In vivo effects of renal Npt2a inhibition

Abstract

Hyperphosphatemia is common in patients with chronic kidney disease and associated with increased mortality. Oral phosphate binders and dietary phosphate restriction are the current management protocols for patients with hyperphosphatemia; however, their effectiveness is insufficient. In the kidney, the sodium‐phosphate cotransporter Npt2a is responsible for bulk uptake of phosphate in the proximal tubule. Recently, an orally bioavailable selective Npt2a inhibitor (Npt2a‐I, PF‐06869206) has been described to reduce phosphate uptake in HEK cells transfected with mouse or rat Npt2a. So far, its physiological in vivo function has not been tested. Based on in vitro IC50 concentrations, we chose to study 30 mg/kg (oral gavage, 1% of body weight) in short‐term (3 hours) metabolic cage experiments in C57BL/6J mice. Compared to vehicle (n=14), bolus administration of Npt2a‐I (n=12) caused significantly higher (~4‐fold) urinary phosphate excretion (104±8 vs 27±6 μmol*min−1, P<0.05). Similarly, urinary phosphate/creatinine ratios were also significantly higher (32±2 vs 8±2 mmol*mmol−1, P<0.05). In addition, Npt2a‐I caused higher urinary excretion of calcium (9±1 vs 3±1 μmol*min−1, P<0.05), sodium (316±37 vs 113±24 μmol*min−1, P<0.05), and chloride (277±31 vs 91±24 μmol*min−1, P<0.05), as well as their respective creatinine ratios (Ca2+: 2.5±0.2 vs 0.8±0.1; Na+: 92±9 vs 31±6; Cl−: 81±8 vs 25±6 mmol*mmol−1; all P<0.05). In contrast, urinary flow rate, urinary potassium excretion, potassium/creatinine ratio, and urinary pH were not significantly different between vehicle and Npt2a‐I. In a different set of mice, we studied the effect of Npt2a‐I on plasma phosphate and calcium. Under baseline conditions, plasma phosphate and calcium levels were not significantly different between the vehicle and Npt2a‐I groups. Oral bolus administration of vehicle did not significantly change plasma phosphate (Δ 0.06±0.08 mmol/L, NS) or calcium (Δ −0.04±0.02 mmol/L, NS) 3 hours after application. In contrast, administration of Npt2a‐I caused a significant decrease in plasma phosphate (Δ −0.5±0.05 mmol/L, P<0.05) without affecting plasma calcium (Δ 0.01±0.03 mmol/L, NS). In summary, our study demonstrates for the first time that in vivo application of a novel Npt2a inhibitor efficiently increases urinary phosphate excretion leading to a decrease in plasma phosphate levels. Thus, inhibiting Npt2a might be a useful treatment strategy for hyperphosphatemia.Support or Funding InformationTR is supported by the National Institute of Diabetes and Digestive and Kidney Diseases (1R01DK110621). JX was supported by an American Heart Association Predoctoral Fellowship (18PRE33990236) and LT by a Postdoctoral Fellowship (19POST34400026).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.