortality. Higher phosphate levels in serum have been associated with increased risk for cardiovascular disease (CVD) in CKD patients, but also in the general population. The causal connections between phosphate and CVD are currently incompletely understood. An assumed link between phosphate and cardiovascular risk is the development of medial vascular calcification, a process actively promoted and regulated by a complex mechanistic interplay involving activation of pro-inflammatory signalling. Emerging evidence indicates a link between disturbances in phosphate homeostasis and inflammation. The present review focuses on critical interactions of phosphate homeostasis, inflammation, vascular calcification and CVD. Especially, pro-inflammatory responses mediating hyperphosphatemia-related development of vascular calcification as well as FGF23 as a critical factor in the interplay between inflammation and cardiovascular alterations, beyond its phosphaturic effects, are addressed.

PF-06869206 is a selective inhibitor of renal Pi transport: evidence from in vitro and in vivo studies

Plasma phosphate (P_i) levels are tightly controlled, and elevated plasma P_i levels are associated with an increased risk of cardiovascular complications and death. Two renal transport proteins mediate the majority of P_i reabsorption: Na⁺-phosphate cotransporters Npt2a and Npt2c, with Npt2a accounting for 70-80% of P_i reabsorption. The aim of the present study was to determine the in vitro effects of a novel Npt2a inhibitor (PF-06869206) in opossum kidney (OK) cells as well as determine its selectivity in vivo in Npt2a knockout (Npt2a^-/-) mice. In OK cells, Npt2a inhibitor caused dose-dependent reductions of Na⁺-dependent P_i uptake (IC₅₀: ~1.4 μmol/L), whereas the unselective Npt2 inhibitor phosphonoformic acid (PFA) resulted in an ~20% stronger inhibition of P_i uptake. The dose-dependent inhibitory effects were present after 24 h of incubation with both low- and high-P_i media. Michaelis-Menten kinetics in OK cells identified an ~2.4-fold higher K_m for P_i in response to Npt2a inhibition with no significant change in apparent V_max. Higher parathyroid hormone concentrations decreased P_i uptake equivalent to the maximal inhibitory effect of Npt2a inhibitor. In vivo, the Npt2a inhibitor induced a dose-dependent increase in urinary P_i excretion in wild-type mice (ED₅₀: ~23 mg/kg), which was completely absent in Npt2a^-/- mice, alongside a lack of decrease in plasma P_i. Of note, the Npt2a inhibitor-induced dose-dependent increase in urinary Na⁺ excretion was still present in Npt2a^-/- mice, a response possibly mediated by an off-target acute inhibitory effect of the Npt2a inhibitor on open probability of the epithelial Na⁺ channel in the cortical collecting duct.